Heavy ions impinging on thin silicon dioxide layers generate a dense plasma of electrons and holes. Under particular conditions, such as in thin oxide layer surrounding isolated conductive lines, this plasma can act as a conductive medium, able to carry current for very short times (10−14s). We studied this phenomenon by using large data set obtained on state-of-the art floating gate memory arrays. Floating gates hit by ions experience a charge loss linearly dependent on ion linear energy transfer and on the electric field across the tunnel oxide. Despite its absolute low value, reaching at most thousands of electrons, charge lost from floating gate exceeds by orders of magnitude that expected from relatively simple models, such as generation and recombination. The model we are proposing is fully consistent with a broad range of theoretical and experimental results, and has excellent fitting capabilities.

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We have addressed the problem of threshold voltage ( TH ) variation in flash memory cells after heavy-ion irradiation by using specially designed array structures and test instruments.After irradiation, low TH tails appear in TH distributions, growing with ion linear energy transfer (LET) and fluence. In particular, high LET ions, such as iodine used in this paper, can produce a bit flip. Since the existing models cannot account for large charge losses from the floating gate, we propose a new mechanism, based on the excess of positive charge produced by a single ion, temporarily lowering the tunnel oxide barrier (positive charge assisted leakage current) and enhancing the tunneling current. This mechanism fully explains the experimental data we present.

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