Impact of Substrate Thickness on the Degradation in Multicrystalline Silicon

Abstract

Light and elevated-temperature-induced degradation (LeTID) is a well-known phenomenon that reduces the bulk lifetime in silicon wafers. The cause of this degradation mechanism is still under investigation. However, a wide range of empirical trends that correlate LeTID with multiple physical and processing parameters have been reported, including the observation that wafers thinner than 120 μm do not show significant LeTID. In this work, we extend that study by varying the thickness of the wafers, the temperature of the firing step, and testing LeTID at the accelerated stability testing conditions. We demonstrate that the extent of degradation reduces with the thickness of the wafer, in agreement with the earlier work. However, silicon wafers with a thickness below 120 μm still suffer from LeTID when fired at sufficiently high temperatures, demonstrating that thinner wafers are not inherently immune to LeTID. By performing accelerated testing using a high-intensity laser and fitting the degradation and regeneration data, we observe that thinner wafers do not necessarily exhibit a faster recovery, as suggested earlier. However, their reduced degradation extent could be a consequence of relatively higher out-diffusion of hydrogen per unit volume in thinner wafers during firing. We further report that the method used for thinning the wafers results in a variation in the surface morphology of the samples, and that may partly be responsible for the observed correlation between the thickness of the wafers and LeTID extent. Finally, we discuss how these new findings can be explained by the involvement of hydrogen and other impurities in LeTID.