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

the C-C backbone of the PFTS molecule, or loss of fluorine from the molecule, disappearance of
the C-F3 peak might be expected, or changes in the relative intensities of the C-F2 and C-H
peaks.
Intensity (arb. units)
Fig. 7.8. Detailed XPS spectra for elements present in PFTS films, normalized to constant total
intensity by element.
7.4.5 Friction measurements
695 690 685 540 536 532 528 295 290 285 110 105 100 95
F1s (eV) O1s (eV) C1s (eV) Si2p (eV)
C-F3 C-F2
Static friction measurements were performed on ODTS films in the as-deposited condition
and after exposure to heating in the presence of water vapor, to examine the effects of decrease
in water contact angle on frictional behavior. The results are shown in Figure 7.9, plotted as
displacement as a function of the square of voltage applied to the actuator. For the electrostatic
comb actuators used in the sidewall friction device (Figure 7.1), the output force does not depend
on the length of engagement of the comb fingers, and the resulting displacement should be
proportional to the voltage squared [7.6]. Therefore, in the absence of friction losses the
displacement should vary linearly with V 2 , with an intercept of zero. Measurement of
displacement in the absence of frictional contact is in fact used to determine the proportionality
constant between displacement and V 2 during calibration of the electrostatic actuators, so that
forces can be estimated [7.6]. Any lag in the displacement of the actuator with V 2 when the
beam is in contact with the post is due to friction between the beam and the post. Figure 7.9
shows that both the as-deposited and exposed ODTS-coated friction devices exhibit some lag in
displacement with voltage. The voltage at which the device first slips can be used to calculate
the static friction coefficient that the actuator must overcome. In the case of as-deposited ODTS,
the static friction coefficient was 0.12, while for ODTS heated in 13% RH air, the static friction
coefficient increased to 0.23. After the beam slips, it will achieve a new position based on the
electrostatic force and the restoring force of springs in the actuator. As the voltage continues to
increase, the beam will slip again when the static friction is overcome. If the static friction
coefficient is high, this will be seen as a “stick-slip” motion of the beam, rather than smooth
sliding. This behavior can be seen in Figure 7.9, in the displacement of the device heated in
water vapor.
71
C-H
500 krad air
as-dep, post XPS
as-deposited