Photoelastic Characterization of Residual Strain Distribution in Commercial Off-Axis SiC Substrates

Abstract

Two-dimensional distribution of strain-induced birefringence has been selectively characterized in commercial 4° off-axis wafers of 4H-silicon carbide (SiC) crystal by using an originally-developed imaging polariscope. The natural birefringence, which is often observed in uniaxial crystals, was eliminated by optimizing the light incident angle so that the light propagates parallel to the c-axis inside the crystal. The eliminated natural birefringence was at least twice as large as that of the strain-induced ones, which demonstrates the validity of the elimination process. A typical retardation map under optimal incidence revealed various distributions of strain-induced birefringence, such as local concentrations due to the structural crystal defects and gradual increase or relief due to the thermal stress during the crystal growth and cooling processes. The maximum retardation was 0.28, which corresponds to 7.6 × 10−5 in strain terms, and 30 MPa in stress terms. Because of its sufficient sensitivity to the strain-induced birefringence and elimination capability in the natural birefringence, it is concluded that our technique is useful to characterize the residual strain distribution in commercial off-axis SiC substrates.