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Design and Application of Space-Based MEMS 333

operation. These coatings provide a hydrophobic surface on which water cannot
condense. Therefore, the most important stiction force by capillary condensation
will not occur. 25

15.4.3 CREEP
Reliability of components due to creep properties of materials is important to
structural integrity. Reliability of MEMS devices will greatly be affected by creep
of components that operate at high temperatures. The reliability will also suffer
when MEMS components are made of materials, which creep at room temperature.
Electrothermal microactuators, considered as the driver components for micromo-
tors, are examples of structures prone to creep deformation upon actuation. Add-
itionally, components made of polymers, such as polyimides, will undergo creep at
room temperature. 26 Creep behavior of all materials exposed to thermal cycling,
including solders and other attached materials should be reviewed.

15.4.4 FATIGUE
MEMS are often chosen for their long life and intrinsic strength. High cyclic fatigue
failure results tend to be impressive. Results from a research team at Pennsylvania
State University provide the most comprehensive, high-cycle, endurance data for
designers of polysilicon micromechanical components available to date. These
researchers evaluated the long-term durability properties of materials for MEMS.
The stress-life cyclic fatigue behavior of a 2-mm thick polycrystalline silicon film
was evaluated in laboratory air using an electrostatically actuated notched canti-
lever beam resonator. A total of 28 specimens were tested for failure under high-
frequency (40 kHz) cyclic loads with lives ranging from about 8 sec to 34 days
5 11
(3 10 to 1.2 10 cycles) over fully reversed, sinusoidal stress amplitudes
varying from 2.0 to 4.0 GPa. The thin-film polycrystalline silicon cantilever beams
exhibited a time-delayed failure that was accompanied by a continuous increase in
the compliance of the specimen. This apparent cyclic fatigue effect resulted in
9
endurance strength at greater than 10 cycles, similar to 2 GPa, that is, roughly one-
half of the (single cycle) fracture strength. Based on experimental and numerical
results, the fatigue process is attributed to a novel mechanism involving the
environmentally assisted cracking of the surface oxide film (termed reaction-layer
fatigue). 27 In silicon, a fatigue-like phenomenon has been observed, but it occurs
only at very high stress intensity levels, at which it is hardly a good idea to use
brittle materials anyway. On the other hand, sudden fracture due to a short ‘‘over-
load’’ condition below the yield strength is likely to destroy brittle materials
(containing small flaws), but not tough materials like metals, although the ultimate
fracture strength of a metal components of a MEMS structure may well be lower
than that of its brittle counterpart. 25 In accelerated life testing analysis, thermal
cycling is commonly treated as a low-cycle fatigue problem, using the inverse
power law relationship. Coffin and Manson suggested that the number of cycles-
to-failure of a metal subjected to thermal cycling is given by: 28

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