Bulk micro-defect detection with low-angle illumination

The detection of oxygen precipitates, voids, and other defects is critical for semiconductor wafer makers. One of the industry standard techniques for detecting these Bulk Micro-Defects (BMDs) is Semilab's Light Scattering Tomograph (LST) system. In this measurement, unpatterned wafers are nominally cleaved in half. Illumination is applied to the front surface of the sample, and the light scattered off of the defects is collected via the cleaved surface. This technique had been limited to the measurement of unpatterned wafers, but device makers show significant interest in measuring BMD distributions on patterned wafers using scattering-based techniques. A pattern on the surface of the wafer can cause significant scattering, making the standard LST technique unsuitable for this task. We present a solution for patterned wafer BMD measurements by an addition of a low-angle illumination unit to the standard LST system. This new illumination unit focuses the light into the bulk of the wafer via the cleaved surface, which enables measurement on patterned samples. The new system is called "light scattering tomograph enhanced by low-angle illumination." Excellent correlation was found between the detected defect densities obtained by the low-angle and the standard LST illumination mode.