Smart erase verify test to detect slow-erasing blocks of memory cells

A method, comprising:

• performing a specified erase-verify iteration of one or more successive erase-verify iterations for the block in an erase operation for a block of memory cells, the memory cells are arranged in NAND strings and connected to a set of word lines, each NAND string comprising a channel, the performing the specified erase-verify iteration comprises charging up the channels of the NAND strings and performing a first verify test comprising testing an erased level of the NAND strings relative to a first verify voltage to determine whether the block is slow-erasing and a second verify test comprising testing an erased level of the NAND strings relative to a second verify voltage which is lower than the first verify voltage to detect whether the erase operation is completed;
• and
• during the one or more successive erase-verify iterations, charging up the channels of the NAND strings and performing the second verify test but not the first verify test.

An apparatus, comprising:

• a block of memory cells, the memory cells are arranged in NAND strings and connected to a set of word lines, each NAND string comprising a channel;
• and
• a control circuit, the control circuit is configured to sense currents in the NAND strings while applying a first verify voltage to a first set of alternate word lines but not a second set of alternate word lines of the set of word lines in response to receipt of a first command to determine whether the block is a slow-erasing block and to sense currents in the NAND strings while applying a second verify voltage, lower than the first verify voltage, to the first set of alternate word lines of the set of word lines and to sense currents in the NAND strings while separately applying the second verify voltage to the second set of alternate word lines of the set of word lines in response to receipt of a second command to determine whether the block completes an erase operation within a maximum allowable number of erase-verify iterations.