Optimal design of multilayered polysilicon films for prescribed curvature

Abstract

LPCVD polysilicon films exhibit tensile or compressive residual stresses and stress gradients, depending on deposition temperature. The stresses, which are a result of an “intrinsic” growth eigenstrain, ε^g, produce undesired curvatures in released structures. A combined experimental/computational design procedure is presented for controlling the curvature of thin films comprised of tensile layers, deposited at 570°C, alternated with compressive layers deposited at 615°C. Experimental measurements are first used to calculate the through-the-thickness variation of ε^g for both temperatures. This information is in turn incorporated into a mechanical model that predicts, for prescribed parameters that define the geometry, elastic moduli, and eigenstrain distribution, the stress distribution before release, and the curvature upon release, of multilayered films. Comparisons of predicted and measured average stress before release of a number of films, and of the curvature upon release of a circular micromirror device, provide a preliminary assessment of the semi-empirical model, which when combined with optimization algorithms can be used to develop recipes (thicknesses of the individual layers of a multiplayer device) that will achieve prescribed curvature according to given constraints.