Alkene based monolayer films as anti-stiction coatings for polysilicon MEMS

Abstract

This paper describes a new class of anti-stiction coatings for polysilicon MEMS. This class of molecular film is based on the free radical reaction of a primary alkene (e.g. 1-octadecene C 16H 33CHCH 2) with hydrogen terminated silicon [1,2]. The new coating has several key advantages over the previously reported octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosilane (FDTS) based self-assembled monolayers (SAM) [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 charge-trapping 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 re-oxidation step is entirely 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 structures have many 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 has been evaluated in several ways, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle analysis, 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 more than 4 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. Based upon the data presented here, we find that the anti-stiction properties of films produced with the alkene chemistry are indeed comparable to those produced with the chlorosilane SAMs, but without many of the limitations imposed by the chlorosilane chemistry.