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MEMS Fabrication 3-3

more recently become commercially viable. Atypical structure fashioned in a Si bulk micromachining
process is shown in Figure 3.1.It was this type of piezoresistive membrane structure, a likely base for a
pressure sensor, that demonstrated that batch fabrication of miniature components need not be limited
to integrated circuits (ICs). This chapter’s emphasis is on the wet etching process itself. Two other
machining steps typically used in conjunction with wet bulk micromachining, additive processes and
bonding processes, are covered in Madou (2002).
After a short historical note on wet bulk micromachining, we will begin with an introduction to the
crystallography of single-crystal Si and a listing of its properties clarifying the importance of Si as a sen-
sor material. Some empirical data on wet etching will be reviewed, and different models for anisotropic
and isotropic etching behavior will follow. Etch stop techniques, which catapulted micromachining into
an industrial manufacturing process, are then discussed. Subsequently, a discussion of problems associ-
ated with bulk micromachining, such as IC incompatibility, extensive real-estate usage, and issues
involving corner compensation, are presented. We conclude with some example applications of wet bulk
micromachining.

3.2 Historical Note

The earliest instance of wet etching of a substrate using a mask (wax) and etchants (acid-base) appears
in the late 15th or early 16th century for decorating armor [Harris, 1976; Durant, 1957] (Inset 3.1).
Engraving hand tools were not hard enough to work the armor, and more powerful acid-based processes
took over. By the early 17th century, etching to decorate arms and armor was a well established process.
Some pieces stemming from that period have been found in which the chemical milling was accurate to
within 0.5mm. The masking in traditional chemical milling was accomplished by cutting the maskant with
a scribing tool and peeling off the maskant where etching was desired. Harris (1976) describes in detail the
improvements that by the mid-1960s made this type of chemical milling a valuable and reliable method
of manufacturing especially popular in the aerospace industry. The method enabled manufacturers to
produce many parts more easily and cheaply than by other means and, in many cases, provided design
and production configurations not previously possible. Through the introduction of photosensitive

INSET 3.1 Decorating armor.

© 2006 by Taylor & Francis Group, LLC