Micromachining of Advanced Materials

Wayne N.P. Hung and Mike Corliss

## Abstract

Market needs often require miniaturized products for portability, size/weight reduction while increasing product capacity. Utilizing additive manufacturing to achieve a complex and functional metallic part has attracted considerable interests in both industry and academia. However, the resulted rough surfaces and low tolerances of as-printed parts require additional steps for microstructure modification, physical and mechanical properties enhancement, and improvement of dimensional/form/surface to meet engineering specifications. Micromachining can (i) produce miniature components or microfeatures on a larger component, and (ii) enhance the quality of additively manufactured metallic components. This chapter suggests the necessary requirements for successful micromachining and cites the research studies on micromachining of metallic materials fabricated by either traditional route or additive technique. Micromachining by nontraditional techniques—e.g., ion/electron beam machining—are beyond the scope of this chapter. The chapter is organized as following: Section 1: Introduction; Section 2: Requirement for successful micromachining: cutting tools, tool coating, machine tools, tool offset measuring methods, minimum quantity lubrication, and size effect; Section 3: Effect of materials: material defects, ductile regime machining, crystalline orientation, residual stress, and microstructure; Section 4: Micromachining: research works from literature, process monitoring, and process parameters; Section 4.1: Micromilling; Section 4.2: Microdrilling; Section 4.3: Ultraprecision turning; Section 5: Summary; and References.

Keywords: micromilling, microdrilling, ultraprecision turning, minimum quantity lubrication, additive manufacturing

## 1. Introduction

Recent technological advancement and market need for product miniaturization demand suitable processes to mass produce three-dimensional (3D) microcomponents. Although microelectronic manufacturing techniques can produce two-dimensional (2D) microdevices using silicon and other semiconducting materials, silicon is neither robust enough for demanding engineering applications nor biocompatible for biomedical applications. Biocompatible materials and superalloys are traditionally fabricated in bulk quantity by forging, casting, or extrusion. The recent explosion of additive manufacturing innovations has led to several revolutionary fabrication methods of engineering devices. Powder bed fusion techniques using energy beams or binding polymers to consolidate powders in

sequential layers are commonly used for metals. As in casting and welding, fabrication of a complex product by fusing re-solidified layers would introduce point, line, and volume defects in the part: dislocation entanglement, porosity, solidification shrinkage, microcrack, significant residual stress, anisotropy, rough surface finish, distortion, and undesirable microstructure are among key issues for metallic components fabricated by additively manufacturing route.

highest possible tool stiffness. The two most important geometries that affect the microtool stiffness are the tool diameter and flute length assuming the number of flutes have been chosen. It can be shown that the torsional stiffness of a mill/drill is

dimension, we must adjust the milling/drilling strategy accordingly to avoid tool

If we select a drill diameter of 0.2 mm instead of 0.5 mm, then such 60% reduction of diameter will result in a reduction in torsional stiffness ΔE of:

4

<sup>4</sup> <sup>¼</sup> <sup>0</sup>:<sup>24</sup> � <sup>0</sup>:5<sup>4</sup>

Similarly, if we choose the flute length of 1.2 mm instead of 1.0 mm, this 20%

ð Þ <sup>L</sup><sup>1</sup> �<sup>2</sup> <sup>¼</sup> <sup>1</sup>:2�<sup>2</sup> � <sup>1</sup>:0�<sup>2</sup>

Machining parameters that are successfully used in macromachining are not necessarily applicable for micromachining. A published literature recommends milling speed of 178 m/min and chip load of 0.1 mm/tooth for end milling 316L

• Macromachining: to have the said surface speed for an Ø25.4 mm end mill, the

• Micromachining: using the same surface speed for an Ø0.1 mm micromill, the

A machine tool with spindle speed exceeding 500,000 rpm is rare or simply not commercially available at this time. Applying the recommended macro chip load of 0.1 mm/tooth for an Ø0.1 mm micromill would break the fragile tool since the

The tool edge radius is critical in micromachining. If the depth of cut (or chip load) is too shallow, the tool simply plows the material and pushes it away elastically. This elastic material layer just springs back after the tool passing. If the depth of cut (or chip load) is substantial, then a chip is formed and a new machined surface is generated. Typical fine grain carbide tools are first sintered from submicron carbide particles in a cobalt matrix, and then ground and lapped to final

<sup>π</sup>ð Þ� rad=rev <sup>25</sup>:4 mm ð Þ � 1000 mm ð Þ¼ <sup>=</sup><sup>m</sup> 2230 rpm (3)

<sup>π</sup>ð Þ� rad=rev <sup>0</sup>:1 mm ð Þ � 1000 mm ð Þ¼ <sup>=</sup><sup>m</sup> 555, 600 rpm (4)

<sup>4</sup> � ð Þ <sup>D</sup><sup>1</sup>

change in flute length will lead to a decrease in torsional stiffness ΔE of:

ð Þ D<sup>1</sup>

<sup>Δ</sup><sup>E</sup> <sup>¼</sup> ð Þ <sup>L</sup><sup>2</sup> �<sup>2</sup> � ð Þ <sup>L</sup><sup>1</sup> �<sup>2</sup>

. For a specific mill/drill tool

<sup>0</sup>:<sup>54</sup> ¼ �97% (1)

<sup>1</sup>:0�<sup>2</sup> ¼ �30% (2)

proportional to (tool diameter)<sup>4</sup> and (flute length)�<sup>2</sup>

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

<sup>Δ</sup><sup>E</sup> <sup>¼</sup> ð Þ <sup>D</sup><sup>2</sup>

stainless steel using uncoated carbide tool.<sup>1</sup>

<sup>π</sup><sup>D</sup> <sup>¼</sup> 178 mð Þ <sup>=</sup> min

<sup>π</sup><sup>D</sup> <sup>¼</sup> 178 mð Þ <sup>=</sup> min

feed/tooth is as large as the microtool diameter.

Machinery's Handbook, 28 ed., Industrial Press, 2008.

required spindle speed is:

new spindle speed is:

<sup>N</sup> <sup>¼</sup> <sup>V</sup>

<sup>N</sup> <sup>¼</sup> <sup>V</sup>

2.2 Tool sharpness

1

5

breakage.

Micromachining techniques can be applied to successfully fabricate engineering components—either in meso or micro scales—from robust or biocompatible bulk materials. Micromachining is also among the key post processing techniques to enhance the quality of additively built metallic components [1–3]. This book chapter provides necessary requirements for micromachining, and cites research studies on micromachining of metallic materials fabricated by traditional or additive techniques.

## 2. Requirement for successful micromachining

To obtain the same surface speed as in macromachining, a machine tool must:


Success of micromachining depends on tool quality and precision of a machine tool. Machine spindle runout, tool concentricity and tool positioning accuracy must be in the neighborhood of 1/100 of a microtool diameter or less for successful operation. Tolerance stack up for spindle runout, tool eccentricity, and wandering of a microtool cause cyclic bending of a tool that leads to a catastrophic failure. At a low rotating speed, the displacement of a spindle can be monitored with a sensitive mechanical indicator. However, this option is not applicable for machines that operates at few thousands rpm or above. Other non-contact techniques using capacitance, magnetism, or light would be more appropriate. A laser beam can be focused on a rotating precision plug gage. The spindle displacement is then recorded on a computer for further analysis and is displayed in either frequency or time domain. Commercial laser systems can provide displacement reading to 10 nm resolution.

## 2.1 Size effect

The parameters for machining and tooling that are successfully applied in macromachining do not necessarily scale down linearly for micromachining. It is relatively easy to have a rigid turning or facing microtool, but it would require careful planning to maintain rigidity of a high aspect ratio micromill or a microdrill. Geometries of macroscale and microscale drilling/milling tools are the same: tool diameter, number of cutting flutes, point included angle for microdrill, helix angle, web thickness, clearance angle, flute length, shank diameter, and overall length. A careful selection of microtools must consider the intended machined features and Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

sequential layers are commonly used for metals. As in casting and welding, fabrication of a complex product by fusing re-solidified layers would introduce point, line, and volume defects in the part: dislocation entanglement, porosity, solidification shrinkage, microcrack, significant residual stress, anisotropy, rough surface finish, distortion, and undesirable microstructure are among key issues for metallic com-

Micromachining techniques can be applied to successfully fabricate engineering components—either in meso or micro scales—from robust or biocompatible bulk materials. Micromachining is also among the key post processing techniques to enhance the quality of additively built metallic components [1–3]. This book chapter provides necessary requirements for micromachining, and cites research studies on micromachining of metallic materials fabricated by traditional or

To obtain the same surface speed as in macromachining, a machine tool must:

c. Have very robust mechanical and thermal structure that does not affect by

Success of micromachining depends on tool quality and precision of a machine tool. Machine spindle runout, tool concentricity and tool positioning accuracy must be in the neighborhood of 1/100 of a microtool diameter or less for successful operation. Tolerance stack up for spindle runout, tool eccentricity, and wandering of a microtool cause cyclic bending of a tool that leads to a catastrophic failure. At a low rotating speed, the displacement of a spindle can be monitored with a sensitive mechanical indicator. However, this option is not applicable for machines that operates at few thousands rpm or above. Other non-contact techniques using capacitance, magnetism, or light would be more appropriate. A laser beam can be focused on a rotating precision plug gage. The spindle displacement is then recorded on a computer for further analysis and is displayed in either frequency or time domain. Commercial laser systems can provide displacement reading to 10 nm

The parameters for machining and tooling that are successfully applied in macromachining do not necessarily scale down linearly for micromachining. It is relatively easy to have a rigid turning or facing microtool, but it would require careful planning to maintain rigidity of a high aspect ratio micromill or a microdrill. Geometries of macroscale and microscale drilling/milling tools are the same: tool diameter, number of cutting flutes, point included angle for microdrill, helix angle, web thickness, clearance angle, flute length, shank diameter, and overall length. A careful selection of microtools must consider the intended machined features and

d. Have high resolution tool positioning and feeding mechanisms.

a. Be capable to rotate a workpiece or tool at high rotation speeds at 25,000 rpm

ponents fabricated by additively manufacturing route.

2. Requirement for successful micromachining

b. Control spindle runout to submicron level,

vibration or thermal drift, and

additive techniques.

Micromachining

or above,

resolution.

4

2.1 Size effect

highest possible tool stiffness. The two most important geometries that affect the microtool stiffness are the tool diameter and flute length assuming the number of flutes have been chosen. It can be shown that the torsional stiffness of a mill/drill is proportional to (tool diameter)<sup>4</sup> and (flute length)�<sup>2</sup> . For a specific mill/drill tool dimension, we must adjust the milling/drilling strategy accordingly to avoid tool breakage.

If we select a drill diameter of 0.2 mm instead of 0.5 mm, then such 60% reduction of diameter will result in a reduction in torsional stiffness ΔE of:

$$
\Delta E = \frac{\left(D\_2\right)^4 - \left(D\_1\right)^4}{\left(D\_1\right)^4} = \frac{0.2^4 - 0.5^4}{0.5^4} = -97\% \tag{1}
$$

Similarly, if we choose the flute length of 1.2 mm instead of 1.0 mm, this 20% change in flute length will lead to a decrease in torsional stiffness ΔE of:

$$
\Delta E = \frac{\left(L\_2\right)^{-2} - \left(L\_1\right)^{-2}}{\left(L\_1\right)^{-2}} = \frac{\mathbf{1.2}^{-2} - \mathbf{1.0}^{-2}}{\mathbf{1.0}^{-2}} = -\mathbf{30}\,\%\tag{2}
$$

Machining parameters that are successfully used in macromachining are not necessarily applicable for micromachining. A published literature recommends milling speed of 178 m/min and chip load of 0.1 mm/tooth for end milling 316L stainless steel using uncoated carbide tool.<sup>1</sup>

• Macromachining: to have the said surface speed for an Ø25.4 mm end mill, the required spindle speed is:

$$N = \frac{V}{\pi D} = \frac{178 \text{ (m/min)}}{\pi (\text{rad/rev}) \times 25.4 \text{ (mm)}} \times 1000 \text{ (mm/m)} = 2230 \text{ rpm} \tag{3}$$

• Micromachining: using the same surface speed for an Ø0.1 mm micromill, the new spindle speed is:

$$N = \frac{V}{\pi D} = \frac{178 \text{ (m/min)}}{\pi (\text{rad/rev}) \times 0.1 \text{ (mm)}} \times 1000 \text{ (mm/m)} = 555,600 \text{ rpm} \tag{4}$$

A machine tool with spindle speed exceeding 500,000 rpm is rare or simply not commercially available at this time. Applying the recommended macro chip load of 0.1 mm/tooth for an Ø0.1 mm micromill would break the fragile tool since the feed/tooth is as large as the microtool diameter.

#### 2.2 Tool sharpness

The tool edge radius is critical in micromachining. If the depth of cut (or chip load) is too shallow, the tool simply plows the material and pushes it away elastically. This elastic material layer just springs back after the tool passing. If the depth of cut (or chip load) is substantial, then a chip is formed and a new machined surface is generated. Typical fine grain carbide tools are first sintered from submicron carbide particles in a cobalt matrix, and then ground and lapped to final

<sup>1</sup> Machinery's Handbook, 28 ed., Industrial Press, 2008.

geometry. Optimal edge radii of 1–4 μm are typically designed for sintered tools to balance edge sharpness and edge strength. Only single crystalline diamond tools can be ground and lapped to form edge radii within nanometer range.

polycrystalline diamond (PCD), and single crystalline diamond (SCD). The HSS is not used in micromachining of metal since it does not have required hardness and strength to resist plastic deformation. A SCD tool is available for microturning, but not for microdrilling or micromilling. Carbide and cermet, having properties between HSS and diamond, are most suitable for microcutting tools. They are sintered from random abrasive grains in either cobalt or nickel binder with a small addition of molybdenum or chromium. A higher binder content increases the tool toughness and crack resistance, but reduces the bulk tool hardness. Having ultrafine grain (submicron size) abrasives in a lesser amount of binder is the optimal solution since a tool with a submicron carbide grains can maintain a high hardness while improving its crack resistance against chattering, interrupted cut, or cyclic

Microtool failure modes include shearing, chipping, and wear. To minimize shearing and catastrophic tool failure, a tool should be made from a high hardness substrate and its geometry would be suitable for micromachining, i.e., large

included angle and sharp cutting edge. A tool with smaller than minimum included

Coating of microtool is still a technical challenge due to conflicting constraints for tool performance. Chemical or physical vapor deposition (CVD or PVD) techniques have been developed to coat cutting tools with mono/multiple layers of intermetallic or ceramic compounds (Table 1). Criteria for acceptable tool coating are numerous: uniformity of coating thickness, high hardness, high toughness, low friction, high wear resistance, surface smoothness, high chemical/diffusion resistance, and high temperature stability at a reasonable cost [6]. Although a coating thickness of 2–4 μm is acceptable for a macrotool, coating thickness on a microtool should be thinner, in the range 1–2 μm, to minimize fracture and peeling of the coating (Figure 2). Both CVD and PVD processes not only add the coating thickness to an edge radius, but they also increase the radius due to extra coating at a sharp corner. This is unfortunate since the thicker coating reduces the tool sharpness by enlarging the tool edge radius and causes an unfavorable plowing effect with negative effective rake angle. An uncoated microtool might perform satisfactorily, but the same machining parameter can be devastating to an over-coated microtool. Although the Calo destructive test can be used to measure coating thickness on a large object [8], it is more practical and convenient to measure coating thickness on an expensive microtool nondestructively.

> Coefficient of friction

TiN Monolayer 24 0.55 1–5 600 TiCN Gradient 37 0.20 1–4 400 TiAlCN Gradient 28 0.30 1–4 500 TiAlN Multilayer 28 0.60 1–4 700 AlTiN Gradient 38 0.70 1–3 900 ZrN Monolayer 20 0.40 1–4 550 CrN Monolayer 18 0.30 1–4 700 Diamond like Gradient 20 0.15 0.5–1.5 400 AlTiN/Si3N4 Nanocomposite 45 0.45 1–4 1200 AlCrN/Si3N4 Nanocomposite 42 0.35 1–5 1100

Coating thickness (μm)

Maximum temperature (°C)

deflection due to spindle runout.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

angle will be deformed and fractured in service.

Coating Structure Hardness

Table 1.

7

Commercial coating for microtools.

(GPa)

The threshold depth has been investigated theoretically and verified experimentally by many researchers. It varies from 5 to 40% of the tool edge radius depending on the workpiece material and original rake angles. A depth of cut (or chip load), therefore, can be conservatively set to be 50% of the tool edge radius. When machining below this threshold, a microtool just rubs the surface and deforms it elastically during the first pass. When machining with depth of cut below the critical level, the material is then being plowed at negative effective rake angle. This results in high cutting force, high specific energy, fast tool wear, rough surface finish, and significant burr [4]. In subsequent passes when the cumulative depth is above the critical depth of cut, then a tool can remove materials as chips and the cycle repeats.

It is crucial to verify the tool edge radius before deciding on cutting parameters. Measuring of tool edge radius, however, is not trivial. A tool edge radius can be estimated from a scanning electron microscopic picture when the cutting edge is parallel to the electron beam axis [5], or scanning probe microscopic picture using a probe to scan the neighborhood of a cutting edge (Figure 1), or by scanning the edge on an optical microscope profiler in different views to reconstruct a 3D image of an tool edge before finding its radius.

## 2.3 Tool materials

Having the right microtool is essential for micromachining. A microtool that successfully drills through holes on a plastic printed circuit board does not necessarily be able to drill deep blind holes on titanium alloys. Understand the requirement and select the right microtool for each condition would save time, money, and frustration.

It has been theoretically derived and experimentally proven that the smaller the chip is, then the higher the required stress will be. Microcutting tools, therefore, have to be designed for higher stress with extreme geometrical constraints. When depth of cut is smaller than the average grain size of a workpiece, each grain with different orientation generates different stress on a cutting edge and eventually fatigues the tool.

Microtools as small as 25 μm are commercially available. Common tool materials are high speed steel (HSS), cermet, carbide, cubic boron nitride (CBN),

Tool edge radii of (a) 750 nm on a new polycrystalline diamond tool and (b) 10 nm on a new single crystalline diamond tool.

## Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

geometry. Optimal edge radii of 1–4 μm are typically designed for sintered tools to balance edge sharpness and edge strength. Only single crystalline diamond tools can

The threshold depth has been investigated theoretically and verified experimentally by many researchers. It varies from 5 to 40% of the tool edge radius depending on the workpiece material and original rake angles. A depth of cut (or chip load), therefore, can be conservatively set to be 50% of the tool edge radius. When machining below this threshold, a microtool just rubs the surface and deforms it elastically during the first pass. When machining with depth of cut below the critical level, the material is then being plowed at negative effective rake angle. This results in high cutting force, high specific energy, fast tool wear, rough surface finish, and significant burr [4]. In subsequent passes when the cumulative depth is above the critical depth of cut, then a tool can remove materials as chips and the

It is crucial to verify the tool edge radius before deciding on cutting parameters. Measuring of tool edge radius, however, is not trivial. A tool edge radius can be estimated from a scanning electron microscopic picture when the cutting edge is parallel to the electron beam axis [5], or scanning probe microscopic picture using a probe to scan the neighborhood of a cutting edge (Figure 1), or by scanning the edge on an optical microscope profiler in different views to reconstruct a 3D image

Having the right microtool is essential for micromachining. A microtool that successfully drills through holes on a plastic printed circuit board does not necessarily be able to drill deep blind holes on titanium alloys. Understand the requirement and select the right microtool for each condition would save time, money, and

It has been theoretically derived and experimentally proven that the smaller the chip is, then the higher the required stress will be. Microcutting tools, therefore, have to be designed for higher stress with extreme geometrical constraints. When depth of cut is smaller than the average grain size of a workpiece, each grain with different orientation generates different stress on a cutting edge and eventually

Microtools as small as 25 μm are commercially available. Common tool materials

Tool edge radii of (a) 750 nm on a new polycrystalline diamond tool and (b) 10 nm on a new single crystalline

are high speed steel (HSS), cermet, carbide, cubic boron nitride (CBN),

be ground and lapped to form edge radii within nanometer range.

cycle repeats.

Micromachining

2.3 Tool materials

frustration.

fatigues the tool.

Figure 1.

6

diamond tool.

of an tool edge before finding its radius.

polycrystalline diamond (PCD), and single crystalline diamond (SCD). The HSS is not used in micromachining of metal since it does not have required hardness and strength to resist plastic deformation. A SCD tool is available for microturning, but not for microdrilling or micromilling. Carbide and cermet, having properties between HSS and diamond, are most suitable for microcutting tools. They are sintered from random abrasive grains in either cobalt or nickel binder with a small addition of molybdenum or chromium. A higher binder content increases the tool toughness and crack resistance, but reduces the bulk tool hardness. Having ultrafine grain (submicron size) abrasives in a lesser amount of binder is the optimal solution since a tool with a submicron carbide grains can maintain a high hardness while improving its crack resistance against chattering, interrupted cut, or cyclic deflection due to spindle runout.

Microtool failure modes include shearing, chipping, and wear. To minimize shearing and catastrophic tool failure, a tool should be made from a high hardness substrate and its geometry would be suitable for micromachining, i.e., large included angle and sharp cutting edge. A tool with smaller than minimum included angle will be deformed and fractured in service.

Coating of microtool is still a technical challenge due to conflicting constraints for tool performance. Chemical or physical vapor deposition (CVD or PVD) techniques have been developed to coat cutting tools with mono/multiple layers of intermetallic or ceramic compounds (Table 1). Criteria for acceptable tool coating are numerous: uniformity of coating thickness, high hardness, high toughness, low friction, high wear resistance, surface smoothness, high chemical/diffusion resistance, and high temperature stability at a reasonable cost [6]. Although a coating thickness of 2–4 μm is acceptable for a macrotool, coating thickness on a microtool should be thinner, in the range 1–2 μm, to minimize fracture and peeling of the coating (Figure 2). Both CVD and PVD processes not only add the coating thickness to an edge radius, but they also increase the radius due to extra coating at a sharp corner. This is unfortunate since the thicker coating reduces the tool sharpness by enlarging the tool edge radius and causes an unfavorable plowing effect with negative effective rake angle. An uncoated microtool might perform satisfactorily, but the same machining parameter can be devastating to an over-coated microtool. Although the Calo destructive test can be used to measure coating thickness on a large object [8], it is more practical and convenient to measure coating thickness on an expensive microtool nondestructively.


## Table 1.

Non-contact techniques, although are generally more expensive, can provide a

• Commercial laser displacement sensor with 20 μs sampling rate (50 kHz) would be sufficient for most cases. Submicron accuracy and be achieved, but the sensor's repeatability depends on the repeatability of multiple axes of a machine tool. Both lateral and axial tool offsets have been successfully used

• Other non-contact techniques using magnetism, capacitance, ultrasound… could be used depending on the required accuracy and the workpiece

Tool damage can be categorized by the relative size of the damage, ranging from submicron to hundreds of microns (Table 2). The tool failure mechanisms include

Mechanical effect is the most common source of tool damage. Abrasive wear is caused by low speed sliding of hard particles from workpiece or tool against the cutting tool surface (Figure 3). At a high cutting speed and lack of sufficient coolant/lubricant, the high temperature at tool cutting edge accelerates the tool wear due to increasing rate of diffusion and/or chemical reaction at the coating layer and the substrate below. Figures 5a and 5b compare the wear of PVD coated TiAlN/TiN layer on WC tool when machining at 180 m/min; the material contrast in scanning electron microscopy highlights the faster wear rate of the coating layer

• Attrition wear is larger than abrasion wear. This happens when one for few grains of the tool are weakened at their grain boundaries and dislodged from

• Microchipping and chipping happen when larger chunks of tool being removed due to mechanical or thermal shocks upon loading and unloading (Figure 4). Machining at optimal parameters and rigid setup would reduce vibration, shock, and mechanical damage to a microtool. Chipping can occur due to high stress when machining at excessive cutting speed and feed [11]. Tool chipping also starts with microcracks due to chemical reaction among tool coating

Microtool damage Damage size (μm) Mechanism Abrasion <1 Mechanical, thermal Attrition 1–3 Mechanical, thermal Peeling 1–3 Mechanical, chemical Microchipping 3–10 Mechanical, adhesion Chipping 10–30 Mechanical Fracture >100 Mechanical

damages due to mechanical, thermal, chemical effects, or adhesion.

when machining 3D printed titanium alloy [10].

material, workpiece material, and coolant/lubricant.

satisfactory accuracy and repeatability.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

with this technique.

materials.

2.5 Tool damage

the tool.

Table 2.

9

Categories of tool damage.

Figure 2. Effect of tool coating thickness on tool life. TiC coated WC tool in interrupted cut [7].

Commercial instruments are available for coating thickness measurement using x-ray, magnetism, Eddy current, or ultrasound. A thin coating less than 1.5 μm following by an edge sharpening process would improve the tool performance, however, at the expense of higher tool cost. Published data indicate that micrograin carbide tools with 1.5 μm-TiN coating is the best for micromilling of H13 tool steel that has been hardened to 45 HRc.

## 2.4 Tool offset and positioning

Tool offset and tool positioning are crucial in micromilling and microdrilling since a high aspect ratio tool is small and extremely vulnerable. Selection of a suitable sensor for tool offsetting and tool positioning depends at least on following criteria:


Contact techniques must be used with care for positioning a microtool. Common shop practices to find tool offsetting and positioning often damage a microtool or workpiece.


Non-contact techniques, although are generally more expensive, can provide a satisfactory accuracy and repeatability.


## 2.5 Tool damage

Commercial instruments are available for coating thickness measurement using x-ray, magnetism, Eddy current, or ultrasound. A thin coating less than 1.5 μm following by an edge sharpening process would improve the tool performance, however, at the expense of higher tool cost. Published data indicate that micrograin carbide tools with 1.5 μm-TiN coating is the best for micromilling of H13 tool steel

Effect of tool coating thickness on tool life. TiC coated WC tool in interrupted cut [7].

Tool offset and tool positioning are crucial in micromilling and microdrilling since

Contact techniques must be used with care for positioning a microtool. Common shop practices to find tool offsetting and positioning often damage a microtool or

• A mechanical edge finder is adequate for most macromachining setup, but it is not suitable for micromachining especially with small and pliable part.

workpiece has been used with moderate success. A pulsed current might spark

• Measuring resistance or current flow when a tool touching a conductive

• An accelerometer can be mounted on either a workpiece or tool spindle housing. The difference in vibration signals indicates contact of tool and workpiece. The vibration signal, however, depends on the material of workpiece and tool, their surface roughness and detection threshold [9].

and damage the sharp edges of a microtool.

a high aspect ratio tool is small and extremely vulnerable. Selection of a suitable sensor for tool offsetting and tool positioning depends at least on following criteria:

• The sensor has better resolution compared to that of machine tool axis.

• The sensor should have a small working zone to cover a microtool.

• The sensor can perform at fast sampling rate for intended tool speed.

that has been hardened to 45 HRc.

2.4 Tool offset and positioning

workpiece.

8

Figure 2.

Micromachining

Tool damage can be categorized by the relative size of the damage, ranging from submicron to hundreds of microns (Table 2). The tool failure mechanisms include damages due to mechanical, thermal, chemical effects, or adhesion.

Mechanical effect is the most common source of tool damage. Abrasive wear is caused by low speed sliding of hard particles from workpiece or tool against the cutting tool surface (Figure 3). At a high cutting speed and lack of sufficient coolant/lubricant, the high temperature at tool cutting edge accelerates the tool wear due to increasing rate of diffusion and/or chemical reaction at the coating layer and the substrate below. Figures 5a and 5b compare the wear of PVD coated TiAlN/TiN layer on WC tool when machining at 180 m/min; the material contrast in scanning electron microscopy highlights the faster wear rate of the coating layer when machining 3D printed titanium alloy [10].



Table 2. Categories of tool damage.

diffuses to steel due to the steel's lower carbon content and its high affinity to carbon. A tool would extend its useful life by applying proper coolant to reduce thermal damage, or having a protective coating that blocks undesirable thermal diffusion from/to a tool surface. At higher cutting speed, the thermal/diffusion wear is the main tool wear mechanism. Combination of abrasive and thermal wear can be present when both high cutting speed and material hardness are combined. After machining at a high cutting speed of 180 m/min, severe coating tool wear (Figure 5b) is observed after cutting the harder selective-laser-melted titanium alloy compared to

that when machining the same but softer extruded material (Figure 5a).

leads to peeling and chipping of the coating.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

BUE while reducing its weldability to tool surface.

Figure 6.

11

Built-up-edge and its effects [12].

its environment like air, cutting fluid, or workpiece material. Tool oxidation is common when cutting in air at high speed. An oxidation reaction is accelerated with temperature, but can be eliminated when using inert gas to shield the cutting tool from surrounding oxygen. A chain reaction can also occur and further degrade a tool. For example, iron in steel is first oxidized at high cutting temperature to form iron oxide; this iron oxide then weakens the aluminum oxide coating of a tool and

Chemical damage of a tool is due to chemical reaction between tool material and

Adhesion tool damage happens when a built-up-edge (BUE) welds strongly to a tool surface and then breaks away with a minute amount of tool material. Some of the BUE deposits on the back of a chip, but some can be on the machined surface thus degrading the workpiece quality (Figure 6). When machining soft materials, a chip tends to adhere to the tool and grows in size (Figure 7). When such cumulative BUE is large and becomes unstable, it is removed with chip while shearing off part of the cutting tool due to the higher adhesion strength of BUE and tool interface than the inter-grain binding strength. Stainless steel, nickel and titanium alloys are known for causing adhesion wear on carbide microtools (Figure 8). Adhesion damage can be reduced by using proper lubricant to reduce friction between chip and tool, by coating tool with a smooth and low friction layer, by reducing tool edge radius, or by increasing cutting speed to raise tool surface temperature and soften

A thick tool coating, although possessing a thicker diffusion barrier, can fail prematurely due to excessive shear stress at the interface (Figure 9). A 5 μm thick coating is common for large carbide insert, but 1–2 μm thin coating is recommended for microtools (Figure 2). Failure of microtools can happen due to combination of the above mechanisms. For example, peeling of tool coating might be due to coating defects or mechanical mechanism when a large gradient of stress exists across a

Figure 3. Abrasive wear on a WC microdrill.

## Figure 4.

Chipping of cutting edge. AlTiN coated micromilling tool.

#### Figure 5.

Wear of coating tool after machining Ti 6Al 4V at 180 m/min on (a) wrought material and (b) selective laser melted material [10].

Thermal effect is the second cause of tool damage. A cutting tool edge is softened at high machining temperature, deformed plastically, and removed from the tool. Both high speed steel tool and carbide tool with high cobalt content are vulnerable to thermal damage. High temperature also promotes diffusion, i.e., atoms from the tool and workpiece move mutually across their interfaces, therefore degrade their properties and cause diffusion wear. Diamond with a carbon-rich matrix, or diamond-like coated tool, cannot be used with low-carbon ferrous alloy like steels or stainless steels since diamond carbonizes at temperature exceeding 600°C and

## Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

diffuses to steel due to the steel's lower carbon content and its high affinity to carbon. A tool would extend its useful life by applying proper coolant to reduce thermal damage, or having a protective coating that blocks undesirable thermal diffusion from/to a tool surface. At higher cutting speed, the thermal/diffusion wear is the main tool wear mechanism. Combination of abrasive and thermal wear can be present when both high cutting speed and material hardness are combined. After machining at a high cutting speed of 180 m/min, severe coating tool wear (Figure 5b) is observed after cutting the harder selective-laser-melted titanium alloy compared to that when machining the same but softer extruded material (Figure 5a).

Chemical damage of a tool is due to chemical reaction between tool material and its environment like air, cutting fluid, or workpiece material. Tool oxidation is common when cutting in air at high speed. An oxidation reaction is accelerated with temperature, but can be eliminated when using inert gas to shield the cutting tool from surrounding oxygen. A chain reaction can also occur and further degrade a tool. For example, iron in steel is first oxidized at high cutting temperature to form iron oxide; this iron oxide then weakens the aluminum oxide coating of a tool and leads to peeling and chipping of the coating.

Adhesion tool damage happens when a built-up-edge (BUE) welds strongly to a tool surface and then breaks away with a minute amount of tool material. Some of the BUE deposits on the back of a chip, but some can be on the machined surface thus degrading the workpiece quality (Figure 6). When machining soft materials, a chip tends to adhere to the tool and grows in size (Figure 7). When such cumulative BUE is large and becomes unstable, it is removed with chip while shearing off part of the cutting tool due to the higher adhesion strength of BUE and tool interface than the inter-grain binding strength. Stainless steel, nickel and titanium alloys are known for causing adhesion wear on carbide microtools (Figure 8). Adhesion damage can be reduced by using proper lubricant to reduce friction between chip and tool, by coating tool with a smooth and low friction layer, by reducing tool edge radius, or by increasing cutting speed to raise tool surface temperature and soften BUE while reducing its weldability to tool surface.

A thick tool coating, although possessing a thicker diffusion barrier, can fail prematurely due to excessive shear stress at the interface (Figure 9). A 5 μm thick coating is common for large carbide insert, but 1–2 μm thin coating is recommended for microtools (Figure 2). Failure of microtools can happen due to combination of the above mechanisms. For example, peeling of tool coating might be due to coating defects or mechanical mechanism when a large gradient of stress exists across a

Figure 6. Built-up-edge and its effects [12].

Thermal effect is the second cause of tool damage. A cutting tool edge is softened at high machining temperature, deformed plastically, and removed from the tool. Both high speed steel tool and carbide tool with high cobalt content are vulnerable to thermal damage. High temperature also promotes diffusion, i.e., atoms from the tool and workpiece move mutually across their interfaces, therefore degrade their properties and cause diffusion wear. Diamond with a carbon-rich matrix, or diamond-like coated tool, cannot be used with low-carbon ferrous alloy like steels or stainless steels since diamond carbonizes at temperature exceeding 600°C and

Wear of coating tool after machining Ti 6Al 4V at 180 m/min on (a) wrought material and (b) selective laser

Figure 3.

Micromachining

Figure 4.

Figure 5.

10

melted material [10].

Abrasive wear on a WC microdrill.

Chipping of cutting edge. AlTiN coated micromilling tool.

tool wear, while the nickel supper alloy Inconel 718 damages tools by all wear

while evacuating tiny chips from the machined surface.

Shearing of workpiece material and relative motion between tool and chip generate a significance of heat during machining. This thermal energy could change the microstructure, plastically deform the subsurface, degrade the part quality and wear a cutting tool quickly. Cutting fluids, either oil based for lubrication or water based for cooling, should be applied appropriately for effective micromachining

• Dry machining. Although simple, dry machining is not appropriate since it neither reduce the heat, extend tool life, nor removing chips that may interfere

• Flood cooling. A large amount of fluid can cover a tool and workpiece, but it is not effective since the bulk liquid cannot penetrate the air boundary layer surrounding a rapidly rotate micro drilling/milling tool-typically in the range of 30,000–120,000 rpm. Increasing the flood cooling pressure, as in jet cooling, simply deflect a fragile microtool and affect the machining quality.

• Minimum quantity lubrication (MQL). A mixture of oil and compressed air is very effective for micromachining when operating at high pressure above 4 bars (400 kPa, 60 psi). The micron-sized oil droplets can be propelled at high speed to penetrate the air boundary layer, adhere to workpiece/tool zones, spread out by surface tension to effectively cool and lubricate the tool/chip interface. Correct applications of MQL extend tool life while reducing burr as reported in published literature. Advanced MQL systems include additives (lignin, nano-sized diamond particle, graphene, etc.) can further enhance the

• Cryogenic cooling. Rapid freezing of most metals at liquid nitrogen

Workpiece materials must meet certain conditions for successful

materials with very fine and uniform grain sizes should be chosen for

temperature (196°C) would embrittle the materials, reduce the required energy for machining and burr formation at the expense of tool wear [15]. This expensive technique, however, requires proper insulation of tooling and

Micromachining is often utilized to fabricate components for miniaturized sensors, medical, optical, and electronic devices. Common engineering materials for these applications include stainless steel, aluminum, titanium, copper, and tool steel

micromachining. Unlike in macromachining, a micromachining tool is subjected to fluctuating cutting force when encounters each grain since microtool size is comparable to material grain size. A microtool is more vulnerable to fatigue fracture and the resulted surface—if the tool survives—would be rough due to different springback protrusion from each grain due to different crystallographic orientations of the grains, and direction-dependent properties of the material. Homogenous workpiece

mechanisms [13]

2.6 Cutting fluid

with machining action.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

effectiveness of MQL [14].

fixture surrounding a workpiece.

3. Effect of materials

for miniature molds and dies.

13

Figure 7.

Figure 8. Adhesion wear of a micromilling tool.

Figure 9. Delamination of AlTiN/Si3N4 coating on a WC microdrilling tool.

thick coating layer; the loosen coating particles then rub and cause mechanical abrasive wear on a tool. Thermal mechanism may cause workpiece atoms to diffuse, weaken, and dislodge several tool grains as microchipping. The quenched and tempered 4140 steel fails tools by abrasion, the 304 stainless steel causes adhesion

tool wear, while the nickel supper alloy Inconel 718 damages tools by all wear mechanisms [13]

## 2.6 Cutting fluid

Shearing of workpiece material and relative motion between tool and chip generate a significance of heat during machining. This thermal energy could change the microstructure, plastically deform the subsurface, degrade the part quality and wear a cutting tool quickly. Cutting fluids, either oil based for lubrication or water based for cooling, should be applied appropriately for effective micromachining while evacuating tiny chips from the machined surface.


## 3. Effect of materials

Micromachining is often utilized to fabricate components for miniaturized sensors, medical, optical, and electronic devices. Common engineering materials for these applications include stainless steel, aluminum, titanium, copper, and tool steel for miniature molds and dies.

Workpiece materials must meet certain conditions for successful micromachining. Unlike in macromachining, a micromachining tool is subjected to fluctuating cutting force when encounters each grain since microtool size is comparable to material grain size. A microtool is more vulnerable to fatigue fracture and the resulted surface—if the tool survives—would be rough due to different springback protrusion from each grain due to different crystallographic orientations of the grains, and direction-dependent properties of the material. Homogenous workpiece materials with very fine and uniform grain sizes should be chosen for

thick coating layer; the loosen coating particles then rub and cause mechanical abrasive wear on a tool. Thermal mechanism may cause workpiece atoms to diffuse, weaken, and dislodge several tool grains as microchipping. The quenched and tempered 4140 steel fails tools by abrasion, the 304 stainless steel causes adhesion

Delamination of AlTiN/Si3N4 coating on a WC microdrilling tool.

Built-up-edge at the cutting edge on a microdrilling tool (left) and micromilling tool (right).

Figure 8.

Figure 9.

12

Figure 7.

Micromachining

Adhesion wear of a micromilling tool.

micromachining. Inclusions and large precipitates should be minimized to avoid damage to a fragile tool edge.

## 3.1 Ductile regime micro/nano machining

The concept of ductile-regime machining has been investigated since 1960s for amorphous brittle materials such as glasses. Silicon, germanium, and glasses have become strategic materials that are widely used to fabricate intricate components in microelectronics, optics, defense industries, and recently as micro optical-electricalmechanical systems. Silicon and other brittle materials are known for their low machinability unless they are machined in the ductile-regime conditions. When utilized at the optimal machining conditions, only minimum effort is required for the subsequent etching, grinding, or polishing to remove the damaged subsurface. This section summarizes the theory and provides practical guidance for ductileregime machining.

The mechanism of ductile-regime machining has been studied by many researchers. Using fracture mechanics approach, it can be shown that there is a threshold below which ductile regime prevailed:

$$d\_c = \frac{\text{plastic flow energy}}{\text{fraction energy}} = A \left(\frac{E}{H}\right) \left(\frac{K\_c}{H}\right)^2\tag{5}$$

same speed, depth of cut, and coolant produces ductile machined surfaces in

• Cutting fluid changes the surface properties of materials (Kc, E, and H) and affects conditions for ductile regime micromachining. When micromachining the (100) plane of germanium using a single crystalline diamond tool, the critical depth of cut changes from 0.13 μm with distill water as cutting fluid to

• Tool geometry also affects the results. Plowing and fracture of material occurs when depth of cut is less than approximately half of the tool cutting edge radius. Tool with negative top rake angle is usually utilized since a negative rake causes compressive zone in the workpiece ahead and below the tool and

The evolution of 3D printing allows metallic parts to be printed in different

• Power beam fusion. Laser or electron beam are used to melt either metal wire or powder particles to form layers, then fuse these layers to form a complex

• Material jetting. Metal powder is fed at the energy beam focused point to melt

• Jet binding. An organic binding material is sprayed and bind metal powder in

i. Is warped due to high thermal induced residual stress, or non-uniform

layers. The "green" part is then sintered to form the final shape.

Using any of the above techniques, an as-built part:

one direction but brittle machined surfaces on others.

0.29 μm in dry machining.

Perfect ductile regime machining of (001) silicon [16].

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

suppresses microcrack formation.

3.2 Additively manufactured metals

shrinkage during sintering,

methods.

15

Figure 10.

shaped part.

and form a part.

where dc: critical depth of cut (m); A: constant. A = 0.15 for microscratching, A = 0.60 for micromachining; E: Young modulus (Pa); KC: surface fracture toughness (Pa m0.5); H: surface microhardness (Pa).

