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           COLLOQUIUM OF THE COMPUTATIONAL MATERIALS SCIENCE CENTER
                              College of Science
                       (CDS Department CSI 898-Sec 001)
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    Rough surface adhesion in dry and wet environments

                    Frank del Rio

          Materials Science and Engineering Laboratory
National Institute of Standards and Technology, Gaithersburg, MD

Microelectromechanical systems (MEMS) are currently used in industrial
applications such as accelerometers, gyroscopes, pressure sensors, and
digital micromirror devices as a result of inherent performance enhancements
and manufacturing cost reductions.  One of the major hurdles preventing a
larger number of MEMS-based products from entering the mainstream is
unwanted adhesion, commonly called stiction, which prevents relative motion
between structures.  MEMS are particularly vulnerable to adhesion as a
result of the large surface-to-volume ratio, small surface separations, and
highly compliant components.  The purpose of this work is to understand the
role of the various interfacial forces via microcantilever experiments as a
function of surface roughness and relative humidity (RH) and independent
calculations using the measured surface topography.

In dry ambients, van der Waals dispersion forces are the dominant adhesion
mechanism.  While the average surface separation Dave is governed by the
contacting (highest) summits, the adhesion is mainly due to van der Waals
dispersion forces acting across extensive non-contacting areas and is
related to 1/Dave2.  In wet ambients, capillary condensation of water has a
significant effect on rough surface adhesion.  Above a threshold RH, which
is a function of the surface roughness, the adhesion jumps due to meniscus
formation at the interface and increases rapidly towards the upper limit of
144 mJ/m2.  A detailed model based on the measured surface topography
qualitatively agrees with the experimental data only when the topographic
correlations between the upper and lower surfaces are considered.

In addition to van der Waals and capillary attractions, particulates can
also strongly influence the interfacial adhesion between rough surfaces by
changing their average separation.  Above a threshold density, the particles
introduce a topography that is more significant than the intrinsic surface
roughness.  As a result, the interfacial separation is governed by the
particle size and the adhesion is lower but stochastic in nature.  Based on
the composition and mechanical properties, we determined that the particles
on our micromachined surfaces are silicon carbide (SiC).  High temperature
annealing in the fabrication process allows residual carbon in the
sacrificial oxide layer to migrate to the polysilicon surface and form the
SiC particles.

                        Monday, February 2, 2009
                                  4:30 pm
                     Room 301, Research I, Fairfax Campus

Refreshments will be served at 4:15 PM.
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Find the schedule at www.cmasc.gmu.edu/seminar/schedule.html
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