Flash Memory Refresh Techniques Triggered by Controlled Scrub Data Reads

• US 20080239808A1
• Filed: 03/28/2007
• Published: 10/02/2008
• Est. Priority Date: 03/28/2007
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Lin, Jason T.
• Granted Patent: US 7,477,547 B2
• Time in Patent Office: Days
• US Class Current: 365/185.09
• CPC Class Codes: G06F 11/106 Correcting systematically all correctable errors, i.e. scrubbing
• CPC Class Codes: G11C 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 16/3431 Circuits or methods to detect disturbed nonvolatile memory cells, e.g. which still read as programmed but with threshold less than the program verify threshold or read as erased but with threshold greater than the erase verify threshold, and to reverse the disturbance via a refreshing programming or erasing step

Abstract

The quality of data stored in individual blocks of memory cells of a flash memory system is monitored by a scrub read of only a small portion of a block, performed after data are read from less than all of a block in response to a read command from a host or memory controller. The small portion is selected for the scrub read because of its greater vulnerability than other portions of the block to being disturbed as a result of the commanded partial block data read. This then determines, as the result of reading a small amount of data, whether at least some of the data in the block was disturbed by the command data read to a degree that makes it desirable to refresh the data of the block.

Claims

1. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in strings that individually contain a plurality of series connected memory cells between ends of the strings

identify those memory cells within the group adjacent ends of the strings to have a higher degree of vulnerability to being disturbed by a data read than other memory cells of the group not adjacent the ends of the strings,

recognize when at least some of the memory cells of the group that are not adjacent the ends of the strings have been accessed and the data stored therein read in response to a command,

subsequently scrub read data from at least some of the memory cells adjacent the ends of the strings of memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in at least some of the memory cells of the group that are adjacent ends of the strings.

14. The method of claim 1, wherein the command to read data is received from outside of the memory system.

15. The method of claim 1, wherein the command to read data is generated within the memory system.

2. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells, subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate,

refresh the data stored in all the memory cells of the group.

3. The method of claim 2, wherein refreshing the data includes reading the data stored in all the memory cells of the group and rewriting the read data into another group of memory cells that have previously been erased.

4. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein determining a level of quality of the read data includes determining a number of bit errors in the scrub read data, and further wherein determining whether the quality of the scrub read data is adequate includes comparing the number of bit errors with a preset threshold.

5. A method operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein determining a level of quality of the scrub read data includes determining a first number of bit errors in at least some of the command read data and a second number of bit errors in at least some of the scrub read data, and further wherein determining whether the quality of the read data is adequate includes comparing the first and second numbers of bit errors.

6. The method of claim 5, wherein comparing the first and second numbers of bit errors includes calculating a ratio of the first and second numbers, and then comparing the ratio with a pre-defined threshold.

7. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein identifying memory cells within the group that have a higher degree of vulnerability to being disturbed includes a consideration of the number of memory cells of the group accessed in response to the command and a sequence in which the memory cells of the group are accessed.

8. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein identifying memory cells within the group that have a higher degree of vulnerability to being disturbed includes a consideration of physical characteristics of the memory cells in different physical locations within the group.

9. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein identifying memory cells within the group that have a higher degree of vulnerability to being disturbed includes a consideration of patterns of data stored in the memory cells of the group.

10. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein identifying memory cells within the group that have a higher degree of vulnerability to being disturbed includes distinguishing between the memory cells that have been accessed in response to the command and other memory cells within the group.

11. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein it is determined that there are no memory cells within the group that have a higher degree of vulnerability to being disturbed by the commanded data read, and in response omits all of the subsequent scrub read, determination of the level of quality of the scrub read data and the data refresh.

12. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein recognizing when less than all the memory cells of the group are accessed includes recognizing when less than all the memory cells of the group are accessed for the purpose of reading data stored therein by a host or application being executed within the memory system.

13. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein the at least one group of memory cells are concurrently erasable prior to new data being written therein.

16. A method of operating a reprogrammable non-volatile memory system having at least one group of charge storage memory cells connected in a manner that accessing less than all of the memory cells in the group can disturb the levels of charge stored in other memory cells in the group, comprising:

recognize when less than all the memory cells of the group have been accessed and the data stored therein read in response to a command,

determine whether some of the memory cells within the group have a higher degree of vulnerability to being disturbed by the commanded data read than other memory cells of the group, and, if so, identify the more vulnerable memory cells,

subsequently scrub read data from at least some of the more vulnerable memory cells,

determine a level of quality of the scrub read data and whether it is adequate, and

if the level of quality of the scrub read data is determined not to be adequate, refresh the data stored in the memory cells of the group,

wherein the refresh of the data stored in the memory cells of the group includes reading all the data stored in the group of memory cells, make any necessary corrections of erroneous bits of the read so read, and rewrite the corrected read data into another group of memory cells that has been erased.

