AVS 49th International Symposium
    Microelectromechanical Systems (MEMS) Wednesday Sessions
       Session MM-WeP

Paper MM-WeP8
Micromirror Coatings with Low-stress, High Reflectivity

Wednesday, November 6, 2002, 11:00 am, Room Exhibit Hall B2

Session: Poster Session
Presenter: Y.N. Picard, University of Michigan--Ann Arbor
Authors: Y.N. Picard, University of Michigan--Ann Arbor
D.P. Adams, Sandia National Laboratories
O.B. Spahn, Sandia National Laboratories
S.M. Yalisove, University of Michigan--Ann Arbor
D.J. Dagel, Sandia National Laboratories
Correspondent: Click to Email
While thin film coatings can greatly improve the reflectivity of micromirrors used in optical MEMS devices, such coatings can yield a moderate compressive or tensile stress, leading to a significant change in the micromirror curvature. This work seeks to develop highly reflective optical coatings exhibiting near-zero average film stress and minimal through-thickness stress. In this study, multilayer thin films consisting of Cr, Ti, Au, Si and Si@sub 3@N@sub 4@ are deposited on blank Si (100) substrates using DC planar magnetron sputtering. Au is greater than 90% reflective over a range of infrared wavelengths, and either Cr or Ti can be employed as an adhesion promoter between Au and Si. The residual stress of multilayer films is determined through curvature based measurements using laser-scanning and applying Stoneys equation. The influence of sputter gas pressure and deposition rate on residual film stress is assessed for a variety of multilayer systems. Also assessed is stress aging at room temperature over a period of one year. Using optical interferometry, we have already demonstrated that low stress Au/Ti films deposited on micromirrors 125-500 µm in size induce less than @lambda@/40 change in bow. This work seeks to extend these initial results by combining low stress Au/Ti films with Si@sub 3@N@sub 4@/Si Bragg reflectors to achieve a near-zero stress multilayer exhibiting both a ~99% reflectivity for a target wavelength of radiation while inducing minimal curvature changes when deposited on pre-released polysilicon micromirrors. Since thin film microstructural defects and interfacial roughness can contribute to optical absorption, examination of both is conducted using cross-sectional transmission electron microscopy. Surface roughness is also measured using atomic force microscopy. Spectral reflectivity of thin film coatings is measured using an optical spectrum analyzer.