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Bulk Micromachining
 
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 south west.

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 be made.
 
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 mass.
 
 
 
 
 
 

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