Page 22 - MEMS Mechanical Sensors

2.3 Fabrication Techniques 11

foundation on which a micromachined device is built, in which case the substrate
material may be unimportant and need only be compatible with the processing
equipment used. Both quartz and Pyrex can be obtained in forms suitable for proc-
essing using standard silicon processing equipment. Sometimes, however, the
device is formed in the substrate itself, in which case the material properties
become important.

2.2.2 Additive Materials
The materials deposited on the substrates include all those associated with inte-
grated circuit processing. These are either epitaxial, polycrystalline, or amorphous
silicon, silicon nitride, silicon dioxide, silicon oxynitride, or a variety of metals and
metallic compounds, such as Cu, W, Al, Ti, and TiN, deposited by chemical (CVD)
or physical vapor deposition (PVD) processes. Organic polymer resists with thick-
nesses up to the order of a few micrometers are deposited by optical or electron
beam lithography.
Additional materials used in MEMS mechanical sensors are: ceramics (e.g., alu-
mina, which can be sputtered or deposited by a sol-gel process); polymers, such as
polyimides and thick X-ray resists and photoresists; a host of other metals and
metallic compounds (e.g., Au, Ni, ZnO) deposited either by PVD, electroplating, or
CVD; and alloys (e.g., SnPb) deposited by cosputtering or electroplating. Some
alloys, such as TiNi, have a shape memory effect that causes the material to return
to a predetermined shape when heated. This is caused by atomic shuffling within the
material during phase transition. At low temperatures the phase is martensite,
which is ductile and can be easily deformed. By simply heating, the phase of the
deformed material changes to austenite and the deformation induced at low tem-
perature can be fully recovered. The transition temperature depends on the impurity
concentration, which can be controlled to give values between –100°C and 100°C.
Therefore, by repeated deformation and heating the shape memory alloy (SMA) can
be incorporated in a useful mechanical device. For micromechanical devices the
high power-to-weight ratio, large achievable strain, low voltage required for heat-
ing, and large mean time between failure suggest that SMAs have the potential for
superior actuators. The maximum frequency of operation, however, is only of the
order of 100 Hz [2]. Diamond and silicon carbide deposited by CVD have some
potentially useful mechanical and thermal properties. Each has high wear resistance
and hardness, is chemically inert, and has excellent heat resistance. Neither has been
extensively explored for their use in MEMS sensors.
It is safe to say that, unless there is an issue of contamination or the sensors are
integrated with circuitry, it is possible to deposit almost any material on the sub-
strate. The issues that are likely to need addressing, however, are how well does it
adhere to the substrate, are there any stresses in the deposited layer that may cause it
to deform, and can it be patterned and etched using lithographic techniques?

2.3 Fabrication Techniques

The fabrication techniques used in MEMS consist of the conventional tech-
niques developed for integrated circuit processing and a variety of techniques