Programming Differently Sized Margins and Sensing with Compensations at Select States for Improved Read Operations in Non-Volatile Memory

• US 20070291556A1
• Filed: 06/19/2006
• Published: 12/20/2007
• Est. Priority Date: 06/19/2006
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Kamei, Teruhiko
• Granted Patent: US 7,606,084 B2
• Time in Patent Office: Days
• US Class Current: 365/189.09
• CPC Class Codes: G11C 11/5628 Programming or writing circuits; Data input circuits
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output 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 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data

Abstract

Non-volatile memory read operations compensate for floating gate coupling when the apparent threshold voltage of a memory cell may have shifted. A memory cell of interest can be read using a reference value based on a level of charge read from a neighboring memory cell. Misreading the neighboring cell may have greater effects in particular programming methodologies, and more specifically, when reading the neighboring cell for particular states or charge levels in those methodologies. In one embodiment, memory cells are programmed to create a wider margin between particular states where misreading a neighboring cell is more detrimental. Further, memory cells are read in one embodiment by compensating for floating gate coupling based on the state of a neighboring cell when reading at certain reference levels but not when reading at other reference levels, such as those where a wider margin has been created.

Claims

1. A method of reading non-volatile storage, comprising:

receiving a request to read a first non-volatile storage element;

reading a second non-volatile storage element in response to said request, said second non-volatile storage element adjacent said first non-volatile storage element and capable of storing data in at least four physical states;

applying a first reference to read said first non-volatile storage element at a level between a first programmed state and a second programmed state;

applying a second reference to read said first non-volatile storage element at a level between said second programmed state and said third programmed state;

determining data of said first non-volatile storage element using a result of applying said first reference at a first level and a result of applying said second reference at a second level when said second non-volatile storage element is in a first subset of said physical states; and

determining data of said first non-volatile storage element using a result of applying said first reference at said first level and a result of applying said second reference at a third level when said second non-volatile storage element is in a second subset of said physical states.

2. The method of claim 1, wherein:

applying said first reference at said first level does not compensate for floating gate coupling between said first non-volatile storage element and said second non-volatile storage element;

applying said second reference at said second level does not compensate for floating gate coupling between said first non-volatile storage element and said second non-volatile storage element; and

applying said second reference at said third level compensates for floating gate coupling between said first non-volatile storage element and said second non-volatile storage element.

3. The method of claim 2, wherein said determining data of said first non-volatile storage element when said second non-volatile storage element is in said second subset of said physical states includes:

applying a first voltage corresponding to said first reference at said first level to a control gate of said first non-volatile storage element and sensing conduction of said first non-volatile storage element;

applying a second voltage corresponding to said second reference at said second level to said control gate of said first non-volatile storage element and sensing conduction of said first non-volatile storage element;

applying a third voltage corresponding to said second reference at said third level to said control gate of said first non-volatile storage element and sensing conduction of said first non-volatile storage element, said third voltage is equal to said second voltage plus an offset;

determining said data of said first non-volatile storage element by selecting said result of applying said first voltage, selecting said result of applying said third voltage, and disregarding said result of applying said second voltage.

4. The method of claim 2, wherein:

said offset is substantially equal to an apparent change in a threshold voltage of said first non-volatile storage element based on said floating gate coupling between said first non-volatile storage element and said second non-volatile storage element.

5. The method of claim 1, wherein said first non-volatile storage element is part of a set of non-volatile storage elements coupled to a first word line, said method further comprising:

programming said set of non-volatile storage elements to a plurality of physical states including said first programmed state, said second programmed state, and said third programmed state;

verifying whether non-volatile storage elements of said set to be programmed to said first state have reached a first target level corresponding to said first state;

verifying whether non-volatile storage elements of said set to be programmed to said second state have reached a second target level corresponding to said second state, said second target level spaced from said first target level by a first amount;

verifying whether non-volatile storage elements of said set to be programmed to said third state have reached a third target level corresponding to said third state, said third target level spaced from said second target level by a second amount smaller than said first amount.

6. The method of claim 1, wherein:

said first subset of physical states includes said first programmed state and said third programmed state; and

said second subset of physical states includes said second programmed state and an erased state.

7. The method of claim 6, wherein:

said first programmed state is adjacent said erased state and said second programmed state; and

said second programmed state is adjacent said first programmed state and said third programmed state.

