Physical levels deterioration based determination of thresholds useful for converting cell physical levels into cell logical values in an array of digital memory cells

• US 9,104,550 B2
• Filed: 11/19/2012
• Issued: 08/11/2015
• Est. Priority Date: 12/05/2007
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: DensBits Technologies Ltd. (Broadcom, Inc.)
• Inventors: Weingarten, Hanan, Levy, Shmuel, Katz, Michael
• Primary Examiner(s): Kim, Hong
• Application Number:
  US13/681,246
• Publication Number:
  US 20130080691A1
• Time in Patent Office: 995 Days
• Field of Search: 711/103, 365/185.01
• US Class Current: 1/1
• CPC Class Codes: G06F 12/0246 in block erasable memory, e.g. flash memory
• CPC Class Codes: G06F 2212/1032 Reliability improvement, data loss prevention, degraded operation etc
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles

Abstract

A method for converting a measured physical level of a cell into a logical value, in an array of memory cells storing physical levels which diminish over time, the method may include: determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in the array; and reading the individual cell including reading a physical level in said cell and converting said physical level into a logical value using at least some of said thresholds, wherein said determining extent of deterioration comprises storing predefined physical levels rather than data-determined physical levels in each of a plurality of cells and determining extent of deterioration by computing deterioration of said predefined physical levels.

Claims

1. A method for converting a measured physical level of a cell into a logical value, in an array of memory cells storing physical levels which diminish over time, the method comprising:

determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in the array; and

reading the individual cell including reading a physical level in said cell and converting said physical level into a logical value using at least some of said thresholds, wherein said determining extent of deterioration comprises storing predefined physical levels rather than data-determined physical levels in each of a plurality of cells and determining extent of deterioration by computing deterioration of said predefined physical levels.

2. The method according to claim 1 wherein said predefined physical levels are maximum physical levels.

3. The method according to claim 1 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming that said deterioration is a known function of said physical levels.

4. The method according to claim 3 wherein said known function comprises a linear function.

5. The method according to claim 1 wherein said storing comprises storing said predefined physical levels in cells whose reliability is no different than the reliability of those cells in the array which are not used for storing said predefined physical levels.

6. The method according to claim 1 wherein said array comprises more than one erase sector, each of which includes a subset of cells which together undergo cycles affecting said extent of deterioration;

and wherein said determining of said extent of deterioration is performed no more than once per erase sector.

7. The method according to claim 1 wherein said array of digital memory cells comprises a plurality of pages characterized in that the cells within each page are written onto as a single operation and wherein said determining is performed no more than once per page.

8. The method according to claim 1 wherein said determining comprises computing extent of deterioration by determining deterioration of said predefined physical levels in each of a plurality of cells.

9. The method according to claim 1 wherein said reading comprises binary reading and wherein said predefined physical levels comprise an ascending sequence of physical levels.

10. The method according to claim 9 wherein said ascending sequence is evenly spaced.

11. The method according to claim 1 wherein said plurality of cells comprises a number of cells selected to as to achieve a level of reading errors which does not exceed a predetermined acceptable level.

12. The method according to claim 1 wherein said physical levels comprise charge levels.

13. The method according to claim 1 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of program/erase cycle number.

14. The method according to claim 1 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of address of said predefined physical levels.

15. A method for determining thresholds useful for converting cell physical levels into cell logical values in an array of digital memory cells storing physical levels which diminish over time, the method comprising:

determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in said array; and

reading said individual cell including using reading a physical level in said cell and converting said physical level into a logical value using said thresholds.

16. The method according to claim 15 wherein said array comprises more than one erase sector, each of which includes a subset of cells which together undergo cycles effecting said extent of deterioration;

wherein said determination of extent of deterioration comprises storing predefined physical levels rather than data physical levels in each of a plurality of cells and wherein said storing is performed no more than once per erase sector.

