Electrical Measurement of Cell-to-Cell Variation of Critical Charge in SRAM and Sensitivity to Single-Event Upsets by Low-Energy Protons

Abstract

The distribution of single-event upsets (SEUs) in commercial off-the-shelf (COTS) static random access memory (SRAM) has generally been thought to be uniform in a device. However, process-induced variation within a device gives rise to variation within the cell-level electrical characteristic known as the data retention voltage (<inline-formula> <tex-math notation= LaTeX >$V_{\\mathrm{ DR}}$ </tex-math></inline-formula>). Furthermore, <inline-formula> <tex-math notation= LaTeX >$V_{\\mathrm{ DR}}$ </tex-math></inline-formula> has been shown to directly influence the critical charge (<inline-formula> <tex-math notation= LaTeX >$Q_{\\mathrm{ C}}$ </tex-math></inline-formula>) necessary for stored data to upset. Low-energy proton irradiation of COTS SRAMs exhibits a preference toward upsetting cells with lower <inline-formula> <tex-math notation= LaTeX >$Q_{\\mathrm{ C}}$ </tex-math></inline-formula>. An electrical procedure is presented to map relative cell critical charge values without knowledge of the underlying circuitry, allowing customized device usage. Given cell-level <inline-formula> <tex-math notation= LaTeX >$Q_{\\mathrm{ C}}$ </tex-math></inline-formula> knowledge, a device can have its cells “screened” such that those with low <inline-formula> <tex-math notation= LaTeX >$Q_{\\mathrm{ C}}$ </tex-math></inline-formula> are not used to store data. This work presents the results such a screening process has on the SEU per bit cross section.