Erase algorithm for multi-level bit flash memory
Methods of erasing a sector of multi-level flash memory cells (MLB) having three or more data states to a single data state are provided. The present invention employs an interactive sector erase algorithm that repeatedly erases, verifies, soft programs, and programs the sector in two or more erase phases to achieve highly compact data state distributions. In one example, the algorithm essentially erases all the MLB cells of the sector to an intermediate state and corresponding threshold voltage value using interactive erasing, soft programming and programming pulses in a first phase. Then in a second phase, the algorithm further erases all the MLB cells of the sector using additional interactive erasing and soft programming pulses until a final data state is achieved corresponding to a desired final threshold voltage value of the cells. Optionally, the algorithm may include one or more additional phases of similar operations that successively bring the memory cells of the sector to a compacted common erased state in preparation for subsequent programming operations. In one aspect of the method, the actual threshold values and/or data states chosen for these phases may be predetermined and input to the memory device by the user.
1. A method of erasing a sector of memory cells to a single data state, the memory cells having three or more data states corresponding to three or more threshold voltage values, the method comprising:
performing a block erase operation on the sector of memory cells until all cells of the sector are erased to a threshold voltage generally corresponding to an intermediate value;
performing a soft programming operation on an over-erased memory cell of the sector until the threshold voltage of the over-erased memory cell generally corresponds to a final value;
programming memory cells of the sector determined to be at or near a threshold voltage corresponding to the final value until the threshold voltage of the memory cells generally correspond to the intermediate value;
re-performing the block erase operation on the sector until all cells of the sector are erased to the threshold voltage generally corresponding to the final value; and
re-performing a soft programming operation on an over-erased memory cell of the sector until the threshold voltage corresponding to the final value is attained.
2. The method of claim 1 , wherein the performing a block erase operation on the sector of memory cells until all cells of the sector are erased to a threshold voltage generally corresponding to an intermediate value further comprises:
verifying the block erase operation of the memory cells by identifying whether the threshold voltage of the memory cells generally correspond to the intermediate value; and
re-performing the block erase operation on the sector of memory cells until all cells of the sector are determined to have a threshold voltage generally corresponding to the intermediate value.
3. The method of claim 2 , wherein verifying the block erase operation includes identifying erased memory cells by measuring threshold voltage values generally corresponding to bits of respective memory cells of the sector and comparing the measured values with a minimum erase threshold voltage value.
4. The method of claim 1 , wherein the performing a soft programming operation on an over-erased memory cell of the sector until the threshold voltage of the over-erased memory cell generally corresponds to the intermediate value comprises:
identifying an over-erased memory cell in the sector;
performing a soft programming operation on the over-erased memory cell;
verifying the soft programming operation of the over-erased memory cell by identifying whether the threshold voltage of the over-erased memory cell corresponds to the final value; and
re-performing the soft programming operation on the over-erased memory cell until the over-erased memory cell of the sector is determined to have a threshold voltage corresponding to the final value.
5. The method of claim 1 , wherein the programming memory cells of the sector determined to be at or near a threshold voltage corresponding to the final value until the threshold voltage of the memory cells generally correspond to the intermediate value comprises:
identifying a group of remaining memory cells in the sector having a threshold voltage corresponding to the final value;
programming all remaining memory cells of the sector determined to be at or near a threshold voltage generally corresponding to the final value;
verifying the programming operation of the memory cells by identifying whether the threshold voltage of the memory cells generally correspond to the intermediate value; and
re-programming the remaining memory cells of the sector determined to be at a threshold voltage corresponding to the final value until the memory cells are determined to have a threshold voltage generally corresponding to the intermediate value.
6. The method of claim 1 , wherein the re-performing the block erase operation on the sector until all cells of the sector are erased to the threshold voltage generally corresponding to the final value comprises:
verifying the block erase operation of the memory cells by identifying whether the threshold voltage of the memory cells generally correspond to the final value; and
re-performing the block erase operation on the sector of memory cells until all cells of the sector are determined to have a threshold voltage generally corresponding to the final value.
