Coding method for mask ROM

A method of manufacturing mask read-only-memory, comprising the steps of:

• providing a substrate;
• forming a plurality of buried bit lines in the substrate and a plurality of word lines over the substrate, wherein the word lines are perpendicular to the buried bit lines, a gate is formed in the substrate between any two buried bit lines underneath each word line and a dielectric layer is formed between the gates and the word lines;
• forming a pre-coding layer over the substrate;
• forming a plurality of pre-coding openings in the pre-coding layer, wherein the pre-coding openings correspond in position to the plurality of coding regions in the substrate underneath the gates;
• forming a filler layer inside the pre-coding openings;
• forming a coding mask over the substrate;
• forming at least one coding opening in the coding mask, wherein the coding opening at least exposes the filler layer inside a first pre-coding openings of the pre-coding openings;
• removing the filler layer inside the first pre-coding opening;
• removing the coding mask;
• and
• conducting an ion implantation using the pre-coding layer and the filler layer inside the pre-coding openings as a mask so that ions are implanted into the code regions through the first pre-coding openings.