AVS 50th International Symposium
    Microelectromechanical Systems (MEMS) Tuesday Sessions
       Session MM-TuM

Paper MM-TuM11
Pointwise Strain Mapping a Multilayer MEMS Mirror Using Synchrotron Radiation

Tuesday, November 4, 2003, 11:40 am, Room 320

Session: Development and Characterization of MEMS and NEMS Materials
Presenter: Y.N. Picard, University of Michigan, Ann Arbor
Authors: Y.N. Picard, University of Michigan, Ann Arbor
S.M. Yalisove, University of Michigan, Ann Arbor
E. Dufresne, University of Michigan, Ann Arbor
C. Cionca, University of Michigan, Ann Arbor
J. Guzman, University of Michigan, Ann Arbor
R. Clarke, University of Michigan, Ann Arbor
D. Walko, Argonne National Laboratory
O.B. Spahn, Sandia National Laboratories
D.P. Adams, Sandia National Laboratories
Correspondent: Click to Email
Precise control over surface curvature of micromirror devices is critical for developing communications and power delivery applications. Furthermore, the control of this curvature must be maintained over time and in a range of operating conditions. Curvature control ultimately requires understanding of how stress in reflective coatings and thermal-stress compensation layers affect the ultimate performance of a variety of micromirror designs (different geometries, thickness, clamping arrangements, etc). Highly localized, non-destructive strain measurement techniques are required to assess variations in stress across and through micromirror coating layers on the actual device. We present results of strain mapping across a metal-coated polysilicon micromirror using a micron-sized x-ray beam at the Advanced Photon Source. Prior to x-ray analysis, a high reflectivity, low stress film of 10 nm Ti/150 nm Au was deposited by DC planar magnetron sputtering on a 2.25 micron thick, 500 micron diameter polysilicon mirror that had been etch-released prior to film deposition. A 10keV x-ray beam was focused down to a 5.3x12.8 micron spot size using two bendable Kirkpatrick-Baez mirrors and then used for point-by-point detection of Au and Si diffraction peaks. The peak positions were then measured and used to determine strain in the respective thin film after comparison to a standard powder sample. Because the freestanding micromirror was still clamped to the substrate, variations in strain were anticipated and indeed detected. Results of measured in-plane and out-of-plane strain for both the Au film and the polysilicon mirror will be presented, where an up to 23% variation in strain is detected from the center to the constrained edges of the micromirror. We also discuss strain resolution by this method and estimate that 3-5 MPa of stress can be resolved point-to-point within each material layer.