Soft errors in DRAMs and SRAMs have been evaluated using nuclear microprobes and neutron sources. Direct observation of soft errors in a DRAM such as soft error mapping indicates both cell-mode and bit- line-mode soft errors at local positions. Well engineering parameters, such as retrograde wells by high energy ion implantation in DRAMs and SRAMs, are directly evaluated by ion beam induced current (IBIC) using nuclear microprobes. Neutron induced soft errors caused by boron in BPSG (borophosphocilicate) layers have been clarified. Floating body effect in partially depleted SOI(silicon-on-insulator) MOSFETs and the effectiveness of body-tie structures in SOI MOSFETs have been directly observed using nuclear microprobe irradiation.