**1. Introduction**

Scanning probe microscopy (SPM) over the former 20 years has been intensively used by many research groups due to its applicability in extensive filed of materials. In past 50 years, the most significant advancement in AFM was ranked at second place [1–8]. This is because the application of AFM not limited to the semiconductor field but also covers many wide range fields like chemical group identification [9], cell biological [10], semiconductor to study the properties of the materials at nanoscale [11]. Although, applications and techniques of SPM are diverse in nature, but they do share a common feature, i.e., probe to sense localized or confined signal, because in many cases the probe that confines the spatial accuracy.

AFM [12] is the most common form of SPM in which the probe is usually in the form of sharp, rigid and having long-life time mounted on the cantilever or tuning fork to transduce the tip-sample force. However, depending upon the application of AFM probe, it might have different shapes, length but the constant geometry over a long-time is common thing in all types of probes. As the probe scanning over the sample, the topography of the surface is constructed depending upon the mode of operation of AFM. The ensued image depicts the geometry of tip corresponding to the surface geometry.

etched tungsten probe and modification using FIB treatment. In the complete process several steps are involved as shown in **Figure 1**. The tuning fork sensor is characterized by the length of carbon nanotube attached, radius of the ball capped on the top of MWCNT and orientation (angle with respect to the surface). All these

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High…*

parameters define the resolution, severity and image appearances of the tip.

A 85% potassium hydroxide (KOH) was purchased from Sigma Aldrich® Company. A 2 molar concentration of KOH aqueous solution was prepared by dissolving KOH pallets in deionized water. After that, solution was agitated thoroughly to make it homogenous and left for 1 h. A gold wire with 200 μm diameter and 10 mm length was used as cathode. Tungsten (W) wire with diameter 50 and 25 mm length, with 99.95% purity, was purchased from Alfa Aesar® and used as anode. To improve the crystallinity, rapid thermal annealing (RTA) was done at 800oC for 40 s. DC voltages (typically 3–5 volts) are applied between gold and Tungsten

*Fabrication of tuning fork sensor followed by ball capped nanotube (a) Tungsten wire was annealed at* 800oC *for 40 s to improve the crystallinity. (b) Crystallinity of tungsten wire has been improved as the atoms are arranged at the lattice points. (c) Electrochemical etching using KOH produce very sharp tip of the order 65 nm. This diameter is ideal to attach all three types of CNTs (110–170 nm, 50–80 nm, 20 nm). (d) Small droplet of*

*functionalized MWCNTTs is picked using micropipette and poured on gold ring electrode. Using Micromanipulator tip brings closer to droplet and meniscus is formed which covers the edge part of tungsten probe. AC voltages are applied across the electrodes (gold electrode and tungsten probe). SEM image of CNT attached at the apex of tungsten probe. Scale bar is 500 nm. (e) Tungsten probe followed by MWCNT is fixed on substrate using carbon tape and cutting, welding and capping was done using FIB. Carbon is deposited at the junction of MWCNT and tungsten so that it is firmly attached (welded) with tungsten. For stability purpose, excess length of CNT is cut-off. Gallium ions are used to cut the excess length of CNT. (f) Carbon atoms are deposited on the top of CNT to form a ball-capped shape to make it suitable for scanning of via holes. (g)*

*Attachment of FIB treated tungsten probe with the quartz tuning fork.*

**2.1 Fabrication of tungsten tips**

*DOI: http://dx.doi.org/10.5772/intechopen.92739*

**Figure 1.**

**259**

Noteworthy improvement and advancement in the instrumentation, correction of artifacts and scanning algorithms has been accomplished. In addition to this, different type of probes has been prepared to obtain the deep trench (DT) [13] and sidewall roughness (SWR) [14], tracing vertical sidewalls [15], overhang and undercut features [16], etc. In addition to this, many useful techniques such as reflectometry [17] to measure the depth of via holes, X-Ray methods [18] and atomic force microscopy (AFM). Each method has its own pros and cons. For examples, conventional AFM is unable to image the features having high aspect ratio such a silicon pillars or deep via holes. In addition to this, reflectometry is also a useful and non-destructive technique. However, its resolution is limited to optical microscopy. Moreover, in some of the measurement instruments prior knowledge of sample surface is required.

