**2. Optical MEMS technologies**

Similar to optical MEMS devices, there is no single standard processing technology for optical MEMS fabrication. Silicon based optical MEMS is dominant materials system and different micromachining processes are being used as the most appropriate fabrication techniques. Also, conventional IC processes (lithography, depositions, implantation, dry etching, etc.) are often used in microstructure formation.

Bulk micromachining has been used for a long time for realization of 3D optomechanical structures on Si substrate for aligning optical fibres or forming optical MEMS devices. Single crystal Si has excellent mechanical properties and low-cost, high-purity Si substrates are available from IC industry. Si bulk micromachining is the process that impacts the substrate. Precise removal of the designated part of silicon substrate can be achieved by anisotropic etchants. Large difference in anisotropic etch rates between the <111> plane and other crystal‐ lographic planes in Si, enables pattern formation on either front-side or backside of the substrate. The etching rate of anisotropic etchants such as potassium hydroxide (KOH), aqueous solution of ethylene diamine and pyrocatechol (EDP) and tetramethylammonium hydroxide (TMAH), is much slower in <111> direction than in <100> and <110> directions [1]. Selectivity for such anisotropic etchants can be higher than 100 allowing creation of 3D optomechanical structures with high precision. Basic properties of commonly used anisotropic etchants are listed in Table 1.


**Table 1.** Basic properties of common anisotropic etchants

V-shaped grooves commonly used for precision positioning of optic fibres, are an example of this processing technology. The (100) Si substrate is first masked with an etch-resistance surface layer (deposited Si3N4 for KOH or thermally grown SiO2 for EDP) and then the Si is etched. Slower rate of <111> planes etch enables V-groove formation by etching <100> oriented planes. V-groove depth can be very well controlled by lithography because {111} planes are effective stop etching planes. Schematic of V-shaped groove formation is shown in Figure 1. By etching through square openings, pyramidal-shaped holes can also be formed that are being used for holding ball lenses. V-grooves and pyramidal-shaped holes are the basis of conventional microoptical benches. Bulk optical components are placed on the etched Si substrate and precisely positioned by holes of various geometries. Vertical micromirrors can be formed by anisotropic etching on a (110) Si substrate. Atomically smooth {111} planes are perpendicular to the surface of the substrate. Large-area semitransparent, optical-quality surfaces are provided. These micromirrors can be also used as beam splitters. In addition to the {111} stop etch planes, some etchants exhibit reduced etch rate in regions that are heavily doped with boron. This allows more flexibility in shapes of final structures: membranes, suspended beams, support beams for vertical micromirrors etched on (110) substrate, etc. Besides boron, other doping materials can be used but doping involves high temperatures and has side effects such as lattice shrinkage and introduction of large tensile stresses in parts formed this way [2].

**Figure 1.** V-shaped grooves formed by bulk silicon etch with wet chemistry

Fusion bonding of glass to bulk micromachined Si substrates allows formation of encapsulated structures as shown in Figure 2. Also, multilayer structures may be formed by bonding Si substrates together. In this way, the range of devices that can be manufactured using bulk micromachining is greatly extended.

**Figure 2.** Wafer bonding

ing technologies are summarized. Then, functional optical MEMS devices for optical network infrastructure are discussed. Finally, the key issues of various MEMS device failure mecha‐ nisms and design, processing and packaging implications are presented. At the closing subsection, the brief summary of the topic is presented with an emphasis on the importance of the research of relevant reliability issues that stand in the way of successful commerciali‐

Similar to optical MEMS devices, there is no single standard processing technology for optical MEMS fabrication. Silicon based optical MEMS is dominant materials system and different micromachining processes are being used as the most appropriate fabrication techniques. Also, conventional IC processes (lithography, depositions, implantation, dry etching, etc.) are often

Bulk micromachining has been used for a long time for realization of 3D optomechanical structures on Si substrate for aligning optical fibres or forming optical MEMS devices. Single crystal Si has excellent mechanical properties and low-cost, high-purity Si substrates are available from IC industry. Si bulk micromachining is the process that impacts the substrate. Precise removal of the designated part of silicon substrate can be achieved by anisotropic etchants. Large difference in anisotropic etch rates between the <111> plane and other crystal‐ lographic planes in Si, enables pattern formation on either front-side or backside of the substrate. The etching rate of anisotropic etchants such as potassium hydroxide (KOH), aqueous solution of ethylene diamine and pyrocatechol (EDP) and tetramethylammonium hydroxide (TMAH), is much slower in <111> direction than in <100> and <110> directions [1]. Selectivity for such anisotropic etchants can be higher than 100 allowing creation of 3D optomechanical structures with high precision. Basic properties of commonly used anisotropic

**Etch Masks Etch stop Main characteristics**

B~7×1019/cm3 Lots of masks,

low AR

greatest selectivity, makes vertical sidewalls

slow etch rate, low AR

lowest Boron doping etch stop,

zation of optical MEMS devices.

**2. Optical MEMS technologies**

100 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

used in microstructure formation.

etchants are listed in Table 1.

