FIELD-DEPENDENCE OF INTERFACE-TRAP BUILDUP IN POLYSILICON AND METAL GATE MOS DEVICES

The electric field dependence of radiation-induced interface- and oxide-trap charge (ΔVot and ΔVit) generation for polysilicon- and metal-gate MOS transistors is investigated at electric fields (Eox) from −4.2 MV/cm to +4.7 MV/cm. If electron-hole recombination effects are taken into account, the absolute value of ΔVot and the saturated value of ΔVit for both polysilicon- and metal-gate transistors are shown to follow an approximate E−1/2 field dependence for Eox ≥ 0.4 MV/cm. An E−1/2 dependence for the saturated value of ΔVit was also observed for negative-bias irradiation followed by a constant positive-bias anneal. This field dependence does not appear to be consistent with interface-trap formation due to hydrogen ion (H+) release in the bulk of the oxide and subsequent drift to the Si/SiO2 interface, proposed by McLean to be the likely cause of interface-trap buildup in metal-gate capacitors. The E−1/2 field dependence observed in this work suggests that the total number of interface traps created in these devices may be determined by hole trapping near the Si/SiO2 interface for positive-bias irradiation, or near the gate/SiO2 interface for negative bias irradiation, though H+ drift remains the likely rate-limiting step in the process. Based on these results, we propose a hole-trapping/hydrogen transport (HT)2 model—involving hole trapping and subsequent near-interfacial H+ release, transport, and reaction at the interface—as a possible explanation of ΔVit buildup in these polysilicon- and metal-gate transistors. © 1990 IEEE