Method of manufacturing semiconductor device

A method of manufacturing a power semiconductor device having a back surface electrode, comprising:nforming an insulated gate bipolar transistor on a semiconductor wafer having a front surface and a back surface;polishing the back surface of the semiconductor wafer;forming a first metal layer on said back surface of said semiconductor wafer after polishing;executing thermal processing to create an ohmic contact between said semiconductor wafer and said first metal layer at a temperature between of 300° C. and 470° C;forming a second metal layer of Ni on said first metal layer, the second metal layer being formed without further thermal processing; andreducing warping in the semiconductor wafer by maintaining a temperature of the semiconductor wafer at or below 80° C. during forming of the second metal layer.