Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device, in which both a high-breakdown-voltage MOS transistor and a low-breakdown-voltage MOS transistor having different drain breakdown voltages are formed on an identical semiconductor substrate, the manufacturing method comprising the steps of:

• (a) forming a gate electrode on a first dielectric film created above the substrate, creating a second dielectric film over surface of the substrate including the gate electrode, and etching the second dielectric film, so as to define a side wall of the second dielectric film on a side face of the gate electrode;
• and
• (b) implanting an impurity to specify a drain area and a source area, the step (b) comprising the sub-steps of:
• (b-1) forming a first mask that keeps open at least a drain-source forming region in the high-breakdown-voltage MOS transistor, which is expected to form a drain area and a source area, and covers over at least an offset forming region in the high-breakdown-voltage MOS transistor, which is expected to form an offset area between the gate electrode and either one of the drain area and the source area;
• (b-2) using the first mask and etching off a dielectric film part at least on the drain-source forming region, out of the dielectric film created on the substrate;
• and
• (b-3) continuously using the same first mask and implanting the impurity into the drain-source forming region.