5. Microfabrication techniques

#### 5.1. Material selection

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Silicon Carbide: A Biocompatible Semiconductor Used in Advanced Biosensors and BioMEMS/NEMS http://dx.doi.org/10.5772/51811 357


the control samples. Blood and membrane proteins have similar band-gaps, because the electronic properties depend mainly on the periodicity of the amino acids, and the proteins %""!.z+\*(5z%\*z0\$!z% z/!-1!\*!\_z\*+0z%\*z0\$!%.z/0.101.(z,!.%+ %%05^z,,.!\*0(5\_z/%)%(.z.!¥

A-SiC: H has superior hemocompatibility; its clotting time is 200 percent longer than to that of titanium and pyrolytic carbon. Furthermore, it has been shown that small variations in the preparation conditions cause a significant change in hemocompatibility. Therefore, it is of paramount importance to know the exact physical properties of the material in use. Amorphous silicon carbide can be deposited on any substrate material which is resistant to temperatures of approximately 250 °C. This property makes amorphous silicon carbide a suitable coating material for all hybrid designs of biomedical devices. The substrate material \*z!z"%00! z0+z0\$!z)!\$\*%(z\*!! /\_z %/.!#. %\*#z%0/z\$!)++),0%%(%05\_z3\$!.!/z0\$!z+0¥ %\*#z!\*/1.!/z0\$!z\$!)++),0%%(%05z+"z0\$!z !2%!^z+//%(!z,,(%0%+\*/z.!z0\$!0!./z+.z/!\*¥

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clusion, the hemocompatibility of SiC was demonstrated [5].

devices are designed to work in harsh environments.

5. Microfabrication techniques

356 Physics and Technology of Silicon Carbide Devices

5.1. Material selection

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z\*z!z0\$+1#\$0z+"z/z\*z!\*(%\*#z0!\$\*+(+#5z0\$0z((+3/z"+.z0\$!z !2!(+,¥ ment of what we commonly call smart or intelligent systems, which operate without the \*!! z"+.z!40!.\*(z+),10%\*#z.!/+1.!/^z\$%/z%\*0!#.0! z)%.+!(!0.+\*%/z\*z,.+!//z0\$!z%\*¥ "+.)0%+\*z !.%2! z".+)z0\$!z/!\*/+./z\* z0\$.+1#\$z/+)!z !%/%+\*w)'%\*#z,.+!//z %.!0z01¥ tors to respond by moving, positioning, regulating, pumping, and/or filtering, thereby controlling the environment for some desired outcome or purpose. Because MEMS devices are manufactured using batch fabrication techniques similar to those used over multiple decades in the integrated circuit industry, we see unprecedented levels of functionality, reliability, and sophistication being placed on small silicon chips at a relatively low cost. Therefore, there is significant potential for MEMS technologies.

Although crystalline SiC is a polymorphic material that exists in well over 100 distinct polytypes, only the cubic 3C-SiC, and the hexagonal 4H-SiC and 6H-SiC polytypes are technologically relevant for MEMS applications since they are the only configurations that can be produced as high-quality substrates and/or thin epitaxial films. At present, 6H-5iC and 4H-SiC are the only polytypes that are commercially available in large-area, integrated circuit (IC)-grade wafer form suitable for epitaxial growth of single crystalline films. In contrast, 3C-SiC is not widely available as bulk substrates, but single crystalline films can be epitaxially grown directly on Si wafers despite a significant mismatch in both lattice constant and thermal coefficient of expansion.

Single crystalline 3C-SiC piezoresistive pressure sensors have been fabricated using bulk micromachining for high temperature gas turbine applications. Bare silicon exhibits inadequate tribological performance. It needs to be coated with a solid and/or liquid overcoat or be surface treated (e.g., oxidation and ion implantation, commonly used in semiconductor manufacturing), which exhibits lower friction and wear. SiC films exhibit good tribological performance. Studies have been conducted on undoped polysilicon film, heavily doped (n+type) polysilicon film, heavily doped (p+-type) single-crystal Si (100) and 3C-SiC (cubic or b-SiC) film [10].