In silicon industry, many improvements in the instrumentation has been done to measure the dimensions up to several hundred nanometer scale [19], lithography, manipulation of individual etc. In nanotechnology, AFM always is the essential contrivance especially when it comes to the height measurement of the feature. Because, measuring the deep, narrow via holes always been difficult and mainly unsolved problem. Measuring the depth of the hole is not enough and situation become worse and complicated when the sidewalls of the hole needs to be scanned. Many other groups also tried to increase the functionality and versatility of AFM either by combining the optical instrument [20] with atomic force microscopy but that is just limited to scan the trench to smaller dimension or limited to visible spectrum. In addition to this, flair shape probe [21], trident probe [22], bi-pod, capped probe [23], Osborne [22], hammerhead [24] and many other shaped probes are designed to address problem related to scanning. However, there is not onesize-fit-all answer. Seo et al. [25] has proposed general algorithm for scanning the deep via holes. However, that general algorithm has not been implemented on samples. One of the major problem associated with the algorithm was it does not stored the motion of probe when it find the sidewall wall in either direction which is necessary to keep following the boundary of the sample. Also, in case of no wall around, the tip moved one step further along the previous direction of motion. In this case, at some point, tip will lost the boundary of sample.

In this work we methodically explorer the scanning algorithm to scan the different types of features present above (protrusion) or below (holes) the surface and removed the problem associated with [24]. We have used the single MWCNT as a scanning probe attached by dielectrophoresis and FIB treatment. In addition to this, first time we have successfully implemented the non-destructive technique with high aspect ratio on real samples to obtain the three-dimension topography, depth measurement of holes, position of via holes by force-distance mapping for not only holes as well as protrusion.

#### **2. Experimental method**

There are mainly two aspects of fabrication of quartz tuning fork sensor followed by ball capped nanotube: nanotube attachment on electrochemically *Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High… DOI: http://dx.doi.org/10.5772/intechopen.92739*

etched tungsten probe and modification using FIB treatment. In the complete process several steps are involved as shown in **Figure 1**. The tuning fork sensor is characterized by the length of carbon nanotube attached, radius of the ball capped on the top of MWCNT and orientation (angle with respect to the surface). All these parameters define the resolution, severity and image appearances of the tip.

## **2.1 Fabrication of tungsten tips**

AFM [12] is the most common form of SPM in which the probe is usually in the form of sharp, rigid and having long-life time mounted on the cantilever or tuning fork to transduce the tip-sample force. However, depending upon the application of AFM probe, it might have different shapes, length but the constant geometry over a long-time is common thing in all types of probes. As the probe scanning over the sample, the topography of the surface is constructed depending upon the mode of operation of AFM. The ensued image depicts the geometry of tip corresponding to

Noteworthy improvement and advancement in the instrumentation, correction of artifacts and scanning algorithms has been accomplished. In addition to this, different type of probes has been prepared to obtain the deep trench (DT) [13] and sidewall roughness (SWR) [14], tracing vertical sidewalls [15], overhang and undercut features [16], etc. In addition to this, many useful techniques such as reflectometry [17] to measure the depth of via holes, X-Ray methods [18] and atomic force microscopy (AFM). Each method has its own pros and cons. For examples, conventional AFM is unable to image the features having high aspect ratio such a silicon pillars or deep via holes. In addition to this, reflectometry is also a useful and non-destructive technique. However, its resolution is limited to optical microscopy. Moreover, in some of the

In silicon industry, many improvements in the instrumentation has been done to measure the dimensions up to several hundred nanometer scale [19], lithography, manipulation of individual etc. In nanotechnology, AFM always is the essential contrivance especially when it comes to the height measurement of the feature. Because, measuring the deep, narrow via holes always been difficult and mainly unsolved problem. Measuring the depth of the hole is not enough and situation become worse and complicated when the sidewalls of the hole needs to be scanned. Many other groups also tried to increase the functionality and versatility of AFM either by combining the optical instrument [20] with atomic force microscopy but that is just limited to scan the trench to smaller dimension or limited to visible spectrum. In addition to this, flair shape probe [21], trident probe [22], bi-pod, capped probe [23], Osborne [22], hammerhead [24] and many other shaped probes are designed to address problem related to scanning. However, there is not onesize-fit-all answer. Seo et al. [25] has proposed general algorithm for scanning the deep via holes. However, that general algorithm has not been implemented on samples. One of the major problem associated with the algorithm was it does not stored the motion of probe when it find the sidewall wall in either direction which is necessary to keep following the boundary of the sample. Also, in case of no wall around, the tip moved one step further along the previous direction of motion. In

measurement instruments prior knowledge of sample surface is required.

this case, at some point, tip will lost the boundary of sample.

holes as well as protrusion.

