Abstract: A method of forming multi-state mask ROM cells on a semiconductor substrate is disclosed. The method comprises following steps. Firstly, a pad oxide layer is formed on a semiconductor substrate. Then the pad oxide layer is patterned so as to form a plurality of first coding oxide regions. Thereafter, another photoresist pattern is formed to define buried bit line regions. A plurality of predetermined buried bit line regions are defined amid the first coding oxide regions. Then, a first ion implant by implanting n-type impurities into the semiconductor substrate using the photoresist pattern as a mask. After stripping the photoresist pattern, a first thermal oxidation is performed to grow oxide layer and driving the n-type impurities into the semiconductor substrate. Three types of oxide layer are formed with different thickness. Thereafter, a conductive layer are formed and then patterned as word lines. Subsequently, a photoresist pattern is formed to define second coding region.

Abstract: A method of forming multi-state mask ROM cells on a semiconductor substrate is disclosed. The method comprises following steps. Firstly, a pad oxide layer is formed on a semiconductor substrate. Then the pad oxide layer is patterned so as to form a plurality of first coding oxide regions. Thereafter, another photoresist pattern is formed to define buried bit line regions. A plurality of predetermined buried bit line regions are defined amid the first coding oxide regions. Then, a first ion implant by implanting n-type impurities into the semiconductor substrate using the photoresist pattern as a mask. After stripping the photoresist pattern, a first thermal oxidation is performed to grow oxide layer and driving the n-type impurities into the semiconductor substrate, Three types of oxide layer are formed with different thickness. Thereafter, a conductive layer are formed and then patterned as word lines. Subsequently, a photoresist pattern is formed to define second coding region.