17. In a reprogrammable non-volatile memory system having an array of memory cells with charge storage elements connected in series in a plurality of strings, the strings being individually connectable at first and second ends thereof to respective first and second voltage sources and having control gates connected in a direction extending across the plurality of strings to individually define pages of data storage capacity, the strings further being grouped into blocks of memory cell strings that are concurrently erasable, a method of operating the memory system, comprising:

recognize when data are read from one or more but less than all of the pages of a given block in response to a command,

identify one or more pages of the given block that have a higher degree of vulnerability to being disturbed by the command data read than other pages of the given block,

scrub read data stored in the identified one or more pages of the given block but not in the other pages of the given block,

measure a number of bit errors in at least the scrub read data,

determine from the number of bit errors whether a refresh of at least the scrub read data is necessary, and

if a refresh of at least the scrub read data is determined to be necessary, refresh all the pages of the given block.

18. The method of claim 17, wherein identifying one or more pages of the given block that have a higher degree of vulnerability to being disturbed by the data read than other pages of the given block includes considering at least one of two pages of the given block defined by control gates connected in a direction extending across the plurality of strings at their ends.

19. The method of claim 17, identifying one or more pages of the given block that have a higher degree of vulnerability to being disturbed by the data read than other pages of the given block includes considering relative patterns of data stored in the pages of the given block.

20. The method of claim 17, wherein

recognizing when data are read from one or more but less than all of the pages of the given block includes noting the identity of the pages command read, and

identifying one or more pages of the given block that have a higher degree of vulnerability to being disturbed by the data read than other pages of the given block includes considering at least one page of the given block not noted to have been command read but which is immediately adjacent to one or more of the pages from which data have been noted to have been command read.

21. The method of claim 17, wherein

recognizing when data are read from one or more but less than all of the pages of the given block includes noting the identity of the pages so command read, and

identifying one or more pages of the given block that have a higher degree of vulnerability to being disturbed by the data read than other pages of the given block includes considering the location of pages noted to have been command read and the order in which data of these pages have been command read.

22. The method of claim 17, wherein recognizing when data are read from one or more but less than all of the pages of the given block includes noting the identity of the pages so command read, and

identifying one or more pages of the given block that have a higher degree of vulnerability to being disturbed by the command read than other pages of the given block includes considering whether individual pages of the given block been command read or not.

23. The method of claim 17, wherein determining whether data in the block are to be refreshed includes comparing the number of bit errors of at least the scrub read data with a preset threshold.

24. The method of claim 17, additionally comprising measuring a number of bit errors in the data command read from one or more pages of the given block, and wherein determining whether data in the block are to be refreshed includes comparing the number of bit errors in the scrub read data with the number of bit errors in the command read data.

25. The method of claim 24, wherein comparing the data bit errors includes calculating a ratio of the number of bit errors in the scrub read data and the number of bit errors in the command data read, and thereafter comparing the ratio with a pre-defined threshold.

26. The method of claim 17, therein the command to read data from one or more but less than all of the pages of the given block is received from outside of the memory system.

27. The method of claim 17, therein the command to read data from one or more but less than all of the pages of the given block generated within the memory system.

28. In a reprogrammable non-volatile memory system having an array of memory cells organized into NAND strings and a system controller connected thereto, the memory cells individually including a transistor having a source and drain, a charge storage element and a control gate, each NAND string having a source end and a drain end and being formed by a series of charge storage transistors daisy-chained by the drain of one cells to the source the adjacent charge storage transistor and switchable to the source end by a source select transistor and switchable to the drain end by a drain select transistor, a method of operating the memory system, comprising:

identify the memory cells of the NAND strings to be in either a first group or a second group, the memory cells of the second group being adjacent to either the source select transistor or the drain select transistor and the memory cells of the first group being the complement of the second group,

recognizing when, in response to a command, data are read from memory cells of the first group but not at least some of the second group,

in response to such a recognition, scrub read data from at least some of the unread memory cells of the second group,

thereafter measure a number of errors in the data read from memory cells of the second group,

thereafter determine from the number of measured data errors whether a refresh of the scrub read data is necessary, and

if so, refresh the data of memory cells in both of the first and second groups.

29. The method as in claim 28, wherein said storing is by programming in parallel a page of memory cells with a common word line extending across a plurality of NAND strings.

30. The method as in claim 28, wherein the array is divided into blocks that individually include a plurality of NAND strings whose memory cells are concurrently erasable prior to programming data therein, and the first and second groups of memory cells are located in a given block of memory cells.