**2. Experimental method**

**258**

In this work we methodically explorer the scanning algorithm to scan the different types of features present above (protrusion) or below (holes) the surface and removed the problem associated with [24]. We have used the single MWCNT as a scanning probe attached by dielectrophoresis and FIB treatment. In addition to this, first time we have successfully implemented the non-destructive technique with high aspect ratio on real samples to obtain the three-dimension topography, depth measurement of holes, position of via holes by force-distance mapping for not only

There are mainly two aspects of fabrication of quartz tuning fork sensor followed by ball capped nanotube: nanotube attachment on electrochemically

the surface geometry.

*21st Century Surface Science - a Handbook*

A 85% potassium hydroxide (KOH) was purchased from Sigma Aldrich® Company. A 2 molar concentration of KOH aqueous solution was prepared by dissolving KOH pallets in deionized water. After that, solution was agitated thoroughly to make it homogenous and left for 1 h. A gold wire with 200 μm diameter and 10 mm length was used as cathode. Tungsten (W) wire with diameter 50 and 25 mm length, with 99.95% purity, was purchased from Alfa Aesar® and used as anode. To improve the crystallinity, rapid thermal annealing (RTA) was done at 800oC for 40 s. DC voltages (typically 3–5 volts) are applied between gold and Tungsten

#### **Figure 1.**

*Fabrication of tuning fork sensor followed by ball capped nanotube (a) Tungsten wire was annealed at* 800oC *for 40 s to improve the crystallinity. (b) Crystallinity of tungsten wire has been improved as the atoms are arranged at the lattice points. (c) Electrochemical etching using KOH produce very sharp tip of the order 65 nm. This diameter is ideal to attach all three types of CNTs (110–170 nm, 50–80 nm, 20 nm). (d) Small droplet of functionalized MWCNTTs is picked using micropipette and poured on gold ring electrode. Using Micromanipulator tip brings closer to droplet and meniscus is formed which covers the edge part of tungsten probe. AC voltages are applied across the electrodes (gold electrode and tungsten probe). SEM image of CNT attached at the apex of tungsten probe. Scale bar is 500 nm. (e) Tungsten probe followed by MWCNT is fixed on substrate using carbon tape and cutting, welding and capping was done using FIB. Carbon is deposited at the junction of MWCNT and tungsten so that it is firmly attached (welded) with tungsten. For stability purpose, excess length of CNT is cut-off. Gallium ions are used to cut the excess length of CNT. (f) Carbon atoms are deposited on the top of CNT to form a ball-capped shape to make it suitable for scanning of via holes. (g) Attachment of FIB treated tungsten probe with the quartz tuning fork.*

electrodes using 2 M KOH aqueous solution. Keithley 32,220 function generator is used to apply DC voltage across the electrodes. By applying the DC voltage, redox reaction occurs at the electrodes and tungsten tip begins to narrow. The shape of the neck depends upon the meniscus formed at the air-liquid interface. The experimental setup for electrochemical etching is shown in **Figure A3**. Reaction takes place can be described by the following equation [26].

At anode

$$6H\_2O + 6e^- \rightarrow 3H\_2(g) + 6OH^-$$

At cathode

$$\text{W}(\text{s}) + \text{8OH}^- \rightarrow \text{WO}\_4^{2-} + 4\text{H}\_2\text{O} + \text{6e}^-$$

Overall, reaction can be summarized as:

$$2\text{ W}(\text{s}) + 2\text{OH}^- + 2\text{H}\_2\text{O} \rightarrow \text{WO}\_4^{2-} + \text{3H}\_2\text{ (g)}\tag{1}$$

investigated by Fourier Transform Infrared spectroscopy (FTIR). **Figure 2** shows the FTIR spectrum of PVA treated MWCNTs in which peaks appears at 3425 and 1390 cm<sup>1</sup> are identified with O-H bond bending and stretching. The frequency associated with these vibrations is due to hydrogen bonding linked with functional groups. The peak observed at 1260 cm<sup>1</sup> is due to C–O bond stretching vibrations in phenol and alcohol. Another peak appeared at 1663 cm<sup>1</sup> is due to C=O stretching of functional group (COOH) considered at the surface of MWCNTs. In addition, C–H stretch vibrations correspond to 2856 and 2924 cm<sup>1</sup> band. Thus, FTIR is a powerful tool to see the chemical changes in the carbon system. MWCNTs are attached to the tungsten tips not only to increase the spatial resolution but also to increase the aspect ratio as well. According the literature survey, many groups have used CNTs as a scanning probe. Some of them used direct grown of CNT on cantilever or by

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High…*

*DOI: http://dx.doi.org/10.5772/intechopen.92739*

picking the single CNT using special system installed within the SEM.

