SEU/SRAM as a process monitor

The SEU/SRAM is a 4-kbit static random access memory (SRAM) designed to detect single-event upsets (SEU´s) produced by high-energy particles, either in space or on the ground. This device was used to determine the distribution of the memory cell spontaneous flip potential. The variance in this potential was determined to be due to the variation in the threshold voltage of one of the n-FET´s found in the memory cell. For a 1.2-μm CMOS process, the standard deviation of the n-FET VT was found to be 8 mV. This structure provides substantial statistical data; 4096 data points are measured. Using cumulative distribution and residual plots, outlier cells, such as stuck cells and nonnormally distributed cells, are easily identified