Post-Irradiation Effects in Field-Oxide Isolation Structures

We have studied experimentally the time dependence of leakage currents in six CMOS (complementary metaloxide semiconductor) processes using LOCOS (local oxidation of silicon) isolation structures. These six process lines represent six different U. S. semiconductor companies. In their radiation response, these processes range from very hard to very soft. In the softer processes, the radiation-induced leakage currents are due to the turning on of a leakage path either under the thick field-oxide or along the transistor edge (bird's beak) region. In the hardest process, the field-oxide did not turn on, and the leakage was entirely due to subthreshold current in the gate region. These different mechanisms have qualitatively different time dependences, which we describe and discuss. We also discuss the implications of our results for hardness assurance testing.