An accurate method for calibrating photoluminescence-based lifetime images on multi-crystalline silicon wafers

Abstract

We present a method for converting photoluminescence images into carrier lifetime images for silicon wafers with inhomogeneous lifetime distributions, such as multi-crystalline silicon wafers, based on a calibration factor extracted from a separate, homogeneous, mono-crystalline calibration wafer and simple optical modelling of the photoluminescence signal from both the calibration wafer and the test wafer. The method is applicable to planar wafers with uniform carrier profiles depth-wise. A multi-crystalline wafer is used to demonstrate the difference between the conventional calibration approach, where the photoluminescence signal is calibrated against a quasi-steady-state photoconductance measurement on the test sample itself, and our proposed method. The lifetimes calibrated by our method are consistent, in contrast with the lifetime calibrated by the conventional approach, in which the magnitude and injection-dependence of the lifetime is observed to be sensitive to the choice of reference area. The error in the conventional calibration method mainly originates from measurement artifacts in the quasi-steady-state photoconductance measurements on multi-crystalline wafers, which we propose to be mainly due to minority carrier trapping, radial sensitivity of the quasi-steady-state photoconductance sensor coil and overestimation of the carrier mobility sum. We also show that the proposed new method is effectively insensitive to the lifetime, doping density, reflectance and wafer thickness of the calibration wafer (provided it is below 500 μm).