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

governed by electrochemistry, the sidewall morphology appears to be controlled by the PMMA
mold. Factors such as molecular weight of the PMMA, residual stresses, and x-ray exposure
conditions are believed to be the major factors influencing the sidewall morphology of LIGA
fabricated parts. At high enough local stresses, the asperities on the sidewalls would deform
plastically during the initial run-in period. However, at low contact pressures, mechanical
interlocking of asperities on the sidewalls of surfaces described here would have a significant
influence on the adhesion of LIGA microsystems.
6.4 LIGA Adhesion Probe Tips and Pull-Off Force Measurements
The first task was to design an adhesion probe tip suitable to perform adhesive pull-off
measurements with commercial nanomechanical testers. The probe tips were made by LIGA
processing so that the sidewall morphology of the probe closely mimics the sidewall morphology
of real LIGA parts. A tip with a radius of curvature of 500 µm is shown in Fig. 6.4.
Fig. 6.4. SEM of a LIGA Ni adhesion probe tip.
An x-ray mask with a two dimensional array of probe tips was first prepared. Poly
methylmethacrylate was used as the mold material, and synchrotron radiation was used to expose
PMMA and create the LIGA molds. Mold filling in this particular case was performed by
conventional electrodepostion of nickel from sulfamate baths. Probe tips of different geometries
and from other electroplatable metals or alloys can be prepared.
A fixture was designed and fabricated to attaché the probe tip (Fig. 6.4) to a commercial
MTS Nanoindenter XP unit in place of the standard Berkovich diamond indenter. The probe
was brought into contact with planar surface of a metallographically polished Ni disk. The disk
was also prepared by electroplating Ni into 10 mm x 10 mm square molds. The nanoindenter
was programmed to collect data in the negative load in the negative load regime until the
surfaces are completely separated during the unloading cycle. A typical load-displacement data
for the contact of the Ni probe tip on metallographically polished Ni surface is given in Fig. 6.5.
The negative load or pull-off force can be used to guide and validate analytical models to predict
adhesion between LIGA fabricated parts.
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