A shallow depth of cut, therefore, would energetically promote plastic flow rather than brittle fracture in the substrate and the chips. Table 3 tabulates properties of some brittle materials and their experimental critical depths of cut.

The constant A in Eq. (5) varies in the range 0.1–0.6 due to measuring uncertainty of surface toughness Kc, elastic modulus E, and microhardness H in a testing environment. These properties depend on crystalline orientation of the materials, surface conditions, and tool geometry. An example of ductile regime machining on single crystal silicon wafer is shown in Figure 10.

• The critical resolved shear stress, on a crystalline plane due to the cutting action, is directly proportional to the Schmid factor (cosλ)(cosϕ), where ϕ and λ are the orientations of the slip plane and slip direction. An ideal ductile mode machining would happen when the cutting shear stress is parallel to both the slip plane and the slip direction, otherwise a pseudo ductile mode with micro cleavages occurs. True ductile-regime machining happens only along certain crystalline orientation, but brittle machining occurs at other crystalline orientation. This explains why micromachining a crystalline specimen at the


Table 3. Properties of selected brittle materials.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

micromachining. Inclusions and large precipitates should be minimized to avoid

The concept of ductile-regime machining has been investigated since 1960s for amorphous brittle materials such as glasses. Silicon, germanium, and glasses have become strategic materials that are widely used to fabricate intricate components in microelectronics, optics, defense industries, and recently as micro optical-electricalmechanical systems. Silicon and other brittle materials are known for their low machinability unless they are machined in the ductile-regime conditions. When utilized at the optimal machining conditions, only minimum effort is required for the subsequent etching, grinding, or polishing to remove the damaged subsurface. This section summarizes the theory and provides practical guidance for ductile-

The mechanism of ductile-regime machining has been studied by many researchers. Using fracture mechanics approach, it can be shown that there is a

fracture energy <sup>¼</sup> <sup>A</sup> <sup>E</sup>

where dc: critical depth of cut (m); A: constant. A = 0.15 for microscratching, A = 0.60 for micromachining; E: Young modulus (Pa); KC: surface fracture tough-

A shallow depth of cut, therefore, would energetically promote plastic flow rather than brittle fracture in the substrate and the chips. Table 3 tabulates proper-

The constant A in Eq. (5) varies in the range 0.1–0.6 due to measuring uncertainty of surface toughness Kc, elastic modulus E, and microhardness H in a testing environment. These properties depend on crystalline orientation of the materials, surface conditions, and tool geometry. An example of ductile regime machining on

• The critical resolved shear stress, on a crystalline plane due to the cutting

Fracture toughness (MPa m0.5)

α-Al2O3 275–393 3.85–5.90 19.6–20.1 1.0 SiC 382–475 2.50–3.50 24.5–25.0 0.2 Silicon 168 0.6 10 0.5

action, is directly proportional to the Schmid factor (cosλ)(cosϕ), where ϕ and λ are the orientations of the slip plane and slip direction. An ideal ductile mode machining would happen when the cutting shear stress is parallel to both the slip plane and the slip direction, otherwise a pseudo ductile mode with micro cleavages occurs. True ductile-regime machining happens only along certain crystalline orientation, but brittle machining occurs at other crystalline orientation. This explains why micromachining a crystalline specimen at the

ties of some brittle materials and their experimental critical depths of cut.

H Kc

H <sup>2</sup>

Knoop hardness (GPa)

Critical depth of cut (μm)

(5)

dc <sup>¼</sup> plastic flow energy

damage to a fragile tool edge.

Micromachining

regime machining.

3.1 Ductile regime micro/nano machining

threshold below which ductile regime prevailed:

ness (Pa m0.5); H: surface microhardness (Pa).

single crystal silicon wafer is shown in Figure 10.

Materials Young modulus

Properties of selected brittle materials.

Table 3.

14

(GPa)

Figure 10. Perfect ductile regime machining of (001) silicon [16].

same speed, depth of cut, and coolant produces ductile machined surfaces in one direction but brittle machined surfaces on others.


## 3.2 Additively manufactured metals

The evolution of 3D printing allows metallic parts to be printed in different methods.


Using any of the above techniques, an as-built part:

i. Is warped due to high thermal induced residual stress, or non-uniform shrinkage during sintering,

costly to perform micromilling by just scaling down a milling cutter, or parameters

Tool material. Carbide tools should be sintered from fine grains, and precisely

Milling direction. Down milling is the preferred mode since a micromill will engages a workpiece and removes a wedge shape chip with decreasing chip thickness. In contrast, a tool in up milling would rub on the workpiece until the effective chip thickness is greater than 0.5 cutting edge radius. Down milling also produces

micromachining. A nozzle should be as closed as possible and is positioned to let the cutting flute pulls the mist into the cutting zone. Tool and workpiece should be arranged to avoid stagnant zone, or being blocked or interfered by chips [17].

Tool vibration. Avoid unnecessary disengaging then engaging of microtool and workpiece in a milling program. Vibration and bending of a microtool at starting

Commercial micromills are available for diameter of 25 μm and above. Optional

As mentioned above, the BUEs on a cutting tool surface can break and deposit on a machined surface as shown in Figure 13. Measuring area surface finish Sa would combine the roughness contributed from milling parameters (e.g., speed, feed), tool and machine condition (e.g., vibration), and surface irregularities (e.g., pore and BUE). The BUEs is reduced when an optimal condition of MQL is used, low chip load, and higher speed. In a study of micromilling 316L stainless steel, the amount of BUE is significantly diminished when cutting at speed higher than

Vibration of a micromill when engaging and disengaging a workpiece. Carbide mill Ø1 mm, 316L stainless

steel, 25,000 rpm, 10 μm/tooth feed, 0.348 mm axial depth, 0.558 mm radial depth.

Lubrication. Minimum quantity lubrication should be used with all

and ending could fatigue and shorten tool life of a microtool (Figure 12).

• Flute lengths: standard or extended length (10–80% longer)

from macroscale milling.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

less amount of burr.

geometries include:

Figure 12.

17

• Number of flutes: 2, 3, or 4

• Helix angles: 25°, 30°, 50°

• End configuration: hemisphere or flat.

ground to obtain a micron-level cutting edge radius.

Figure 11.

Surface topography of an Inconel 718 block after selective laser melting. (a) Oblique view, and (b) viewing along the building z-axis.


Post processing of the as-built metal parts must be performed so that they can meet required engineering criteria for surface finish or dimensional/form tolerances. Micromachining is the most effective post processing technique to control the surface and dimension of local areas of additively manufactured metals due to its high removal rate and well-established computer numerical controlled (CNC) industry.

## 4. Micromachining

Micromachining refers to removal of material subtractively in micron scale. The process can be done by (i) conventional processes, i.e., removing material mechanically with hard tools in contact with a workpiece and removing minute amount of material as chips, or (ii) non-conventional processes, i.e., removing material by other physical mechanisms such as optical, thermal, chemical, electrical, or combinations of these. The following section focuses only on the three major conventional techniques, namely micromilling, microdrilling, and microturning of advanced materials.

#### 4.1 Micromilling

Micromilling is among the most versatile microfabrication processes. Although alternative nontraditional processes to produce microfeatures (e.g., laser micromachining, electrical discharge micromachining, electrochemical micromachining, chemical microetching, electron/ion beam micromachining) are available, these processes are either cost prohibitive, or inferior when comparing resulted surface and subsurface integrity, anisotropic aspect ratio, material removal rate, or feature quality. Successful micromilling requires new tool geometry, tool material, machining parameters, and machining skills. It is technically incorrect and

## Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

costly to perform micromilling by just scaling down a milling cutter, or parameters from macroscale milling.

Tool material. Carbide tools should be sintered from fine grains, and precisely ground to obtain a micron-level cutting edge radius.

Milling direction. Down milling is the preferred mode since a micromill will engages a workpiece and removes a wedge shape chip with decreasing chip thickness. In contrast, a tool in up milling would rub on the workpiece until the effective chip thickness is greater than 0.5 cutting edge radius. Down milling also produces less amount of burr.

Lubrication. Minimum quantity lubrication should be used with all micromachining. A nozzle should be as closed as possible and is positioned to let the cutting flute pulls the mist into the cutting zone. Tool and workpiece should be arranged to avoid stagnant zone, or being blocked or interfered by chips [17].

Tool vibration. Avoid unnecessary disengaging then engaging of microtool and workpiece in a milling program. Vibration and bending of a microtool at starting and ending could fatigue and shorten tool life of a microtool (Figure 12).

Commercial micromills are available for diameter of 25 μm and above. Optional geometries include:


ii. Has very rough surface (typically 15–20 μm Ra), and

(e.g., porosity, inclusion, shrinkage cavity, etc.).

industry.

Figure 11.

Micromachining

along the building z-axis.

materials.

16

4.1 Micromilling

4. Micromachining

iii. Is filled with surface defects (e.g., microcrack, shrinkage cavity, partially welded powder particles, etc.) as shown in Figure 11, and volume defects

Surface topography of an Inconel 718 block after selective laser melting. (a) Oblique view, and (b) viewing

Post processing of the as-built metal parts must be performed so that they can meet required engineering criteria for surface finish or dimensional/form tolerances. Micromachining is the most effective post processing technique to control the surface and dimension of local areas of additively manufactured metals due to its high removal rate and well-established computer numerical controlled (CNC)

Micromachining refers to removal of material subtractively in micron scale. The process can be done by (i) conventional processes, i.e., removing material mechanically with hard tools in contact with a workpiece and removing minute amount of material as chips, or (ii) non-conventional processes, i.e., removing material by other physical mechanisms such as optical, thermal, chemical, electrical, or combinations of these. The following section focuses only on the three major conventional techniques, namely micromilling, microdrilling, and microturning of advanced

Micromilling is among the most versatile microfabrication processes. Although

micromachining, chemical microetching, electron/ion beam micromachining) are available, these processes are either cost prohibitive, or inferior when comparing resulted surface and subsurface integrity, anisotropic aspect ratio, material removal rate, or feature quality. Successful micromilling requires new tool geometry, tool material, machining parameters, and machining skills. It is technically incorrect and

alternative nontraditional processes to produce microfeatures (e.g., laser micromachining, electrical discharge micromachining, electrochemical


As mentioned above, the BUEs on a cutting tool surface can break and deposit on a machined surface as shown in Figure 13. Measuring area surface finish Sa would combine the roughness contributed from milling parameters (e.g., speed, feed), tool and machine condition (e.g., vibration), and surface irregularities (e.g., pore and BUE). The BUEs is reduced when an optimal condition of MQL is used, low chip load, and higher speed. In a study of micromilling 316L stainless steel, the amount of BUE is significantly diminished when cutting at speed higher than

#### Figure 12.

Vibration of a micromill when engaging and disengaging a workpiece. Carbide mill Ø1 mm, 316L stainless steel, 25,000 rpm, 10 μm/tooth feed, 0.348 mm axial depth, 0.558 mm radial depth.

Figure 13.

Built-up-edges deposit on machined Ti 6Al 4V surface. Micromilling at 9.6 m/min, 0.1 μm/tooth, 10 μm axial depth. Dry [18].

30 m/min (Figure 14). Perhaps the high temperature at high cutting speed improves the material plasticity and reduces the weldability of BUEs on cutting tool tip.

Lack of BUEs on coated tool also results in better surface finish of micromilled channels on 304/316L stainless steels, NiTi alloy, A36 low carbon steel, 6061-T6 aluminum, and Ti-6Al-4V titanium alloy. The AlTiN coating effectively increases tool life while reducing burrs significantly when micromilling 304 stainless steel. The incompatibility of the coating on specific material workpiece prevent BUE formation, therefore, shear the materials as chips rather than deforming it as burr (Figure 15b). The effect of micromilling mode is also shown when up-milling tends to generate more burr than down-milling (Figure 15a).

The theoretical surface finish of machined surface after milling with a flat-end tool can be shown to be:

$$R\_d = \frac{\mathbf{S}}{\mathbf{18}} f\_{\text{lama}} \tag{6}$$

And that for a ball-end milling tool is:

Rearranging Eq. (7) to have:

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Figure 15.

MQL [19].

and degrade the machined surface.

num alloy [21].

conventional metals.

19

Ra <sup>¼</sup> <sup>0</sup>:<sup>2423</sup> ft

Effect of tool coating on resulting burrs: (a) uncoated ϕ152 μm tool, milling 304 stainless steel, 24 m/min, 0.1 μm/tooth, MQL; and (b) AlTiN coated ϕ198 μm tool, milling 304 stainless steel, 24 m/min, 0.1 μm/tooth,

D � Ra ¼ 0:2423f

ball-end milling tool (μm); α: concavity angle, or end-cutting-edge angle (°).

manufactured Ti 6Al 4V alloys. At a very low chip load of 0.1 μm/tooth, the presence of significant BUEs on machined surface degrades the surface quality as indicated by high surface roughness Ra. The surface improves at higher chip loads, by reduction of BUEs, but is gradually increased with chip load as predicted by Eq. (6). Similar surface roughness result was reported for machining 7075 alumi-

where Ra: average surface finish (μm); ft: chip load (μm/tooth); D: diameter of

Both Eqs. (6) and (8) predict the dependent of chip load on surface finish Ra. When plotting Eq. (8) using experimental data from different tool diameters and different chip loads, then Eq. (8) is confirmed with data in macromilling when chip load >100 μm but not with smaller chip load for micromilling (Figure 16). The reason for a higher surface finish is the intermittent BUEs, although small, smear

Similar experimental results is shown in Figure 17 for micromilling of additively

Post processing by micromachining of additively manufactured metals have been investigated. Few studies have compared machinability of selected metals produced by conventional route (e.g., casting, extrusion, rolling, etc.) and by additive manufacturing route (e.g., powder bed fusion, direct energy deposition, etc.). Limited machining investigations on Inconel 625, Inconel 718, Ti 6Al 4V, H13 tool steel, Ti 48Al 2Nb 2Cr alloy, 17Cr 4Ni stainless steel, and 316L stainless steel have concluded that the AM metals in general have lower machinability compared to the

Machinability is affected by microstructural changes in a material. A quick comparison between the microstructure of extruded and SLM'ed Inconel 718 shows the contrast of the same materials after different manufacturing routes. Uniform

2

2

<sup>D</sup> : (7)

<sup>t</sup> (8)

Figure 14.

Effect of cutting speed and chip load on area surface roughness Sa. Each range plot shows the maximum, minimum, and average of 15 measurements. Micromilling 316L stainless steel [12].

Figure 15.

30 m/min (Figure 14). Perhaps the high temperature at high cutting speed improves the material plasticity and reduces the weldability of BUEs on cutting tool tip.

Built-up-edges deposit on machined Ti 6Al 4V surface. Micromilling at 9.6 m/min, 0.1 μm/tooth, 10 μm axial

to generate more burr than down-milling (Figure 15a).

tool can be shown to be:

Figure 13.

depth. Dry [18].

Micromachining

Figure 14.

18

Lack of BUEs on coated tool also results in better surface finish of micromilled channels on 304/316L stainless steels, NiTi alloy, A36 low carbon steel, 6061-T6 aluminum, and Ti-6Al-4V titanium alloy. The AlTiN coating effectively increases tool life while reducing burrs significantly when micromilling 304 stainless steel. The incompatibility of the coating on specific material workpiece prevent BUE formation, therefore, shear the materials as chips rather than deforming it as burr (Figure 15b). The effect of micromilling mode is also shown when up-milling tends

The theoretical surface finish of machined surface after milling with a flat-end

tanα (6)

Ra <sup>¼</sup> <sup>5</sup> <sup>18</sup> ft

Effect of cutting speed and chip load on area surface roughness Sa. Each range plot shows the maximum,

minimum, and average of 15 measurements. Micromilling 316L stainless steel [12].

Effect of tool coating on resulting burrs: (a) uncoated ϕ152 μm tool, milling 304 stainless steel, 24 m/min, 0.1 μm/tooth, MQL; and (b) AlTiN coated ϕ198 μm tool, milling 304 stainless steel, 24 m/min, 0.1 μm/tooth, MQL [19].

And that for a ball-end milling tool is:

$$R\_d = 0.2423 \frac{{\cal f}\_t}{D}.\tag{7}$$

Rearranging Eq. (7) to have:

$$D \cdot R\_d = 0.2423 f\_t^2 \tag{8}$$

where Ra: average surface finish (μm); ft: chip load (μm/tooth); D: diameter of ball-end milling tool (μm); α: concavity angle, or end-cutting-edge angle (°).

Both Eqs. (6) and (8) predict the dependent of chip load on surface finish Ra. When plotting Eq. (8) using experimental data from different tool diameters and different chip loads, then Eq. (8) is confirmed with data in macromilling when chip load >100 μm but not with smaller chip load for micromilling (Figure 16). The reason for a higher surface finish is the intermittent BUEs, although small, smear and degrade the machined surface.

Similar experimental results is shown in Figure 17 for micromilling of additively manufactured Ti 6Al 4V alloys. At a very low chip load of 0.1 μm/tooth, the presence of significant BUEs on machined surface degrades the surface quality as indicated by high surface roughness Ra. The surface improves at higher chip loads, by reduction of BUEs, but is gradually increased with chip load as predicted by Eq. (6). Similar surface roughness result was reported for machining 7075 aluminum alloy [21].

Post processing by micromachining of additively manufactured metals have been investigated. Few studies have compared machinability of selected metals produced by conventional route (e.g., casting, extrusion, rolling, etc.) and by additive manufacturing route (e.g., powder bed fusion, direct energy deposition, etc.). Limited machining investigations on Inconel 625, Inconel 718, Ti 6Al 4V, H13 tool steel, Ti 48Al 2Nb 2Cr alloy, 17Cr 4Ni stainless steel, and 316L stainless steel have concluded that the AM metals in general have lower machinability compared to the conventional metals.

Machinability is affected by microstructural changes in a material. A quick comparison between the microstructure of extruded and SLM'ed Inconel 718 shows the contrast of the same materials after different manufacturing routes. Uniform

#### Figure 16.

Average surface finish at center of milled microchannels. Ball-end milling tools ϕ152–9525 μm, workpiece materials 6061-T6, A36 steel, NiTi, 304/316L stainless steels, in MQL condition [19].

Conflicting literature data are probably due to different process parameters for

Effect of chip load and cutting speed on surface finish. Micromilling of Ti 6Al 4V fabricated by electron beam

Advanced cutting fluids and techniques have been applied for micromilling of advanced materials. Poor results are reported when using a pressurized jet to flood cool during micromilling of Ti 6Al 4V. The fragile tool (200 μm diameter, AlTiN coated, 1.25 μm/tooth feed, 30,000 rpm rotating speed, 20 μm depth of cut) being deflected and vibrated under pressurized jet, generates large burr and rough surface (Figure 22a), while cutting a large slot width (Figure 22b). Applying MQL with flow in the feeding direction solves these problems. When using MQL at high air pressure above 5.5 bar, a nozzle with rough internal surface breaks the lubricant into smaller droplets and effectively improves tool life of micromilling tools [24].

AM metals and different scanning strategies. For example, the precipitation hardenable 17Cr 4Ni stainless steel was reported to have near fully martensitic structure and high yield strength than the cast alloy, but the opposite conclusion

Uniform microstructure of extruded Inconel 718. Viewing along the extruding direction.

was found in another study.

Figure 17.

Figure 18.

21

melting, and direct metal laser sintering [20].

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

microstructure of the extruded specimen is expected (Figure 18). The mechanical properties of extruded specimen could be slightly different in longitudinal and transversal directions due to preferred grain orientation along the extrusion direction. In contrast, the fast heating and cooling rate of SLM'ed Inconel 718 creates alternate layers along the laser paths and hard particles in the microstructure (Figure 19). The obvious changes in microstructure result in different mechanical properties, therefore, affecting machinability. Fast cooling in SLM'ed Inconel 718 also forms brittle Laves particles and traps pores near an edge (Figure 20a). During machining, some particles are broken, smeared along the tool path and probably chipped the cutting tool. Porosity is unavoidable in AM metals. The burr on top of those micron-size pores after micromachining is difficult to remove mechanically (Figure 20b). Although hot isostatic pressing (HIPping) can eliminate porosity, some materials (e.g., Inconel 718) are aged at HIPping temperature; the resulted precipitates increase the material strength but reduce its machinability (Figure 21).

## Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

#### Figure 17.

Effect of chip load and cutting speed on surface finish. Micromilling of Ti 6Al 4V fabricated by electron beam melting, and direct metal laser sintering [20].

Figure 18. Uniform microstructure of extruded Inconel 718. Viewing along the extruding direction.

Conflicting literature data are probably due to different process parameters for AM metals and different scanning strategies. For example, the precipitation hardenable 17Cr 4Ni stainless steel was reported to have near fully martensitic structure and high yield strength than the cast alloy, but the opposite conclusion was found in another study.

Advanced cutting fluids and techniques have been applied for micromilling of advanced materials. Poor results are reported when using a pressurized jet to flood cool during micromilling of Ti 6Al 4V. The fragile tool (200 μm diameter, AlTiN coated, 1.25 μm/tooth feed, 30,000 rpm rotating speed, 20 μm depth of cut) being deflected and vibrated under pressurized jet, generates large burr and rough surface (Figure 22a), while cutting a large slot width (Figure 22b). Applying MQL with flow in the feeding direction solves these problems. When using MQL at high air pressure above 5.5 bar, a nozzle with rough internal surface breaks the lubricant into smaller droplets and effectively improves tool life of micromilling tools [24].

microstructure of the extruded specimen is expected (Figure 18). The mechanical properties of extruded specimen could be slightly different in longitudinal and transversal directions due to preferred grain orientation along the extrusion direction. In contrast, the fast heating and cooling rate of SLM'ed Inconel 718 creates alternate layers along the laser paths and hard particles in the microstructure (Figure 19). The obvious changes in microstructure result in different mechanical properties, therefore, affecting machinability. Fast cooling in SLM'ed Inconel 718 also forms brittle Laves particles and traps pores near an edge (Figure 20a). During machining, some particles are broken, smeared along the tool path and probably chipped the cutting tool. Porosity is unavoidable in AM metals. The burr on top of those micron-size pores after micromachining is difficult to remove mechanically (Figure 20b). Although hot isostatic pressing (HIPping) can

Average surface finish at center of milled microchannels. Ball-end milling tools ϕ152–9525 μm, workpiece

materials 6061-T6, A36 steel, NiTi, 304/316L stainless steels, in MQL condition [19].

eliminate porosity, some materials (e.g., Inconel 718) are aged at HIPping temperature; the resulted precipitates increase the material strength but reduce its machin-

ability (Figure 21).

20

Figure 16.

Micromachining

#### Figure 19.

Microstructure of SLM'ed Inconel 718 by scanning electron microscopy in (a) secondary electron mode, and (b) back scattered electron mode. Notice the different layers across the laser scanning paths.

Effort was made to produce an environmentally friendly cutting fluid while enhancing the cutting fluid performance. Lignin is a biodegradable product from wood. It can be mixed in alcohol then TRIM water soluble as cutting fluid. Micromilling tests are performed with 396 μm diameter milling tool on 6061 aluminum and 1018 steel. The optimal concentration of 0.015% lignin seems to provide the best lubricating and cooling effects when reducing the cutting forces on both

Effect if different cooling methods on (a) burr formation, and (b) resulted slot width. Micromilling of Ti 6Al

Cryogenic precooling can be used to reduce BUE formation and its effect on part quality. Liquid nitrogen, dispensed in front of a microtool in a micromilling test on Inconel 718. The tool (760 μm diameter, AlCrN coated) is used at 48 m/min speed, 1.25–5 μm/flute chip load, 0.1–0.2 mm depth of cut for a constant distance of 120 mm. The cryogenic condition embrittles the Inconel material so it can be micromilled in brittle mode with minimum plastically deformed burr and BUEs. The result is the low surface roughness at different chip loads (Figure 24a) and depth of cuts (Figure 24b). However, the brittle chip debris are abundant and might interfere with subsequent machining passes. No chip debris is seen when

The positive results of cryogenic cooling on Inconel 718, however, was not confirmed in a similar study on micromilling of electron beam melted Ti 6Al 4V. The 300 μm cutting tools were utilized at 63–145 m/min speed, 0.1–3.0 μm/tooth chip load, and 30 μm axial depth of cut. The temperature of the workpiece was at 155 5°C with liquid nitrogen. No brittle chip debris are seen, ductile burr are visible (Figure 25a), and the surface profile (Ra finish) are similar for dry, MQL,

micromilling at cryogenic condition as shown in the nanohardness results below the surface (Figure 25b). This implies the material is not truly embrittled as planned.

and cryogenic conditions. The machined surface is work-hardened after

materials (Figure 23).

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Figure 22.

4V [23].

MQL is used.

23

#### Figure 20.

Irregularities in selective laser Inconel 718. (a) Sheared Laves particles after micromilling, (b) A machined micropore with burr.

#### Figure 21.

Micromilling of SLM'ed Inconel 718. Change of slot width with milling distance. Minimum quantity lubrication, 50 μm depth of cut [22].

Figure 22. Effect if different cooling methods on (a) burr formation, and (b) resulted slot width. Micromilling of Ti 6Al 4V [23].

Effort was made to produce an environmentally friendly cutting fluid while enhancing the cutting fluid performance. Lignin is a biodegradable product from wood. It can be mixed in alcohol then TRIM water soluble as cutting fluid. Micromilling tests are performed with 396 μm diameter milling tool on 6061 aluminum and 1018 steel. The optimal concentration of 0.015% lignin seems to provide the best lubricating and cooling effects when reducing the cutting forces on both materials (Figure 23).

Cryogenic precooling can be used to reduce BUE formation and its effect on part quality. Liquid nitrogen, dispensed in front of a microtool in a micromilling test on Inconel 718. The tool (760 μm diameter, AlCrN coated) is used at 48 m/min speed, 1.25–5 μm/flute chip load, 0.1–0.2 mm depth of cut for a constant distance of 120 mm. The cryogenic condition embrittles the Inconel material so it can be micromilled in brittle mode with minimum plastically deformed burr and BUEs. The result is the low surface roughness at different chip loads (Figure 24a) and depth of cuts (Figure 24b). However, the brittle chip debris are abundant and might interfere with subsequent machining passes. No chip debris is seen when MQL is used.

The positive results of cryogenic cooling on Inconel 718, however, was not confirmed in a similar study on micromilling of electron beam melted Ti 6Al 4V. The 300 μm cutting tools were utilized at 63–145 m/min speed, 0.1–3.0 μm/tooth chip load, and 30 μm axial depth of cut. The temperature of the workpiece was at 155 5°C with liquid nitrogen. No brittle chip debris are seen, ductile burr are visible (Figure 25a), and the surface profile (Ra finish) are similar for dry, MQL, and cryogenic conditions. The machined surface is work-hardened after micromilling at cryogenic condition as shown in the nanohardness results below the surface (Figure 25b). This implies the material is not truly embrittled as planned.

Figure 19.

Micromachining

Figure 20.

Figure 21.

22

lubrication, 50 μm depth of cut [22].

micropore with burr.

Microstructure of SLM'ed Inconel 718 by scanning electron microscopy in (a) secondary electron mode, and

Irregularities in selective laser Inconel 718. (a) Sheared Laves particles after micromilling, (b) A machined

Micromilling of SLM'ed Inconel 718. Change of slot width with milling distance. Minimum quantity

(b) back scattered electron mode. Notice the different layers across the laser scanning paths.

Figure 23. Effect of atomized cutting fluid with lignin on micromilling of (a) 6061 aluminum and (b) 1018 steel [25].

Another effective additive in MQL is nano-sized diamond particles. This application, applied to microdrilling, will be presented in the next section.

Figure 24.

Figure 25.

25

axial cutting depth (Ucun, 2014).

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Effect of different cooling strategies on micromilling of Inconel 718 (a) effect of chip loads, and (b) effect of

Effect of cryogenic cooling on micromilling of electron beam melted Ti 6Al 4V [26].

#### 4.2 Microdrilling

Microdrilling is a more complex operation comparing to turning or milling. Chip removal and effectively supplying of cutting fluid are easy with the latter, but not with microdrilling due to extremely limited space around a microdrill.

Tool material. As with a micromill, a carbide microdrill should be sintered from fine grains, and ground to small cutting edge radius.

Hole quality. Spindle runout, tool eccentricity, and wandering of a microdrill cause cyclic bending of a tool and could lead to a catastrophic failure. To control drill wandering, precision pre-drilling of a center hole can be tried, or the workpiece surface must be ground to minimize deflection of a slender drill when starting on an irregular surface.

Micromist with fixed nozzle pointing to the drill tip and making an angle of 60–70° with the tool axis is recommended. This way, the chip is blown away after each pecking cycle and the microdrill is re-lubricated before re-entering into the hole. Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Figure 24.

Another effective additive in MQL is nano-sized diamond particles. This appli-

Effect of atomized cutting fluid with lignin on micromilling of (a) 6061 aluminum and (b) 1018 steel [25].

Microdrilling is a more complex operation comparing to turning or milling. Chip removal and effectively supplying of cutting fluid are easy with the latter, but not

Tool material. As with a micromill, a carbide microdrill should be sintered from

Hole quality. Spindle runout, tool eccentricity, and wandering of a microdrill cause cyclic bending of a tool and could lead to a catastrophic failure. To control drill wandering, precision pre-drilling of a center hole can be tried, or the workpiece surface must be ground to minimize deflection of a slender drill when starting on an

Micromist with fixed nozzle pointing to the drill tip and making an angle of 60–70° with the tool axis is recommended. This way, the chip is blown away after each pecking cycle and the microdrill is re-lubricated before re-entering into the hole.

cation, applied to microdrilling, will be presented in the next section.

with microdrilling due to extremely limited space around a microdrill.

fine grains, and ground to small cutting edge radius.

4.2 Microdrilling

Figure 23.

Micromachining

irregular surface.

24

Effect of different cooling strategies on micromilling of Inconel 718 (a) effect of chip loads, and (b) effect of axial cutting depth (Ucun, 2014).

Figure 25. Effect of cryogenic cooling on micromilling of electron beam melted Ti 6Al 4V [26].

#### Micromachining

High aspect ratio. Pecking is essential for microhole drilling since chips have to be extracted and cutting fluid must penetrate into a small and deep microhole. The pecking depth can be substantial in the beginning, but it must be gradually reduced when drilling at deeper depths. One can start with an initial pecking depth of (2\*drill diameter) and gradually reduce it to (0.5\*diameter) at the hole depth of (10\*diameter). It is convenient to program pecking cycles in microdrilling following the equations below.

$$\frac{P}{D} = \frac{1}{9}(-1.5R + 19.5) \quad \text{for } R \le 10\tag{9}$$

$$\frac{P}{D} = 0.5 \quad \text{ for } R > 10\tag{10}$$

the drill shank is rigid, axial thrust force and torsional torque can fail a drill in either buckling mode or torsional mode. Transverse shear strength in three-point bending test of a sintered carbide tool is estimated to be half of the material tensile strength. This study used commercially pure (CP) titanium, 316L stainless steel, 6061-T6 aluminum, PEEK plastic, and Nitinol (51 wt% Ni 49% Ti) shape memory alloy. Some surfaces were faced milled on a milling machine, others were milled, hand ground and then finish polished with 1 μm diamond paste. Microdrilling was performed with Ø100–150 μm drill diameter, 135° point angle, 30° rake angle, 40–44° helix angle, 2 flutes, and 1.50–3.50 mm flute length. Some were coated with

The classical Taylor's equation has been applied for macromachining, micromachining, and is used for microdrilling to show the effects of chip load and tool coating. For the same cutting speed of 20 m/min and comparable drilling distance of about 35 mm, the CP titanium can be microdrilled 400% faster than 316L stainless steel since the chip load for the former is 0.1 μm and that for the latter is 0.02 μm. Also, AlTiN coated drills improve tool life by at least 122%. This drilling operation is stopped after drilling all possible holes on the test blocks (Figures 27 and 28). Negligible tool wear are observed when drilling 6061-T6

ute to the hole quality. During the initial engagement of drill and workpiece surface, a slight lateral motion of the drill chisel edge is sufficient to bend and misguide the slender microdrill. Drill wandering refers to the deviation of a drilled hole from its intended position. A machined surface is rough enough to cause wandering of microdrills. A sloped ridge on a rough surface bends and deviates the drill axis from intended position. Such deviation causes drill wandering, significant burrs, and irregular hole diameters. The hole quality is improved significantly when drilling on a polished surface at the same or more aggressive drilling parameters. An improvement of 27% in hole center deviation is achieved for the polished surface of CP titanium (Figure 29). Similarly, an improvement of 260% on hole deviation is

Burr, hole size, hole position, and work hardening around a drilled hole contrib-

Vickers microhardness near a drilled surface is obtained to study the level of plastic deformation and work hardening below a drilled surface. The hardness near the drilled surface is found to be 15% higher than those at the unmachined zone (Figure 30). The work hardening effect is caused by plastic deformation of the

Tool life plot for microdrilling of CP titanium. Progressive pecking, tool life criterion 8 μm [28].

aluminum and PEEK plastic, therefore, no modeling is necessary.

AlTiN or AlTiN/Si3N4 under the trade name Nanotek.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

achieved when drilling a polished PEEK block.

Figure 27.

27

where P: incremental pecking depth (mm); D: drill diameter (mm); R: progressive drill aspect ratio = current hole depth/drill diameter.

Although application of MQL in micromachining is necessary, other researchers have found ways to improve its effectiveness. Nanoparticles are mixed in MQL oil to improve its performance in microdrilling. The nano-sized particles of CNT-C60, TiO2, Al2O3, MoS2, and diamond would increase the thermal conductivity of the fluid thus prolong the tool life while reducing burr. In an experimental microdrilling study, uncoated microdrills of 100–500 μm diameters are used at 10–15 mm/min, 30,000–60,000 rpm spindle speeds, while varying the concentration of 30 nm diamond particles in the range 0–4%. The MQL system is used at 3-bar air pressure to drill a constant 0.3 mm depth. The optimal conditions for low torque and thrust force are experimentally obtained to be 60,000 rpm and 2% concentration.

Inspection of a microdrilled and tapped hole would be difficult. Destructive technique by sectioning a part is time consuming, expensive, and error prone since the internal features might be distorted by releasing of residual stress. X-ray computed tomography (CT) has been utilized to evaluated drilled and tapped Ti 6Al 4V dental implant fabricated by direct metal laser sintering. The best microdrilled hole quality—cylindricity and perpendicularity—is achieved at the lowest testing drill speed of 60 m/min and the lowest chip load of 10 μm/flute. Figure 26a shows the sectional view of the CT image of a drilled and tapped dental implant. Detailed observation and measurement can then be performed (Figure 26b).

Finite element modeling of a microdrill was done to find the limiting drilling parameters that would fracture a microdrill catastrophically. It was assumed that

#### Figure 26.

Inspection of drilled and tapped microhole by X-ray computed tomography. Ti 6Al 4V dental implants fabricated by direct metal laser sintering [27].

## Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

High aspect ratio. Pecking is essential for microhole drilling since chips have to be extracted and cutting fluid must penetrate into a small and deep microhole. The pecking depth can be substantial in the beginning, but it must be gradually reduced when drilling at deeper depths. One can start with an initial pecking depth of (2\*drill diameter) and gradually reduce it to (0.5\*diameter) at the hole depth of (10\*diameter). It is convenient to program pecking cycles in microdrilling following

where P: incremental pecking depth (mm); D: drill diameter (mm); R: progres-

Although application of MQL in micromachining is necessary, other researchers have found ways to improve its effectiveness. Nanoparticles are mixed in MQL oil to improve its performance in microdrilling. The nano-sized particles of CNT-C60, TiO2, Al2O3, MoS2, and diamond would increase the thermal conductivity of the fluid thus prolong the tool life while reducing burr. In an experimental microdrilling study, uncoated microdrills of 100–500 μm diameters are used at 10–15 mm/min, 30,000–60,000 rpm spindle speeds, while varying the concentration of 30 nm diamond particles in the range 0–4%. The MQL system is used at 3-bar air pressure to drill a constant 0.3 mm depth. The optimal conditions for low torque and thrust force are experimentally obtained to be 60,000 rpm and 2% concentration. Inspection of a microdrilled and tapped hole would be difficult. Destructive technique by sectioning a part is time consuming, expensive, and error prone since the internal features might be distorted by releasing of residual stress. X-ray computed tomography (CT) has been utilized to evaluated drilled and tapped Ti 6Al 4V dental implant fabricated by direct metal laser sintering. The best microdrilled hole quality—cylindricity and perpendicularity—is achieved at the lowest testing drill speed of 60 m/min and the lowest chip load of 10 μm/flute. Figure 26a shows the sectional view of the CT image of a drilled and tapped dental implant. Detailed

<sup>9</sup> ð Þ �1:5<sup>R</sup> <sup>þ</sup> <sup>19</sup>:<sup>5</sup> for <sup>R</sup>≤<sup>10</sup> (9)

<sup>D</sup> <sup>¼</sup> <sup>0</sup>:5 for <sup>R</sup>><sup>10</sup> (10)

the equations below.

Micromachining

Figure 26.

26

fabricated by direct metal laser sintering [27].

P <sup>D</sup> <sup>¼</sup> <sup>1</sup>

sive drill aspect ratio = current hole depth/drill diameter.

P

observation and measurement can then be performed (Figure 26b).

Finite element modeling of a microdrill was done to find the limiting drilling parameters that would fracture a microdrill catastrophically. It was assumed that

Inspection of drilled and tapped microhole by X-ray computed tomography. Ti 6Al 4V dental implants

the drill shank is rigid, axial thrust force and torsional torque can fail a drill in either buckling mode or torsional mode. Transverse shear strength in three-point bending test of a sintered carbide tool is estimated to be half of the material tensile strength. This study used commercially pure (CP) titanium, 316L stainless steel, 6061-T6 aluminum, PEEK plastic, and Nitinol (51 wt% Ni 49% Ti) shape memory alloy. Some surfaces were faced milled on a milling machine, others were milled, hand ground and then finish polished with 1 μm diamond paste. Microdrilling was performed with Ø100–150 μm drill diameter, 135° point angle, 30° rake angle, 40–44° helix angle, 2 flutes, and 1.50–3.50 mm flute length. Some were coated with AlTiN or AlTiN/Si3N4 under the trade name Nanotek.

The classical Taylor's equation has been applied for macromachining, micromachining, and is used for microdrilling to show the effects of chip load and tool coating. For the same cutting speed of 20 m/min and comparable drilling distance of about 35 mm, the CP titanium can be microdrilled 400% faster than 316L stainless steel since the chip load for the former is 0.1 μm and that for the latter is 0.02 μm. Also, AlTiN coated drills improve tool life by at least 122%. This drilling operation is stopped after drilling all possible holes on the test blocks (Figures 27 and 28). Negligible tool wear are observed when drilling 6061-T6 aluminum and PEEK plastic, therefore, no modeling is necessary.

Burr, hole size, hole position, and work hardening around a drilled hole contribute to the hole quality. During the initial engagement of drill and workpiece surface, a slight lateral motion of the drill chisel edge is sufficient to bend and misguide the slender microdrill. Drill wandering refers to the deviation of a drilled hole from its intended position. A machined surface is rough enough to cause wandering of microdrills. A sloped ridge on a rough surface bends and deviates the drill axis from intended position. Such deviation causes drill wandering, significant burrs, and irregular hole diameters. The hole quality is improved significantly when drilling on a polished surface at the same or more aggressive drilling parameters. An improvement of 27% in hole center deviation is achieved for the polished surface of CP titanium (Figure 29). Similarly, an improvement of 260% on hole deviation is achieved when drilling a polished PEEK block.

Vickers microhardness near a drilled surface is obtained to study the level of plastic deformation and work hardening below a drilled surface. The hardness near the drilled surface is found to be 15% higher than those at the unmachined zone (Figure 30). The work hardening effect is caused by plastic deformation of the

Figure 27. Tool life plot for microdrilling of CP titanium. Progressive pecking, tool life criterion 8 μm [28].

Figure 28.

Tool life plot for microdrilling of 316L stainless steel. Progressive pecking, tool life criterion 15 μm. Drilling with AlTiN coated drills were stopped due to shortage of materials [28].

4.3 Ultraprecision turning

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Figure 30.

the ductile-regime mode.

Product miniaturization and demand for ultraprecision products drives the rapid development of micro/nano scale turning or ultraprecision turning. This technology produces polished and high quality spherical, aspherical parts from metals, ceramics, semiconductors, and polymers that cannot be economically produced by traditional grinding, lapping, or polishing processes. Micro/nano turning also produces intricate shape with low or no subsurface damage since it operates in

Vickers microhardness below drilled surface of the 10th hole. 12.7 mm drilling distance, 14 m/min, 0.035 μm/

flute chip load on 316L stainless steel. Microhardness test at 50 g load, 14 s dwell time [28].

Commercial lathe systems for ultraprecision machining are available. Although tool and axes motions can be in the nanometer ranges, it is an engineering challenge to control the thermal drift issue of a large system. Having a very compact 200-mm

• Have single crystalline structure that allows a sharp cutting edge as small as

• Have highest thermal conductivity among all engineering materials,

• Possess high elastic and shear moduli to resist plastic deformation, and

A diamond tool, however, is costly and brittle. A tool with zero or negative rake angle (i) improves its edge strength, and (ii) forms a hydrostatic compressive stress field in the material just in front and below a tool, therefore, minimizes crack initiation. The single crystal diamond typically has (110) crystal plane as rake face

Not any material can be successfully micro/nano turned with a diamond tool.

