Erase Operation For 3D Non-Volatile Memory With Controllable Gate-Induced Drain Leakage Current

• US 20130279257A1
• Filed: 04/18/2012
• Published: 10/24/2013
• Est. Priority Date: 04/18/2012
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: Sandisk Technologies Incorporated (Western Digital Corporation)
• Inventors: Costa, Xiying, Li, Haibo, Higashitani, Masaaki, Mui, Man L.
• Granted Patent: US 9,019,775 B2
• Time in Patent Office: Days
• US Class Current: 365/185.17
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/06 Auxiliary circuits, e.g. for writing into memory
• CPC Class Codes: G11C 16/14 Circuits for erasing electrically, e.g. erase voltage switching circuits
• CPC Class Codes: G11C 16/3445 Circuits or methods to verify correct erasure of nonvolatile memory cells
• CPC Class Codes: G11C 16/3472 Circuits or methods to verify correct erasure of nonvolatile memory cells whilst erasing is in progress, e.g. by detecting onset or cessation of current flow in cells and using the detector output to terminate erasure
• CPC Class Codes: G11C 5/02 Disposition of storage elements, e.g. in the form of a matrix array
• CPC Class Codes: H10B 43/27 the channels comprising vertical portions, e.g. U-shaped channels
• CPC Class Codes: H10B 43/35 with cell select transistors, e.g. NAND

Abstract

An erase operation for a 3D stacked memory device applies an erase pulse which includes an intermediate level (Vgidl) and a peak level (Verase) to a set of memory cells, and steps up Vgidl in erase iterations of the erase operation. Vgidl can be stepped up when a specified portion of the cells have reached the erase verify level. In this case, a majority of the cells may have reached the erase verify level, such that the remaining cells can benefit from a higher gate-induced drain leakage (GIDL) current to reached the erase verify level. Verase can step up before and, optionally, after Vigdl is stepped up, but remain fixed while Vgidl is stepped. Vgidl can be stepped up until a maximum allowed level, Vgidl_max, is reached. Vgidl may be applied to a drain-side and/or source-side of a NAND string via a bit line or source line, respectively.

Claims

1. A method for performing an erase operation in a 3D stacked non-volatile memory device, comprising:

performing each erase iteration of a plurality of erase iterations of the erase operation for a set of memory cells in at least one NAND string, the at least one NAND string comprises a drain-side end in communication with a bit line, and a select gate, drain (SGD) transistor, the SGD transistor comprises a drain in communication with the drain-side end, and the SGD transistor comprises a control gate, the performing each erase iteration comprising;

raising a voltage of the bit line from a starting level (Vss) to an intermediate level (Vgidl);

raising the voltage of the bit line from the intermediate level (Vgidl) to a peak level (Verase);

when the voltage of the bit line is at the starting level and at the intermediate level, controlling the control gate of the SGD transistor to provide a gate-to-drain voltage of the SGD transistor which is sufficiently high to generate gate-induced drain leakage (GIDL) current in the at least one NAND string; and

stepping up the intermediate level in at least one erase iteration of the plurality of erase iterations.

2. The method of claim 1, wherein:

the intermediate level is stepped up in successive erase iterations of the plurality of erase iterations until the intermediate level reaches a respective maximum allowed level or until the erase operation is concluded, whichever occurs first.

3. The method of claim 1, wherein:

the intermediate level is not stepped up in at least one erase iteration of the plurality of erase iterations before the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up.

4. The method of claim 1, further comprising:

stepping up the peak level in at least one erase iteration of the plurality of erase iterations before the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up; and

discontinuing the stepping up of the peak level during the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up.

5. The method of claim 4, further comprising:

discontinuing the stepping up of the intermediate level and resuming the stepping up of the peak level in a subsequent erase iteration of the plurality of erase iterations, after the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up.

6. The method of claim 5, wherein:

the stepping up of the peak level continues after the resuming until the peak level reaches a respective maximum allowed level or until the erase operation is concluded, whichever occurs first.

7. The method of claim 5, wherein:

the discontinuing of the stepping up of the intermediate level is responsive to the intermediate level reaching a respective maximum allowed level.

8. The method of claim 1, further comprising:

performing a verify test for the set of memory cells; and

determining a number of the memory cells which do not pass the verify test, the stepping up of the intermediate level in the at least one erase iteration of the plurality of erase iterations occurs if the number of the memory cells which do not pass the verify test is below one maximum allowable number (N2).

9. The method of claim 8, further comprising:

concluding the erase operation for the set of memory cells if the number of the memory cells which do not pass the verify test is below another maximum allowable number (N1), which is less than the one maximum allowable number (N2).

10. The method of claim 1, further comprising:

stepping up the peak level in successive erase iterations of the plurality of erase iterations until the peak level reaches a respective maximum allowed level, the stepping up of the intermediate level in the at least one erase iteration of the plurality of erase iterations occurs in response to the peak level reaching the respective maximum allowed level.

