Mechanics and Tribology of MEMS Materials - prod.sandia.gov ...
Mechanics and Tribology of MEMS Materials - prod.sandia.gov ...
Mechanics and Tribology of MEMS Materials - prod.sandia.gov ...
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The increase in static friction for the ODTS sample heated in the presence <strong>of</strong> water vapor,<br />
measured using a <strong>MEMS</strong> device operating in air, shows that degradation <strong>of</strong> hydrophobicity for<br />
alkylsilane films can result in increased friction coefficient. Since the devices were operated in<br />
air, hydrophilic surfaces facilitate adsorption <strong>of</strong> water <strong>and</strong> meniscus formation at the asperity<br />
contacts. This increases capillary adhesion between the beam <strong>and</strong> the post. Capillary films can<br />
increase the normal contact force between the beam <strong>and</strong> post, <strong>and</strong> must also be sheared in order<br />
to slide the beam tangentially. Both effects result in an increase in the force necessary to move<br />
the beam, <strong>and</strong> thus an increase in the static friction coefficient.<br />
7.6 CONCLUSIONS<br />
ODTS <strong>and</strong> PFTS films on Si(100) <strong>and</strong> polycrystalline silicon <strong>MEMS</strong> devices have been<br />
exposed to radiation <strong>and</strong> thermal treatments in controlled environments. Neither film exhibited a<br />
decrease in hydrophobic character upon exposure to a dose <strong>of</strong> 500 krad from Co-60 radiation.<br />
However, PFTS films on Si(100) exhibited degradation after XPS analysis, where the equivalent<br />
dose was estimated to be several megarads.<br />
Although the radiation dose estimated for the samples during XPS analysis seems very<br />
high, it is important to note that the lower energy photons result in significant dose enhancement<br />
due to <strong>prod</strong>uction <strong>of</strong> photoelectrons in the near surface <strong>of</strong> the samples. Therefore, modest x-ray<br />
exposure <strong>of</strong> devices could result in significant doses at surfaces treated with alkylsilanes,<br />
potentially resulting in degradation <strong>of</strong> the films. Due to their increased sensitivity to radiolysis,<br />
fluorinated alkylsilane films would be expected to undergo more degradation than hydrocarbon<br />
films in radiation environments.<br />
ODTS films heated in air containing 4261 ppm water vapor exhibited significant decrease<br />
in water contact angle. This is in agreement with previous work showing increased degradation<br />
<strong>of</strong> ODTS in air compared to PFTS [7.4], although the water vapor concentration in the previous<br />
work was not reported. XPS analysis suggests that this degradation is accompanied by an<br />
increase in contributions to the C1s spectrum from C-O bonds. Hydrolysis <strong>of</strong> the ODTS<br />
molecule results in loss <strong>of</strong> carbon from the treated surface. Although water vapor can clearly<br />
cause degradation <strong>of</strong> ODTS films, this work has shown that heating for up to 60 minutes in<br />
nitrogen, <strong>and</strong> for 10 minutes in air containing less than 33 ppm H2O, caused no measurable<br />
changes in hydrophobicity <strong>of</strong> ODTS films. The PFTS films resisted degradation during heating<br />
in all <strong>of</strong> these environments.<br />
Changes in static friction coefficient for ODTS heated in water vapor were significant.<br />
The static friction coefficient increased by a factor <strong>of</strong> two compared to the as-deposited film.<br />
Such changes in the friction force that must be overcome to move <strong>MEMS</strong> structures could have<br />
significant impact on the reliability <strong>of</strong> devices exposed to elevated temperature in the presence <strong>of</strong><br />
water vapor. Additional work is needed to investigate the effects <strong>of</strong> lower concentrations <strong>of</strong> H2O<br />
on hydrolysis <strong>of</strong> ODTS, <strong>and</strong> the kinetics <strong>of</strong> degradation in low water vapor atmospheres.<br />
7.7 ACKNOWLEDGEMENTS<br />
The authors wish to thank Frere MacNamara in the <strong>Materials</strong> Characterization department<br />
for performing the XPS analyses, <strong>and</strong> Liz Sorroche in the Microsystem <strong>Materials</strong>, <strong>Tribology</strong> <strong>and</strong><br />
Technology department for assistance with fixturing to control the atmosphere during irradiation<br />
<strong>and</strong> thermal treatment. The authors also acknowledge the help <strong>of</strong> Specialty Glass Co.,<br />
Albuquerque, NM. S<strong>and</strong>ia is a multiprogram laboratory operated by S<strong>and</strong>ia Corporation, a<br />
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