Nonvolatile memory and method for compensating during programming for perturbing charges of neighboring cells
Shifts in the apparent charge stored on a charge storing element of a non-volatile memory cell can occur because of the coupling of an electric field based on the charge stored in adjacent charge storing elements. To compensate for this coupling, the programming process for a given memory cell can take into account the target programmed state of one or more adjacent memory cell. The amount of programming is verified after each programming pulse and the standard verify level for the programming cell is dependent on the target state. The verify level is further offset lower dependent on the amount of perturbation from neighboring cells, determinable by their target states. The verify level is preferably virtually offset by biasing adjacent word lines instead of actually offsetting the standard verify level. For soft-programming erased cells, neighboring cells on both adjacent word lines are taken into account.
1. A method of programming a group of memory cells in parallel with reduced error due to perturbing electric fields from neighboring memory cells, comprising:
(a) selecting a group of memory cells to be programmed in parallel to a given target state;
(b) applying a predetermined dose of programming waveform voltage to the group of memory cells in parallel to increase a threshold voltage of each memory cell under programming;
(c) verifying the programmed states of the group of memory cells relative to the given target state by determining the threshold voltage of the memory cell being verified relative to a predetermined verify level, the predetermined verify level being a function of the given target state and the target states of memory cells neighboring the memory cell being verified;
(d) inhibiting from further programming memory cells in the group that have been verified; and
repeating (b) to (d) until the memory cells of the group have all been verified to the given target memory state.
2. The method as in claim 1 , wherein the function yields a predetermined verify level for programming a given memory cell to the target state such that the correct target state can still be read from the given memory cell when the neighboring memory cells are programmed after the given memory cell.
3. The method as in claim 1 , wherein the memory cells are accessible by word lines and bit lines, and the neighboring memory cells include ones sharing a common word line as the memory cell being verified.
4. The method as in claim 1 , wherein the memory cells are accessible by word lines and bit lines, and the neighboring memory cells include ones sharing a common bit line as the memory cell being verified.
5. The method as in claim 1 , wherein the memory cells are accessible by word lines and bit lines, and the verifying step further comprises:
sensing with a voltage given by the predetermined verify level on the word line of the memory cell being verified.
6. The method as in claim 1 , wherein the memory cells are accessible by word lines and bit lines, and the verifying step further comprises:
sensing with a voltage given by a target verify level on the word line of the memory cell being verified, said target verify level being a function of the target state, and with a bias voltage on an adjacent word line, said bias voltage being a function of the target states of memory cells neighboring the memory cell being verified.
7. The method as in claim 5 , wherein the predetermined verify level on the word line of the memory cell being verified is a decreasing function of the amount of perturbation from neighboring memory cells.
8. The method as in claim 6 , wherein the predetermined verify level on the word line of the memory cell being verified is a decreasing function of the amount of perturbation from neighboring memory cells.
9. In a nonvolatile memory having an array of memory cells accessible by word lines and bit lines, a method of programming a group of memory cells in parallel with reduced error due to perturbing electric fields from neighboring memory cells, comprising:
(a) designating a group of memory cells to be programmed in parallel to a given target state;
(b) applying a dose of programming waveform voltage to the group of memory cells in parallel to increase a threshold voltage of each memory cell under programming;
(c) applying a predetermined verify voltage level to a word line accessing the group of memory cells, the predetermined verify voltage level being a first function of the given target state;
(d) applying a predetermined bias voltage level to an adjacent word line, the predetermined bias voltage being a second function of target states of memory cells adjacent the memory cell being verified;
(e) sensing to verify the group of memory cells under programming;
(f) inhibiting from further programming memory cells in the group that have been verified to have been programmed to the given target state; and
repeating (b) to (f) until the memory cells of the group have all been verified to be in the given target state.
10. The method as in claim 9 , wherein the second function yields a predetermined bias level for verifying so that the programming results in the correct target state being read from a programmed memory cell of the group even when the neighboring memory cells are programmed subsequently.
11. The method as in claim 9 , wherein the neighboring memory cells include ones sharing a common word line as the memory cell being verified.
12. The method as in claim 9 , wherein the neighboring memory cells include ones sharing a common bit line as the memory cell being verified.
13. The method as in claim 12 , wherein the second function is a decreasing function of the amount of perturbation from neighboring memory cells.
14. In a nonvolatile memory having an array of memory cells accessible by word lines and bit lines, a method of erasing a group of memory cells in parallel with reduced error due to perturbing electric fields from neighboring memory cells, comprising:
(a) erasing a group of memory cells substantially beyond a predetermined threshold level demarcating an erased state;
(b) applying a dose of programming waveform voltage to the group of erased memory cells in parallel to increase a threshold voltage of each memory cell under programming;
(c) applying the predetermined threshold level voltage to the word line accessing the group of memory cells;
(d) applying a first predetermined bias voltage level to a first adjacent word line, the first predetermined bias voltage being a second function of the target state of a first neighboring memory cell adjacent the memory cell being verified, the first neighboring memory cell being accessible by the first adjacent word line;
(e) applying a second predetermined bias voltage level to a second adjacent word line, the second predetermined bias voltage being a second function of the target state of a second neighboring memory cell adjacent the memory cell being verified, the second neighboring memory cell being accessible by the second adjacent word line;
(f) sensing to verify the group of erased memory cells that has been programmed relative to the predetermined threshold level;
(g) inhibiting from further programming memory cells in the group that have been verified; and
repeating (b) to (g) until the memory cells of the group have all been verified.
15. A method as in claim 14 , wherein the second function yields a predetermined bias level for verifying so that the programming results in the correct target state being read from a memory cell of the group even when the neighboring memory cells are programmed after the memory cell.
16. A method as in claim 14 , wherein the neighboring memory cells include ones sharing a common word line as the memory cell being verified.
17. A method as in claim 14 , wherein the neighboring memory cells include ones sharing a common bit line as the memory cell being verified.
18. A method as in claim 17 , wherein the second function is a decreasing function of the amount of perturbation from neighboring memory cells.