Residual oxygen impurities are eliminated from silicon wafers pulled from a crucible (Czochralski silicon). A multitude of trenches are etched into the back side of the crucible-pulled silicon wafer and the wafer is subsequently heat-treated at about 1100° C. The very large surface area at the front side of the silicon wafer allows oxygen impurities to diffuse out effectively. After the diffusion has been carried out, the trenches are filled with heavily doped polysilicon without leaving gaps.