Analysis of multiple bit upsets (MBU) in CMOS SRAM

Multiple Bit Upsets (MBU) have been studied in a 256 k CMOS static RAM irradiated at normal incidence and grazing angle. In normal incidence the sensitive areas have been identified with pulsed laser irradiation. The laser power thresholds have been determined for single to quadruple upsets in adjacent cells. Both experimental data and 3D simulations emphasize the role of delayed charge collection, by diffusion, and charge sharing between sensitive areas. Upset tracks have been recorded at grazing angle and used to determine the charge collection depth. These data revealed the existence of an LET threshold for MBU at grazing angle. As the ion LET increases different types of tracks are observed and correlated to the topological pattern in adjacent memory cells. This phenomenon is due to an unexpected charge collection mechanism, which couples adjacent sensitive areas and results in charge transfer between memory cells. The comparison with previous data on the same device indicates a strong influence of both ion energy and angle of incidence on the cross section, emphasizing the intrinsic limitation of standard characterizations with low energy ions. These results indicate that the basic assumption of a rectangular parallelepipedic volume does not take into account coupling phenomena, such as occurs in MBUs, and is no longer valid at grazing angle.