Method of forming high density multi-state mask rom cells

A method of forming multi-state mask ROM cells on a semiconductor substrate, said method comprising the steps of:

• forming a pad oxide layer on said semiconductor substrate;
• patterning said pad oxide layer so as to form a plurality of first coding oxide regions spaced from each other;
• forming a photoresist pattern to overlay said first coding oxide regions and a plurality of regions amid said first coding oxide regions so as to form a plurality of first openings;
• performing first ion implant by implanting first conductive impurities through said first openings into said semiconductor substrate so as to form first doping regions using said photoresist pattern as a mask;
• stripping said photoresist pattern;
• performing first thermal oxidation so as to grow first, second, and third type of oxide layer on said semiconductor substrate and driving said first conductive impurities into said semiconductor substrate, said first type of oxide layer being formed at first doping regions, said second type of oxide layer being regions with first coding oxide layer thereon, said third type of oxide layer being remnant regions;
• forming a first conductive layer on said oxide layers;
• patterning said first conductive layer to define word lines;
• forming a photoresist pattern on entire surface to define second coding region, said defined second coding regions having either said first coding oxide regions or said third type oxide layer thereon;
• and
• performing second ion implant by implanting second conductive impurities through said first conductive layer into said semiconductor substrate so as to form second coding regions using said photoresist pattern as a mask.