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

Fig. 7.2. MEMS die sitting on a flat ground in Pyrex rod, and this inside a Pyrex tube (a), and
these components inside a vial for radiation and thermal exposure in controlled
environments (b).
then calculated the contact angle on both sides of the drop. These two numbers were usually
identical, but in some cases differed by up to 2 degrees. The average of the two numbers was
recorded as the water contact angle for the sample. Two samples were measured for each
experimental condition. This technique was used as a primary screening tool, to identify
samples of interest for analysis using other techniques.
Surface chemical information was obtained using x-ray photoelectron spectroscopy
(XPS). A PHI achromatic XPS system (Physical Electronics, Eden Prairie, MN) was used for
analysis. Spectra were digitally acquired using an Al Kα x-ray source (1486.6 eV). A survey
spectrum was collected on each sample, followed by detailed scans using analysis regions that
captured the Si2p, C1s, O1s and F1s (for the PFTS) peaks. Si, O and F peaks were acquired at
0.2 eV/step, while C was acquired at 0.1 eV/step. After analysis, the spectra were smoothed
using a 19-point Savitsky-Golay algorithm, satellite subtraction was performed, and the spectra
were shifted to provide a constant Si2p3/2 peak position at 99.3 eV. Atomic concentrations were
calculated from the detailed scans using handbook sensitivity factors. For comparison of peak
shapes and offsets due to chemical changes in the films, the peaks for a given element were
normalized by peak area and plotted with a baseline intensity offset to stagger the peaks.
7.3.3 Friction measurements
(a) (b)
Static friction was determined using the MEMS devices shown in Figure 7.1. All
measurements were conducted in laboratory ambient atmosphere (23°C, 13% relative humidity)
using bare die and drive signals applied with probes making contact with electrical contact pads
on the device. A DC voltage was applied to the loading actuator to bring the beam into contact
with the post (Figure 7.1b). Normal load is estimated at 10 µN based on the applied voltage and
displacement of the beam. With the load applied, the voltage on the oscillation actuator was
slowly ramped up from zero while acquiring images of the beam via the probe station
microscope. In this configuration, the beam remains stationary against the post until sufficient
force is applied by the actuator to overcome the static friction between the beam and the post. At
this point, the beam slips. Image processing was used to calculate the displacement of the beam
as a function of voltage applied to the oscillation actuator. The force at which slip takes place
can be calculated as described by Senft and Dugger [7.6], using the lateral stiffness of springs in
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