Mechanics and Tribology of MEMS Materials - prod.sandia.gov ...

9 Conclusions and Recommendations
The Weibull strength distribution of SMM polysilicon was evaluated for each of the five
structural silicon layers in the SUMMiT TM process over a wide range of stressed surface areas
(200 to 20,000 µm 2 ), thereby revealing the previously elusive Weibull size effect, as well as a
strong layer-dependence. The layer dependence, thought to be at least partially due to
microstructural variations, was consistent with a recently developed polycrystal elasticity model
based on EBSD orientation maps of SUMMiT TM crystallography. New techniques were
developed and applied to characterize the role of temperature and process steps (i.e. sacox cuts)
on the observed strength behavior. Finally, a next generation of MEMS test structures, including
compact tension fracture toughness structures and bend structures were fabricated and evaluated.
Surface micromachined (SMM) structures that contain isolated tribological contacts have
been used to investigate interface performance of alkylsilane monolayers after storage in inert
environments, and after exposure to a variety of thermal and radiation environments. Results
show that both octadecyltrichlorosilane (ODTS) and perfluorodecyltrichlorosilane (PFTS)
exhibit little change in hydrophobicity or friction after Co-60 radiation exposures at a total dose
of up to 500 krad. However, exposure to temperature cycles consistent with packaging
technologies, in the presence of low levels of water vapor, produces degradation of
hydrophobicity and increase in static friction for ODTS films while producing no significant
degradation in PFTS films. In addition, a range of new friction measurement tools for SMM
structures have been developed. These include devices that permit a wider range of applied
forces to be generated between contacting surfaces, and structures where the contacts may be
latched into position to retain load without application of power. The latter are important
structures for examining the effects of aging on the interface behavior of contacts under load.
New methodologies have been developed to study wear and subsurface deformation in
electroplated metals for microsystems (LIGA). The methodology involves the application of
focused ion beam (FIB) techniques to prepare cross sections of wear tracks. Electron backscatter
diffraction (EBSD) analysis of wear scars on electroformed Ni revealed the formation two
subsurface zones, each with its own characteristic features which are different from that of the
bulk microstructure. Bending of columns in the direction of sliding, breakdown of columnar
grains into equiaxed fine grain structure, and formation of low angle grain boundaries have been
revealed. Formation of low-angle grain boundaries and spread in the orientation of pole figures
were also observed. This work has demonstrated that focused ion beam techniques are ideally
suited for preparing cross sections of shallow wear scars generated under low loads.
We have demonstrated the feasibility of characterizing adhesion between electroplated
LIGA materials using a commercial MTS Nanoindenter XP with a LIGA fabricated adhesion
probe tip in place of the standard Berkovich diamond indenter. It is well recognized that under
the action of surface forces, the surfaces are drawn together and a finite area of contact is
established for zero applied load. If an external load is applied, the contact area increases but on
reducing the load it decreases reversibly. When the applied external load is removed, an
additional load or a pull-off force is required to fully separate the two surfaces. Two widely
referred models, the Johnson-Kendall-Roberts (JKR) model and the Derjaguin-Muller-Toporov
(DMR) model, account for small but finite contact area at zero applied load in the presence of
surface forces. In our measurements, the nanoindenter was programmed to collect data in the
negative load regime until the surfaces were completely separated. LIGA Ni probe tips with a
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