Bias-temperature instabilities and radiation effects in MOS devices

We report the combined effects of irradiation and bias temperature stress (BTS) on MOS capacitors with HfO₂ dielectrics. Irradiation is found to enhance BTS-induced degradation in these devices; significant differences in the amounts of enhancement are observed for different irradiation biases. Zero-bias or positive-bias irradiation followed by negative BTS (NBTS) leads to much worse degradation than either irradiation or NBTS alone. This is primarily due to the formation of dipoles during irradiation and the electrostatic repulsion of electrons from the oxide during NBTS. In an integrated circuit application, the worst-case response for these gate stacks will be pMOS transistors irradiated in their "off" states, and annealed in their "on" states. The effects of irradiation and BTS are also reported for Al₂O₃-based MOS capacitors with Al gates and SiO₂-based MOS capacitors with NiSi gates for comparison with HfO₂. Somewhat less sensitivity to combined irradiation and BTS is observed for the Al₂O₃-based devices, and significantly less sensitivity to combined effects is observed for the thermal oxides with NiSi gates.