Ferrous alloys and silicon carbide (SiC) are not suitable for diamond turning

• Retain high strength and hardness at high temperature,

and is brazed onto a steel shank of different shape and size.

system, however, would compromise the required resolution for precision microturning [29]. Diamonds are commonly used for micro/nano turning. Polycrystalline diamond tools are sintered from micron-sized diamond grains. It is less expensive but with limited capability due to large edge radius (few hundred nanometers) and lower edge strength due to attrition wear. Single crystalline diamond

tools are best for micro/nano turning since they:

few nanometers (Figure 1b),

• Exhibit a low coefficient of friction.

29

#### Figure 29.

Composite optical images showing drill wandering and hole accuracy on (a) milled CP titanium surface; 4 m/min, 1 μm/flute, 2:1 aspect ratio, and (b) polished CP titanium surface; 12 m/min, 0.05 μm/flute, 10:1 aspect ratio [28].

surface by a worn tool, and smearing of BUE on the drill wall. Similar work hardening effect is reported while drilling austenitic stainless steel leading to a higher resistance near the chisel edge of the drill. Ideally, the drill cutting edges should not shear the workpiece within the work hardened layer from a previous cut. Since the work hardening zone is about 30 μm, it is impractical to microdrill 316L stainless steel at an aggressive chip load more than 30 μm/flute since a fragile microdrill would simply fracture.

Figure 30.

Vickers microhardness below drilled surface of the 10th hole. 12.7 mm drilling distance, 14 m/min, 0.035 μm/ flute chip load on 316L stainless steel. Microhardness test at 50 g load, 14 s dwell time [28].

## 4.3 Ultraprecision turning

Product miniaturization and demand for ultraprecision products drives the rapid development of micro/nano scale turning or ultraprecision turning. This technology produces polished and high quality spherical, aspherical parts from metals, ceramics, semiconductors, and polymers that cannot be economically produced by traditional grinding, lapping, or polishing processes. Micro/nano turning also produces intricate shape with low or no subsurface damage since it operates in the ductile-regime mode.

Commercial lathe systems for ultraprecision machining are available. Although tool and axes motions can be in the nanometer ranges, it is an engineering challenge to control the thermal drift issue of a large system. Having a very compact 200-mm system, however, would compromise the required resolution for precision microturning [29]. Diamonds are commonly used for micro/nano turning. Polycrystalline diamond tools are sintered from micron-sized diamond grains. It is less expensive but with limited capability due to large edge radius (few hundred nanometers) and lower edge strength due to attrition wear. Single crystalline diamond tools are best for micro/nano turning since they:


A diamond tool, however, is costly and brittle. A tool with zero or negative rake angle (i) improves its edge strength, and (ii) forms a hydrostatic compressive stress field in the material just in front and below a tool, therefore, minimizes crack initiation. The single crystal diamond typically has (110) crystal plane as rake face and is brazed onto a steel shank of different shape and size.

Not any material can be successfully micro/nano turned with a diamond tool. Ferrous alloys and silicon carbide (SiC) are not suitable for diamond turning

surface by a worn tool, and smearing of BUE on the drill wall. Similar work hardening effect is reported while drilling austenitic stainless steel leading to a higher resistance near the chisel edge of the drill. Ideally, the drill cutting edges should not shear the workpiece within the work hardened layer from a previous cut. Since the work hardening zone is about 30 μm, it is impractical to microdrill 316L stainless steel at an aggressive chip load more than 30 μm/flute since a fragile microdrill

Composite optical images showing drill wandering and hole accuracy on (a) milled CP titanium surface; 4 m/min, 1 μm/flute, 2:1 aspect ratio, and (b) polished CP titanium surface; 12 m/min, 0.05 μm/flute, 10:1

Tool life plot for microdrilling of 316L stainless steel. Progressive pecking, tool life criterion 15 μm. Drilling with

AlTiN coated drills were stopped due to shortage of materials [28].

would simply fracture.

Figure 29.

28

aspect ratio [28].

Figure 28.

Micromachining

Micromist is required to lubricate and cool both tool and machined surface. A micromist nozzle should move with a tool while blowing micro/nano chips away

An experimental study was done to compare machinability of Ti 6Al 4V produced by casting or selective laser melting. TiAlN coated microturning tools with 8° rake angle, 1.3 μm edge radius were used. Microturning at orthogonal condition was performed at 6–600 m/min, 1–20 μm/rev, and 500 μm depth of cut. The chip morphologies are similar for both materials and there is no significant effect on microstructure; however, the cutting and feeding forces on AM alloy is about 3–24% higher than those for cast alloy when varying the cutting speed (Figure 32a) or changing the feed (Figure 32b). Such higher forces would shorten the tool life

The demanding for product miniaturization and increasing part precision has fueled the development of micromachining. Recent explosion of product innovation with printing 3D metal parts also escalate the post processing studies due to inher-

micromilling, microdrilling, and microturning—with additive processes for metals —power bed fusion, material jetting, binder jetting, and direct energy deposition will enable the successful manufacturing of complex metal parts to meet strict

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

ent defects of 3D printed metals. The synergy of subtractive processes—

\* and Mike Corliss<sup>2</sup>

1 Texas A&M University, College Station, Texas, USA

\*Address all correspondence to: hung@tamu.edu

provided the original work is properly cited.

from the machined surface.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

5. Summary

engineering requirement.

Author details

31

Wayne N.P. Hung<sup>1</sup>

2 KGSBO, Katy, Texas, USA

and subsequently degrade the surface quality.

Figure 31.

(a) Microchip from CA 173 showing a beryllide inclusion, (b) deep scratch on machined surface by a broken beryllide.

because of diffusion from highly concentrated carbon in diamond tool to a lower concentration zone of carbon in workpiece materials when the cutting zone is at high temperature during machining. Selected materials that can be successfully machined with a diamond tool are shown in Table 4. These material should be homogeneous and contain few if no impurities. The hard inclusions might either damage a sharp diamond edge or being sheared off and smearing against the machined surface. Figure 31 shows the hard beryllides in beryllium copper CA173 that plow and smear the mirror finish surface.


#### Table 4.

Examples of diamond machinable materials.

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Micromist is required to lubricate and cool both tool and machined surface. A micromist nozzle should move with a tool while blowing micro/nano chips away from the machined surface.

An experimental study was done to compare machinability of Ti 6Al 4V produced by casting or selective laser melting. TiAlN coated microturning tools with 8° rake angle, 1.3 μm edge radius were used. Microturning at orthogonal condition was performed at 6–600 m/min, 1–20 μm/rev, and 500 μm depth of cut. The chip morphologies are similar for both materials and there is no significant effect on microstructure; however, the cutting and feeding forces on AM alloy is about 3–24% higher than those for cast alloy when varying the cutting speed (Figure 32a) or changing the feed (Figure 32b). Such higher forces would shorten the tool life and subsequently degrade the surface quality.

## 5. Summary

because of diffusion from highly concentrated carbon in diamond tool to a lower concentration zone of carbon in workpiece materials when the cutting zone is at high temperature during machining. Selected materials that can be successfully machined with a diamond tool are shown in Table 4. These material should be homogeneous and contain few if no impurities. The hard inclusions might either damage a sharp diamond edge or being sheared off and smearing against the machined surface. Figure 31 shows the hard beryllides in beryllium copper CA173

Semiconductor Metal Ceramic Plastics

Comparison of cutting forces in orthogonal microturning of cast and selective laser melted Ti 6Al 4V [30].

Aluminum oxide Zirconium oxide Optical glasses Quartz

Acrylic Fluoroplastics Nylon Polycarbonate Polymethylmethacrylate

Propylene Styrene

Aluminum alloys Copper alloys Electroless nickel

Gold Magnesium Silver Zinc

Examples of diamond machinable materials.

(a) Microchip from CA 173 showing a beryllide inclusion, (b) deep scratch on machined surface by a broken

that plow and smear the mirror finish surface.

Figure 31.

Micromachining

beryllide.

Cadmium telluride Gallium arsenide Germanium Lithium niobate Silicon Silicon nitride Zinc selenide Zinc sulphide

Table 4.

Figure 32.

30

The demanding for product miniaturization and increasing part precision has fueled the development of micromachining. Recent explosion of product innovation with printing 3D metal parts also escalate the post processing studies due to inherent defects of 3D printed metals. The synergy of subtractive processes micromilling, microdrilling, and microturning—with additive processes for metals —power bed fusion, material jetting, binder jetting, and direct energy deposition will enable the successful manufacturing of complex metal parts to meet strict engineering requirement.

## Author details

Wayne N.P. Hung<sup>1</sup> \* and Mike Corliss<sup>2</sup>

1 Texas A&M University, College Station, Texas, USA

2 KGSBO, Katy, Texas, USA

\*Address all correspondence to: hung@tamu.edu

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## References

[1] Chu WS, Kim CS, Lee HT, Choi JO, Park JI, Song JH, et al. Hybrid manufacturing in micro/nano scale: A review. International Journal of Precision Engineering and Manufacturing-Green Technology. 2014;1(1):75-92

[2] Kaynak Y, Kitay O. The effect of post-processing operations on surface characteristics of 316L stainless steel produced by selective laser melting. Additive Manufacturing. 2019;26:84-93

[3] Kaynak Y, Tascioglu E. Finish machining-induced surface roughness, microhardness and XRD analysis of selective laser melted Inconel 718 alloy. Procedia CIRP. 2018;71:500-504

[4] Jagadesh T, Samual GL. Investigation into cutting forces and surface roughness in micro turning of titanium alloy using coated carbide tool. Procedia Materials Science. 2014;5:2450-2457

[5] Afazov AM, Rachev SM, Segal J. Modelling and simulation of micromilling cutting forces. Journal of Materials Processing Technology. 2010; 210:2154-2162

[6] Rahim EA. Tool failure modes and wear mechanism of coated carbide tools when drilling Ti-6Al-4V. International Journal of Precision Technology. 2007;1 (1):30-39

[7] Zhou L, Ni J, He Q. Study on failure mechanism of the coated carbide tool. International Journal of Refractory Metals and Hard Materials. 2007;25:1-5

[8] Rutherford KL, Hutchings IM. A micro-abrasive wear test, with particular application to coated systems. Surface and Coatings Technology. 1996; 79:231-239

[9] Kumar M, Dotson K, Melkote SN. An experimental technique to detect tool–workpiece contact in micromilling. Journal of Manufacturing Processes. 2010;12:99-105

Engineering Manufacture. 2015;229

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Journal of Manufacturing Processes.

[25] Zhang Y, Jun MBG. Feasibility of lignin as additive in metalworking fluids

of Manufacturing Processes. 2014;16:

[26] Bruschi S, Tristo G, Rysava Z, Bariani PF, Umbrello D, De Chiffre L. Environmentally clean micromilling of electron beam melted Ti6Al4V. Journal of Cleaner Production. 2016;133:932-941

[27] Rysava Z, Bruschi S, Carmignato S, Medeossi F, Savio E, Zanini F. Microdrilling and threading of the Ti6Al4V titanium alloy produced through additive manufacturing. Procedia CIRP.

[28] Mohanty S, Wells S, Hung NP. Microdrilling of Biocompatible Materials. IMECE2012-87523, Proceedings, ASME International Mechanical Engineering Congress & Exposition; Houston, Texas. 2012

[29] Lu Z, Yoneyama T. Micro cutting in

the micro lathe turning system. International Journal of Machine Tools and Manufacture. 1999;39:1171-1183

[30] Coz GL, Fischer M, Piquard R, D'Acunto A, Laheurte P, Dudzinski D. Micro cutting of Ti-6Al-4V parts produced by SLM process. Procedia

CIRP. 2017;58:228-232

2018;34(Part B):750-757

for micro-milling. Journal

503-510

2016b;46:583-586

[17] Kajaria S, Chittipolu S, Adera S, Hung NP. Micromilling in minimum quantity lubrication. Machining Science and Technology. 2012;16:524-546

[18] Ziberov M, Bacci da Silva M, Jackson M, Hung NP. Effect of cutting fluid on micromilling of Ti-6Al-4V titanium alloy, NAMRC 44-129.

Surface finish of ball-end milled microchannels. Micro- and Nano-Manufacturing. 2014;2(0411005):1-10

Procedia Manufacturing. 2016;5:332-347

[19] Berestovskyi D, Hung NP, Lomeli P.

[20] Rysava Z, Bruschi S. Comparison between EBM and DMLS Ti6Al4V machinability characteristics under dry micro-milling conditions. Materials Science Forum. 2016a;836-837:177-184

[21] Kuram E, Ozcelik B. Multi-objective optimization using Taguchi based grey relational analysis for micro-milling of Al 7075 material with ball nose end mill. Measurement. 2013;46:1849-1864

[22] Sadiq M, Hoang MN, Valencia N, Obeidat S, Hung NP. Experimental study of micromilling selective laser melted Inconel 718 superalloy. Procedia Manufacturing. 2018;26:983-992

[23] Vazquez E, Gomar J, Ciurana J, Rodríguez CA. Analyzing effects of cooling and lubrication conditions in micromilling of Ti6Al4V. Journal of Cleaner Production. 2015;87:906-913

[24] Khan WA, Hoang MN, Tai B, Hung NP. Through-tool minimum quantity lubrication and effect on machinability.

33

[16] Hung NP, Fu YQ. Effect of crystalline orientation in the ductileregime machining of silicon. Journal of Advanced Manufacturing Technology.

(12):2134-2143

2000;16:871-876

[10] Shunmugavel M, Polishetty A, Nomani J, Goldberg M, Littlefair G. Metallurgical and machinability characteristics of wrought and selective laser melted Ti-6Al-4V. Journal of Metallurgy. 2016;2016:1-10. Article ID 7407918. Available from: http://dx. doi.org/10.1155/2016/7407918

[11] Gu J, Barber G, Tung S, Gu RJ. Tool life and wear mechanism of uncoated and coated milling inserts. Wear. 1999; 225–229:273-284

[12] Wang Z, Kovvuria V, Araujo A, Bacci M, Hung NP, Bukkapatnam STS. Built-up-edge effects on surface deterioration in micromilling processes. Journal of Manufacturing Processes. 2016;24:321-327

[13] Klocke F, Maßmann T, Gerschwiler K. Combination of PVD tool coatings and biodegradable lubricants in metal forming and machining. Wear. 2005; 259:1197-1206

[14] Nam JS, Kim DH, Chung H, Lee SW. Optimization of environmentally benign micro-drilling process with nanofluid minimum quantity lubrication using response surface methodology and genetic algorithm. Journal of Cleaner Production. 2015;102: 428-436

[15] Ucun I, Aslantasx K, Bedir F. The effect of minimum quantity lubrication and cryogenic pre-cooling on cutting performance in the micro milling of Inconel 718. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of

Micromachining of Advanced Materials DOI: http://dx.doi.org/10.5772/intechopen.89432

Engineering Manufacture. 2015;229 (12):2134-2143

References

Micromachining

2014;1(1):75-92

[1] Chu WS, Kim CS, Lee HT, Choi JO,

[9] Kumar M, Dotson K, Melkote SN. An experimental technique to detect tool–workpiece contact in micromilling. Journal of Manufacturing Processes.

[10] Shunmugavel M, Polishetty A, Nomani J, Goldberg M, Littlefair G. Metallurgical and machinability

laser melted Ti-6Al-4V. Journal of Metallurgy. 2016;2016:1-10. Article ID 7407918. Available from: http://dx.

doi.org/10.1155/2016/7407918

characteristics of wrought and selective

[11] Gu J, Barber G, Tung S, Gu RJ. Tool life and wear mechanism of uncoated and coated milling inserts. Wear. 1999;

[12] Wang Z, Kovvuria V, Araujo A, Bacci M, Hung NP, Bukkapatnam STS. Built-up-edge effects on surface

deterioration in micromilling processes. Journal of Manufacturing Processes.

[13] Klocke F, Maßmann T, Gerschwiler K. Combination of PVD tool coatings and biodegradable lubricants in metal forming and machining. Wear. 2005;

[14] Nam JS, Kim DH, Chung H, Lee SW. Optimization of environmentally benign micro-drilling process with nanofluid minimum quantity lubrication using response surface methodology and genetic algorithm. Journal of Cleaner Production. 2015;102:

[15] Ucun I, Aslantasx K, Bedir F. The effect of minimum quantity lubrication and cryogenic pre-cooling on cutting performance in the micro milling of Inconel 718. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of

2010;12:99-105

225–229:273-284

2016;24:321-327

259:1197-1206

428-436

manufacturing in micro/nano scale: A review. International Journal of Precision Engineering and

Manufacturing-Green Technology.

[2] Kaynak Y, Kitay O. The effect of post-processing operations on surface characteristics of 316L stainless steel produced by selective laser melting. Additive Manufacturing. 2019;26:84-93

[3] Kaynak Y, Tascioglu E. Finish machining-induced surface roughness, microhardness and XRD analysis of selective laser melted Inconel 718 alloy. Procedia CIRP. 2018;71:500-504

into cutting forces and surface

[4] Jagadesh T, Samual GL. Investigation

roughness in micro turning of titanium alloy using coated carbide tool. Procedia Materials Science. 2014;5:2450-2457

[5] Afazov AM, Rachev SM, Segal J. Modelling and simulation of micromilling cutting forces. Journal of Materials Processing Technology. 2010;

[6] Rahim EA. Tool failure modes and wear mechanism of coated carbide tools when drilling Ti-6Al-4V. International Journal of Precision Technology. 2007;1

[7] Zhou L, Ni J, He Q. Study on failure mechanism of the coated carbide tool. International Journal of Refractory Metals and Hard Materials.

[8] Rutherford KL, Hutchings IM. A micro-abrasive wear test, with

particular application to coated systems. Surface and Coatings Technology. 1996;

210:2154-2162

(1):30-39

2007;25:1-5

79:231-239

32

Park JI, Song JH, et al. Hybrid

[16] Hung NP, Fu YQ. Effect of crystalline orientation in the ductileregime machining of silicon. Journal of Advanced Manufacturing Technology. 2000;16:871-876

[17] Kajaria S, Chittipolu S, Adera S, Hung NP. Micromilling in minimum quantity lubrication. Machining Science and Technology. 2012;16:524-546

[18] Ziberov M, Bacci da Silva M, Jackson M, Hung NP. Effect of cutting fluid on micromilling of Ti-6Al-4V titanium alloy, NAMRC 44-129. Procedia Manufacturing. 2016;5:332-347

[19] Berestovskyi D, Hung NP, Lomeli P. Surface finish of ball-end milled microchannels. Micro- and Nano-Manufacturing. 2014;2(0411005):1-10

[20] Rysava Z, Bruschi S. Comparison between EBM and DMLS Ti6Al4V machinability characteristics under dry micro-milling conditions. Materials Science Forum. 2016a;836-837:177-184

[21] Kuram E, Ozcelik B. Multi-objective optimization using Taguchi based grey relational analysis for micro-milling of Al 7075 material with ball nose end mill. Measurement. 2013;46:1849-1864

[22] Sadiq M, Hoang MN, Valencia N, Obeidat S, Hung NP. Experimental study of micromilling selective laser melted Inconel 718 superalloy. Procedia Manufacturing. 2018;26:983-992

[23] Vazquez E, Gomar J, Ciurana J, Rodríguez CA. Analyzing effects of cooling and lubrication conditions in micromilling of Ti6Al4V. Journal of Cleaner Production. 2015;87:906-913

[24] Khan WA, Hoang MN, Tai B, Hung NP. Through-tool minimum quantity lubrication and effect on machinability.

Journal of Manufacturing Processes. 2018;34(Part B):750-757

[25] Zhang Y, Jun MBG. Feasibility of lignin as additive in metalworking fluids for micro-milling. Journal of Manufacturing Processes. 2014;16: 503-510

[26] Bruschi S, Tristo G, Rysava Z, Bariani PF, Umbrello D, De Chiffre L. Environmentally clean micromilling of electron beam melted Ti6Al4V. Journal of Cleaner Production. 2016;133:932-941

[27] Rysava Z, Bruschi S, Carmignato S, Medeossi F, Savio E, Zanini F. Microdrilling and threading of the Ti6Al4V titanium alloy produced through additive manufacturing. Procedia CIRP. 2016b;46:583-586

[28] Mohanty S, Wells S, Hung NP. Microdrilling of Biocompatible Materials. IMECE2012-87523, Proceedings, ASME International Mechanical Engineering Congress & Exposition; Houston, Texas. 2012

[29] Lu Z, Yoneyama T. Micro cutting in the micro lathe turning system. International Journal of Machine Tools and Manufacture. 1999;39:1171-1183

[30] Coz GL, Fischer M, Piquard R, D'Acunto A, Laheurte P, Dudzinski D. Micro cutting of Ti-6Al-4V parts produced by SLM process. Procedia CIRP. 2017;58:228-232

**35**

**Chapter 2**

**Abstract**

**1. Introduction**

Texturing

Pico- and Femtosecond Laser

The pico- and femtosecond laser micromachining has grown up as a reliable tool for precise manufacturing and electronic industries to make fine drilling and machining into hard metals and ceramics as well as soft plastic and to form various nano- and microtextures for improvement of surface functions and properties in products. The ultrashort-pulse laser machining systems were developed to describe the fine microdrilling and microtexturing behavior for various materials. Accuracy in circularity and drilled depth were evaluated to discuss the effect of substrate materials on the laser microdrilling. Accuracy in unit geometry and alignment was also discussed for applications. A carbon base mold substrate was micromachined to transcribe its microtextures to transparent plastics and oxide glasses. Three practical examples were introduced to demonstrate the effectiveness of nano-/ microtexturing on the improvement of microjoinability, the reduction in friction and wear of mechanical parts and tools, and the surface property control. The fastrate laser machinability, the spatial resolution in laser microtexturing as well as the laser micromanufacturing capacity were discussed to aim at the future innovations

**Keywords:** picosecond laser micromachining, femtosecond laser micromachining, microdrilling, microtexturing, nano-/microtexturing, laser micropatterning

The laser technology for manufacturing is classified into two categories; e.g., thermal and athermal processings [1]. CO2-laser with continuous power supply and fiber-lasers with use of short pulses are typical processing for welding, machining, and joining by formation of thermally hot spots [2]. Various fiber lasers [3] have been developed and applied to laser welding, laser machining, laser marking, and so on [3]. Most of them utilize the nanosecond solid-state oscillators and make thermal machining of materials. In recent, pico- and femtosecond laser machining [4–6] is widely utilized for athermal removal of materials with high dimensional accuracy in practice. There are two keywords to classify the laser processing; i.e., the wave length of light and the pulse duration time. CO2 laser has the longest wave length of 10.6 μm, while excimer laser by KrF, 248 nm. Most of laser wave length (λ) ranges from the far ultra-violet regime less than 200 nm to infra-red regime more than 20 μm. Since every material has its own relaxation rime (τ0), most of laser power can be absorbed by the material having the equivalent τ0 to λ. Then, this targeting work material is athermally

Micromachining for Surface

*Tatsuhiko Aizawa and Tadahiko Inohara*

in manufacturing toward the sustainable society.

## **Chapter 2**

## Pico- and Femtosecond Laser Micromachining for Surface Texturing

*Tatsuhiko Aizawa and Tadahiko Inohara*

## **Abstract**

The pico- and femtosecond laser micromachining has grown up as a reliable tool for precise manufacturing and electronic industries to make fine drilling and machining into hard metals and ceramics as well as soft plastic and to form various nano- and microtextures for improvement of surface functions and properties in products. The ultrashort-pulse laser machining systems were developed to describe the fine microdrilling and microtexturing behavior for various materials. Accuracy in circularity and drilled depth were evaluated to discuss the effect of substrate materials on the laser microdrilling. Accuracy in unit geometry and alignment was also discussed for applications. A carbon base mold substrate was micromachined to transcribe its microtextures to transparent plastics and oxide glasses. Three practical examples were introduced to demonstrate the effectiveness of nano-/ microtexturing on the improvement of microjoinability, the reduction in friction and wear of mechanical parts and tools, and the surface property control. The fastrate laser machinability, the spatial resolution in laser microtexturing as well as the laser micromanufacturing capacity were discussed to aim at the future innovations in manufacturing toward the sustainable society.

**Keywords:** picosecond laser micromachining, femtosecond laser micromachining, microdrilling, microtexturing, nano-/microtexturing, laser micropatterning

## **1. Introduction**

The laser technology for manufacturing is classified into two categories; e.g., thermal and athermal processings [1]. CO2-laser with continuous power supply and fiber-lasers with use of short pulses are typical processing for welding, machining, and joining by formation of thermally hot spots [2]. Various fiber lasers [3] have been developed and applied to laser welding, laser machining, laser marking, and so on [3]. Most of them utilize the nanosecond solid-state oscillators and make thermal machining of materials. In recent, pico- and femtosecond laser machining [4–6] is widely utilized for athermal removal of materials with high dimensional accuracy in practice.

There are two keywords to classify the laser processing; i.e., the wave length of light and the pulse duration time. CO2 laser has the longest wave length of 10.6 μm, while excimer laser by KrF, 248 nm. Most of laser wave length (λ) ranges from the far ultra-violet regime less than 200 nm to infra-red regime more than 20 μm. Since every material has its own relaxation rime (τ0), most of laser power can be absorbed by the material having the equivalent τ0 to λ. Then, this targeting work material is athermally

#### **Figure 1.**

*Typical characteristics of ultra-short pulse laser machining. (a) Significant increase of laser power by reduction of Δt down to 1 ps, (b) ablation as an athermal removal of materials, and (c) laser intensity profile.*

machined by selecting the laser with suitable wave length; otherwise, the work is only thermally cut or drilled. A micromachining essentially requires for fast-rate removal of materials with sufficient accuracy in dimension and geometry; the repetition frequency as well as the wave length must be optimally selected to make suitable laser micromachining to each work-material. With use of second harmonic generator (SHG), third harmonic generator (THG), and forth harmonic generator (FHG), the fundamental wavelength of 1064 nm is controllable to be 532, 355 and 266 nm, respectively.

The pulse duration time (Δt) is important for short-pulse laser micromachining. As shown in **Figure 1a**, the pulse power increases significantly with reduction of Δt. When the laser energy with Δt = 1 ms is 1 mJ, the laser power (P) is only 1 W; P reaches to 1 GW only by shortening Δt down to 1 ps.

Under high power laser irradiation, most of materials are athermally removed, or, ablazed, as depicted in **Figure 1b**. The dimensional accuracy in laser micromachining is determined by focusing the laser spot for this ablation process. This laser irradiation has a finite spot size which is dependent on λ and Δt. The laser intensity distributes even in the focused spot; e.g., the well-controlled laser intensity distributes in Gaussian profile as depicted in **Figure 1c**.

In the following, our developing ultrashort pulse laser machining systems are employed to make microdrilling and microtexturing into various kinds of work materials. In particular, the laser microtexturing technology is applied to microjoining process of dissimilar polymers, and to microdimple formation for friction control of sliding parts and components and for reservoir of wear debris during dry cutting. Further applications including the surface property control by using the nano-/microtexturing are discussed in this chapter.

## **2. Pico- and femtosecond laser micromachining system**

Our developed pico- and femtosecond laser machining systems are stated with some comments on their capacity and configuration.

**37**

**Figure 3.**

**Figure 2.**

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

A single picosecond is equivalent to the relaxation time of molecular bonding stage; its pulsed power is readily absorbed by most of work materials. Three types of picosecond laser machining systems were developed; a standard system and its configuration are shown in **Figure 2**. The machining speed is dependent on the repletion frequency and average power. The dimensional accuracy in machining is determined by the beam spot size. To be discussed later, this spot size depends on the optical system; e.g., the minimum spot size can be controlled down to 1 μm when every lens is fixed on the stage. However, when the lens position is controlled during machining, the spot size becomes wider; e.g., it is limited by 10 μm when using the galvanometer.

A single femtosecond or subpicosecond lasers are developed for innovative research and development; most industrial applications stand on this laser machining in the order of 100 femtoseconds. Our developed system and its configuration

tion, how to scan the beam spot becomes more important when using this laser machining system. Higher repetition frequency of laser beams as well as higher scanning speed result in fast-rate dimensionally accurate machining. At present, a laser oscillator with the repetition frequency of 40 MHz has been already developed for machining. How to make fast control of this short pulse laser becomes an

*Our developed picosecond laser machining system and its capacity and configuration.*

*Our developed femtosecond laser machining system and its capacity and configuration.*

Since the focused spot of work materials is subjected to ultra-high power irradia-

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

**2.1 Picosecond laser micromachining system**

**2.2 Femtosecond laser micromachining system**

essential issue in laser machining design.

are shown in **Figure 3**.

## **2.1 Picosecond laser micromachining system**

*Micromachining*

**Figure 1.**

machined by selecting the laser with suitable wave length; otherwise, the work is only thermally cut or drilled. A micromachining essentially requires for fast-rate removal of materials with sufficient accuracy in dimension and geometry; the repetition frequency as well as the wave length must be optimally selected to make suitable laser micromachining to each work-material. With use of second harmonic generator (SHG), third harmonic generator (THG), and forth harmonic generator (FHG), the fundamental wavelength of 1064 nm is controllable to be 532, 355 and 266 nm, respectively.

*Typical characteristics of ultra-short pulse laser machining. (a) Significant increase of laser power by reduction of Δt down to 1 ps, (b) ablation as an athermal removal of materials, and (c) laser intensity profile.*

The pulse duration time (Δt) is important for short-pulse laser micromachining. As shown in **Figure 1a**, the pulse power increases significantly with reduction of Δt. When the laser energy with Δt = 1 ms is 1 mJ, the laser power (P) is only 1 W; P

Under high power laser irradiation, most of materials are athermally removed, or, ablazed, as depicted in **Figure 1b**. The dimensional accuracy in laser micromachining is determined by focusing the laser spot for this ablation process. This laser irradiation has a finite spot size which is dependent on λ and Δt. The laser intensity distributes even in the focused spot; e.g., the well-controlled laser intensity distrib-

In the following, our developing ultrashort pulse laser machining systems are employed to make microdrilling and microtexturing into various kinds of work materials. In particular, the laser microtexturing technology is applied to microjoining process of dissimilar polymers, and to microdimple formation for friction control of sliding parts and components and for reservoir of wear debris during dry cutting. Further applications including the surface property control by using the

Our developed pico- and femtosecond laser machining systems are stated with

reaches to 1 GW only by shortening Δt down to 1 ps.

utes in Gaussian profile as depicted in **Figure 1c**.

nano-/microtexturing are discussed in this chapter.

some comments on their capacity and configuration.

**2. Pico- and femtosecond laser micromachining system**

**36**

A single picosecond is equivalent to the relaxation time of molecular bonding stage; its pulsed power is readily absorbed by most of work materials. Three types of picosecond laser machining systems were developed; a standard system and its configuration are shown in **Figure 2**. The machining speed is dependent on the repletion frequency and average power. The dimensional accuracy in machining is determined by the beam spot size. To be discussed later, this spot size depends on the optical system; e.g., the minimum spot size can be controlled down to 1 μm when every lens is fixed on the stage. However, when the lens position is controlled during machining, the spot size becomes wider; e.g., it is limited by 10 μm when using the galvanometer.

## **2.2 Femtosecond laser micromachining system**

A single femtosecond or subpicosecond lasers are developed for innovative research and development; most industrial applications stand on this laser machining in the order of 100 femtoseconds. Our developed system and its configuration are shown in **Figure 3**.

Since the focused spot of work materials is subjected to ultra-high power irradiation, how to scan the beam spot becomes more important when using this laser machining system. Higher repetition frequency of laser beams as well as higher scanning speed result in fast-rate dimensionally accurate machining. At present, a laser oscillator with the repetition frequency of 40 MHz has been already developed for machining. How to make fast control of this short pulse laser becomes an essential issue in laser machining design.


#### **Figure 2.**

*Our developed picosecond laser machining system and its capacity and configuration.*


**Figure 4.**

*Typical two optical control units for laser micromachining. (a) Optical control unit with use of galvanometer and (b) beam rotator.*

## **2.3 Optical unit control**

In parallel with the development of laser oscillators and machining unit, the optical unit design is also important for accurate laser machining. Two unit designs are introduced in **Figure 4**; e.g., an optical control unit with use of galvanometer to distribute the laser beam as designed, and a beam rotator for laser drilling with accurate circularity. The former unit is a standard approach for laser machining with moderate rate; new controller must be developed to make much faster rate laser machining. The latter is a powerful tool to rotate the optical units and to move the laser beam in the axisymmetric manner.

Various controlling tools of laser beam can be designed and developed for each application of laser machining.

## **3. Fine microdrilling into metallic alloys and ceramics**

These pico- and femtoseconds with the pulse duration in the order of 10<sup>−</sup><sup>12</sup> and 10<sup>−</sup>15 s provide a reliable means to drill the through-holes into the ceramics, the metallic alloys, and the plastics [7]. Compared to the micromilling and the microelectrical discharge machining (micro-EDM), finer through-holes with higher circularity are formed without residuals at the inlet of holes and without deterioration on their inner surfaces [8]. In addition, no micromilling tools and no thin EDM wires are needed to drill the lots of through-holes onto the relatively large area. In this laser drilling process, the surface quality of through-holes as well as their circularity is strongly dependent on the laser beam control, as summarized in [9]. In the conventional fiber-laser machining, the inlet of through-holes is deteriorated by the redeposits and the residuals [10]. Even when using the picosecond pulse lasers, the through-hole shape is also damaged by the unstable laser beams [11, 12]. Typical damage of through-holes comes from the branching from the straight hole drilled in the initial stage to two holes. The deviation of beam focusing and positioning directly induces these defects [7, 8, 13]. Our developed picosecond laser machining system for industrial applications is applied to drill the through-holes into the ceramic plates. The beam rotator is used as a trepanning system for laser drilling. The alumina plate with the thickness of 1 mm is employed as a substrate. Scanning electron microscopy (SEM) is used to measure the diameter of throughholes as well as their aspect ratio. The replica method is also utilized to describe the

**39**

**Figure 6.**

*and, (b) Picosecond laser drilling.*

**Figure 5.**

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

geometric alignment and homogeneity of through-holes. The drilled through-holes with the uniform diameter of 50 μm and the aspect ratio of 10.0 are accurately aligned into the alumina plate. The present trepanning device works to control the diameter of irradiation for fine drilling of the tapered and inversely tapered

The alumina plate is prepared for the laser drilling under the experimental setup in **Figure 5**. The beam rotator as well as projection lens unit is utilized to improve the focused beam quality. Through the CCD and display, the microdrilling process

Let us first evaluate on the difference of drilling behavior between the fiber lasers and the picosecond laser. The through-hole with the diameter of 50 μm is drilled into alumina plate. When using the normal fiber lasers, the surroundings of hole are completely damaged with deposits on them **Figure 6a**. While, the accurate hole with circularity of 1 μm is drilled by the picosecond laser without damage and

No residuals or no redeposits at the vicinity of through-hole inlets prove that the present picosecond lase drilling is free from the thermal effects to deteriorate the surface quality of work specimen. The picosecond laser drilling is utilized to

*Comparison of the drilled through-hole between fiber lasers and the picosecond laser. (a) Fiber laser drilling,* 

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

**3.1 Microdrilling into ceramic substrates**

through-holes into steels.

is monitored during operation.

deposits, as shown in **Figure 6b**.

*A typical experimental setup for laser microdrilling.*

## *Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

geometric alignment and homogeneity of through-holes. The drilled through-holes with the uniform diameter of 50 μm and the aspect ratio of 10.0 are accurately aligned into the alumina plate. The present trepanning device works to control the diameter of irradiation for fine drilling of the tapered and inversely tapered through-holes into steels.

## **3.1 Microdrilling into ceramic substrates**

*Micromachining*

**2.3 Optical unit control**

*and (b) beam rotator.*

**Figure 4.**

the laser beam in the axisymmetric manner.

**3. Fine microdrilling into metallic alloys and ceramics**

application of laser machining.

In parallel with the development of laser oscillators and machining unit, the optical unit design is also important for accurate laser machining. Two unit designs are introduced in **Figure 4**; e.g., an optical control unit with use of galvanometer to distribute the laser beam as designed, and a beam rotator for laser drilling with accurate circularity. The former unit is a standard approach for laser machining with moderate rate; new controller must be developed to make much faster rate laser machining. The latter is a powerful tool to rotate the optical units and to move

*Typical two optical control units for laser micromachining. (a) Optical control unit with use of galvanometer* 

Various controlling tools of laser beam can be designed and developed for each

These pico- and femtoseconds with the pulse duration in the order of 10<sup>−</sup><sup>12</sup> and 10<sup>−</sup>15 s provide a reliable means to drill the through-holes into the ceramics, the metallic alloys, and the plastics [7]. Compared to the micromilling and the microelectrical discharge machining (micro-EDM), finer through-holes with higher circularity are formed without residuals at the inlet of holes and without deterioration on their inner surfaces [8]. In addition, no micromilling tools and no thin EDM wires are needed to drill the lots of through-holes onto the relatively large area. In this laser drilling process, the surface quality of through-holes as well as their circularity is strongly dependent on the laser beam control, as summarized in [9]. In the conventional fiber-laser machining, the inlet of through-holes is deteriorated by the redeposits and the residuals [10]. Even when using the picosecond pulse lasers, the through-hole shape is also damaged by the unstable laser beams [11, 12]. Typical damage of through-holes comes from the branching from the straight hole drilled in the initial stage to two holes. The deviation of beam focusing and positioning directly induces these defects [7, 8, 13]. Our developed picosecond laser machining system for industrial applications is applied to drill the through-holes into the ceramic plates. The beam rotator is used as a trepanning system for laser drilling. The alumina plate with the thickness of 1 mm is employed as a substrate. Scanning electron microscopy (SEM) is used to measure the diameter of throughholes as well as their aspect ratio. The replica method is also utilized to describe the

**38**

The alumina plate is prepared for the laser drilling under the experimental setup in **Figure 5**. The beam rotator as well as projection lens unit is utilized to improve the focused beam quality. Through the CCD and display, the microdrilling process is monitored during operation.

Let us first evaluate on the difference of drilling behavior between the fiber lasers and the picosecond laser. The through-hole with the diameter of 50 μm is drilled into alumina plate. When using the normal fiber lasers, the surroundings of hole are completely damaged with deposits on them **Figure 6a**. While, the accurate hole with circularity of 1 μm is drilled by the picosecond laser without damage and deposits, as shown in **Figure 6b**.

No residuals or no redeposits at the vicinity of through-hole inlets prove that the present picosecond lase drilling is free from the thermal effects to deteriorate the surface quality of work specimen. The picosecond laser drilling is utilized to

**Figure 5.** *A typical experimental setup for laser microdrilling.*

#### **Figure 6.**

*Comparison of the drilled through-hole between fiber lasers and the picosecond laser. (a) Fiber laser drilling, and, (b) Picosecond laser drilling.*

## *Micromachining*

fabricate a series of holes with periodically aligned into alumina substrate. **Figure 7a** depicts the through-holes drilled into the alumina. Each through-hole is aligned with the pitch of 300 μm as programmed by the CAM data mining through the positioning control of beams. As had been discussed in [11], the inner surface quality of through-holes is sensitive to the instability during the laser drilling. **Figure 7b** also demonstrates that the straight through-hole inner surfaces are formed to have constant diameter without any geometric damages by the picosecond laser drilling. This is because the laser beam is well profiled through the trepanning system before fine control by the galvanometer, and is controlled to move into the depth of work materials. The above straightforwardness of through-holes is also demonstrated by using the replica method. In this method, the silicone-based polymers are infiltrated into each through-hole. The frozen polymers are used as a replica to reproduce the drilled through-hole shape. **Figure 7c** depicts the alignment of replicas in correspondence to a series of laser-drilled through-holes. Three through-holes were laser-drilled down to the same depth in the alumina plate. Since the first three polymer pillars have the same height as 150 μm, the successive series of throughholes are accurately machined into the alumina with the same depth.

These straight through-holes with high aspect ratio provide a solution to the demand for the probe-cards to make accurate inspection of the semiconductor

#### **Figure 7.**

*Picosecond laser drilling of through-holes into the alumina plate. (a) Alignment of through-holes, (b) inner surface of through-hole with the diameter of 50 μm, and (c) demonstration of the homogeneous laser drilling by using the replica method.*

#### **Figure 8.**

*Picosecond laser drilling of through-holes with higher aspect ratio. (a) Drilled through-holes into alumina plate and (b) drilled through-holes into partially stabilized zirconia (PSZ).*

**41**

**Figure 10.**

**Figure 9.**

*the picosecond laser.*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

chips. The probe-pins are pierced through the straight through-holes of alumina or PSZ substrates for inspection. These through-holes must have higher aspect ratio than 10 to preserve the sufficient working space. **Figure 8a** depicts the through-hole with the diameter of 50 μm machined into the alumina plate with the thickness of 1 mm; the aspect ratio reaches to 20. This high aspect ratio is also attained even when laser drilling PSZ in **Figure 8b**. This demonstrates that the trepanned laser drilling enables to make through-holes with higher aspect ratio than 20 under the

In the die and mold industries, the case-hardened and plasma-treated steels are often utilized for high proof of dimensional accuracy. Let us also compare the laser drilling performance between the fiber-lasers and the picosecond laser. **Figure 9** compared the drilled through-holes between two lasers. The large heat-affected zones as well as damages surround the drilled hole by fiber laser in **Figure 9a**. While, the clean and accurate through-hole is drilled into the case-hardened steels

Without use of the beam rotation control, the tapering is difficult or nearly impossible in the laser drilling. In the present setup, the pair of lenses in the beam rotator in **Figure 5** is radially adjusted to directly control the diameter of irradiated

*Comparison of the drilled through-holes into the case-hardened steels. (a) Using the fiber lasers and (b) using* 

*Picosecond laser drilling of the tapered and inversely tapered through-holes into the case-hardened steels.*

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

well-structured setup in laser machining.

