High resolution lifetime scan maps of silicon wafers

High resolution lifetime mappings of excess minority carriers in silicon wafers are obtained by means of the phase-shift technique at practically constant injection level. This technique is contactless, based on reflected microwave power variations which occur when the sample is illuminated by a focused near infrared light. It measures the phase shift φ between a sinusoidal modulation of the excitation and the reflection power of microwaves. Surfaces are passivated by means of an aqueous iodine solution, whose passivating efficiency is remarkably constant. Scan maps are obtained with a lateral resolution of 50 μm by means of a coaxial cable which directs 9.4-GHz microwaves onto the wafer and a fiber coupled laser diode which generates minority carriers in excess. Lifetime mappings are in agreement with minority carrier diffusion length mappings obtained by the light beam induced current technique, especially for multicrystalline wafers.