Atmospheric corrosion of chemical vapor deposited tungsten films.

The performance and reliability of silicon-based microelectromechanical (MEMS) devices is dictated in part by the wear characteristics of the contacting surfaces. Chemical processes at the polycrystalline Si surfaces can dramatically change the static and dynamic coefficients of friction as well as the actual contacting surface area (asperity shape and areal density). Increased friction and wear can lead to particle production and eventually device failure. Closely spaced surfaces in a MEMS device represent potential occluded volume regions where capillary condensation of moisture becomes likely at higher relative humidity. In an effort to address these concerns, numerous materials have been proposed and studied as potential candidates for improving wear resistance in these devices. Tungsten appears as a particularly attractive coating because of its hardness and an ability to deposit very thin films in non-line-of-sight geometries using low temperature chemical vapor deposition and atomic layer deposition techniques (2,3). The disadvantage to using tungsten is that it is not a highly stable metal when exposed to water and oxygen at near neutral pH resulting in soluble tungstate (WO4 ) formation. In addition, little is known about the extent to which nanocrystallinity contributes to an atmospheric corrosion process in W. In this paper, we report on our attempts to characterize the changing morphology and composition of CVD W thin film surfaces subjected to a range of controlled humidity levels at room temperature.