Systems and methods to avoid false verify and false read
View Patent ↗In a nonvolatile NAND memory array, a NAND block may be falsely determined to be in an erased condition because of the effect of unwritten cells prior to the erase operation. Such cells may be programmed with dummy data prior to erase, or parameters used for a verify operation may be modified to compensate for such cells. Read operations may be similarly modified to compensate for unwritten cells.
1. A method of performing an adaptive erase verification during erase of a block in a 3-D NAND memory, comprising:
determining an amount of unwritten space remaining in the block in the 3-D NAND memory;
if the amount of unwritten space is less than a predetermined amount then performing erase verification using a default set of verification parameters; and
if the amount of unwritten space exceeds the predetermined amount then performing erase verification using a modified set of verification parameters that is different to the default set of erase parameters.
2. The method of claim 1 wherein the modified set of verification parameters is selected from a plurality of sets of modified verification parameters according to the amount of unwritten space.
3. The method of claim 1 wherein the amount of unwritten space is determined from a binary search of memory cells of the block.
4. The method of claim 3 wherein the binary search is performed by logic circuits that are on the same integrated circuit chip as the NAND memory.
5. The method of claim 1 wherein the 3-D NAND memory is formed of two or more layers of memory cells disposed above a substrate surface.
6. The method of claim 5 wherein the 3-D NAND memory includes vertical NAND strings with a first end at the substrate surface and a second end on top.
7. The method of claim 5 wherein the 3-D NAND memory includes U-shaped NAND strings.
8. The method of claim 1 wherein the block includes two or more vertical NAND strings connected to a common bit line.
9. A method of performing an adaptive erase verification during erase of a block in a NAND memory, comprising:
determining an amount of unwritten space remaining in the block;
if the amount of unwritten space is less than a predetermined amount then performing erase verification using a default set of verification parameters;
if the amount of unwritten space exceeds the predetermined amount then performing erase verification using a modified set of verification parameters that is different to the default set of erase parameters; and
wherein the amount of unwritten space is determined from a write pointer.
10. The method of claim 9 wherein the write pointer is maintained by a memory controller that is connected to the NAND memory.