Memory with cell population distribution assisted read margining

• US 7,716,538 B2
• Filed: 09/27/2006
• Issued: 05/11/2010
• Est. Priority Date: 09/27/2006
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Gonzalez, Carlos J., Guterman, Daniel C.
• Primary Examiner(s): Baker; Stephen M
• Application Number:
  US11/535,879
• Publication Number:
  US 20080077842A1
• Time in Patent Office: 1,322 Days
• Field of Search: 714/709, 365/189.07, 365/189.09
• US Class Current: 714/709
• CPC Class Codes: G11C 16/04 using variable threshold transistors, e.g. FAMOS
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles
• CPC Class Codes: G11C 2029/0411 Online error correction

Abstract

A memory using techniques to extract the data content of its storage elements, when the distribution of stored states is degraded, is presented. If the distribution of stored states has degraded, secondary evaluations of the memory cells are performed using modified read conditions. Based upon the results of these supplemental evaluations, the memory device determines the read conditions at which to best decide the data stored.

Claims

1. A memory device comprising:

a array of memory cells each storing one of at least two data states;

read circuitry connectable to the array to evaluate the memory cells at a first reference condition and at a plurality of secondary reference conditions; and

logic and control circuitry connectable to the read circuitry to establish a read condition at which to establish the memory cells that contain data corresponding to a selected one of the data states based on comparing the number of memory cells evaluated at the first reference condition and at the second reference conditions, wherein the read condition is established based on the rate of change of number of memory cells evaluated at the plurality of reference conditions.

2. The memory device of claim 1, wherein the established read condition is one of said secondary reference conditions.

3. The memory device of claim 1, wherein the established read condition is a reference condition other than one of said first and secondary reference conditions.

4. The memory device of claim 1, wherein said memory device includes a memory containing the memory cells and a controller, wherein the logic and control circuitry is within the memory.

5. The memory device of claim 1, wherein said memory device includes a memory containing the memory cells and a controller, wherein a substantial portion of said logic and control circuitry is on the controller.

6. The memory device of claim 1, further comprising a plurality of reference cells, wherein said first reference condition is established by one or more of the reference cells.

7. The memory device of claim 1, wherein said logic and control circuitry is further connectable to the read circuitry to determine whether the evaluation result at the first reference condition has unacceptable error, wherein the read circuitry evaluates the memory cells at the plurality of secondary reference conditions is in response to the evaluation result at the first reference point having unacceptable error.

8. The memory device of claim 7, further comprising error correction code circuitry, wherein said determining whether the evaluation result at the first reference condition has unacceptable error is based on an error correction code result.

9. The memory device of claim 1, wherein the first reference condition is a first set of bias conditions and the plurality of secondary reference conditions are a plurality of secondary sets of bias conditions.

10. The memory device of claim 9, wherein the first set of bias conditions and the secondary sets of bias conditions are distinguished from each other by a differing control gate voltage.

11. The memory device of claim 1, wherein said memory cells are non-volatile memory cells.

12. The memory device of claim 11, wherein said memory cells are charge storing devices.

13. The memory device of claim 12, wherein said memory device is a flash memory.

14. The memory device of claim 1, wherein the plurality of evaluation conditions use the same bias conditions, but differing reference points.

15. The memory device of claim 14, wherein said reference points are current levels.

16. The memory device of claim 14, wherein said reference points are voltage levels.

17. The memory device of claim 14, wherein said reference points are time values.

18. A memory device comprising:

a memory array having a plurality of memory cells;

read circuitry connectable to the memory array to evaluate the memory cells using standard read conditions and a plurality of secondary read conditions; and

control and logic circuitry connectable to the read circuitry to determine whether an evaluation using standard read conditions has an unacceptable level of error and, in response to the evaluation using standard read conditions having an unacceptable level of error, to perform evaluations of the memory cells using a plurality of secondary read conditions, extract information on the distribution of programmed state populations of the memory cells based on the results of the evaluations using the standard read conditions and the plurality of secondary read conditions, and establish modified read conditions differing from the standard read conditions at which to evaluate the memory cells to determine their data content based on said information on the distribution of programmed state populations.

19. The memory device of claim 18, wherein the modified read conditions are one of said secondary read conditions.

20. The memory device of claim 18, wherein the modified read conditions are read conditions other than one of said standard and secondary read conditions.

21. The memory device of claim 18, wherein said memory device includes a memory containing the memory cells and a controller, wherein the control and logic circuitry is within the memory.

22. The memory device of claim 18, wherein said memory device includes a memory containing the memory cells and a controller, wherein a substantial portion of said control and logic circuitry is on the controller.

23. The memory device of claim 18, further comprising a plurality of reference cells, wherein said first reference condition is established by one or more of the reference cells.

24. The memory device of claim 18, wherein the control and logic circuitry extracts information on the distribution of programmed state populations of the memory cells by comparing the number of memory cells evaluated using the standard read conditions, the number of memory cells evaluated using the plurality of secondary read conditions, and the rate of change of the number of memory cells evaluated at the standard and secondary read conditions.

25. The memory device of claim 24, further comprising error correction code circuitry, wherein the determination of whether the evaluation using standard read conditions has an unacceptable level of error is based on an error correction code result.

26. The memory device of claim 18, wherein the standard read conditions are a first set of bias conditions and the plurality of secondary read conditions are a plurality of secondary sets of bias conditions.

27. The memory device of claim 26, wherein the first set of bias conditions and the secondary sets of bias conditions are distinguished from each other by a differing control gate voltage.

28. The memory device of claim 18, wherein said memory cells are non-volatile memory cells.

29. The memory device of claim 28, wherein said memory cells are charge storing devices.

30. The memory device of claim 29, wherein said memory device is a flash memory.

31. The memory device of claim 18, wherein the plurality of read conditions use the same bias conditions, but differing reference points.

32. The memory device of claim 31, wherein said reference points are current levels.

33. The memory device of claim 31, wherein said reference points are voltage levels.

34. The memory device of claim 31, wherein said reference points are time values.