ADJUSTING RESISTANCE OF NON-VOLATILE MEMORY USING DUMMY MEMORY CELLS

• US 20080273388A1
• Filed: 03/21/2007
• Published: 11/06/2008
• Est. Priority Date: 03/21/2007
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Chin, Henry, Zhao, Dengtao, Mokhlesi, Nima
• Granted Patent: US 7,535,764 B2
• Time in Patent Office: Days
• US Class Current: 365/185.17
• CPC Class Codes: G11C 11/5628 Programming or writing circuits; Data input circuits
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/10 Programming or data input circuits
• CPC Class Codes: G11C 16/3454 Arrangements for verifying correct programming or for detecting overprogrammed cells

Abstract

In some non-volatile storage systems, a block of data memory cells is manufactured with a dummy word line at the bottom of the block, at the top of the block, and/or at other locations. By selectively programming memory cells on the dummy word line(s), the resistances associated with the data memory cells can be changed to account for different programmed data patterns.

Claims

1. A method for operating non-volatile storage, comprising:

determining information indicative of a resistance characteristic for a set of non-volatile storage elements, said set of non-volatile storage elements includes data non-volatile storage elements and dummy non-volatile storage elements;

programming said data non-volatile storage elements; and

setting one or more dummy non-volatile storage elements based on said determined information indicative of a resistance characteristic.

2. A method according to claim 1, wherein:

said information indicative of a resistance characteristic can include absolute resistance, relative resistance, change in resistance, threshold voltage data that can be used to determine information about resistance, or stored data that can be used to determine information about resistance; and

said information indicative of a resistance characteristic can pertain to all of said data non-volatile storage elements or a subset of said data non-volatile storage elements.

3. A method according to claim 1, wherein:

said setting one or more dummy non-volatile storage elements changes said resistance characteristic.

4. A method according to claim 1, wherein:

said setting one or more dummy non-volatile storage elements changes a group resistance for said set of non-volatile storage elements.

5. A method according to claim 1, wherein:

said determining information indicative of a resistance characteristic includes determining a deviation of programmed data from a predefined distribution of data.

6. A method according to claim 1, wherein:

said setting one or more dummy non-volatile storage elements includes adjusting a threshold voltage for a dummy non-volatile storage element to a threshold voltage range associated with a data state.

7. A method according to claim 1, wherein:

said setting one or more dummy non-volatile storage elements includes adjusting a threshold voltage for a dummy non-volatile storage element to an analog value.

8. A method according to claim 1, wherein said determining information indicative of a resistance characteristic comprises:

determining deviation of change in resistance for at least a subset of said data non-volatile storage elements with respect to a predefined change, for a given data non-volatile storage element said change in resistance is based on pre and post programming of other data non-volatile storage elements;

identifying a minimum deviation based on said determining deviation;

identifying a maximum deviation based on said determining deviation; and

determining a condition for one or more of said dummy non-volatile storage elements that alter said minimum deviation and said maximum deviation in accordance with an error reduction criteria.

9. A method according to claim 8, wherein:

said setting one or more dummy non-volatile storage elements includes adjusting resistance for said set of non-volatile storage elements by setting said one or more of said dummy non-volatile storage elements to said condition.

10. A method according to claim 8, wherein:

said predefined change pertains to an even distribution of data.

11. A method according to claim 1, wherein:

said setting one or more dummy non-volatile storage elements includes setting a first dummy non-volatile storage element after completing programming of a first subset of data non-volatile storage elements and prior to completing programming of all data non-volatile storage elements; and

said setting one or more dummy non-volatile storage elements further includes setting a second dummy non-volatile storage element after completing programming of all data non-volatile storage elements.

12. A method according to claim 1, wherein:

said setting one or more dummy non-volatile storage elements includes setting a first dummy non-volatile storage element after completing programming of a first subset of data non-volatile storage elements and prior to completing programming of all data non-volatile storage elements; and

said setting one or more dummy non-volatile storage elements further includes setting a second dummy non-volatile storage element after completing programming of a second subset of data non-volatile storage elements and prior to completing programming of all data non-volatile storage elements.

13. A method according to claim 1, wherein:

said setting one or more dummy non-volatile storage elements includes setting a first dummy non-volatile storage element and a second dummy non-volatile storage element after completing programming of all data non-volatile storage elements.

