AVS 45th International Symposium
    The Science of Micro-Electro-Mechanical Systems Topical Conference Tuesday Sessions
       Session MM+NS+SS-TuM

Paper MM+NS+SS-TuM9
Adhesion Hysteresis of Polysilicon Beams in Controlled Humidity Ambients

Tuesday, November 3, 1998, 11:00 am, Room 324/325

Session: Micro-Science and Tribology of MEMS
Presenter: T.A. Michalske, Sandia National Laboratories
Authors: M.P. de Boer, Sandia National Laboratories
T.A. Michalske, Sandia National Laboratories
M.R. Tabbara, Sandia National Laboratories
R. Maboudian, University of California, Berkeley
T.M. Mayer, Sandia National Laboratories
Correspondent: Click to Email
Auto-adhesion, or spontaneous sticking between MEMS structures, is currently a major limitation in bringing this new class of engineering devices to the broader market. MEMS are particularly susceptible to auto-adhesion because the structural members: 1) are constructed in close proximity to each other, 2) are highly compliant due to their extreme length to thickness aspect ratio and, 3) have large surface to volume ratios which increase the relative importance of adhesive surface forces. If the miniature structural members are brought together by surface (capillary, electrostatic) or inertial (shock, rapid air flow) forces, they may remain adhered after the external force is removed. If the structures remain adhered, bonding may increase over time, giving rise to the phenomena known as adhesion hysteresis. In this work we develop mechanical analysis for and report on measurements of adhesion hysteresis in surface micromachined polysilicon beams subject to dry and wet ambients. The electrostatically activated beams used in this study were tested directly after supercritical drying or after the application of hydrophobic molecular coatings such as octadecyltrichlorosilane (ODTS) or perfluorodecyltrichlorosilane (FDTS). Results indicate that both uncoated and coated beams show strong increase in adhesion after an incubation period in humid environments. This incubation time is shorter and occurrs at lower RH for uncoated beams than coated beams. For the case of uncoated beams, we are able to show that a model based on individual asperity contact forces can be used to predict the overall adhesion behavior in micromachined beams. The behavior of coated beams is compared with ellipsometric measurements indicating water adsorption on these nominaly hydrophobic surfaces after extended exposure at high RH conditions.