Relationship between IBICC imaging and SEU in CMOS ICs

Ion-beam-induced charge-collection imaging (IBICC) has been used to study the single event upset (SEU) mechanisms of the Sandia TA670 16-kb static random-access memory (SRAM). Quantitative charge-collection spectra from known regions of the memory cell have been derived using this technique. For 2.4-MeV He ions at normal incidence, charge collection depth for a reverse-biased p+ drain strike is estimated to be 4.8±0.4 μm. Heavy-ion strikes to the reverse-biased p-well result in nearly complete collection of deposited charge to a depth of 5.5±0.5 μm. A charge amplification effect in the n-on drain is identified and is due to either bipolar amplification or a shunt effect in the parasitic vertical npn bipolar transistor associated with the n+/n substrate, p-well, and n+drain. This effect is present only when the n+drain is at 0 V bias. When coupled with previous SEU-imaging, these results strongly suggest that the dominant SEU mechanism in this SRAM is a heavy-ion strike to the n-on transistor drain