Measuring Stress Birefringence in Small Si Samples

We report stress birefringence measurements for small (up to 150 mm x 150 mm) samples such as Si slabs, wafers and small ingot segments. Measured stress birefringence in an Si ingot segment exhibits an interesting four-fold symmetry that may be caused by the crystal growth process or subsequent squaring or grinding of the ingot. Stress birefringence map for an axially cut and mechanically polished cross section of the seed end of a CZ-grown Si ingot shows striations that could be a visualization of the solid-liquid interface of silicon crystals. Two instruments are used for measuring these small Si samples. The first instrument is a near infrared (NIR) Exicor birefringence measurement system that employs photoelastic modulator (PEM) technology. This is a point measurement instrument. The second instrument is a camera-based NIR imaging polariscope that uses a fixed circular polarizer and a rotating analyzer. Both instruments are designed primarily for Si material research at significantly lower cost than the previously reported system for measuring large, pseudo-square, PV Si ingots. The two instruments described in this paper complement each other for different applications.