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Design and Application of Space-Based MEMS 331

An approach in the same vein emphasizing process monitoring and quality
assurance methods has been applied to MEMS components. These methods include
techniques to study:

(1) Process bias
(2) Material microstructure and mechanical properties
(3) Mechanical response of spring-supported structures, and
(4) Actuator performance

Characterization of the as-produced components and materials serves as the
starting point for future studies of reliability of MEMS components and systems.
Extensive process monitoring is performed at every step. 13
In recent years, the POF approach has been used for new and emerging
technologies such as multichip modules for insertion into space flight applica-
tions. 14 The POF approach has been applied to MEMS reliability by representatives
from the French Space Agency, Centre National d’Etudes Spatiales 4,6 and at JPL,
Caltech 15 among others.

15.4 MEMS FAILURE MECHANISMS
MEMS reliability and failure mechanisms concerns for the space environment
include: material mismatches, fracture and fatigue, adhesion and stiction, friction
and wear, electrostatic interference, radiation damage, and thermal effects.

15.4.1 MATERIAL INCOMPATIBILITIES

Process incompatibilities, materials issues, and fabrication limitations still present
formidable challenges to any practical commercialization of most developmental
16
microsystems. Processing may induce thermomechanical stresses due to mismatch
of the coefficients of thermal expansion (CTE) of the base material. Due to the unique
structure and small scale of MEMS, residual stresses during the deposition processes
can have a profound effect on the functionality of the structures. Typically, material
properties of thin films used in surface micromachining are not controlled during
deposition. The residual stress, for example, tends to vary significantly for different
deposition methods. Currently, few techniques are available to measure the residual
stress in MEMS devices. Differences in stress between the multiple metal and
dielectric layers may cause vertical stress gradients and curl. Additionally, misalign-
ment between layers may cause lateral stress gradients and curl. This curl that may
also be induced through stresses in plating and postprocessing yields, an effect often
termed ‘‘potato chipping.’’
At all steps, the concerns of material compatibilities will need to be addressed.
During packaging of MEMS, stresses will be distributed within the die attach, with
die and substrate contributing to the reliability or lack of reliability of the packaging
structure. Numerous studies in the literature are available relative to the potential of
decreasing or inducing stress during processing steps. 17–20

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