**(110) µm/min**

**AR**

EDP 1,25 35 SiO2, Si3N4,

**Table 1.** Basic properties of common anisotropic etchants

**{100}/{111}**

KOH 1,4 400 Si3N4, SiO2 B>1020/cm3 Fastest,

Ta, Au, Cr, Ag, Cu

TMAH 1 30 Si3N4 B~4×1020/cm3 Smooth surface,

**Etchant Etch rate**

Another process commonly used in optical MEMS fabrication is surface micromachining. While, in bulk micromachining, substrates materials are being removed in order to create 3D structures, surface micromachined structures are constructed from deposited thin films. Alternating layers of structural and sacrificial materials are deposited and patterned on the substrate. The sacrificial layers can be selectively removed by an etchant that attacks only the sacrificial materials. In this way suspended beams, cantilevers, diaphragms and cavities can be realized. Because of its excellent mechanical properties, polysilicon is being used as structural material and SiO2 as the sacrificial material because of the high selectivity of sacrificial etching with hydrofluoric acid. Figure 3. illustrates polysilicon surface machining process. The complexity of the surface machining process is determined by the number of structural and sacrificial layers. Two structural layers allow formation of free moving me‐ chanical gears, springs, sliders, etc. The main advantage of surface micromachining over bulk micromachining is that many different devices can be realized using common fabrication process. By changing patterns on the photomask layouts different devices are being fabricated simultaneously on the same substrate. For that reason, the surface micromachining process is often referred to as an IC process that allows formation of multilayer structures usually with two to five polysilicon levels.

**Figure 3.** Polysilicon surface micromachining

Often, it is desirable to fabricate structures thicker than those achievable using polysilicon. An alternative micromachining process uses lithographic exposure of thick photoresist, followed by electroplating to build on chip high aspect ratio 3D structures. In the LIGA (lithography, electroplating and moulding) process synchotron radiation is used as the exposure source that can achieve feature heights of the order of 500µm. Cheaper alternatives use excimer lasers or UV mask aligners that achieve feature heights of the order of 200µm and 20µm, respectively [2]. Parts are usually plated in nickel after removal of the resist as illustrated in Figure 4. The released metal layer can be used in various applications including optical MEMS devices.

Suspended single crystal Si structures, with lower stress and more reproducible properties than polysilicon, are formed using process based on BSOI (bonded silicon-on-insulator). Si wafer is thermally bonded to an oxidized Si substrate. Desired thickness (usually 5 to 200µm [3]) of the bonded wafer is achieved by polishing and the bonded layer is structured by deep reactive ion etching (DRIE) that has high etch rates and anisotropy to form very deep features with almost vertical sidewalls (Figure 5.). Movable parts can be made by removal of the buried oxide and one of the typical applications of this technique is realization of vertical mirrors for optical switching.

**Figure 5.** Deep reactive ion etching (DRIE) of bonded silicon-on-insulator (BSOI)

DRIE has also allowed Si micromolding techniques, such as HexSil process, to be developed [4]. DRIE is used to etch narrow trenches into the substrate. Trenches are fraction of a millimetre deep. After that, a sacrificial oxide layer is deposited, followed by the polysilicon structural layer that fills the trenches. As shown in Figure 6., deep suspended structures are being made by releasing the polysilicon.

**Figure 6.** HexSil process

sacrificial etching with hydrofluoric acid. Figure 3. illustrates polysilicon surface machining process. The complexity of the surface machining process is determined by the number of structural and sacrificial layers. Two structural layers allow formation of free moving me‐ chanical gears, springs, sliders, etc. The main advantage of surface micromachining over bulk micromachining is that many different devices can be realized using common fabrication process. By changing patterns on the photomask layouts different devices are being fabricated simultaneously on the same substrate. For that reason, the surface micromachining process is often referred to as an IC process that allows formation of multilayer structures usually with

102 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Often, it is desirable to fabricate structures thicker than those achievable using polysilicon. An alternative micromachining process uses lithographic exposure of thick photoresist, followed by electroplating to build on chip high aspect ratio 3D structures. In the LIGA (lithography, electroplating and moulding) process synchotron radiation is used as the exposure source that can achieve feature heights of the order of 500µm. Cheaper alternatives use excimer lasers or UV mask aligners that achieve feature heights of the order of 200µm and 20µm, respectively [2]. Parts are usually plated in nickel after removal of the resist as illustrated in Figure 4. The released metal layer can be used in various applications including optical MEMS devices.

Suspended single crystal Si structures, with lower stress and more reproducible properties than polysilicon, are formed using process based on BSOI (bonded silicon-on-insulator). Si wafer is thermally bonded to an oxidized Si substrate. Desired thickness (usually 5 to 200µm [3]) of the bonded wafer is achieved by polishing and the bonded layer is structured by deep reactive ion etching (DRIE) that has high etch rates and anisotropy to form very deep features with almost vertical sidewalls (Figure 5.). Movable parts can be made by removal of the buried oxide and one of the typical applications of this technique is realization of vertical mirrors for

two to five polysilicon levels.

**Figure 3.** Polysilicon surface micromachining

**Figure 4.** Metal micromachining

optical switching.

All described techniques involve surface patterning processes and therefore realized microstruc‐ tures are quasi 3D. Very often out-of-plane structures with high aspect ratios are required for free-space optical systems. Anisotropic etching or deep dry etching can provide such struc‐ tures but it is difficult to pattern their side walls. Fully 3D structures can be formed using microhinge technology [5]. Surface micromachined polysilicon planes are patterned by photolithography and then folded into 3D structures. Figure 7. shows schematic cross section of the microhinge that consists of hinge pin and a confining staple. After selective etching of the sacrificial SiO2, the polysilicon plate connected to the hinge pin is free to rotate out of the substrate plane and become perpendicular to the substrate. Polysilicon plate can also achieve other angles. This technology allows monolithic integration of 3D structures with surface micromachined actuators. It is of the special interest for fabrication of integrable free-space microoptical elements.

**Figure 7.** Schematic of surface-micromachined microhinge

The full potential of surface micromachining, bulk micromachining and wafer/chip bonding techniques is still being explored. The key activities are continued development of masks and etches that can yield high aspect ratio structures and the development of deposition techni‐ ques. Special attention is being paid to the development of techniques for creating fully 3D structures.