11. The method of claim 1, further comprising:

maintaining the gate-to-drain voltage of the SGD transistor below a maximum allowed voltage by controlling a voltage of the control gate of the SGD transistor when the voltage of the bit line is at the intermediate level and the peak level.

12. The method of claim 1, wherein:

the stepping up of the intermediate level in the at least one erase iteration of the plurality of erase iterations occurs at a predetermined erase iteration of the plurality of erase iterations.

13. The method of claim 1, wherein:

the at least one NAND string comprises a source-side end in communication with a source, and a select gate, source (SGS) transistor, the SGS transistor comprises a source in communication with the source-side end, and the SGS transistor comprises a control gate, the performing each erase iteration further comprising;

raising a voltage of the source line from a respective starting level (Vss) to a respective intermediate level (Vgidl);

raising the voltage of the source line from the respective intermediate level (Vgidl) to a respective peak level (Verase);

when the voltage of the source line is at the respective starting level and at the respective intermediate level, controlling the control gate of the SGS transistor to provide a gate-to-drain voltage of the SGS transistor which is sufficiently high to generate gate-induced drain leakage (GIDL) current in the at least one NAND string; and

stepping up the respective intermediate level in at least one erase iteration of the plurality of erase iterations.

14. A 3D stacked non-volatile memory device, comprising:

a bit line;

a set of memory cells in at least one NAND string, the at least one NAND string comprises a drain-side end in communication with the bit line, and a select gate, drain (SGD) transistor, the SGD transistor comprises a drain in communication with the drain-side end, and the SGD transistor comprises a control gate; and

a control circuit, the control circuit, to perform each erase iteration of a plurality of erase iterations of an erase operation for a set of memory cells;

raises a voltage of the bit line from a starting level (Vss) to an intermediate level (Vgidl);

raises the voltage of the bit line from the intermediate level (Vgidl) to a peak level (Verase);

when the voltage of the bit line is at the starting level and at the intermediate level, controls the control gate of the SGD transistor to provide a gate-to-drain voltage of the SGD transistor which is sufficiently high to generate gate-induced drain leakage (GIDL) current in the at least one NAND string; and

the control circuit steps up the intermediate level in at least one erase iteration of the plurality of erase iterations.

15. The 3D stacked non-volatile memory device of claim 14, wherein:

the intermediate level is stepped up in successive erase iterations of the plurality of erase iterations until the intermediate level reaches a respective maximum allowed level or until the erase operation is concluded, whichever occurs first; and

the intermediate level is not stepped up in at least one erase iteration of the plurality of erase iterations before the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up.

16. The 3D stacked non-volatile memory device of claim 14, wherein the control circuit:

performs a verify test for the set of memory cells; and

determines a number of the memory cells which do not pass the verify test, the step up of the intermediate level in the at least one erase iteration of the plurality of erase iterations occurs if the number of the memory cells which do not pass the verify test is below one maximum allowable number (N2).

17. The 3D stacked non-volatile memory device of claim 14, wherein the control circuit:

steps up the peak level in at least one erase iteration of the plurality of erase iterations before the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up; and

discontinues the step up of the peak level during the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up.

18. The 3D stacked non-volatile memory device of claim 17, wherein the control circuit:

discontinues the step up of the intermediate level and resuming the stepping up of the peak level in a subsequent erase iteration of the plurality of erase iterations, after the at least one erase iteration of the plurality of erase iterations in which the intermediate level is stepped up.

19. A method for performing an erase operation in a 3D stacked non-volatile memory device, comprising:

performing an erase iteration of a plurality of erase iterations of the erase operation for a set of memory cells in at least one NAND string, the performing the erase iteration comprises raising a voltage of a bit line in communication with the at least one NAND string from a starting level (Vss) to an intermediate level (Vgidl), raising the voltage of the bit line from the intermediate level (Vgidl) to a peak level (Verase);

performing a verify test for the set of memory cells;

determining whether a verify condition is met based on the verify test, the verify condition indicating a progress of the set of memory cells in the erase operation;

if the verify condition is met, stepping up the intermediate level in a next erase iteration of the plurality of erase iterations; and

if the verify condition is not met, not stepping up the intermediate level in the next erase iteration of the plurality of erase iterations.

20. The method of claim 19, further comprising:

determining a number of the memory cells which do not pass the verify test, the verify condition is met of the number of the memory cells which do not pass the verify test is below a maximum allowable number (N2).

21. The method of claim 19, further comprising:

if the verify condition is met, stepping up the intermediate level in successive erase iterations of the plurality of erase iterations until a respective maximum allowed level is reached.

22. The method of claim 21, further comprising:

performing an additional erase iteration of the plurality of erase iterations after the respective maximum allowed level is reached.