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Residual Stress Dependency on Wafer Location of Thin Film PECVD Silicon Nitride

J.R. Godin, S.-H.P. Won P.M. Nieva, L.N. Phong
University of Waterloo, CA

Keywords: thin film silicon nitride, residual stress, bent beam strain sensors, wafer level characterization

Abstract:

The residual stress of use plasma enhanced chemical vapor deposition (PECVD) silicon nitride has been investigated in the past. The majority of these studies utilize bulk stress measurements through wafer curvature, assuming that the residual stress of PECVD silicon nitride is constant. We demonstrate that the variations of residual stress of nanometer thick PECVD silicon nitride films are significant across the wafer and in addition, are greatly affected by post-deposition etching processes. Bent-beam strain gauges were uniformly distributed over the entire wafer to analyze the stress distribution. The deflections of the strain gauges were analytically modeled to extract the stress values, and the results were verified with a FEM model. Measurements of residual stress calculated prior to the patterning and release of the bent-beam strain gauges ranged between ±10MPa, but stresses measured at different locations on a quarter of a wafer using bent-beam strain gauges indicated a mean stress of the wafer of about 69.04 MPa with a standard deviation of 60.89 MPa, which indicates that significant contribution of residual stress is induced by the etching process rather than the deposition process. Finally, we show that the residual stress decreases as the distance from the wafer center increases.
 
 
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