Enhancing the Reliability of MLC NAND Flash Memory Systems by Read Channel Optimization

Abstract

NAND flash memory is not only the ubiquitous storage medium in consumer applications but has also started to appear in enterprise storage systems as well. MLC and TLC flash technology made it possible to store multiple bits in the same silicon area as SLC, thus reducing the cost per amount of data stored. However, at current sub-20nm technology nodes, MLC flash devices fail to provide the levels of raw reliability, mainly cycling endurance, that are required by typical enterprise applications. Advanced signal processing and coding schemes are needed to improve the flash bit error rate and thus elevate the device reliability to the desired level. In this article, we report on the use of adaptive voltage thresholds and cell-to-cell interference cancellation in the read operation of NAND flash devices. We discuss how the optimal read voltage thresholds can be determined and assess the benefit of cancelling cell-to-cell interference in terms of cycling endurance, data retention, and resilience to read disturb.