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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8 Friction <strong>and</strong> Wear <strong>of</strong> Selective Tungsten Coatings for Surface<br />
Micromachined Silicon Devices<br />
8.1 Introduction<br />
Wear <strong>of</strong> silicon surfaces in microelectromechanical devices limits their use in applications<br />
requiring long life. Several surface modification strategies have been explored for reducing<br />
friction <strong>and</strong> wear in <strong>MEMS</strong> devices, including adsorbed organic molecules [8.1, 8.2], hard<br />
coatings <strong>and</strong> solid lubricants [8.2], <strong>and</strong> films deposited by atomic layer deposition [8.3].<br />
Selective tungsten has also been examined to treat surfaces <strong>of</strong> silicon <strong>MEMS</strong> devices [8.4], <strong>and</strong><br />
has the advantage that the basic process was developed in the 1980’s <strong>and</strong> is well understood,<br />
semiconductor fabrication equipment exists to deposit the coatings, <strong>and</strong> this process is<br />
compatible with most CMOS fabrication facilities. Prior work has focused on the deposition<br />
process for the coating applied to microsystems, adhesion <strong>and</strong> basic device functionality<br />
measurements determined from treated <strong>MEMS</strong> devices [8.4]. The purpose <strong>of</strong> this study was to<br />
examine the surface composition <strong>of</strong> selective tungsten films as a function <strong>of</strong> age after deposition,<br />
quantify the friction performance <strong>of</strong> this treatment in a <strong>MEMS</strong> sliding contact, <strong>and</strong> identify the<br />
surface species responsible for the observed friction <strong>and</strong> wear performance.<br />
8.2 Experimental Approach<br />
8.2.1 Treatment <strong>of</strong> SMM devices with selective tungsten<br />
Tungsten is deposted on silicon device surfaces using chemical vapor deposition. A WF6<br />
plasma is used, <strong>and</strong> silicon at the surface is replaced by tungsten according to one <strong>of</strong> the<br />
following reactions.<br />
2WF6 + 3Si ↔ 2W+ 3SiF4 ↑<br />
WF6 + 3Si ↔ W+ 3SiF2 ↑<br />
The reaction that proceeds is a function <strong>of</strong> the reaction temperatere where processing takes place.<br />
In order for the WF6 to react with the silicon surface, the silicon must be clean <strong>and</strong> free <strong>of</strong> any<br />
residual oxide. In order to develop structures for friction <strong>and</strong> wear measurements, silicon<br />
SUMMiT TM die were first released in HF:HCl, rinsed in water <strong>and</strong> then dried using supercritical<br />
CO2. This allows the silicon structural elements to be exposed from the sacrificial oxide with no<br />
surface treatment applied <strong>and</strong> without having structures collapse under meniscus forces during<br />
drying <strong>of</strong> the devices. Although this process results in a nominally uncoated surface, there are<br />
residual organic contaminants present from the CO2 process, <strong>and</strong> a natural oxide also forms<br />
during the post-release rinse <strong>and</strong> any time spent exposed to air. These materials must be<br />
removed prior to the selective tungsten process, since the reactions shown above will only<br />
proceed in the presence <strong>of</strong> clean silicon surface.<br />
8.2.2 Surface Chemical Analysis<br />
Surface chemical information was obtained using x-ray photoelectron spectroscopy<br />
(XPS). A PHI achromatic XPS system (Physical Electronics, Eden Prairie, MN) was used for<br />
analysis. Spectra were digitally acquired using an Al Kα x-ray source (1486.6 eV). A survey<br />
spectrum was collected on each sample, followed by detailed scans using analysis regions that<br />
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