Method for programming of multi-state non-volatile memory using smart verify

• US 7,301,817 B2
• Filed: 10/27/2005
• Issued: 11/27/2007
• Est. Priority Date: 10/27/2005
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Li, Yan, Pham, Long
• Primary Examiner(s): Ho; Hoai V.
• Application Number:
  US11/260,658
• Publication Number:
  US 20070097749A1
• Time in Patent Office: 761 Days
• Field of Search: 365/185.03, 365/185.22, 365/185.24, 365/185.28
• US Class Current: 365/185.22
• 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 16/12 Programming voltage switching circuits
• CPC Class Codes: G11C 16/3454 Arrangements for verifying correct programming or for detecting overprogrammed cells
• CPC Class Codes: G11C 16/3459 Circuits or methods to verify correct programming of nonvolatile memory cells
• CPC Class Codes: G11C 2211/5621 Multilevel programming verification

Abstract

In a non-volatile memory, the initiation of program verification is adaptively set so that programming time is decreased. In one approach, non-volatile storage elements are programmed based on a lower page of data to have a voltage threshold (V_TH) that falls within a first V_TH distribution or a higher, intermediate V_TH distribution. Subsequently, the non-volatile storage elements with the first V_TH distribution either remain there, or are programmed to a second V_TH distribution, based on an upper page of data. The non-volatile storage elements with the intermediate V_TH distribution are programmed to third and fourth V_TH distributions. The non-volatile storage elements being programmed to the third V_TH distribution are specially identified and tracked. Verification of the non-volatile storage elements being programmed to the fourth V_TH distribution is initiated after one of the identified non-volatile storage elements transitions to the third V_TH distribution from the intermediate V_TH distribution.

Claims

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

programming, in a set of non-volatile storage elements, at least first and second subsets of non-volatile storage elements having respective threshold voltages that fall within a first threshold voltage distribution, so that a threshold voltage of at least a first non-volatile storage element of the first subset transitions to a second threshold voltage distribution which overlaps with the first threshold voltage distribution, and a threshold voltage of at least a second non-volatile storage element of the second subset transitions to a third threshold voltage distribution which is outside of the first and second threshold voltage distributions;

tracking the threshold voltage of the at least a first non-volatile storage element to determine when it has transitioned to the second threshold voltage distribution; and

determining when to initiate a verification process for verifying when the threshold voltage of the at least a second non-volatile storage element has transitioned to the third threshold voltage distribution, responsive to the tracking.

2. The method of claim 1, wherein:

the verification process is initiated after a predetermined number of programming voltage pulses have been applied to the second subset of non-volatile storage elements, after it is determined that the threshold voltage of the at least a first non-volatile storage element has transitioned to the second threshold voltage distribution.

3. The method of claim 1, wherein:

the tracking comprises tracking the threshold voltage of the at least a first non-volatile storage element past a lower threshold voltage, which is below the second threshold voltage distribution, and then past a higher threshold voltage, which is in the second threshold voltage distribution, the verification process is initiated when the threshold voltage of the at least a first non-volatile storage element passes the higher threshold voltage.

4. The method of claim 3, further comprising:

programming the at least a first non-volatile storage element in a fast program mode when the threshold voltage thereof is below the lower threshold voltage, and in a slow program mode when the threshold voltage thereof is between the lower and higher threshold voltages.

5. The method of claim 3, wherein:

the higher threshold voltage exceeds a lower boundary of the second threshold voltage distribution by a margin.

6. The method of claim 5, wherein:

the margin comprises at least one of a sensing margin and a noise margin.

7. The method of claim 1, wherein:

the tracking comprises identifying the at least a first non-volatile storage element by determining which of the non-volatile storage elements of the first subset have a threshold voltage which is below the second threshold voltage distribution.

8. The method of claim 1, wherein:

the set of non-volatile storage elements are arranged in a plurality of NAND strings.

9. The method of claim 1, wherein:

the second and third threshold voltage distributions represent different binary data states.

10. The method of claim 1, wherein:

the verification process is initiated responsive to a determination that the threshold voltage of any of the non-volatile storage elements of the first subset has transitioned to the second threshold voltage distribution.

11. A method for programming non-volatile storage, comprising:

programming, in a set of non-volatile storage elements, at least first and second subsets of non-volatile storage elements having respective threshold voltages that fall within a first threshold voltage distribution, so that a threshold voltage of at least a first non-volatile storage element of the first subset transitions to a second threshold voltage distribution which overlaps with the first threshold voltage distribution, and a threshold voltage of at least a second non-volatile storage element of the second subset transitions to a third threshold voltage distribution which is outside of the first and second threshold voltage distributions;

determining when the threshold voltage of the at least a first non-volatile storage element has transitioned to the second threshold voltage distribution; and

after a predetermined number of programming voltage pulses have been applied to the at least a second non-volatile storage element after it is determined that the threshold voltage of the at least a first non-volatile storage element has transitioned to the second threshold voltage distribution, initiating a verification process for verifying when the threshold voltage of the at least a second non-volatile storage element has transitioned to the third threshold voltage distribution.

12. The method of claim 11, wherein:

the determining comprises determining when the threshold voltage of the at least a first non-volatile storage element exceeds a lower threshold voltage, which is below the second threshold voltage distribution, and then exceeds a higher threshold voltage, which is in the second threshold voltage distribution.

13. The method of claim 11, wherein:

the set of non-volatile storage elements are arranged in a plurality of NAND strings.

14. The method of claim 11, wherein:

the second and third threshold voltage distributions represent different binary data states.

15. A method for programming non-volatile storage, comprising:

programming, in a set of non-volatile storage elements, at least first and second subsets of non-volatile storage elements having respective threshold voltages that fall within a first threshold voltage distribution, so that a threshold voltage of at least a first non-volatile storage element of the first subset transitions to a second threshold voltage distribution, and a threshold voltage of at least a second non-volatile storage element of the second subset transitions to a third threshold voltage distribution which exceeds the first and second threshold voltage distributions;

determining when the threshold voltage of the at least a first non-volatile storage element has transitioned to the second threshold voltage distribution; and

a) prior to when it is determined that the threshold voltage of the at least a first non-volatile storage element has transitioned to the second threshold voltage distribution, programming of the first and second subsets of non-volatile storage elements is performed without verifying whether their respective threshold voltages have transitioned to the third threshold voltage distribution, and b) after it is determined that the threshold voltage of the at least a first non-volatile storage element has transitioned to the second threshold voltage distribution, programming of the first and second subsets of non-volatile storage elements is performed with verifying of whether their respective threshold voltages have transitioned to the third threshold voltage distribution.

16. The method of claim 15, wherein:

the programming of the first and second subsets of non-volatile storage elements is performed with the verifying of whether their respective threshold voltages have transitioned to the third threshold voltage distribution, after a predetermined number of programming voltage pulses have been applied to the second subset of non-volatile storage elements after it is determined that the threshold voltage of the at least a first non-volatile storage element has transitioned to the second threshold voltage distribution.

17. The method of claim 15, wherein:

the determining comprises determining when the threshold voltage of the at least a first non-volatile storage element passes a lower threshold voltage, which is below the second threshold voltage distribution, and then passes a higher threshold voltage, which is in the second threshold voltage distribution.

18. The method of claim 15, wherein:

the set of non-volatile storage elements are arranged in a plurality of NAND strings.

19. The method of claim 15, wherein:

the second and third threshold voltage distributions represent different binary data states.

20. The method of claim 15, wherein:

the first and second threshold voltage distributions overlap.