The use of multi-step deposition of poly-silicon thin film to increase silicon wafer external gettering ability and reduce warpage

This invention is to increase the silicon wafers external gettering ability and to reduce the wafer warpage commonly observed in the deposition of a thick polycrystalline silicon layer in the back-side of a wafer for external gettering purpose. By using the multiple-step deposition techniques developed in this invention, the interfacial internal stress between the first polycrystalline layer and the silicon substrate can be counteracted by the interfacial internal stress between the second polycrystalline layer and the first polycrystalline layer. Therefore, the warpage of the wafer can be significantly reduced. At the same time, due to a finer polycrystalline grain size obtained from the multiple-step depositions and the additional interfaces, the external gettering ability was also increased.