| 摘要 |
PURPOSE:To provide a method of manufacturing a mask ROM, which increases the memory capacity without increasing its dimensions as well as the read-out margin easily. CONSTITUTION:When ions are implanted two times so as to differ-set different threshold voltages for the first, second, third and fourth memory cell transistors a0, b0, c0, d0 after finishing the first time ion implantation, the interlayer insulating films 17 on the second and fourth memory cell transistors b0, d0 parts are made thinner by etching away step using the same mask. Next, the other new masks 20 for opening the third and fourth memory cell transistors c0, d0 parts are formed to start the second time ion implantation for differentiating the ion transmittivity due to the difference between the thickness of the interlayer insulating films 17 of both memory cell transistors c0, d0 so that ions 21 may be sufficiently implanted in the channel part 14 of the fourth memory cell transistor do thereby enabling the threshold value voltage to be distributed at equi- intervals. |
