Method of restoring data in non-volatile semiconductor memory

Abstract

Disclosed is a method of restoring data in a non-volatile semiconductor memory which has a memory cell array that a plurality of electrically programmable memory cells to be set of several threshold voltages are matrix-disposed, a data erasing means that divides the memory cell array into several blocks and erases in the lump data stored in memory cells included in the divided block, a data writing means that writes data into the memory cells, a data reading means that reads out data from the memory cells, and a control means that controls the operations of the data erasing means, data writing means and data reading means, wherein the non-volatile semiconductor memory has a function that detects the deterioration of data stored in the memory cells and then restores it, the method having the steps of: detecting the data-deterioration state of memory cells included in the block; conducting a light erasing operation with a threshold voltage change smaller than that of an ordinary erasing operation to an erase block including a memory cell with a data-deterioration state that cannot be restored by additional writing; then detecting the data-deterioration state of the memory cells again; and restoring data of a memory cell that a data deterioration is detected.