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Material Selection for Applications of MEMS 319

applications, as silicon structures will only undergo elastic deformation allowing for
high mechanical stability. The mechanical properties of silicon are anisotropic and
dependent on crystal orientation.
SiliconhasmanyusefulpropertiesforMEMSdevices.Onthemicroscaleitisoften
likened to stainless steel on the macroscale. It has higher yield strength than stainless
and has a density lower than aluminum. The hardness of Si is slightly better than
stainless steel. Silicon’s specific strength, defined as the ratio of yield strength to
density,issignificantlyhigherthanformostcommonmaterialsasshowninTable14.9.
Single-crystal silicon cleaves, or forms cracks, along its crystal planes. The
more continuous the surface and edges of a piece of silicon are the less likely it will
crack under mechanical stress. Sharp corners and edges can serve as crack initiation
points when the material is stressed. Smooth curves and highly polished surfaces
resist cracking best. Etching processes can also damage or alter silicon surfaces,
affecting the material’s mechanical properties.
14.6.2 POLYSILICON

Polysilicon or polycrystalline silicon is also widely used in the semiconductor indus-
try. It is a crystalline material, but instead of being made up of a single crystal like bulk
silicon, it is made up of many small crystal grains. ICs use polysilicon for resistors,
gates,emitters, andohmiccontacts.Itiswidelyusedasastructuralmaterialforsurface
micromachining, heaters, and piezoresistive sensors in MEMS devices. Like single-
crystal silicon it can be doped to change its conductivity. Polysilicon is typically
deposited by low-pressure chemical vapor deposition (LPCVD), plasma-enhanced
chemical vapor deposition (PECVD), or sputtering. The structure of polysilicon and
hence its material properties change with deposition process, dopants, and tempera-
ture. After deposition thermal annealing can be used to drive dopants, recrystallize the
structure to reduce the grain size, and reduce stress. Since the material properties of
polysilicon reported in the literature vary widely, it is best to characterize the material
to be used in order to extract the proper properties for the design process.

14.6.3 SILICON NITRIDE
Silicon nitride is a dielectric material with good passivation properties. It is often used
as the topmost barrier layer on ICs, and forms a good barrier for H 2 O and ions. It is
also used as a capacitor dielectric material, etch mask, and wear-resistant coating.
Silicon nitride can be deposited by LPCVD, PECVD, physical sputtering, or reactive
sputtering. Again the deposition process greatly influences the material properties of
these films. LPCVD is a high-temperature process (700 to 8008C), which produces
the best quality stoichiometric Si x N y films. By controlling the amount of silicon in the
film, the refractive index and stress can be changed. Increasing the Si content in
silicon nitride films lowers the tensile film stress, increases transparency, and im-
proves HF etch resistance. PECVD systems can control stress by deposition frequen-
cies, power, and pressure and etch resistance by impurities. Sputtering systems can
also be tuned to control the stress of the film by altering the temperature, power,
pressure, and gas flow rates of depositions.

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