8. The method of claim 1, wherein:

said second non-volatile storage element stores upper page data and lower page data;

reading said second non-volatile storage element in response to said request comprises reading said upper page data for said second non-volatile storage element;

said first and second levels for said second reference are based on said upper page data and not on said lower page data for said second non-volatile storage element; and

said first subset of physical states corresponds to said second non-volatile storage element storing first data for said upper page; and

said second subset of physical states corresponds to said second non-volatile storage element storing second data for said upper page.

9. The method of claim 1, wherein:

said first non-volatile storage element stores data for a first logical page and a second logical page;

said second non-volatile storage element stores data for a third logical page and a fourth logical page;

said data for said second logical page stored by said first non-volatile storage element is programmed after programming said data for said third logical page stored by said second non-volatile storage element and before programming said data stored for said fourth logical page by said second non-volatile storage element.

10. The method of claim 1, wherein:

said first non-volatile storage element is connected to a first word line;

said second non-volatile storage element is connected to a second word line adjacent said first world line;

wherein programming data for non-volatile storage elements connected to said first word line begins before programming data for non-volatile storage elements connected to said second word line begins.

11. The method of claim 1, wherein:

said first non-volatile storage element is a multi-state NAND flash memory device.

12. The method of claim 1, wherein:

said first non-volatile storage element is a multi-state NAND flash memory device.

13. The method of claim 1, wherein:

said first non-volatile storage element is part of an array of flash memory devices;

said array is removable from a host system.

14. A method of reading a first non-volatile storage element capable of storing data in a plurality of states, comprising:

reading charge information from a second non-volatile storage element;

if said charge information indicates one or more predetermined levels of programming of said second non-volatile storage element, reading data from said first non-volatile storage element without using compensation for coupling between said first non-volatile storage element and said second non-volatile storage element when reading at a level between a first programmed state and a second programmed state and using compensation for coupling between said first and second non-volatile storage elements when reading at a level between said second programmed state and a third programmed state;

if said charge information does not indicate said one or more predetermined levels of programming of said second non-volatile storage element, reading data from said first non-volatile storage element without using compensation for coupling between said first and second non-volatile storage elements when reading at a level between said first and second programmed states and without using compensation for coupling between said first and second non-volatile storage elements when reading at said level between said second and third programmed states.

15. The method of claim 14, wherein:

reading data from said first non-volatile storage element if said charge information indicates one or more predetermined levels of programming includes;

reading at said level between said first programmed state and said programmed state without using compensation by applying a first reference voltage to a control gate of said first non-volatile storage element and sensing conduction of said first non-volatile storage element;

reading at said level between said second programmed state and said third programmed state using compensation by applying a second reference voltage and sensing conduction of said first non-volatile storage element and applying said second reference voltage plus an offset and sensing conduction of said first non-volatile storage element;

reading data from said first non-volatile storage element if said charge information does not indicate one or more predetermined levels of programming includes;

reading at said level between said first programmed state and said programmed state without using compensation by applying said first reference voltage to said control gate of said first non-volatile storage element and sensing conduction of said first non-volatile storage element;

reading at said level between said second programmed state and said third programmed state without using compensation by applying said second reference voltage and sensing conduction of said first non-volatile storage element.

16. A method of programming non-volatile storage, comprising:

programming a first group of non-volatile storage elements from a set of non-volatile storage elements to a first programmed state, said first programmed state associated with a first range of threshold voltages;

programming a second group of non-volatile storage elements from said set to a second programmed state, said second programmed state adjacent said first programmed state and associated with a second range of threshold voltages, said second range of threshold voltages is separated from said first range of threshold voltages by a first margin; and

programming a third group of non-volatile storage elements from said set to a third programmed state, said third programmed state adjacent said second programmed state and associated with a third range of threshold voltages, said third range of threshold voltages separated from said second range of threshold voltages by a second margin smaller than said first margin.

17. The method of claim 16, wherein:

programming said first group of memory cells to said first programmed state includes verifying whether said first group of memory cells has reached a first verify level for said first programmed state;

programming said second group of memory cells to said second programmed state includes verifying whether said second group of memory cells has reached a second verify level for said second programmed state, said second verify level is spaced from said first verify level by a first amount;

programming said third group of memory cells to said third programmed state includes verifying whether said third group of memory cells has reached a third verify level for said third programmed state, said third verify level is spaced from said second verify level by a second amount less than said first amount.

