Method of manufacturing mask ROM devices with self-aligned coding implant

A method for forming buried bit line mask read-only memories, said method comprises:

• forming a gate oxide layer on a semiconductor substrate;
• forming a polysilicon layer on said gate oxide layer;
• forming a silicon nitride layer on said polysilicon layer;
• patterning said silicon nitride layer and said polysilicon layer to define gate structures;
• forming silicon oxide spacers on sidewalls of said patterned gate structures;
• Implanting first conductive type dopant into said semiconductor substrate by using said silicon nitride layer and said silicon oxide spacers as a mask to form buried bit lines in said semiconductor substrate between said gate structures;
• coating a borophosphosilicate glass(BPSG) layer on said semiconductor substrate;
• removing a portion of said BPSG layer on a top surface of said gate structures by using chemical mechanism polishing(CMP) process;
• removing said silicon nitride layer;
• forming a conductive layer along a surfaces of said residual BPSG layer, silicon oxide spacers and polysilicon layer;
• and
• defining word lines by etching said conductive layer.