Effects of Surface Constraints on Stresses in Heteroepitaxial Films Grown on Compliant Substrates
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Effects of Surface Constraints on Stresses in Heteroepitaxial Films Grown on Compliant Substrates Zhaohua Feng,1 Edward G. Lovell,1 Roxann L. Engelstad,1 Thomas F. Kuech2 1 Computational Mechanics Center, Mechanical Engineering Department 2 Department of Chemical Engineering University of Wisconsin, Madison, WI 53706, U.S.A. ABSTRACT As films heteroepitaxially grow on substrates, lattice mismatch strain at the interfaces causes stresses in the films and substrates. These stresses can have deleterious effects on film quality. To facilitate an understanding of defect production and control during growth of films on compliant substrates, transient finite element models were developed to simulate the complete mechanical stress and strain fields. Effects of constraints between the template and handle wafer on the film stresses were examined. The investigation showed that different types of constraints caused stress variations over a large range. Other factors affecting the stresses, such as lattice mismatch strain, wafer radius, film and template thickness, were also assessed. INTRODUCTION During film heteroepitaxy on a thick substrate, lattice mismatch strain on the interfaces can cause high stresses in the film. Such stresses result in high-density dislocation fields. 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]. It is not easy, however, to realize this goal, since the freestanding thin template is mechanically unstable. In practice, a handle or support wafer is needed. The details of the interlayer bonding the template to the handle wafer are critical. An ideal interlayer should limit the unnecessary degrees of freedom of the template and keep it stable, but not decrease its deformation compliance. Accurate models simulating the stress development and subsequent variation are essential for selecting the constraints connecting the template and handle wafer, optimizing fabrication parameters, controlling the film growth processes and reducing film defects. Analytical models with freestanding supports were developed by Lo [1] and Freund [2]. These models clearly describe the stress generation and illustrate the mechanism of substrate compliance, but cannot be used to investigate the effects of different types of constraints or to select the parameters of a potential interlayer. To demonstrate the influence of the template constraints on film stress development, axisymmetric three-dimensional finite element (FE) models were created to simulate the film and template, as well as the constraints connecting the template to the handle wafer. Transient stress analyses provided accurate values for all stresses in the film and template during the entire film growth process. Effects of constraint types and related factors were investigated in detail to determine their influence on film stress magnitudes.
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FINITE ELEMENT MODEL Figure 1 shows a schem
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