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Material Selection for Applications of MEMS 321

14.6.7 SILICON CARBIDE
Silicon carbide has many properties that make it well suited for MEMS
applications, due to its chemical resistance and good mechanical properties.
Its high-temperature resistance, radiation resistance, electronics capability,
extreme hardness, and high stiffness make it a good choice for missions in harsh
environments. SiC is much stiffer than Si and thus makes good resonant struc-
tures.
The properties of SiC can vary significantly depending upon how it is grown
and processed. This is because SiC is polymorphic, and exists in many polytypes.
The three most common crystal types are cubic, hexagonal, and rhombehedral. Of
these cubic 3C-SiC, and hexagonal 4H-SiC and 6H-SiC are the most common. SiC
does not have a defined melting point; however it breaks down at 28308C where it
decomposes into graphite and a silicon-rich melt. It is typically grown using
APCVD or LPCVD processes at 13008C. Polycrystalline SiC can be grown at
temperatures as low as 5008C using APCVD, LPCVD, PECVD, or reactive sputter-
ing processes. Silicon carbide is chemically resistant but can be etched electro-
chemically or with a plasma process.
Silicon carbide is a wide-bandgap semiconductor material with a bandgap
of approximately 3 eV. The exact bandgap depends on the crystal structure.
Silicon carbide also oxidizes readily above 6008C to form a native silicon dioxide.
SiC is a better natural insulator than Si or GaAs, but can be doped with aluminum or
boron to form p-type material or nitrogen or phosphorus to form n-type materials. SiC
electronics have the potential to operate at temperatures of 400 to 6008C, which may
make them attractive for future missions with high-temperature constraints such as
Venus.

14.6.8 POLYMERS AND EPOXIES
Organic polymers can be deposited via evaporation, CVD, plasma deposition, spin
on, and spray techniques. Nonreactive and nonoutgassing polymers are required for
space missions. Polymers, epoxies, and polyimides must pass an acceptance criteria
of <0.1% CVCM and <1.0% TML. Teflon, kapton, and mylar have been used
extensively in space. Polyvinylchloride materials should be avoided due to outgas-
sing in vacuum and temperature constraints.

14.6.9 SU-8
SU-8, an EPON 1 epoxy-based negative photoresist, originally developed at IBM.
It is known for its high thickness films and is often used as a poor man’s
LIGA technique for electroplating metal MEMS. Thicknesses up to 2 mm can be
achieved with aspect ratios greater than 20. Since the stiffness and strength are low
and thermal coefficient of expansion is high it is primarily used as a mask. It has
been used as a structural post in thermal switches scheduled to fly on Midstar 1 due
to its 5 better thermal insulating performance over silicon dioxide and 100
improvement over silicon nitride. 27 Material properties of SU-8 are given in
Table 14.7.

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