Determination of stress components in 4H-SiC power devices via Raman spectroscopy

Abstract

The stress dependencies of the phonon modes in a 4H silicon carbide (SiC) crystal were investigated. The deformation potentials of the A1(TO), E2, and E1(TO) modes were determined on the basis of uniaxial stress tests. The A1(TO) mode was sensitive to stress along the c axis, whereas the E1(TO) and E2 modes exhibited larger dependencies on the stress perpendicular to the c axis than the A1(TO) mode as expected from their atomic displacements. The coefficient of the frequency shift in the E2 mode for an isotropic biaxial stress in the c plane was determined to be −323 MPa/cm−1. We applied cross-sectional Raman measurements to SiC metal-oxide-semiconductor field-effect transistors. The horizontal and vertical stress components, σ11 and σ33, were derived from the frequency shifts of the E2 and A1(TO) modes using the deformation-potential constants. A compressive horizontal stress was observed near the interface between the SiC chip and the metals. The observed compressive stress increased as the temperature decreased. The main cause for the compressive horizontal stress at low temperature is considered to be the difference in the coefficients of thermal expansion of the SiC chip and the metals. The results suggest that the temperature of the fabrication process is one of the key factors for achieving a reduction in the thermal stress in the SiC power devices.