Optimized erase operation for non-volatile memory with partially programmed block

Abstract

In connection with an erase operation of a block of non-volatile storage elements, a determination is made as to whether the block is partially but not fully programmed. A degree of partial programming can be determined by a pre-erase read operation which determines a highest programmed word line, or which determines whether there is a programmed storage element in a subset of word lines above a small subset of source side word lines. Since a partially programmed block will pass an erase-verify test more easily than a fully programmed block, a measure is taken to ensure that the block is sufficiently deeply erased. In one approach, an erase-verify test is made stricter by adjusting a sensing parameter when the block is partially programmed. In another approach, the block can be programmed before being erased. Or, an extra erase pulse which is not followed by an erase-verify test can be applied.