Non-volatile memory with efficient testing during erase

Abstract

When erasing multiple sub-blocks of a block, erase verify is performed for memory cells connected to even word lines to generate even results and for memory cells connected to odd word lines to generate odd results. The even results and the odd results are used to determine that the erase verify process successfully completed. For each NAND string of a first sub-block, a last even result for the NAND string is compared to a last odd result for the NAND string. Despite the determination that the first sub-block successfully completed erase verify, the erasing failed for the first sub-block because the number of NAND strings that have the last even result different than the last odd result is greater than a limit. The system determines that one or more additional sub-blocks also failed erasing based on and in response to determining that the first sub-block failed erasing.