#### 5.2. Surface micromachining

Surface micromachining involves the monolithic fabrication of suspended microscale structures by selective removal of underlying thin film sacrificial layers. Surface micromachining is inherently an additive process utilizing thin film deposition techniques to produce both structural and sacrificial layers. As such, the primary function of the substrate is to provide mechanical support for the resulting device. Patterning of the thin film layers involves wet and dry etching techniques that are sensitive only to the chemical properties of the materials, and not their microstructure or crystallinity, thereby enabling a high degree of flexibility with respect to planar designs. In concept, there is no restriction on the structural and sacrificial materials to be used in the fabrication of a particular device as long as the materials are compatible with each other during the tabrication process. As such, surface micromachining is not constrained by the properties of the substrate and, thus, can accommodate an extremely wide range of materials, including SiC.

Like silicon, SiC thin films can be deposited by chemical vapor deposition (CVD), making it particularly well adapted as a "plug-and-play" substitute for polysilicon in surface micromachining. SiC films can be deposited by low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), and plasma-enhanced chemical vapor deposition (PECVD). Even for non- MEMS applications, CVD is the by far most common method to deposit SiC due at least in part to the availability of precursor gases as well as numerous process and equipment similarities to silicon CVD. The silicon carbide analog to polysilicon is polycrystalline 3C-SiC, hereafter referred to simply as poly-SiC. Poly-SiC is actually more versatile than polysilicon in that it can be deposited directly on SiO2 and polysilicon. In essence, the process of fabricating MEMS structures in poly-SiC by surface micromachining mirrors that of polysilicon. The main differences are process used to deposit the SiC films, the selection of sacrificial layer material, and the etch recipes used to pattern the structural films. A significant breakthrough in the advancement of SiC surface micromachining was the development of reactive ion etching techniques that are highly selective to SiC, which when combined with MEMS-friendly SiC deposition techniques, allow SiC surface micromachining to follow directly from polysilicon micromachining.

A wide range of micromachined structures, such as lateral resonators, flow sensors, capacitive pressure sensors, micromotors, and microbridge resonators can be fabricated using the deposition, patterning, etching, and sacrificial release techniques commonly used in polysilicon surface micromachining [1]. Several groups have demonstrated surface micromachining using a-SiC films as structural layers. Examples include RF switches and accelerometers [12]. Amorphous-SiC films deposited by PECVD generally exhibit a very wide range of residual stress (typically compressive in as-deposited films) that exhibit a strong dependence on deposition conditions [13].

#### 5.3. Bulk micromachining

Bulk micromachining can generally be defined as a process to fabricate suspended structures by selective bulk removal of the supporting substrate. Bulk micromachined structures can be comprised of the substrate material itself or thin films that are deposited directly onto the substrate. Unlike surface micromachining, the substrate in bulk micromachined devices is not merely a solid mechanical support, but rather forms a key component of the device structure.

In Si MEMS, direct wafer bonding has proven to be a key enabling process in the production of silicon-on-insulator (SOI) wafers for the realization of single crystalline Si MEMS devices. To create a SiC-on-insulator substrate, the Si water that was originally used as the substrate for SiC growth is removed by etching. The principal factor affecting yield is wafer bowing due to high tensile residual stress in the 3C-SiC films. The techniques developed for Si bulk micromachining, including photoelectrochemical etching, DRIE, and laser micromachining, have been successfully adapted for SiC albeit typically with much lower etch rates [13]. Among the first SiC MEMS structures to be routinely fabricated were diaphragms, cantilever beams, and related structures fabricated out of single crystalline 3C-SiC films by silicon anisotropic etching. Although conventional wet chemical techniques are not effective in etching structures into SiC substrates, several electrochemical etch processes have been demonstrated and used in the fabrication of bulk micromachined SiC MEMS devices from 6H- and 4H-SiC substrates. Examples of such structures include pressure sensors [14], accelerometers [15], and more recently, biosensors [16]. It is worth mentioning that, the good biocompatibility of devices made with common micromachining technologies allows the exploration of these technologies.

