READ DISTURB MITIGATION IN NON-VOLATILE MEMORY

• US 20090323412A1
• Filed: 06/30/2008
• Published: 12/31/2009
• Est. Priority Date: 06/30/2008
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: Sandisk Technologies Incorporated (Western Digital Corporation)
• Inventors: Mokhlesi, Nima, Schuegraf, Klaus
• Granted Patent: US 7,808,831 B2
• Time in Patent Office: Days
• US Class Current: 365/185.02
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 16/3427 Circuits or methods to prevent or reduce disturbance of the state of a memory cell when neighbouring cells are read or written
• CPC Class Codes: G11C 29/00 Checking stores for correct operation ; Subsequent repair; Testing stores during standby or offline operation

Abstract

Read disturb is reduced in non-volatile storage. In one aspect, when a read command is received from a host for reading a selected word line, a word line which is not selected for reading is randomly chosen and its storage elements are sensed to determine optimized read compare levels for reading the selected word line. Or, a refresh operation may be indicated for the entire block based on an error correction metric obtained in reading the storage elements of the chosen word line. This is useful especially when the selected word line is repeatedly selected for reading, exposing the other word lines to additional read disturb. In another aspect, when multiple data states are stored, one read compare level is obtained from sensing, e.g., from a threshold voltage distribution, and other read compare levels are derived from a formula.

Claims

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

receiving a read command requesting to read data from at least one selected non-volatile storage element in a set of non-volatile storage elements, the at least one selected non-volatile storage element is associated with a selected word line in a set of word lines;

in response to the read command, selecting another word line in the set of word lines, performing sense operations on non-volatile storage elements which are associated with the another word line, and, based on the sense operations, determining at least one read compare level; and

reading the at least one selected non-volatile storage element using the at least one read compare level.

2. The method of claim 1, wherein:

the sense operations provide a threshold voltage distribution which bridges at least two data states, and the at least one read compare level is determined by identifying a valley in the threshold voltage distribution.

3. The method of claim 1, further comprising:

selecting the another word line randomly from the set of word lines.

4. The method of claim 1, further comprising:

selecting the another word line randomly from the set of word lines, excluding at least one specified word line in the set of word lines.

5. The method of claim 1, wherein:

the another word line is adjacent to the selected word line.

6. The method of claim 1, further comprising:

determining multiple read compare levels for the non-volatile storage elements which are associated with the another word line, based on the sense operations; and

reading the at least one selected non-volatile storage element using the multiple read compare levels.

7. The method of claim 1, wherein the at least one read compare level comprises a first read compare level which distinguishes adjacent states of a first pair of data states from one another, the method further comprising:

determining a second read compare level which distinguishes adjacent states of a second pair of data states from one another, based on the first read compare level, and without performing sense operations for distinguishing the adjacent states of the second pair of data states from one another.

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

receiving a read command requesting to read data from at least one selected non-volatile storage element in a set of non-volatile storage elements, the at least one selected non-volatile storage element is associated with a selected word line in a set of word lines;

in response to the read command, selecting another word line in the set of word lines;

sensing data from non-volatile storage elements which are associated with the another word line, including performing an error correction process;

determining a metric based on the error correction process;

if the metric is below a threshold, reading the at least one selected non-volatile storage element using at least one read compare level which is determined based on the data sensed from the non-volatile storage elements which are associated with the another word line; and

if the metric is above the threshold, further investigating integrity of the data in the set of non-volatile storage elements and/or performing a refresh operation for the data in the set of non-volatile storage elements.

9. The method of claim 8, wherein:

the metric comprises a number of bit errors.

10. The method of claim 8, wherein:

the metric comprises a decoding time needed by an error correction code engine to decode the data.

11. The method of claim 8, wherein:

the metric comprises a number of iterations needed by an error correction code engine to decode the data.

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

receiving a read command requesting to read data from at least one selected non-volatile storage element in a set of non-volatile storage elements, the set of non-volatile storage elements stores data in multiple data states, and the at least one selected non-volatile storage element is associated with a selected word line in a set of word lines;

in response to the read command, selecting another word line in the set of word lines, performing sense operations on non-volatile storage elements which are associated with the another word line to distinguish adjacent states of a first pair of data states of the multiple data states from one another, and, based on the sense operations, determining a first read compare level which is between the adjacent data states of the first pair of data states;

determining a second read compare level which is between adjacent data states of a second pair of data states of the multiple data states, based on the first read compare level, and without performing sense operations for distinguishing the adjacent states of the second pair of data states from one another; and

reading the at least one selected non-volatile storage element using at least the second read compare level.

13. The method of claim 12, wherein the determining the second read compare level comprises:

determining a first threshold voltage shift based on a difference between the first read compare level and an associated baseline read compare level;

determining a second threshold voltage shift based on the first threshold voltage shift; and

modifying an associated baseline read compare level of the second read compare level by the second threshold voltage shift.

14. The method of claim 12, further comprising:

reading the at least one selected non-volatile storage element using the second read compare level.

15. The method of claim 12, further comprising:

determining a third read compare level which is between adjacent data states of a third pair of data states, based on the first read compare level, without performing sense operations to distinguish the adjacent states of the third pair of data states from one another.

16. The method of claim 12, wherein:

the multiple data states extend from a lower range to a higher range; and

the adjacent data states of the first pair of data states are in the higher range, and the adjacent data states of the second pair of data states are in the lower range.

17. The method of claim 12, wherein:

the determining the second read compare level is also based on a number of write/erase cycles experienced by the set of non-volatile storage elements.

18. The method of claim 12, wherein:

the determining the second read compare level is also based on a position of the selected word line in the set of word lines.

19. The method of claim 12, wherein:

the determining the second read compare level is also based on a temperature dependency.

20. The method of claim 12, wherein:

the determining the second read compare level is also based on a power supply level dependency.

21. The method of claim 12, further comprising:

the another word line is selected randomly from the set of word lines.

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

receiving a read command requesting to read data from a set of non-volatile storage elements, the set of non-volatile storage elements stores data in multiple data states;

in response to the read command, performing sense operations to distinguish adjacent states of a first pair of data states of the multiple data states from one another, and, based on the sense operations, determining a first read compare level which is between the adjacent data states of the first pair of data states;

determining a second read compare level which is between adjacent data states of a second pair of data states of the multiple data states, based on the first read compare level, and without performing sense operations for distinguishing the adjacent states of the second pair of data states from one another; and

reading at least one selected non-volatile storage element using at least the second read compare level.

23. The method of claim 22, wherein:

the multiple data states extend from a lower range to a higher range; and

the adjacent data states of the first pair of data states are in the higher range, and the adjacent data states of the second pair of data states are in the lower range.

24. The method of claim 22, wherein:

the determining the second read compare level is also based on a number of write/erase cycles experienced by the set of non-volatile storage elements.

25. The method of claim 22, wherein:

the determining the second read compare level is also based on a position of the selected word line in the set of word lines.

26. The method of claim 22, wherein:

the determining the second read compare level is also based on a temperature-dependency.