Influences on soft error rates in static RAMs

Alpha-particle-induced soft error rates (SERs) in RAMs were measured by exposing commercial chips, with lid and protective coating removed, to an Americium-241 alpha source. These measurements have shown that, under normal operating conditions, resistive load SRAMs can be as sensitive as DRAMs. Measurements of variations of SER with cycle time and supply voltage were in broad agreement with a model previously suggested by other workers, and explanations are proposed for the different regions of operation. The effect of an alpha-particle hit on SRAM cell is simulated using SPICE with typical process parameters. A simple analysis of the model highlights the features that determine the critical charge. The behavior predicted by this model is different in a small but significant way from that predicted by previous workers: it was confirmed by SPICE simulation. Various parameters in the cell model were changed in order to investigate the effects on the alpha-particle sensitivity, and the results of these simulations suggest a mechanism by which the sensitivity could most economically be improved. By combining the result of the simulations and experiments, quantitative estimates of expected SER improvements are presented.