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

6 Novel Techniques for Measurement of Adhesion in LIGA
Contacts
6.1 Introduction
Real surfaces--however flat they may be-- are rough on an atomic scale, and contact
occurs where the asperities of one surface touch the other. It is in these regions that adhesional
and tribological action takes place. When the size scale of structures approach microscopic
dimensions (~ 0.1 to 100 µm), adhesion between contacting surfaces becomes a critical issue. A
consequence of this is that adhesion becomes a major concern for the performance and reliability
of microelectromachnical systems, MEMS. Since the surface area to volume ratio of a
mechanical element varies as the reciprocal of its characteristic length, the surface area to
volume ratio in microsystems is several orders of magnitude larger than in macroscale
mechanical systems. Combined with the fact that available motive forces via electrostatics,
differential thermal expansion, fluid pressure, etc. are small, this leads to the result that adhesive
forces, which are typically uncontrolled and ignored in macrosystems, play a major role in the
behavior of microsystems. Further, due to the planar nature of these fabrication methods, forces
tend to be transmitted between structural elements in the plane of the device, leading to the result
that sliding interactions are predominantly between sidewalls. For example, see the gear train
and rack mechanism, assembled from parts fabricated by LIGA, in Fig. 6.1. LIGA processing
produces unique sidewall morphologies [6.1,6.2], and the existing analytical models (Derjaguin-
Muller-Toporov, DMT and Johnson-Kendall-Roberts, JKR) do not account for the effect of
unique morphology found on LIGA sidewalls on adhesion. The objective of this study was to
develop an experimental technique to measure adhesive pull-off forces between LIGA fabricated
structures in micro- and milli-newton load regimes.
Fig. 6.1. A fully assembled LIGA gear train and track, showing several sidewall-to-sidewall
sliding contacts. The aluminum substrate was machined conventionally to accept pressfit
steel gage pins on which the keyed bushings and gears were assembled.
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