**3.2 Microdrilling into metallic alloys**

by the picosecond laser in **Figure 9b**.

## *Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

chips. The probe-pins are pierced through the straight through-holes of alumina or PSZ substrates for inspection. These through-holes must have higher aspect ratio than 10 to preserve the sufficient working space. **Figure 8a** depicts the through-hole with the diameter of 50 μm machined into the alumina plate with the thickness of 1 mm; the aspect ratio reaches to 20. This high aspect ratio is also attained even when laser drilling PSZ in **Figure 8b**. This demonstrates that the trepanned laser drilling enables to make through-holes with higher aspect ratio than 20 under the well-structured setup in laser machining.

## **3.2 Microdrilling into metallic alloys**

In the die and mold industries, the case-hardened and plasma-treated steels are often utilized for high proof of dimensional accuracy. Let us also compare the laser drilling performance between the fiber-lasers and the picosecond laser. **Figure 9** compared the drilled through-holes between two lasers. The large heat-affected zones as well as damages surround the drilled hole by fiber laser in **Figure 9a**. While, the clean and accurate through-hole is drilled into the case-hardened steels by the picosecond laser in **Figure 9b**.

Without use of the beam rotation control, the tapering is difficult or nearly impossible in the laser drilling. In the present setup, the pair of lenses in the beam rotator in **Figure 5** is radially adjusted to directly control the diameter of irradiated

#### **Figure 9.**

*Micromachining*

**40**

**Figure 8.**

**Figure 7.**

*by using the replica method.*

*Picosecond laser drilling of through-holes with higher aspect ratio. (a) Drilled through-holes into alumina plate* 

fabricate a series of holes with periodically aligned into alumina substrate. **Figure 7a** depicts the through-holes drilled into the alumina. Each through-hole is aligned with the pitch of 300 μm as programmed by the CAM data mining through the positioning control of beams. As had been discussed in [11], the inner surface quality of through-holes is sensitive to the instability during the laser drilling. **Figure 7b** also demonstrates that the straight through-hole inner surfaces are formed to have constant diameter without any geometric damages by the picosecond laser drilling. This is because the laser beam is well profiled through the trepanning system before fine control by the galvanometer, and is controlled to move into the depth of work materials. The above straightforwardness of through-holes is also demonstrated by using the replica method. In this method, the silicone-based polymers are infiltrated into each through-hole. The frozen polymers are used as a replica to reproduce the drilled through-hole shape. **Figure 7c** depicts the alignment of replicas in correspondence to a series of laser-drilled through-holes. Three through-holes were laser-drilled down to the same depth in the alumina plate. Since the first three polymer pillars have the same height as 150 μm, the successive series of through-

holes are accurately machined into the alumina with the same depth.

These straight through-holes with high aspect ratio provide a solution to the demand for the probe-cards to make accurate inspection of the semiconductor

*Picosecond laser drilling of through-holes into the alumina plate. (a) Alignment of through-holes, (b) inner surface of through-hole with the diameter of 50 μm, and (c) demonstration of the homogeneous laser drilling* 

*and (b) drilled through-holes into partially stabilized zirconia (PSZ).*

*Comparison of the drilled through-holes into the case-hardened steels. (a) Using the fiber lasers and (b) using the picosecond laser.*

## **Figure 10.**

*Picosecond laser drilling of the tapered and inversely tapered through-holes into the case-hardened steels.*

region. When this diameter is narrowed from the inlet to the outlet with the constant velocity, the uniformly tapered through-hole is drilled to have a constant tapered angle up to the specified positive skew angle. On the other hand, the inversely tapered through-hole is also machined by enlarging this diameter with the constant velocity in the similar way down to the negative skew angle. These tapering or inversely tapering processes from the inlet to outlet of the through-hole are automatically programmed. After CAM data in the present laser drilling, this diameter of irradiation is narrowed from the inlet by 100 μm to the outlet by 30 μm with the constant feeding velocity. Then, the tapered through-hole is built into the alumina plate with the constant angle of +30° and the higher aspect ratio than 10.0 in **Figure 10**. In the similar way, the inversely tapered through-hole is formed by enlarging the diameter of irradiation from the inlet by 100 μm to the outlet by 180 μm also with the constant velocity. The inversely tapered through-hole is also drilled into the alumina with the thickness of 1 mm. The inversely tapered angles are also constant by −25°. In both cases, the inner surfaces of holes are finely shaped with less roughness [14].

## **4. Fine microtexturing onto metallic alloys and polymers**

Microtextures with the size in the order of 1–100 μm on the solid surface and interface work to reduce the friction and wear, to assist the joinability, and to functionalize the surfaces and interfaces [15]. Micromilling [16] and microelectrical discharge machining (micro-EDM) [17] have been utilized to make microtexturing onto the steel surfaces. Due to the limitation on the machining tool shape and their controllability for machining, their application is also limited in practice. Shortpulse laser machining is employed to make microtexturing onto the metallic and ceramic surfaces.

## **4.1 Microtexturing of dimples onto the surfaces**

A circular dimple is formed on the various metallic surfaces as an aligned structure. **Figure 11** depicts four microtexturing cases. The unit-geometry of microdimples, their alignment on the surfaces, and the finished surface quality are preserved with less roughing during laser processing. For an example, the circular microdimples with the diameter of 95 μm and the depth of 26 μm are formed on the AISI430 surface in the pitch of 110 μm as shown in **Figure 11d**. No difference

#### **Figure 11.**

*Laser microtexturing of circular dimples aligned on the metallic substrates. (a) Aluminum, (b) copper, (c) nickel, and (d) AISI430.*

**43**

**Figure 13.**

*and (d) AISI430.*

**Figure 12.**

*(c) AISI304, and (d) boron-silicate glass.*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

**4.2 Microtexturing of embosses onto the surfaces**

in microdimple size and shape and in its alignment is noticed for various kinds of

The initial geometric data in CAD and CAM for laser microdimple texturing are data-transformed from positive to negative; this transformed CAD and CAM data are automatically built for laser microemboss formation. In practice, the concave patterning to form the microdimples changes itself to the convex patterning to form the microembosses onto the substrate surfaces. **Figure 12** depicts four microembossing cases. The dimensional and geometric accuracies are preserved in the similar manner of microdimple formation. For an example, the circular microembosses with the diameter of 250 μm and the depth of 125 μm are formed on the

boron-silicate glass surface in the pitch of 280 μm, as shown in **Figure 12d**.

With use of femtosecond lasers, finer microtextures are formed as a threedimensional structure on the metallic surfaces. **Figure 13** depicts the threedimensional microstructures formed on the steel surfaces. In particular, the

*Laser microembossing of circular embosses aligned on the metallic and ceramic substrates. (a) AISI430, (b) Ni,* 

*Laser microtexturing of three-dimensional structures onto the surfaces. (a) AISI410, (b) SISI304, (c) AISI430,* 

**4.3 Microtexturing of 3D-lattice structures onto the surfaces**

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

metallic substrates.

*Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

in microdimple size and shape and in its alignment is noticed for various kinds of metallic substrates.

## **4.2 Microtexturing of embosses onto the surfaces**

The initial geometric data in CAD and CAM for laser microdimple texturing are data-transformed from positive to negative; this transformed CAD and CAM data are automatically built for laser microemboss formation. In practice, the concave patterning to form the microdimples changes itself to the convex patterning to form the microembosses onto the substrate surfaces. **Figure 12** depicts four microembossing cases. The dimensional and geometric accuracies are preserved in the similar manner of microdimple formation. For an example, the circular microembosses with the diameter of 250 μm and the depth of 125 μm are formed on the boron-silicate glass surface in the pitch of 280 μm, as shown in **Figure 12d**.

## **4.3 Microtexturing of 3D-lattice structures onto the surfaces**

With use of femtosecond lasers, finer microtextures are formed as a threedimensional structure on the metallic surfaces. **Figure 13** depicts the threedimensional microstructures formed on the steel surfaces. In particular, the

#### **Figure 12.**

*Micromachining*

with less roughness [14].

ceramic surfaces.

**42**

**Figure 11.**

*(c) nickel, and (d) AISI430.*

*Laser microtexturing of circular dimples aligned on the metallic substrates. (a) Aluminum, (b) copper,* 

region. When this diameter is narrowed from the inlet to the outlet with the constant velocity, the uniformly tapered through-hole is drilled to have a constant tapered angle up to the specified positive skew angle. On the other hand, the inversely tapered through-hole is also machined by enlarging this diameter with the constant velocity in the similar way down to the negative skew angle. These tapering or inversely tapering processes from the inlet to outlet of the through-hole are automatically programmed. After CAM data in the present laser drilling, this diameter of irradiation is narrowed from the inlet by 100 μm to the outlet by 30 μm with the constant feeding velocity. Then, the tapered through-hole is built into the alumina plate with the constant angle of +30° and the higher aspect ratio than 10.0 in **Figure 10**. In the similar way, the inversely tapered through-hole is formed by enlarging the diameter of irradiation from the inlet by 100 μm to the outlet by 180 μm also with the constant velocity. The inversely tapered through-hole is also drilled into the alumina with the thickness of 1 mm. The inversely tapered angles are also constant by −25°. In both cases, the inner surfaces of holes are finely shaped

**4. Fine microtexturing onto metallic alloys and polymers**

**4.1 Microtexturing of dimples onto the surfaces**

Microtextures with the size in the order of 1–100 μm on the solid surface and interface work to reduce the friction and wear, to assist the joinability, and to functionalize the surfaces and interfaces [15]. Micromilling [16] and microelectrical discharge machining (micro-EDM) [17] have been utilized to make microtexturing onto the steel surfaces. Due to the limitation on the machining tool shape and their controllability for machining, their application is also limited in practice. Shortpulse laser machining is employed to make microtexturing onto the metallic and

A circular dimple is formed on the various metallic surfaces as an aligned structure. **Figure 11** depicts four microtexturing cases. The unit-geometry of microdimples, their alignment on the surfaces, and the finished surface quality are preserved with less roughing during laser processing. For an example, the circular microdimples with the diameter of 95 μm and the depth of 26 μm are formed on the AISI430 surface in the pitch of 110 μm as shown in **Figure 11d**. No difference

*Laser microembossing of circular embosses aligned on the metallic and ceramic substrates. (a) AISI430, (b) Ni, (c) AISI304, and (d) boron-silicate glass.*

#### **Figure 13.**

*Laser microtexturing of three-dimensional structures onto the surfaces. (a) AISI410, (b) SISI304, (c) AISI430, and (d) AISI430.*

**Figure 14.** *Laser microtexturing of fine periodic structures onto the surfaces. (a) Al, (b) AISI304, (c) AISI304, and (d) AISI304.*

Gaussian-shaped pillar array with the height of 20 μm and the pitch of 20 μm is machined into the AISI430 substrate as shown in **Figure 13c**.

## **4.4 Microtexturing of periodic structures onto the surfaces**

Three-dimensional periodic microstructures have a capability to functionalize the metallic surfaces for optical reflection and diffraction devices and for stamping die and injection mold to transcribe their negative textures onto metallic and polymer sheets. **Figure 14** depicts the periodic microstructures formed on the aluminum and AISI304 steel substrates, respectively. **Figure 14a** is a stepwise terrace structure machined into aluminum with each layer thickness of 5 μm by decreasing the diameter from 450 μm down to 50 μm with the step of 100 μm.

## **5. Fine microtexturing into carbon-base molds**

Two- and three-dimensional microtexturing becomes much important in preparation of mold-dies for mold-stamping of optical elements [18]. The most popular microtexture is a Fresnel pattern for optical lens; circumferential patterns with steep surfaces must be imprinted onto the surface of substrate materials. V-lettershaped micropatterns are laser-machined onto the glassy carbon substrate to discuss the dimensional accuracy and to investigate the depth profile for different aspect ratio. Furthermore, our developing microstamping system [8, 19, 20] is utilized to duplicate these micropatterns onto optical polymers by using the patterned glassy carbon mold-dies and to discuss the accuracy by this imprinting.

## **5.1 Microtexturing into glassy carbon die substrate**

In the two-dimensional microtexturing, a unit pattern like a groove, a dimple, or a wedge is machined with the specified regularity onto the substrate by using X-Y positioning control. Here, a microgroove is employed as a standard unit pattern to fabricate the microtextured mold-die. Glassy carbon (GC) substrate is employed to make V-letter-shaped microgrooving with the pitch of 35 μm, the V-shaped wedge width of 10 μm, and the depth of 10 μm in design. **Figure 15a** shows the optical micrograph of V-shaped grooving pattern on GC substrate. One groove is laser-machined twice on the same designed machining path. This micropattern is formed onto the GC substrate with the area of 25 × 25 mm2 for 40 min or 2.4 ks. As shown in **Figure 15b**, a sharp

**45**

**Figure 17.**

**Figure 15.**

**Figure 16.**

*and (b) SEM image of microgrooves.*

*Depth profile across the V-letter shaped microgrooves in GC substrate.*

*Relationship between the designed and measured microgroove depths.*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

wedge of microgroove is imprinted onto the multilayered GC substrate. The microgroove has 10 μm in width, and 35 μm in pitch. The geometric dimensions specified in CAM program are accurately reproduced in the actual laser microtexturing. The depth profile of V-letter-shaped microgrooves is directly measured to investigate the accuracy of depth in the two-dimensional texturing. **Figure 16** depicts the measured depth profile by precise surface profilometer. Deviation of depth ranges from −1 to +2 μm around the average depth of 10 μm. This proves that regular patterns could be machined by the present approach. In order to investigate the controllability of microgrooving in depth, the designed depth parameter is varied with the laser beam power kept constant. **Figure 17** compares the relationship between the designed and

*Laser microtexturing of V-letter shaped grooves into GC substrate. (a) Microscopic image of microgrooved GC* 

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

## *Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

wedge of microgroove is imprinted onto the multilayered GC substrate. The microgroove has 10 μm in width, and 35 μm in pitch. The geometric dimensions specified in CAM program are accurately reproduced in the actual laser microtexturing. The depth profile of V-letter-shaped microgrooves is directly measured to investigate the accuracy of depth in the two-dimensional texturing. **Figure 16** depicts the measured depth profile by precise surface profilometer. Deviation of depth ranges from −1 to +2 μm around the average depth of 10 μm. This proves that regular patterns could be machined by the present approach. In order to investigate the controllability of microgrooving in depth, the designed depth parameter is varied with the laser beam power kept constant. **Figure 17** compares the relationship between the designed and

#### **Figure 15.**

*Micromachining*

**Figure 14.**

*(d) AISI304.*

Gaussian-shaped pillar array with the height of 20 μm and the pitch of 20 μm is

*Laser microtexturing of fine periodic structures onto the surfaces. (a) Al, (b) AISI304, (c) AISI304, and* 

Three-dimensional periodic microstructures have a capability to functionalize the metallic surfaces for optical reflection and diffraction devices and for stamping die and injection mold to transcribe their negative textures onto metallic and polymer sheets. **Figure 14** depicts the periodic microstructures formed on the aluminum and AISI304 steel substrates, respectively. **Figure 14a** is a stepwise terrace structure machined into aluminum with each layer thickness of 5 μm by decreasing

Two- and three-dimensional microtexturing becomes much important in preparation of mold-dies for mold-stamping of optical elements [18]. The most popular microtexture is a Fresnel pattern for optical lens; circumferential patterns with steep surfaces must be imprinted onto the surface of substrate materials. V-lettershaped micropatterns are laser-machined onto the glassy carbon substrate to discuss the dimensional accuracy and to investigate the depth profile for different aspect ratio. Furthermore, our developing microstamping system [8, 19, 20] is utilized to duplicate these micropatterns onto optical polymers by using the patterned glassy

In the two-dimensional microtexturing, a unit pattern like a groove, a dimple, or a wedge is machined with the specified regularity onto the substrate by using X-Y positioning control. Here, a microgroove is employed as a standard unit pattern to fabricate the microtextured mold-die. Glassy carbon (GC) substrate is employed to make V-letter-shaped microgrooving with the pitch of 35 μm, the V-shaped wedge width of 10 μm, and the depth of 10 μm in design. **Figure 15a** shows the optical micrograph of V-shaped grooving pattern on GC substrate. One groove is laser-machined twice on the same designed machining path. This micropattern is formed onto the GC substrate

for 40 min or 2.4 ks. As shown in **Figure 15b**, a sharp

machined into the AISI430 substrate as shown in **Figure 13c**.

**4.4 Microtexturing of periodic structures onto the surfaces**

the diameter from 450 μm down to 50 μm with the step of 100 μm.

carbon mold-dies and to discuss the accuracy by this imprinting.

**5.1 Microtexturing into glassy carbon die substrate**

**5. Fine microtexturing into carbon-base molds**

**44**

with the area of 25 × 25 mm2

*Laser microtexturing of V-letter shaped grooves into GC substrate. (a) Microscopic image of microgrooved GC and (b) SEM image of microgrooves.*

**Figure 16.** *Depth profile across the V-letter shaped microgrooves in GC substrate.*

**Figure 17.** *Relationship between the designed and measured microgroove depths.*

#### **Figure 18.**

*Transcription of the V-letter wedge microtexture on GC to the V-letter bump microtexture via the mold stamping. (a) Multimicrogrooved PMMA sheet, (b) V-letter bump microtextures on PMMA, and (c) formation of microbump by inclusion of melt PMMA into V-letter wedge on GC mold.*

measured depths in this microgrooving. Up to 20 μm, the average depth of microgrooves is accurately controlled by the present laser machining system.

## **5.2 Mold stamping into optical plastics**

The above microtextured GC substrate is used as a mold-die for warm moldstamping. PMMA sheet with the thickness of 1 mm is employed as a work material for this mold-stamping just above its glass-transition temperature of 383 K (or 110°C). **Figure 18a** showed the V-letter-shaped grooving patterns, which are imprinted onto PMMA by the load of 1 kN for 60 s. The V-letter-shaped concave patterns in **Figure 15b** are accurately imprinted onto PMMA as the convex micropattern as shown in **Figure 18b**. That is, a series of microwedge fins are fabricated by this mold-stamping with use of microtextured mold in **Figure 15**. In the mold-stamping, the filling process of work materials into the micropatterns on the mold-die is essential for accurate imprinting. Precise observation with higher magnification in SEM is made to investigate this filling behavior at the initial stage of mold-stamping. **Figure 18c** depicted a convex bump with the width of 10 μm and the height of 3.5 μm. This bump formation is just the initial stage of filling process for viscous PMMA to infiltrate into the V-letter-shaped groove by mold-stamping. In case of mold-stamping just above the glass transition temperature, viscosity of plastic materials is so high as to reduce the filling velocity. This reflects on the slow shearing along the side faces of microgroove.

## **6. Microjoining of dissimilar polymers by laser microtexturing**

Most of mobile cellular phones are not water-proven so as to be diminished in the accident where those were dropped into water. To be free from these damages, there have been done many efforts to install the perfect waterproof into them [21]; e.g., a silicone rubber ring was sandwiched between plastic cover cases to prevent from water penetration through clearance. This fixture might work well just after shipping; it could be useless at the presence of dirt on the interface or through its misalignment by users in daily use of mobile phones. As the first remedy, a liquid silicone rubber with adhesives is fixed onto their polymer case by the liquid injection molding (LIM) process [22]. Since adhesives invoked in the silicone are responsible for joining, delamination might occur in partial after repetitive opening-and-closing operations in daily use of mobile phones. This difficulty requests us to reconsider the joining process between flexible rubber and hard plastic case in the mobile phone.

**47**

**Figure 19.**

*steel substrate.*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

**6.1 Microgrooving into the mold for injection molding**

The microgrooves are formed into the stainless steel mold for injection molding [23, 24]. Silicone rubber is joined with the polycarbonate plate as a specimen for joining strength test. The measured joining strength is constant by 4 N/mm at the presence of fine microgrooves, where the thinnest silicon rubber fractures without interfacial delamination. This joinability is common to the mobile phone model. The waterproof testing demonstrates that this joined interface has sufficient

The picosecond laser microtexturing with use of the galvanometer is employed to form the microgroove textures onto the AISI martensitic stainless steel mold. **Figure 19** depicts four microgrooved AISI420 molds with varying widths of 100, 75, 45, and 20 μm, respectively. The groove depth is constant by 10 μm. Each microgroove is shaped to have Gaussian profile irrespectively; the beam intensity profile directly reflects on this microgroove geometry. This mold is inserted into the die-set for injection molding. Polycarbonate (PC) is employed as a work material to imprint these microgroove textures onto the work surface. **Figure 20** depicts the transcribed microbump textures onto PC from the microgroove on the AISI420 mold. Both the groove width and pitch are accurately preserved through this injection molding.

In the LIM process, adhesive primer is deposited onto the interface before infiltration of silicone melt in the mold-die. Since intermission between two processes is less than 2–3 s, adhesion takes place between silicon and PC-plate under the cooling stage. **Figure 21a** depicts the PC plate specimen with a silicone square ring after joining in the inside of mold-die during LIM process. In the following test, only the joined section in the width of 80 mm is used for tensile adhesive strength testing. A uniaxial

*Microgroove textures with various widths from 100 to 20 μm and constant depth of 10 μm into AISI420 stainless* 

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

integrity at high pressure state by 15 kPa.

**6.2 Joining strength test**

*Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

The microgrooves are formed into the stainless steel mold for injection molding [23, 24]. Silicone rubber is joined with the polycarbonate plate as a specimen for joining strength test. The measured joining strength is constant by 4 N/mm at the presence of fine microgrooves, where the thinnest silicon rubber fractures without interfacial delamination. This joinability is common to the mobile phone model. The waterproof testing demonstrates that this joined interface has sufficient integrity at high pressure state by 15 kPa.

## **6.1 Microgrooving into the mold for injection molding**

The picosecond laser microtexturing with use of the galvanometer is employed to form the microgroove textures onto the AISI martensitic stainless steel mold. **Figure 19** depicts four microgrooved AISI420 molds with varying widths of 100, 75, 45, and 20 μm, respectively. The groove depth is constant by 10 μm. Each microgroove is shaped to have Gaussian profile irrespectively; the beam intensity profile directly reflects on this microgroove geometry. This mold is inserted into the die-set for injection molding. Polycarbonate (PC) is employed as a work material to imprint these microgroove textures onto the work surface. **Figure 20** depicts the transcribed microbump textures onto PC from the microgroove on the AISI420 mold. Both the groove width and pitch are accurately preserved through this injection molding.

#### **6.2 Joining strength test**

*Micromachining*

**Figure 18.**

*Transcription of the V-letter wedge microtexture on GC to the V-letter bump microtexture via the mold stamping. (a) Multimicrogrooved PMMA sheet, (b) V-letter bump microtextures on PMMA, and (c) formation of microbump by inclusion of melt PMMA into V-letter wedge on GC mold.*

measured depths in this microgrooving. Up to 20 μm, the average depth of micro-

The above microtextured GC substrate is used as a mold-die for warm moldstamping. PMMA sheet with the thickness of 1 mm is employed as a work material for this mold-stamping just above its glass-transition temperature of 383 K (or 110°C). **Figure 18a** showed the V-letter-shaped grooving patterns, which are imprinted onto PMMA by the load of 1 kN for 60 s. The V-letter-shaped concave patterns in **Figure 15b** are accurately imprinted onto PMMA as the convex micropattern as shown in **Figure 18b**. That is, a series of microwedge fins are fabricated by this mold-stamping with use of microtextured mold in **Figure 15**. In the mold-stamping, the filling process of work materials into the micropatterns on the mold-die is essential for accurate imprinting. Precise observation with higher magnification in SEM is made to investigate this filling behavior at the initial stage of mold-stamping. **Figure 18c** depicted a convex bump with the width of 10 μm and the height of 3.5 μm. This bump formation is just the initial stage of filling process for viscous PMMA to infiltrate into the V-letter-shaped groove by mold-stamping. In case of mold-stamping just above the glass transition temperature, viscosity of plastic materials is so high as to reduce the filling velocity. This reflects on the slow

grooves is accurately controlled by the present laser machining system.

**6. Microjoining of dissimilar polymers by laser microtexturing**

Most of mobile cellular phones are not water-proven so as to be diminished in the accident where those were dropped into water. To be free from these damages, there have been done many efforts to install the perfect waterproof into them [21]; e.g., a silicone rubber ring was sandwiched between plastic cover cases to prevent from water penetration through clearance. This fixture might work well just after shipping; it could be useless at the presence of dirt on the interface or through its misalignment by users in daily use of mobile phones. As the first remedy, a liquid silicone rubber with adhesives is fixed onto their polymer case by the liquid injection molding (LIM) process [22]. Since adhesives invoked in the silicone are responsible for joining, delamination might occur in partial after repetitive opening-and-closing operations in daily use of mobile phones. This difficulty requests us to reconsider the joining process between flexible rubber and hard plastic case in

**5.2 Mold stamping into optical plastics**

shearing along the side faces of microgroove.

**46**

the mobile phone.

In the LIM process, adhesive primer is deposited onto the interface before infiltration of silicone melt in the mold-die. Since intermission between two processes is less than 2–3 s, adhesion takes place between silicon and PC-plate under the cooling stage. **Figure 21a** depicts the PC plate specimen with a silicone square ring after joining in the inside of mold-die during LIM process. In the following test, only the joined section in the width of 80 mm is used for tensile adhesive strength testing. A uniaxial

**Figure 19.**

*Microgroove textures with various widths from 100 to 20 μm and constant depth of 10 μm into AISI420 stainless steel substrate.*

**Figure 20.**

*Transcription from the microgrooves on the AISI420 mold to the microbumps on the PC specimen.*

#### **Figure 21.**

*Joining strength testing. (a) Microbump textured PC specimen joined with silicone rubber and (b) fatal fracture of silicone without interfacial delamination.*

tensile testing system with the dynamic video monitoring is used to measure the loading behavior till the final fracture with in situ observation on the deformation of silicone. As shown in **Figure 21b**, when the microgroove width is less than the intrinsic microcavity width of 100 μm, the fatal fracture occurs in the tensile silicone rubber without any delamination of interface between PC and silicone. This joining strength reaches 4 N/mm irrespective of the joined length and size even if the microcavities are present on the interface. This implies that microtextures on the joined interface could control the cavitation process to be free from interfacial delamination.

## **6.3 Waterproof test of cellular-phone model**

The skewed microgrooves with their width and depth of 20 μm are lasermachined into the AISI420 die insert. In the similar way to preparation for the PC-specimen with the silicone rubber ring, the injection molding is used to transcribe the microgrooves into the PC-cover case; LIM process is also utilized to make

**49**

**Figure 22.**

**Figure 23.**

*on the interface.*

with aid of microbump textures.

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

in situ joining of silicone rubber ring onto the PC-cover case via the microbump textures on PC. **Figure 22** depicts the mobile phone model, fabricated in the above procedure. Each interface between the PC-cover case and silicone rubber has microbump textures. The Hamron leakage testing is employed to perform the waterproof test; e.g., this test aims at the quality check of significant deformation by small leaks under the applied pressure for 5 min. This model is dipped into a water pool, pressurized up to 15 kPa and held for 5 min. As shown in **Figure 23**, the PC-cover case deforms by pressuring it up to 15 kPa; no further deformation is detected during the holding duration. This demonstrates the perfect waterproof on the jointed interface

*Waterproof test to demonstrate the integrity of mobile phones under the pressure of 15 kPa.*

*A mobile phone PC-model joined with the silicone rubber through the microtexture with the width of 20 mm* 

Superhydrophilicity and superhydrophobicity have grown up as a key surface engineering to keep clean and fresh surface of products and to control the liquid

**7. Surface property control by laser nano-/microtexturing**

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

#### **Figure 22.**

*Micromachining*

**Figure 20.**

**Figure 21.**

**48**

tensile testing system with the dynamic video monitoring is used to measure the loading behavior till the final fracture with in situ observation on the deformation of silicone. As shown in **Figure 21b**, when the microgroove width is less than the intrinsic microcavity width of 100 μm, the fatal fracture occurs in the tensile silicone rubber without any delamination of interface between PC and silicone. This joining strength reaches 4 N/mm irrespective of the joined length and size even if the microcavities are present on the interface. This implies that microtextures on the joined interface could

*Joining strength testing. (a) Microbump textured PC specimen joined with silicone rubber and (b) fatal* 

*Transcription from the microgrooves on the AISI420 mold to the microbumps on the PC specimen.*

control the cavitation process to be free from interfacial delamination.

The skewed microgrooves with their width and depth of 20 μm are lasermachined into the AISI420 die insert. In the similar way to preparation for the PC-specimen with the silicone rubber ring, the injection molding is used to transcribe the microgrooves into the PC-cover case; LIM process is also utilized to make

**6.3 Waterproof test of cellular-phone model**

*fracture of silicone without interfacial delamination.*

*A mobile phone PC-model joined with the silicone rubber through the microtexture with the width of 20 mm on the interface.*

#### **Figure 23.**

*Waterproof test to demonstrate the integrity of mobile phones under the pressure of 15 kPa.*

in situ joining of silicone rubber ring onto the PC-cover case via the microbump textures on PC. **Figure 22** depicts the mobile phone model, fabricated in the above procedure. Each interface between the PC-cover case and silicone rubber has microbump textures. The Hamron leakage testing is employed to perform the waterproof test; e.g., this test aims at the quality check of significant deformation by small leaks under the applied pressure for 5 min. This model is dipped into a water pool, pressurized up to 15 kPa and held for 5 min. As shown in **Figure 23**, the PC-cover case deforms by pressuring it up to 15 kPa; no further deformation is detected during the holding duration. This demonstrates the perfect waterproof on the jointed interface with aid of microbump textures.

## **7. Surface property control by laser nano-/microtexturing**

Superhydrophilicity and superhydrophobicity have grown up as a key surface engineering to keep clean and fresh surface of products and to control the liquid

flow on the product surfaces. The oxide-glass lens as well as metallic-glass, optical elements are a typical targeting product to have their surface hydrophilic or superhydrophilic for liquid film formation, and to have it hydrophobic or superhydrophobic for well-defined water repellency [25]. The high energy surface had higher attractive capacity to other material atoms and molecules; those are adherent to each other to form a wet film on the surface. While, the low energy surface had lower attractive capacity to other material atoms and molecules; those are isolated from each other to form the droplets on the surface.

There are two modifications to control this surface state; e.g., the chemical and physical treatments. The chemical treatment is a general tool to modify the surface condition; e.g., fluorine-based coating increases the contact angle up to 130–150° in [26]. On the other hand, the idea of lotus effect has been discussed as a physical approach to form hydrophobic surface [27]. This lotus effect works in nature since the water droplets are supported by the air gap through the fine fibrous lotus leaf; this idea suggests that wettability might be widely controlled by the micro-/ nanotexturing [28]. As has been reported in [29–31], the femtosecond laser micro-/ nanotexturing methods have been developed to tune the surface wettability from superhydrophilic to superhydrophobic states. In particular, the micro-/submicro textures are formed on any materials by the laser-induced periodic surface structuring (LIPSS), where the incident and reflected lights have interaction with the scattered and diffracted lights at the vicinity of surface roughness [32]. Among several approaches to design this LIPSS, the authors proposed the micro-/submicrotexturing design by LIPSS with the use of fundamental wavelets and high-frequency ripples [33, 34]. Here, LIPSS is formed onto the AISI304 stainless steel substrates by using the femtosecond laser texturing. Both the superhydrophilic and superhydrophobic surfaces can be formed by the present laser nano-/microtexturing. The geometric effect of surface geometry on the superhydrophobicity is discussed to optimize the laser surface profile control.

## **7.1 LIPSS by femtosecond laser texturing**

With reduction of the pulse duration, the optical interaction with irradiated materials localizes in the wavelength range. When irradiating the materials in the fundamental mode, this interaction field is limited within the submicrometer range. LIPSS is a typical local interaction, occurring at the site of material surface roughness in the order of micrometer. **Figure 24** depicts the LIPSS formed on the austenitic stainless steel type 304 by the present femtosecond laser texturing. Nanotexturing alignment angulates itself across the microtexture in **Figure 24a**

#### **Figure 24.**

*LIPSS formed on AISI304 substrate surface by the present femtosecond laser texturing. (a) Microtextured angulation and nanotextures and (b) fine alignment of nanotextures.*

**51**

**Figure 26.**

**Figure 25.**

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

the metallic surface by the femtosecond laser treatment.

works as a major geometric item in surface quality.

**7.3 Superhydrophobic surface formation**

with the size of 25 × 25 × 3 mm3

since optical interaction is affected by the surface profile in micrometer range. As shown in **Figure 24b**, the spatial periodicity of these nanotextures is constant by 250 nm. This reveals that fine nanotextures with constant periodicity are formed on

After the classical theory on the surface wettability [35], the hydrophilic or the hydrophobic surfaces are modified to have superhydrophilic or superhydrophobic states, respectively. This is because the geometric item works to decrease the contact

angle for hydrophilic surface or to increase it for hydrophobic one. **Figure 25** depicts the wettability of nanotextured AISI304 surface by the femtosecond laser surface modification. The measured contact angle reaches down to 8°; it is superhydrophilic. This reveals that the classical theory is true to describe the geometric nanotexture effect on the contact angle when the spatial periodicity of nanotextures

In addition to the nanotexturing surface modification, the microtexturing angulation is taken into account as the geometric item. AISI304 stainless steel sheets

of surface wettability by this processing. **Figure 26** compares the droplets swelling on the specimen before and after this micro/submicrolaser texturing. The contact angle of pure water on the bare stainless steels is 70–75°, corresponding to the normal wettability of metals [36]. Through the present texturing, the contact angle

*Modification of hydrophilic surface to have superhydrophilic state by laser nanotexturing.*

*Modification of wettability on the AISI304 substrate from the original hydrophilic state to the* 

*superhydrophobic one by laser nano-/microtexturing.*

are nano-/microtextured to investigate the change

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

**7.2 Superhydrophilic surface formation**

## *Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

since optical interaction is affected by the surface profile in micrometer range. As shown in **Figure 24b**, the spatial periodicity of these nanotextures is constant by 250 nm. This reveals that fine nanotextures with constant periodicity are formed on the metallic surface by the femtosecond laser treatment.

## **7.2 Superhydrophilic surface formation**

*Micromachining*

**50**

**Figure 24.**

*LIPSS formed on AISI304 substrate surface by the present femtosecond laser texturing. (a) Microtextured* 

With reduction of the pulse duration, the optical interaction with irradiated materials localizes in the wavelength range. When irradiating the materials in the fundamental mode, this interaction field is limited within the submicrometer range. LIPSS is a typical local interaction, occurring at the site of material surface roughness in the order of micrometer. **Figure 24** depicts the LIPSS formed on the austenitic stainless steel type 304 by the present femtosecond laser texturing. Nanotexturing alignment angulates itself across the microtexture in **Figure 24a**

flow on the product surfaces. The oxide-glass lens as well as metallic-glass, optical elements are a typical targeting product to have their surface hydrophilic or superhydrophilic for liquid film formation, and to have it hydrophobic or superhydrophobic for well-defined water repellency [25]. The high energy surface had higher attractive capacity to other material atoms and molecules; those are adherent to each other to form a wet film on the surface. While, the low energy surface had lower attractive capacity to other material atoms and molecules; those are isolated

There are two modifications to control this surface state; e.g., the chemical and physical treatments. The chemical treatment is a general tool to modify the surface condition; e.g., fluorine-based coating increases the contact angle up to 130–150° in [26]. On the other hand, the idea of lotus effect has been discussed as a physical approach to form hydrophobic surface [27]. This lotus effect works in nature since the water droplets are supported by the air gap through the fine fibrous lotus leaf; this idea suggests that wettability might be widely controlled by the micro-/ nanotexturing [28]. As has been reported in [29–31], the femtosecond laser micro-/ nanotexturing methods have been developed to tune the surface wettability from superhydrophilic to superhydrophobic states. In particular, the micro-/submicro textures are formed on any materials by the laser-induced periodic surface structuring (LIPSS), where the incident and reflected lights have interaction with the scattered and diffracted lights at the vicinity of surface roughness [32]. Among several approaches to design this LIPSS, the authors proposed the micro-/submicrotexturing design by LIPSS with the use of fundamental wavelets and high-frequency ripples [33, 34]. Here, LIPSS is formed onto the AISI304 stainless steel substrates by using the femtosecond laser texturing. Both the superhydrophilic and superhydrophobic surfaces can be formed by the present laser nano-/microtexturing. The geometric effect of surface geometry on the superhydrophobicity is discussed to

from each other to form the droplets on the surface.

optimize the laser surface profile control.

**7.1 LIPSS by femtosecond laser texturing**

*angulation and nanotextures and (b) fine alignment of nanotextures.*

After the classical theory on the surface wettability [35], the hydrophilic or the hydrophobic surfaces are modified to have superhydrophilic or superhydrophobic states, respectively. This is because the geometric item works to decrease the contact angle for hydrophilic surface or to increase it for hydrophobic one. **Figure 25** depicts the wettability of nanotextured AISI304 surface by the femtosecond laser surface modification. The measured contact angle reaches down to 8°; it is superhydrophilic. This reveals that the classical theory is true to describe the geometric nanotexture effect on the contact angle when the spatial periodicity of nanotextures works as a major geometric item in surface quality.

## **7.3 Superhydrophobic surface formation**

In addition to the nanotexturing surface modification, the microtexturing angulation is taken into account as the geometric item. AISI304 stainless steel sheets with the size of 25 × 25 × 3 mm3 are nano-/microtextured to investigate the change of surface wettability by this processing. **Figure 26** compares the droplets swelling on the specimen before and after this micro/submicrolaser texturing. The contact angle of pure water on the bare stainless steels is 70–75°, corresponding to the normal wettability of metals [36]. Through the present texturing, the contact angle

**Figure 25.**

*Modification of hydrophilic surface to have superhydrophilic state by laser nanotexturing.*

#### **Figure 26.**

*Modification of wettability on the AISI304 substrate from the original hydrophilic state to the superhydrophobic one by laser nano-/microtexturing.*

#### **Figure 27.**

*Effect of the longitudinal aspect ratio on the measured contact angle among 30 nano-/microtextured AISI304 substrates.*

increases up to 156°. This proves that nano-/microlaser texturing provides a tool to modify the wettability of stainless steel surfaces from hydrophobic to superhydrophobic state. This finding is completely against the classical theory; if more geometric items are put into laser texturing, the material surface quality can be widely controlled by geometric design.

#### **7.4 Optimization of surface geometric configuration**

There are two geometric items affecting on the surface property; the fractal dimension and the aspect ratio for nanotextures [37]. The former influences on the complexity of surface geometry; the latter, on the local angulation of geometry. Thirty AISI304 stainless steel sheets with the size of 10 × 10 × 0.1 t mm3 are laser nano-/microtextured to investigate the effect of microtexture pitch and height on the measured wettability. **Figure 27** describes the relationship between the aspect ratio of nanotexture width to height on the measured contact angle. When this aspect ratio is less than 0.1 or more than 0.3, almost all measured contact angles are less than 155°; the micro-/submicrotextured AISI304 specimens are only hydrophobic. Higher contact angle up to 170° is attained when tuning this aspect ratio between 0.2 and 0.3; e. g., when using the microtextures with the width of 20 μm, their height might well be 2–6 μm. This implies that local angulation of surface geometry has significant influence on the controllability of hydrophobicity.

## **8. Friction control of tools by laser microtexturing**

Under the strong demand for reduction of environmental burdens in manufacturing, every productive line must be energy-saving and highly material-efficient with less emission to environments [38]. In past, the huge amount of lubricating oils has been utilized to reduce the friction and wear not only in automobile industries but also in machining, metal forming, and so on [39]. In order to reduce this amount down to

**53**

**Figure 28.**

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

the effect of microdimpled cutting tool on the reduction of tool wear.

the minimum quantity, the contact surface of mechanical parts and tool surfaces are microtextured to reduce the friction coefficient and wear rate under minimum quantity lubrication (MQL) [40]. Microdimples on the working interfaces and surfaces play as a lubricating oil pocket to form a thin lubricating oil film on the interface between sliding parts and between work materials and tools [41]. The depth profile of each microdimple reflects on the local pressure distribution; this interfacial lubricating film works as a pressure boundary to support the sufficient film thickness to lubrication under MQL [42]. In addition, these microdimples work as a reservoir to store the wear debris of work materials and tool chips during the semidry machining and metal forming [43]. Here, the microdimples are formed by the picosecond laser texturing onto the dies and tools. The pin-on-ball method is employed to evaluate on the reduction of friction for the microdimpled die. The normal milling test is also utilized to describe

The picosecond laser microtexturing is employed to form the circular microdimples onto the AISI420 stainless steel dies, with the diameter of 50 and 100 μm and the depth of 10 μm in the regular lattice alignment with the pitch of 100 and 200 μm, respectively, for tribotesting. While, the isosceles triangular microdimples with the bottom edge of 155 μm, the height of 80 μm and the depth of 5 μm are machined onto the WC (Co) cutting tools in the zigzag alignment. **Figure 28** depicts these microdimpled specimens and tool together with the SEM-image and

The pin-on-ball testing is employed to measure the time evolution of frictional force under the constant normal load. In this testing, the counter material ball is on contact with the die material under the applied normal weight as depicted in **Figure 29**. The frictional force is directly measured by load sensor attached to the arm. In the following tests, SUJ2 hard balls are utilized as a counter material. The friction coefficient is calculated by division of the measured friction force to the applied normal load. **Figure 30** depicts the transients of friction coefficient with increasing

*Laser microdimple texturing. (a) Microdimpled stainless steel die and (b) microdimpled WC (Co) cutting tool.*

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

**8.1 Laser microtexturing of dimples**

three-dimensional profile of microdimples.

