Cmos process

Abstract

A process is disclosed for fabricating complementary insulated gate field effect transistors including doped field isolation regions and optional punch through protection. In one embodiment of invention, a silicon substrate is provided which has N-type and P-type surface regions. First and second masks are formed overlying active areas of the two surface regions. A third mask is then formed overlying the first region and the first mask. P-type impurities are implanted into the second region with an implant energy which is sufficient to penetrate through the second mask but insufficient to penetrate through the third mask. A second P-type implant is performed with an implant energy insufficient to penetrate through either mask. The first implant will aid in preventing punch through while the second implant dopes the field region. A fourth mask is then formed overlying the second region and the second mask. A first N-type implant is performed at energy sufficient to penetrate through the first mask but insufficient to penetrate through the fourth mask. This implant provides punch through protection for P channel transistors to be formed later. A second N-type impurity is implanted into the surface at an implant energy insufficient to penetrate through the first mask to provided field doping. The silicon substrate is then oxidized to form a field oxide at portions of the first and second surface regions which are not covered by the first and second masks.