7. The method of claim 1 , wherein the re-performing a soft programming operation on an over-erased memory cell of the sector until the threshold voltage generally corresponding to the final value is attained comprises:
verifying the soft programming operation of the over-erased memory cells by identifying whether the threshold voltage of the over-erased memory cells generally correspond to the final value; and
re-performing the soft programming operation on the over-erased memory cells until the over-erased memory cells of the sector are determined to have a threshold voltage generally corresponding to the final value.
8. The method of claim 1 , wherein the threshold voltage of the intermediate value is a higher voltage than the threshold voltage of the final value.
9. The method of claim 1 , wherein the intermediate and final threshold voltage values individually correspond to one of the three or more data states generally corresponding to the three or more threshold voltage values.
10. The method of claim 1 , wherein the memory cells having three or more data states generally corresponding to three or more threshold voltage values correspond to memory cells having four data states and four threshold voltage values.
11. The method of claim 1 , further comprising additional erasing and soft programming operations to the memory cells for bringing the threshold voltage of the memory cells to one or more additional threshold voltage values between the intermediate and final threshold voltages to provide further bit compacting.
12. The method of claim 1 , wherein the intermediate and final threshold voltage values are predetermined by a memory device user.
13. The method of claim 12 , wherein the intermediate and final threshold voltage values are input by the user to circuitry associated with the memory cells.
14. The method of claim 12 , wherein the final threshold voltage value is about half the voltage of the intermediate threshold voltage value.
15. The method of claim 1 , further comprising pre-programming a portion of the memory cells prior to performing the block erase operation.
16. The method of claim 1 , wherein the block erase operation reduces threshold voltage values generally corresponding to bits of respective memory cells of the sector.
17. The method of claim 1 , wherein different portions of the sector of memory cells are initially programmed to different threshold voltage values.
18. The method of claim 1 , wherein the sector of memory cells comprises a portion of a larger array of memory cells, wherein different portions of the array of memory cells are initially programmed to different threshold voltage values.
19. The method of claim 1 , wherein one or more sectors of memory cells comprise an array of memory cells, wherein different sectors of the array are initially programmed to different threshold voltage values.
20. The method of claim 1 , wherein the sector of memory cells is erased to an L 1 data state, the memory cells have four data states L 1 , L 2 , L 3 , and L 4 corresponding to four respective threshold voltage values, and wherein the intermediate threshold voltage value corresponds to the L 2 data state and the final threshold voltage value corresponds to the L 1 data state.
21. The method of claim 1 , wherein one or more voltages used in the erase, program, and soft-program operations are adjusted to further optimize the method.
22. A method of erasing a group of memory cells having three or more threshold voltage values, the method comprising:
performing a block erase operation on the group of memory cells until all cells of the group are erased to a threshold voltage corresponding to an intermediate value;
performing a soft programming operation on each over-erased memory cell of the group until the threshold voltage of the over-erased memory cells generally correspond to a final value;
programming all remaining memory cells of the group determined to be at a threshold voltage corresponding to the final value until the threshold voltage of the remaining memory cells correspond to the intermediate value;
re-performing the block erase operation on the group until all cells of the group are erased to the threshold voltage corresponding to the final value; and
re-performing a soft programming operation on each over-erased memory cell of the group until the threshold voltage corresponding to the final value is attained.
23. The method of claim 22 , wherein the performing a block erase operation on the group of memory cells until all cells of the group are erased to a threshold voltage generally corresponding to an intermediate value further comprises:
verifying the block erase operation of the memory cells by identifying whether the threshold voltage of the memory cells corresponds to the intermediate value; and
re-performing the block erase operation on the group of memory cells until all cells of the group are determined to have a threshold voltage corresponding to the intermediate value.
24. The method of claim 23 , wherein verifying the block erase operation includes identifying erased memory cells by measuring threshold voltage values corresponding to bits of respective memory cells of the group and comparing the measured values with a minimum erase threshold voltage value.
