Alkene Based Monolayer Films as Anti-stiction Coatings for Polysilicon Mems

This paper describes a new class of anti-stiction coatings for silicon MEMS that is based on the free radical reaction of a primary alkene (e.g., 1-octadecene, C16H33CH=CH2) with hydrogen terminated silicon [1,2]. The new monolayer coating has several key advantages over the previously reported octadecyltrichlorosilane (OTS) and 1H,1H,2H,2Hperfluorodecyltrichlorosilane (FDTS) based self-assembled monolayers (SAMs) [3,4]: 1) The coating does not produce HCl at any stage in the monolayer formation whereas chlorosilane-based chemistry does. 2) The coating does not require the formation of an intervening oxide layer. 3) The film formation procedure for alkene-based monolayers is simpler than for chlorosilane-based SAMs for two main reasons. First, the surface oxidation step is eliminated. Second, the coating solution does not need to be conditioned before use, since water is not a reagent in this process. 4) The coating process is much more robust since it is essentially insensitive to relative humidity. 5) The coated surfaces have much fewer particulates in comparison to those coated with OTS. 6) The coating process can be made selective to coat only exposed silicon by generating radicals using a radical initiator. The coating is evaluated in several ways, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle analysis (CAA), work of adhesion by cantilever beam array technique and coefficient of static friction using a sidewall testing device. The octadecene film is compared to the OTS SAM with respect to anti-stiction properties. XPS data confirm the absence of oxygen in both freshly prepared samples and in samples aged for four months in laboratory ambient. Water and hexadecane contact angles, and work of adhesion data are similar to those of OTS. AFM shows that the samples which receive 1-octadecene films accumulate far fewer particles during processing than those which receive the OTS SAM treatment.