**8.2 Tribotesting**

*Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

the minimum quantity, the contact surface of mechanical parts and tool surfaces are microtextured to reduce the friction coefficient and wear rate under minimum quantity lubrication (MQL) [40]. Microdimples on the working interfaces and surfaces play as a lubricating oil pocket to form a thin lubricating oil film on the interface between sliding parts and between work materials and tools [41]. The depth profile of each microdimple reflects on the local pressure distribution; this interfacial lubricating film works as a pressure boundary to support the sufficient film thickness to lubrication under MQL [42]. In addition, these microdimples work as a reservoir to store the wear debris of work materials and tool chips during the semidry machining and metal forming [43]. Here, the microdimples are formed by the picosecond laser texturing onto the dies and tools. The pin-on-ball method is employed to evaluate on the reduction of friction for the microdimpled die. The normal milling test is also utilized to describe the effect of microdimpled cutting tool on the reduction of tool wear.

## **8.1 Laser microtexturing of dimples**

The picosecond laser microtexturing is employed to form the circular microdimples onto the AISI420 stainless steel dies, with the diameter of 50 and 100 μm and the depth of 10 μm in the regular lattice alignment with the pitch of 100 and 200 μm, respectively, for tribotesting. While, the isosceles triangular microdimples with the bottom edge of 155 μm, the height of 80 μm and the depth of 5 μm are machined onto the WC (Co) cutting tools in the zigzag alignment. **Figure 28** depicts these microdimpled specimens and tool together with the SEM-image and three-dimensional profile of microdimples.

## **8.2 Tribotesting**

*Micromachining*

**Figure 27.**

*substrates.*

controlled by geometric design.

**7.4 Optimization of surface geometric configuration**

**8. Friction control of tools by laser microtexturing**

*Effect of the longitudinal aspect ratio on the measured contact angle among 30 nano-/microtextured AISI304* 

increases up to 156°. This proves that nano-/microlaser texturing provides a tool to modify the wettability of stainless steel surfaces from hydrophobic to superhydrophobic state. This finding is completely against the classical theory; if more geometric items are put into laser texturing, the material surface quality can be widely

There are two geometric items affecting on the surface property; the fractal dimension and the aspect ratio for nanotextures [37]. The former influences on the complexity of surface geometry; the latter, on the local angulation of geometry.

nano-/microtextured to investigate the effect of microtexture pitch and height on the measured wettability. **Figure 27** describes the relationship between the aspect ratio of nanotexture width to height on the measured contact angle. When this aspect ratio is less than 0.1 or more than 0.3, almost all measured contact angles are less than 155°; the micro-/submicrotextured AISI304 specimens are only hydrophobic. Higher contact angle up to 170° is attained when tuning this aspect ratio between 0.2 and 0.3; e. g., when using the microtextures with the width of 20 μm, their height might well be 2–6 μm. This implies that local angulation of surface geometry has significant influence on the controllability of hydrophobicity.

Under the strong demand for reduction of environmental burdens in manufacturing, every productive line must be energy-saving and highly material-efficient with less emission to environments [38]. In past, the huge amount of lubricating oils has been utilized to reduce the friction and wear not only in automobile industries but also in machining, metal forming, and so on [39]. In order to reduce this amount down to

are laser

Thirty AISI304 stainless steel sheets with the size of 10 × 10 × 0.1 t mm3

**52**

The pin-on-ball testing is employed to measure the time evolution of frictional force under the constant normal load. In this testing, the counter material ball is on contact with the die material under the applied normal weight as depicted in **Figure 29**. The frictional force is directly measured by load sensor attached to the arm. In the following tests, SUJ2 hard balls are utilized as a counter material. The friction coefficient is calculated by division of the measured friction force to the applied normal load. **Figure 30** depicts the transients of friction coefficient with increasing

#### **Figure 28.** *Laser microdimple texturing. (a) Microdimpled stainless steel die and (b) microdimpled WC (Co) cutting tool.*

the sliding distance for three die specimens; e.g., a bare AISI420 die without microdimples, and two microdimpled dies with the microdimple diameter (D) of 100 μm and its pitch (p) of 200 μm and with D = 50 μm and p = 100 μm, respectively. In case of bare die, the friction coefficient increases monotonically with sliding distance up to 0.15. When using the microdimpled die with D = 50 μm and p = 100 μm, lower friction coefficient than 0.07 is preserved during this tribotesting.

## **8.3 Machining test**

When milling the aluminum alloys by WC (Co) tools, the tool face is inevitably subjected to adhesion of work material. Microtexturing into the tool face enables to reduce this adhesion by storing the wear debris and cutting chips into these pockets on it. In this experiment, AA5052 aluminum alloy is employed as a work material for normal milling with use of the bare WC (Co) and microtextured one as shown in **Figure 28b**. **Figure 31** compares the adhesion process of work material onto the tool face at the milling distance (L) of 900 and 1800 m, respectively, between the bare and microdimpled tools. Without microdimples, the adhesive area and thickness of work materials onto the tool face enlarges with increasing L; e.g., when

**Figure 29.**

*The ball-on-disc method for measurement of friction coefficient during the sliding conditions.*

#### **Figure 30.**

*Variation of the friction coefficient with increasing the sliding distance for three cases; the bare die without microdimples, and the microdimpled dies with D = 100 μm and p = 200 μm and D = 50 μm and p = 100 μm, respectively.*

**55**

**Figure 32.**

*microtextured piston skirt.*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

L = 1800 m, nearly the whole face is covered by these work adhesives with their film thickness of 10 μm around the tool edge. On the other hand, little adhesion to microdimpled face is noted even after milling up to 1800 m. This significant reduction of adhesion by microtexturing comes from the storing mechanism where the wear debris and cutting chips are reserved into each microdimple. This reduction of adhesion influences on the cutting force; e.g., the cutting force becomes relatively

Low friction and wear is indispensable for most of automotive parts and manufacturing tools. They have curved surfaces, the friction coefficient of which must be reduced to save the energy waste and to improve the fuel efficiency. In particular,

*Comparison of work material adhesion to tool face with increasing the milling distance between the bare and* 

*Microtexturing into the inner surfaces of automotive parts. (a) Microtextured piston cylinder and (b)* 

insensitive to cutting distance when using these microtextured tools.

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

**8.4 Applications**

**Figure 31.**

*microdimpled WC (Co) tools.*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

L = 1800 m, nearly the whole face is covered by these work adhesives with their film thickness of 10 μm around the tool edge. On the other hand, little adhesion to microdimpled face is noted even after milling up to 1800 m. This significant reduction of adhesion by microtexturing comes from the storing mechanism where the wear debris and cutting chips are reserved into each microdimple. This reduction of adhesion influences on the cutting force; e.g., the cutting force becomes relatively insensitive to cutting distance when using these microtextured tools.

## **8.4 Applications**

*Micromachining*

**8.3 Machining test**

**54**

**Figure 30.**

**Figure 29.**

*respectively.*

*Variation of the friction coefficient with increasing the sliding distance for three cases; the bare die without microdimples, and the microdimpled dies with D = 100 μm and p = 200 μm and D = 50 μm and p = 100 μm,* 

*The ball-on-disc method for measurement of friction coefficient during the sliding conditions.*

the sliding distance for three die specimens; e.g., a bare AISI420 die without microdimples, and two microdimpled dies with the microdimple diameter (D) of 100 μm and its pitch (p) of 200 μm and with D = 50 μm and p = 100 μm, respectively. In case of bare die, the friction coefficient increases monotonically with sliding distance up to 0.15. When using the microdimpled die with D = 50 μm and p = 100 μm, lower

When milling the aluminum alloys by WC (Co) tools, the tool face is inevitably subjected to adhesion of work material. Microtexturing into the tool face enables to reduce this adhesion by storing the wear debris and cutting chips into these pockets on it. In this experiment, AA5052 aluminum alloy is employed as a work material for normal milling with use of the bare WC (Co) and microtextured one as shown in **Figure 28b**. **Figure 31** compares the adhesion process of work material onto the tool face at the milling distance (L) of 900 and 1800 m, respectively, between the bare and microdimpled tools. Without microdimples, the adhesive area and thickness of work materials onto the tool face enlarges with increasing L; e.g., when

friction coefficient than 0.07 is preserved during this tribotesting.

Low friction and wear is indispensable for most of automotive parts and manufacturing tools. They have curved surfaces, the friction coefficient of which must be reduced to save the energy waste and to improve the fuel efficiency. In particular,

#### **Figure 31.**

*Comparison of work material adhesion to tool face with increasing the milling distance between the bare and microdimpled WC (Co) tools.*

#### **Figure 32.**

*Microtexturing into the inner surfaces of automotive parts. (a) Microtextured piston cylinder and (b) microtextured piston skirt.*

## *Micromachining*

the piston cylinder as well as piston skirt are important sliding-part. **Figure 32a** shows the microdimpled AISI316L inner surface of cylinder with the size of 30 × 500 μm2 and the depth of 5 μm in the pitch of 1 mm in the circumferential direction and 0.5 mm along the length. This wedge-shaped microdimples improve the fuel efficiency significantly. The AA7075 piston skirt is also microdimpled to have circular dimples with the diameter of 30 μm, the depth of 3 μm and the pitch of 120 μm, respectively, as shown in **Figure 32b**.

## **9. Discussion**

The spatial resolution in this laser machining is first discussed to find out the way to improve its dimensional accuracy. Through the practical survey on the micromachining and texturing into curved surfaces, the feasible applications are understood to search for bio-medical laser processing. In particular, future trend of fast-rate laser technologies is discussed for further improvement of micromachining.

## **9.1 Spatial resolution in laser micromachining**

Laser drilling of circular holes is employed as a benchmark test to discuss the dimensional accuracy of 25 × 25 holes in square structure with the diameter of 30 mm and the pitch of 50 mm, as depicted in **Figure 33a**. Silicon nitride plate with the thickness of 125 mm is used as a work material. **Figure 33b** and **c** shows the

### **Figure 33.**

*Benchmark test to investigate the dimensional accuracy in the laser microdrilling. (a) Test-drilling, (b) deviation map, measured at the inlet, and (c) deviation map, measured at the outlet.*

### **Figure 34.**

*Microtexturing into the metallic tube. (a) Microdrilling of holes with the diameter of 100 μm into a AISI304 stainless steel pipe with Dout = 0.7 mm and Dιν = 0.58 mm, (b) microgrooving of shallow grooves with the width of 25 μm and the depth of 3 μm into the same pipe as (a), and (c) microgrooving of lateral grooves with the depth of 30 μm into the same pipe as (a).*

**57**

**Figure 36.**

**Figure 35.**

*30 × 500 μm2*

*of 65 × 50 μm<sup>2</sup>*

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

X- and Y-deviation maps at the inlet diameter and outlet diameter for 625 holes. Since both maps are nearly coincident to each other, the straightness and circularity are preserved to be within 1/125 μm ~ 0.7°, and within 2 μm, respectively. The

Without specially designed jigs and fixtures, both the micromilling and micro-EDM are difficult or nearly impossible to microdrill the holes and grooves. Laser microdrilling has little constraint in the manufacturing setup; it is readily applied to make direct drilling. AISI304 stainless steel pipe with its outer diameter (Dout) of 0.7 mm and its inner diameter (Din) of 0.58 mm is employed as a work to make microdrilling the holes and grooves. **Figure 34** shows three microtexturing cases; e.g., a microdrilled pipe, a spiral-grooved pipe, and a laterally grooved pipe. The designed textures can be accommodated to the curved surfaces by this laser microdrilling.

Another feature of laser microtexturing is developed by changing the beam control. A thin spring is structured from a pipe in **Figure 35a**. A wide slit is structured into a pipe as depicted in **Figure 35b**. Any shaped short-cuts are equipped into a pipe as shown in **Figure 35c**. This suggests that complex microstructure can be built

Let us discuss how to make laser-structuring a micropart from commercial components. A polylactic acid (PLA) pipe is employed as a starting component to fabricate the PLA-stents for medical usage. **Figure 35** depicts three PLA-stents fabricated from the same PLA-pipe by the laser microtexturing. These three can be selectively made from PLA-pipe only by varying the slit width (Ws) to be 154,

*Fabrication of the geometrically functionalized parts by laser microtexturing. (a) Structuring a spring with the pitch of 150 μm from thin brass pipe with Dout = 160 μm and Din = 80 μm, (b) structuring a slit with the size of* 

*Laser micropart formation of stents from PLA pipe with Dout = 2.55 mm, Din = 2.20 mm, and the length of* 

 *into AISI304 stainless steel pipe with Dout = 100 μm, and (c) fabrication of short-cuts with the size* 

spatial resolution of hole diameter is within 2.5 μm in the 2σ-reliability.

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

in the micromachine and micromember.

**9.3 Laser micropart formation from pipes**

 *from the same pipe as (b).*

*25 m. (a) Ws = 154 μm, (b) Ws = 156 μm, and (c) Ws = 160 μm.*

**9.2 Laser micromachining into curved surfaces**

X- and Y-deviation maps at the inlet diameter and outlet diameter for 625 holes. Since both maps are nearly coincident to each other, the straightness and circularity are preserved to be within 1/125 μm ~ 0.7°, and within 2 μm, respectively. The spatial resolution of hole diameter is within 2.5 μm in the 2σ-reliability.

## **9.2 Laser micromachining into curved surfaces**

Without specially designed jigs and fixtures, both the micromilling and micro-EDM are difficult or nearly impossible to microdrill the holes and grooves. Laser microdrilling has little constraint in the manufacturing setup; it is readily applied to make direct drilling. AISI304 stainless steel pipe with its outer diameter (Dout) of 0.7 mm and its inner diameter (Din) of 0.58 mm is employed as a work to make microdrilling the holes and grooves. **Figure 34** shows three microtexturing cases; e.g., a microdrilled pipe, a spiral-grooved pipe, and a laterally grooved pipe. The designed textures can be accommodated to the curved surfaces by this laser microdrilling.

Another feature of laser microtexturing is developed by changing the beam control. A thin spring is structured from a pipe in **Figure 35a**. A wide slit is structured into a pipe as depicted in **Figure 35b**. Any shaped short-cuts are equipped into a pipe as shown in **Figure 35c**. This suggests that complex microstructure can be built in the micromachine and micromember.

## **9.3 Laser micropart formation from pipes**

Let us discuss how to make laser-structuring a micropart from commercial components. A polylactic acid (PLA) pipe is employed as a starting component to fabricate the PLA-stents for medical usage. **Figure 35** depicts three PLA-stents fabricated from the same PLA-pipe by the laser microtexturing. These three can be selectively made from PLA-pipe only by varying the slit width (Ws) to be 154,

#### **Figure 35.**

*Micromachining*

500 μm2

**9. Discussion**

micromachining.

**56**

**Figure 34.**

*depth of 30 μm into the same pipe as (a).*

**Figure 33.**

*Benchmark test to investigate the dimensional accuracy in the laser microdrilling. (a) Test-drilling, (b) deviation map, measured at the inlet, and (c) deviation map, measured at the outlet.*

the piston cylinder as well as piston skirt are important sliding-part. **Figure 32a** shows the microdimpled AISI316L inner surface of cylinder with the size of 30 ×

120 μm, respectively, as shown in **Figure 32b**.

**9.1 Spatial resolution in laser micromachining**

 and the depth of 5 μm in the pitch of 1 mm in the circumferential direction and 0.5 mm along the length. This wedge-shaped microdimples improve the fuel efficiency significantly. The AA7075 piston skirt is also microdimpled to have circular dimples with the diameter of 30 μm, the depth of 3 μm and the pitch of

The spatial resolution in this laser machining is first discussed to find out the way to improve its dimensional accuracy. Through the practical survey on the micromachining and texturing into curved surfaces, the feasible applications are understood to search for bio-medical laser processing. In particular, future trend of fast-rate laser technologies is discussed for further improvement of

Laser drilling of circular holes is employed as a benchmark test to discuss the dimensional accuracy of 25 × 25 holes in square structure with the diameter of 30 mm and the pitch of 50 mm, as depicted in **Figure 33a**. Silicon nitride plate with the thickness of 125 mm is used as a work material. **Figure 33b** and **c** shows the

*Microtexturing into the metallic tube. (a) Microdrilling of holes with the diameter of 100 μm into a AISI304 stainless steel pipe with Dout = 0.7 mm and Dιν = 0.58 mm, (b) microgrooving of shallow grooves with the width of 25 μm and the depth of 3 μm into the same pipe as (a), and (c) microgrooving of lateral grooves with the* 

*Fabrication of the geometrically functionalized parts by laser microtexturing. (a) Structuring a spring with the pitch of 150 μm from thin brass pipe with Dout = 160 μm and Din = 80 μm, (b) structuring a slit with the size of 30 × 500 μm<sup>2</sup> into AISI304 stainless steel pipe with Dout = 100 μm, and (c) fabrication of short-cuts with the size of 65 × 50 μm<sup>2</sup> from the same pipe as (b).*

#### **Figure 36.**

*Laser micropart formation of stents from PLA pipe with Dout = 2.55 mm, Din = 2.20 mm, and the length of 25 m. (a) Ws = 154 μm, (b) Ws = 156 μm, and (c) Ws = 160 μm.*

156 and 160 micro-meter, respectively. The topological geometry of stents can be designed and fabricated for each medical treatment by tuning the laser microtexturing parameters as shown in **Figure 36**.

#### **9.4 Future trends in laser micromanufacturing**

Various geometric transformations can be realized only by the laser processing such as the micromachining, microtexturing, and microstructuring in the above. Through the fusion of other manufacturing treatments with the laser processing, further advancement is expected to propose the innovative procedures. With combination of laser nano-/microtexturing with laser polishing, the surface property is selectively controlled to be superhydrophilic or superhydrophobic by tuning the LIPSS-conditions. With combination of laser microtexturing with the mechanical milling, a multimaterial part as well as a structural member with large area can be functionalized as a complex-shaped part or as a functionalized component.

Among the engineering issues related to ultrashort pulse laser processing, how to put the fast-rate microtexturing into practice is one of the important targets. In addition to increase of repetition frequency in laser oscillation, new optical control must be developed to transform the spatial geometry and topology in shape into time sequence of scanning in beam technology.

## **10. Conclusion**

The picosecond and femtosecond laser processing is designed to be tools for advanced manufacturing; laser microdrilling, laser microtexturing, laser nano-/ microtexturing, laser microstructuring, and so on. The dimensional accuracy, the spatial resolution as well as the circularity approaches to 1 μm or less than; every micropart, every microstructure, and every microtexture is fabricated in the product size of 10 to 100 μm range. Most of engineering issues related to surface and interface are well defined in this laser processing to find an optimum solution to each problem. Reduction of friction and wear in tools and works is attained by microtexturing onto the tool and part surfaces. Reliable joining between dissimilar materials and parts is put into practice by chemical adhesion with aid of microtextures on their interface. Surface and interface properties are also controllable by optimization of nano-/ microtextures.

Sustainable manufacturing requires for the well-designed processing to support the efficient circulation of products, parts, and materials in addition to recycle and reuse of second hands. Laser micromachining is useful to prolong the tool life, to revise the product surfaces and interfaces for multiple use and to assist the multimaterialization for second-hand products and parts.

Further research and development on the unknown features of laser processing is necessary to advance new steps in innovative technology and medical engineering to further improve the sustainability in future society.

## **Acknowledgements**

The authors would like to express their gratitude to Mr. T. Hasegawa, T. Miyagawa (SIT), Dr. K. Wasa (TecDia, Co. Ltd.), Mr. T. Omata, and Mr. K. Sanbongi (LPS-works, Co. Ltd.) for their help in experiments. The present study was financially support in part by the METI with Supporting-Industry Projects in Japanese Government from 2010 to 2017.

**59**

**Author details**

Tatsuhiko Aizawa1

Japan

provided the original work is properly cited.

2 LPS Works, Co. Ltd., Tokyo, Japan

*Pico- and Femtosecond Laser Micromachining for Surface Texturing*

The authors declared no conflict of interest.

*DOI: http://dx.doi.org/10.5772/intechopen.83741*

**Conflict of interest**

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

1 Surface Engineering Design Laboratory, Shibaura Institute of Technology, Tokyo,

\* and Tadahiko Inohara2

\*Address all correspondence to: taizawa@sic.shibaura-it.ac.jp

*Pico- and Femtosecond Laser Micromachining for Surface Texturing DOI: http://dx.doi.org/10.5772/intechopen.83741*

## **Conflict of interest**

*Micromachining*

**10. Conclusion**

microtextures.

**Acknowledgements**

ing parameters as shown in **Figure 36**.

**9.4 Future trends in laser micromanufacturing**

time sequence of scanning in beam technology.

materialization for second-hand products and parts.

to further improve the sustainability in future society.

Projects in Japanese Government from 2010 to 2017.

156 and 160 micro-meter, respectively. The topological geometry of stents can be designed and fabricated for each medical treatment by tuning the laser microtextur-

Various geometric transformations can be realized only by the laser processing such as the micromachining, microtexturing, and microstructuring in the above. Through the fusion of other manufacturing treatments with the laser processing, further advancement is expected to propose the innovative procedures. With combination of laser nano-/microtexturing with laser polishing, the surface property is selectively controlled to be superhydrophilic or superhydrophobic by tuning the LIPSS-conditions. With combination of laser microtexturing with the mechanical milling, a multimaterial part as well as a structural member with large area can be functionalized as a complex-shaped part or as a functionalized component.

Among the engineering issues related to ultrashort pulse laser processing, how to put the fast-rate microtexturing into practice is one of the important targets. In addition to increase of repetition frequency in laser oscillation, new optical control must be developed to transform the spatial geometry and topology in shape into

The picosecond and femtosecond laser processing is designed to be tools for advanced manufacturing; laser microdrilling, laser microtexturing, laser nano-/ microtexturing, laser microstructuring, and so on. The dimensional accuracy, the spatial resolution as well as the circularity approaches to 1 μm or less than; every micropart, every microstructure, and every microtexture is fabricated in the product size of 10 to 100 μm range. Most of engineering issues related to surface and interface are well defined in this laser processing to find an optimum solution to each problem. Reduction of friction and wear in tools and works is attained by microtexturing onto the tool and part surfaces. Reliable joining between dissimilar materials and parts is put into practice by chemical adhesion with aid of microtextures on their interface. Surface and interface properties are also controllable by optimization of nano-/

Sustainable manufacturing requires for the well-designed processing to support the efficient circulation of products, parts, and materials in addition to recycle and reuse of second hands. Laser micromachining is useful to prolong the tool life, to revise the product surfaces and interfaces for multiple use and to assist the multi-

Further research and development on the unknown features of laser processing is necessary to advance new steps in innovative technology and medical engineering

The authors would like to express their gratitude to Mr. T. Hasegawa, T. Miyagawa (SIT), Dr. K. Wasa (TecDia, Co. Ltd.), Mr. T. Omata, and

Mr. K. Sanbongi (LPS-works, Co. Ltd.) for their help in experiments. The present study was financially support in part by the METI with Supporting-Industry

**58**

The authors declared no conflict of interest.

## **Author details**

Tatsuhiko Aizawa1 \* and Tadahiko Inohara2

1 Surface Engineering Design Laboratory, Shibaura Institute of Technology, Tokyo, Japan

2 LPS Works, Co. Ltd., Tokyo, Japan

\*Address all correspondence to: taizawa@sic.shibaura-it.ac.jp

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[32] Gečys P, Vinčiūnas A, Gedvilas M, Kasparaitis A, Lazdinas R, Račiukaitis G. Ripple formation by femtosecond laser pulses for enhanced absorptance of stainless steel. Journal of Laser Micro/ Nanoengineering. 2015;**10**:129-133

[33] Hasegawa T, Aizawa T, Inohara T, Wasa K. Fabrication and control of super-hydrophobic surfaces by the femto-second laser machining. In: Proc. 1st World Congress on Micro and Nano Manufacturing. 2017. pp. 381-382

[34] Hasegawa T, Aizawa T, Inohara T, Wasa K. Engineering design for super-hydrophobic surfaces via the femto-second laser machining. In: Proc. 10th Asian Workshop on Micro/Nano Forming Technology. 2017. pp. 33-34

[35] Kawase T. Super-hydrophobic surface. In: SEM'I GAKKAI. Vol. 65. 2009. pp. 200-207

[36] Kam DH, Bhattacharya S, Mazumder J. Control of the wetting properties of an AISI 316L stainless steel surface by femtosecond laser induced surface modification. Journal of Micromechanics and Microengineering. 2012;**22**:1-6

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**60**

*Micromachining*

**References**

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machining—A review. International Journal of Machine Tools and Manufacture. 2008;**48**:609-628

engineering. The Japan Society of Mechanical Engineers. 2006;**5**:111

[11] Doering S, Richter S, Molte S, Tuennermann A. In-situ observation

[12] Doering S, Richter S, Nolte S, Tuennermann A. In-situ imaging of hole shape evolution in ultrashort pulse drilling. Optics Express.

2010;**18**:20395-20400

103. 2017. pp. 1-2

2018;**9**:147-1-147-10

ICOMM. 2012. pp. 85-89

[13] Ashkenasi D, Mueller N, Kaszemeikat T, Illing G. Advanced laser micro machining using a novel trepanning system. Journal of Laser Micro/Nanoengineering. 2011;**6**(1):1-5

[14] Aizawa T, Inohara T. Pico-second laser drilling of high-aspect ratio through-holes with and without tapering. In: Proc. 1st WCMNM. Vol.

[15] Aizawa T, Wasa K, Tamagaki H. A DLC-punch array to fabricate the microtextured aluminum sheet for boiling heat transfer control. Micromachines.

[16] Denkena B, Koehler J, Kaestner J. Efficient machining of micro-dimples for friction reduction. In: Proc. 7th

[17] Jiang Y, Zhao WS, Kang XM, Gu L. Adaptive control for micro-hole EDM process with wavelet transform detecting method. In: Proc. 6th ICOMM. 2011. pp. 207-211

[18] Aizawa T, Fukuda T. Oxygen plasma etching of diamond-like carbon coated mold-die for micro-texturing. Surface and Coating Technology.

2013;**215**:364-368

of the hole formation during deep drilling with ultra-short pulses. Proceedings of SPIE. 2011;**7925**:792517:1-792517:8

[2] Davim P. Nontraditional Machining

[3] Schmid K. Manufacturing Processes for Engineering Materials. 5th ed. New

[4] Aizawa T, Iohara T. Japan Patent.

[5] Gaertner E, Polise V, Tagliaferri F, Palumno B. Laser micro machining of alumina by a picosecond laser. Journal of Laser Micro/Nanoengineering.

[6] Amer MS, Ei-Ashry MA, Dosser LR,

Femtosecond bversus nanosecond laser machining: Comparison of induced stresses and structural changes in silicon wafers. Applied Surface Science.

[7] Nasrollahi V, Penchev P, Dimov SS. A new laser drilling method for producing high aspect ratio micro holes. In: Proc.

[8] Aizawa T, Inohara T. Micro-texturing onto glassy carbon substrates by multiaxially controlled pico-second laser machining. In: Proc. 7th ICOMM. 2012.

Hix KE, Maguire JF, Irwin B.

11th 4M Conf. 2016. pp. 15-18

[9] Aizawa T, Inohara T. Multidimensional micro-patterning onto ceramics by pico-second laser machining. Research Reports of Shibaura Institute of Technology. Natural Sciences and Engineering. 2012;**56**(1):17-26

[10] Tokura K. Machining process. Japanese textbook for mechanical

Processes: Research Advances. New York: Springer; 2013

Jersy: Prentice Hall; 2008

2011-212046; 2011

2018;**13**:76-84

2005;**242**:162-167

pp. 66-73

plastics and glasses with transcription of super-hydrophobic surfaces. Procedia Manufacturing. 2018;**13**:1437-1444

[38] Allwood JM, Cullen JM. Sustainable Materials. Cambridge: UIT; 2012

[39] Kataoka S. Influence of lubricants on global environment. Journal of the Japan Society for Technology of Plasticity. 2005;**46**:4-10

[40] Czichos H, Habig K-H. Tribologie-Handbuch (Tribology handbook). 2nd ed. Wiesbaden: Vieweg Verlag; 2003

[41] Aizawa T, Morita H. Dry progressive stamping of copper-alloy snaps by the plasma nitrided punches. Materials Science Forum. 2018;**920**:28-33

[42] Aizawa T, Morita H. Tribological design and engineering in surface treatment for semi-dry and dry stamping. In: Proc. ICTMP. 2016. pp. 14-28

[43] Sugihara T, Enomoto T. Performance of cutting tools with dimple textured surfaces: Comparative study of different texture patterns. Journal of the International Societies for Precision Engineering and Nanotechnology. 2017;**49**:52-60

**63**

**Chapter 3**

**Abstract**

**1. Introduction**

*Jiye Yang and Tao Wu*

Silicon-Based Micromachining

Process for Flexible Electronics

In this chapter, we introduce silicon-based micromachining process and devices for flexible electronics application. Silicon-based flexible electronics have the unique advantage over other polymer-based process that leverage the traditional standard CMOS process and can be integrated with scalable IC technology. While integrating with CMOS process, special considerations must be taken into account, such as release process, transfer process, and process integration, in order to produce siliconbased flexible electronics. Several efforts and process developments will be illustrated in this chapter with the highlights of imager and wearable electronics application.

**Keywords:** silicon-based flexible electronics, silicon-on-insulator (SOI), deep reactive-ion etching (DRIE), release process, chemical mechanical polishing (CMP), atomic layer deposition (ALD), complementary metal-oxide semiconductor (CMOS),

through-silicon via (TSV), frontside-release process, backside-release process

Over the past decade, an enthusiastic pursuit for flexible electronics, employing both organic and inorganic semiconductor materials, with continuously improved performance has been observed [1]. The material like polymer, carbon nanotubes (CNTs), and silicon (Si) membrane are popular candidates for flexible electronics. Compared with other materials, monocrystalline Si nanomembrane released from siliconon-insulator (SOI) emerges as one of the best choices due to its high carrier mobility, commercial availability at relatively lower cost, and mature fabrication techniques [2]. Recently, nanostructured silicon has been widely used to produce flexible electronic devices like flexible solar cells, thermal electricity, and piezoelectric generators.

Functional part of flexible electronics based on silicon can be fabricated in a standard complementary metal-oxide semiconductor (CMOS) technology. A standard CMOS technology includes photolithography, etch, deposition, and doping. Moreover, frontside- and backside-release process, transfer process, and bonding process for flexible substrate are developed to generate flexible silicon membrane with functional part. For frontside-release process, deep reactive-ion etching (DRIE), buffered oxide etcher (BOE), or xenon difluoride (XeF2) etching are used to release membrane structures, while for backside-release process, lapping, chemical mechanical polishing (CMP), or XeF2 etching are employed to thin the Si substrate. After fabricating thin Si membranes with functional devices, special transfer process is up required to stick released devices on a flexible substrate like PDMS or Kapton® tape. After the release process, the released Si membrane is

transferred and bonded to a flexible substrate (**Figure 1**).

## **Chapter 3**

*Micromachining*

plastics and glasses with transcription of super-hydrophobic surfaces. Procedia Manufacturing. 2018;**13**:1437-1444

[38] Allwood JM, Cullen JM. Sustainable

[39] Kataoka S. Influence of lubricants on global environment. Journal of the Japan Society for Technology of

[40] Czichos H, Habig K-H. Tribologie-Handbuch (Tribology handbook). 2nd ed. Wiesbaden: Vieweg Verlag; 2003

[41] Aizawa T, Morita H. Dry progressive stamping of copper-alloy snaps by the plasma nitrided punches. Materials Science Forum. 2018;**920**:28-33

[42] Aizawa T, Morita H. Tribological design and engineering in surface treatment for semi-dry and dry stamping. In: Proc. ICTMP. 2016.

[43] Sugihara T, Enomoto T. Performance of cutting tools with dimple textured surfaces: Comparative study of different texture patterns. Journal of the International Societies for Precision Engineering and Nanotechnology.

Materials. Cambridge: UIT; 2012

Plasticity. 2005;**46**:4-10

pp. 14-28

2017;**49**:52-60

**62**

## Silicon-Based Micromachining Process for Flexible Electronics

*Jiye Yang and Tao Wu*

## **Abstract**

In this chapter, we introduce silicon-based micromachining process and devices for flexible electronics application. Silicon-based flexible electronics have the unique advantage over other polymer-based process that leverage the traditional standard CMOS process and can be integrated with scalable IC technology. While integrating with CMOS process, special considerations must be taken into account, such as release process, transfer process, and process integration, in order to produce siliconbased flexible electronics. Several efforts and process developments will be illustrated in this chapter with the highlights of imager and wearable electronics application.

**Keywords:** silicon-based flexible electronics, silicon-on-insulator (SOI), deep reactive-ion etching (DRIE), release process, chemical mechanical polishing (CMP), atomic layer deposition (ALD), complementary metal-oxide semiconductor (CMOS), through-silicon via (TSV), frontside-release process, backside-release process

## **1. Introduction**

Over the past decade, an enthusiastic pursuit for flexible electronics, employing both organic and inorganic semiconductor materials, with continuously improved performance has been observed [1]. The material like polymer, carbon nanotubes (CNTs), and silicon (Si) membrane are popular candidates for flexible electronics. Compared with other materials, monocrystalline Si nanomembrane released from siliconon-insulator (SOI) emerges as one of the best choices due to its high carrier mobility, commercial availability at relatively lower cost, and mature fabrication techniques [2]. Recently, nanostructured silicon has been widely used to produce flexible electronic devices like flexible solar cells, thermal electricity, and piezoelectric generators.

Functional part of flexible electronics based on silicon can be fabricated in a standard complementary metal-oxide semiconductor (CMOS) technology. A standard CMOS technology includes photolithography, etch, deposition, and doping. Moreover, frontside- and backside-release process, transfer process, and bonding process for flexible substrate are developed to generate flexible silicon membrane with functional part. For frontside-release process, deep reactive-ion etching (DRIE), buffered oxide etcher (BOE), or xenon difluoride (XeF2) etching are used to release membrane structures, while for backside-release process, lapping, chemical mechanical polishing (CMP), or XeF2 etching are employed to thin the Si substrate. After fabricating thin Si membranes with functional devices, special transfer process is up required to stick released devices on a flexible substrate like PDMS or Kapton® tape. After the release process, the released Si membrane is transferred and bonded to a flexible substrate (**Figure 1**).

#### **Figure 1.**

*Schematic diagram of Si-based nanostructures that are served as flexible thermoelectric generator, solar cells, ICs, and piezoelectric generators. As shown in the middle, Si nanowire array on plastic substrate and its cross section under scanning electron microscopy (SEM) are flexible. Images at the bottom: Reproduced with permission [3]. Copyright 2012, American Chemical Society. Images in the middle: Reproduced with permission [4]. Copyright 2011, American Chemical Society. Images in the middle right: Reproduced with permission [5]. Copyright 2011, American Chemical Society. Images in the middle left: Reproduced with permission [6]. Copyright 2011, American Chemical Society. Images at the top: Reproduced with permission [7]. Copyright 2008, nature publishing group [8].*

## **2. Silicon-based micromachining process**

Many researchers have demonstrated flexible electronics based on polymers [9, 10] and CNT [11, 12]; however, the micromachining process with those materials is largely limited by the process temperature and compatible chemicals. Moreover, the devices are typically not scalable or almost impossible to integrate with current advanced IC technology. Compared to polymers and CNT, Si-based flexible electronics can employ the matured CMOS fabrication techniques such as photolithography, atomic layer deposition (ALD), physical vapor deposition (PVD), chemical vapor deposition (CVD), Hydrofluoric Acid (HF) etching, reactive-ion etching (RIE), etc. However, in order to produce flexible electronics using traditional Si-based COMS process, development of release process, transfer process, and bonding process are essential for the production of flexible thin silicon membrane. To realize flexibility the oft used processes are DRIE, XeF2 dry etching, transfer through polymer stamp, process for bonding to flexible substrate, etc. On the whole, the fabrication of silicon-based flexible electronics consists of two major steps: the fabrication of functional part like photodiode, metal-oxide semiconductor field-effect transistor (MOSFET), fin field-effect transistor (FinFET), ferroelectric RAM (FeRAM), etc. and the thinning of device to realize the flexibility. In this chapter, micromachining processes are introduced and described.

#### **2.1 Traditional silicon CMOS process**

CMOS process is a standard process used to produce integrated circuits (ICs) and form electronic circuits and system in large scale. CMOS process involves various basic fabrication processes such as wafer manufacturing, oxidation, photolithography, doping, deposition, etching, and CMP.

**65**

**Figure 2.**

*(a) Traditional bulk silicon wafer. (b) FD-SOI starting wafer.*

*Silicon-Based Micromachining Process for Flexible Electronics*

ness of ultra thin BOX ranges from 10 nm to 30 nm [14].

oxidation is used to produce thicker and slightly porous layers.

Silicon wafers are produced from raw material sand by purifying and crystallizing. The purified silicon is held in molten state at about 1500°C, and after dipping a seed crystal into the melt, the silicon ingot can be produced by gradually extracting the rod. In addition, the silicon can be lightly doped by inserting doping material into the crucible. The fabricated silicon material is used to produce the CMOS device such as MOSFET and FinFET. **Figure 2a** shows the fabricated traditional bulk Si wafer. Nowadays the most advanced transistors are FinFET or fully depleted silicon-on-insulator (FD-SOI) planar transistor technology that is developed at the scale smaller than 25 nm. In the fabrication of FD-SOI transistor, instead of the traditional bulk Silicon wafer, the new more expensive material called SOI wafer is employed. The SOI wafer is fabricated by either separation by implantation of oxygen (SIMOX) process or Smart-Cut process [13]. **Figure 2b** shows SOI wafer for FD-SOI transistor. The thickness of the silicon film is in the ranges from 10 nm to 30 nm; while the standard thickness of BOX is approximately 145 nm and the thick-

In the CMOS fabrication, silicon dioxide layer is used as an insulating material between different conducting layers or acts as a mask or protective layer against diffusion and high-energy ion implantation. The oxidation is performed by a chemical reaction between oxygen (dry oxidation) or water vapor (wet oxidation), and the silicon slice surface is heated in a high-temperature furnace at about 1000°C [15]. Dry oxidation is often used to produce thin and robust oxide layers, while wet

With the help of mask, the photolithography is employed to create patterned layers of different materials on the silicon wafer. Photolithography involves several steps. At first, a photosensitive emulsion (photoresist) film is coated on wafer surface using spin coat. Following that, the wafer is exposed to a pattern of intense light with the help of mask. For positive photoresist (PR), the exposed regions are soluble in the developer, while for negative photoresist, the unexposed regions are soluble in the developer. Tetramethylammonium hydroxide (TMAH) is a widely used developer to remove unwanted photoresist regions. After development, the etching is performed to remove the unwanted regions that are not protected by photoresist. In projection systems, the resolution is limited by the wavelength of the light and the ability of the reduction lens system to capture enough diffraction orders from the illuminated

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

*2.1.1 Wafer manufacturing*

*2.1.2 Oxidation*

*2.1.3 Photolithography*

## *2.1.1 Wafer manufacturing*

*Micromachining*

**Figure 1.**

**2. Silicon-based micromachining process**

*permission [7]. Copyright 2008, nature publishing group [8].*

Many researchers have demonstrated flexible electronics based on polymers

*Schematic diagram of Si-based nanostructures that are served as flexible thermoelectric generator, solar cells, ICs, and piezoelectric generators. As shown in the middle, Si nanowire array on plastic substrate and its cross section under scanning electron microscopy (SEM) are flexible. Images at the bottom: Reproduced with permission [3]. Copyright 2012, American Chemical Society. Images in the middle: Reproduced with permission [4]. Copyright 2011, American Chemical Society. Images in the middle right: Reproduced with permission [5]. Copyright 2011, American Chemical Society. Images in the middle left: Reproduced with permission [6]. Copyright 2011, American Chemical Society. Images at the top: Reproduced with* 

[9, 10] and CNT [11, 12]; however, the micromachining process with those materials is largely limited by the process temperature and compatible chemicals. Moreover, the devices are typically not scalable or almost impossible to integrate with current advanced IC technology. Compared to polymers and CNT, Si-based flexible electronics can employ the matured CMOS fabrication techniques such as photolithography, atomic layer deposition (ALD), physical vapor deposition (PVD), chemical vapor deposition (CVD), Hydrofluoric Acid (HF) etching, reactive-ion etching (RIE), etc. However, in order to produce flexible electronics using traditional Si-based COMS process, development of release process, transfer process, and bonding process are essential for the production of flexible thin silicon membrane. To realize flexibility the oft used processes are DRIE, XeF2 dry etching, transfer through polymer stamp, process for bonding to flexible substrate, etc. On the whole, the fabrication of silicon-based flexible electronics consists of two major steps: the fabrication of functional part like photodiode, metal-oxide semiconductor field-effect transistor (MOSFET), fin field-effect transistor (FinFET), ferroelectric RAM (FeRAM), etc. and the thinning of device to realize the flexibility. In this

chapter, micromachining processes are introduced and described.

