METHOD FOR IMPROVING DATA RETENTION OF MULTI LAYER CELL FLASH MEMORY

摘要

PURPOSE: A method for improving data retention of a multi layer cell(MLC) flash memory is provided, which assures an enough window between states and performs refresh automatically to prevent the degradation of data retention. CONSTITUTION: According to the method, a window size between Vt levels is increased by moving a main cell erase voltage(Vt) of a MLC flash memory to a negative region. And the movement of Vt is monitored, and when Vt is moved by a fixed amount, a main cell is refreshed. A stress condition is made worse than the main cell by making Vt of af an erase reference cell lower than the erase Vt of the main cell. And the stress condition is made worse than the main cell by making Vt of a write reference cell higher than a write Vt of the main cell.