Tetra-state mask read only memory

摘要

The ROM is fabricated by providing a semiconductor substrate having a gate oxide layer, a first polysilicon layer, a plurality of source/drain regions, and a plurality of channel regions. By using a first photoresist to dope the channel regions, a first coding step is performed to obtain a transistor having 2 different threshold voltages. A thin film transistor oxide layer is formed over the substrate, patterned and etched to expose the source/drain regions. A second polysilicon layer is formed on the thin film transistor oxide, and a mask used to dope a part of it to form a plurality of doped regions. Using a second photoresist layer as a mask, a part of the second polysilicon layer corresponding to the channel regions is doped and a second coding step performed.