AVS 46th International Symposium
    The Science of Micro-Electro-Mechanical Systems Topical Conference Wednesday Sessions
       Session MM-WeA

Paper MM-WeA5
Adhesion Performance of Silane Coupling Agents at High Humidity Levels

Wednesday, October 27, 1999, 3:20 pm, Room 620

Session: Micro-Science and Tribology
Presenter: M.P. de Boer, Sandia National Laboratories
Authors: M.P. de Boer, Sandia National Laboratories
T.M. Mayer, Sandia National Laboratories
T.A. Michalske, Sandia National Laboratories
R.W. Carpick, Sandia National Laboratories
R. Maboudian, University of California, Berkeley
U. Srinivasan, University of California, Berkeley
Correspondent: Click to Email
We have measured the effect of humidity on autoadhesion of polycrystalline silicon cantilever beams fabricated by surface micromachining, and coated with silane coupling agents. To make the measurements, we designed and constructed an environmental microprobing station with interferometric capability, and automated the system to enable measurement of beam deflections in-situ. We quantified adhesion by applying a fracture mechanics equilibrium to each adhered beam. For both ODTS (C@sub 18@H@sub 37@SiCl@sub 3@) and FDTS (C@sub 8@F@sub 17@C@sub 2@H@sub 4@SiCl@sub 3@) coatings, the effect of relative humidity (RH) is negligible for RH up to approximately 80%. For ODTS coatings at 99% RH after a 40 hour exposure, adhesion increases only moderately by a factor of two. For FDTS coatings at 90% RH, adhesion increases dramatically by a factor of 100 after seven hours, with further subsequent increases at higher RH values. This is a surprising result, considering that FDTS has a higher contact angle with water than does ODTS, and exhibits lower adhesion at low RH. We believe that defect formation is responsible for the adhesion increase. To support this assertion, we conducted water absorption experiments and obtained atomic force microscopy images revealing agglomerated coupling agent on films exposed to high RH. ODTS is less susceptible to this mechanism than FDTS because of its greater chain length and smaller chain diameter. Our results contrast with experiments on fatty acid monolayers using the surface force apparatus, where uniform swelling of the film is responsible for a monotonic adhesion increase with RH.