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332 MEMS and Microstructures in Aerospace Applications

Contamination from outgassing may bind to other materials present in the
environment, leading to clogging or build up of material. Depending on the
configuration, the device may become inoperable. Contamination binding with
other materials or allowing a build up have been found to cause device failure
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when in crucial active areas.
15.4.2 STICTION
With their small dimensions, MEMS structures are dominated by surface forces,
especially the van der Waals force, that cause microscopic structures to stick
together. Van der Waals forces bonding two clean surfaces together are a result
of instantaneous dipole moments of atoms. If two flat parallel surfaces become
separated by less than a characteristic distance of z 0 , which is approximately 20 nm,
the attractive pressure will be given by:
A
P vdW ¼ (15:1)
6pd 2

where
A is Hamaker constant (1.6 eV for Si) and
d the separation between the surfaces.

While this equation ignores the repulsive part of the surface forces and over-
estimates the force of adhesion by at least a factor of two, it allows for an order of
approximations for adhesive forces. Typical values of d are in the order of several
angstroms. 2,22 As soon as a flexible structure comes close enough to another surface
so that this force is stronger than the elastic force retracting the structure, the two
surfaces will almost permanently stick together.
The probability of stiction occuring may be reduced with designs where surfaces
that can contact other surfaces are minimized, for example, by using small dimples
which hold the structures at a distance. Forces that can cause stiction in MEMS
devices are capillary force and electrostatic force. Causes of stiction also include
shock-induced stiction (mechanical overstress) and voltage overstress, which can
both result in large areas in contact and allow stiction to occur. 21
Processing techniques, such as critical drying after the release, may reduce the
potential for stiction to occur as a result of the capillary forces. The ability to
successfully release a MEMS device is a critical processing step of a MEMS device.
Due to the inherent proximity of the moving structure and the surrounding surfaces,
the final drying process on a surface micromachined polysilicon structure can lead
to permanent stiction of the structure dependent upon the various drying techniques
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employed. Stiction induced by capillary forces during the postrelease drying step
of MEMS fabrication can substantially limit the functional yield of complex
devices. Supercritical CO 2 drying provides a method to remove liquid from the
device surface without creating a liquid or vapor interface, thereby mitigating
stiction. 24 Fluoro- or hydrocarbon coatings can be used on MEMS surfaces after
they are released to lower the surface interaction energy and prevent stiction during

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