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Handling and Contamination Control for Critical Space Applications 301

FIGURE 13.2 Example of a resonator defect due to particulate contamination: A small particle
betweentwo fingersthat does notfusethe fingersand hence the inter-fingercapacitanceisgreatly
increased due to the significant gap reduction. (Courtesy: CMU S. Blanton.)

The digital micromirror device (DMD) is a microchip consisting of a superstruc-
ture array of Al micromirrors functionally located over CMOS memory cells. The
mirrors are hermetically sealed beneath nonreflecting glass to prevent contamination-
induced failure. For reliability, the device must be isolated in a hermetic package. The
key question lies in when the device is isolated since the damage often occurs before
packaging. Also, while hermeticity specifications are defined in terms of leakage in
and out of a ‘‘sealed’’ cavity, the issue is more complex. A hermetic package prevents
the diffusion of gases, moisture, and outgassed hydrocarbons through its walls. A
robust contamination solution must also stop permeation, which occurs when con-
taminants diffuse through the seal over time, and outgassing, where materials internal
to the hermetic cavity (such as polymers or epoxies) release trace quantities of gases
or vapors, which contaminate active surfaces of the device.
Particular attention should be paid to the protection of devices that are not
hermetically packaged such as environmental sensors; however, hermetic parts are
also susceptible to contamination problems. Any contaminant once sealed in a
hermetic package has a wonderful ‘‘growth medium’’ that has accelerators such
as voltage and temperature. Of particular concern is the presence of liquid, vapor,
gases, particles, or other contaminants. Controls for packaging cleanliness used in
the microcircuit industry are not adequate for the MEMS world as MEMS devices
are affected by particles, especially nonmetallics, which might not affect an IC.
Modifications may have to be made to standard assembly equipment, assembly
handling methods and tooling, and equipment environments to accommodate the
intensive handling and particle control requirements for packaging microstructures.

13.6.5 MEMS POSTPACKAGE CONTAMINATION CONTROL
Postpackaged MEMS devices must be considered as contamination-sensitive flight
hardware and handled accordingly. Additional contamination control precautions

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