Method of manufacturing semiconductor device with high and low breakdown transistors

Abstract

Among first and second oxide films and formed on a substrate the oxide film in a low-breakdown-voltage transistor area LV is all etched off, while the whole surface of the oxide film in a high-breakdown-voltage transistor area HV is left intact. A sixth oxide film to define a side wall is subsequently formed on the whole surface of the substrate and a resist R is formed over the whole high-breakdown-voltage transistor area HV. Over-etching of the low-breakdown-voltage transistor area LV is carried out to make the surface of the substrate exposed and to define the side wall only in the low-breakdown-voltage transistor area LV. The oxide film is made to remain in the high-breakdown-voltage transistor area HV. Non-required portions of the oxide films and are then etched off with a resist R B. This causes a drain-source forming region, which is expected to form a drain area and a source area, to be open in an element forming region in a high-breakdown-voltage nMOS area HVn. The resist R B is not removed but is used continuously, and an n-type impurity ion is implanted into the open drain-source forming region. This arrangement enables both a high-breakdown-voltage MOS transistor and a low-breakdown-voltage MOS transistor to be formed efficiently on an identical substrate without damaging the characteristics of the respective MOS transistors.