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Thermoplastic Deformation and Residual Stress Topography of 4H-SiC Wafers

Published online by Cambridge University Press:  15 March 2011

Robert. S. Okojie  and
Ming Zhang
Robert. S. Okojie
Affiliation:
NASA Glenn Research Center, 21000 Brookpark Road, M/S 77-1, Cleveland, OH 44135, USA
Ming Zhang
Affiliation:
Department of Materials Science & Engineering, CWRU, Cleveland, OH 44106, USA
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Abstract

We have measured thermoplastic deformation in as-received, single-side polished, 4H-SiC wafers and also residual stresses in homoepitaxially grown epilayers on wafers by radius curvature measurements. The wafers studied had n-type resistivities of 0.010-0.011 ω-cm and p-type resistivities of 4.42, 4.72, 9.57 ω-cm. In a first thermal excursion to 900 °C in vacuum, the bow height of the bare substrates in all cases decreased with temperature. Upon cooling down, however, the bow heights remained largely unchanged from their values at 900 °C. A second cyclic excursion to 900 °C did not yield any significant change in the curvature, thus indicating that the substrates had thermoplastically deformed in the first heating cycle. Epilayers having nitrogen doping between 5 × 1017 and 2 × 1019 cm−3 grown on the n- and p-type substrates resulted in compressive stresses ranging between 190 and 400 MPa in the epilayers. Transmission electron microscopy (TEM) examination of the n-type epilayer (with doping levels of 5 × 1017 cm−3 and 5 × 1018 cm−3) on the n-type substrate, revealed bands of stacking faults (SFs) confined within the epilayers after the bicrystals were further annealed at 1150°C in nitrogen for thirty minutes. These doping levels are approximately one and two orders of magnitude below the reported threshold value of 3 × 1019 cm−3 previously suggested for the onset generation of SFs in annealed n-type 4H-SiC epilayers. The calculated residual stresses in all the epilayers were above the critical stress for the motion of dislocations above 1000 °C in 4H-SiC. Thus the SFs that form by glide of pre-existing partial dislocations may actually be stress induced and occur across a much wider range of doping levels. Therefore, it is possible that a significant mechanism for formation of the stacking faults and 3C bands observed in thermally treated 4H-SiC wafer is stress relief via the generation and motion of new and pre-existing partial dislocations on the basal planes of 4H-SiC.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 815 , 2004 , J6.2
Copyright
Copyright © Materials Research Society 2004

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