14. A method according to claim 1, wherein:

said programming includes verify operations, said verify operations uses a first overdrive voltage for unselected data non-volatile storage elements and a second overdrive voltage for dummy data non-volatile storage elements, said first overdrive voltage is higher than said second overdrive voltage;

said setting one or more dummy non-volatile storage elements includes adjusting threshold voltages for said setting one or more dummy non-volatile storage elements to be within a range of threshold voltages near charge-neutrality for said dummy non-volatile storage elements; and

said method includes reading said data non-volatile storage elements, said reading includes using said first overdrive voltage for unselected data non-volatile storage elements and said second overdrive voltage for dummy data non-volatile storage elements.

15. A method according to claim 1, wherein:

said determining information indicative of a resistance characteristic includes physically measuring resistance information.

16. A method according to claim 1, wherein:

said data non-volatile storage elements are multi-state flash memory devices.

28. A method according to claim 1, wherein:

said set of non-volatile storage elements are part of a NAND string; and

said determining information indicative of resistance includes determining a deviation of programmed data from a predefined set of data.

17. A method for operating non-volatile storage, comprising:

determining a deviation of programmed data for a set of non-volatile storage elements as compared to a predefined distribution of data, said set of non-volatile storage elements includes data non-volatile storage elements and dummy non-volatile storage elements;

programming said data non-volatile storage elements; and

adjusting a group resistance of said set of non-volatile storage elements by changing a characteristic of said one or more dummy non-volatile storage elements based on said determined deviation.

18. A method according to claim 17, wherein:

said changing a characteristic of said one or more dummy non-volatile storage elements includes adjusting a threshold voltage for at least one of said dummy non-volatile storage elements.

19. A method for operating non-volatile storage, comprising:

performing an erase operation on a group of non-volatile storage elements, said group includes a plurality of NAND strings, each of said NAND strings includes a set of data memory cells and one or more dummy memory cells;

programming said data memory cells;

determining information indicative of a resistance characteristic of at least a subset of said data memory cells; and

setting at least a subset of said dummy memory cells based on said determined information.

20. A method according to claim 19, wherein:

said determining information indicative of a resistance characteristic includes determining a deviation of programmed data from a predefined distribution of data.

21. A method according to claim 19, wherein:

said setting at least a subset of said dummy memory cells changes resistance for NAND strings that include said dummy memory cells.

22. A method for operating non-volatile storage, comprising:

determining deviation of change in resistance for at least a subset of a set of data non-volatile storage elements with respect to a predefined change, said set of non-volatile storage elements includes data non-volatile storage elements and one or more dummy non-volatile storage elements, for a given data non-volatile storage element said change in resistance is based on pre and post programming of other data non-volatile storage elements;

identifying a minimum deviation based on said determining deviation;

identifying a maximum deviation based on said determining deviation; and

determining a condition for one or more of said dummy non-volatile storage elements that alter said minimum deviation and said maximum deviation in accordance with an error reduction criteria; and

altering resistance for said set of non-volatile storage elements by setting said one or more of said dummy non-volatile storage elements to said condition.

23. A method according to claim 22, wherein:

said set of non-volatile storage elements are part of a NAND string; and

said altering resistance for said set of non-volatile storage elements includes altering resistance for said NAND string by changing threshold voltages for one or more of said dummy non-volatile storage elements.

24. A method for operating non-volatile storage, comprising:

determining information indicative of a resistance characteristic for a set of non-volatile storage elements, said set of non-volatile storage elements includes data non-volatile storage elements and dummy non-volatile storage elements;

programming said data non-volatile storage elements, said programming includes verify operations, said verify operations uses a first overdrive voltage for unselected data non-volatile storage elements and a second overdrive voltage for dummy data non-volatile storage elements, said first overdrive voltage is higher than said second overdrive voltage; and

setting one or more dummy non-volatile storage elements to a range of threshold voltages near charge-neutrality for said dummy non-volatile storage elements based on said determined resistance information.

25. A method according to claim 24, wherein said method further comprises:

reading said data non-volatile storage elements, said reading includes using said first overdrive voltage for unselected data non-volatile storage elements and said second overdrive voltage for dummy data non-volatile storage elements.

26. A method for operating non-volatile storage, comprising:

determining information indicative of resistance for a set of non-volatile storage elements, said set of non-volatile storage elements includes user data non-volatile storage elements and non-user data non-volatile storage elements;

programming said user data non-volatile storage elements; and

setting one or more non-user data non-volatile storage elements based on said determined information indicative of resistance, said setting one or more non-user data non-volatile storage elements changes said resistance for said set of non-volatile storage elements.

27. A method according to claim 26, wherein:

said set of non-volatile storage elements are connected together.