Rejuvenation of analog memory cells

• US 8,248,831 B2
• Filed: 12/30/2009
• Issued: 08/21/2012
• Est. Priority Date: 12/31/2008
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Rotbard, Barak, Sommer, Naftali, Winter, Shai, Shalvi, Ofir, Sokolov, Dotan, Ordentlich, Or, Anholt, Micha
• Primary Examiner(s): Hoang, Huan
• Application Number:
  US12/649,382
• Publication Number:
  US 20100165689A1
• Time in Patent Office: 965 Days
• Field of Search: 365/45, 365/185.03, 365/222, 365/185.25, 365/185.33
• US Class Current: 365/45
• CPC Class Codes: G11C 11/56 using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
• CPC Class Codes: G11C 11/5621 using charge storage in a floating gate
• CPC Class Codes: G11C 13/00 Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
• CPC Class Codes: G11C 13/0033 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 13/0035 Evaluating degradation, retention or wearout, e.g. by counting writing cycles
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 16/3431 Circuits or methods to detect disturbed nonvolatile memory cells, e.g. which still read as programmed but with threshold less than the program verify threshold or read as erased but with threshold greater than the erase verify threshold, and to reverse the disturbance via a refreshing programming or erasing step
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles
• CPC Class Codes: G11C 2029/0409 Online test
• CPC Class Codes: G11C 2029/5002 Characteristic
• CPC Class Codes: G11C 27/005 with non-volatile charge storage, e.g. on floating gate or MNOS
• CPC Class Codes: G11C 29/00 Checking stores for correct operation ; Subsequent repair; Testing stores during standby or offline operation
• CPC Class Codes: G11C 29/52 Protection of memory contents; Detection of errors in memory contents

Abstract

A method for data storage in a memory that includes multiple analog memory cells fabricated using respective physical media, includes identifying a group of the memory cells whose physical media have deteriorated over time below a given storage quality level. A rejuvenation process, which causes the physical media of the memory cells in the group to meet the given storage quality level, is applied to the identified group. Data is stored in the rejuvenated group of the memory cells.

Claims

1. A method for data storage, comprising:

in a memory that includes multiple analog memory cells fabricated using respective physical media, identifying a group of the memory cells whose physical media have deteriorated over time below a given storage quality level;

applying to the identified group a rejuvenation process, which repairs the physical media of the memory cells in the group and thus causes the physical media of the memory cells in the group to meet the given storage quality level; and

storing data in the rejuvenated group of the memory cells.

2. The method according to claim 1, wherein applying the rejuvenation process comprises causing removal of electrical charge that is trapped in the physical media of the memory cells in the group.

3. The method according to claim 1, wherein applying the rejuvenation process comprises inhibiting programming of the memory cells in the group for a given time period.

4. The method according to claim 1, wherein applying the rejuvenation process comprises heating the memory cells in the group.

5. The method according to claim 4, wherein heating the memory cells comprises activating a heating device that is thermally coupled to the group of the memory cells.

6. The method according to claim 4, wherein heating the memory cells comprises applying memory access operations to some of the memory cells in the memory.

7. The method according to claim 4, wherein heating the memory cells comprises preventing heat removal from the group.

8. The method according to claim 4, wherein heating the memory cells comprises directing to the memory cells in the group heat generated by another component that is not part of the group.

9. The method according to claim 1, wherein applying the rejuvenation process comprises attempting to erase or program the memory cells in the group.

10. The method according to claim 1, wherein applying the rejuvenation process comprises copying the data from the group to an alternative location in the memory before rejuvenating the group.

11. The method according to claim 1, wherein applying the rejuvenation process comprises heating the memory cells in the group while periodically refreshing the data stored in the memory cells in the group.

12. The method according to claim 11, wherein periodically refreshing the data comprises refreshing the data during the rejuvenation process at a rate that is higher than a nominal refresh rate used for refreshing the data in the memory.

13. The method according to claim 1, wherein identifying the group comprises programming respective storage values into the memory cells in the group, measuring a shift that develops in the programmed storage values over a predefined time period, and determining, based on the measured shift, that the physical media have deteriorated below the given storage quality level.

14. The method according to claim 13, and comprising copying the data from the group to another group of memory cells before programming the respective storage values into the memory cells in the group.

