Impact of Irradiation Side on Neutron-Induced Single-Event Upsets in 65-nm Bulk SRAMs

The impact of the irradiation side on the cross sections of single-event upsets (SEUs) induced by neutrons was investigated by performing neutron irradiation measurements and simulations. A test board equipped with 65-nm bulk 6-T CMOS static random access memories was irradiated by quasi-monoenergetic neutrons, and the number of SEUs was counted. The number of SEUs obtained by the board-side irradiation was approximately 20% to 30% smaller than that obtained by irradiation on the plastic package side. We also investigated the impact of irradiation side on the soft error rates (SERs) obtained with by the terrestrial neutron energy spectrum via a Monte Carlo simulation. The SER obtained from the plastic package side irradiation was approximately twice that obtained for the board side irradiation, indicating that SERs can be reduced by equipping the device with the package side facing downward. Additionally, based on the simulation, the atomic composition of the material placed in front of the memory chip has a considerable influence on the SER because production yields and angular distributions of secondary H and He ions (the main causes of SEUs) depend on the composition. In particular, the existence of hydrides, such as plastic, considerably increases the SER because of the higher production yields of secondary H ions that are generated via elastic scattering of neutrons with hydrogen atoms.