A complementary metal oxide semiconductor (CMOS) retrograde twin well including a buried layer was fabricated using MeV ion implantation. A profiled retrograde twin well was fabricated using multiple MeV ion implantations by controlling ion energy and dose independently. For the formation of a buried layer 1.5 MeV B- implantation was conducted at various ion doses, MeV implanted buried layers were observed to show greatly improved characteristics of latchup suppression. Junction leakage current, however, showed a critical behavior as a function of ion dose. The density of secondary defects had no correlation with leakage current, Instead, threading dislocations were observed to be responsible for the leakage current. The gettering efficiency of background oxygen by secondary defects formed near the projected range (R-p) seemed to be related to the formation of threading dislocations,