17. The method according to claim 16 wherein said determination of the extent of deterioration is performed no more than once per erase sector and said threshold determining is performed once for each of a plurality of pages included in each erase sector.

18. A computer program product comprising a non-transitory computer useable medium having computer readable program code having embodied therein a method for determining thresholds useful for converting cell physical levels into cell logical values in an array of memory cells storing physical levels which diminish over time, said computer program product comprising:

computer readable program code for;

determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in said array; and

reading said individual cell including using reading a physical level in said cell and converting said physical level into a logical value using said thresholds, wherein said determining comprises storing predefined physical levels rather than data-determined physical levels in each of a plurality of cells and computing extent of deterioration by determining deterioration of said predefined physical levels.

19. The computer program product of claim 18 wherein said predefined physical levels are maximum physical levels.

20. The computer program product of claim 18 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming that said deterioration is a known function of said physical levels.

21. The computer program product of claim 20 wherein said known function comprises a linear function.

22. The computer program product of claim 18 wherein said storing comprises storing said predefined physical levels in cells whose reliability is no different than the reliability of those cells in the array which are not used for storing said predefined physical levels.

23. The computer program product of claim 18 wherein said array comprises more than one erase sector, each of which includes a subset of cells which together undergo cycles affecting said extent of deterioration;

and wherein said determining of said extent of deterioration is performed no more than once per erase sector.

24. The computer program product of claim 18 wherein said array of digital memory cells comprises a plurality of pages characterized in that the cells within each page are written onto as a single operation and wherein said determining is performed no more than once per page.

25. The computer program product of claim 18 wherein said determining comprises computing extent of deterioration by determining deterioration of said predefined physical levels in each of a plurality of cells.

26. The computer program product of claim 18 wherein said reading comprises binary reading and wherein said predefined physical levels comprise an ascending sequence of physical levels.

27. The computer program product of claim 26 wherein said ascending sequence is evenly spaced.

28. The computer program product of claim 18 wherein said physical levels comprise charge levels.

29. The computer program product of claim 18 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of program/erase cycle number.

30. The computer program product of claim 18 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of address of said predefined physical levels.

31. A system for determining thresholds useful for converting cell physical levels into cell logical values in an array of memory cells storing physical levels which diminish over time, the system comprising:

deterioration determining apparatus determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in said array; and

reading circuitry operative to read said individual cell including reading a physical level in said cell and converting said physical level into a logical value using said thresholds, wherein said deterioration determining apparatus is operative to store predefined physical levels rather than data-determined physical levels in each of a plurality of cells and to compute extent of deterioration by determining deterioration of said predefined physical levels.

32. The system of claim 31 wherein said predefined physical levels are maximum physical levels.

33. The system of claim 31 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming that said deterioration is a known function of said physical levels.

34. The system of claim 33 wherein said known function comprises a linear function.

35. The system of claim 31 wherein said storing comprises storing said predefined physical levels in cells whose reliability is no different than the reliability of those cells in the array which are not used for storing said predefined physical levels.

36. The system of claim 31 wherein said array comprises more than one erase sector, each of which includes a subset of cells which together undergo cycles affecting said extent of deterioration;

and wherein said determining of said extent of deterioration is performed no more than once per erase sector.

37. The system of claim 31 wherein said array of digital memory cells comprises a plurality of pages characterized in that the cells within each page are written onto as a single operation and wherein said determining is performed no more than once per page.

38. The system of claim 31 wherein said determining comprises computing extent of deterioration by determining deterioration of said predefined physical levels in each of a plurality of cells.

39. The system of claim 31 wherein said reading comprises binary reading and wherein said predefined physical levels comprise an ascending sequence of physical levels

40. The system of claim 39 wherein said ascending sequence is evenly spaced.

41. The system of claim 31 wherein said physical levels comprise charge levels.

42. The system of claim 31 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of program/erase cycle number.

43. The system of claim 31 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of address of said predefined physical levels.