| 摘要 |
PURPOSE:To diminish PR and implantation processes to contrive to shorten the term of manufacture and to reduce a manufacturing cost by forming a P-type well region on a semiconductor substrate by ion-implanting an N-type impurity in the whole surface to form an N-type well region and to lower the effective concentration of the P-type well region, and by adjusting the surface concentration of a desired P-type well region. CONSTITUTION:A P-well 3 is formed by the ion implantation of trivalent atom. Then, an N-well 4 is formed by the ion implantation of pentavalent atom. Further, after a thick oxide film region 5 and diffusion region are formed, trivalent atom is ion-implanted in the region of the N-well forming a P-channel type MOS transistor so that the threshold voltage of a transistor is brought to a desired value. When the ion implantation is conducted also in the region of the P-well forming an N-channel type MOS transistor other than the region constituting ROM code, the depression type region of the P-well is changed into an enhancement type to form a depression transistor 6 constituting the ROM code. |
