Dose and energy dependence of interface trap formation in cobalt-60 and X-ray environments

The radiation-induced buildup of interface traps is examined on radiation-hardened 20-nm-gate-oxide n-channel transistors irradiated with ⁶⁰Co and 10 keV X-rays. A variation in the fundamental dependence of interface-trap (ΔD_it) growth on dose from linear with dose to a square-root dependence was observed. The samples with the largest low-dose interface-trap buildup showed the slowest rate of growth (D^1/2), while the samples with the smallest low-dose ΔD_it showed the fastest buildup (linear with dose). A possible qualitative explanation of this behavior based on cluster centers is given. The interface-trap density growth was observed to tend toward saturation at doses above 3 Mrad (SiO ₂). A photon energy dependence of D_it formation between ⁶⁰Co and X-ray irradiation was not observed