Column erasing in non-volatile memory strings

Patent metadata

Field	Value
Publication number	US-11037631-B2
Application number	US-201916252300-A
Country	US
Kind code	B2
Filing date	Jan 18, 2019
Priority date	Jul 6, 2018
Publication date	Jun 15, 2021
Grant date	Jun 15, 2021

Abstract

Strings of non-volatile memory cells include one or more joint regions adjacent to dummy non-volatile memory cells. During erase operations, different voltage levels are used for different dummy word lines coupled to respective dummy non-volatile memory cells. For example, a selection circuit may set a voltage level of a particular dummy word line to a voltage level greater than a different dummy word line. In another example, the selection circuit may determine a voltage level for a given dummy word line based on a distance between a non-volatile memory cell coupled to the given dummy word line and a selection device included in a string of non-volatile memory cells. Electron holes generated using the dummy word lines during erase operations may neutralize undesired trapped charges in a non-volatile memory string, thereby reducing disparity in erase times for different strings in the non-volatile memory circuit.

First claim

What is claimed is:

1. An apparatus, comprising:
a plurality of NAND strings each NAND string coupled to a source line and including:
a plurality of non-volatile memory cells;
a joint region positioned between ends of the NAND strings;
a selection device coupled to the source line and a bit line; and
a plurality of dummy non-volatile memory cells each adjacent to the joint region; and

a control circuit configured to set a plurality of dummy word lines to different voltage levels as part of executing an erase command, wherein each of the plurality of dummy word lines is coupled to a respective one of the plurality of dummy non-volatile memory cells, and to select voltage levels for each particular dummy word line based on a distance from a corresponding dummy non-volatile memory cell coupled to the particular dummy word line to the selection device, wherein the selected voltage level is greater as the distance is greater.

2. The apparatus of claim 1 , wherein to select the voltage level for each particular dummy word line, the control circuit is further configured to decode a plurality of previously programmed data bits associated with the particular dummy word line, wherein a value for the voltage level for the particular dummy word line is encoded in the plurality of previously programmed data bits.

3. The apparatus of claim 1 , wherein the control circuit is further configured to set a plurality of word lines to a particular voltage level greater than each of the respective voltage levels of the dummy word lines, wherein each of the plurality of word lines is coupled to a respective one of the plurality of non-volatile memory cells.

4. The apparatus of claim 3 , wherein the control circuit is further configured to, prior to setting the plurality of dummy word lines to the respective voltage levels:
float each of the plurality of word lines and each of the plurality of dummy word lines;
float first and second control lines coupled to a select gate drain device and a select gate source device, respectively, wherein the select gate drain device is coupled to a bit line coupled to the NAND string, and the select gate source device is coupled to a different select gate source device; and
set a voltage level of a third control line coupled to the different select gate source device to a given voltage level, wherein the different select gate source device is coupled to the source line.

5. The apparatus of claim 4 , wherein the control circuit is further configured, after a period of time has elapsed since the plurality of word lines were floated, to set the plurality of word lines and the plurality of dummy word lines to ground.

6. A method, comprising:
receiving an erase command for a plurality of NAND strings included in a non-volatile memory circuit, wherein each NAND string includes a plurality of non-volatile memory cells and a plurality of dummy non-volatile memory cells; and
executing the erase command, at least in part by:
setting a plurality of word lines to a particular voltage level, wherein each word line of the plurality of word lines is coupled to a respective one of the plurality of non-volatile memory cells; and
setting a plurality of dummy word lines to respective voltage levels each of which is different from the particular voltage level, wherein each of the plurality of dummy word lines is coupled to a respective one of the plurality of dummy non-volatile memory cells, wherein a voltage level of each particular dummy word line is based on a distance from a corresponding dummy non-volatile memory cell coupled to the particular dummy word line to a selection device included in the particular one of the plurality of NAND strings, wherein the voltage level of each particular dummy word line is greater as the distance is greater.

7. The method of claim 6 , wherein the selection device included in the particular one of the plurality of NAND strings is coupled to a bit line.

8. The method of claim 7 , wherein information indicative of the distance is encoded in a plurality of data bits previously programmed in a register included in the non-volatile memory circuit.

9. The method of claim 6 , further comprising, in response to executing the erase command and prior to setting the plurality of word lines to the particular voltage level and setting the plurality of dummy word lines to the respective voltage levels:
decoupling each of the plurality of word lines and the plurality of dummy word lines from respective power supply signals;
decoupling a first control line coupled to a select gate drain device and a second control line coupled to a first select gate source device from respective power supply signals; and
setting a voltage level of a third control line coupled to a second select gate source device to a given voltage level, wherein the second select gate source device is coupled to the first select gate source device and a select line.

10. The method of claim 9 , further comprising, setting, after a period of time has elapsed since the plurality of word lines were decoupled from the respective power supply signals, the plurality of word lines and the plurality of dummy word lines to ground potential prior to continuing with executing the erase command.

11. The method of claim 6 , wherein the each of the respective voltage levels is less than the particular voltage level.

12. The method of claim 6 , wherein each one of the plurality of dummy non-volatile memory cells is adjacent to a particular one of a plurality of joint regions included in the particular one of the plurality of NAND strings.

13. A system, comprising:
a plurality of non-volatile memory cell strings including a non-volatile memory cell string that includes a plurality of dummy non-volatile memory cells and a plurality of joint regions adjacent to the plurality of dummy non-volatile memory cells, and separating sets of non-volatile memory cells, wherein each set is coupled to a plurality of word lines;
a control circuit configured to
receive an erase command; and
initiate a two-phase erase operation of a particular non-volatile memory cell string of the plurality of non-volatile memory cell strings, wherein an initial phase of the two-phase erase operation includes a pre-erase phase and a subsequent phase includes at least one erase phase; and
reduce a number of trapped charge carriers in the particular non-volatile memory cell string by setting a plurality of dummy word lines to respective voltage levels in response to the erase operation included in the subsequent phase, wherein each of the plurality of dummy word lines is coupled to a respective one of the plurality of dummy non-volatile memory cells.

14. The system of claim 13 , wherein non-volatile memory cell string further includes a selection device coupled to a bit line, and wherein the control circuit is further configured to select a voltage level for a particular dummy word line based on a distance from a corresponding dummy non-volatile memory cell coupled to the particular dummy word line to the selection device.

15. The system of claim 14 , further comprising a register configured to store a plurality of data bits including a subset of the plurality of data bits that whose values are indicative of a voltage level of the particular dummy word line to be used during the erase operation included in the subsequent phase, and wherein to select the voltage level for the particular dummy word line, the control circuit is further configured to decode the subset of the plurality of data bits.

16. The system of claim 13 , wherein each dummy non-vo

Assignees

• Sandisk Technologies Llc

Inventors

• Pachamuthu Jayavel
• Desai Amul Dhirajbhai
• Babariya Ankitkumar

Classifications

• G11C16/16 Primary

  for erasing blocks, e.g. arrays, words, groups · CPC title

• G11C16/3427

  Circuits or methods to prevent or reduce disturbance of the state of a memory cell when neighbouring cells are read or written · CPC title

• G11C16/0483

  comprising cells having several storage transistors connected in series · CPC title

• G11C16/3445

  Circuits or methods to verify correct erasure of nonvolatile memory cells · CPC title