Effects of process parameter distributions and ion strike locations on SEU cross-section data [CMOS SRAMs]

The effect of statistical parameter distributions and stochastic ion strike locations in hardened memory arrays on observed SEU (single event upset) cross-section data is discussed. Application of numerical analysis to the parasitic bipolar gain distributions in SIMOX (separation by implantation of oxygen) SRAMs (static random-access memories) and the introduction of an effective critical charge based on sensitivities to ion strike locations explain the nonsaturating behavior of measured cross-section curves in SOI (silicon-on-insulator) and other hardened memories. A direct, analytical relationship between the measured cross-section curves seen in contemporary SOI CMOS SRAMs and the distribution of the parasitic bipolar β values controlling each cell critical charge is presented. Results of this technique clearly indicate that distributed processing parameters and random ion strike locations lead to nonideal cross-section curves. The results also indicate that if the distribution for a critical parameter is known, the LET (linear energy transfer) threshold and shape of the upset cross-section curves can be anticipated