#### 5.4. Encapsulation and protection

In addition to using capsules or shells for providing a hermetic or vacuum package for MEMS, it is increasingly attractive to use thin films to provide the necessary protection or encapsulation [17-19] (Fig.2). Thin films are attractive because they occupy a very small area, \*z!z "+.)! z1/%\*#zz2.%!05z+"z 0!\$\*%-1!/\_z\* z.!z+),0%(!z3%0\$z3"!.w(!2!(z,.+!//¥ %\*#^z \*z %0%+\*\_z0\$!5z\*z0'!z\*5z/\$,!z+.z"+.)^z+3!2!.\_z)+/0z0\$%\*z"%()z)0!.%(/z.!z!%¥ ther not hermetic, or are so thin that they can be compromised easily when exposed to the environmental conditions MEMS typically experience. Two categories of thin film materials can be identified: organic and inorganic materials. Organic materials include such films as epoxies, silicones, a variety of polymers including polyimides, polyurethanes, Parylene-C, etc. The majority of these films can be deposited at low temperatures, are quite conformal and their characteristics can be modified for different applications. However, most of these films are not hermetic and most are prone to moisture penetration, or can be attacked in harsh environments. In spite of this, these materials have found widespread use because 0\$!5z \*z !z /!(!0%2!(5z 1/! z%\*z ,,(%0%+\*/z3\$%\$z)5z \*+0z .!-1%.!z 2!.5z(+\*#w0!.)z +,!.¥ tion, or where the conditions are controlled, or where the performance specifications are not \$%#\$(5z.!/0.%0%2!^z \*z "0\_z,+(5)!./z.!z,!.\$,/z 0\$!z)+/0z3% !(5z1/! z)0!.%(z "+.z,'#¥ ing, albeit not hermetic or vacuum packaging. The second category of materials used for ,'#%\*#z\* z,.+0!0%+\*z%/z%\*w+.#\*%z)0!.%(/^z\$!/!z)0!.%(/z%\*(1 !z"%()/z/1\$z/z/%(%¥ con nitride, silicon carbide, polycrystalline diamond, metal thin films, tantalum oxide, or thin films of other materials that are resistant to environmental parameters. Semiconductor materials such as silicon or silicon carbide are quite attractive because they can be deposited readily and are resistant to many corrosive environments. The main challenge in using these materials is that they typically require a high temperature to achieve a reasonable deposition .0!\_z\* z%\*z/+)!z%\*/0\*!/z0\$!z"%()/z.!z\*+0z-1%0!z/z+\*"+.)(z/z.!-1%.! z5z/+)!z,,(%¥ tions. Therefore, they have not been widely used for hermetic packaging, especially where hybrid components are involved. The discussion is limited to inorganic thin films since it is not possible to discuss the broad category of organic materials that are used nowadays in the encapsulation and packaging of microdevices [20].

6. Implantable BioMEMS

using MEMS technology.

7. Biosensing

7.1. Biosensors

by allow evaluation of an individual's medical condition.

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Silicon Carbide: A Biocompatible Semiconductor Used in Advanced Biosensors and BioMEMS/NEMS

http://dx.doi.org/10.5772/51811

361

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z)0!.%(/z+"z+\*/0.10%+\*z"+.z%),(\*0¥ able medical devices: (1) single crystal silicon (Si), (2) polycrystalline silicon (polysilicon), (3) silicon oxide (SiO2), (4) silicon nitride (Si3N4), (5) single crystal cubic silicon carbide (3C-SiC or b-SiC), (6) titanium (Ti), and (7) SU-8 epoxy photoresist. Of these, polysilicon, Si3N4, and 3C-SiC were deposited by chemical vapor deposition (CVD), SiO2 by thermal oxidation of Si, Ti by physical vapor deposition (PVD), and SU-8 by spin coating [23]. Many of the earlier studies examining the biocompatibility of SiC employed materials generated by fabrication methods suited to other implantable applications, such as RF sputtering. The results of the biocompatibility tests for these fabrication methods may not apply to materials fabricated

The Kotzar et al. study [6] results for SiC show that when the material is generated using MEMS fabrication techniques, it elicited no significant non-biocompatible responses to the test battery employed in this series. The biocompatibility testing discussed by Kotzar et al. % z\*+0z1\*+2!.z\*5z

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Figure 2. Structure of a MEMS package formed using a thin film capsule or shell, compared with a package formed using a bonded capsule or shell [20].
