Influence of defects in gate-oxide peripheral regions on the electrical characteristics of MOS structures

Radiation-induced defect formation is studied experimentally in the gate-insulator layer and at the semiconductor-insulator interface of NMOS and PMOS structures differing in perimeter-to-area ratio. The structures are fabricated by CMOS technology on the same n-Si wafer, the NMOS structures being formed in a p-well. Heavily phosphorus doped polysilicon and noncrystalline silicon dioxide are used as the gate and insulator materials, respectively. The devices considered are MOS varactors, MOS diodes, and MOSFETs. Capacitance-voltage characteristics are measured on the MOS varactors and diodes. The gate-voltage dependence is examined of surface conduction for the MOSFETs and the surface-recombination emitter-current component for the MOS diodes. The results are used to characterize defect formation in peripheral gate-oxide regions and the lightly doped part of the source (emitter) and the drain, as well as in the central gate-oxide region and at the Si/SiO₂ interface. The peripheral oxide regions are found to have a two-sided influence on the performance of the MOS structures. On the one hand, they act as a drain of uncombined hydrogen from the gate oxide, so that the effectiveness of defect deactivation by hydrogen depends on the perimeter-to-area ratio. On the other hand, the peripheral regions, particularly their corners, may have an elevated density of latent process-induced defects that can be activated by radiation, voltage, or thermal stress.