Combined simultaneous sensing of multiple wordlines in a post-write read (PWR) and detection of NAND failures
Techniques for a post-write read are presented. In an exemplary embodiment, a combined simultaneous sensing of multiple word lines is used in order to identify a problem in one or more of these word lines. That is, sensing voltages are concurrently applied to the control gates of more than one memory cell whose resultant conductance is measured on the same bit line. The combined sensing result is use for measuring certain statistics of the cell voltage distribution (CVD) of multiple word lines and comparing it to the expected value. In case the measured statistics are different than expected, this may indicate that one or more of the sensed word lines may exhibit a failure and more thorough examination of the group of word lines can be performed.
1. A method of operating a memory device including a plurality of memory cells formed along word lines and bit lines, the method comprising:
performing a write operation including the programming of a plurality of memory cells along a corresponding plurality of selected word lines and one or more selected bit lines; and
subsequently performing a composite sensing operation, including:
concurrently applying a plurality of first sensing voltages to distinguish between levels of programming to the corresponding plurality of selected word lines; and
concurrently performing, for each of the one or more selected bit lines, a first combined sensing operation of the corresponding plurality of memory cells along the plurality of selected word lines while the plurality of first sensing voltages are applied thereto, where the result of the first combined sensing operation is dependent upon the state of the plurality of memory cells along the selected bit line.
2. The method of claim 1 , further comprising:
performing a comparison of the result of the composite sensing operation with an expected result; and
based upon the comparison, making a determination of the integrity of the sensed memory cells.
3. The method of claim 2 , wherein the composite sensing operation further includes, subsequent to applying the first set of sensing voltages and concurrently performing a first combined sensing operation:
concurrently applying a corresponding plurality of second sensing voltages to distinguish between levels of programming to the corresponding plurality of selected word lines, where the arrangement of the second sensing voltages differs from the arrangement of the first sensing voltages;
concurrently performing, for each of the selected bit lines, a second combined sensing operation of the corresponding plurality of memory cells along the plurality of selected word lines while the plurality of second sensing voltages are applied thereto, where the result of the second combined sensing operation is dependent upon the state of the plurality of memory cells along the selected bit line,
wherein the comparison is based upon a combination of the first and second combined sensing operation.
4. The method of claim 3 , wherein the first sensing voltages includes a plurality of different levels and second sensing voltages is the same levels as the first sensing voltages, but applied to the selected word lines in a different combination.
5. The method of claim 3 , wherein the first sensing voltages are all equal to a first level, the second sensing voltages are all equal to a second level, and the first and second levels are different from each other.
6. The method of claim 3 , wherein said comparison includes computing a metric that is a function of the difference of each of the combined sensing operations from an expected value thereof, and said determination is based upon the value of the computed metric.
7. The method of claim 3 , wherein said write operation programs a block of memory cells and the method further comprises:
subsequently performing a composite sensing operation for one or more additional sets of selected word lines and performing an additional comparison of the result of the composite sensing operation for each of the one or more additional sets of selected word lines with a corresponding expected result,
wherein said determination is further based upon the additional comparisons.
8. The method of claim 7 , wherein said determination includes:
determining which of the comparisons indicates the largest difference between the composite sensing operation and the corresponding expected result.
9. The method of claim 8 , wherein said determination further includes:
for the comparison with the largest difference between the composite sensing operation and the corresponding expected result, individually examining the word lines of the corresponding selected word lines.
10. The method of claim 2 , wherein the composite sensing operation, comparison and determination are performed as part of a post-write verification process.
11. The method claim 2 , further comprising:
in response to the determination finding that the sensed memory cells have insufficient integrity, performing corrective operations.
12. The method of claim 1 , wherein the number of said one or more selected bit lines is the number of bit lines corresponding to a page.
13. The method claim 1 , wherein the memory device can store more than one bit of data in each of the memory cells.
14. The method of claim 13 , wherein the plurality of first sensing voltages concurrently applied to the corresponding plurality of selected word lines includes at least two distinct levels.
15. The method of claim 14 , wherein the number of selected word lines is greater than two.
16. The method of claim 13 , wherein the plurality of first sensing voltages concurrently applied to the corresponding plurality of selected word lines includes at least two levels that are the same.
17. The method of claim 1 , wherein the number of selected word lines is greater than two.
18. The method of claim 1 , wherein the memory device is formed according to a NAND-type architecture and the sensed memory cells along each of the selected bit lines are from a common NAND string, and wherein the composite sensing operation further includes applying a voltage to the non-selected word-lines of the common NAND strings whereby the memory cells there along do not contribute to the result of the combined sensing operation.
19. The method of claim 18 , wherein the voltage applied to the non-selected word-lines is greater than the first sensing voltages.
20. The method of claim 1 , wherein one or more of the first sensing voltages is offset from the sensing voltages used in a standard read operation.
21. The method of claim 1 , wherein the expected result is based on the data previously written into the sensed memory cells.
22. The method of claim 1 , wherein the expected result is based on an expected statistical distribution of the data written into the sensed memory cells.