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CAMS IN MICROELECTROMECHANICAL SYSTEMS 507

plays, light modulators, switches, gas chromatographs, etc., are some of the many devices
that appeared in that period. Some of these pioneering works are described in Trimmer
(1990). An excellent review article (Petersen, 1982) also has a good description of the
early applications of MEMS and the manufacturing techniques used to make them. Today,
commercial applications of MEMS are in inertial measurement devices such as accelerom-
eters and gyroscopes; microoptical devices such as displays; telecommunication devices
such as mechanical filters, aligners, and switches; microfluidic devices such as valves,
pumps, flowchannels for biochemical lab-on-a-chip devices, and ink-jet printer heads, to
name a few. Many other applications have emerged and are likely to enter the commer-
cial market in the near future.
From a mechanical perspective, the principle of operation of most MEMS devices is
rather simple. For example, a micromechanical accelerometer consists of an inertial mass
suspended with an elastic structure. It can be as simple as a cantilever with a mass attached
to its free end. The movement of the mass is measured by one of a variety of means
and is used to estimate the acceleration of the body on which the device is mounted. A
typical MEMS pressure sensor consists of a membrane that deflects in response to
increased pressure. Its deformation is measured using a transduction technique to estimate
the pressure. The same membrane when actuated in a controlled manner can be used
as a pump. A system of vibrating masses attached to each other with beams serves as a
mechanical filter. Flat structures attached to beams that twist act as micromechanical
mirrors used in projection displays. The majority of MEMS devices developed until now
use very simple mechanical structures (beams, membranes, and plates) that elastically
deform. Thus, the significance of such devices lies not in their mechanical structure but
in the transduction scheme based on physical, chemical, and biochemical phenomena
and of course micromachining. A sourcebook on micromachined transducers (Kovacs,
1998) describes them comprehensively. In contrast, the emphasis in this chapter is on
those MEMS devices in which there is relative motion and motion and force transmis-
sion among rigid bodies with intermittent or sustained contact. Whether elastic deforma-
tion is used or rigid-body motion is used, materials and the processing techniques play a
crucial role in the development of MEMS. These two are discussed in the following two
sections.

15.3 MATERIALS FOR MEMS

Silicon has turned out to be an excellent mechanical material, and it has played a key role
in the development of MEMS technology so far. A number of other materials are also
appearing in MEMS devices. The choice of a material depends largely on the fabrication
technology used to make MEMS devices. In this respect, the development of MEMS
materials and fabrication processes can be divided into four phases (NRC, 1997). The first
phase consisted of the early years (1965 to 1985) when “old” materials and “old” processes
of microelectronics were used to make MEMS devices. These materials were silicon,
oxide and nitride, quartz, and some metals. The processes were deposition and
photolithography-based etching of thin films. In the second phase, microelectronic
processes were developed further into new micromachining processes but still used the
old materials. The new processes include a sacrificial layer that enable the fabrication of
releasable structures, bulk etching techniques that can also create releasable structures,
some wafer-bonding techniques, and deep lithography and micromolding techniques. The
third phase used “new” materials such as polymers, ceramics, metals, and some active
materials but processed them with existing “old” techniques. The fourth phase will be to
develop new processes for these new materials. The MEMS field is now in the third phase