Flash memory with data refresh triggered by controlled scrub data reads

• US 7,573,773 B2
• Filed: 03/28/2007
• Issued: 08/11/2009
• 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.
• Primary Examiner(s): Luu; Pho M.
• Assistant Examiner(s): BUI, THA-O H
• Application Number:
  US11/692,829
• Publication Number:
  US 20080239851A1
• Time in Patent Office: 867 Days
• Field of Search: 365/189.02, 365/185.04, 365/185.07, 365/185.17, 365/185.24, 365/185.03, 365/185.18, 365/185.21
• US Class Current: 365/222
• 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 reprogrammable non-volatile memory system, comprising:

a plurality of charge storage memory cells connected in a plurality of strings that individually contain a plurality of series connected memory cells, the strings being individually connectable at first and second ends thereof to respective first and second voltage sources and having word lines extending in a direction across the plurality of strings to individually define pages of memory cells, and

a controller connected with the charge storage memory cells and that operates to;

designate at least those pages of memory cells immediately adjacent the first and second ends of the strings to be vulnerable to being disturbed during execution of a commanded data read from pages of memory cells not immediately adjacent the ends of the strings,

recognize when pages of memory cells not immediately adjacent the ends of the strings have been accessed and data stored in the accessed pages of memory cells read in response to a command,

subsequently initiate a scrub read of data from at least some of the pages of memory cells immediately adjacent the first and second ends of the strings,

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

if the level of quality of the read data is determined not to be adequate, refresh the data stored in the at least some of the pages of memory cells immediately adjacent the first and second ends of the strings.

2. The memory system of claim 1, wherein the plurality of strings are formed in groups of memory cells that individually contain a plurality of the pages of memory cells, the memory cells of individual groups being erasable concurrently.

3. The memory system of claim 1, wherein the controller further operates to:

determine the level of quality of the read data by determining a number of bit errors in the scrub read data, and

determine the level of quality of the scrub data by comparing the number of bit errors with a preset threshold.

4. The memory system of claim 1, wherein the controller further operates to:

determine a level of quality of the scrub read data by 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

determine the level of quality of the scrub read data by comparing the first and second numbers of bit errors.

5. The memory system of claim 4, wherein the controller further operates to:

compare the first and second numbers of bit errors by calculating a ratio of the first and second numbers, and

thereafter comparing the ratio with a pre-defined threshold.

6. A reprogrammable non-volatile memory system, comprising:

a plurality of groups 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, and

a controller connected with the charge storage memory cells and that operates to;

recognize when less than all the memory cells of the group have been accessed and the data stored therein have been 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 memory cells within the group that have the higher degree of vulnerability to being disturbed by considering 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,

subsequently initiate a scrub read of data from at least some of the memory cells that have been identified to have the higher degree of vulnerability,

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

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

7. A reprogrammable non-volatile memory system, comprising:

a plurality of groups 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, and

a controller connected with the charge storage memory cells and that, operates to;

recognize when less than all the memory cells of the group have been accessed and the data stored therein have been 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 memory cells within the group that have the higher degree of vulnerability to being disturbed by considering physical characteristics of the memory cells in different physical locations within the groups,

subsequently initiate a scrub read of data from at least some of the memory cells that have been identified to have the higher degree of vulnerability,

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

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

8. A reprogrammable non-volatile memory system, comprising:

a plurality of groups 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, and

a controller connected with the charge storage memory cells and that operates to;

recognize when less than all the memory cells of the group have been accessed and the data stored therein have been 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 memory cells within the group that have the higher degree of vulnerability to being disturbed by considering patterns of data stored in the memory cells of the group,

subsequently initiate a scrub read of data from at least some of the memory cells that have been identified to have the higher degree of vulnerability,

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

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

9. A reprogrammable non-volatile memory system, comprising:

a plurality of groups 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, and

a controller connected with the charge storage memory cells and that operates to;

recognize when less than all the memory cells of the group have been accessed and the data stored therein have been 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 memory cells within the group that have the higher degree of vulnerability to being disturbed by distinguishing between the memory cells that have been accessed in response to the command and other memory cells within the group,

subsequently initiate a scrub read of data from at least some of the memory cells that have been identified to have the higher degree of vulnerability,

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

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