Patent ID: 7079421

Claim:
A method of improving a data retention ability of a semiconductor memory device having; a plurality of nonvolatile memory cells storing a plurality of memory states, the method comprising the steps of: (a) selecting from the plurality of nonvolatile memory cells a first memory group of nonvolatile memory cells each of which accumulates charges higher in level than a first threshold; (b) selecting from the nonvolatile memory cells in the first memory group a first sub-group of nonvolatile memory cells each of which accumulates charges lower in level than a second threshold; and (c) programming the nonvolatile memory cells in the first sub-group until each of the nonvolatile memory cells in the first subgroup accumulates charges higher in level than the second threshold, wherein each of the plurality of nonvolatile memory cells includes a gate electrode formed on a semiconductor layer via a gate insulating film, a channel region disposed under the gate electrode, diffusion regions disposed on both sides of the channel regions and having a conductive type opposite to that of the channel region, and memory functional units formed on both sides of the gate electrode and having the function of retaining charges.