AABSTRACTINTRODUCTIONEXPERIMENTALRESULTS AND DISCUSSIONCONCLUSIONS AND FUTURE WORKREFERENCESALKENE BASED MONOLAYER FILMS AS ANTI-STICTION COATINGS FORPOLYSILICON MEMSW. Robert Ashurst, Christina Yau, Carlo Carraro, Roger T. Howe*, and Roya MaboudianBerkeley Sensor & Actuator CenterDepartment of Chemical Engineering* Departments of Electrical Engineering and Computer Sciences and Mechanical EngineeringUniversity of California at Berkeley, Berkeley CA 94720-1492 USAtel (510) 643-3489, fax (510) 642-4778, [email protected] paper describes a new class of anti-stiction coatings forsilicon MEMS that is based on the free radical reaction of aprimary alkene (e.g., 1-octadecene, C16H33CH=CH2) withhydrogen terminated silicon [1,2]. The new monolayer coating hasseveral key advantages over the previously reportedoctadecyltrichlorosilane (OTS) and 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) based self-assembledmonolayers (SAMs) [3,4]: 1) The coating does not produce HCl atany stage in the monolayer formation whereas chlorosilane-basedchemistry does. 2) The coating does not require the formation ofan intervening oxide layer. 3) The film formation procedure foralkene-based monolayers is simpler than for chlorosilane-basedSAMs for two main reasons. First, the surface oxidation step iseliminated. Second, the coating solution does not need to beconditioned before use, since water is not a reagent in this process.4) The coating process is much more robust since it is essentiallyinsensitive to relative humidity. 5) The coated surfaces have muchfewer particulates in comparison to those coated with OTS. 6) Thecoating process can be made selective to coat only exposed siliconby generating radicals using a radical initiator.The coating is evaluated in several ways, including X-rayphotoelectron spectroscopy (XPS), atomic force microscopy(AFM), contact angle analysis (CAA), work of adhesion bycantilever beam array technique and coefficient of static frictionusing a sidewall testing device. The octadecene film is comparedto the OTS SAM with respect to anti-stiction properties. XPS dataconfirm the absence of oxygen in both freshly prepared samplesand in samples aged for four months in laboratory ambient. Waterand hexadecane contact angles, and work of adhesion data aresimilar to those of OTS. AFM shows that the samples whichreceive 1-octadecene films accumulate far fewer particles duringprocessing than those which receive the OTS SAM treatment.INTRODUCTIONThe large surface-area-to-volume ratios of surface and bulkmicromachined micromechanisms bring the role of stiction into theforeground, as adhesion of these mechanisms to adjacent surfacesis a major failure mode for MEMS [5,6]. Stiction is a term that hasbeen applied to the unintentional adhesion of compliantmicrostructure surfaces when restoring forces are unable toovercome interfacial forces such as capillary, van der Waals andelectrostatic attractions. Release stiction, the adhesion of surface-micromachined structures to the underlying substrate following thefinal sacrificial layer etch, is caused primarily by liquid capillaryforces. Engineering solutions to this problem include a variety oftechniques which have been reviewed elsewhere [5-7]. Thesetechniques, however, do not prevent adhesion from occurringduring micromachine operation. Microstructure surfaces maycome into contact unintentionally through acceleration orelectrostatic forces, or intentionally in applications where surfacesimpact or shear against each other. When adhesive attractionsexceed restoring forces, surfaces permanently adhere to each othercausing device failure -- a phenomenon known as in-use stiction.In addition, it is known that on the microscale, friction is stronglydependent upon adhesion [8].In order to alleviate these adhesion-related problems, both thetopography and the chemical composition of the contactingsurfaces must be controlled [5,6]. Several approaches have beendeveloped to address these tribological problems and have beenreviewed elsewhere. Our group has investigated both OTS andFDTS SAMs for alleviating adhesion in polycrystalline siliconmicrostructures. These SAMs have been shown to effectivelyeliminate release stiction, and significantly reduce in-use stiction,friction, and to some extent wear [9-11].Although these SAM coatings have been shown to effectivelyalleviate both release and in use stiction, they possess a number oflimitations intrinsically related to their chemistry. As with anychlorosilane-based SAM precursor molecule, the first step in thereaction sequence for binding the molecule to the substrate is thehydrolysis of one or more Si-Cl bonds. This hydrolysis thusresults in one equivalent of HCl for each Si-Cl bond that ishydrolyzed. The presence of HCl in the SAM coating solutionposes a threat to exposed aluminum interconnects, as well as othermetals or metal compounds that may be present. Additionally,silicon surfaces must first be oxidized in order for any chlorosilaneSAM to form. This is usually accomplished by growing a thinchemical oxide (~20 Å) with an oxidizing agent such as hydrogenperoxide. This oxidation step further complicates the coatingprocess and poses a barrier for any strategy to achieve selectivecoatings. Another limitation arises from the ability of theprecursor molecule to polymerize. As long as the precursormolecule has a functionality greater than one, bulk polymerizationcan occur, producing particulates that are up to several microns indiameter. These particulates can mechanically interfere with thedevice operation and pose reliability concerns. Unfortunately,there is no satisfactory method for removal of the polymerizedclusters once they have agglomerated on the surfaces of thesubstrate or micromachines.Other limitations of the chlorosilane SAM coatings arise fromthe coating procedure. The coating process is somewhatcumbersome in that the SAM solution must be freshly made andappropriately conditioned immediately before each coating. Thisis due to the sensitivity of the SAM solution to ambient humidity,and the ability of the SAM precursors to polymerize. Asmentioned earlier, a hydrolysis step is required for SAMformation. The water, which is a reagent, necessary for this step isintroduced to the organic SAM solution by mass transfer from theambient air. Hence it is necessary to control the ambient humiditysuch that there will be enough water present in solution to