Implications of Dopant-Fluctuation-Induced $V_{\rm t}$ Variations on the Radiation Hardness of Deep Submicrometer CMOS SRAMs

Accurately analyzing the single-event (SE) vulnerability of static random-access memory (SRAM) cells leads to precisely calculated soft-error rates (SERs). Random dopant-fluctuation-induced V_t variations affect the SE vulnerability of these memory cells and increase the intercell spread in critical charge (Q_crit), which cause SE upsets. This might consequently lead to higher SERs than would be calculated, assuming a single critical charge. Monte Carlo simulations in the IBM 130- and 90-nm technologies quantify this spread in Q_crit and in SRAM soft-error cross sections with increasing variance. For a radiation-tolerant design, a statistical-design methodology must be used to validate existing hardening schemes and to obtain the expected tolerance levels.