Limitations in the Use of Linear System Theory for the Prediction of Hardened-MOS Device Response in Space Satellite Environments

The applicability of widely used linear system theory techniques for predicting CMOS and VMOS device response in low-dose-rate space satellite environments is examined. To apply these techniques, the transient annealing response of several MOS capacitors with hardened gate oxides is measured following short pulsed LINAC irradiation. These responses were found to deviate at late times (0>105 s) from a generally observed linear-with-ln(t) behavior. If device response is assumed to be linear with ln(t) for all times of interest, a smaller radiation response is predicted than is actually measured following low-dose-rate 60Co irradiations at 145 and 0.2 rad(SiO2)/s. To improve on the prediction of device response using linear (with dose) system theory, a numerical integration was performed of the measured transient annealing curve in the ionizing radiation environment. Although this approach provides an excellent fit to the data for the 145-rad(SiO2)/s 60Co irradiation, it yields relatively poor agreement for the 0.2-rad(SiO2)/s 60Co irradiation. 60Co irradiations at 0.2 rad(SiO2)/s indicate an enhanced response from that predicted based on 60Co measurements at 145 rad(SiO2)/s. This dependence of MOS capacitor response on dose rate has been observed for several different gate oxides. Testing at higher dose rates may therefore suggest devices are acceptable that will in fact fail in a low dose-rate environment.