Program and read operations for 3D non-volatile memory based on memory hole diameter
Techniques are provided for programming and reading memory cells in a 3D stacked non-volatile memory device by compensating for variations in a memory hole diameter. The memory hole diameter is smaller at the bottom of the stack, resulting in more severe read disturb. To compensate, programming of memory cells at the lower word line layers is modified. In one approach, threshold voltage (Vth) distributions of one or more data states are narrowed during programming so that a lower read pass voltage can be used in a subsequent sensing operation. A sufficient spacing is maintained between the read pass voltage and the upper tail of the highest data state. The Vth distributions can be downshifted as well. In another approach, the read pass voltage is not lowered, but the lowest programmed state is upshifted to provide spacing from the upper tail of the erased state.
1. A method for programming a 3d non-volatile memory device, comprising:
selecting a set of memory cells in one word line layer of a plurality of word line layers to store data, the plurality of word line layers are arranged alternatingly with dielectric layers in a stack, and memory cells in the set of memory cells in the one word line layer are arranged in respective memory holes which extend through the stack, the respective memory holes having respective widths which vary along the memory holes; and
programming the set of memory cells in the one word line layer, the programming is adjusted based on a position of the one word line layer in the stack.
2. The method of claim 1 , wherein:
the programming is adjusted to provide a relatively narrower threshold voltage distribution for a highest programmed data state of a plurality of programmed data states when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
3. The method of claim 1 , wherein:
the programming is adjusted to provide a relatively lower upper tail of a threshold voltage distribution for a highest programmed data state of a plurality of programmed data states when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
4. The method of claim 1 , wherein:
the respective widths are progressively smaller closer to a bottom of the stack.
5. The method of claim 1 , wherein:
the programming is adjusted by programming one subset of the set of memory cells to a highest programmed data state of a plurality of programmed data states using a verify level which is relatively lower when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
6. The method of claim 1 , wherein:
the programming is adjusted by programming one subset of the set of memory cells to a lowest programmed data state of a plurality of programmed data states, above an erased data state, using a verify level which is relatively higher when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
7. The method of claim 1 , wherein:
the programming is adjusted by programming one subset of the set of memory cells to a highest programmed data state of a plurality of programmed data states at a rate which is relatively slower when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
8. The method of claim 7 , wherein:
the programming of the set of memory cells comprises applying a series of program pulses to the set of memory cells; and
the programming of the one subset of the set of memory cells at the rate which is relatively slower is achieved by adjusting bit line voltages for the one subset of the set of memory cells during one or more of the program pulses.
9. The method of claim 8 , wherein:
the adjusting of the bit line voltages begins partway through the series of program pulses based on a fixed program pulse number in the series of program pulses or based on an adaptive decision which is made according to a program progress of the set of memory cells.
10. The method of claim 7 , wherein:
the programming of the set of memory cells comprises applying a series of program pulses to the set of memory cells; and
the programming of the one subset of the set of memory cells at the rate which is relatively slower comprises reducing a step size of one or more of the program pulses to a zero or non-zero value.
11. The method of claim 10 , wherein:
the reducing of the step size begins partway through the series of program pulses based on a fixed program pulse number in the series of program pulses or based on an adaptive decision which is made according to a program progress of the set of memory cells.
12. The method of claim 1 , further comprising:
after the programming of the set of memory cells, sensing another set of memory cells in another word line layer of the plurality of word line layers, the sensing the another set of memory cells comprises applying a sense voltage to the another word line layer while applying a read pass voltage to the one word line layer, the read pass voltage is set based on the position of the one word line layer in the stack, and the read pass voltage is relatively lower when the position of the one word line layer in the stack is adjacent to relatively narrower portions of the memory holes.
13. The method of claim 1 , wherein:
the memory holes are columnar and extend at least from a top word line layer of the plurality of word line layers to a bottom word line layer of the plurality of word line layers.
14. A 3d non-volatile memory device, comprising:
a plurality of word line layers arranged alternatingly with dielectric layers in a stack;
a plurality of memory cells arranged in NAND strings and in communication with the plurality of word line layers, the NAND strings are arranged in respective memory holes which extend through the stack, the respective memory holes having respective widths which vary along the memory holes; and
a control circuit, the control circuit is configured to select a set of memory cells in one word line layer of the plurality of word line layers to store data, and programs the set of memory cells in the one word line layer, the programming is adjusted based on a position of the one word line layer in the stack.
15. The 3d non-volatile memory device of claim 14 , wherein:
the programming is adjusted to provide a relatively narrower threshold voltage distribution for a highest programmed data state of a plurality of programmed data states when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
16. The 3d non-volatile memory device of claim 14 , wherein:
the programming is adjusted by programming one subset of the set of memory cells to a highest programmed data state of a plurality of programmed data states using a verify level which is relatively lower when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
17. The 3d non-volatile memory device of claim 14 , wherein:
the programming is adjusted by programming one subset of the set of memory cells to a lowest programmed data state of a plurality of programmed data states, above an erased data state, using a verify level which is relatively higher when the position of the one word line layer is adjacent to relatively narrower portions of the memory holes.
18. The 3d non-volatile memory device of claim 14 , wherein:
the control circuit, after the programming of the set of memory cells, is configured to sense another set of memory cells in another word line layer of the plurality of word line layers, the sensing the another set of memory cells comprises applying a sense voltage to the another word line layer while applying a read pass voltage to the one word line layer, the read pass voltage is set based on the position of the one word line layer in the stack, the read pass voltage is relatively lower when the position of the one word line layer in the stack is adjacent to relatively narrower portions of the memory holes.
19. The 3d non-volatile memory device of claim 14 , wherein:
the plurality of word line layers have a uniform thickness.
20. A method for programming a 3d non-volatile memory device, comprising:
selecting a set of memory cells in one word line layer of a plurality of word line layers to sense data, the plurality of word line layers are arranged alternatingly with dielectric layers in a stack, and memory cells in the set of memory cells in the one word line layer are arranged in respective memory holes which extend through the stack, the respective memory holes having respective widths which vary along the memory holes; and
sensing the set of memory cells in the one word line layer, the sensing comprises applying a sense voltage to the one word line layer while applying read pass voltages to remaining word line layers of the plurality of word line layers, the read pass voltages are set based on relative positions of the remaining word line layers in the stack.
21. The method of claim 20 , wherein:
for at least one of the remaining word line layers, the read pass voltage is set relatively lower when the position of the at least one of the remaining word line layers in the stack is adjacent to relatively narrower portions of the memory holes.
22. The method of claim 20 , wherein:
memory cells of the at least one of the remaining word line layers are programmed to a plurality of programmed data states; and
a highest programmed data state of the plurality of programmed data states has a relatively lower upper tail of a threshold voltage distribution when the position of the at least one of the remaining word line layers in the stack is adjacent to relatively narrower portions of the memory holes.