ANALYSIS OF MULTIPLE BIT UPSETS (MBU) IN A CMOS SRAM

Multiple Bit Upsets (MBU) have been studied in a 256k 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 quadru ple upsets in adjacent cells. Both experimental data and 3D simulation s emphasize the role of delayed charge collection, by diffusion, and c harge sharing between sensitive areas. Upset tracks have been recorded at grazing angle and used to determine the charge collection depth. T hese data revealed the existence of an LET threshold for MBU at grazin g angle. As the ion LET increases different types of tracks are observ ed 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 de vice indicates a strong influence of both ion energy and angle of inci dence on the cross section, emphasizing the intrinsic limitation of st andard characterizations with low energy ions. These results indicate that the basic asumption of a rectangular parallelepipedic volume does not take into account coupling phenomena, such as occurs in MBUs, and is no longer valid at grazing angle.