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36 MEMS and Microstructures in Aerospace Applications

Society at the California Institute of Technology (Caltech), which was first pub-
5
lished in the 1960 issue of Caltech’s Engineering and Science magazine, and it has
since been reprinted several times. 6,7 In the talk, Feynman conceptually presented,
motivated, and challenged people with the desire, and advantages of exploring
engineered devices at a small scale. This talk is frequently cited as the conceptual
beginnings of the fields of microelectromechanical systems (MEMS), and nano-
technology. Feynman offered two US $1000 prizes for the following achievements:

. Building a working electric motor no larger than a 1/64th-in. (400-mm) cube
. Printing text at a scale that the Encyclopedia Britannica could fit on the head
of a pin.

In less than a year, a Caltech engineer, William McLellan, constructed a 250-mm,
8
2000-rpm electric motor using 13 separate parts to collect his prize. This illustrated
that technology was constantly moving toward miniaturization and that
some aspects of the technology already existed. However, the second prize was
not awarded until 1985, when T. Newman, and R.F.W. Pease used e-beam lithog-
raphy to print the first page of A Tale of Two Cities within a 5.9-mm square. 9
The achievement of the second prize was enabled by the developments of the
microelectronics industry in the ensuing 25 years. The first indication that MEMS
can be realized came in the early days of microelectronic development. The
fabrication processes that were being developed for microelectronics would
eventually be utilized and further developed for MEMS fabrication at the micron
scale. Sense mechanisms such as the piezoresistive properties of the micro-
electronic materials (silicon, germanium) 10 and the mechanical motion of a
11
silicon transistor gate provided the indication that MEMS sensors could be
developed. During the 1960s and 1970s, initial devices (strain gauges, pressure
sensors, accelerometers) that utilized piezoresistive sensing were developed. How-
ever, it was not until the early 1980s that the field of MEMS was launched in
earnest, stimulated in part by the consideration of silicon as a mechanical mater-
ial. 12 In the ensuing years, the development of fabrication processes for MEMS, the
demonstration of MEMS devices, and the commercialization of MEMS devices
have occurred.

3.2 MEMS FABRICATION TECHNOLOGIES

MEMS fabrication technologies are a part of a spectrum of fabrication technologies
that also include traditional precision machining processes. Traditional machining
processes can utilize a large variety of materials, fabricate complex three-dimen-
sional devices, and produce precise devices. MEMS fabrication processes are
generally more limited in the materials utilized, but they can produce functional
devices with micron-scale dimensions. Table 3.1 is a comparison of the MEMS
fabrication processes and conventional machining processes.

© 2006 by Taylor & Francis Group, LLC