**Figure A2**.

**Figure 2.**

**261**

**2.3 MWCNTs welding and cutting using FIB**

However, these instruments are costly or require special techniques to attach single MWCNT on AFM probe. Here, we have used dielectrophoresis method which is not only the cost-effective process but also a time saving and a simple process to attach the CNT. To attach the single MWCNTs on chemically etched tungsten probe, sharpness of the tip, applied frequency and diameter of the tip are the key parameters. One electrode is the etched tungsten wire while other is gold ring electrode. Using micropipette small droplet of functionalized MWCNTs is dropped into the gold ring electrode and with the help of micro-manipulator, the tip is brought closer to MWCNTs solution droplet and form a meniscus which cover only the edge part of etched tungsten tip. Using this technique, possibility to attach the CNT at the edge is increased. Otherwise, CNTs may attach at the surface of tungsten tip. Alternating voltages (6 volts at 2.2 MHz) were applied to the electrodes causes to generate the electric filed and CNTs were began to align parallel and eventually attached to the sharp edge of tungsten tip. However, at this stage MWCNT cannot be used for scanning purpose because CNT is attached with tungsten probe using Van der Waals and not sufficient to hold it during scanning. The homemade 3D AFM setup along with all electronics components is shown in

We have utilized a focused beam of gallium ions (FIB) to align and attach the CNT on tungsten tip. The FIB system used (QUNTA 3D FEG) is a dual beam type

*Functionalization of MWCNT. (a) FTIR spectra depicts attachment of functional groups in CNTs: Peaks reveal 1200–3500 confirm the presence of functional group in purified CNTs. Inset image clearly shows the peaks at 2856 and 2926. (b) Comparison between the PVA-functionalized and as grown MWCNT.*

The hydrogen gas bubbles produced during the etching should be avoided as it disturbs the meniscus which affects the shape of probe. Due to this, glass slide is placed between the electrodes. During the etching process, variation in the thickness of tungsten wire and current was monitored using the CCD camera and oscilloscope. When the current drops below the set point, shut-off circuit trigger the solid state relay (shut-off time is 0.5 ns), sharp edge of the tungsten wire is formed and hence in this way over-etching does not happen. The home-built automatic cutoff circuit to avoid over-etching is used as shown in **Figure A1**. After the wire has been etched, it was washed with the hot distilled water (D.I) to reduce the oxide layer around the edge and examined under the scanning electron microscope. Tips with the sharp edges are reserved for next step (attachment of MWCNT with tungsten probe) as these are the best candidates used for D.E.P.

#### **2.2 Functionalization and multiwall carbon nanotubes attachment with tungsten tip using dielectrophoresis**

We have used commercially available MWCNTs (Sigma Aldrich). First two MWCNTs samples having 110–170 nm (10 μm in length) and 60 nm (15 μm in length) diameter are grown by chemical vapor deposition (CVD) and 20 nm (30 μm in length) diameter MWCNTs was grown by arc discharge method. The reason why we used the un-purified carbon nanotubes is to reveal that anyone can easily make the carbon nanotubes tips in one's laboratory, because MWCNTs made by arc discharge are very difficult to purify. To obtain a single MWCNT, fictionalization of MWCNTs is good choice. Other than that, it is difficult to attach single MWCNT on tungsten tip. Firstly, to functionalize the MWCNTs (with different diameter as 100–170 nm, 60 nm, 20 nm), 1 g of PVA was completely dissolved in 60 ml of water at room temperature. After that, 0.2 mg of MWCNTs was added in to the above-mentioned solution and mixture was stirred at 70oC for four 12 h. Secondly, centrifugation machine was used to remove the impurities or particles from the CNT-PVA solution. Centrifugation at 4000 RPM for 30 min was done several times unless the solution becomes clear and no CNT left in solution. Thirdly, to remove excess PVA from PVA-CNT solution, filter paper Anodisc 47 (Whatman) with the pore size 200 nm and diameter 47 mm is used. Filtered CNT-PVA are then collected and dried under vacuum.

