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320 MEMS and Microstructures in Aerospace Applications

14.6.4 SILICON DIOXIDE
Silicon dioxide is the native oxide of silicon and one of the most common MEMS
materials. It is used as a dielectric insulator, etch mask, or as part of a mechanical
structure. It can be thermally grown on silicon, or deposited by LPCVD, PECVD, or
sputtering. Thermal oxides require high temperature (~10008C), so low temperature
oxide (LTO) LPCVD or PECVD processes are used to coat over metals. PECVD and
sputtering processes can be tuned to control the stress of SiO 2 -deposited films.

14.6.5 METALS
There are a wide number of metal materials which can be used in MEMS as electrical
conductors, structural material, and low emissivity coatings. These materials can be
deposited by evaporation, sputtering, CVD, laser deposition, or electroplating.
Metals are ductile materials which will plastically deform when stressed past their
yield strength. In this section we will cover some of the most commonly used metals.
Aluminum is one of the most commonly used IC conductor materials and
therefore also one of the most common MEMS materials. It is either evaporated
or sputtered to form surface micromachined structures. Its native oxide is Al 2 O 3 ,
which is a hard and chemically resistant material. Aluminum adheres well to silicon
dioxide by forming Al 2 O 3 –SiO 2 bonds.
Gold is a soft material which is used as a conductor or emissivity coating. Gold
along with platinum are fairly inert and do not oxidize. Being nonreactive they have a
hard time adhering to materials. A thin layer (10–50 nm) of a reactive metal such as Ti
or Cr is typically used between surfaces to promote adhesion. Silicon will diffuse into
Au at temperatures of 1008C. To prevent this, a barrier layer is needed between the Si
and Au interface. Typically a thin layer of SiO 2 will suffice to prevent Si diffusion.
There are a number of metals to avoid in space applications:

. Tin can form whiskers in vacuum environments and can undergo a low
temperature ( 40 to 608C) transformation from a stable tetragonal structure
to a crack-prone powdery diamond cubic structure.
. Silver can also form whiskers and easily corrodes in sulfur-rich environ-
ments. Silver is also susceptible to galvanic corrosion with other metals.
. Mercury, cadmium, zinc, magnesium, selenium, tellurium sublime in vacuum
and can redeposit potentially causing shorting or damaging optics.

14.6.6 POLYCRYSTALLINE DIAMOND
Polycrystalline diamond films have a high hardness and high thermal conductivity.
This makes them an attractive material for high wear and high-temperature
environments. Diamond is naturally hydrophobic and structures made from diamond
are inherently striction resistant. Diamond films are typically grown using a hot-
filament CVD or microwave plasma system at ~9008C. It is a wide bandgap material
with a bandgap of 5.5 eV. P-type regions can be formed by doping with boron.
Diamond also has the highest stiffness amongst material, making it ideal for high Q
resonators. It is typically etched using an O 2 ion source or reactive ion etch (RIE).

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