fabrication of suspended microstructures by the selective etching of silicon sub-
strates. One early example is the air-gap transistor with a suspended metal gate
cantilever which was successfully fabricated at Westinghouse Research Labs in the
mid-1960s [1]. Early micro-scale devices primarily used single crystal silicon as a
mechanical structure since fabrication processes were well developed for silicon in
order to manufacture microelectronic circuits on a monolithic platform [2–4]. At
that time, anisotropic etching of the silicon was the dominant process used to create
microstructures on silicon substrates. Boron-doped silicon was used as an etch-stop
layer to precisely control the depth of etching to form micro-scale devices on the
thin films. For example, at Berkeley in 1980, composite cantilever beams, com-
posed of silicon dioxide (SiO
2
) and doped silicon, were released by anisotropic
etching of a silicon substrate [5].
Polycrystalline silicon is another material which is attractive for MEMS appli-
cations. Poly-silicon (poly-Si) was first used as a structural material for resonant
cantilever sensors [6]. Since 1960s, poly-Si has been widely used for electrical contact
in the gates of the metal-oxide-semiconductor field-effect transistors [7]. Low pressure
chemical-vapor-deposition (LPCVD) process was used to deposit poly-Si [8] and
surface micromachining was utilized to fabricate poly-Si cantilever beams with SiO
2
as a sacrificial layer [9]. However, non-uniform internal stress on the poly-Si layer
caused the beams to curve away from the substrate. This suggested that the residual
stress on the poly-Si layer needed to be controlled in order to achieve flat cantilever
beams. This was a major research topic in the MEMS community for over a decade
and tremendous progress was made. Several viable solutions, including heavily doping
the poly-Si layer [10, 11], annealing after deposition [12], and a multipoly process
[13], were suggested to reduce the residual stress and achieve flat cantilever beams.
Conformal deposition of poly-Si followed by thermal oxidation provided the
capability of achieving a uniform coating of poly SiO
2
layer in deep trenches [14].
This was an important step towards achieving a conformal etch-resistant layer as
well as electrical isolation. Conformal coating of poly-Si paved the way for fabri-
cation of more-sophisticated structures such as bearings, linkages, and motors. In
the late 1980s, micro-motors based on poly-Si were demonstrated by Professor
Muller and his collaborators at Berkeley [15, 16]. Their impressive results drew
large amounts of attention to the field.
1.2.2 Early devices and systems
The first market for optical MEMS devices was display technology. In the early
1970s, the fabrication of micromachined spatial light modulators (SLMs) gave rise
to projection displays [17, 18]. In the 1980s, the appearance of the digital mirror
device (DMD) made a significant impact on high-quality digital light processing
(DLP) projections [19]. Mechanically actuated micromirrors, which were deve-
loped and commercialized by Texas Instruments, are now widely used in cinemas,
TVs, and portable projectors. As shown in Figure 1.1, the DMD consists of an array
of highly reflective microscopic mirrors corresponding to the pixels of the images.
The bistable mirrors can be tilted 12
to ON and OFF states (Figure 1.1(a)). When
2 Optical MEMS for chemical analysis and biomedicine