Analysis of grain structure evolution based on optical measurements of mc Si wafers

Abstract

Grain structure and grain competition have a strong impact on bulk lifetime in multicrystalline (mc) silicon. A fast and thorough characterization of grain structure is crucial in order to improve industrial crystal growth. This work introduces key parameters of grain structure, extracted with a newly developed image processing tool. Four bricks grown with different concepts were chosen to investigate the value of the identified key parameters and to identify characteristic developments along the brick. Optical measurements on as-cut wafers from these bricks serve as a basis to extract grain structure properties, i.e., size, shape, homogeneity and distribution of grain size. By connecting the 2D-information over brick height, a statistical insight into the entire brick is gained. Weighted percentiles of grain area offer a robust measure to characterize grain size distribution. As twinning has a large impact on grain competition, twinned grains are detected via grain shape. Additionally, regions with strong grain competition are highlighted for investigations on grain overgrowth.It is found that the share of twin grains increases with brick height in high-performance mc (HPM) silicon with fine-granular seeds from almost zero up to about 15% whereas it remains rather constant over the whole brick height in standard mc-Si. The results of the investigated bricks show clearly that towards the brick top, the material differences in grain size decrease. This suggests that an energetically favorable state may exist for grain structure development.