Patent ID: 6929994

Claim:
A method for manufacturing a semiconductor device, the method comprising: selectively forming an anti-oxidation layer having a mask function against oxidation on a semiconductor substrate of a first conductivity type, wherein the anti-oxidation layer has a first opening and a second opening; introducing an impurity of a second conductivity type in the semiconductor substrate using the anti-oxidation layer as a first mask to form a first impurity layer in the first opening and an independent second impurity layer in the second opening; forming a resist layer on the anti-oxidation layer and in the first opening, wherein the resist layer has a third opening that corresponds to the second opening of the anti-oxidation layer, and wherein the third opening is larger than the second opening; further introducing an impurity of the second conductivity type in a region of the second impurity layer using the anti-oxidation layer and the resist layer as a second mask to form a third impurity layer; and conducting a heat treatment to diffuse impurities of the first impurity layer and the third impurity layer to form a first well of the second conductivity type and a second well of the second conductivity type, the second well having an impurity concentration higher than that of the first well.