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2010
Conference Paper
Titel
Mechanical characterization of epitaxial wafer equivalents from block casting to thin film deposition
Abstract
The epitaxial wafer equivalent concept promises to reduce the cost for photovoltaic solar energy conversion. For this purpose, less expensive substrates of upgraded metallurgical silicon (UMG-Si) are used as a mechanical support for the epitaxial cell. In the presented paper the mechanical strength of substrates coming from different positions of an UMG-Si ingot was investigated. The investigation of mechanical strength was performed after block crystallization, multi-wire sawing of the wafers and after depositing a 20 µm crystalline thin film until the production of an epitaxial wafer equivalent. UMG-Si substrates and epitaxial wafer equivalents were tested with the ring on ring bending test. To interpret the results IR-transmission and scanning electron microscopy, impurity concentration measurements and residual stress measurements were done on selected wafers. It could be shown that the presence of precipitates in the silicon matrix has a strong influence on the mechanical strength of the wafers. UMG-Si as well as industry standard Cz-Si and mc-Si showed less mechanical strength after epitaxial processes. This could be due either to a change in microstructure of the substrate or to thermal stresses introduced during the high temperature process. The deposition of the crystalline silicon thin film did not decrease the mechanical strength of the epitaxial wafer equivalent. It could be concluded that the UMG-Si investigated in this work shows sufficient mechanical strength for manufacturing epitaxial wafer equivalents.
Author(s)