Finally, CNT-PVA solution is sonicated in an ultrasonic bath for 6 h to uniformly disperse in solution. After attachment of carboxylic group, MWCNTs were

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High… DOI: http://dx.doi.org/10.5772/intechopen.92739*

investigated by Fourier Transform Infrared spectroscopy (FTIR). **Figure 2** shows the FTIR spectrum of PVA treated MWCNTs in which peaks appears at 3425 and 1390 cm<sup>1</sup> are identified with O-H bond bending and stretching. The frequency associated with these vibrations is due to hydrogen bonding linked with functional groups. The peak observed at 1260 cm<sup>1</sup> is due to C–O bond stretching vibrations in phenol and alcohol. Another peak appeared at 1663 cm<sup>1</sup> is due to C=O stretching of functional group (COOH) considered at the surface of MWCNTs. In addition, C–H stretch vibrations correspond to 2856 and 2924 cm<sup>1</sup> band. Thus, FTIR is a powerful tool to see the chemical changes in the carbon system. MWCNTs are attached to the tungsten tips not only to increase the spatial resolution but also to increase the aspect ratio as well. According the literature survey, many groups have used CNTs as a scanning probe. Some of them used direct grown of CNT on cantilever or by picking the single CNT using special system installed within the SEM.

However, these instruments are costly or require special techniques to attach single MWCNT on AFM probe. Here, we have used dielectrophoresis method which is not only the cost-effective process but also a time saving and a simple process to attach the CNT. To attach the single MWCNTs on chemically etched tungsten probe, sharpness of the tip, applied frequency and diameter of the tip are the key parameters. One electrode is the etched tungsten wire while other is gold ring electrode. Using micropipette small droplet of functionalized MWCNTs is dropped into the gold ring electrode and with the help of micro-manipulator, the tip is brought closer to MWCNTs solution droplet and form a meniscus which cover only the edge part of etched tungsten tip. Using this technique, possibility to attach the CNT at the edge is increased. Otherwise, CNTs may attach at the surface of tungsten tip. Alternating voltages (6 volts at 2.2 MHz) were applied to the electrodes causes to generate the electric filed and CNTs were began to align parallel and eventually attached to the sharp edge of tungsten tip. However, at this stage MWCNT cannot be used for scanning purpose because CNT is attached with tungsten probe using Van der Waals and not sufficient to hold it during scanning. The homemade 3D AFM setup along with all electronics components is shown in **Figure A2**.

#### **2.3 MWCNTs welding and cutting using FIB**

We have utilized a focused beam of gallium ions (FIB) to align and attach the CNT on tungsten tip. The FIB system used (QUNTA 3D FEG) is a dual beam type

#### **Figure 2.**

electrodes using 2 M KOH aqueous solution. Keithley 32,220 function generator is used to apply DC voltage across the electrodes. By applying the DC voltage, redox reaction occurs at the electrodes and tungsten tip begins to narrow. The shape of the neck depends upon the meniscus formed at the air-liquid interface. The experimental setup for electrochemical etching is shown in **Figure A3**. Reaction takes

� ! 3*H*2ð Þþ *g* 6*OH*�

<sup>4</sup> þ 4*H*2*O* þ 6*e*

�

<sup>4</sup> þ 3*H*<sup>2</sup> ð Þ*g* (1)

place can be described by the following equation [26].

*21st Century Surface Science - a Handbook*

Overall, reaction can be summarized as:

**tungsten tip using dielectrophoresis**

6*H*2*O* þ 6*e*

*W s*ðÞþ <sup>8</sup>*OH*� ! *WO*2�

tungsten probe) as these are the best candidates used for D.E.P.

Filtered CNT-PVA are then collected and dried under vacuum.

