Measurement of Viscoelastic Stress Relief in Patterned Silicon-on-Insulator Composite Structures With Raman Spectroscopy
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MEASUREMENT OF VISCOELASTIC STRESS RELIEF IN PATTERNED SILICON-ON-INSULATOR COMPOSITE STRUCTURES WITH RAMAN SPECTROSCOPY T.J. LETAVIC*, E.W. MABY*, R.J. GUTMANN* AND J. PETRUZZELLO** *Department of Electrical and Computer Engineering and the Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12180 "**Philips Laboratories, North American Philips Corporation, 345 Scarborough Road, Briarcliff Manor, New York 10510
ABSTRACT Raman spectroscopy has been utilized to measure room-temperature residual strain in the active device layer of laser-recrystallized silicon-on-insulator (SOI) composite structures. The SOI composite structures were fabricated on synthetic fused-silica substrates, and the composites contained a phosphosilicate glass (PSG) layer to provide high-temperature stress relief. Conventional masking and etching techniques were used to selectively pattern the polycrystalline silicon layer into isolated square islands prior to recrystallization. The biaxial in-plane stress in recrystallized films was calculated from the measured strain-induced first-order Stokes Raman wavenumber shifts, and the results indicate that 200- lim-square recrystallized silicon islands have significantly lower in-plane stress values than continuous recrystallized silicon films. These measurements provide a preliminary confirmation of the dependence of the time constant for viscoelastic stress relief on the in-plane pattern dimension.
INTRODUCTION AND TECHNICAL BACKGROUND The zone-melting recrystallization technique (ZMR) has been extensively utilized as a means of obtaining high-quality silicon films on an insulating substrate for the fabrication of novel semiconductor device structures. Conceptually, the SOI laminar composite is fabricated by the sequential deposition of various thin films onto a thick insulating substrate, and one of the thin surface films (usually polycrystalline silicon) is recrystallized through the application of a primary heating source to the surface of the SOI structure. The selection of a primary heating source is determined by the desired energy absorption depth and the characteristics of the liquid-solid interface during ZMR. Resistively heated graphite strips [1], continuous-wave lasers [2], and focussed electron beams [3] have been used as heating sources for ZMR. The choice of the substrate material is generally device-application driven, and dielectric materials which include fused-silica [4], single-crystal sapphire [5], and various low-temperature glasses have been investigated [6]. While the dissimilar properties of the adjacent layers in typical SOI composite structures can be utilized to realize unique device structures, they can also result in fundamental mechanical limitations which degrade the mechanical integrity of the recrystallized layer. During the thermal cycling which is inherent to the ZMR process, mechanical in-plane stresses are induced within each layer of the laminar SOI composite as a result of strain imbalances at the adjacent layer interfaces. T
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