CMOS process is a standard process used to produce integrated circuits (ICs) and form electronic circuits and system in large scale. CMOS process involves various basic fabrication processes such as wafer manufacturing, oxidation, photo-

**2.1 Traditional silicon CMOS process**

lithography, doping, deposition, etching, and CMP.

**64**

Silicon wafers are produced from raw material sand by purifying and crystallizing. The purified silicon is held in molten state at about 1500°C, and after dipping a seed crystal into the melt, the silicon ingot can be produced by gradually extracting the rod. In addition, the silicon can be lightly doped by inserting doping material into the crucible. The fabricated silicon material is used to produce the CMOS device such as MOSFET and FinFET. **Figure 2a** shows the fabricated traditional bulk Si wafer. Nowadays the most advanced transistors are FinFET or fully depleted silicon-on-insulator (FD-SOI) planar transistor technology that is developed at the scale smaller than 25 nm. In the fabrication of FD-SOI transistor, instead of the traditional bulk Silicon wafer, the new more expensive material called SOI wafer is employed. The SOI wafer is fabricated by either separation by implantation of oxygen (SIMOX) process or Smart-Cut process [13]. **Figure 2b** shows SOI wafer for FD-SOI transistor. The thickness of the silicon film is in the ranges from 10 nm to 30 nm; while the standard thickness of BOX is approximately 145 nm and the thickness of ultra thin BOX ranges from 10 nm to 30 nm [14].

## *2.1.2 Oxidation*

In the CMOS fabrication, silicon dioxide layer is used as an insulating material between different conducting layers or acts as a mask or protective layer against diffusion and high-energy ion implantation. The oxidation is performed by a chemical reaction between oxygen (dry oxidation) or water vapor (wet oxidation), and the silicon slice surface is heated in a high-temperature furnace at about 1000°C [15]. Dry oxidation is often used to produce thin and robust oxide layers, while wet oxidation is used to produce thicker and slightly porous layers.

## *2.1.3 Photolithography*

With the help of mask, the photolithography is employed to create patterned layers of different materials on the silicon wafer. Photolithography involves several steps. At first, a photosensitive emulsion (photoresist) film is coated on wafer surface using spin coat. Following that, the wafer is exposed to a pattern of intense light with the help of mask. For positive photoresist (PR), the exposed regions are soluble in the developer, while for negative photoresist, the unexposed regions are soluble in the developer. Tetramethylammonium hydroxide (TMAH) is a widely used developer to remove unwanted photoresist regions. After development, the etching is performed to remove the unwanted regions that are not protected by photoresist. In projection systems, the resolution is limited by the wavelength of the light and the ability of the reduction lens system to capture enough diffraction orders from the illuminated

**Figure 2.** *(a) Traditional bulk silicon wafer. (b) FD-SOI starting wafer.*

mask. Nowadays, the most advanced CMOS photolithography is at the scale of about 7 nm, but for flexible electronics photolithography at the scale of about 1 μm is enough for most application such as imager, temperature sensor, and humidity sensor.

## *2.1.4 Doping*

Doping is used to produce electronic components such as diode and various transistors. After masking some area of the silicon surface, doping can be done in exposed regions. Doping can be performed by either diffusion method or ion implantation. There are two basic steps for diffusion method: predeposition and drive-in. In the predeposition step, the wafer is heated in a furnace to a certain temperature (about 1000°C), and carrier gas such as nitrogen and argon with the desired dopant such as phosphine PH3 or diborane B2H6 flow to the silicon wafer. The diffusion of dopant atoms takes place onto the surface of the silicon, and in this step we can control the dose of dopant atoms. In the drive-in step, the wafer is heated in an inert atmosphere for few hours to distribute the atoms more uniformly and to a higher depth [15]. For ion implantation method, charged dopants (ions) are accelerated in an electric field and penetrated into the wafer. The penetration depth can be precisely controlled by reducing or increasing the voltage that needed to accelerate the ions. Following ion implantation, a drive-in step is also performed to achieve uniform distribution of the ions and increase the depth of penetration. **Figure 3** shows two phosphorous doping processes for SOI wafer, in which secondary ion mass spectrometry (SIMS) was used to analyze the doping profiles under two implantation conditions: one has energy/ dose of 12 keV*/*1 × 1016 cm−<sup>3</sup> , and the other has 150 keV/4 × 1015 cm−<sup>3</sup> [1].

## *2.1.5 Deposition*

For MOS Fabrication, various deposition methods are used to form conducting insulating and passivation layers with a variety of materials. There are three

#### **Figure 3.**

*SIMS results of phosphorous doping profiles of two implantation conditions: 12 keV/1 × 1016 cm<sup>−</sup><sup>3</sup> (a) before and (b) after annealing; 150 keV/4 × 1015 cm<sup>−</sup><sup>3</sup> (c) before and (d) after annealing. Reproduced with permission [1]. Copyright 2012, IOP.*

**67**

**Figure 4.**

*Silicon-Based Micromachining Process for Flexible Electronics*

able for TSVs with aspect ratios larger than 10:1 [17].

*2.1.6 Etching*

main deposition processes: PVD, CVD, and ALD. CVD is widely used to deposit conducting layers such as polysilicon and insulating layers such as SiO2 and Si3N4. ALD is used to deposit gate dielectrics with high-k material such as hafnium dioxide HfO2 and tantalum pentoxide Ta2O5 that are necessary for FET at scale smaller than 25 nm. PVD is an established method of depositing metal contacts, barriers, and interconnects used in ICs [16]. In the advanced CMOS, a 3D stack chip structure is used to further improve the integration by using solder flip chip and throughsilicon vias (TSVs). For the fabrication of TSVs, the depositions of passivation layer such as silicon nitride (SiN) and metal layer such as copper (Cu) are necessary. But some deposition processes are not more suitable for TSVs with aspect ratios more than 10:1; the capability of several deposition processes to coat the sidewalls of TSVs is limited as shown in **Figure 4**. Compared with ALD, the deposition coverage of CVD decreases below 20% for aspect ratios exceeding 10:1, and for aspect ratios larger than 2.5:1 the deposition coverage of PVD is already less than 20%. Moreover, molecular vapor deposition (MVD) is an alternative deposition process that is suit-

Etching process is used to remove unwanted material and to create desired pattern. There are two types of etching methods: wet etching and dry etching. For wet etching, the wafer is immersed in a suitable etching solution, which can remove the exposed material leaving the material beneath the protective layer intact. For example, potassium hydroxide (KOH) is used to etch silicon, while hydrofluoric acid (HF) is used to etch SiO2. In addition, the etching mask should not dissolve or at least be etched much slower in the etchant. For example, SiO2 and Si3N4 can serve as mask for Si etching in KOH, while Si3N4 and metal are usually used as SiO2 wet etching mask. Dry etching, usually called plasma etching or reactive-ion etching (RIE), is used to remove the materials by chemical reactions (using chemical reactive gases or plasma) and by purely physical methods (e.g., sputtering and ion beam-induced etching) or with a combination of both chemical reaction and physical bombardment (e.g., RIE). For instance, SF6 and CF4 can be utilized to etch silicon anisotropically, while XeF2 etches silicon isotropically with pure chemical reaction. Depending on the selectivity and how much materials need to be etched,

PR, SiO2, or metal can be used as the mask for silicon etching [19, 20].

*Schematic graph of deposition coverage in comparison of PVD, CVD, and ALD deposition processes. Reproduced with permission [18]. Copyright 2016, springer international publishing Switzerland.*

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

#### *Silicon-Based Micromachining Process for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.83347*

main deposition processes: PVD, CVD, and ALD. CVD is widely used to deposit conducting layers such as polysilicon and insulating layers such as SiO2 and Si3N4. ALD is used to deposit gate dielectrics with high-k material such as hafnium dioxide HfO2 and tantalum pentoxide Ta2O5 that are necessary for FET at scale smaller than 25 nm. PVD is an established method of depositing metal contacts, barriers, and interconnects used in ICs [16]. In the advanced CMOS, a 3D stack chip structure is used to further improve the integration by using solder flip chip and throughsilicon vias (TSVs). For the fabrication of TSVs, the depositions of passivation layer such as silicon nitride (SiN) and metal layer such as copper (Cu) are necessary. But some deposition processes are not more suitable for TSVs with aspect ratios more than 10:1; the capability of several deposition processes to coat the sidewalls of TSVs is limited as shown in **Figure 4**. Compared with ALD, the deposition coverage of CVD decreases below 20% for aspect ratios exceeding 10:1, and for aspect ratios larger than 2.5:1 the deposition coverage of PVD is already less than 20%. Moreover, molecular vapor deposition (MVD) is an alternative deposition process that is suitable for TSVs with aspect ratios larger than 10:1 [17].

## *2.1.6 Etching*

*Micromachining*

*2.1.4 Doping*

dose of 12 keV*/*1 × 1016 cm−<sup>3</sup>

*2.1.5 Deposition*

mask. Nowadays, the most advanced CMOS photolithography is at the scale of about 7 nm, but for flexible electronics photolithography at the scale of about 1 μm is enough for most application such as imager, temperature sensor, and humidity sensor.

Doping is used to produce electronic components such as diode and various transistors. After masking some area of the silicon surface, doping can be done in exposed regions. Doping can be performed by either diffusion method or ion implantation. There are two basic steps for diffusion method: predeposition and drive-in. In the predeposition step, the wafer is heated in a furnace to a certain temperature (about 1000°C), and carrier gas such as nitrogen and argon with the desired dopant such as phosphine PH3 or diborane B2H6 flow to the silicon wafer. The diffusion of dopant atoms takes place onto the surface of the silicon, and in this step we can control the dose of dopant atoms. In the drive-in step, the wafer is heated in an inert atmosphere for few hours to distribute the atoms more uniformly and to a higher depth [15]. For ion implantation method, charged dopants (ions) are accelerated in an electric field and penetrated into the wafer. The penetration depth can be precisely controlled by reducing or increasing the voltage that needed to accelerate the ions. Following ion implantation, a drive-in step is also performed to achieve uniform distribution of the ions and increase the depth of penetration. **Figure 3** shows two phosphorous doping processes for SOI wafer, in which secondary ion mass spectrometry (SIMS) was used to analyze the doping profiles under two implantation conditions: one has energy/

, and the other has 150 keV/4 × 1015 cm−<sup>3</sup>

For MOS Fabrication, various deposition methods are used to form conducting insulating and passivation layers with a variety of materials. There are three

*SIMS results of phosphorous doping profiles of two implantation conditions: 12 keV/1 × 1016 cm<sup>−</sup><sup>3</sup>*

[1].

 *(a) before* 

 *(c) before and (d) after annealing. Reproduced with permission* 

**66**

**Figure 3.**

*[1]. Copyright 2012, IOP.*

*and (b) after annealing; 150 keV/4 × 1015 cm<sup>−</sup><sup>3</sup>*

Etching process is used to remove unwanted material and to create desired pattern. There are two types of etching methods: wet etching and dry etching. For wet etching, the wafer is immersed in a suitable etching solution, which can remove the exposed material leaving the material beneath the protective layer intact. For example, potassium hydroxide (KOH) is used to etch silicon, while hydrofluoric acid (HF) is used to etch SiO2. In addition, the etching mask should not dissolve or at least be etched much slower in the etchant. For example, SiO2 and Si3N4 can serve as mask for Si etching in KOH, while Si3N4 and metal are usually used as SiO2 wet etching mask. Dry etching, usually called plasma etching or reactive-ion etching (RIE), is used to remove the materials by chemical reactions (using chemical reactive gases or plasma) and by purely physical methods (e.g., sputtering and ion beam-induced etching) or with a combination of both chemical reaction and physical bombardment (e.g., RIE). For instance, SF6 and CF4 can be utilized to etch silicon anisotropically, while XeF2 etches silicon isotropically with pure chemical reaction. Depending on the selectivity and how much materials need to be etched, PR, SiO2, or metal can be used as the mask for silicon etching [19, 20].

#### **Figure 4.**

*Schematic graph of deposition coverage in comparison of PVD, CVD, and ALD deposition processes. Reproduced with permission [18]. Copyright 2016, springer international publishing Switzerland.*

#### *Micromachining*

Etching can be isotropic or anisotropic and therefore can form different etching profiles. Isotropic etching has the same etch rate in all directions and, anisotropic etching has different etch rates in the lateral and vertical directions. For example, silicon can be etched anisotropically by using CF4 or SF6 and can be etched isotropically using XeF2 or HF:HNO3:H2O.

## *2.1.7 Chemical mechanical polishing (CMP)*

CMP process is a combination of mechanical and chemical actions, and it has been widely used to polish and thin silicon substrate. A CMP process could be significantly influenced by many factors such as abrasives, pH, and polishing temperature [21]. The schema of CMP tool and process to polish wafer are shown in **Figure 5** [22]. A wafer is firstly held by the polishing head using a vacuum and then the polishing head starts to rotate, resulting in the rotation of held wafer on the polishing pad [22]. The slurry used in the CMP process is dispensed through a slurry arm with the help of polishing pad conditioner, and the polishing pad surface is refreshed for each polishing process so that global planarization and polishing can be achieved [22]. During CMP process, the wafer is polished through abrasive and chemistry, and the complicate interaction between pad asperity, slurry, and wafer surface is described in **Figure 5** in a microscale observation [22]. For CMP process, readers are directed to Refs. [21, 22] for more details.

## **2.2 Frontside-release process for flexible silicon membrane**

Frontside-release process utilizes SOI wafers and, in general, consists of active device fabrication, frontside-release hole, or structure patterning, releasing protection coating and release etching. Two release etching strategies are usually employed: one way is to remove BOX layer in SOI and fully release the device layer. RIE or DRIE is used to etch the Si device layer depending on the required etching depth and expose the BOX layer to HF etchant for releasing.

The other approach is to remove bulk silicon carrier in SOI and fully release the structures consisting of both device and BOX layers. Therefore, Si isotropically etching is required for releasing, and XeF2 is mostly employed. Pure SF6 plasma can also etch silicon isotropically.

Different from CMOS process, the thickness of FD-SOI device layer for MOSFET is approximately in the order of 100 nm, and the thickness of SOI device layer for flexible electronics can be as thick as 10 μm in many applications. Therefore DRIE is essential to form high aspect ratio trenches for exposure of BOX or silicon substrate to etchant. Following DRIE, the release can be achieved by either removing BOX or undercutting silicon substrate from frontside. HF etchant is used to remove BOX

**69**

*Silicon-Based Micromachining Process for Flexible Electronics*

and XeF2 is used to undercut silicon. Moreover, for release by undercutting silicon below BOX layer, a protective layer is necessary to protect other silicon parts from etching. Deposition of the protective layer onto the sidewall of trenches must be performed. Compared to PVD and CVD, ALD method can deposit high conformal and continuous protective layer inside the trench. The protective layer can be made of silicon oxide or alumina. In the following sections, we first introduce DRIE and XeF2 RIE processes, and then we will describe how these two technique are utilized in the

DRIE is an extension of the traditional RIE process and is a highly anisotropic etch process. Different from traditional RIE, DRIE can be used to create vertical (90°) etch profiles, deep penetration, and holes with high aspect ratios. So far it has been used to fabricate capacitors for deep trench DRAM, TSVs, and microphotonic structures. With the help of novel thermal budget and by-product redeposition management, DRIE can pattern more than 5 μm silicon or even thru the wafer with

1.Plasma-induced deposition of a polymeric layer as passivation layer using C4F8

2.Anisotropic removal of passivation layer on the bottom followed by an isotropic Si chemical etch, and SF6 is usually employed as working gas for etching

The DRIE process is shown in **Figure 6** and consists of six steps. At first, the polymeric passivation layer is coated overall to protect the sidewalls from chemical attack in the etching step (**Figure 6a**). Following that is the etching step; first the passivation layer on vertical surface (trench bottom) is removed through electrical field-accelerated ions (**Figure 6b**), and after the removal of passivation layer on trench bottom, the trench bottom is isotropically etched (**Figure 6c**). This isotropic etching usually lasts a few seconds, and the working gas mostly is a fluorine-based gas such as SF6. Followed by the etching step, a deposition step is performed for a few seconds to coat the overall polymeric passivation layer (**Figure 6d**), which is similar to the first step. Then the etching step is repeated (**Figure 6e**), which is similar to step 2 and step 3 [18]. The removal of passivation layer on vertical surfaces is much faster than on horizontal, since the ions are accelerated in vertical direction. After the removal of passivation layer on the trench bottom, the further etchants start etching the trench bottom, and simultaneously polymeric passivation layer of the sidewall slows the lateral etch rate [18]. To achieve the desired depth of TSVs, these etching and deposition steps are repeated several times (**Figure 6f**) [18]. The DRIE process involves six steps, and the performance of each step is controlled by a significant number of parameters such as gas flows, the power of the inductively

There are quite a few parameters that can significantly influence the DRIE process profile, such as gas flows, the power of the inductively coupled plasma or the platen source, time, etc. For 1 μm line and hole, scallops were deeper in the top (~40 nm) and none in the middle (<5 nm) and minimal (~20 nm) in the bottom for both holes and lines for the optimized recipe [23]. **Figure 7** shows how the process parameters influence the DRIE process properties. Scallops in top and in the bottom

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

*2.2.1 Deep reactive-ion etching and XeF2 RIE*

frontside-release processes.

*2.2.1.1 Deep reactive-ion etching*

cycling of two processes:

of Si [18].

as working gas [18].

coupled plasma or the platen source, time, etc. [18].

#### **Figure 5.**

*The schema of conventional CMP process [22]. Copyright (2018) with permission from IntechOpen.*

## *Silicon-Based Micromachining Process for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.83347*

and XeF2 is used to undercut silicon. Moreover, for release by undercutting silicon below BOX layer, a protective layer is necessary to protect other silicon parts from etching. Deposition of the protective layer onto the sidewall of trenches must be performed. Compared to PVD and CVD, ALD method can deposit high conformal and continuous protective layer inside the trench. The protective layer can be made of silicon oxide or alumina. In the following sections, we first introduce DRIE and XeF2 RIE processes, and then we will describe how these two technique are utilized in the frontside-release processes.

## *2.2.1 Deep reactive-ion etching and XeF2 RIE*

## *2.2.1.1 Deep reactive-ion etching*

*Micromachining*

cally using XeF2 or HF:HNO3:H2O.

*2.1.7 Chemical mechanical polishing (CMP)*

readers are directed to Refs. [21, 22] for more details.

**2.2 Frontside-release process for flexible silicon membrane**

depth and expose the BOX layer to HF etchant for releasing.

also etch silicon isotropically.

Etching can be isotropic or anisotropic and therefore can form different etching profiles. Isotropic etching has the same etch rate in all directions and, anisotropic etching has different etch rates in the lateral and vertical directions. For example, silicon can be etched anisotropically by using CF4 or SF6 and can be etched isotropi-

CMP process is a combination of mechanical and chemical actions, and it has been widely used to polish and thin silicon substrate. A CMP process could be significantly influenced by many factors such as abrasives, pH, and polishing temperature [21]. The schema of CMP tool and process to polish wafer are shown in **Figure 5** [22]. A wafer is firstly held by the polishing head using a vacuum and then the polishing head starts to rotate, resulting in the rotation of held wafer on the polishing pad [22]. The slurry used in the CMP process is dispensed through a slurry arm with the help of polishing pad conditioner, and the polishing pad surface is refreshed for each polishing process so that global planarization and polishing can be achieved [22]. During CMP process, the wafer is polished through abrasive and chemistry, and the complicate interaction between pad asperity, slurry, and wafer surface is described in **Figure 5** in a microscale observation [22]. For CMP process,

Frontside-release process utilizes SOI wafers and, in general, consists of active

The other approach is to remove bulk silicon carrier in SOI and fully release the structures consisting of both device and BOX layers. Therefore, Si isotropically etching is required for releasing, and XeF2 is mostly employed. Pure SF6 plasma can

Different from CMOS process, the thickness of FD-SOI device layer for MOSFET is approximately in the order of 100 nm, and the thickness of SOI device layer for flexible electronics can be as thick as 10 μm in many applications. Therefore DRIE is essential to form high aspect ratio trenches for exposure of BOX or silicon substrate to etchant. Following DRIE, the release can be achieved by either removing BOX or undercutting silicon substrate from frontside. HF etchant is used to remove BOX

device fabrication, frontside-release hole, or structure patterning, releasing protection coating and release etching. Two release etching strategies are usually employed: one way is to remove BOX layer in SOI and fully release the device layer. RIE or DRIE is used to etch the Si device layer depending on the required etching

*The schema of conventional CMP process [22]. Copyright (2018) with permission from IntechOpen.*

**68**

**Figure 5.**

DRIE is an extension of the traditional RIE process and is a highly anisotropic etch process. Different from traditional RIE, DRIE can be used to create vertical (90°) etch profiles, deep penetration, and holes with high aspect ratios. So far it has been used to fabricate capacitors for deep trench DRAM, TSVs, and microphotonic structures. With the help of novel thermal budget and by-product redeposition management, DRIE can pattern more than 5 μm silicon or even thru the wafer with cycling of two processes:


The DRIE process is shown in **Figure 6** and consists of six steps. At first, the polymeric passivation layer is coated overall to protect the sidewalls from chemical attack in the etching step (**Figure 6a**). Following that is the etching step; first the passivation layer on vertical surface (trench bottom) is removed through electrical field-accelerated ions (**Figure 6b**), and after the removal of passivation layer on trench bottom, the trench bottom is isotropically etched (**Figure 6c**). This isotropic etching usually lasts a few seconds, and the working gas mostly is a fluorine-based gas such as SF6. Followed by the etching step, a deposition step is performed for a few seconds to coat the overall polymeric passivation layer (**Figure 6d**), which is similar to the first step. Then the etching step is repeated (**Figure 6e**), which is similar to step 2 and step 3 [18]. The removal of passivation layer on vertical surfaces is much faster than on horizontal, since the ions are accelerated in vertical direction. After the removal of passivation layer on the trench bottom, the further etchants start etching the trench bottom, and simultaneously polymeric passivation layer of the sidewall slows the lateral etch rate [18]. To achieve the desired depth of TSVs, these etching and deposition steps are repeated several times (**Figure 6f**) [18]. The DRIE process involves six steps, and the performance of each step is controlled by a significant number of parameters such as gas flows, the power of the inductively coupled plasma or the platen source, time, etc. [18].

There are quite a few parameters that can significantly influence the DRIE process profile, such as gas flows, the power of the inductively coupled plasma or the platen source, time, etc. For 1 μm line and hole, scallops were deeper in the top (~40 nm) and none in the middle (<5 nm) and minimal (~20 nm) in the bottom for both holes and lines for the optimized recipe [23]. **Figure 7** shows how the process parameters influence the DRIE process properties. Scallops in top and in the bottom

#### **Figure 6.**

*Bosch process scheme. (a) Deposition of a conformal C4F8 passivation layer, (b) directed removal of the passivation layer by ions, (c) isotropic etching with SF6, (d) deposition of a conformal C4F8 passivation layer, (e) passivation removal and isotropic etching, and (f) alternating steps (b)–(e). reproduced with permission [18]. Copyright 2016, springer international publishing Switzerland.*

**Figure 7.** *DRIE profile scalloping prediction and desired profiles [23].*

showed that a lower etch time results in less scalloping. Those etches are isotropic, so lowering the time lowers the etch distance in all directions. It also appears to be a weak but somewhat significant evidence for dependence on temperature. However, these trends are in the opposite directions, so there is no optimal temperature for the minimal scalloping. SF6 flow shows no measured statistical significance to the scalloping or undercut.

## *2.2.1.2 XeF2 RIE*

Xenon difluoride (XeF2), bromine trifluoride (BrF3), chlorine trifluoride (ClF3), and fluorine (F2) are widely used to etch silicon [24]. Compared to other silicon etchants, XeF2 has unique advantages like gas-phase isotropic etching, high selectivity for silicon, and ease of operation [24]. At room temperature and atmospheric

**71**

**Figure 8.**

*Silicon-Based Micromachining Process for Flexible Electronics*

pressure, XeF2 is white and in solid state [24]. However, when XeF2 is at a pressure smaller than 4 torr, the XeF2 solid will transform into a gas state [24]. Since the gas etching process is simple to operate, XeF2 etching process is widely performed by using the pulse etching system [24]. The XeF2 pulse etching process can be controlled by process parameters such as XeF2 pressure, etching time for a single cycle, and the number of etch cycles [24]. **Figure 8** shows the micromachining mechanism

In this release process, the SOI BOX layer is patterned and exposed using RIE or DRIE, followed by HF wet etching and critical point dry to fully release the struc-

This release process involves three steps. In the first step, trenches are formed through RIE or DRIE to expose BOX (**Figure 9b**). When the silicon layer is thicker than 10 μm and aspect ratio is more than 10:1, DRIE is essential to expose BOX. For thin SOI, the exposure can also be performed by RIE. The second step is the deposition of a protective layer (**Figure 9b**). The Protective layer can protect other parts of silicon oxide from damage by etching, and the materials such as Si3N4 and PR can be used as a protective layer in this process. In the last step, the wafer is immersed in a HF-contained solution, which removes the exposed BOX. **Figure 9c** shows that the BOX is already partially removed, and **Figure 9d** shows that the BOX is fully removed through HF-contained solution. After the release process, the wafer will be transferred to a flexible substrate, and a bonding process will be performed to bond

*Schema of interaction between XeF2 and Si by using XeF2 RIE to etch Si. (1) XeF2 gas diffused from the reactor to the external surface of the etching window. (2) XeF2 gas diffused from the etching window through the etched Si cavity to the silicon surface. (3) adsorption of XeF2 at the silicon surface. (4) dissociation of XeF2 molecule into fluorine atoms (F) and xenon (Xe) gas. (5) formation of Si-F bond and adsorption of SiF4 at the silicon surface. (6) SiF4 at the external surface is desorpted from Si surface. (7) the products are transferred from the* 

*wafer surface to the reactor. Reproduced with permission [24]. Copyright 2012, IEEE.*

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

*2.2.2 Release through removal of BOX*

tures above the BOX layer.

wafer to flexible substrate.

of XeF2 etching.

pressure, XeF2 is white and in solid state [24]. However, when XeF2 is at a pressure smaller than 4 torr, the XeF2 solid will transform into a gas state [24]. Since the gas etching process is simple to operate, XeF2 etching process is widely performed by using the pulse etching system [24]. The XeF2 pulse etching process can be controlled by process parameters such as XeF2 pressure, etching time for a single cycle, and the number of etch cycles [24]. **Figure 8** shows the micromachining mechanism of XeF2 etching.

## *2.2.2 Release through removal of BOX*

*Micromachining*

**Figure 6.**

**70**

scalloping or undercut.

*2.2.1.2 XeF2 RIE*

**Figure 7.**

showed that a lower etch time results in less scalloping. Those etches are isotropic, so lowering the time lowers the etch distance in all directions. It also appears to be a weak but somewhat significant evidence for dependence on temperature. However, these trends are in the opposite directions, so there is no optimal temperature for the minimal scalloping. SF6 flow shows no measured statistical significance to the

*Bosch process scheme. (a) Deposition of a conformal C4F8 passivation layer, (b) directed removal of the passivation layer by ions, (c) isotropic etching with SF6, (d) deposition of a conformal C4F8 passivation layer, (e) passivation removal and isotropic etching, and (f) alternating steps (b)–(e). reproduced with permission* 

*[18]. Copyright 2016, springer international publishing Switzerland.*

*DRIE profile scalloping prediction and desired profiles [23].*

Xenon difluoride (XeF2), bromine trifluoride (BrF3), chlorine trifluoride (ClF3),

and fluorine (F2) are widely used to etch silicon [24]. Compared to other silicon etchants, XeF2 has unique advantages like gas-phase isotropic etching, high selectivity for silicon, and ease of operation [24]. At room temperature and atmospheric

In this release process, the SOI BOX layer is patterned and exposed using RIE or DRIE, followed by HF wet etching and critical point dry to fully release the structures above the BOX layer.

This release process involves three steps. In the first step, trenches are formed through RIE or DRIE to expose BOX (**Figure 9b**). When the silicon layer is thicker than 10 μm and aspect ratio is more than 10:1, DRIE is essential to expose BOX. For thin SOI, the exposure can also be performed by RIE. The second step is the deposition of a protective layer (**Figure 9b**). The Protective layer can protect other parts of silicon oxide from damage by etching, and the materials such as Si3N4 and PR can be used as a protective layer in this process. In the last step, the wafer is immersed in a HF-contained solution, which removes the exposed BOX. **Figure 9c** shows that the BOX is already partially removed, and **Figure 9d** shows that the BOX is fully removed through HF-contained solution. After the release process, the wafer will be transferred to a flexible substrate, and a bonding process will be performed to bond wafer to flexible substrate.

#### **Figure 8.**

*Schema of interaction between XeF2 and Si by using XeF2 RIE to etch Si. (1) XeF2 gas diffused from the reactor to the external surface of the etching window. (2) XeF2 gas diffused from the etching window through the etched Si cavity to the silicon surface. (3) adsorption of XeF2 at the silicon surface. (4) dissociation of XeF2 molecule into fluorine atoms (F) and xenon (Xe) gas. (5) formation of Si-F bond and adsorption of SiF4 at the silicon surface. (6) SiF4 at the external surface is desorpted from Si surface. (7) the products are transferred from the wafer surface to the reactor. Reproduced with permission [24]. Copyright 2012, IEEE.*

#### *Micromachining*

Zhou et al. utilized this release process to release their strained nanomembrane. In their paper for fast flexible electronics with strained silicon nanomembrane, the strips are released in a 4:1 diluted HF (49% HF) solution in which the BOX layer is selectively etched away [25]. **Figure 10** shows the process for release of silicon nanomembrane from Si handling substrate.

## *2.2.3 Release through undercut of silicon*

This release process is achieved by undercutting silicon substrate under the BOX through XeF2 isotropic etching. DRIE is usually used to pattern top silicon device layer followed by protective coating and removal of BOX layer in RIE.

This process involves four steps, and we use **Figure 11** to describe this release process. At first, an oxide film such as PECVD SiO2 is deposited atop the device as an etching buffer layer (**Figure 11a**). Following that, with the help of a PR mask, the exposed oxide layers are removed through RIE, and then the exposure of silicon

**Figure 9.**

*The process flow for release through removal of BOX.*

#### **Figure 10.**

*(a) Atomic lattice schematic diagram showing the strain sharing principle. Optical images show the strained NM during release and after finishing release. (b) Process flow to implement the strain sharing principle and the release. Reproduced with permission [25]. Copyright 2013, nature publishing group.*

**73**

the flexible FinFET wafer [34].

**Figure 11.**

thin the silicon substrate.

**2.3 Backside-release process for flexible silicon membrane**

*2.3.1 Comparison between CMP, RIE, and lapping*

*Silicon-Based Micromachining Process for Flexible Electronics*

under the BOX is performed by using DRIE or RIE (**Figure 11b**). Following exposure septs, protective layer is coated overall to protect other parts of silicon from damage through etching (**Figure 11c**), and the materials such as Al, PR, GaN, and SiO2 can be used as protective layer in this process. After that, a RIE etching is performed to remove the protective layer at the bottom of the trenches. At last, the wafer is placed in XeF2 RIE to etch the silicon under the BOX. Once the undercuts meet with each other, the SOI and the BOX is completely released from the bulk substrate (**Figure 11d**). After the release process, the wafer will be transferred to flexible substrate, and a bonding process will be performed to bond wafer to flexible substrate. Wu et al. (2016) employed this release process to fabricate a silicon-based flexible imager [26–33], and **Figure 11** shows the process flow to release an imager from the carrier substrate and the fabricated mounted monocentric imager. Sevilla et al. used this release process to fabricate a silicon-based flexible FinFET. **Figure 12** shows the basic steps to release FinFET from the carrier substrate, the fabricated FinFET, and

*DRIE process flow and fabricated monocentric imager: (a) after fabrication of photodiode circuitry, (b) pattern tessellated structures thru Si device and buried oxide layers, (c) sidewall passivation, (d) released device by XeF2 etching, (e) a released and curved device transferred into a hemispherical fixture, and (f) a mounted monocentric imager. Reproduced with permission [35]. Copyright 2016, nature publishing group.*

Instead of etching the BOX and undercutting silicon substrate under the BOX, backside-release process etches the silicon substrate from the backside. Lapping, CMP, and RIE can be employed in this release process. The mechanisms of CMP and XeF2 RIE are already introduced in this chapter, and in this section the comparison between CMP, XeF2 RIE, and lapping is described. Following that, the process flow is described through the example in which lapping, CMP, and RIE are employed to

CMP, RIE, and lapping are used to realize backside-release, and we compare these tree fabrication processes to know how to choose suitable fabrication process. The mechanisms of CMP and XeF2 RIE are already introduced in previous sections, and lapping is a mechanical process in which a pad is used with polishing liquid to remove excess silicon from a wafer substrate. Lapping takes place between

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

*Silicon-Based Micromachining Process for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.83347*

#### **Figure 11.**

*Micromachining*

nanomembrane from Si handling substrate.

*2.2.3 Release through undercut of silicon*

*The process flow for release through removal of BOX.*

Zhou et al. utilized this release process to release their strained nanomembrane. In their paper for fast flexible electronics with strained silicon nanomembrane, the strips are released in a 4:1 diluted HF (49% HF) solution in which the BOX layer is selectively etched away [25]. **Figure 10** shows the process for release of silicon

This release process is achieved by undercutting silicon substrate under the BOX through XeF2 isotropic etching. DRIE is usually used to pattern top silicon device

This process involves four steps, and we use **Figure 11** to describe this release process. At first, an oxide film such as PECVD SiO2 is deposited atop the device as an etching buffer layer (**Figure 11a**). Following that, with the help of a PR mask, the exposed oxide layers are removed through RIE, and then the exposure of silicon

*(a) Atomic lattice schematic diagram showing the strain sharing principle. Optical images show the strained NM during release and after finishing release. (b) Process flow to implement the strain sharing principle and* 

*the release. Reproduced with permission [25]. Copyright 2013, nature publishing group.*

layer followed by protective coating and removal of BOX layer in RIE.

**72**

**Figure 10.**

**Figure 9.**

*DRIE process flow and fabricated monocentric imager: (a) after fabrication of photodiode circuitry, (b) pattern tessellated structures thru Si device and buried oxide layers, (c) sidewall passivation, (d) released device by XeF2 etching, (e) a released and curved device transferred into a hemispherical fixture, and (f) a mounted monocentric imager. Reproduced with permission [35]. Copyright 2016, nature publishing group.*

under the BOX is performed by using DRIE or RIE (**Figure 11b**). Following exposure septs, protective layer is coated overall to protect other parts of silicon from damage through etching (**Figure 11c**), and the materials such as Al, PR, GaN, and SiO2 can be used as protective layer in this process. After that, a RIE etching is performed to remove the protective layer at the bottom of the trenches. At last, the wafer is placed in XeF2 RIE to etch the silicon under the BOX. Once the undercuts meet with each other, the SOI and the BOX is completely released from the bulk substrate (**Figure 11d**). After the release process, the wafer will be transferred to flexible substrate, and a bonding process will be performed to bond wafer to flexible substrate.

Wu et al. (2016) employed this release process to fabricate a silicon-based flexible imager [26–33], and **Figure 11** shows the process flow to release an imager from the carrier substrate and the fabricated mounted monocentric imager. Sevilla et al. used this release process to fabricate a silicon-based flexible FinFET. **Figure 12** shows the basic steps to release FinFET from the carrier substrate, the fabricated FinFET, and the flexible FinFET wafer [34].

#### **2.3 Backside-release process for flexible silicon membrane**

Instead of etching the BOX and undercutting silicon substrate under the BOX, backside-release process etches the silicon substrate from the backside. Lapping, CMP, and RIE can be employed in this release process. The mechanisms of CMP and XeF2 RIE are already introduced in this chapter, and in this section the comparison between CMP, XeF2 RIE, and lapping is described. Following that, the process flow is described through the example in which lapping, CMP, and RIE are employed to thin the silicon substrate.

#### *2.3.1 Comparison between CMP, RIE, and lapping*

CMP, RIE, and lapping are used to realize backside-release, and we compare these tree fabrication processes to know how to choose suitable fabrication process.

The mechanisms of CMP and XeF2 RIE are already introduced in previous sections, and lapping is a mechanical process in which a pad is used with polishing liquid to remove excess silicon from a wafer substrate. Lapping takes place between

**Figure 12.**

*(a) Spin coat of thick (7 μm) photoresist and hole patterning, (b) cavern formation beneath BOX due to XeF2 etchant, (c) top view of fins after the gate etch process, which is a complex task performed with a combination of reactive-ion etching and wet cleans, and (d) FinFET silicon fabric at minimum device scale bending radius (5 mm). Reproduced with permission [34]. Copyright 2014, John Wiley & Sons, Inc.*

two counter-rotating cast iron plates and either an abrasive film or slurry. To adjust the penetration of the film/slurry, the wafers either spin faster or experience a heavier load to fit the target specification.

The surface roughness of silicon is about 50 nm by using CMP, and through lapping the surface roughness of silicon can achieve 1 μm. The surface roughness of silicon by using RIE is worst about 10 μm. The cost of RIE is the most expensive, because this process needs also a working gas and vacuum environment. The CMP process is more expensive compared to lapping, because CMP consumes chemicals while lapping involves mechanical polish only. For thinning substrate, man can chose suitable process depending on the surface roughness and the cost.

**Figure 13** shows three present backside thinning processes by using CMP, RIE, or lapping. For thinning through lapping and XeF2 RIE (**Figure 13a**), at first the substrate is reduced to exact thickness (usually about 50 μm) by lapping for cost saving, and after that the resulting surface micro-crack damages induced during the lapping process are removed by XeF2 etching processes with the buried oxide layer as the etch stop layer. Thinning process through anisotropic RIE (**Figure 13b**) possesses advantage high etch rate about 20 μm/min and disadvantage high surface roughness. At first the wafer is turned upside down, and then the substrate is thinned through RIE. The thickness of substrate is controlled by a measurement. Besides RIE and lapping, backside-release can be also performed through CMP, and **Figure 13c** shows this thinning process. Compared with RIE and lapping, the etch rate of CMP is much lower about 0.5 μm/min, and the surface roughness is best at about 50 nm. Usually for cost saving, before CMP process, the substrate can be thinned through lapping, and after lapping process expensive and more precise CMP is performed to further thin the substrate.

### *2.3.2 Process flow of backside-release process*

## *2.3.2.1 Backside-release process using lapping and XeF2 RIE*

Lapping and XeF2 etching can thin the SOI wafer from the backside all the way to the BOX layer with a clean surface finish due to high selectivity between SiO2 and Si in XeF2 RIE. This process involves three steps. At first the wafer is coated by protective layer to protect the parts of wafer that shall not be etched from damage through isotropic XeF2 RIE. Following that, the substrate can be thinned to certain thickness by lapping. At last, the rest of Si substrate is removed by XeF2 RIE, and

**75**

**Figure 13.**

*Silicon-Based Micromachining Process for Flexible Electronics*

the BOX serves as stop layer for XeF2 RIE. With the help of XeF2 RIE and stop layer, the resulting surface micro-crack damages induced during the lapping process can be removed. After thinning process, the wafer is transferred to a flexible substrate, and a bonding process will be performed to bond wafer to flexible substrate.

*The schema for three present backside thinning processes by using CMP, RIE, or lapping. (a) Thinning through lapping (first step) and XeF2 RIE (second step), the BOX serves as stop layer for XeF2 RIE, (b) thinning through RIE by using working gas such as CF4 and SF6, and (c) thinning through lapping (first step) and CMP (second step).*

Hsieh et al. used this process to fabricate a biocompatible flexible IC. At first a wafer lapping machine is used to thin the Si wafer substrate, and the thickness is reduced to ∼50 μm [28]. Following lapping, a dry XeF2 etching process with BOX as etch stop layer is performed to remove the surface micro-crack damages that are caused by the lapping process [28]. Liu et al. employed this process to fabricate a spherical flexible CMOS retina chip. They thinned the backside Si to a thickness of around 50 μm by mechanical lapping, and after that a dry etching such as XeF2 or RIE is used to etch the Si substrate down to around 10μm thickness and remove the

Instead of lapping and XeF2, RIE can be directly used to thin the backside bulk

This backside-release process consists of deposition of photoresist to protect wafer during etching and RIE etching with thickness measurement of the substrate. In this process, the substrate can be traditional bulk substrate or SOI substrate, because the thickness is controlled by a measurement instead of a stop layer.

