Page 348 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c015 Final Proof page 341 1.9.2005 12:53pm

Design and Application of Space-Based MEMS 341

TABLE 15.3
Environmental Effects and the Principal Failures Induced on MEMS Devices —
Continued
Environment Principal Effects Typical Failures Induced

Dissociated gases Chemical reactions Alteration of physical and
electrical properties
Contamination
Reduced dielectric strength Insulation breakdown and arc-
over
Acceleration Mechanical stress Structural collapse
Separation from substrate
Vibration Mechanical stress Loss of mechanical strength
Interference with function
Increased wear
Fatigue Structural collapse
Magnetic fields Induced magnetization Interference with function
Alteration of electrical properties
Induced heating

15.5.2 THERMAL EFFECTS
High temperatures impose a severe stress on most electronic items including
MEMS devices, since it can cause catastrophic failure. High temperature also
causes progressive deterioration of reliability due primarily to chemical degradation
effects. The nature of MEMS design requires small sizes, often with high part
densities. This generally requires a cooling system to provide a path of low thermal
resistance from heat-producing elements to an ultimate heat sink of reasonably low
temperature. Reliability improvement techniques for high-temperature stress in-
clude the use of heat dissipation devices, cooling systems, thermal insulation, and
heat-withstanding materials.
Low temperatures experienced by MEMS can cause reliability problems. These
problems usually are associated with mechanical system elements. They include
mechanical stresses produced by differences in the coefficients of expansion (con-
traction) of metallic and nonmetallic materials, embrittlement of nonmetallic com-
ponents, mechanical forces caused by freezing of entrapped moisture, stiffening of
liquid constituents, etc. Typical examples include cracking, delaminations, binding
of mechanical linkages, and excessive viscosity of lubricants. Reliability improve-
ment techniques for low-temperature stress include the use of heating devices,
thermal insulation, and cold-withstanding materials.
Additional stresses are produced when MEMS are exposed to sudden changes
of temperature or rapidly changing thermal cycling conditions. These conditions
generate large internal mechanical stresses in structural elements, particularly when
dissimilar materials are involved. Effects of thermal shock-induced stresses include

© 2006 by Taylor & Francis Group, LLC