A NAND flash memory is well known as a kind of an electrically rewritable nonvolatile semiconductor memory device. A multi-level cell (MLC) storage system in which a threshold distribution of a memory cell is segmented to be able to store plural bits in the memory cell is used as a technology of increasing the storage capacity of the NAND flash memory.
In the memory cell of the NAND flash memory, a write voltage is applied to a control gate of the memory cell, and electrons are injected in a charge storage layer using a potential difference between the control gate and a substrate, thereby writing data. Because of the circuit configuration of the NAND flash memory, the write operation is performed as a unit of all the memory cells connected to one word line.
When the electrons are injected in the charge storage layer of the memory cell, sometimes a threshold voltage of an adjacent memory cell in which the data is already written is shifted by the influence of capacitive coupling. Additionally, a shift amount of the threshold voltage varies according to a data pattern of the adjacent memory cell due to the shrinking of the memory cell, which affects the threshold distribution. Therefore, sometimes the data of the memory cell is incorrectly read, namely, a frequency of erroneous read is increased to degrade the reliability of the NAND flash memory.