15. The method according to claim 1, wherein identifying the group comprises making a prediction that the physical media of the group of the memory cells is expected to deteriorate below the given storage quality level, so as to apply the rejuvenation process responsively to the prediction.

16. The method according to claim 15, wherein making the prediction comprises estimating an integral of a temperature of the memory cells in the group over time, and initiating the rejuvenation process responsively to the integral.

17. The method according to claim 15, wherein making the prediction comprises estimating one or more statistical properties of analog values stored in the memory cells in the group, and initiating the rejuvenation process responsively to the estimated statistical properties.

18. The method according to claim 17, wherein estimating the statistical properties comprises predicting a number of errors in the group of the memory cells based on the statistical properties.

19. Apparatus for data storage, comprising:

an interface for communicating with a memory that includes multiple analog memory cells fabricated using respective physical media; and

circuitry, which is configured to identify a group of the memory cells whose physical media have deteriorated over time below a given storage quality level, to apply to the identified group a rejuvenation process, which repairs the physical media of the memory cells in the group and thus causes the physical media of the memory cells in the group to meet the given storage quality level, and to store data in the rejuvenated group of the memory cells.

20. The apparatus according to claim 19, wherein the rejuvenation process causes removal of electrical charge that is trapped in the physical media of the memory cells in the group.

21. The apparatus according to claim 19, wherein the circuitry is configured to apply the rejuvenation process by inhibiting programming of the memory cells in the group for a given time period.

22. The apparatus according to claim 19, wherein the circuitry is configured to apply the rejuvenation process by heating the memory cells in the group.

23. The apparatus according to claim 22, wherein the circuitry comprises a heating device that is thermally coupled to the group of the memory cells, and is configured to heat the memory cells in the group by activating the heating device.

24. The apparatus according to claim 22, wherein the circuitry is configured to heat the memory cells in the group by applying memory access operations to some of the memory cells in the memory.

25. The apparatus according to claim 22, wherein the circuitry is configured to heat the memory cells in the group by preventing heat removal from the group.

26. The apparatus according to claim 22, wherein the circuitry is configured to heat the memory cells in the group by directing to the memory cells in the group heat generated by another component that is not part of the group.

27. The apparatus according to claim 19, wherein the circuitry is configured to apply the rejuvenation process by attempting to erase or program the memory cells in the group.

28. The apparatus according to claim 19, wherein the circuitry is configured to copy the data from the group to an alternative location in the memory before rejuvenating the group.

29. The apparatus according to claim 19, wherein the circuitry is configured to apply the rejuvenation process by heating the memory cells in the group while periodically refreshing the data stored in the memory cells in the group.

30. The apparatus according to claim 25, wherein the circuitry is configured to refresh the data during the rejuvenation process at a rate that is higher than a nominal refresh rate used for refreshing the data in the memory.

31. The apparatus according to claim 19, wherein the circuitry is configured to identify the group by programming respective storage values into the memory cells in the group, measuring a shift that develops in the programmed storage values over a predefined time period, and determining, based on the measured shift, that the physical media have deteriorated below the given storage quality level.

32. The apparatus according to claim 31, wherein the circuitry is configured to copy the data from the group to another group of memory cells before programming the respective storage values into the memory cells in the group.

33. The apparatus according to claim 19, wherein the circuitry is configured to make a prediction that the physical media of the group of the memory cells is expected to deteriorate below the given storage quality level, and to apply the rejuvenation process responsively to the prediction.

34. The apparatus according to claim 33, wherein the circuitry is configured to estimate an integral of a temperature of the memory cells in the group over time, and to initiate the rejuvenation process responsively to the integral.

35. The apparatus according to claim 33, wherein the circuitry is configured to estimate one or more statistical properties of analog values stored in the memory cells in the group, and to initiate the rejuvenation process responsively to the estimated statistical properties.

36. The apparatus according to claim 35, wherein the circuitry is configured to predict a number of errors in the group of the memory cells based on the statistical properties.

37. A method for data storage, comprising:

operating a memory, which includes a plurality of Flash memory cells and generates internal heat;

heating at least some of the Flash memory cells by applying external heat to the memory, in addition to the internal heat generated by the memory; and

storing data in the heated Flash memory cells.