Method of improving data retention ability of semiconductor memory device, and semiconductor memory device

A method of improving a data retention ability of a semiconductor memory device having nonvolatile memory cells storing a plurality of memory states, the method comprising the steps of, in the case where a first plurality of the nonvolatile memory cells comprises memory cells in an erasing state each of which accumulates charges lower in level than a predetermined erasure reference level and a second plurality of the memory cells in a programming state each of which accumulates the charges higher in level than a predetermined program reference level are present:

• (a) selecting a third plurality of the nonvolatile memory cells each of which accumulates charges higher than the erasure reference level and lower than the program reference level;
• and
• (b) programming the selected nonvolatile memory cells until each of the selected nonvolatile memory cells accumulates charges higher than the program reference level, wherein
• each of the 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.

A method of improving a data retention ability of a semiconductor memory device having nonvolatile memory cells storing a plurality of memory states, the method comprising the steps of, in the case where a first plurality of the nonvolatile memory cells comprises nonvolatile memory cells in an erasing state each of which accumulates charges lower in level than a predetermined erasure reference level and a second plurality of memory cells in a programming state each of which accumulates charges higher in level than a predetermined program reference level are present:

• (a) reading first data from a certain nonvolatile memory cell group using a reference voltage level corresponding to the program reference level;
• (b) reading second data from the certain nonvolatile memory cell group using a reference voltage level corresponding to a level obtained by adding a guard band level to the erasure reference level;
• (c) comparing the first data obtained in the step (a) with the second data obtained in the step (b);
• and
• (d) programming at least one nonvolatile memory cell group corresponding to a difference between the first data and the second data, wherein
• each of the 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.

A semiconductor memory device comprising:

• a memory array;
• a command register constituted to receive a data retention command for performing a data retention processing including a step of programming a nonvolatile memory cell that accumulates charges belonging to an intermediate charge region between an erasing state and a programming state;
• and
• a memory array control circuit executing the data retention processing to the memory array in response to the data retention command, wherein
• the memory array includes a plurality of nonvolatile memory cells storing a plurality of memory states, and
• each of the nonvolatile memory cell 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.