Erase and soft program for vertical NAND flash

1. A method comprising:

erasing a selected block of NAND memory comprising two or more sub-blocks;

performing a soft program cycle on the selected block of NAND memory;

determining a subset of the two or more sub-blocks;

validating sub-blocks of the subset by:

checking threshold voltages of the two or more sub-blocks against a target minimum voltage; and

including sub-blocks that have a threshold voltage lower than the target minimum voltage in the subset;

executing an other soft program cycle on the selected block of NAND memory if none of the sub-blocks of the subset pass validation;

repeating said validating and said executing until at least one of the sub-blocks of the subset passes validation;

verifying the two or more sub-blocks in the subset until a sub-block fails to verify;

stopping the verification in response to the failed verify;

performing another erase cycle on all of the selected block of NAND memory; and

re-starting to verify the two or more sub-blocks at the sub-block that failed to verify.

2. The method of claim 1 , wherein the sub-blocks of the subset are validated serially.

3. The method of claim 1 , further comprising:

(a) validating the sub-blocks of the subset again;

(b) performing an other soft program cycle on the selected block of NAND memory; and

repeating (a) and (b) until all of the sub-blocks of the subset pass validation;

wherein said (a) validating the sub-blocks of the subset again skips a sub-block after a sub-block being validated has passed validation.

4. The method of claim 1 , further comprising programming one or more dummy cells of the two or more sub-blocks of the selected block of NAND memory to have a positive threshold voltage.