Effects of Angle of Incidence on Proton and Neutron-Induced Single-Event Latchup

The effect of proton angle of incidence on proton-induced single-event latchup (SEL) is investigated in detail at room and elevated temperatures in present-day SRAMs. SRAMs from seven different vendors were irradiated at proton energies from 50 to 200 MeV, at temperatures of 25 degC and 75 degC, and at angles of incidence from 0deg (normal) to 85deg (grazing). The effects of angle of incidence were also investigated for neutron-induced SEL. The angle of incidence can significantly impact SEL hardness. For one SRAM at a temperature of 75 degC, characterizing SEL cross section at grazing angle resulted in a 16 times increase in SEL cross section. Large increases in SEL cross section with angle were also observed for other SRAMs characterized at room temperature. These increases in SEL cross section with angle of incidence are much larger than those measured previously for older SRAM technologies. The mechanism for the effect of angle of incidence on SEL cross section is not due simply to the deposition of more energy in the sensitive volume caused by an increase in path length as the angle of incidence is increased. To investigate possible mechanisms nuclear scattering calculations were performed and combined with device simulations. Simulation results suggest that the mechanism is a consequence of the linear energy transfer (LET) and range distributions of secondary ions produced by proton-material (or neutron-material) interactions coupled with an increase in SEL sensitivity (decrease in LET threshold) as angle of incidence is increased. These results have significant impact on SEL hardness assurance testing, especially for system applications where latchups cannot be tolerated. To best ensure that SEL hardness requirement are met, SRAMs should be characterized at both grazing and normal angles of incidence, and at maximum temperature, voltage, and proton energy