Ultraviolet Raman stress mapping in silicon

We report micro-Raman stress imaging in silicon structures using 351 nm excitation. This excitation has an extremely shallow optical penetration depth (≈5 nm) in silicon. It should also ultimately provide higher spatial resolution compared to visible wavelengths used in micro-Raman studies. As a test we examine a wafer which consists of a patterned nitride/polycrystalline silicon/oxide/substrate “stack” with neighboring regions of device-quality silicon covered only by a thin oxide layer. We obtain two-dimensional images with spatial resolutions ranging from 1 to 4 μm. The smallest Raman shift reliably detectable is found to be ±0.07 cm−1, corresponding to a stress magnitude of 35 MPa. Stresses on the order of 140 MPa are observed beneath the stack region. The contrasting linewidths between the substrate and polycrystalline silicon provide a complementary image.