Moreover, the protective layer against etchant is not needed for this process, because an anisotropic RIE is used to thin substrate. The PR is coated to protect the ultrathin wafer from mechanical damage such as scrape and fracture. Working gases such as

Sevilla et al. have used this approach to fabricate a flexible nanoscale high performance FinFET [36]. **Figure 14a** shows wafer with FinFET before the release process. First step for this process is deposition of PR that servers as protect layer against mechanical damage (**Figure 14b**). After deposition of PR, the wafer is turned upside down, and the substrate is thinned through RIE (**Figure 14c**). The thickness of substrate is controlled by measurement; when the thickness is the same to the plan, the thinning is finished. Otherwise, the wafer is placed in RIE again for further reduction of substrate. **Figure 14d** shows the thinned wafer, and

surface micro-crack damages induced during the lapping process [33].

*2.3.2.2 Backside-release process using RIE*

silicon substrate, although it results in high roughness.

CF4 and SF6 are used to etch the silicon in RIE.

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

*Silicon-Based Micromachining Process for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.83347*

#### **Figure 13.**

*Micromachining*

**Figure 12.**

two counter-rotating cast iron plates and either an abrasive film or slurry. To adjust the penetration of the film/slurry, the wafers either spin faster or experience a

*(a) Spin coat of thick (7 μm) photoresist and hole patterning, (b) cavern formation beneath BOX due to XeF2 etchant, (c) top view of fins after the gate etch process, which is a complex task performed with a combination of reactive-ion etching and wet cleans, and (d) FinFET silicon fabric at minimum device scale bending radius* 

*(5 mm). Reproduced with permission [34]. Copyright 2014, John Wiley & Sons, Inc.*

The surface roughness of silicon is about 50 nm by using CMP, and through lapping the surface roughness of silicon can achieve 1 μm. The surface roughness of silicon by using RIE is worst about 10 μm. The cost of RIE is the most expensive, because this process needs also a working gas and vacuum environment. The CMP process is more expensive compared to lapping, because CMP consumes chemicals while lapping involves mechanical polish only. For thinning substrate, man can chose suitable process depending on the surface roughness and the cost. **Figure 13** shows three present backside thinning processes by using CMP, RIE, or lapping. For thinning through lapping and XeF2 RIE (**Figure 13a**), at first the substrate is reduced to exact thickness (usually about 50 μm) by lapping for cost saving, and after that the resulting surface micro-crack damages induced during the lapping process are removed by XeF2 etching processes with the buried oxide layer as the etch stop layer. Thinning process through anisotropic RIE (**Figure 13b**) possesses advantage high etch rate about 20 μm/min and disadvantage high surface roughness. At first the wafer is turned upside down, and then the substrate is thinned through RIE. The thickness of substrate is controlled by a measurement. Besides RIE and lapping, backside-release can be also performed through CMP, and **Figure 13c** shows this thinning process. Compared with RIE and lapping, the etch rate of CMP is much lower about 0.5 μm/min, and the surface roughness is best at about 50 nm. Usually for cost saving, before CMP process, the substrate can be thinned through lapping, and after lapping process expensive and more precise CMP is performed to further

Lapping and XeF2 etching can thin the SOI wafer from the backside all the way to the BOX layer with a clean surface finish due to high selectivity between SiO2 and Si in XeF2 RIE. This process involves three steps. At first the wafer is coated by protective layer to protect the parts of wafer that shall not be etched from damage through isotropic XeF2 RIE. Following that, the substrate can be thinned to certain thickness by lapping. At last, the rest of Si substrate is removed by XeF2 RIE, and

heavier load to fit the target specification.

**74**

thin the substrate.

*2.3.2 Process flow of backside-release process*

*2.3.2.1 Backside-release process using lapping and XeF2 RIE*

*The schema for three present backside thinning processes by using CMP, RIE, or lapping. (a) Thinning through lapping (first step) and XeF2 RIE (second step), the BOX serves as stop layer for XeF2 RIE, (b) thinning through RIE by using working gas such as CF4 and SF6, and (c) thinning through lapping (first step) and CMP (second step).*

the BOX serves as stop layer for XeF2 RIE. With the help of XeF2 RIE and stop layer, the resulting surface micro-crack damages induced during the lapping process can be removed. After thinning process, the wafer is transferred to a flexible substrate, and a bonding process will be performed to bond wafer to flexible substrate.

Hsieh et al. used this process to fabricate a biocompatible flexible IC. At first a wafer lapping machine is used to thin the Si wafer substrate, and the thickness is reduced to ∼50 μm [28]. Following lapping, a dry XeF2 etching process with BOX as etch stop layer is performed to remove the surface micro-crack damages that are caused by the lapping process [28]. Liu et al. employed this process to fabricate a spherical flexible CMOS retina chip. They thinned the backside Si to a thickness of around 50 μm by mechanical lapping, and after that a dry etching such as XeF2 or RIE is used to etch the Si substrate down to around 10μm thickness and remove the surface micro-crack damages induced during the lapping process [33].

#### *2.3.2.2 Backside-release process using RIE*

Instead of lapping and XeF2, RIE can be directly used to thin the backside bulk silicon substrate, although it results in high roughness.

This backside-release process consists of deposition of photoresist to protect wafer during etching and RIE etching with thickness measurement of the substrate. In this process, the substrate can be traditional bulk substrate or SOI substrate, because the thickness is controlled by a measurement instead of a stop layer. Moreover, the protective layer against etchant is not needed for this process, because an anisotropic RIE is used to thin substrate. The PR is coated to protect the ultrathin wafer from mechanical damage such as scrape and fracture. Working gases such as CF4 and SF6 are used to etch the silicon in RIE.

Sevilla et al. have used this approach to fabricate a flexible nanoscale high performance FinFET [36]. **Figure 14a** shows wafer with FinFET before the release process. First step for this process is deposition of PR that servers as protect layer against mechanical damage (**Figure 14b**). After deposition of PR, the wafer is turned upside down, and the substrate is thinned through RIE (**Figure 14c**). The thickness of substrate is controlled by measurement; when the thickness is the same to the plan, the thinning is finished. Otherwise, the wafer is placed in RIE again for further reduction of substrate. **Figure 14d** shows the thinned wafer, and

at last the PR layer is removed (**Figure 14e**). If the SOI is not very thin and the surface is hard, this PR layer is not anymore necessary for this release process.

## *2.3.2.3 Backside-release process using CMP*

Besides RIE and lapping, backside-release can be also performed through pure CMP process. The etch rate of CMP is much lower than RIE and lapping, but the surface roughness is the best and in the order of 10 nm or less. Since CMP process is very slow, it usually starts with a thin substrate, for example, 100–200 μm or after a lapping process with reduced thickness for cost saving purpose.

Dumas et al. use CMP to fabricate curved focal plane detector array for wide field cameras. To spherically curve the device, they used CMP to thin the substrate. In their experiment, the process is designed to obtain a component thickness of 50 μm [37]. They have demonstrated that 10 × 10 mm2 silicon samples thinned down to 50 μm could be curved in concave and convex shapes, down to a bending radius of 40 mm [37]. The curved detector is showed in **Figure 15**.

## **2.4 Transfer processes and bond technique**

After the release, the released membranes are transferred to flexible substrate and then bonded to flexible substrate. Now we introduce transfer process and bond technique for silicon-based flexible electronics.

## *2.4.1 Transfer through polymer stamp*

In this transfer process, a photoresist or similar polymer layer is deposited, and then a flat piece of polymer such as poly(dimethylsiloxane) PDMS serves as stamp, which conformally contacts the top surface of the wafer. When the stamp is in contact with the PR, it is carefully peeled up with the released thin membrane. The interface between stamp and photoresist must be strongly bonded, and the wafer is transferred on a flexible substrate. The flexible substrate can be polyimide substrate or liquid crystal polymer (LCP) substrate, and the polyimide adhesion

## **Figure 14.**

*Process flow for the fabrication of flexible FinFET: (a) produced FinFET devices on SOI substrate (90 nm SOI with 150 nm BOX); (b) deposition of PR to protect chip from damages induced by back etch process; (c) FinFET devices etched from backside using RIE process; (d) Si substrate thinned to 50 μm; and (e) removal of PR. Reproduced with permission [36]. Copyright 2014, American Chemical Society.*

**77**

**Figure 16.**

**Figure 15.**

*The Optical Society.*

*Silicon-Based Micromachining Process for Flexible Electronics*

promoter is spin coated on substrate; once the wafer is brought to polyimide, the wafer is baked to cure polyimide adhesion promoter. At last the PR and stamp are striped. This transfer process is the same to the process shown in **Figure 16a**. **Figure 16a** shows a transfer process developed by Menard et al. In this process, bendable single crystal silicon thin film transistors are printed on plastic substrates. At first they brought a flat piece of PDMS that served as stamp into conformal contact

*(Color online) Pictures of curved microbolometer. (a) The thinned curved component on a glass holder. (b) This curved bolometer is bonded onto an electrical board. Reproduced with permission [31]. Copyright 2012,* 

*(a) Process flow for transfer released silicon (μs-Si) ribbons to a plastic flexible substrate. Reproduced with permission [38]. (b) the schematic structure of a flexible thin film transistor with high performance, which was transferred on a PET substrate. The bottom insets show optical images of a device array. In insets, we can see that each device consists of four interconnected microstrips of μs-Si (100 nm thick). Reproduced with permission [38]. Copyright 2005, American Institute of Physics. (c) Process flow for the fabrication of the ACF-packaged flexible Si NAND flash memory using roll-based thermo-compression bonding. (i) Fabricated NAND flash on an SOI wafer was bonded with transfer glass. Release processes are realized by wet etching of the BOX. (ii) NAND chip bonded on transfer glass was separated by dicing. (iii) the released memory was transferred and interconnected on FPCB by using roll-based transfer. (iv) fabricated ACF-packaged Si f-NAND. As shown in inset, the adhesion and interconnection of the electrode bumps of the device and the FPCB are realized by thermosetting resin and conductive particles, respectively. Reproduced with permission [39]. Copyright 2016, John Wiley & Sons, Inc. (d) Photograph of the highly compliant ACF-packaged f-NAND wrapped on a glass rod (diameter of 7 mm). The OM image of the electrode area (left) and the active device area (right) are* 

*shown in insets. Reproduced with permission [39]. Copyright 2016, John Wiley & Sons, Inc***.**

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

## *Silicon-Based Micromachining Process for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.83347*

promoter is spin coated on substrate; once the wafer is brought to polyimide, the wafer is baked to cure polyimide adhesion promoter. At last the PR and stamp are striped. This transfer process is the same to the process shown in **Figure 16a**.

**Figure 16a** shows a transfer process developed by Menard et al. In this process, bendable single crystal silicon thin film transistors are printed on plastic substrates. At first they brought a flat piece of PDMS that served as stamp into conformal contact

#### **Figure 15.**

*Micromachining*

*2.3.2.3 Backside-release process using CMP*

at last the PR layer is removed (**Figure 14e**). If the SOI is not very thin and the surface is hard, this PR layer is not anymore necessary for this release process.

Besides RIE and lapping, backside-release can be also performed through pure CMP process. The etch rate of CMP is much lower than RIE and lapping, but the surface roughness is the best and in the order of 10 nm or less. Since CMP process is very slow, it usually starts with a thin substrate, for example, 100–200 μm or after a

Dumas et al. use CMP to fabricate curved focal plane detector array for wide field cameras. To spherically curve the device, they used CMP to thin the substrate. In their experiment, the process is designed to obtain a component thickness of

down to 50 μm could be curved in concave and convex shapes, down to a bending

After the release, the released membranes are transferred to flexible substrate and then bonded to flexible substrate. Now we introduce transfer process and bond

In this transfer process, a photoresist or similar polymer layer is deposited, and then a flat piece of polymer such as poly(dimethylsiloxane) PDMS serves as stamp, which conformally contacts the top surface of the wafer. When the stamp is in contact with the PR, it is carefully peeled up with the released thin membrane. The interface between stamp and photoresist must be strongly bonded, and the wafer is transferred on a flexible substrate. The flexible substrate can be polyimide substrate or liquid crystal polymer (LCP) substrate, and the polyimide adhesion

*Process flow for the fabrication of flexible FinFET: (a) produced FinFET devices on SOI substrate (90 nm SOI with 150 nm BOX); (b) deposition of PR to protect chip from damages induced by back etch process; (c) FinFET devices etched from backside using RIE process; (d) Si substrate thinned to 50 μm; and (e) removal of* 

*PR. Reproduced with permission [36]. Copyright 2014, American Chemical Society.*

silicon samples thinned

lapping process with reduced thickness for cost saving purpose.

radius of 40 mm [37]. The curved detector is showed in **Figure 15**.

50 μm [37]. They have demonstrated that 10 × 10 mm2

**2.4 Transfer processes and bond technique**

technique for silicon-based flexible electronics.

*2.4.1 Transfer through polymer stamp*

**76**

**Figure 14.**

*(Color online) Pictures of curved microbolometer. (a) The thinned curved component on a glass holder. (b) This curved bolometer is bonded onto an electrical board. Reproduced with permission [31]. Copyright 2012, The Optical Society.*

#### **Figure 16.**

*(a) Process flow for transfer released silicon (μs-Si) ribbons to a plastic flexible substrate. Reproduced with permission [38]. (b) the schematic structure of a flexible thin film transistor with high performance, which was transferred on a PET substrate. The bottom insets show optical images of a device array. In insets, we can see that each device consists of four interconnected microstrips of μs-Si (100 nm thick). Reproduced with permission [38]. Copyright 2005, American Institute of Physics. (c) Process flow for the fabrication of the ACF-packaged flexible Si NAND flash memory using roll-based thermo-compression bonding. (i) Fabricated NAND flash on an SOI wafer was bonded with transfer glass. Release processes are realized by wet etching of the BOX. (ii) NAND chip bonded on transfer glass was separated by dicing. (iii) the released memory was transferred and interconnected on FPCB by using roll-based transfer. (iv) fabricated ACF-packaged Si f-NAND. As shown in inset, the adhesion and interconnection of the electrode bumps of the device and the FPCB are realized by thermosetting resin and conductive particles, respectively. Reproduced with permission [39]. Copyright 2016, John Wiley & Sons, Inc. (d) Photograph of the highly compliant ACF-packaged f-NAND wrapped on a glass rod (diameter of 7 mm). The OM image of the electrode area (left) and the active device area (right) are shown in insets. Reproduced with permission [39]. Copyright 2016, John Wiley & Sons, Inc***.**

## *Micromachining*

with PR layer on the surface of the wafer and then carefully peeled back to pick up the released wafer with silicon (μs-Si) ribbons [38]. The interaction between the PR and the PDMS must be strong enough to bond them together for removal, with good efficiency [38]. A 180 μm thick polyethylene terephthalate (PET) plastic sheet coated with a 100 nm thick indium tin oxide (ITO) was used as the flexible substrate [38]. A dielectric layer of epoxy was used to enhance the adhesion between released wafer and flexible substrate and was spin coated on flexible substrate [38]. Bringing the PDMS with the μs-Si on its surface into contact with the warm epoxy layer and then peeling back the PDMS led to the transfer of the μs-Si to the epoxy [38].

Kim et al. demonstrate simultaneous roll transfer and interconnection of Si-based flexible NAND flash memory (f-NAND) based on highly productive rollto-plate ACF packaging [39]. This process is described in **Figure 16b**.

## *2.4.2 Bonding SOI to flexible substrate*

When thinned die or wafer is transferred on the flexible substrate, the bonding between die and flexible substrate must be performed in order to realize the electrical connection between die and other devices.

**Figure 17.**

*(a) Process flow for flip-chip bond and (b) process flow for adhesion method.*

### **Figure 18.**

*(a) Illustration of polyimide or LCP substrate and solder assembly approach. Reproduced with permission [40]. Copyright 2008, IEEE. (b) Illustration of thinned Si die embedded in polyimide with thin film interconnect using adhesion method. Reproduced with permission [40]. Copyright 2008, IEEE.*

**79**

*Silicon-Based Micromachining Process for Flexible Electronics*

Flip-chip bond and adhesion method can be used to bond released dies to flexible substrate. For the flip-chip bond, polyimide or liquid crystal polymer (LCP) can be used as flexible substrate. For adhesion method, the substrate is made of polyimide, and the interconnection of die is formed through wire bonding.

Flip-chip bond consists of four steps. First, the bumps and pads are fabricated on flexible substrate. Following that, the die is placed on the flexible substrate and aligned. Once the die and bumps are in contact, the bumps are heated at melting temperature, and then die is bonded to substrate. At last an underfill is performed. The materials such as SnPb and SnAg can be used as

For the adhesion method, a polyimide adhesion promoter such as a dielectric layer of epoxy is applied, so that the die can adhere to polyimide substrate. The polyimide adhesion promoter is spin coated on substrate, and once the wafer is brought to polyimide, the wafer is baked to cure polyimide adhesion promoter. At

Holland et al. [40] used flip-chip bonding to bond die to substrate. Different from our bonding process, they used immersion bump. **Figure 18a** shows thinned die flip-chip bonded on polyimide or LCP substrate. Moreover Holland et al. [40] used also adhesion method to bond die to substrate. Different from our adhesion

Menard et al. used a dielectric layer of epoxy as polyimide adhesion promoter that was spin coated on substrate to bond the die to polyimide substrate through

Theoretically, all devices such as transistor circuit, DRAM, NAND flash, and sensors that were fabricated through traditional Si-based CMOS process can also be fabricated in flexible forms by using appropriate release processes and transfer technique. We have mainly described two types of release processes: frontside- and backside-release. The frontside-release is realized by etching the BOX or undercutting silicon under the BOX in SOI wafer. The BOX layer etching is achieved in wet etching with HF-contained etchant, and the bulk silicon undercutting is achieved by XeF2 isotropic etching. The backside-release process etches the Si substrates through CMP, lapping, or RIE. After releasing, the Si thin membrane with active devices is transferred to a flexible substrate. Polymer stamp transfer, flip-chip bond, or adhesion method can be used to bond released dies to a flexible substrate. By leveraging those silicon-based micromachining processes, flexible electronics can be achieved on top of current standard CMOS process and scale to large volume manufacturing.

last the interconnection is formed through wire bonding (**Figure 17**).

method, they embed the thinned Si die in Polyimide (**Figure 18b**).

*DOI: http://dx.doi.org/10.5772/intechopen.83347*

bumps.

adhesion method [38].

**3. Conclusion**

*Silicon-Based Micromachining Process for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.83347*

Flip-chip bond and adhesion method can be used to bond released dies to flexible substrate. For the flip-chip bond, polyimide or liquid crystal polymer (LCP) can be used as flexible substrate. For adhesion method, the substrate is made of polyimide, and the interconnection of die is formed through wire bonding.

Flip-chip bond consists of four steps. First, the bumps and pads are fabricated on flexible substrate. Following that, the die is placed on the flexible substrate and aligned. Once the die and bumps are in contact, the bumps are heated at melting temperature, and then die is bonded to substrate. At last an underfill is performed. The materials such as SnPb and SnAg can be used as bumps.

For the adhesion method, a polyimide adhesion promoter such as a dielectric layer of epoxy is applied, so that the die can adhere to polyimide substrate. The polyimide adhesion promoter is spin coated on substrate, and once the wafer is brought to polyimide, the wafer is baked to cure polyimide adhesion promoter. At last the interconnection is formed through wire bonding (**Figure 17**).

Holland et al. [40] used flip-chip bonding to bond die to substrate. Different from our bonding process, they used immersion bump. **Figure 18a** shows thinned die flip-chip bonded on polyimide or LCP substrate. Moreover Holland et al. [40] used also adhesion method to bond die to substrate. Different from our adhesion method, they embed the thinned Si die in Polyimide (**Figure 18b**).

Menard et al. used a dielectric layer of epoxy as polyimide adhesion promoter that was spin coated on substrate to bond the die to polyimide substrate through adhesion method [38].

## **3. Conclusion**

*Micromachining*

with PR layer on the surface of the wafer and then carefully peeled back to pick up the released wafer with silicon (μs-Si) ribbons [38]. The interaction between the PR and the PDMS must be strong enough to bond them together for removal, with good efficiency [38]. A 180 μm thick polyethylene terephthalate (PET) plastic sheet coated with a 100 nm thick indium tin oxide (ITO) was used as the flexible substrate [38]. A dielectric layer of epoxy was used to enhance the adhesion between released wafer and flexible substrate and was spin coated on flexible substrate [38]. Bringing the PDMS with the μs-Si on its surface into contact with the warm epoxy layer and then

peeling back the PDMS led to the transfer of the μs-Si to the epoxy [38].

to-plate ACF packaging [39]. This process is described in **Figure 16b**.

*2.4.2 Bonding SOI to flexible substrate*

cal connection between die and other devices.

*(a) Process flow for flip-chip bond and (b) process flow for adhesion method.*

*(a) Illustration of polyimide or LCP substrate and solder assembly approach. Reproduced with permission [40]. Copyright 2008, IEEE. (b) Illustration of thinned Si die embedded in polyimide with thin film interconnect using adhesion method. Reproduced with permission [40]. Copyright 2008, IEEE.*

Kim et al. demonstrate simultaneous roll transfer and interconnection of Si-based flexible NAND flash memory (f-NAND) based on highly productive roll-

When thinned die or wafer is transferred on the flexible substrate, the bonding between die and flexible substrate must be performed in order to realize the electri-

**78**

**Figure 18.**

**Figure 17.**

Theoretically, all devices such as transistor circuit, DRAM, NAND flash, and sensors that were fabricated through traditional Si-based CMOS process can also be fabricated in flexible forms by using appropriate release processes and transfer technique. We have mainly described two types of release processes: frontside- and backside-release. The frontside-release is realized by etching the BOX or undercutting silicon under the BOX in SOI wafer. The BOX layer etching is achieved in wet etching with HF-contained etchant, and the bulk silicon undercutting is achieved by XeF2 isotropic etching. The backside-release process etches the Si substrates through CMP, lapping, or RIE. After releasing, the Si thin membrane with active devices is transferred to a flexible substrate. Polymer stamp transfer, flip-chip bond, or adhesion method can be used to bond released dies to a flexible substrate. By leveraging those silicon-based micromachining processes, flexible electronics can be achieved on top of current standard CMOS process and scale to large volume manufacturing.

*Micromachining*

## **Author details**

Jiye Yang and Tao Wu\* School of Information Science and Technology, ShanghaiTech University, China

\*Address all correspondence to: wutao@shanghaitech.edu.cn

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**81**

*Silicon-Based Micromachining Process for Flexible Electronics*

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*DOI: http://dx.doi.org/10.5772/intechopen.83347*

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**Author details**

Jiye Yang and Tao Wu\*

provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

School of Information Science and Technology, ShanghaiTech University, China

\*Address all correspondence to: wutao@shanghaitech.edu.cn

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Chapter 4

Abstract

are reported.

1. Introduction

compatibility and less toxicity.

83

Applications

CMOS Compatible Wet Bulk

Wet bulk micromachining of silicon is a convenient and economical method for realizing various silicon-based microsensors and actuators. Tetramethylammonium hydroxide (TMAH) based anisotropic wet etching is popular due to it being less toxic and CMOS compatible. The etch rate of TMAH depends on the wafer's crystal plane orientation and temperature/concentration of solution. While using TMAH to

underetching, causing a deviation in the intended size of the diaphragm, inducing variation in the designed characteristics of the device. It is necessary to estimate and minimize these deviations. Experiments were designed and the rate of etching for (100) and (111) planes using 25 wt.% TMAH have been determined at different temperatures. Linear fit equations are obtained from experimental data to relate the underetch per unit depth to the solution temperature. These findings are extremely useful in the fabrication of silicon diaphragms with precise dimensions. While using anisotropic wet etchants to realize proof mass for accelerometers, the etchants attack the convex corners. This necessitates a suitable design of compensating structure while realizing microstructures with sharp convex corners. Experimental studies are carried out to protect convex corners from undercutting and the results

realize a pressure sensor diaphragm, the etching of {111} planes causes

Keywords: TMAH etching, piezoresistive pressure sensor, diaphragm,

The process of bulk micromachining is carried out in order to etch out a significant portion of silicon from silicon substrate resulting in structures created out of silicon for different applications. Some of these structures include: diaphragms (for pressure sensors) [1], proof-mass (for accelerometers) [2], nozzle (for inkjet printer) [3], random pyramids on Si surface (for solar cells) [4], and cantilevers [5]. Aqueous TMAH is often used for realization of structures in silicon using wet anisotropic bulk micromachining. It offers an attractive low-cost alternative to dry bulk micromachining technique, which uses gases and expensive equipment. TMAH is favoured choice over other alternatives like KOH, EDP etc. for wet anisotropic etching due to complementary metal-oxide-semiconductor (CMOS)

underetching, convex corner compensation

Micromachining for MEMS

S. Santosh Kumar and Ravindra Mukhiya

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[35] Wu T et al. Design and fabrication of silicon-tessellated structures for monocentric imagers. Microsystems & Nanoengineering. 2016;**2**(1)

[36] Torres Sevilla GA et al. Flexible nanoscale high-performance FinFETs. ACS Nano. 2014;**8**(10):9850-9856

[37] Dumas D et al. Curved focal plane detector array for wide field cameras. Applied Optics. 2012;**51**(22):5419-5424

[38] Menard E, Nuzzo RG, Rogers JA. Bendable single crystal silicon thin film transistors formed by printing on plastic substrates. Applied Physics Letters. 2005;**86**(9):093507

[39] Kim DH et al. Simultaneous roll transfer and interconnection of flexible silicon NAND flash memory. Advanced Materials. 2016;**28**(38):8371-8378

[40] Holland B et al. Ultra-Thin, Flexible Electronics. Piscataway, New Jersey, US: IEEE; 2008. pp. 1110-1116

## Chapter 4

*Micromachining*

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[21] Hong J et al. Removal rate and surface quality of the GLSI silicon substrate during the CMP process. Microelectronic

inorganic materials. Advanced Materials. 2010;**22**(19):2108-2124

2012;**37**(4):653-655

Technology Office

2014;**24**(4):045025

2014;**26**(18):2794-2799

Nanoengineering. 2016;**2**(1)

[31] Dumas D et al. Infrared camera based on a curved retina. Optics Letters.

[32] Blake T et al. Utilization of a curved local surface Array in a 3.5 m wide field of view telescope. Arlington; 2013, Defense Advanced Research Projects Agency Arlington Va Tactical

[33] Liu C-Y et al. A contact-lensshaped IC chip technology. Journal of Micromechanics and Microengineering.

[34] Sevilla GA et al. Flexible and transparent silicon-on-polymer based sub-20 nm non-planar 3D FinFET for brain-architecture inspired computation. Advanced Materials.

[35] Wu T et al. Design and fabrication of silicon-tessellated structures for monocentric imagers. Microsystems &

[36] Torres Sevilla GA et al. Flexible nanoscale high-performance FinFETs. ACS Nano. 2014;**8**(10):9850-9856

[37] Dumas D et al. Curved focal plane detector array for wide field cameras. Applied Optics. 2012;**51**(22):5419-5424

[38] Menard E, Nuzzo RG, Rogers JA. Bendable single crystal silicon thin film transistors formed by printing on plastic substrates. Applied Physics Letters.

[39] Kim DH et al. Simultaneous roll transfer and interconnection of flexible silicon NAND flash memory. Advanced Materials. 2016;**28**(38):8371-8378

[40] Holland B et al. Ultra-Thin, Flexible Electronics. Piscataway, New Jersey, US:

IEEE; 2008. pp. 1110-1116

2005;**86**(9):093507

[22] Kim HJ. In: Rudawska A, editor. Abrasive for Chemical Mechanical Polishing, Abrasive Technology. Rijeka:

[23] Hamann ACAS. Smooth Sidewall

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Etching in the PT-DSE. 2014

2012;**21**(6):1436-1444

2013;**3**:1291

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[24] Xu D et al. Isotropic silicon etching with XeF2 gas for wafer-level micromachining applications. Journal of Microelectromechanical Systems.

[25] Zhou H et al. Fast flexible electronics with strained silicon nanomembranes. Scientific Reports.

[27] Rim S-B et al. The optical

[26] Dinyari R et al. Curving monolithic silicon for nonplanar focal plane array applications. Applied Physics Letters.

advantages of curved focal plane arrays. Optics Express. 2008;**16**(7):4965-4971

[28] Hsieh C et al. A flexible mixedsignal/RF CMOS technology for implantable electronics applications. Journal of Micromechanics and Microengineering. 2010;**20**(4):045017

[29] Iwert O, Delabre B. The challenge of highly curved monolithic imaging detectors. In: High Energy, Optical, and Infrared Detectors for Astronomy IV. Germany: International Society for

Optics and Photonics; 2010

[30] Kim DH et al. Stretchable, curvilinear electronics based on

IntechOpen; 2018

**82**

## CMOS Compatible Wet Bulk Micromachining for MEMS Applications

S. Santosh Kumar and Ravindra Mukhiya

## Abstract

Wet bulk micromachining of silicon is a convenient and economical method for realizing various silicon-based microsensors and actuators. Tetramethylammonium hydroxide (TMAH) based anisotropic wet etching is popular due to it being less toxic and CMOS compatible. The etch rate of TMAH depends on the wafer's crystal plane orientation and temperature/concentration of solution. While using TMAH to realize a pressure sensor diaphragm, the etching of {111} planes causes underetching, causing a deviation in the intended size of the diaphragm, inducing variation in the designed characteristics of the device. It is necessary to estimate and minimize these deviations. Experiments were designed and the rate of etching for (100) and (111) planes using 25 wt.% TMAH have been determined at different temperatures. Linear fit equations are obtained from experimental data to relate the underetch per unit depth to the solution temperature. These findings are extremely useful in the fabrication of silicon diaphragms with precise dimensions. While using anisotropic wet etchants to realize proof mass for accelerometers, the etchants attack the convex corners. This necessitates a suitable design of compensating structure while realizing microstructures with sharp convex corners. Experimental studies are carried out to protect convex corners from undercutting and the results are reported.

Keywords: TMAH etching, piezoresistive pressure sensor, diaphragm, underetching, convex corner compensation

## 1. Introduction

The process of bulk micromachining is carried out in order to etch out a significant portion of silicon from silicon substrate resulting in structures created out of silicon for different applications. Some of these structures include: diaphragms (for pressure sensors) [1], proof-mass (for accelerometers) [2], nozzle (for inkjet printer) [3], random pyramids on Si surface (for solar cells) [4], and cantilevers [5]. Aqueous TMAH is often used for realization of structures in silicon using wet anisotropic bulk micromachining. It offers an attractive low-cost alternative to dry bulk micromachining technique, which uses gases and expensive equipment. TMAH is favoured choice over other alternatives like KOH, EDP etc. for wet anisotropic etching due to complementary metal-oxide-semiconductor (CMOS) compatibility and less toxicity.

#### Micromachining

In industrial microelectronics process line or CMOS-MEMS processes, it is preferable to replace KOH with TMAH, to avoid contamination due to Potassium (K+) ions. However, owing to a high undercutting ratio of TMAH, it entails a dedicated study on compensation structures. This has inspired us to carry out experimental studies using TMAH.

TMAH etching rate is generally governed by: the orientation of the Si wafer, the temperature of aq. TMAH solution and the TMAH concentration in aqueous solution [6]. Although TMAH etches aluminium when it comes in contact with it, it is possible to carry out post CMOS bulk micromachining with aluminium metallization using TMAH by dissolving silicon and oxidizers like ammonium peroxodisulphate into the solution [7, 8].

The undercutting of convex corners and non (111) planes in Si is helpful in realization of freely suspended microstructures using wet anisotropic etchants [9, 10]. However, when mesa type structures having convex corners are desired, these effect of undercutting of convex corners have to be checked and avoided, if possible. It has been observed that TMAH shows a high undercutting ratio in comparison to KOH. In this chapter, experimental studies and analysis to protect convex corners and non {111} crystal planes from undercutting during TMAH etching is described.

minimize the deviation, especially in square diaphragms, where precise placement of piezoresistors is of prime importance for optimum performance of the device.

Cross-section of diaphragm of pressure sensor showing the displacement of position of piezoresistors due to

CMOS Compatible Wet Bulk Micromachining for MEMS Applications

DOI: http://dx.doi.org/10.5772/intechopen.88487

For the purpose of this study, experiments are designed and the etch rates in the

The cross-section of the etch profile obtained during etching, while fabricating diaphragm structures on (100) Si substrate using TMAH is shown in Figure 2. The

underetching below the mask due to etching in {111} planes. Alternatively, we can

3. Experimental evaluation of TMAH etching for pressure sensor

{100} and {111} planes for 25 wt.% TMAH have been determined at different temperatures. Through these experiments, the underetching per unit depth of etching is also determined. This data can be used for selection of proper dimensions (based on the selected temperature of etching during fabrication) in the mask sets, compensating for the change in dimensions caused due to underetching. This method will ensure that diaphragms with accurate dimension are realized after

figure distinctly depicts anisotropic etching by TMAH and also shows the

diaphragm

etching by TMAH in (111) planes.

Figure 1.

etching.

Figure 2.

85

TMAH etch profile depicting etching in (100) and (111) planes.

In this chapter, we present the studies related to realization of diaphragm (for piezoresistive pressure sensor) and proof-mass (for accelerometer) using aq. 25 wt. % TMAH solution. The importance of precise etching of diaphragm in piezoresistive pressure sensor is discussed in Section 2. Experimental evaluation of TMAH etching is carried out to determine the etch rate in (100) and (111) planes of silicon at different temperatures and is discussed in Section 3. The underetching of silicon diaphragm during TMAH etching is also determined to design proper dimensions of openings in mask sets. Corner compensation structures for TMAH, experimental details, and design analysis and discussion are presented in Sections 4–6, respectively.

## 2. Piezoresistive pressure sensor

Usually, a piezoresistive pressure sensor has four piezoresistors on a silicon diaphragm, close to the edges of the diaphragm, connected in a Wheatstone configuration. When the diaphragm is subjected to a pressure load, the deflection of the diaphragm leads to stress generation on the diaphragm. As the piezoresistors are placed near the surface of the diaphragm, they experience these stresses, leading to a change in the resistance of the piezoresistors. Prior to fabricating the sensor, the placement of piezoresistors on the diaphragm is optimized using finite element method (FEM) based tools.

Diaphragm in (100) silicon can be fabricated using TMAH by exploiting its anisotropic etching property. TMAH etches different silicon planes with different etch rates but it has a high etch rate in <100> direction. Subsequent to the groundbreaking paper by Tabata et al. [11], much research has been dedicated to analyse the etch rate of silicon for different crystallographic directions [6, 12, 13]. While realizing diaphragms using aq. TMAH etching in (100) silicon wafers, the expected position of the piezoresistors (determined using FEM based design simulations) may shift due to etching in {111} planes. This leads to the phenomenon of underetching, where the TMAH etches under the etching mask in a (100) wafer. Underetching may cause the piezoresistors to be shifted from their pre-planned position on the diaphragm, as shown in Figure 1. This induces variation in the designed characteristics of the device. Therefore, it is necessary to estimate and

CMOS Compatible Wet Bulk Micromachining for MEMS Applications DOI: http://dx.doi.org/10.5772/intechopen.88487

Figure 1.

In industrial microelectronics process line or CMOS-MEMS processes, it is preferable to replace KOH with TMAH, to avoid contamination due to Potassium (K+) ions. However, owing to a high undercutting ratio of TMAH, it entails a dedicated study on compensation structures. This has inspired us to carry out experimental

TMAH etching rate is generally governed by: the orientation of the Si wafer, the temperature of aq. TMAH solution and the TMAH concentration in aqueous solution [6]. Although TMAH etches aluminium when it comes in contact with it, it is possible to carry out post CMOS bulk micromachining with aluminium metallization using TMAH by dissolving silicon and oxidizers like ammonium peroxodi-

The undercutting of convex corners and non (111) planes in Si is helpful in realization of freely suspended microstructures using wet anisotropic etchants [9, 10]. However, when mesa type structures having convex corners are desired, these effect of undercutting of convex corners have to be checked and avoided, if possible. It has been observed that TMAH shows a high undercutting ratio in comparison to KOH. In this chapter, experimental studies and analysis to protect convex corners and non {111} crystal planes from undercutting during TMAH

In this chapter, we present the studies related to realization of diaphragm (for piezoresistive pressure sensor) and proof-mass (for accelerometer) using aq. 25 wt. % TMAH solution. The importance of precise etching of diaphragm in piezoresistive pressure sensor is discussed in Section 2. Experimental evaluation of TMAH etching is carried out to determine the etch rate in (100) and (111) planes of silicon at different temperatures and is discussed in Section 3. The underetching of silicon diaphragm during TMAH etching is also determined to design proper dimensions of openings in mask sets. Corner compensation structures for TMAH, experimental

details, and design analysis and discussion are presented in Sections 4–6,

Usually, a piezoresistive pressure sensor has four piezoresistors on a silicon diaphragm, close to the edges of the diaphragm, connected in a Wheatstone configuration. When the diaphragm is subjected to a pressure load, the deflection of the diaphragm leads to stress generation on the diaphragm. As the piezoresistors are placed near the surface of the diaphragm, they experience these stresses, leading to a change in the resistance of the piezoresistors. Prior to fabricating the sensor, the placement of piezoresistors on the diaphragm is optimized using finite element

Diaphragm in (100) silicon can be fabricated using TMAH by exploiting its anisotropic etching property. TMAH etches different silicon planes with different etch rates but it has a high etch rate in <100> direction. Subsequent to the groundbreaking paper by Tabata et al. [11], much research has been dedicated to analyse the etch rate of silicon for different crystallographic directions [6, 12, 13]. While realizing diaphragms using aq. TMAH etching in (100) silicon wafers, the expected position of the piezoresistors (determined using FEM based design simulations) may shift due to etching in {111} planes. This leads to the phenomenon of underetching, where the TMAH etches under the etching mask in a (100) wafer. Underetching may cause the piezoresistors to be shifted from their pre-planned position on the diaphragm, as shown in Figure 1. This induces variation in the designed characteristics of the device. Therefore, it is necessary to estimate and

studies using TMAH.

Micromachining

etching is described.

respectively.

84

2. Piezoresistive pressure sensor

method (FEM) based tools.

sulphate into the solution [7, 8].

Cross-section of diaphragm of pressure sensor showing the displacement of position of piezoresistors due to etching by TMAH in (111) planes.

minimize the deviation, especially in square diaphragms, where precise placement of piezoresistors is of prime importance for optimum performance of the device.

## 3. Experimental evaluation of TMAH etching for pressure sensor diaphragm

For the purpose of this study, experiments are designed and the etch rates in the {100} and {111} planes for 25 wt.% TMAH have been determined at different temperatures. Through these experiments, the underetching per unit depth of etching is also determined. This data can be used for selection of proper dimensions (based on the selected temperature of etching during fabrication) in the mask sets, compensating for the change in dimensions caused due to underetching. This method will ensure that diaphragms with accurate dimension are realized after etching.

The cross-section of the etch profile obtained during etching, while fabricating diaphragm structures on (100) Si substrate using TMAH is shown in Figure 2. The figure distinctly depicts anisotropic etching by TMAH and also shows the underetching below the mask due to etching in {111} planes. Alternatively, we can

Figure 2. TMAH etch profile depicting etching in (100) and (111) planes.

also say that underetching is dependent on etch rate of TMAH in <110> direction. If we consider etching for time t, the <100> direction etch rate (ER) can be calculated by determining the etch depth (D) and dividing it by t. The <111> direction etch rate (x/t) can be calculated by determining the underetching (δ/2) in the mask and subsequently applying the formulas as shown in Eqs. (1)–(3) [14].

$$
\frac{\infty}{\delta/2} = \sin 54.74 = \frac{\sqrt{2}}{\sqrt{3}}, \delta = \sqrt{6}\infty \tag{1}
$$

$$\text{ER of } \{ 111 \} \text{ planes}: \frac{\varkappa}{t} = \frac{\delta}{\sqrt{6}t}, t = \text{etch time} \tag{2}$$

$$\text{Underetch per unit depth}: \frac{\delta/2}{D} \tag{3}$$

It is evident from the table that the rate of etching increases both in the <100> and <111> direction with increase in temperature. The underetch per unit depth is determined by the ratio of etching in <110> direction to etching in <100> direction. It can be observed that the underetch per unit depth decreases with increased temperature. Alternatively stated, for the same etch depth in <100> direction, lesser underetching is obtained at higher temperatures. A minor deviation in this upward trend is observed between temperature of 73 and 78°C. This may be attributed to some error in experimentally observed data due to temperature variation in the TMAH solution. It may also be attributed to error in the measurement of the diaphragm size from SEM images. The plot of etch rate in <100> direction and underetch per unit depth with solution temperature is shown in Figures 5 and 6,

Etch rates of Si in TMAH [14]. Copyright 2014 by springer nature (used with permission).

(a) SEM image of a diaphragm and (b) hanging oxide after TMAH etching [14] copyright 2014 by springer

 10.02 1.15 8.71 0.141 13.80 1.29 10.69 0.114 17.98 1.40 12.84 0.095 21.62 1.73 12.49 0.098 30.38 1.89 16.07 0.076 38.90 2.00 19.45 0.063

Etch rate in <111> (μm/h) Etch rate ratio <100>/<111> Underetch per unit depth

Etch rate in <100> (μm/h)

CMOS Compatible Wet Bulk Micromachining for MEMS Applications

DOI: http://dx.doi.org/10.5772/intechopen.88487

Based on the above observations, the following empirical equations (linear fit) are proposed for the etching rates in <100> and <111> direction in 25% wt. aq.

A new empirical linear fit equation is obtained from the data in Table 1 to relate the underetch per unit depth of etching in (100) plane to the solution temperature,

<100>etch rate ¼ 1:13 � T–63:21 (4) < 111>etch rate ¼ 0:0365 � T–1:176 (5)

respectively.

