| 摘要 |
A method of providing a multilevel programmed Mask ROM is disclosed that results in a substantially higher manufacturing yield. This method begins by fabricating an unprogrammed mask ROM. Each of the transistors in the mask ROM array is then programmed with one of a plurality of threshold voltages using low concentration ion implantation. As a consequence of the low concentration ion implantation, the method minimizes the leakage current during reading of the array from at least the transistors programmed with first level threshold voltage. Minimization of the leakage current may include biasing at least the first level threshold voltage programmed transistors to inhibit leakage current, which may be accomplished by applying a negative voltage to the gates of those transistors. Minimization of leakage current may further (or alternatively) include implanting at least standard ion implantatin dosages in regions surrounding at least the first level threshold voltage programmed transistors. Another disclosed alternative to minimizing the desirably resulting leakage current involves selecting an inhibiting source voltage that substantially eliminates leakage current from at least the first level threshold voltage programmed transistors and then reading the array by applying the inhibiting source voltage to the sources and the inhibiting source voltage plus approximately 1.2 volts to the drains of the transistors. |
