Analysis of local and global transient effects in a CMOS SRAM

We have studied the sensitivity of a 256 kbit CMOS/epi SRAM to transient phenomena (protons and ions SEU, dose rate). Local and global failure mechanisms are observed and discussed. For single particles, or at low dose rate, cell upsets are uncorrelated. When the dose rate is increased, correlated failures are due to both cell photocurrent summation (rail span collapse) and supply voltage drop in peripheral circuits. The transition between uncorrelated to correlated failures is interpreted, on the basis of device structure and pattern influence.