Stress Generation and Relaxation during Film Heteroepitaxy on a Compliant Substrate with a Viscoelastic Glass Interlayer
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Stress Generation and Relaxation during Film Heteroepitaxy on a Compliant Substrate with a Viscoelastic Glass Interlayer Zhaohua Feng,1 Edward G. Lovell,1 Roxann L. Engelstad,1 Peter D. Moran,2 Thomas F. Kuech,3 Computational Mechanics Center, Mechanical Engineering Department 3 Department of Chemical Engineering University of Wisconsin, Madison, WI 53706, U.S.A. 2 Materials Science and Engineering Department Michigan Technological University, Houghton, MI 49931, U.S.A 1
ABSTRACT Lattice mismatch strain between films and substrates causes stresses in each and degrades the film quality. Compliant substrates can decrease the stresses and dislocation density in the film. A particular type of compliant substrate, which consists of a thin template, a handle wafer and a glass interlayer, is discussed here. Three-dimensional axisymmetric finite element models were developed to simulate the film-substrate structure and analyze stress generation and relaxation. The materials of film and template were considered as elastic but the glass interlayer was viscoelastic at the film growth temperature. Factors affecting stress generation and relaxation are reported. INTRODUCTION During semiconductor film heteroepitaxy on a conventional substrate, the lattice mismatch between the film and substrate can lead to large stresses in the film. The stresses generate a high-density of dislocations which adversely affects the film quality. If a film grows on a freestanding thin template, excessive stresses and high dislocation densities are generated in the template, but stresses in the film can be very low, so that a defect-free film could be produced [1, 2]. It is not easy, however, to realize this goal, since the freestanding thin template is mechanically unstable. In practice, a handle or support wafer and an interlayer bonding the template to the handle wafer are needed. The template, interlayer and handle wafer form a compliant substrate. An ideal interlayer should limit the unnecessary degrees of freedom of the template and keep it stable, but not decrease its deformation compliance. Glass with low viscoelastic characteristics at high temperature is a potential interlayer material. In order to understand how compliant substrates with a glass interlayer reduce defects in the film, transient stress analysis is required. Since it is very difficult to directly measure the film stresses when stress relaxation is in progress, numerical techniques were developed to simulate the filmcompliant substrate system and analyze the complete stress and strain fields during the entire film growth process. Influences of viscosity, geometry parameters and effective mismatch strain on stress generation and relaxation were investigated. SIMULATION MODELS The model of the compliant substrate was composed of three layers: a very thin template, a mechanical handle wafer and a borosilicate glass interlayer (Figure 1). N3.19.1
Table I. Material properties and geometric parameters of the Base Case.
y
r0
θ
Template GaAs
Interlayer Glass
E (GPa)
67.38
1
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