A semiconductor device including a semiconductor substrate in which a diode region and an IGBT region are formed has been known. In the diode region, an n-type cathode layer is formed on a lower surface side of the semiconductor substrate, while in the IGBT region, a p-type collector layer is formed on the lower surface side of the semiconductor substrate. The collector layer and the cathode layer are formed in general by doping impurity ions to the lower surface side of the semiconductor substrate. For example, after p-type impurity ions are doped to the entire lower surface of the semiconductor substrate, the region which becomes the collector layer on the lower surface is covered by a mask and n-type impurity ions are doped, and the impurities are diffused by an annealing process. In this case, in the cathode layer, if a dose amount of the n-type impurity ions is not high enough to compensate for a dose amount of the p-type impurity ions, voltage/current characteristics (VI characteristics) of the semiconductor device deteriorates, and a snapback phenomenon can easily occur.
In order to avoid this problem, in Japanese Patent Application Publication No 2011-507299 (Patent Literature 1), the region on the lower surface of the semiconductor substrate where the cathode layer is to be formed is covered by a mask, the p-type semiconductor layer which becomes a collector layer is deposited on an opening portion of the mask, and a metal film is formed on the lower surface thereof. After that, the mask is removed, and the n-type impurity ions are doped to the region on the lower surface of the semiconductor substrate where the cathode layer is to be formed by using the metal film as a mask.
Moreover, in Japanese Patent Application Publication No. 2011-507300 (Patent Literature 2), after the p-type impurity ions are doped to the entire lower surface of the semiconductor substrate, the semiconductor substrate in the region where a cathode layer is to be formed is removed by etching or the like using the mask. After the layer to which the p-type impurity ions are doped is removed, the n-type impurity ions are doped so as to form a cathode layer.