AVS 52nd International Symposium
    MEMS and NEMS Monday Sessions
       Session MN-MoM

Paper MN-MoM1
Mechanical Properties of Polysilicon Thin Films using Micromachined Membranes and a Design of Experiments Methodology

Monday, October 31, 2005, 8:20 am, Room 207

Session: Processing & Characterization of Materials for MEMS & NEMS
Presenter: C.A. Zorman, Case Western Reserve University
Authors: A.J. Fleischman, The Cleveland Clinic Foundation
A. Dubnisheva, The Cleveland Clinic Foundation
R.S. Butler, The Cleveland Clinic Foundation
R. Rosenblum, Case Western Reserve University
C.A. Zorman, Case Western Reserve University
S. Roy, The Cleveland Clinic Foundation
Correspondent: Click to Email
The Youngs modulus, residual stress, and burst pressure of 4 micron-thick polysilicon films were determined from the load-deflection characteristics of suspended membranes. Specimens were prepared by the deposition of undoped amorphous Si films onto Si3N4 coated, Si wafers by LPCVD. The wafers (called Poly2) were annealed at 1100C for 1 hour to crystallize the films and lower the residual stress. For roughly 50% of the samples (called Poly1), a second identical annealing step was performed to evaluate changes in mechanical properties when such films are used in multilayer devices. Membranes that were 1.2 x 1.2 mm2 in area were fabricated by KOH-based anisotropic etching, using the underlying nitride film as an etch stop. Each test chip was subjected to pressure cycling tests using an interferometric load-deflection setup developed specifically for membrane evaluation. Data were collected using two test chip mounting schemes: (1) epoxy mounting, and (2) mechanical slot screw clamping. We found that mechanical clamping was sufficient to achieve the desired results with the added benefit that the specimens could be demounted without damage. A design-of-experiments testing methodology and one way ANOVA analysis at the 95% confidence interval was employed to factor out environmental and testing setup variables associated with the measurement technique. We found the Poly1 samples had an average Youngs modulus of 163 GPa and a residual stress of 121 MPa, while those for Poly 2 were 141 GPa and 23 MPa. The membranes were also evaluated for burst pressure in a setup capable of pressurizing the membranes well above the lowest average burst pressure of 78 psi. SEM analysis was used to examine the membranes after bursting. We found that the membranes did not delaminate, but rather failed at sites near the membrane edges. The presentation will detail the testing and data analysis procedures, as well as the use of these data in designing MEMS structures subject to failure.