**260**

**2.2 Functionalization and multiwall carbon nanotubes attachment with**

We have used commercially available MWCNTs (Sigma Aldrich). First two MWCNTs samples having 110–170 nm (10 μm in length) and 60 nm (15 μm in length) diameter are grown by chemical vapor deposition (CVD) and 20 nm (30 μm in length) diameter MWCNTs was grown by arc discharge method. The reason why we used the un-purified carbon nanotubes is to reveal that anyone can easily make the carbon nanotubes tips in one's laboratory, because MWCNTs made by arc discharge are very difficult to purify. To obtain a single MWCNT, fictionalization of MWCNTs is good choice. Other than that, it is difficult to attach single MWCNT on tungsten tip. Firstly, to functionalize the MWCNTs (with different diameter as 100–170 nm, 60 nm, 20 nm), 1 g of PVA was completely dissolved in 60 ml of water at room temperature. After that, 0.2 mg of MWCNTs was added in to the above-mentioned solution and mixture was stirred at 70oC for four 12 h. Secondly, centrifugation machine was used to remove the impurities or particles from the CNT-PVA solution. Centrifugation at 4000 RPM for 30 min was done several times unless the solution becomes clear and no CNT left in solution. Thirdly, to remove excess PVA from PVA-CNT solution, filter paper Anodisc 47 (Whatman) with the pore size 200 nm and diameter 47 mm is used.

Finally, CNT-PVA solution is sonicated in an ultrasonic bath for 6 h to uniformly

disperse in solution. After attachment of carboxylic group, MWCNTs were

*W s*ðÞþ <sup>2</sup>*OH*� <sup>þ</sup> <sup>2</sup>*H*2*<sup>O</sup>* ! *WO*<sup>2</sup>�

The hydrogen gas bubbles produced during the etching should be avoided as it disturbs the meniscus which affects the shape of probe. Due to this, glass slide is placed between the electrodes. During the etching process, variation in the thickness of tungsten wire and current was monitored using the CCD camera and oscilloscope. When the current drops below the set point, shut-off circuit trigger the solid state relay (shut-off time is 0.5 ns), sharp edge of the tungsten wire is formed and hence in this way over-etching does not happen. The home-built automatic cutoff circuit to avoid over-etching is used as shown in **Figure A1**. After the wire has been etched, it was washed with the hot distilled water (D.I) to reduce the oxide layer around the edge and examined under the scanning electron microscope. Tips with the sharp edges are reserved for next step (attachment of MWCNT with

At anode

At cathode

*Functionalization of MWCNT. (a) FTIR spectra depicts attachment of functional groups in CNTs: Peaks reveal 1200–3500 confirm the presence of functional group in purified CNTs. Inset image clearly shows the peaks at 2856 and 2926. (b) Comparison between the PVA-functionalized and as grown MWCNT.*

where the electron and ion column are placed vertically (at 0°) and at an angle 52°, respectively. Alternatively, the same viewing angle of another gun can be obtained by tilting the sample back and forth by 52o.

other hand, all these parameters are carefully chosen while considering the CNT diameter under investigation, length and initial orientation. Thirdly, gallium ions are used to shorten the CNT (**Figure 3(c)**). After the selection of suitable length of CNT, i.e. we choose 300, 500, 700 nm and 1μm length, extra length was cut-off using gallium ions (**Figure 3(d)**). **Figure 3** shows the welding, alignment, cutting

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High…*

After that, the tip is shorten to less than 0.5 mm and attached to one end of quartz tuning fork (QTF) using the Torr Seal®. Quality factor, Q, of 45,000500 in air was routinely obtained. Q factor of TF should be as higher as possible to obtain the highest sensitivity and resonance frequency, *f*, was shifted from 32,768 Hz to 32,310 Hz. QTF was excited at 15 mV which corresponds to 2.85 nm free oscillation

Herein, we have validated the working of the algorithm to measure the 3D profile of different samples (AAO and silicon pillars). Proposed algorithm enjoys all the advantages of conventional AFM. In addition to this, it has the capability to the measure the high aspect ratio features present above or below the sample. Here, we have used AAO sample to measure the internal morphology of the holes and 3D scanning of silicon pillars. To measure the internal morphology of the hole, firstly, the raster scan is performed using the MWCNTs based AFM probe: MWCNTs probe is attached using the FIB process on the Tungsten (W) apex which is further attached with the one prong of quartz tuning fork. Once raster scan is completed, feedback loop is turned off to start the 3D scanning of the hole. Positions of the holes and protrusion can be clearly seen from the raster scan image. Secondly, the algorithm is executed to scan the internal topography of via hole. Proposed algorithm can be summed up in four steps: Finding the location of hole(s), moving the tip to the bottom of the hole to be scanned internally, finding the first sidewall along the lateral direction, making the motion corresponding to the sidewall detected.

