A novel evaluation technique for soft errors in Mbit DRAMs (dynamic random access memories) has been developed using a 400 keV proton microprobe system. This technique, which is called soft error mapping, consists of a bit-state mapping image and a secondary electron mapping image, and can reveal the correlation between the incident position of protons and susceptibility against soft errors in DRAMs. Soft errors are found to be induced by proton incidence at 400 keV within about 6 μm around the memory cell in the case of DRAMs with a conventional well. The susceptible area against proton incidence is much larger than the memory cell size. It is found that the area within 4 μm around the memory cell is, in particular, highly sensitive to 400 keV protons. A threshold dose to radiation hardness is estimated by deterioration of the DRAMs during soft error mapping. A buried barrier layer, formed by high-energy ion-implantation, was found to control the charge collection of induced carriers and to suppress soft errors by 400 keV proton microprobes
