Edge Word-Line Reliability Problem in 3-D NAND Flash Memory: Observations, Analysis, and Solutions

The 3-D flash memory is gradually becoming the mainstream nonvolatile storage medium due to its high capacity and high performance. However, interlayer interference during 3-D flash programming leads to significant differences in the error characteristics of edge and inner word lines; interlayer interference becomes more pronounced as the number of stacked layers increases, seriously affecting data storage reliability. In this study, many actual tests were conducted on triple-level cell (TLC) and quad-level cell (QLC) flash memory, which are the mainstream storage media in the current consumer market, to obtain the edge and inner word-line threshold voltage data under the interference of different factors, such as retention loss and read disturb; then, the threshold voltage difference between the edge and inner word lines under different conditions was quantitatively analyzed. An edge word-line reliability optimization strategy is proposed based on the read-reference voltage extra offset (RRVEO). Experimental results show that this strategy can reduce the edge word-line raw bit error rate (RBER) by more than 90% and eliminate the reliability difference between inner and edge word lines without significant overhead, thus significantly improving the data storage reliability of flash memory.