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302 MEMS and Microstructures in Aerospace Applications

are needed for nonhermetically packaged MEMS devices that are more susceptible
to contaminants. Measures to protect nonhermetically packaged MEMS devices,
may include temperature control, humidity control, gas purging, and protective
enclosures. In addition, for nonhermetically sealed MEMS devices, especially if
mounted on the skin of the spacecraft, the need to identify the component and ‘‘red
tag’’ the item for special handling is essential.
MEMS postpackage level contamination control is concentrated on maintaining
proper surface cleanliness levels, that is, molecular and particulate contamination
budget. Therefore, the amount of performance degradation that is allowed for
MEMS contamination-sensitive surfaces needs to be established. From this degrad-
ation limit, the amount of contamination that can be tolerated, that is, the contam-
ination allowance, can be established. This allowable degradation should also be
included as a contamination budget stated in CCP.
The contamination budget describes the quantity of contaminant and the deg-
radation that may be expected during various phases in the lifetime of a MEMS
device. The established contamination budget for MEMS devices is monitored as
the program progresses. When the contamination budget exceeded requirements,
MEMS surfaces may be cleaned periodically to reestablish a budget baseline. In
addition, contamination-preventive methods, such as clean rooms and MEMS
device covers, should be included.
The integration and test (I&T) of conventional spacecraft is generally per-
formed in clean rooms with air cleanliness classes ranges from Class 1000 to as
high as Class 100,000. Integration through launch conditions may provide numer-
ous opportunities for gaseous and particulate contaminants to be deposited on
MEMS surfaces. For optical MEMS (MOEMS) gaseous contaminants can degrade
performance by condensing on critical windows or alternatively by absorbing light
along the line-of-sight.
There is a concern for MEMS devices when they are exposed to uncontrolled
ambient humidity. During I&T, MEMS devices with sliding and rotational motion
may experience wear since speeds can approach 1 million rpm in the devices.
According to study results from Sandia National Laboratory, the RH is critical for
proper operations of MEMS devices. Low humidity may increase resistance and
wear of MEMS devices, while high humidity may cause corrosion, wear, and
stiction. The ideal range appears to be somewhere between 20 and 60% for the
I&T of MEMS devices. However, specific RH requirements may depend on distinct
MEMS hardware design and applications.
As stated in Table 13.2, considerable amounts of contaminants may be
generated during launch and on-orbit operations. Microscopic particles can dislodge
or even form during these operations. To prevent contaminants, materials with
a less potential of generating particles should be chosen for fabricating MEMS
devices. Besides particles, material outgassing as a major contamination source is
also a well-recognized fact. Outgassed contaminants are greatly promoted by the
space environments of high vacuum and elevated temperatures. On-orbit degrad-
ation due to contamination can truncate the mission lifetime and degrade data
quality. These degradations may include long-term changes in the optical surfaces

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