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Bulk Micromachining makes micromechanical devices by etching deeply
into the silicon wafer. There are several ways to etch the silicon
wafer. Anisotropic etching uses etchants like KOH that etch different
crystallographic directions at different rates. Certain crystallographic
planes etch extremely slowly, and are called stop planes. Anisotropic
etching usually produces Vee grooves, pyramids, and channels into the surface
of the silicon wafer. Isotropic etching etches all directions in
the silicon wafer with nearly the same rate, and produces rounded depressions
on the surface of the wafer that usually resemble hemispheres and cylinders.
Deep Reactive Ion Etching, RIE or DRIE, uses a plasma to etch straight
walled structures on the wafer that resemble the mesa hills in the American
References are given as (p xxx) which refer to page xxx in the book
"Micromechanics and MEMS"
Bulk Micromachining makes extensive use of "wafer bonding," where different
silicon wafers are brought together and permanently joined. The two
most common bonding techniques are "Anodic Bonding" (p 582) and "Silicon
Fusing Bonding" (p 584). When several bulk micromachined wafers are
bonded together, a wide range and complexity of mechanical structures can
An early use of Reactive Ion Etching to make mechanical components
was demonstrated in the paper "Micro Gears and Turbines Etched from Silicon"
by K Gabriel, W Trimmer, and M Mehregany in the Transducers '87 Proceedings
page 853. One of the etched gears is shown below.
A picture of one of these RIE etched gears is shown held by an ant
in the photograph below (p 100).
Photograph taken by C LaGreca and W Trimmer.
Ant caught by Scott Trimmer (age 8).
A combination of bulk micromachining and wafer bonding is shown
in the schematic diagram of an accelerometer chip below. The lower
wafer is borosilicate glass, with a small relief etched in its top surface.
The top wafer is silicon. Anisotropic etching from the bottom of
the silicon wafer has released the seismic mass. Anisotropic etching
on the top surface of the silicon wafer defines several suspension beams
for the seismic mass.
Two scanning electron micrographs of this accelerometer are given
below. The figure to the right shows a poppy seed on top of the seismic
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