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312 MEMS and Microstructures in Aerospace Applications

selecting MEMS materials including mission duration, thermal swings, radiation,
acceleration, vibration, shocks, and locations such as low-Earth orbit (LEO), geo-
stationary orbit (GEO), deep space, or exploratory planetary environments.

14.4 MATERIAL FAILURES
This section describes some common MEMS material failure mechanisms, but the
coverage is limited to the issues specific to the space environment. The reader is
referred to the classic text books for material failure issues due to device fabrication
and standard operation. 7,8

14.4.1 STICTION
Stiction occurs when attractive surface forces cause MEMS components that touch
to stick together. This adhesion can be caused by capillary forces, van der Waals
forces, hydrogen bonding, or electrostatic forces. Stiction is one of the greatest
reason for the failure of MEMS devices. MEMS structures are typically very
smooth and polished causing two surfaces that touch to have many points or surface
area in contact and thus generate large adhesive forces. Rough surfaces have less
surface area in contact and thus lower adhesive forces. Some design solutions that
can help mitigate surface interactions are:

. The addition of bumps or dimples to structures. These surfaces prevent the
whole structure from making contact with the substrate, lessening adhesive
forces.
. The use of self-assembled monolayer (SAM) coatings to reduce surface
9
adhesion. This surface modification uses chemicals to coat the surface
(covalent bonding of monolayers) and change its surface properties. Silox-
ane-based chemistries are used for coating silicon surfaces and thiol chemis-
tries are used for gold.
. The deposit of diamond like carbon (DLC) films. Diamond is hydrophobic
and will prevent capillary forces from causing stiction. 10
. The use of flourinated polymer coatings. 11
. The use of ammonium fluoride coatings. 12
. The use of stiff materials for suspended structures.
. The roughening of contact surfaces. 13
. The use of hermetically sealed packages with getters to prevent stiction due to
humidity. 14
. The use of leaky dielectrics in RF MEMS devices will prevent dielectric
charging. 15

14.4.2 DELAMINATION
Delamination occurs when a material interface loses its adhesive bond.
It can be the result of fatigue, induced by the long-term cycling of structures

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