Method of manufacturing semiconductor device including such input protection transistor

A method of manufacturing a semiconductor device in which an internal circuit including a memory device and a transistor having an LDD structure, and an input protection device for protecting said internal circuit are formed on one semiconductor substrate, comprising the steps of:

• forming an insulator film on a surface of said semiconductor substrate;
• forming a first conductor film as a gate electrode on said insulator film in a region where said memory device is to be formed and in a region where said input protection device is to be formed;
• forming an insulator film on said first conductor film;
• forming a second conductor film as a gate electrode on insulator film on said first conductor film and on said insulator film in said region where said transistor is to be formed;
• forming a resist film on said second conductor film corresponding to said gate forming region in said memory device forming region and in said input protection device forming region, forming said resist film on the overall surface of said second conductor film in said transistor forming region, and then etching with said resist film used as a mask, thereby forming a gate electrode formed of a plurality of conductor layers in said memory device forming region and in said input protection device forming region;
• implanting high concentration impurity ions into the surface of said semiconductor substrate, with said gate electrode and said second conductor film in said transistor forming region used as masks, thereby forming impurity regions to be source and drain regions in said memory device forming region and in said input protection device forming region;
• covering the overall surfaces of said memory device forming region and said input protection device forming region with a resist film, forming said resist film on said second conductor film corresponding to a gate forming region in said transistor forming region, and then etching with said resist film used as a mask, thereby forming a gate electrode in said transistor forming region;
• and
• implanting low concentration impurity ions into the surface of said semiconductor substrate, with said resist film and said gate electrode in said transistor forming region used as masks, forming a sidewall at peripheries of said gate electrode, and then implanting high concentration impurity ions with said sidewall used as a mask, thereby forming source and drain impurity regions having the LDD structure in said transistor forming region.