CMOS Integrated circuit technology utilizing dual implantation of slow and fast diffusing donor ions to form the n-well

To minimize the number of independent masking operations in the manufacture of a CMOS integrated circuit device using twin tub technology, the n-tub is made by separate phosphorus and arsenic implants through a common mask, and the p-tub is made by two separate boron implants through a common mask, complementary to that used for forming the n-tub. One of the boron implants occurs before, the other after, the drive-in heating step. After tub formation, further movement of the implanted ions is kept small by use of a high pressure process for growing the field oxide and by only limited further heating. Transistors are then formed in the tubs.