An SEU Tolerant Memory Cell Derived from Fundamental Studies of SEU Mechanisms in SRAM

A new single event upset (SEU) hardening concept, an LRAM cell, is demonstrated theoretically and experimentally. Decoupling resistors in the LRAM are used only to protect against the short n-channel transient; longer persisting pulses are reduced in magnitude by a voltage divider, a basically new concept for SEU protection. In such a design, smaller resistors provide SEU tolerance, allowing higher performance, hardened memories. As basis for the LRAM idea, techniques were developed to measure time constants for ion induced voltage transients in conventional static random access memories, SRAM. Time constants of 0.8 and 6.3 nsec were measured for transients following strikes at the n- and p-channel drains, respectively--primary areas of SEU sensitivity. These data are the first transient time measurements on full memory chips and the large difference is fundamental to the LRAM concept. Test structures of the new design exhibit equivalent SEU tolerance with resistors 5-to-10 times smaller than currently used in SRAM. Our advanced transport-plus-circuit numerical simulations of the SEU process predicted this result and account for the LRAM experiments, as well as a variety of experiments on conventional SRAM.