Location of the hole can be founded in various ways. One of the easiest ways is do the raster scan over certain area at high resolution. However, due to the high aspect ratio of the AAO holes, the tip wear might occur which is not suitable to do the 3D scanning of the hole. For 3D scanning of the hole, the tip should have perfect symmetry so that the accurate imaging can be done inside of the hole. Other method is to do the force-distance curve or mapping, amplitude-distance curves or mapping over a certain area i.e. doing the amplitude-distance curve at each pixel and recording the coordinates and Z position. Lowest Z scanner value can be marked by red color and highest value with blue. Each individual pixel locally quantifies physical properties and interactions. By this, lot of information (elasticity, adhesion) can be mapped directly of the sample internal topography. In the **Figure 4**, we have done the mapping of certain area of AAO in which location of the can be clearly seen with different color contrast. In **Figure 4(i)**, FD curve is done on solid surface which has sharp change at 450 nm and on contrary side, **Figure 4(ii)** has rapid decrease in amplitude at 201 nm which shows which shows the tip can penetrate up to 249 nm. However, at this stage, tip is restricted not to go beyond the set point level. Later on, once the location of the hole is determined, tip can be pushed deep inside to

and ball shape capping of CNT on tungsten probe.

*DOI: http://dx.doi.org/10.5772/intechopen.92739*

amplitude.

**3. Results and discussion**

**3.1 Finding the location of holes**

measure the exact depth of the hole.

**263**

Using the FIB process, CNT can be welded and shorten to use for AFM scanning. Our Fabrication of MWCNT based AFM probe comprises of four steps. Firstly, using electron beam induced deposition (EBID) hydrocarbon, CNT is welded on the etched part of tungsten tip (**Figure 3(b)**). The carbon atoms are deposited at the interface of tungsten which makes the CNT to attach firmly with tungsten. Secondly, CNT is aligned in the FIB system through the ion beam bending (IBB) [27] (**Figure 3(c)**). FIB is irradiated from the top. In the complete process of CNT alignment, acceleration voltage was set to 30 kV, resolution was 512 pixels 442 pixels and FIB image magnification was 5000. Beam current as well as acceleration voltages can be increased to reduce the alignment time, however, should be optimized to avoid sample damage. We have used this method on 110–170, 50–80 and 20 nm CNT and it has been found that exposure time (for alignment process) to MWCNT is directly proportion to the diameter of MWCNT. For example, MWCNT with the diameter 20 nm takes less exposure time as compared to 50–80 or 110– 170 nm MWCNT. The variables that affect the alignment process includes FIB magnification, exposure time, beam current and accelerating voltage. To produce good welding, many combinations can be made that produce good results. On the

#### **Figure 3.**

*Fabrication of MWCNT AFM probe. (a) As attached using DEP, some impurities are also attached with MWCNT which can be removed using lower current ion beam. (b) After hydrocarbon deposition (welding) on the interface of MWCNT and Tungsten probe. (c) Alignment of MWCNT using IBB process (d) using gallium ion bombardment extra length of CNT is cut off. To give some offset from the sidewall (in case of AAO sample or samples having vertical side walls), ball shape (using hydrocarbon) was deposited on the top of MWCNT. Diameter of MWCNT is 20, 60 and 120 nm. Scale bar is 1 μm.*

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High… DOI: http://dx.doi.org/10.5772/intechopen.92739*

other hand, all these parameters are carefully chosen while considering the CNT diameter under investigation, length and initial orientation. Thirdly, gallium ions are used to shorten the CNT (**Figure 3(c)**). After the selection of suitable length of CNT, i.e. we choose 300, 500, 700 nm and 1μm length, extra length was cut-off using gallium ions (**Figure 3(d)**). **Figure 3** shows the welding, alignment, cutting and ball shape capping of CNT on tungsten probe.

After that, the tip is shorten to less than 0.5 mm and attached to one end of quartz tuning fork (QTF) using the Torr Seal®. Quality factor, Q, of 45,000500 in air was routinely obtained. Q factor of TF should be as higher as possible to obtain the highest sensitivity and resonance frequency, *f*, was shifted from 32,768 Hz to 32,310 Hz. QTF was excited at 15 mV which corresponds to 2.85 nm free oscillation amplitude.
