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

captured the Si2p, C1s, O1s and W4f peaks. Si, O and W 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.
8.2.3 Tribological Measurements of Tungsten-Coated Surfaces
The dynamic friction coefficient of tungsten coated devices was determined using the
MEMS devices shown in Figure 7.1. All measurements were conducted in laboratory ambient
atmosphere (23°C, 20% 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. A square wave
is applied to the oscillation axis to drive the beam back and forth against the post at some
amplitude. A frame grabber is triggered at each end of the stroke of the beam, and images of the
relative position of the movable beam with respect to the post are acquired. Image processing
was used to calculate the displacement of the beam as a function of voltage applied to the
oscillation actuator. The amplitude of the beams motion when in contact with the post,
compared to the motion amplitude in the absence of contact, allows the friction force at the
beam/post interface to be calculated as described by Senft and Dugger [8.5], using the lateral
stiffness of springs in the friction structure and the calibration parameters for displacement
versus voltage.
8.3 Results and Discussion
A polycrystalline structure treated with selective tungsten is shown in Figure 8.1. The
tungsten forms a thin layer that covers the polycrystalline silicon conformally, even in shadowed
surfaces. Transmission electron microscopy revealed that the coating process forms a layer of
tungsten that is self-limiting, and on the order of 20 nm thick [8.4].
Si
polySi
Fig. 8.1 Cross section of a polycrystalline silicon (polySi) anchor on single crystal Si, treated
with selective tungsten. The silicon has been etched back to reveal the thin layer of
tungsten covering all surfaces.
76
W
1 µm