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298 MEMS and Microstructures in Aerospace Applications

The goal of contamination control at the design level is to minimize contamin-
ation sources and to remove contaminants from MEMS devices whenever it is
feasible on-ground or on-orbit. By eliminating contaminants before they ever have
chance to generate, this design level contamination control is not only effective but
also very cost-saving. Unfortunately this critical stage of contamination control is
often neglected due to the lack of the involvement from a contamination engineer.
Material selections for MEMS devices are critical for effective contamination
control. Single-crystal Si, polysilicon, Si 3 N 4 , and SiO 2 , and other materials are
well recognized for constructing MEMS devices. In addition SiC, shape memory
alloy (SMA) metals, permalloy, and high-temperature superconductive materials
are potential candidates. Although these materials have certain unique properties
which are attractive for certain MEMS applications, contamination issues may
result from the usage of these materials. For example, silica material used in fiber
optics is brittle and is prone to fracture including delayed fracture.

13.6.3 CONTAMINATION CONTROLS DURING FABRICATION
Contamination concerns start at the beginning of the MEMS fabrication life.
Problem areas in the foundry can be with both inferior materials and chemicals or
due to inadequate or not followed processing steps. Entire lots due to the homoge-
neous nature of fabrication runs may need to be destroyed due to contamination
related yield losses such as streamers, corrosion, and other results from impurities
or improper processing. The greater concern at the foundry level is allowing
contamination to reside with a lot only to appear at a later date found through
failure of the component. At the foundry level the most common source of con-
tamination is organics that have not been adequately removed. Most foundries ship
product with the photoresists still present, which protect the MEMS from damage,
but are absolutely necessary to be removed prior to release. Other sources of
contamination include those from humans such as finger oils, makeup, human
spittle, and processing materials. Often, dicing films are special adhesives that
must be properly removed. Bubbles forming during the release step can ‘‘protect’’
the material in the sacrificial area yielding a nonfunctioning or only partially
functioning device.
The recommended solvent should be used to assure the complete removal of
organics. Oxygen plasma and piranha etch are often used. Oxygen plasma is just
gaseous oxygen electrically charged into plasma. Organics placed in oxygen plasma
will etch quite thoroughly. Piranha etch is an etching compound formed of 70%
sulfuric acid and 30% hydrogen peroxide that will consume almost all organics, but
leave behind nonorganics. Piranha etch can remove some metal so it is necessary to
test pieces before committing a lot to any particular solution.

13.6.4 MEMS PACKAGE CONTAMINATION CONTROL
The discussion of package level contamination control for MEMS devices for space
flight use must be devoted to controlling contaminants from damaging the devices.
Risk of contamination is present at the bare die level, packaged, and through

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