18. The method of claim 17, wherein:

said first margin is equal to a difference between a largest threshold voltage of any memory cell of said first group programmed to said first programmed state and a lowest threshold voltage of any memory cell of said second group programmed to said second programmed state;

said second margin is equal to a difference between a largest threshold voltage of any memory cell of said second group programmed to said second programmed state and a lowest threshold voltage of any memory cell of said third group programmed to said third programmed state;

said verifying whether said second group of memory cells has reached said second verify level includes verifying that said lowest threshold voltage of any memory cell of said second group is at or above said second verify level to create said first margin with said first amount; and

said verifying whether said third group of memory cells has reached said third verify level includes verifying that said lowest threshold voltage of any memory cell of said third group is at or above said third verify level to create said second margin with said second amount.

19. The method of claim 16, wherein:

said first margin corresponds to a forbidden range of threshold voltages between said first programmed state and said second programmed state; and

said second margin corresponds to a forbidden range of threshold voltages between said second programmed state and said third programmed state.

20. The method of claim 16, further comprising:

programming a fourth group of non-volatile storage elements to a fourth programmed state, said fourth programmed state adjacent said first programmed state and associated with a fourth range of threshold voltages, said fourth range of threshold voltages includes lower threshold voltages than said first range of threshold voltages.

21. The method of claim 16, further comprising:

receiving a request to read said set of non-volatile storage elements;

reading charge information from a second non-volatile storage element in response to said request;

if said charge information indicates one or more predetermined levels of programming of said second non-volatile storage element, reading data from said first non-volatile storage element without using compensation for coupling between said first non-volatile storage element and said second non-volatile storage element when reading at a level between a first programmed state and a second programmed state and using compensation for coupling between said first and second non-volatile storage elements when reading at a level between said second programmed state and a third programmed state; and

if said charge information does not indicate said one or more predetermined levels of programming of said second non-volatile storage element, reading data from said first non-volatile storage element without using compensation for coupling between said first and second non-volatile storage elements when reading at a level between said first and second programmed states and without using compensation for coupling between said first and second non-volatile storage elements when reading at said level between said second and third programmed states.

22. A method of programming non-volatile storage, comprising:

programming a plurality of non-volatile storage elements to a plurality of physical states including an erased state, a first programmed state, a second programmed state, and a third programmed state, said second programmed state corresponds to threshold voltages between threshold voltages corresponding to said first programmed state and threshold voltages corresponding to said third programmed state;

verifying whether non-volatile storage elements of said plurality to be programmed to said first state have reached a first target level corresponding to said first state;

verifying whether non-volatile storage elements of said plurality to be programmed to said second sate have reached a second target level corresponding to said second state, said second target level is spaced from said first target level by a first amount; and

verifying whether non-volatile storage elements of said plurality to be programmed to said third state have reached a third verify level corresponding to said third state, said third target level is spaced from said second target level by a second amount.

23. The method of claim 22, wherein:

said second amount is smaller than said first amount.

24. The method of claim 23, wherein:

verifying whether storage elements to be programmed to said first state have reached said first target level and verifying whether storage elements to be programmed to said second sate have reached a second target level provides a first margin between said first physical state and said second physical state; and

verifying whether storage elements to be programmed to said second sate have reached said second target level and verifying whether storage elements to be programmed to said third state have reached said third verify level provides a second margin of a smaller size between said second physical state and said first physical state.

25. The method of claim 22, wherein said plurality is a first plurality, said method further comprising:

receiving a request to read said first plurality of non-volatile storage elements;

reading a second plurality of non-volatile storage elements adjacent said first plurality;

reading data from each non-volatile storage element of said first plurality when said reading a second plurality indicates one or more predetermined levels of programming of an adjacent non-volatile storage element in said second plurality by;

reading data from said each non-volatile storage element without using compensation for coupling between said each non-volatile storage element and said adjacent non-volatile storage element when reading at a level between said first programmed state and said second programmed state; and

reading data from said each non-volatile storage element using compensation for coupling between said each and adjacent non-volatile storage elements when reading at a level between said second programmed state and a third programmed state;

reading data from each non-volatile storage element of said first plurality when said reading a second plurality does not indicate said one or more predetermined levels of programming of an adjacent non-volatile storage element in said second plurality by;

reading data from said each non-volatile storage element without using compensation for coupling between said each and adjacent non-volatile storage elements when reading at a level between said first and second programmed states; and

reading data from said each non-volatile storage element without using compensation for coupling between said each and adjacent non-volatile storage elements when reading at said level between said second and third programmed states.