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

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 C16H33CH=CH2) With hydrogen terminated silicon [1,2]. The new coating has several key advantages over the previou sly reported octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosila ne (FDTS) based self-assembled monolayers (SAM) [3,4]: (1) the coating does not produce HCl at any stage in the monolayer formation whereas chlorosila ne based chemistry does. (2) The coating does not require the formation of an intervening charge-trapping oxide layer. (3) The film formation procedur e for alkene based monolayers is simpler than for chlorosilane based SAMs f or two main reasons. First, the surface re-oxidation step is entirely elimi nated. 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 humidit y. (5) The coated structures have many fewer particulates in comparison to those coated with OTS. (6) The coating process can be made selective to coa t only exposed silicon by generating radicals using a radical initiator. The coating has been evaluated in several ways, including X-ray photoelectr on spectroscopy (XPS), atomic force microscopy (AFM), contact angle analysi s, work of adhesion by cantilever beam array technique and coefficient of s tatic friction using a sidewall testing device. The octadecene film is comp ared to the OTS SAM with respect to anti-stiction properties. XPS data conf irm the absence of oxygen in both freshly prepared samples and in samples a ged for more than 4 months in laboratory ambient. Water and hexadecane cont act angles, and work of adhesion data are similar to those of OTS. AFM show s 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 antistiction properti es of films produced with the alkene chemistry are indeed comparable to tho se produced with the chlorosilane SAMs, but without many of the limitations imposed by the chlorosilane chemistry. (C) 2001 Elsevier Science B.V. All rights reserved.