6. Implantable BioMEMS

5.4. Encapsulation and protection

360 Physics and Technology of Silicon Carbide Devices

the encapsulation and packaging of microdevices [20].

using a bonded capsule or shell [20].

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

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

+z 0!\_z0\$!z)&+.%05z+"z0\$!z !2!(+,)!\*0z!""+.0/z%\*z0\$!z

z"%!( z\$2!z"+1/! z+\*z/+,\$%/0%¥ cated devices to meet the requirements of industrial applications. However, MEMS devices "+.z)! %(z,,(%0%+\*/z.!,.!/!\*0zz,+0!\*0%(z)1(0%w%((%+\*z +((.z).'!0\_z,.%).%(5z+\*/%/0¥ ing of micro miniature devices with high functionality that are suitable for implantation. These implanted systems could revolutionize medical diagnostics and treatment modalities. Implantable muscle microstimulators for disabled individuals have already been developed. Precision sensors combined with integrated processing and telemetry circuitry can remotely )+\*%0+.z\*5z\*1)!.z+"z,\$5/%(z+.z\$!)%(z,.)!0!./z3%0\$%\*z0\$!z\$1)\*z+ 5z\* z0\$!.!¥ by allow evaluation of an individual's medical condition.

z,.+!//%\*#z 0!\$\*+(+#5z%/z(/+z!%\*#z1/! z 0+z"/\$%+\*z"1\*0%+\*((5z/%),(!z,//%2!z)%¥ crodevices like retinal implants [21, 22]. In the future, in order to improve functionality and reduce size, ever increasing numbers of MEMS devices will have direct patient contact thus requiring that biocompatibility testing be performed on MEMS materials of construction. +06.z!0z(^z/!(!0! z0\$!z"+((+3%\*#z)0!.%(/z/z

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 using MEMS technology.

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

z)0!.%(z 0\$0z3/z\*+0z%++),0%(!z3\$!\*z/1&!0z 0+z 0\$!z,.+¥ essing, packaging, and sterilization methods. Amorphous-SiC (a-SiC) has the longest and most diverse track record of any SiC microstructure used in biomedical microdevices. Early work using a-SiC for medical applications focused on developing a-SiC films as corrosionresistant coatings for macroscale structures, such as Ti alloy-based orthopedic implants and metallic coronary stents [24], thus setting the stage for its use in biomedical microsystems.
