Method of forming high density multi-state mask rom cells

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. The openings of the photoresist pattern located at regions having either first coding oxide regions or the thinnest type oxide layer thereon. Finally, an ion implant using boron ion is performed. Accordingly four states mask ROM is formed.