Method for fabricating mask ROM device

A fabrication method of the mask ROM device, comprising:

• providing a substrate, wherein the substrate comprises a memory cell region and a peripheral circuit region, and the peripheral circuit region already comprises an isolation structure to define an active region;
• forming a buried drain region in the substrate of the memory cell region;
• forming a first dielectric layer on the substrate;
• forming a patterned second dielectric layer on the first dielectric layer, wherein the second dielectric layer in the memory cell region includes a plurality of bar-shaped dielectric structures perpendicular to the buried drain region and the second dielectric layer in the peripheral circuit region exposes the first dielectric layer in the active region;
• removing a portion of the first dielectric layer in the memory cell region to expose the substrate and removing the exposed first dielectric layer in the peripheral circuit region to expose the substrate;
• forming a gate oxide layer on the exposed substrate surface;
• forming a conductive layer on the substrate to cover the second dielectric layer;
• removing a portion of the conductive layer until the second dielectric layer is exposed to form a plurality of coded memory cells, wherein the coded memory cells that comprise a gate oxide layer corresponds to a logic state of “1” while the code memory cells that comprise the first dielectric layer corresponds to a logic state of “0”;
• and
• removing the second dielectric layer.