25. The method of claim 22 , wherein the performing a soft programming operation on each over-erased memory cell of the group until the threshold voltage of the over-erased memory cells generally correspond to a final value comprises:
identifying a set of over-erased memory cells in the group;
performing a soft programming operation on each over-erased memory cell in the group;
verifying the soft programming operation of the over-erased memory cells by identifying whether the threshold voltage of the over-erased memory cells correspond to the final value; and
re-performing the soft programming operation on the over-erased memory cells until the over-erased memory cells of the group are determined to have a threshold voltage corresponding to the final value.
26. The method of claim 22 , wherein the programming all remaining memory cells of the group determined to be at a threshold voltage corresponding to a final value until the threshold voltage of the remaining memory cells correspond to the intermediate value comprises:
identifying a set of remaining memory cells in the group having a threshold voltage corresponding to the final value;
programming all remaining memory cells of the group determined to be at a threshold voltage corresponding to the final value;
verifying the programming operation of the memory cells by identifying whether the threshold voltage of the memory cells correspond to the intermediate value; and
re-programming the remaining memory cells of the group determined to be at a threshold voltage corresponding to the final value until the memory cells are determined to have a threshold voltage corresponding to the intermediate value.
27. The method of claim 22 , wherein the re-performing the block erase operation on the group until all cells of the group are erased to the threshold voltage corresponding to the final value comprises:
verifying the block erase operation of the memory cells by identifying whether the threshold voltage of the memory cells correspond to the final value; and
re-performing the block erase operation on the group of memory cells until all cells of the group are determined to have a threshold voltage corresponding to the final value.
28. The method of claim 22 , wherein the re-performing a soft programming operation on each over-erased memory cell of the group until the threshold voltage corresponding to the final value is attained comprises:
verifying the soft programming operation of the over-erased memory cells by identifying whether the threshold voltage of the over-erased memory cells correspond to the final value; and
re-performing the soft programming operation on the over-erased memory cells until the over-erased memory cells of the group are determined to have a threshold voltage corresponding to the final value.
29. The method of claim 22 , wherein the threshold voltage of the intermediate value is a higher voltage than the threshold voltage of the final value.
30. The method of claim 22 , wherein the intermediate and final threshold voltage values individually correspond to one of the three or more data states generally corresponding to the three or more threshold voltage values.
31. The method of claim 22 , wherein the memory cells having three or more data states generally corresponding to three or more threshold voltage values corresponds to memory cells having four data states and four threshold voltage values.
32. The method of claim 22 , further comprising additional erasing and soft programming operations to the memory cells for bringing the threshold voltage of the memory cells to one or more additional threshold voltage values between the intermediate and final threshold voltages to provide further bit compacting.
33. The method of claim 22 , wherein the intermediate and final threshold voltage values are predetermined by a memory device user.
34. The method of claim 33 , wherein the intermediate and final threshold voltage values are input by the user to circuitry associated with the memory cells.
35. The method of claim 33 , wherein the final threshold voltage value is about half the voltage of the intermediate threshold voltage value.
36. The method of claim 22 , further comprising pre-programming a portion of the memory cells prior to performing the block erase operation.
37. The method of claim 22 , wherein the block erase operation reduces threshold voltage values corresponding to bits of respective memory cells of the group.
38. The method of claim 22 , wherein different portions of the group of memory cells are initially programmed to different threshold voltage values.
39. The method of claim 22 , wherein the group of memory cells comprises a portion of a larger array of memory cells, wherein different portions of the array of memory cells are initially programmed to different threshold voltage values.
40. The method of claim 22 , wherein one or more groups of memory cells comprise an array of memory cells, wherein different groups of the array are initially programmed to different threshold voltage values.
41. The method of claim 22 , wherein the groups of memory cells are erased to an L 1 data state, the memory cells have four data states L 1 , L 2 , L 3 , and L 4 corresponding to four respective threshold voltage values, and wherein the intermediate threshold voltage value corresponds to the L 2 data state and the final threshold voltage value corresponds to the L 1 data state.
42. The method of claim 22 , wherein one or more voltages used in the erase, program, and soft-program operations are adjusted to further optimize the method.