Semiconductor memory device having multilevel memory cell and method of manufacturing the same

Abstract

A semiconductor ROM device which enables to obtain a reference current which can securely distinguish data stored in a memory cell in a multilevel mask ROM for storing multilevel data of three or more levels per memory cell. The device comprises a memory cell in which a threshold voltage is set up corresponding to an amount of ions injected to a channel region of a cell transistor and multilevel data of three or more levels are stored, a reference cell for generating the reference current for comparing with a current read out from the memory cell, and dummy cells disposed adjacent to the reference cell. In the channel region of the reference cell and the channel region of the dummy cell, ions are injected simultaneously to set up the equal threshold voltages both in the reference cell and the dummy cell.