Optimized configurable NAND parameters
Configurable parameters may be used to access NAND flash memory according to schemes that optimize such parameters according to predicted characteristics of memory cells, for example, as a function of certain memory cell device geometry, which may be predicted based on the location of a particular device within a memory array.
1. A method of operating a charge-storage memory cell array comprising:
identifying a pattern of variation of at least one memory cell dimension within the charge-storage memory cell array;
determining a first set of parameters to use in accessing a first plurality of memory cells based on the at least one memory cell dimension of the first plurality of memory cells;
determining a second set of parameters to use in accessing a second plurality of memory cells based on the at least one memory cell dimension of the second plurality of memory cells, the second set of parameters including at least one parameter that is different from the first set of parameters;
subsequently accessing the first plurality of memory cells using the first set of parameters; and
accessing the second plurality of memory cells using the second set of parameters.
2. The method of claim 1 wherein the at least one memory cell dimension is a diameter of a cylindrical memory hole.
3. The method of claim 2 wherein the charge-storage memory cell array is a three-dimensional memory array, the first plurality of memory cells are located in a lower portion of the three-dimensional memory array, the second plurality of memory cells are located in an upper portion of the three-dimensional memory array, and the identified pattern of variation indicates that the diameter of the cylindrical memory hole is greater in the second plurality of memory cells than in the first plurality of memory cells.
4. The method of claim 1 wherein the at least one parameter that is different is a voltage that is applied to program memory cells.
5. The method of claim 1 wherein the at least one parameter that is different is a number of voltage pulses used to program memory cells.
6. The method of claim 1 wherein the at least one parameter that is different is a read voltage that is used to read memory cells.
7. The method of claim 1 wherein the at least one parameter is an erase voltage that is used to erase memory cells.
8. The method of claim 1 further comprising maintaining a write-erase cycle count for blocks of the charge-storage memory array and updating the first and second sets of parameters according to the write erase cycle counts.
9. The method of claim 8 further comprising updating the first set of parameters according to a first update scheme and updating the second set of parameters according to a second update scheme that is different from the first update scheme.
10. A charge-storage memory cell array comprising:
an array of memory cells, each memory cell having a memory cell dimension, the memory cell dimension having a pattern of variation across the array of memory cells; and
memory access circuits that access memory cells of the array of memory cells according to access parameters, the memory access circuits having a first set of parameters for accessing a first plurality of memory cells and a second set of parameters for accessing a second plurality of memory cells, the second set of parameters including at least one parameter that is different from the first set of parameters, the first plurality of memory cells and the second plurality of memory cells defined according to the pattern of variation.
11. The charge-storage memory cell array of claim 10 wherein the charge-storage memory cell array is a three-dimensional memory array.
12. The charge-storage memory cell array of claim 11 wherein the charge-storage memory cell array is a three-dimensional NAND memory array which includes a plurality of NAND strings that extend vertically in a direction that is perpendicular to a substrate surface.
13. The charge-storage memory cell array of claim 12 wherein the pattern of variation across the array of memory cells is a pattern of variation of the memory cell dimension according to vertical distance from the substrate surface.
14. The charge-storage memory cell array of claim 13 wherein the memory cell dimension is a diameter of an opening that extends vertically through word lines with a channel extending vertically through the opening.
15. The charge-storage memory cell array of claim 10 further comprising a write-erase cycle count circuit that maintains write-erase cycle counts for blocks in the charge-storage memory cell array and updates access parameters according to write-erase cycle counts.
16. The charge-storage memory cell array of claim 15 wherein the write-erase cycle count circuit has a first update scheme for updating the first set of parameters and a second update scheme for updating the second set of parameters.