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

A non-volatile storage device, comprising:

• a block formed on a substrate, the block comprises a string of non-volatile storage elements;
• a plurality of word lines connected to the non-volatile storage elements, the word lines extend from a lowest word line at a source side of the block to a highest word line at a drain side of the block;
• and
• a control circuit, the control circuit:
• in connection with an erase operation, makes a determination of a degree of partial programming of the block, the determination is made by one or more read operations to identify a highest programmed word line among predetermined word lines of the plurality of word lines which are progressively further apart from one another, relative to the source side of the block, and in response to the determination of the degree of partial programming of the block, apply an erase voltage to the substrate and subsequently perform an erase-verify test which tests at least one of a resistance or a current in the string of non-volatile storage elements, the erase-verify test is configured according to the degree of partial programming of the block, such that the erase-verify test is relatively stricter when the degree of partial programming of the block is relatively low.

A method for erasing a block, the block is formed on a substrate and comprises a string of non-volatile storage elements, and a plurality of word lines are connected to the non-volatile storage elements, the word lines extend from a lowest word line at a source side of the block to a highest word line at a drain side of the block, the method comprising:

• in connection with an erase operation, performing a sensing operation to determine whether a non-source side adjacent subset of the string is erased, the non-source side adjacent subset of the string is not adjacent to the source side of the block, the performing the sensing operation comprises concurrently:
• applying a demarcation control gate voltage to a plurality of non-volatile storage elements of the non-source side adjacent subset of the string, the demarcation control gate voltage demarcates between an erased state and a programmed state, applying a read control gate voltage to one or more non-volatile storage elements of a source side adjacent subset of the string, the source side adjacent subset of the string is adjacent to the source side of the block, the read control gate voltage renders the one or more non-volatile storage elements of the source side adjacent subset of the string in a conductive state, and sensing a current in the string;
• and
• if the performing the sensing operation determines that the non-source side adjacent subset of the string is erased, determining that the string is partially but not fully programmed.

A non-volatile storage device, comprising:

• a block formed on a substrate, the block comprises a string of non-volatile storage elements;
• a plurality of word lines connected to the non-volatile storage elements, the word lines extend from a lowest word line at a source side of the block to a highest word line at a drain side of the block;
• and
• a control circuit, the control circuit:
• in connection with an erase operation, provides a performance of a sense operation to determine whether a non-source side adjacent subset of the string is erased, the non-source side adjacent subset of the string is not adjacent to the source side of the block, the performance of the sense operation is made by a concurrent:
• application of a demarcation control gate voltage to a plurality of non-volatile storage elements of the non-source side adjacent subset of the string, the demarcation control gate voltage demarcates between an erased state and a programmed state, application of a read control gate voltage to one or more non-volatile storage elements of a source side adjacent subset of the string, the source side adjacent subset of the string is adjacent to the source side of the block, the read control gate voltage renders the one or more non-volatile storage elements of the source side adjacent subset of the string in a conductive state, and a sense operation of a current in the string, and if the performance of the sense operation determines that the non-source side adjacent subset of the string is erased, determines that the string is partially but not fully programmed.