87

Figure 4.

Table 1.

nature (used with permission).

Solution temperature (°C) error = �1

TMAH at different temperatures:

which is given by Eq. (6):

To ascertain the nature of etching in 25 wt.% aq. TMAH for (100) silicon at different temperatures, (100) Si n-type wafers with resistivity values in the range of 5–7 Ω-cm and a thickness of 527 � 2 μm are selected. Subsequently, a thermal SiO2 layer of 1 μm thickness is grown (and afterward patterned) for use as masking layer. A single level mask is used to obtain the required diaphragms. The diaphragms etched using TMAH are than analyzed for feature dimensions to determine the (111) and (100) planes etch rates. Each sample was etched in TMAH at different temperature (with an accuracy of �1°C). For maintaining a constant concentration of TMAH in the aqueous solution, during the course of the experiment, suitable amount of deionized (DI) water was added after each experiment [15]. The etching on each sample was performed for 5 h. Six samples were used for six different temperatures.

The depth of the cavity (behind the diaphragm) for different samples is determined using a surface contact profilometer. The measured value is used to obtain the <100> direction etch rate. To obtain the <111> direction etch rate for different samples, the initial and final sizes of the square alignment mark are compared. The initial size of alignment mark would be equal to the size of cavity opening in the mask set. This determines the underetching by determining the length of hanging oxide. These measurements are carried out using scanning electron microscope (SEM). The underetch (δ/2) obtained from SEM is used to calculate the <111> direction etch rate. A sample SEM image of the alignment marks, overall diaphragm and zoomed view of hanging oxide are shown in Figures 3 and 4a and b respectively. The experiments are performed for six temperatures. The computed <100> and <111> direction etch rates are listed in Table 1.

#### Figure 3.

SEM image of alignment marks used for calculating underetching (a) alignment mark during etching and (b) alignment mark converted to V-groove.

CMOS Compatible Wet Bulk Micromachining for MEMS Applications DOI: http://dx.doi.org/10.5772/intechopen.88487

#### Figure 4.

also say that underetching is dependent on etch rate of TMAH in <110> direction. If we consider etching for time t, the <100> direction etch rate (ER) can be calculated by determining the etch depth (D) and dividing it by t. The <111> direction etch rate (x/t) can be calculated by determining the underetching (δ/2) in the mask and subsequently applying the formulas as shown in Eqs. (1)–(3) [14].

> x <sup>t</sup> <sup>¼</sup> <sup>δ</sup> ffiffiffi 6 <sup>p</sup> <sup>t</sup>

Underetch per unit depth :

To ascertain the nature of etching in 25 wt.% aq. TMAH for (100) silicon at different temperatures, (100) Si n-type wafers with resistivity values in the range of 5–7 Ω-cm and a thickness of 527 � 2 μm are selected. Subsequently, a thermal SiO2 layer of 1 μm thickness is grown (and afterward patterned) for use as masking layer. A single level mask is used to obtain the required diaphragms. The diaphragms etched using TMAH are than analyzed for feature dimensions to determine the (111) and (100) planes etch rates. Each sample was etched in TMAH at different temperature (with an accuracy of �1°C). For maintaining a constant concentration of TMAH in the aqueous solution, during the course of the experiment, suitable amount of deionized (DI) water was added after each experiment [15]. The etching on each sample was performed for 5 h. Six samples were used for six different temperatures. The depth of the cavity (behind the diaphragm) for different samples is determined using a surface contact profilometer. The measured value is used to obtain the <100> direction etch rate. To obtain the <111> direction etch rate for different samples, the initial and final sizes of the square alignment mark are compared. The initial size of alignment mark would be equal to the size of cavity opening in the mask set. This determines the underetching by determining the length of hanging oxide. These measurements are carried out using scanning electron microscope (SEM). The underetch (δ/2) obtained from SEM is used to calculate the <111> direction etch rate. A sample SEM image of the alignment marks, overall diaphragm and zoomed view of hanging oxide are shown in Figures 3 and 4a and b respectively. The experiments are performed for six temperatures. The computed <100>

SEM image of alignment marks used for calculating underetching (a) alignment mark during etching and

ffiffi 2 p ffiffiffi <sup>3</sup> <sup>p</sup> , <sup>δ</sup> <sup>¼</sup> ffiffiffi

6

δ=2

<sup>p</sup> <sup>x</sup> (1)

<sup>D</sup> (3)

, t ¼ etch time (2)

x

Micromachining

and <111> direction etch rates are listed in Table 1.

Figure 3.

86

(b) alignment mark converted to V-groove.

ER of 111 f g planes :

<sup>δ</sup>=<sup>2</sup> <sup>¼</sup> sin 54:<sup>74</sup> <sup>¼</sup>

(a) SEM image of a diaphragm and (b) hanging oxide after TMAH etching [14] copyright 2014 by springer nature (used with permission).


#### Table 1.

Etch rates of Si in TMAH [14]. Copyright 2014 by springer nature (used with permission).

It is evident from the table that the rate of etching increases both in the <100> and <111> direction with increase in temperature. The underetch per unit depth is determined by the ratio of etching in <110> direction to etching in <100> direction. It can be observed that the underetch per unit depth decreases with increased temperature. Alternatively stated, for the same etch depth in <100> direction, lesser underetching is obtained at higher temperatures. A minor deviation in this upward trend is observed between temperature of 73 and 78°C. This may be attributed to some error in experimentally observed data due to temperature variation in the TMAH solution. It may also be attributed to error in the measurement of the diaphragm size from SEM images. The plot of etch rate in <100> direction and underetch per unit depth with solution temperature is shown in Figures 5 and 6, respectively.

Based on the above observations, the following empirical equations (linear fit) are proposed for the etching rates in <100> and <111> direction in 25% wt. aq. TMAH at different temperatures:

$$<100>etch\ rate = 1.13 \times T \text{--} 63.21\tag{4}$$

$$<111>etch\ rate = 0.0365 \times T - 1.176\tag{5}$$

A new empirical linear fit equation is obtained from the data in Table 1 to relate the underetch per unit depth of etching in (100) plane to the solution temperature, which is given by Eq. (6):

of mask sets which have been fabricated without considering the effect of underetching, a higher temperature must be used to minimize the effect of

CMOS Compatible Wet Bulk Micromachining for MEMS Applications

In many applications of silicon microsensor/actuator structures, like accelerometer and bossed diaphragm pressure sensors, it is desired to have truncated pyramid or mesa type of structures to be realized and/or integrated with thin membrane or beams. During the wet chemical anisotropic etching, it is observed that the convex corners of mesa type structures deteriorates very fast and results in undesired shape/structure. This deterioration is mainly due to the fact that different crystal planes intercept at the convex corner and some of the planes are fast etching planes [10, 16]. The fast etching planes dominate over other crystal planes and hence results in the deterioration of the convex corner shape. This phenomenon is referred to as convex corner undercutting. In sensors structures, like accelerometer, it is needed to preserve the shape of mesas/truncated pyramid. These convex corners can be preserved by adding extra structures (known as convex corner compensation structures) at these convex corners, which are removed or etched out during

Some of the most common corner compensation structures are <110> square,

<100> bar type (thin and wide bar structures) and triangle shape structures [17–23]. These compensation structures are shown in Figure 7, along with design parameters. Square compensation structures have edges aligned to <110> wafer flat direction is attached to one of the convex corners. Two kinds of bar structures (thin bar and wide bar or bar1 and bar2), aligned in <100> direction, are shown on

underetching on diaphragm dimensions.

DOI: http://dx.doi.org/10.5772/intechopen.88487

the process of etching.

Figure 7.

89

Corner compensation structures.

4. Corner compensation structures for TMAH

Figure 5. Plot of etch rate in (100) plane vs. solution temperature.

Figure 6. Plot of underetch per unit depth vs. solution temperature.

Underetch per unit depth ¼ 0:31398–0:00286 � T (6)

where T is the solution temperature.

This expression can be used to estimate the dimensions of the masks to be used for obtaining patterns using TMAH etching during fabrication of the pressure sensor. This study provides an important guideline towards making silicon diaphragms with precise dimensional control. Also, the study indicates that in the case of mask sets which have been fabricated without considering the effect of underetching, a higher temperature must be used to minimize the effect of underetching on diaphragm dimensions.

## 4. Corner compensation structures for TMAH

In many applications of silicon microsensor/actuator structures, like accelerometer and bossed diaphragm pressure sensors, it is desired to have truncated pyramid or mesa type of structures to be realized and/or integrated with thin membrane or beams. During the wet chemical anisotropic etching, it is observed that the convex corners of mesa type structures deteriorates very fast and results in undesired shape/structure. This deterioration is mainly due to the fact that different crystal planes intercept at the convex corner and some of the planes are fast etching planes [10, 16]. The fast etching planes dominate over other crystal planes and hence results in the deterioration of the convex corner shape. This phenomenon is referred to as convex corner undercutting. In sensors structures, like accelerometer, it is needed to preserve the shape of mesas/truncated pyramid. These convex corners can be preserved by adding extra structures (known as convex corner compensation structures) at these convex corners, which are removed or etched out during the process of etching.

Some of the most common corner compensation structures are <110> square, <100> bar type (thin and wide bar structures) and triangle shape structures [17–23]. These compensation structures are shown in Figure 7, along with design parameters. Square compensation structures have edges aligned to <110> wafer flat direction is attached to one of the convex corners. Two kinds of bar structures (thin bar and wide bar or bar1 and bar2), aligned in <100> direction, are shown on

Figure 7. Corner compensation structures.

Underetch per unit depth ¼ 0:31398–0:00286 � T (6)

This expression can be used to estimate the dimensions of the masks to be used

for obtaining patterns using TMAH etching during fabrication of the pressure sensor. This study provides an important guideline towards making silicon diaphragms with precise dimensional control. Also, the study indicates that in the case

where T is the solution temperature.

Plot of underetch per unit depth vs. solution temperature.

Plot of etch rate in (100) plane vs. solution temperature.

Figure 5.

Micromachining

Figure 6.

88

two other corners. The convex corner, while the <310> triangle corner compensation structure is shown at the fourth corner. Design parameters and equations of the structures are discussed further in the chapter. In this chapter, we have focused only on <100> bar type structures because these are the ones which can give perfect convex corners with less space. The other structures are reported elsewhere [15].

From the experiments the etching parameters were extracted and summarized

<100> thin bar (structure 1) is longer and has less width. This structure has etching in two sides in <100> direction and from front side in <310> direction. In <100> direction the etching is similar to the (100) plane and in front side it is faster due to fast etch plane {311}. In this compensation structure, the depth is controlled by the width of the bar (twice of the etch depth) and the convex corner shape is preserved by front of the beam. The length should be more than the width. Thus, the convex shape is preserved completely. The convex corner of the final structure

<100> wide bar (structure 2) is shorter in length and wider in width. This structure has also etching in two sides in <100> direction and from front side in <310> direction. In <100> direction the etching is similar to the (100) plane and in front side it is faster due to fast etch plane {311}. In this compensation structure, the depth is controlled by the length of the bar and the convex corner shape is preserved by {310} etch front of the beam. By increasing the width, requirement of the length is reduced. Thus, the convex shape is preserved completely. The convex corner of the final structure obtained by bar2 compensation structure is shown in

Parameters Value (100) plane etch rate: ER (100) 0.67 μm/min (111) plane etch rate: ER (111) 0.048 μm/min (311) plane etch rate: ER (311) 1.494 μm/min Anisotropic ratio: ER (111)/ER (100) 0.071 Anisotropic ratio: ER (311)/ER (100) 2.23

Minimum side separation: Ws 10 μm (for all the structures)

<100> thin bar width: Wb1 860 μm <100> thin bar length: Lb1 1918 μm <100> thin bar minimum window opening: WOb1 1366 μm Alignment angle with <110> primary flat 45°

<100> thin bar width: Wb2 1802 μm <100> thin bar length: Lb2 1499 μm <100> thin bar minimum window opening: WOb2 1070 μm Alignment angle with <110> primary flat 45°

in Table 3, along with compensating structure dimensions.

CMOS Compatible Wet Bulk Micromachining for MEMS Applications

obtained by bar1 compensation structure is shown in Figure 8.

5.2 <100> wide bar structure etching mechanism

<100> thin bar structure design dimensions:

<100> wide bar structure design dimensions:

Etch rate parameters and compensating structure dimensions [23].

Figure 9.

Table 3.

91

5.1 <100> thin bar structure etching mechanism

DOI: http://dx.doi.org/10.5772/intechopen.88487

## 5. Experimental details

Etch profile or undercutting of the convex corners depends on the crystallographic orientation of the wafer, alignment of structure pattern with the crystallographic direction, type of dopant and its concentration, type of etchant/chemical, chemical concentration, and temperature. We have performed experiments with silicon (100) substrate using 25 wt.% TMAH water solution at 90 1°C temperature in a constant temperature bath. The details of experiment parameters are given in Table 2. Thermally grown silicon dioxide has compressive residual stress of 100 MPa, which gives better adhesion and reduces the shape deterioration near the mask [24]. After carrying out thermal oxidation 1 μm, the mask structures, with different compensating structures as shown in Figure 7, were transferred to the silicon substrate with its primary flat aligned with <110> and experiments were performed.

To understand the etch profile and morphology, the samples were removed from the bath, cleaned and examined periodically. Depth was measured using a digital microscope with an accuracy of 2 μm. It was observed that the profile is consistent in repeated experiments and has a front etch angle of 25°, which corresponds to the {311} planes and [310] directions. A square mask aligned to <110> direction without any compensation structure was used to measure the front etch attack angle. The measured data agrees with the reported literature [25, 26]. In KOH {411} planes are responsible for convex corner undercutting [17], while in TMAH {311} planes are responsible [25, 26].


#### Table 2.

Silicon substrate specifications and etching parameters [23].

From the experiments the etching parameters were extracted and summarized in Table 3, along with compensating structure dimensions.

## 5.1 <100> thin bar structure etching mechanism

two other corners. The convex corner, while the <310> triangle corner compensation structure is shown at the fourth corner. Design parameters and equations of the structures are discussed further in the chapter. In this chapter, we have focused only on <100> bar type structures because these are the ones which can give perfect convex corners with less space. The other structures are reported

Etch profile or undercutting of the convex corners depends on the crystallographic orientation of the wafer, alignment of structure pattern with the crystallographic direction, type of dopant and its concentration, type of etchant/chemical, chemical concentration, and temperature. We have performed experiments with silicon (100) substrate using 25 wt.% TMAH water solution at 90 1°C temperature in a constant temperature bath. The details of experiment parameters are given in Table 2. Thermally grown silicon dioxide has compressive residual stress of 100 MPa, which gives better adhesion and reduces the shape deterioration near the mask [24]. After carrying out thermal oxidation 1 μm, the mask structures, with different compensating structures as shown in Figure 7, were transferred to the silicon substrate with its primary flat aligned with <110> and experiments were

To understand the etch profile and morphology, the samples were removed from the bath, cleaned and examined periodically. Depth was measured using a digital microscope with an accuracy of 2 μm. It was observed that the profile is consistent in repeated experiments and has a front etch angle of 25°, which corresponds to the {311} planes and [310] directions. A square mask aligned to <110> direction without any compensation structure was used to measure the front etch attack angle. The measured data agrees with the reported literature [25, 26]. In KOH {411} planes are responsible for convex corner undercutting [17], while in

Masking layer and thickness Silicon dioxide, 1 μm (thermally grown)

Etchant 25 wt.% TMAHW Temperature 90 1°C Stirring No Etch front angle 24–25° Fast etch plane {311} Fast etch direction <310>

Parameters Value Substrate and size Silicon, 4-in. Orientation (100) Type and dopant n-type, phosphorus Resistivity 8–10 Ω-cm Structure alignment <110> direction Substrate thickness 525 μm

elsewhere [15].

Micromachining

performed.

Table 2.

90

5. Experimental details

TMAH {311} planes are responsible [25, 26].

Silicon substrate specifications and etching parameters [23].

<100> thin bar (structure 1) is longer and has less width. This structure has etching in two sides in <100> direction and from front side in <310> direction. In <100> direction the etching is similar to the (100) plane and in front side it is faster due to fast etch plane {311}. In this compensation structure, the depth is controlled by the width of the bar (twice of the etch depth) and the convex corner shape is preserved by front of the beam. The length should be more than the width. Thus, the convex shape is preserved completely. The convex corner of the final structure obtained by bar1 compensation structure is shown in Figure 8.

## 5.2 <100> wide bar structure etching mechanism

<100> wide bar (structure 2) is shorter in length and wider in width. This structure has also etching in two sides in <100> direction and from front side in <310> direction. In <100> direction the etching is similar to the (100) plane and in front side it is faster due to fast etch plane {311}. In this compensation structure, the depth is controlled by the length of the bar and the convex corner shape is preserved by {310} etch front of the beam. By increasing the width, requirement of the length is reduced. Thus, the convex shape is preserved completely. The convex corner of the final structure obtained by bar2 compensation structure is shown in Figure 9.


### Table 3.

Etch rate parameters and compensating structure dimensions [23].

experimental results, equations to design these structures are deduced and reported in Mukhiya et al. [23]. For bar1 structure, design is given by Eqs. (7) and (8), as

WOb<sup>1</sup> ¼ Ws þ 1:414

Wb<sup>2</sup> ≤ 2:07

CMOS Compatible Wet Bulk Micromachining for MEMS Applications

DOI: http://dx.doi.org/10.5772/intechopen.88487

efficient as compared to bar1 for the same etch depth.

For bar2 structure, design is given by Eqs. (9) and (10), as follows [23]:

WOb<sup>2</sup> <sup>≥</sup>Ws <sup>þ</sup> <sup>0</sup>:<sup>351</sup> <sup>þ</sup> <sup>0</sup>:<sup>734</sup> <sup>R</sup>ð Þ <sup>311</sup>

the chapter. In order to obtain perfect convex corners for the bar structures, Eqs. (7)–(10) provide the biggest and smallest dimensions of width of beam and

Rð Þ 311

where De is the etch depth. The remaining symbols have been defined earlier in

From etch profile of both the <100> bar compensation structures, it is observed that they self-align with the [310] direction, which is the fastest etch plane direction. Compensating structure designed with [310] triangle shape will have consistent etch profile but it will require more space. Bar2 structure is �37% more space

With the advancement of technology, it is preferred to have CMOS-MEMS integration. This imposes the challenge of development of CMOS compatible processes and micromachining techniques. Among various micromachining techniques, wet bulk micromachining is a preferred technique because it is easy, costeffective and has well defined characteristics. In spite of extensive research in this field, wet bulk micromachining using TMAH is still an interesting area of research among the researchers and academicians. Post process CMOS compatible wet bulk micromachining is possible with TMAH. By adding some additives, it is also possible to protect Aluminum in TMAH-based etchants. In the presented work, etch rates for different crystal planes have been measured at various temperatures. It is observed that for 25 wt.% TMAH underetching of {111} planes increase with increase in temperature. However, the anisotropy ratio (111/100) decreases with temperature. Etch rate of both the crystal planes, (100) and (111), increases with temperature, as bond becomes weak at higher temperature. Etch rate of (100) plane is faster than the etch rate of (111) because (100) is a low atomic density plane. Empirical design equations have been derived for the etch rates as a function of temperature. As a use case of this anisotropic etching, diaphragm of pressure sensor has been fabricated at the authors' laboratory and its analysis in context of TMAH

It is observed that during wet anisotropic etching using 25 wt.% TMAH, the convex corners etches very fast and deteriorates in shape. This deterioration is known as undercutting, and it is mainly due to the fact that different crystal planes are encountered at the convex corner and some of the crystal planes etch very fast in comparison to other planes. These fast etching planes are responsible for the

Wb<sup>1</sup> ¼ 2De (7)

<sup>R</sup>ð Þ <sup>100</sup> � <sup>0</sup>:<sup>336</sup> De (9)

<sup>R</sup>ð Þ <sup>100</sup> De (10)

De (8)

Rð Þ 311 Rð Þ 100

follows [23]:

window opening, respectively.

7. Conclusions

etching is presented.

93

Figure 8.

Optical photograph of the convex corner realized using the <100> thin bar. [23] Copyright 2006 by IOP Publishing Ltd. (used with permission).

#### Figure 9.

Optical photograph of the convex corner realized using the <100> wide bar. [23] Copyright 2006 by IOP Publishing Ltd. (used with permission).

## 6. Design analysis and discussions

Optical photographs of the realized convex corners using the <100> thin and wide bars are depicted in Figures 8 and 9. As shown in Figure 8, for the dimensions mentioned earlier, a perfect convex corner is obtained for the <100> thin bar with free end at a depth of etching equal to 430 μm. As shown in Figure 9, for the dimensions mentioned earlier, a perfect convex corner is obtained for the <100> wide bar at a depth of etching equal to 485 μm. The thin bar is narrower in terms of width and longer in terms of length than the wide bar structure.

We can infer from Figures 8 and 9 that perfect convex corners are obtained in each of the compensation structures—wide bar and thin bar. Bar2 requires less window opening compared to bar1. On the basis of the examination of the

CMOS Compatible Wet Bulk Micromachining for MEMS Applications DOI: http://dx.doi.org/10.5772/intechopen.88487

experimental results, equations to design these structures are deduced and reported in Mukhiya et al. [23]. For bar1 structure, design is given by Eqs. (7) and (8), as follows [23]:

$$\mathcal{W}\_{b1} = \mathcal{Z}D\_{\epsilon} \tag{7}$$

$$WO\_{b1} = W\_s + 1.414 \frac{R(311)}{R(100)} D\_\epsilon \tag{8}$$

For bar2 structure, design is given by Eqs. (9) and (10), as follows [23]:

$$W\_{b2} \le \left[ 2.07 \, \frac{R(311)}{R(100)} - 0.336 \right] D\_e \tag{9}$$

$$\mathcal{W}\mathcal{O}\_{b2} \ge \mathcal{W}\_{\varepsilon} + \left[ \mathbf{0.351} + \mathbf{0.734} \,\frac{R(\mathbf{311})}{R(\mathbf{100})} \right] D\_{\varepsilon} \tag{10}$$

where De is the etch depth. The remaining symbols have been defined earlier in the chapter. In order to obtain perfect convex corners for the bar structures, Eqs. (7)–(10) provide the biggest and smallest dimensions of width of beam and window opening, respectively.

From etch profile of both the <100> bar compensation structures, it is observed that they self-align with the [310] direction, which is the fastest etch plane direction. Compensating structure designed with [310] triangle shape will have consistent etch profile but it will require more space. Bar2 structure is �37% more space efficient as compared to bar1 for the same etch depth.

## 7. Conclusions

With the advancement of technology, it is preferred to have CMOS-MEMS integration. This imposes the challenge of development of CMOS compatible processes and micromachining techniques. Among various micromachining techniques, wet bulk micromachining is a preferred technique because it is easy, costeffective and has well defined characteristics. In spite of extensive research in this field, wet bulk micromachining using TMAH is still an interesting area of research among the researchers and academicians. Post process CMOS compatible wet bulk micromachining is possible with TMAH. By adding some additives, it is also possible to protect Aluminum in TMAH-based etchants. In the presented work, etch rates for different crystal planes have been measured at various temperatures. It is observed that for 25 wt.% TMAH underetching of {111} planes increase with increase in temperature. However, the anisotropy ratio (111/100) decreases with temperature. Etch rate of both the crystal planes, (100) and (111), increases with temperature, as bond becomes weak at higher temperature. Etch rate of (100) plane is faster than the etch rate of (111) because (100) is a low atomic density plane. Empirical design equations have been derived for the etch rates as a function of temperature. As a use case of this anisotropic etching, diaphragm of pressure sensor has been fabricated at the authors' laboratory and its analysis in context of TMAH etching is presented.

It is observed that during wet anisotropic etching using 25 wt.% TMAH, the convex corners etches very fast and deteriorates in shape. This deterioration is known as undercutting, and it is mainly due to the fact that different crystal planes are encountered at the convex corner and some of the crystal planes etch very fast in comparison to other planes. These fast etching planes are responsible for the

6. Design analysis and discussions

Publishing Ltd. (used with permission).

Figure 8.

Micromachining

Figure 9.

92

Publishing Ltd. (used with permission).

Optical photographs of the realized convex corners using the <100> thin and wide bars are depicted in Figures 8 and 9. As shown in Figure 8, for the dimensions mentioned earlier, a perfect convex corner is obtained for the <100> thin bar with free end at a depth of etching equal to 430 μm. As shown in Figure 9, for the dimensions mentioned earlier, a perfect convex corner is obtained for the <100> wide bar at a depth of etching equal to 485 μm. The thin bar is narrower in terms of

Optical photograph of the convex corner realized using the <100> wide bar. [23] Copyright 2006 by IOP

Optical photograph of the convex corner realized using the <100> thin bar. [23] Copyright 2006 by IOP

We can infer from Figures 8 and 9 that perfect convex corners are obtained in each of the compensation structures—wide bar and thin bar. Bar2 requires less window opening compared to bar1. On the basis of the examination of the

width and longer in terms of length than the wide bar structure.

undercutting. For TMAH, the fast etching planes are found at an angle of 25°, which are {311} plane. In case of TMAH, the underetching and undercutting are found faster than its counterpart KOH. In many applications, where these convex corners are required to be protected, some additional structures are added to protect the convex corner from deterioration. These are known as convex corner compensation structures. Most common compensation structure shave been discussed and optimum structure, i.e., <100> bar type of structures (thin bar and wide bar) have been discussed in detail. Both the structures can protect the mesa and can give perfect convex corner. Bar2 structure is more space efficient than bar1 structure. Using this type of compensation structures, accelerometer proof mask has been fabricated at authors' laboratory and presented. Generalized empirical design equations have also been discussed.

References

115-131

[1] Bhat KN. Silicon micromachined pressure sensors. Journal of the

[2] Sharma A, Mukhiya R, Santosh Kumar S, Gopal R, Pant BD. Dynamic characterization of bulk micromachined accelerometer using laser doppler vibrometer (LDV). Microsystem Technologies. 2015;21:2221-2232. DOI:

10.1007/s00542-014-2316-3

2011. pp. 2093-2096

(00)00156-2

[3] Wei J, Sarro PM, Duc TC. A piezoresistive sensor for pressure monitoring at inkjet nozzle. In:

Proceedings of IEEE Sensors 2010; 1-4 November 2010; USA, New York: IEEE;

[4] You JS, Kim DH, Huh JY, Park HJ, Pak JJ, Kang CS. Experiments on

anisotropic etching of Si in TMAH. Solar Energy Materials and Solar Cells. 2001; 66:37-44. DOI: 10.1016/S0927-0248

[5] Bashir R, Hilt JZ, Elibol O, Gupta A, Peppas NA. Micromechanical cantilever as an ultrasensitive pH microsensor. Applied Physics Letters. 2002;81(16): 3091-3093. DOI: 10.1063/1.1514825

[6] Tokoro K, Uchikawa D, Shikida M, Sato K. Anisotropic etching properties of silicon in KOH and TMAH solutions. In: 25-28 Nov. 1998; Japan, New York:

[7] Yan G-Z, Chan PCH, Hsing I-M, Sharma RK, Sin JKO. An improved Sietching solution without attacking exposed aluminum. Sensors and Actuators A. 2001;89:135-141. DOI: 10.1109/MEMSYS.2000.838579

[8] Fujitsuka N, Hamaguchi K, Funabashi H, Kawasaki E, Fukada T. Aluminum protected silicon anisotropic etching technique using TMAH with an

95

IEEE; 2002. pp. 65-70

Indian Institute of Science. 2007;87(1):

DOI: http://dx.doi.org/10.5772/intechopen.88487

CMOS Compatible Wet Bulk Micromachining for MEMS Applications

oxidizing agent and dissolved Si. R&D Review of Toyota CRDL. 2004;39:34-40

[10] Koide, Sato K, Tanaka S. Simulation of two-dimensional etch profile of silicon during orientation-dependent anisotropic etching. In: Proceedings of the IEEE Microelectromechanical Systems (MEMS) Workshop; Nara:

[11] Tabata O, Asahi R, Funabashi H, Shimaoka K, Sugiyama S. Anisotropic etching of silicon in TMAH solutions. Sensors and Actuators A. 1992;34:51-57. DOI: 10.1016/0924-4247(92)80139-T

[12] Pal P, Sato K, Gosalvez MA, Shikida M. Study of rounded concave and sharp edge convex corners undercutting in CMOS compatible anisotropic etchants. Journal of

0960-1317/17/11/017

80511-0

Micromechanics and Microengineering. 2007;17:2299-2307. DOI: 10.1088/

[13] Thong JTL, Choi WK, Chong CW. TMAH etching of silicon and the interaction of etching parameters. Sensors and Actuators A. 1997;63: 243-249. DOI: 10.1016/S0924-4247(97)

[14] Santosh Kumar S, Pant BD. Design principles and considerations for the 'ideal'silicon piezoresistive pressure sensor: A focused review. Microsystem Technologies. 2014;20:1213-1247. DOI:

Bhattacharyya TK, Lorenzelli L, Zen M. Experimental study and analysis of

10.1007/s00542-014-2215-7

[15] Mukhiya R, Bagolini A,

[9] Seidel H. The mechanism of anisotropic silicon etching and its relevance for micromachining. In: Proc. Transducers '87, Rec. 4th Int. Conf. Solid-State Sensors and Actuators; 2-5 June 1987; Tokyo: Japan; pp. 2093-2096

Japan; 1991. pp. 216-220

## Acknowledgements

Authors would like to acknowledge the generous support of the Director, CSIR-CEERI, Pilani. The authors would also like to thank all the scientific and technical members of Smart Sensors Area at CSIR-CEERI, Pilani. The financial support by CSIR, New Delhi to carry out the research work through various projects is gratefully acknowledged. R Mukhiya acknowledges FBK, Trento, Italy, for the experimental study on corner compensation structures, and DST for the financial support through the ITPAR project.

## Author details

S. Santosh Kumar\* and Ravindra Mukhiya CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan, India

\*Address all correspondence to: santoshkumar.ceeri@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CMOS Compatible Wet Bulk Micromachining for MEMS Applications DOI: http://dx.doi.org/10.5772/intechopen.88487

## References

undercutting. For TMAH, the fast etching planes are found at an angle of 25°, which are {311} plane. In case of TMAH, the underetching and undercutting are found faster than its counterpart KOH. In many applications, where these convex corners are required to be protected, some additional structures are added to protect the convex corner from deterioration. These are known as convex corner compensation structures. Most common compensation structure shave been discussed and optimum structure, i.e., <100> bar type of structures (thin bar and wide bar) have been discussed in detail. Both the structures can protect the mesa and can give perfect convex corner. Bar2 structure is more space efficient than bar1 structure. Using this type of compensation structures, accelerometer proof mask has been fabricated at authors' laboratory and presented. Generalized empirical design equa-

Authors would like to acknowledge the generous support of the Director, CSIR-CEERI, Pilani. The authors would also like to thank all the scientific and technical members of Smart Sensors Area at CSIR-CEERI, Pilani. The financial support by CSIR, New Delhi to carry out the research work through various projects is gratefully acknowledged. R Mukhiya acknowledges FBK, Trento, Italy, for the experimental study on corner compensation structures, and DST for the financial support

CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan, India

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: santoshkumar.ceeri@gmail.com

tions have also been discussed.

Acknowledgements

Micromachining

through the ITPAR project.

Author details

94

S. Santosh Kumar\* and Ravindra Mukhiya

provided the original work is properly cited.

[1] Bhat KN. Silicon micromachined pressure sensors. Journal of the Indian Institute of Science. 2007;87(1): 115-131

[2] Sharma A, Mukhiya R, Santosh Kumar S, Gopal R, Pant BD. Dynamic characterization of bulk micromachined accelerometer using laser doppler vibrometer (LDV). Microsystem Technologies. 2015;21:2221-2232. DOI: 10.1007/s00542-014-2316-3

[3] Wei J, Sarro PM, Duc TC. A piezoresistive sensor for pressure monitoring at inkjet nozzle. In: Proceedings of IEEE Sensors 2010; 1-4 November 2010; USA, New York: IEEE; 2011. pp. 2093-2096

[4] You JS, Kim DH, Huh JY, Park HJ, Pak JJ, Kang CS. Experiments on anisotropic etching of Si in TMAH. Solar Energy Materials and Solar Cells. 2001; 66:37-44. DOI: 10.1016/S0927-0248 (00)00156-2

[5] Bashir R, Hilt JZ, Elibol O, Gupta A, Peppas NA. Micromechanical cantilever as an ultrasensitive pH microsensor. Applied Physics Letters. 2002;81(16): 3091-3093. DOI: 10.1063/1.1514825

[6] Tokoro K, Uchikawa D, Shikida M, Sato K. Anisotropic etching properties of silicon in KOH and TMAH solutions. In: 25-28 Nov. 1998; Japan, New York: IEEE; 2002. pp. 65-70

[7] Yan G-Z, Chan PCH, Hsing I-M, Sharma RK, Sin JKO. An improved Sietching solution without attacking exposed aluminum. Sensors and Actuators A. 2001;89:135-141. DOI: 10.1109/MEMSYS.2000.838579

[8] Fujitsuka N, Hamaguchi K, Funabashi H, Kawasaki E, Fukada T. Aluminum protected silicon anisotropic etching technique using TMAH with an

oxidizing agent and dissolved Si. R&D Review of Toyota CRDL. 2004;39:34-40

[9] Seidel H. The mechanism of anisotropic silicon etching and its relevance for micromachining. In: Proc. Transducers '87, Rec. 4th Int. Conf. Solid-State Sensors and Actuators; 2-5 June 1987; Tokyo: Japan; pp. 2093-2096

[10] Koide, Sato K, Tanaka S. Simulation of two-dimensional etch profile of silicon during orientation-dependent anisotropic etching. In: Proceedings of the IEEE Microelectromechanical Systems (MEMS) Workshop; Nara: Japan; 1991. pp. 216-220

[11] Tabata O, Asahi R, Funabashi H, Shimaoka K, Sugiyama S. Anisotropic etching of silicon in TMAH solutions. Sensors and Actuators A. 1992;34:51-57. DOI: 10.1016/0924-4247(92)80139-T

[12] Pal P, Sato K, Gosalvez MA, Shikida M. Study of rounded concave and sharp edge convex corners undercutting in CMOS compatible anisotropic etchants. Journal of Micromechanics and Microengineering. 2007;17:2299-2307. DOI: 10.1088/ 0960-1317/17/11/017

[13] Thong JTL, Choi WK, Chong CW. TMAH etching of silicon and the interaction of etching parameters. Sensors and Actuators A. 1997;63: 243-249. DOI: 10.1016/S0924-4247(97) 80511-0

[14] Santosh Kumar S, Pant BD. Design principles and considerations for the 'ideal'silicon piezoresistive pressure sensor: A focused review. Microsystem Technologies. 2014;20:1213-1247. DOI: 10.1007/s00542-014-2215-7

[15] Mukhiya R, Bagolini A, Bhattacharyya TK, Lorenzelli L, Zen M. Experimental study and analysis of

corner compensation structures for CMOS compatible bulk micromachining using 25 wt.% TMAH. Microelectronics Journal. 2011;42:127-134. DOI: 10.1016/ j.mejo.2010.08.018

[16] Offereins HL, Kühl K, Sandmaier H. Methods for fabrication of convex corners in anisotropic etching of (100) silicon in aqueous KOH. Sensors and Actuators A. 1990;25:9-13. DOI: 10.1016/0924-4247(90)87002-Z

[17] Mayer GK, Offereings HL, Sandmaier H, Kühl K. Fabrication of non-underetched convex corner in anisotropic etching of (100) silicon in aqueous KOH with respect to novel micromechanical elements. Journal of the Electrochemical Society. 1990;137: 3947-3951. DOI: 10.1149/1.2086334

[18] Enoksson P. New structure for corner compensation in anisotropic KOH etching. Journal of Micromechanics and Microengineering. 1997;7:141-144. DOI: 10.1088/0960-1317/7/3/016

[19] Zhang Q, Liu L, Li Z. A new approach to convex corner compensation for anisotropic etching of (100) Si in KOH. Sensors and Actuators A. 1996;56:251-254. DOI: 10.1016/S0924-4247(96)01312-X

[20] Puers B, Sansen W. Compensation structures for convex corner micromachining in silicon. Sensors and Actuators A. 1990;23:1036-1041. DOI: 10.1016/0924-4247(90)87085-W

[21] Bao M, Burrer C, Esteve J, Bausells J, Marco S. Etching front control of <110> strips for corner compensation. Sensors and Actuators A. 1993;37-38:727-732. DOI: 10.1016/0924-4247(93)80123-X

[22] Wu X-P, Ko WH. Compensating corner undercutting in anisotropic etching of (100) silicon. Sensors and Actuators A. 1989;18:207-215. DOI: 10.1016/0250-6874(89)87019-2

[23] Mukhiya R, Bagolini A, Margesin B, Zen M, Kal S. <100> bar corner compensation for CMOS compatible anisotropic TMAH etching. Journal of Micromechanics and Microengineering. 2006;16:2458-2462. DOI: 10.1088/ 0960-1317/16/11/029

[24] Takao H, Yong C-C, Rajanna K, Ishida M. Shape deterioration of mesa structures in post-CMOS anisotropic etching of silicon microstructures: An experimental study. Sensors and Actuators A. 2000;86:115-121. DOI: 10.1016/S0924-4247(00)00437-4

[25] Tellier CR, Charbonnieras AR. Characterization of the anisotropic chemical attack of (h h l) silicon plates in a TMAH 25 wt.% solution: Micromachining and adequacy of the dissolution slowness surface. Sensors and Actuators A. 2003;105:62-75. DOI: 10.1016/S0924-4247(03)00064-5

[26] Trieu HK, Mokwa W. A generalized model describing corner undercutting by the experimental analysis of TMAH/ IPA. Journal of Micromechanics and Microengineering. 1998;8:80-83. DOI: 10.1088/0960-1317/8/2/009

**97**

**Chapter 5**

**Abstract**

Assistance

Physical Processes and Plasma

Thin-Film Production with Ion

*Elena Kralkina, Andrey Alexandrov, Polina Nekludova,* 

*Aleksandr Nikonov, Vladimir Pavlov, Konstantin Vavilin,* 

The results of the study of the plasma reactor on the combined magnetron discharge and radio-frequency (RF) inductive discharge located in the external magnetic field are presented. Magnetron discharge provides the generation of atoms and ions of the target materials, while the flow of accelerated ions used for the ion assistance is provided by the RF inductive discharge located in an external magnetic field. Approaching the region of resonant absorption of RF power by optimizing the magnitude and configuration of the external magnetic field makes it possible to obtain a uniform within 10% radial distribution of the ion current across the diameter of 150 mm. When the RF power supply power is 1000 W, the ion cur-

rent density on the substrate can be adjusted in the range of 0.1–3 mA/cm2

of ion assisting results in a fundamental change in the structure and properties of

**Keywords:** RF inductive discharge, assisting ions, helicon, Trivelpiece-Gould wave,

At the present time, vacuum plasma methods for the formation of multicomponent thin-film structures based on magnetron or vacuum arc gas discharges are widely used in the industry. These methods allow to obtain a wide class of functional coatings, such as optical, hardening, anticorrosion, antibacterial, etc. A particular case of the known vacuum plasma methods of coating formation is ionassisted deposition [1–8]. The method involves continuous or periodic bombard-

The results of the bombardment of the substrate and the growing films by

. The use

Hybrid Plasma System for

*Vadim Odinokov and Vadim Sologub*

functional coatings, deposited using a magnetron.

magnetic field, magnetron, film deposition

ment of growing thin films by accelerated ions.

**1. Introduction**

accelerated ions are [1–8]:

Parameters in a Radio-Frequency

## **Chapter 5**

corner compensation structures for CMOS compatible bulk micromachining using 25 wt.% TMAH. Microelectronics Journal. 2011;42:127-134. DOI: 10.1016/

[23] Mukhiya R, Bagolini A, Margesin B, Zen M, Kal S. <100> bar corner compensation for CMOS compatible anisotropic TMAH etching. Journal of Micromechanics and Microengineering. 2006;16:2458-2462. DOI: 10.1088/

[24] Takao H, Yong C-C, Rajanna K, Ishida M. Shape deterioration of mesa structures in post-CMOS anisotropic etching of silicon microstructures: An experimental study. Sensors and Actuators A. 2000;86:115-121. DOI: 10.1016/S0924-4247(00)00437-4

[25] Tellier CR, Charbonnieras AR. Characterization of the anisotropic chemical attack of (h h l) silicon plates

Micromachining and adequacy of the dissolution slowness surface. Sensors and Actuators A. 2003;105:62-75. DOI: 10.1016/S0924-4247(03)00064-5

[26] Trieu HK, Mokwa W. A generalized model describing corner undercutting by the experimental analysis of TMAH/ IPA. Journal of Micromechanics and Microengineering. 1998;8:80-83. DOI:

in a TMAH 25 wt.% solution:

10.1088/0960-1317/8/2/009

0960-1317/16/11/029

[16] Offereins HL, Kühl K, Sandmaier H. Methods for fabrication of convex corners in anisotropic etching of (100) silicon in aqueous KOH. Sensors and Actuators A. 1990;25:9-13. DOI: 10.1016/0924-4247(90)87002-Z

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