Compensating for coupling during read operations of non-volatile memory
View Patent ↗Shifts in the apparent charge stored on a floating gate (or other charge storing element) of a non-volatile memory cell can occur because of the coupling of an electric field based on the charge stored in adjacent floating gates (or other adjacent charge storing elements). The problem occurs most pronouncedly between sets of adjacent memory cells that have been programmed at different times. To compensate for this coupling, the read process for a given memory cell will take into account the programmed state of an adjacent memory cell.
1. A method for reading non-volatile storage, comprising:
performing a read process for a set of non-volatile storage elements using a set of one or more read compare points, said read process provides a set of read data;
determining whether said read data has one or more errors;
determining whether an error correction process is able to correct said one or more errors;
correcting said one or more errors using said error correction process if said error correction process is able to correct said one or more errors; and
performing a data recovery process if said error correction process is not able to correct said one or more errors, said data recovery process comprises reading non-volatile storage elements adjacent to said set of non-volatile storage elements, adjusting said one or more read compare points for at least a subset of said set of non-volatile storage elements based on reading of said non-volatile storage elements adjacent to said set of non-volatile storage elements and performing another one or more read processes for said set of non-volatile storage elements using said adjusted one or more read compare points.
2. A method according to claim 1 , wherein:
said set of non-volatile storage elements includes a page of data;
said set of non-volatile storage elements are connected to a first word line;
said non-volatile storage elements adjacent to said set of non-volatile storage elements are connected to a second word line adjacent to said first word line; and
said error correction process uses error correction codes.
3. A method according to claim 1 , wherein:
said adjusting said one or more read compare points includes changing a reference voltage by an offset voltage.
4. A method according to claim 1 , wherein:
said adjusting said one or more read compare points includes changing a reference voltage by one or a set of offsets; and
said set of offsets includes an offset for each programmed data state.
5. A method according to claim 1 , wherein:
said performing another one or more read processes includes performing multiple read processes, each read process uses a different one of a predetermined set of offsets and is performed on all of said set of non-volatile storage elements, there is at least one read process for each offset, each non-volatile storage element provides final data from an appropriate one of said read processes associated with said offset associated with an adjacent non-volatile storage element for the respective non-volatile storage element.
6. A method according to claim 1 , wherein said adjusting said one or more read compare points and said performing another one or more read processes comprise:
performing a first read process without adjusting said one or more read compare points and storing results for one or more non-volatile storage elements having an adjacent non-volatile storage element in a first state; and
performing a second read process with a first adjustment to said one or more read compare points and storing results for one or more non-volatile storage elements having an adjacent non-volatile storage element in a second state.
7. A method according to claim 1 , wherein said adjusting said one or more read compare points and said performing another one or more read processes comprise:
performing a first read process without adjusting said one or more read compare points and storing results for one or more non-volatile storage elements having an adjacent non-volatile storage element in a first state;
performing a second read process with a first adjustment to said one or more read compare points and storing results for one or more non-volatile storage elements having an adjacent non-volatile storage element in a second state;
performing a third read process with a second adjustment to said one or more read compare points and storing results for one or more non-volatile storage elements having an adjacent non-volatile storage element in a third state; and
performing a fourth read process with a third adjustment to said one or more read compare points and storing results for one or more non-volatile storage elements having an adjacent non-volatile storage element in a fourth state.
8. A method according to claim 1 , wherein:
said set of non-volatile storage elements are connected to a first word line;
said non-volatile storage elements adjacent to said set of non-volatile storage elements are connected to a second word line adjacent to said first word line;
said first and second word lines are part of a memory system having N word lines where an Nth word line is programmed last in sequence; and
said data recovery process is performed for all of said word lines except said Nth word line.
9. A method according to claim 1 , further comprising:
reporting results of said data recovery process.
10. A method according to claim 1 , wherein:
said set of non-volatile storage elements are connected to a first word line; and
said set of non-volatile storage elements are connected to consecutive bit lines.
11. A method according to claim 1 , wherein:
said set of non-volatile storage elements are connected to a first word line; and
said set of non-volatile storage elements are connected to every other bit line of a group of bit lines.
12. A method according to claim 1 , wherein:
said set of non-volatile storage elements are flash memory devices.
13. A method according to claim 1 , wherein:
said set of non-volatile storage elements are NAND flash memory devices.
14. A method according to claim 1 , wherein:
said set of non-volatile storage elements are multi-state flash memory devices.
15. A method according to claim 1 , wherein:
said set of non-volatile storage elements include floating gates.
16. A method according to claim 1 , wherein:
said set of non-volatile storage elements each include a dielectric region for storing charge.
17. A method for reading non-volatile storage, comprising:
performing a read process for a set of non-volatile storage elements, said read process includes using a set of reference values to determine a set of read data stored in said set of non-volatile storage elements;
determining that said read data has one or more errors that cannot be corrected by an error correction process; and
performing a data recovery process for said set of non-volatile storage elements that includes using a set of offsets with said reference values to determine a corrected set of read data, said offsets are chosen on an individual basis for said set of non-volatile storage elements based on status of respective adjacent storage elements to said set of non-volatile storage elements.
18. A method according to claim 17 , wherein:
said set of non-volatile storage elements store a page of data;
said set of non-volatile storage elements are connected to a first word line; and
said adjacent storage elements are connected to a second word line adjacent to said first word line.
19. A method according to claim 17 , wherein:
said data recovery processes includes performing multiple read processes, each read process uses a different one of said offsets and is performed on all of said set of non-volatile storage elements, there is at least one read process for each offset, each non-volatile storage element provides final data from an appropriate one of said read processes.
20. A method according to claim 17 , wherein said data recovery processes comprises:
performing a first read process without using an offset and storing results for one or more of said set of non-volatile storage elements having an adjacent non-volatile storage element in a first state;
performing a second read process using a first offset and storing results for one or more of said set of non-volatile storage elements having an adjacent non-volatile storage element in a second state;
performing a third read process using a second offset and storing results for one or more of said set of non-volatile storage elements having an adjacent non-volatile storage element in a third state; and
performing a fourth read process using a third offset and storing results for one or more of said set of non-volatile storage elements having an adjacent non-volatile storage element in a fourth state.
21. A method according to claim 17 , wherein:
said set of non-volatile storage elements are connected to a first word line that is part of a memory system having N word lines where an Nth word line is programmed last in sequence; and
said data recovery process is performed for all of said word lines except said Nth word line.
22. A method according to claim 17 , further comprising:
reporting results of said data recovery process.
23. A method according to claim 17 , wherein:
said set of non-volatile storage elements are NAND flash memory devices.
24. A method according to claim 17 , wherein:
said set of non-volatile storage elements are multi-state flash memory devices.