Blocks of 3D-NAND flash memory are programmed in order of WL. In real usage of 3D-NAND flash memory, a block contains both programmed WLs and not programmed WLs because the programmed data do not always fill the block. In the block, the last data are programmed at 'lastprogrammed WL' where the upper WL is not programmed. The last programmed data are supposed to have high reliability owing to shorter data-retention time. However, when upper WL is not programmed, BER of last-programmed WL largely increases because upper WL has less electrons and causes lateral charge migration. To suppress the errors at last-programmed word-line, this paper proposes Last-programmed Word-line Protection (LWLP). Proposed LWLP suppresses BER by 37% at last-programmed WL and extends data-retention time by more than 4 times.
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