Erase stress and delta erase loop count methods for various fail modes in non-volatile memory

Patent metadata

Field	Value
Publication number	US-9934872-B2
Application number	US-201414528711-A
Country	US
Kind code	B2
Filing date	Oct 30, 2014
Priority date	Oct 30, 2014
Publication date	Apr 3, 2018
Grant date	Apr 3, 2018

Abstract

Techniques are presented for using erase stress and variations in the loop count (number of cycles) for various fail modes in non-volatile memories, including erase disturb and shallow erase. For detection of shallow erase, cells are programmed and then erased, where the variation (delta) in the number of erase loop counts can be used to determine defective blocks. To determine blocks prone to erase disturb, an erase stress is applied to unselected blocks, after which they are programmed: after then erasing one block, the next block can then be read to determine whether it has suffered erase disturb.

First claim

It is claimed:

1. A method of determining defects in a non-volatile flash memory circuit, comprising:
programming memory cells of a first plurality of blocks of an array of the non-volatile flash memory circuit;
subsequently performing an erase operation on the first plurality of blocks, wherein the erase operation includes an alternating series of erase pulses and erase verify operations;
maintaining, for each the first plurality of blocks, a count of the number erase pulses required for the corresponding block to verify as erased in the erase operation;
determining from the counts of a second plurality of blocks from the first plurality of blocks an average erase count;
performing a first defect determination operation on a first block of the first plurality of blocks, including:
comparing the count for first block to the average erase count; and
in response to the count for the first block exceeding the average erase count by a first number of counts, marking the first block as defective; and

subsequent to the first defect determination operation, performing a stress operation on a group of one or more blocks of the first plurality of blocks, comprising:
with the group of blocks unselected, applying an erase stress voltage to the group of blocks; and
subsequently programming the group of blocks; and
subsequent to the stress operation, performing a second defect determination operation, comprising:
performing an erase operation on a first block of the group;
subsequently reading a second, distinct block of the group;
determining whether the quality of the data as read from the second block is degraded; and
in response to determining that the data as read from the second block is degraded, marking the second block as defective.

2. The method of claim 1 , wherein the first defect determination operation further includes:
in response to the count for the first block not exceeding the average erase count by the first number of counts, determining whether the count for the first block exceeds the average erase count by a second number of counts, the second number of counts being less than the first number of counts;
in response to the count for the first block exceeding the average erase count by second number of counts, determining whether the first block had a number failed bits in the verify operation that exceeds a bound; and
in response to the number of failed bits in the verify operation exceeding the bound, marking the first block as defective.

3. The method of claim 1 , wherein the programming memory cells of the first plurality of blocks is performed on a fresh memory circuit.

4. The method of claim 1 , wherein a level and duration of the applied erase voltage is determined as part of a device characterization process.

5. The method of claim 1 , wherein the second block of the group is physically adjacent to the first block of the second group.

6. The method of claim 1 , wherein the group of blocks are formed above a well structure and applying an erase stress voltage to the group of blocks with the group of blocks unselected includes applying an erase voltage to the well structure.

7. The method of claim 6 , wherein the group of blocks being unselected includes floating word lines and select gate of the group blocks.

8. The method of claim 1 , wherein programming the group of blocks includes writing a known data pattern to the blocks, and
wherein determining whether the quality of the data as read from the second block is degraded includes comparing the data as read from the second block to the known data pattern.

9. The method of claim 1 , wherein determining whether the quality of the data as read from the second block is degraded is based on an error checking and correction (ECC) result.

10. The method of claim 1 , wherein the second defect determination operation further comprises:
subsequently performing an erase operation on the second block of the group;
subsequently reading a third, distinct block of the group;
determining whether the quality of the data as read from the third block is degraded; and
in response to determining that the data as read from the third block is degraded, marking the third block as defective.

11. The method of claim 1 , further comprising:
subsequent to performing the second defect determination operation, performing a further defect determination operation, comprising:
performing a further programming operation on one or more blocks of the group, erasing the further programmed blocks, determining whether any of the further programmed blocks are defective based upon the number of erase pulses needed to erase.

12. The method of claim 1 , wherein the non-volatile memory flash circuit is a monolithic three-dimensional semiconductor memory device where the memory cells are arranged in multiple physical levels above a silicon substrate and comprise a charge storage medium.

13. The method of claim 12 , wherein the non-volatile memory flash circuit has a NAND type architecture where the NAND strings run in a vertical direction relative to the substrate, and word lines run in a horizontal direction relative to the substrate.

14. The method of claim 1 , wherein the non-volatile memory circuit is a monolithic two-dimensional semiconductor memory device where the memory cells are arranged in a single physical level.

15. The method of claim 1 , wherein the method is performed as part of a built in self-test (BIST) process.

16. A method of determining defects in a non-volatile flash memory circuit, comprising:
performing a stress operation on a group of one or more blocks of an array of the non-volatile flash memory circuit, comprising:
with the group of blocks unselected, applying an erase stress voltage to the group of blocks; and
subsequently programming the group of blocks; and

subsequent to the stress operation, performing a first defect determination operation, comprising:
performing an erase operation on a first block of the group;
subsequently reading a second, distinct block of the group;
determining whether the quality of the data as read from the second block is degraded; and
in response to determining that the data as read from the second block is degraded, marking the second block as defective; and

subsequent to the first defect determination operation, performing a second defect determination comprising:
subsequently performing an erase operation on the second block of the group;
subsequently reading a third, distinct block of the group;
determining whether the quality of the data as read from the third block is degraded; and
in response to determining that the data as read from the third block is degraded, marking the third block as defective.

17. The method of claim 16 , wherein a level and duration of the applied erase voltage is determined as part of a device characterization process.

18. The method of claim 16 , wherein the second block of the group is physically adjacent to the first block of the second group.

19. The method of claim 16 , wherein the group of blocks are formed above a well structure and applying an erase stress voltage to the group of blocks with the group of blocks unselected includes applying an erase voltage to the well structure.

20. The method of claim 19 , wherein the group of blocks being unselected includes floating word lines and select gate of the group blocks.

21. The method of claim 16 , wherein programming the group of blocks includes writing a known data patte

Assignees

• Sandisk Technologies Llc

Inventors

• Magia Sagar
• Sabde Jagdish
• Pachamuthu Jayavel

Classifications

• G11C29/52 Primary

  Protection of memory contents; Detection of errors in memory contents · CPC title

• G11C16/349

  Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles · CPC title

• G11C2029/0409

  Online test · CPC title

• G06F3/0619

  in relation to data integrity, e.g. data losses, bit errors · CPC title

• G11C29/50004

  of threshold voltage · CPC title