Patent ID: 6468868
Filing Date: 2002-10-22
Classification: H01L

Abstract:
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; forming a photoresist pattern on said first coding regions and said semiconductor substrate 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 form oxide first, second and third layers on said semiconductor substrate including forming an oxide layer on the regions with first coding oxide layer thereon and driving said first conductive impurities into semiconductor substrate so as to form buried bit line 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, said photoresist pattern having second openings so as to define second coding regions; performing second ion implant by implanting second conductive impurities through said first conductive layer in said semiconductor substrate so as to form second coding regions using said photoresist pattern as a mask.