A New Programming Scheme for the Improvement of Program Disturb Characteristics in Scaled nand Flash Memory

This paper investigates the new programming scheme to reduce the program disturb in the NAND Flash memory. Program disturb characteristics are determined by the unwilling electron injections in the floating gate of the unselected cells during programming. Thus, the key point to improve the program disturb characteristics is how to suppress the electron injection in the unselected cells. This requirement can be implemented by reducing the number of electrons in the unselected NAND strings prior to programming. By applying negative bias to all the word lines in the selected block, excess electrons can be removed from the channel and source/drain regions into the bit line or the source line using drift and diffusion mechanisms, and also electrons in the surface states can be recombined with accumulated holes before programming. After the pretreatment of electron reduction in the NAND string, a normal NAND program sequence follows. The advantage of the pretreatment before programming has been verified by measuring the 8-Gb NAND Flash memory with a 50-nm technology node. Significant reduction of the threshold voltage shift was observed even after the severe program disturb stress, which corresponds to around 30 times of the programming of the 2 bit/cell operation.