Threshold Voltage Variability of NROM Memories After Exposure to Ionizing Radiation

Threshold voltage (
V
_th
) behavior of nitride read-only memories (NROMs) was studied after irradiation with photons (γ- and X-rays), light and heavy ions. Both programmed and nonprogrammed single cells were investigated. The data suggest that two main physical phenomena are contributing to
V
_th
variation and that the
V
_th
loss and the variability can be modeled by a Weibull statistics with a shape parameter
k
~ 2.2 regardless of the irradiation species and total dose. The same peculiarities were found in large memory arrays, confirming the results from single-cell studies but with significantly larger statistics. Hence, once the irradiation dose is known, the
V
_th
loss distribution can be obtained, thus providing a predictive model of the radiation tolerance of NROM memory arrays.