Saturation of Threshold Voltage Shift in MOSFET's at High Total Dose

We present the results of an investigation into the buildup of trapped positive oxide charge responsible for a negative component of radiation-induced threshold voltage shift in both hard and soft metaloxide semiconductor (MOS) gate oxides and the processes which limit this buildup. Hole-trapping effects at doses to 15 Mrad(SiO2) were examined in MOS field-effect transistors (MOSFET's) and MOS capacitors with 11- to 27-nm gate oxides. The observed saturation of threshold voltage shift was modeled with the aid of a computer simulation of charge buildup in an MOS structure and was found to be caused by a complex interaction between trap filling and recombination of radiation-generated free electrons with trapped holes, modulated by trapped-hole-induced distortion of the oxide electric field. A supplemental measurement of 10-keV x-ray-induced currents in MOS capacitors produced no evidence for radiation-generated hot electron injection from the Si substrate into SiO2 layers of various thicknesses and also yielded data on x-ray-induced charge generation in the SiO2.