Viscoelastic Stress Relief in Patterned Silicon-on-Insulator Structures
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VISCOELASTIC STRESS RELIEF IN PATT'ERNED SILICON-ON-INSULATOR STRUCTURES Theodore J. Letavic, Edward W. Maby and Ronald J. Gutmann, Department of Electrical, Computer and Systems Engineering and Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180. ABSTRACT A high-temperature viscoelastic stress relief technique has been investigated as a means for reducing in-plane stress encountered during zone-melt recrystallization of patterned silicon-on-insulator structures. This technique incorporates a phosphosilicate glass layer between the silicon film and the insulating substrate to provide a viscous flow mechanism for stress relief within the composite structure. The stress relaxation can be qualitatively described by a mechanical model which couples thermal expansion and viscoelastic flow. The model predicts the time constant for stress relief at high temperatures as a function of pattern size, and the results are useful as a design aid for zone-melt recrystallization experiments. INTRODUCTION AND TECHNICAL BACKGROUND The zone-melt recrystallization (ZMR) process has been extensively utilized as a means for obtaining device-quality semiconductor films on an insulating substrate. Samples which are subjected to the ZMR process are layered composites which are fabricated by the sequential deposition of thin surface films onto a thick insulating substrate. In most studies, the surface film to be recrystallized has been polycrystalline silicon, and substrates such as oxidized silicon [1], fused silica and sapphire [2], alumina [3], and various glasses [4,5] have been employed. ZMR of silicon is necessarily a hightemperature process in which the entire composite is uniformly subjected to temperatures just below the silicon melting point. High-temperature processing of composite structures consisting of materials with different thermophysical properties can lead to excessive stress levels within the constituent films. As the temperature of a layered composite is uniformly changed, in-plane forces within the layers are generated to accommodate elastic strain due to the requirement of point-to-point coherency between adjacent materials. Mechanical failure becomes probable when the magnitude of in-plane stress approaches one tenth of the shear modulus in any one of the films. Observed failure modes during silicon ZMR over thermally mismatched substrates relate to either the substrate (plastic deformation, thermal spalling and fracture) or the overlying thin films (tensile failure and delamination). In order to provide physical insight into the observed failure modes and indicate directions for future experiments, in-plane stresses have been evaluated using a model based upon linear elasticity theory. A cross-sectional view of a typical silicon-on-alumina composite is shown in Fig. 1, and the analysis of this configuration is made subject to two assumptions. First, it is assumed that a simple uniaxial solution will approximate a (less tractable) biaxial solution in regions sufficiently removed from
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