Improvement of soft-error rate in MOS SRAMs

Two techniques which reduce the α-particle-induced soft-error rate (SER) in MOS static RAMs (SRAMs) are described. The mechanism of the soft error is the high-resistive load memory cell is analyzed. It is found that the dependence of SER on the cycle time is caused by the potential drop in the high storage node, which is produced by the threshold current through the driver and access transistors in the memory cell. Improvement methods to suppress the subthreshold current are presented. One method utilizes high-threshold-voltage transistors in the memory cell. The other sets the selected word-line level lower than the supply voltage. Using these methods, the high storage node potential is kept at the supply voltage in spite of the small conductance of the load resistor. The effect is confirmed in 256 kbit CMOS SRAMs. The dependence of SER on the cycle time becomes negligible, and SER is improved by two orders of magnitude