Subpage programming for extending the lifetime of NAND flash memory

During the past decade, the density of NAND flash memory has been increased in many folds. The increase has been driven by storing multiple bits in a cell and scaling down the fabrication process. Such advance in manufacturing technology, however, has been significantly impaired the reliability of flash memory so that it becomes one of the major concerns in use of flash memory. Moreover, as flash memory writes data in the unit of flash page, the trend of the increase in page size worsens the reliability by amplifying a small update to a full flash page programming. In this paper, we propose a new programming method to improve the flash endurance cycle, especially when a small amount of data are written repeatedly. The proposed method, so called “subpage programming”, partitions a page into smaller subpages. A small amount of data can be programmed to one of the subpages while the other subpages are inhibited from the programming by leveraging the mechanisms of flash cell programming. Thus, the number of flash cells that undergo programming is minimized. We evaluated the effect of the proposed subpage programming on real NAND flash memory chips from three different manufacturers. Our evaluation results show that subpage programming improves the flash endurance cycle by up to 258%.