Erase technique for checking integrity of non-data word lines in memory device and corresponding firmware
An erase operation for data memory cells is integrated with a process for detecting dummy memory cells and/or select gate transistors which have an out-of-range threshold voltage. In one aspect, an erase operation is performed for the data memory cells of a block followed by a supplementary verify operation for the dummy memory cells and/or select gate transistors. In another aspect, the verify operation occurs during the erase operation and, optionally, also in a supplementary verify operation. A separate pass/fail status can be set for the erase verify of the data memory cells and the verify of the dummy memory cells and/or select gate transistors operations, where the block is assigned to a potential bad block pool or bad block pool based on a status return combination. The out-of-range dummy memory cells and/or select gate transistors can be adjusted by programming or erasing.
1. An apparatus, comprising:
a control circuit configured to connect to a set of NAND strings in a block, each NAND string in the set of NAND strings comprising a select gate transistor and data memory cells, and the circuit is configured to:
perform an erase operation for the data memory cells;
perform a verify operation for the select gate transistors relative to at least one boundary voltage of a respective allowable range of threshold voltages;
check a status of the erase operation and a status of the verify operation; and
assign the block to a potential bad block pool if the status of the erase operation is a pass status and the status of the verify operation is a fail status, the status of the erase operation is the pass status when the erase operation is successfully completed and the status of the verify operation is the fail status when more than a specified number of the select gate transistors have a threshold voltage outside the respective allowable range of threshold voltages.
2. The apparatus of claim 1 , wherein:
when the block is in the potential bad block pool and a number of blocks in the potential bad block pool exceeds a threshold, the control circuit is configured to:
re-trim threshold voltages of the select gate transistors; and
after the re-trimming:
perform another erase operation for the data memory cells;
perform another verify operation for the select gate transistors relative to at least one boundary voltage of the respective allowable range of threshold voltages;
check a status of the another erase operation and a status of the another verify operation; and
assign the block to a bad block pool if the status of the another verify operation is the fail status; and
assign the block to a good block pool if the status of the another erase operation and the status of the another verify operation are both the pass status.
3. The apparatus of claim 1 , wherein:
during the verify operation for the select gate transistors, the control circuit is configured to store data identifying a subset of the select gate transistors, the subset having threshold voltages outside the respective allowable range of threshold voltages; and
when the block is in the potential bad block pool and a number of blocks in the potential bad block pool exceeds a threshold, the control circuit is configured to re-trim threshold voltages of the subset of the select gate transistors based on the stored data without re-trimming threshold voltages of remaining select gate transistors of the set of NAND strings.
4. The apparatus of claim 1 , wherein:
the control circuit is configured to perform the verify operation for the select gate transistors after the erase operation is completed.
5. The apparatus of claim 1 , wherein:
the control circuit is configured to perform the verify operation for the select gate transistors concurrent with performing an erase-verify test for the data memory cells in one or more erase-verify loops of the erase operation.
6. The apparatus of claim 1 , wherein the control circuit is configured to:
perform the verify operation for the select gate transistors concurrent with applying a read pass voltage to the data memory cells in a first erase-verify loop of the erase operation; and
perform an erase-verify test for the data memory cells in at least one erase-verify loop of the erase operation which follows the first erase-verify loop but not in the first erase-verify loop.
7. The apparatus of claim 1 , wherein:
the control circuit is configured to assign the block to a good block pool if the status of the erase operation and the status of the verify operation for the select gate transistors are both the pass status.
8. The apparatus of claim 1 , wherein:
the erase operation for the data memory cells comprises a plurality of erase-verify loops; and
the status of the erase operation is the pass status if the erase operation is completed within a maximum allowable number of erase-verify loops.
9. The apparatus of claim 1 , wherein:
each NAND string comprises a first drain-side dummy memory cell; and
the control circuit is configured to:
perform a verify operation for the first drain-side dummy memory cells relative to at least one boundary voltage of a respective allowable range of threshold voltages;
check a status of the verify operation for the first drain-side dummy memory cells; and
assign the block to the potential bad block pool if the status of the verify operation for the first drain-side dummy memory cells is the fail status.
