Soft-error immunity of a 256kbit DRAM against quasi-heavy ion irradiation has been evaluated using He single ion microprobe at Waseda University. The technique for hitting micron size area with arbitrary number of He ions enables us to simulate the space environment under irradiation of heavy ions with higher LET than that of a-particles. From the maps of error-sensitive sites obtained by irradiating various number of He ions, the threshold LET of soft-errors, effect of diffused charges on the soft-error cross section, and susceptibility to multi-bit errors have been estimated. The results of measuring single-ion induced noise charges at the PN junction which is considered as equivalent to the storage node of the DRAM have been presented and the origins which determine the soft-error immunity under irradiation with various LET have been discussed.
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