Some challenges in making accurate and reproducible measurements of minority carrier lifetime in high-quality Si wafers

Measurement of the minority carrier lifetime (τ) of high-quality wafers (having bulk minority carrier lifetime, τ_b > few milliseconds) requires surface passivation with very low surface recombination velocity, typically <; 1cm/s. Furthermore, for mapping large (e.g., 156 x156 mm) wafers, the passivation must also be stable and uniform over the entire wafer surfaces. These are very demanding requirements and it is a common experience that they are very difficult to achieve. Yet, they are necessary for performing defect analyses of the current N-type wafers. To understand the problems associated with these measurements, we have studied effect of wafer preparation (cleaning procedures, handling) and the passivation characteristics (stability, sensitivity to light, thickness of the passivation medium required for stable passivation) for many commonly used passivation media-iodine-ethanol (IE), quinhydrone-methanol (QHM), aluminum oxide (Al₂O₃), amorphous-silicon (a-Si), and silicon dioxide (SiO₂). Here, we will discuss main factors that influence the accuracy and repeatability of lifetime measurements.