Page 95 - An Introduction to Microelectromechanical Systems Engineering

74 Processes for Micromachining

Photoresist

Oxide

Silicon substrate
1. Deposit oxide and photoresist 2. Lithography and oxide etch

3. Silicon etch 4. Coat sidewalls with PECVD oxide

Suspended beam Sharp tip

5. Remove oxide at bottom and etch silicon 6. Plasma etch in SF to release structures
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Figure 3.34 Basic steps of the SCREAM process. (After: [37].)

oxide only at the bottom of the trench. A subsequent plasma silicon etch extends the
depth of the trench. A dry isotropic etch step using sulfur hexafluoride (SF ) laterally
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etches the exposed sidewalls near the bottom of the trench, thus undercutting adja-
cent structures and mechanically releasing them. Sputter deposition of aluminum
provides the metal for electrical contacts and interconnects.
This process, known by its SCREAM acronym, was initially developed at
Cornell University. Kionix, Inc., of Ithaca, New York, uses a variation of SCREAM
for the manufacture of accelerometers, micromirrors, and other devices.

Summary

The toolbox of micromachining processes is very large and diverse. The vast
majority of the methods can be condensed into three major categories: