Uncertainty in Photoluminescence Metrology on Multicrystalline Silicon Bricks and Cross Validation With Wafers

Abstract

Bulk lifetime and interstitial iron concentration are two pertinent parameters describing the quality of multicrystalline silicon, which can be measured on silicon bricks using photoluminescence (PL) imaging. Multicrystalline silicon is inherently variable, therefore accurate measurements are valuable for quality control and process improvement. In this study, the uncertainty of PL extracted lifetime and interstitial iron concentration measurements was assessed by consideration of the random and systematic errors present, as well as experimental comparison with other techniques. For PL-based brick measurements, uncertainty was estimated to be ±30% for bulk lifetime and ±43% for interstitial iron concentration, and was comprised mainly of systematic errors rather than random errors from repeatability issues or measurement noise. The validity of PL-based brick measurements is supported by experimental comparison to quasi-steady-state photoconductance, and by directly comparing measurements on an exemplary silicon brick to its resulting wafers after slicing and surface passivation. All techniques report the same trends in lifetime and interstitial iron concentration across the brick and support the idea that brick measurements are indicative of as-cut wafer quality. However, some systematic offsets are present between measurements likely due to the injection dependence of lifetime and impact from processing.