10. The apparatus of claim 9 , wherein:
each NAND string comprises a second drain-side dummy memory cell; and
the control circuit is configured to:
perform a verify operation for the second drain-side dummy memory cells relative to at least one boundary voltage of a respective allowable range of threshold voltages, separately from the verify operation for the first drain-side dummy memory cells;
check a status of the verify operation for the second drain-side dummy memory cells; and
assign the block to the potential bad block pool if the status of the verify operation for the second drain-side dummy memory cells is the fail status.
11. The apparatus of claim 9 , wherein:
the control circuit is configured to perform the verify operation for the first drain-side dummy memory cells concurrent with performing an erase-verify test for the data memory cells relative to an erase-verify voltage in one or more erase-verify loops of the erase operation.
12. The apparatus of claim 9 , wherein the control circuit is configured to:
perform the verify operation for the first drain-side dummy memory cells concurrent with applying a read pass voltage to the data memory cells in a first erase-verify loop of the erase operation; and
perform an erase-verify test for the data memory cells in at least one erase-verify loop of the erase operation which follows the first erase-verify loop but not in the first erase-verify loop.
13. The apparatus of claim 9 , wherein:
the control circuit is configured to assign the block to a good block pool if the status of the erase operation, the status of the verify operation for the select gate transistors, and the status of verify operation for the first drain-side dummy memory cells are all the pass status.
14. A method, comprising:
issuing one or more commands for a block, the block comprising a set of NAND strings, each NAND string in the set of NAND strings comprising a select gate transistor and data memory cells, the one or more commands are for an erase operation of the data memory cells and a verify operation for the select gate transistors relative to at least one boundary voltage of a respective allowable range of threshold voltages;
issuing a check status command to obtain a status of the erase operation and a check status command to obtain a status of the verify operation; and
deciding whether to assign the block to a potential bad block pool based on the status of the erase operation and the status of the verify operation.
15. The method of claim 14 , wherein:
the one or more commands comprise a prefix command followed by an erase block command for the block, the erase block command is for the erase operation and the prefix command is for the verify operation.
16. The method of claim 14 , wherein:
the one or more command comprise a device parameter which enables the erase operation and the verify operation.
17. The method of claim 14 , further comprising:
re-trimming threshold voltages of the select gate transistors in response to the block being identified as being in the potential bad block pool; and
after the re-trimming:
issuing one or more commands for another erase operation of the data memory cells and another verify operation for the select gate transistors relative to at least one boundary voltage of the respective allowable range of threshold voltages
issuing a check status command to obtain a status of the another erase operation and a check status command to obtain a status of the another verify operation; and
deciding whether to assign the block to a bad block pool based on the status of the another erase operation and the status of the another verify operation.
18. The method of claim 17 , further comprising:
identifying the block as being in the bad block pool when the status of the another erase operation and the status of the another verify operation are not both a pass status; and
identifying the block as being in a good block pool when the status of the another erase operation and the status of the another verify operation are both the pass status.
19. An apparatus, comprising:
a control circuit configured to connect to a set of NAND strings in a block, each NAND string in the set of NAND strings comprising a select gate transistor and data memory cells; and
an interface connected to the control circuit, the circuit is configured to issue commands via the interface to:
perform an erase operation for the data memory cells, the performing the erase operation comprises performing a plurality of erase-verify loops for the set of NAND strings, each erase-verify loop comprising an erase portion and a verify portion;
in the erase portion of each erase-verify loop, bias the data memory cells for erase;
in the verify portion of each erase-verify loop, perform a verify operation for the select gate transistors relative to at least one boundary voltage of a respective allowable range of threshold voltages while concurrently performing an erase-verify test for the data memory cells; and
identify the block as being faulty if the erase-verify test for the data memory cells is not successful upon completion of the plurality of erase-verify loops.
20. The apparatus of claim 19 , wherein:
the plurality of erase-verify loops comprise a maximum allowable number of erase-verify loops for the erase operation.