Method for implanting and coding a read-only memory with automatic alignment at four corners

A method for implanting and coding a read-only memory with automatic alignment at four corners for being used in a mask memory;

• the read-only memory comprising a substrate, bit lines on said substrate;
• a gate oxide layer on said substrate;
• and a spaced-striped first polysilicon layer on said gate oxide layer;
• comprising the steps of:
• forming a spacing layer on said gate oxide layer;
• depositing a second polysilicon layer overlying said spacing layer and said spaced-striped first polysilicon layer;
• etching a part of said second polysilicon layer and said spaced-striped first polysilicon layer to expose said gate oxide layer so that the second polysilicon layer becomes a striped second polysilicon layer;
• forming an isolating layer on said etched portion of the spaced-striped first polysilicon layer and said etched portion of the striped second polysilicon layer;
• and
• defining code areas by using the striped second polysilicon layer as a mask for performing the process of implantation and coding.