Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 91

Materials for MEMS
The fact that the mechanical properties of structures of microdimensions can
be very different from those of analogous macrostructures has been recognized
for some time, but the real practical implications are just now being explored
comprehensively. For example, the Materials Research Society has devoted a
symposium to the topic of materials science for MEMS every year for 3 or 4
years. It stands to reason that if the mechanical structures themselves are on the
order of microns, then the grain structure of mechanical materials is very impor-
tant and must be understood and controlled precisely at the submicron scale in
order to optimize the MEMS mechanical function.
The magnitude and nature of built-in strain in the MEMS structures is ex-
tremely important to their practical applications. The grain size and deposition
and annealing history are among the important factors in determining residual
strain and the effect that this will have on the overall MEMS process. To date,
MEMS mechanical properties have been studied most thoroughly in polysilicon 177
and in the sacrificial layers most often used in the MEMS process flows, such as
silicon dioxide and silicon oxynitride films. 178 Also being investigated are the
effects of metal fatigue and anelastic creep on metal MEMS mechanical struc-
tures produced by various processes and involving such materials as Au, Ni, and
Ni alloys. Just as for macrosize metal structures discussed previously, the nano-
structure of metallic materials resulting from the deposition or forming process
used will clearly affect the mechanical properties of MEMS devices. However, in
the MEMS size world, these effects will dominate.
Also very important at the micron scale (but not usually on the macroscale)
are the effects of surface roughness and surface morphology on the mechanical
integrity and strength of MEMS devices. The surface character of the top, bot-
tom, and sidewalls of MEMS structures plays a profound role in mechanical
quality factors and other mechanical figures of merit. The control and optimiza-
tion of all the surfaces of relatively complex mechanical structures throughout the
fabrication and packaging processes have been among the most challenging of
the technical obstacles to MEMS developers to date. 179
Many types of materials have been investigated for MEMS applications,
including variously doped polysilicon, amorphous Si, single-crystal Si, SiC and
other Si alloys, III-V semiconductors, quartz, diamond, and various types of
metals and metal alloys. In a number of applications, the limited durability of the
MEMS mechanical structure is a severe issue for practical applications. Finding
harder, more durable materials than the pervasive silicon and polysilicon is one
avenue that is being pursued. Another is to find ways to apply hard coatings to
MEMS structures that permit them to withstand surface sliding and mechanical
contact. 180 The technical issues involved in hard coatings for MEMS are analo-
gous to those involved in depositing hard coatings on macroscopic structures, but
are even more intertwined with material science at the nanoscale.