Boosting to control programming of non-volatile memory

• US 7,023,733 B2
• Filed: 05/05/2004
• Issued: 04/04/2006
• Est. Priority Date: 05/05/2004
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Mokhlesi, Nima, Guterman, Daniel C., Fong, Yupin
• Primary Examiner(s): Hoang, Huan
• Application Number:
  US10/839,764
• Publication Number:
  US 20050248988A1
• Time in Patent Office: 699 Days
• Field of Search: 365/185.17, 365/185.18, 365/185.19
• 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 16/12 Programming voltage switching circuits
• CPC Class Codes: G11C 16/30 Power supply circuits
• CPC Class Codes: G11C 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 16/3427 Circuits or methods to prevent or reduce disturbance of the state of a memory cell when neighbouring cells are read or written
• CPC Class Codes: G11C 16/3454 Arrangements for verifying correct programming or for detecting overprogrammed cells
• CPC Class Codes: G11C 16/3459 Circuits or methods to verify correct programming of nonvolatile memory cells

Abstract

A system is disclosed for programming non-volatile memory with greater precision. In one embodiment, the system includes applying a first phase of a boosting signal to one or more unselected word lines for a set of NAND strings, applying a programming level to selected bit lines of the NAND strings while applying the first phase of the boosting signal, and applying an inhibit level to unselected bit lines of the NAND strings while applying the first phase of the boosting signal. Subsequently, a second phase of the boosting signal is applied to the one or more unselected word lines and the signal(s) on the selected bit lines are changed by applying the inhibit level to the selected bit lines so that NAND strings associated with the selected bit lines will be boosted by the second phase of the boosting signal. A program voltage signal is applied to a selected word line in order to program storage elements connected to the selected word line.

Claims

1. A method for programming non-volatile storage, comprising:

applying a source of boosting to a first non-volatile storage element;

discouraging boosting of said first non-volatile storage element during a first time period while applying said source of boosting;

allowing boosting of said first non-volatile storage element during a second time period while applying said source of boosting so that said first non-volatile storage element experiences boosting; and

programming said first non-volatile storage element while said first non-volatile storage element is at least partially boosted.

2. A method according to claim 1, wherein:

said first non-volatile storage element is associated with a bit line;

said discouraging boosting includes said bit line being at a programming condition; and

said allowing boosting includes said bit line being at an inhibit condition.

3. A method according to claim 1, wherein:

said first non-volatile storage element is associated with a bit line;

said discouraging boosting includes said bit line being at 0 volts; and

said allowing boosting includes said bit line being at Vdd.

4. A method according to claim 1, wherein:

said applying a source of boosting includes applying a signal to a control gate for said first non-volatile storage element.

5. A method according to claim 1, wherein:

said applying a source of boosting includes applying a signal to a control gate of a different non-volatile storage element, said different non-volatile storage element is in series with said first non-volatile storage element.

6. A method according to claim 5, wherein:

said applying a source of boosting includes raising said signal to a first level and subsequently raising said signal to a second level; and

said allowing boosting starts after said signal is at said first level and prior to raising said signal to said second level.

7. A method according to claim 1, wherein:

said first non-volatile storage element is programmed while its channel has a boosted voltage level, which reduces speed of programming of said first non-volatile storage element.

8. A method according to claim 1, wherein:

said programming includes applying a pulse to a control gate for said first non-volatile storage element; and

said discouraging boosting and allowing boosting are performed prior to said pulse.

9. A method according to claim 1, wherein:

performing coarse mode programming for said first non-volatile storage element; and

performing fine mode programming for said first non-volatile storage element, said step of allowing boosting is performed as part of said fine mode programming.

10. A method according to claim 9, wherein:

said performing fine mode programming for said first non-volatile storage element includes repeating said steps of applying, discouraging, allowing and programming with said first time period being shortened.

11. A method according to claim 1, wherein:

repeating said steps of applying, discouraging, allowing and programming with said first time period being shortened in successive iterations of said steps of applying, discouraging, allowing and programming.

12. A method according to claim 1, wherein:

a length of said first time period being based on a differential between an existing threshold voltage of said first non-volatile storage element and target threshold voltage of said first non-volatile storage element.

13. A method according to claim 1, wherein:

repeating said steps of applying, discouraging, allowing and programming with said first time period being shortened in successive iterations of said steps of applying, discouraging, allowing and programming based on a differential between an existing threshold voltage of said first non-volatile storage element and target threshold voltage of said first non-volatile storage element.

14. A method according to claim 1, wherein:

said first non-volatile storage element includes a NAND flash memory device.

15. A method according to claim 1, wherein:

said first non-volatile storage element includes a multi-state NAND flash memory device.

16. A method for programming NAND flash memory, comprising:

applying a first boosting phase to a set of NAND strings, each NAND string having multiple storage elements;

while applying said first boosting phase, asserting a first bit line condition for a first bit line, said first bit line condition discourages boosting for a first NAND string of said set of NAND strings, said first NAND string is selected for programming;

changing said first bit line from said first bit line condition that discourages boosting to a second bit line condition that encourages boosting for said first NAND string;

applying a second boosting phase to said set of NAND strings after changing said first bit line; and

programming a storage element on said first NAND string while said first NAND string is at least partially boosted from said second boosting phase.

17. A method according to claim 16, wherein:

said asserting a first bit line condition that discourages boosting includes applying 0 voltage to said first bit line;

said changing said first bit line includes applying at least Vdd to said first bit line;

said applying a first boosting phase includes raising one or more word lines for said NAND strings to an intermediate boosting voltage; and

said applying a second boosting phase includes raising said one or more word lines for said NAND strings to a higher boosting voltage.

18. A method according to claim 16, wherein:

performing coarse mode programming for NAND strings; and

performing fine mode programming for NAND strings, said step of changing is performed as part of said fine mode programming.

19. A method according to claim 18, wherein:

said performing fine mode programming includes repeating said steps of applying a first boosting voltage, asserting a first bit line condition, changing said first bit line and programming with said step of changing being performed earlier in time relative to a previous performance of said step of changing.

20. A method according to claim 16, wherein:

repeating said steps of applying a first boosting voltage, asserting a bit line condition, changing said first bit line and programming with said step of changing being performed progressively earlier in time in successive iterations.

21. A method according to claim 16, wherein:

for a set of storage elements on said NAND strings, individual timing for commencing said step of changing with respect to commencement of said applying a first boosting voltage is based on a differential between existing threshold voltages and target threshold voltages.

22. A method for programming NAND flash memory, comprising:

applying a first phase of a boosting signal to one or more unselected word lines for a set of NAND strings;

applying a programming level to selected bit lines of said NAND strings while applying said first phase of said boosting signal;

applying an inhibit level to unselected bit lines of said NAND strings while applying said first phase of said boosting signal;

applying a second phase of said boosting signal to said one or more unselected word lines;

changing said selected bit lines by applying said inhibit level to said selected bit lines so that NAND strings associated with said selected bit lines will be at least partially boosted by said second phase of said boosting signal; and

applying a program voltage signal to a selected word line in order to program storage elements connected to said selected word line and associated with selected bit lines.

23. A method according to claim 22, wherein:

said changing said selected bit lines is performed prior to said applying said second phase of said boosting signal.

24. A method according to claim 22, wherein:

said first phase of said boosting signal includes raising said one or more unselected word lines to an intermediate level voltage; and

said second phase of said boosting signal includes raising said one or more unselected word lines to a higher voltage.

25. A method according to claim 24, wherein:

timing of said changing said selected bit lines is based on a difference between an existing threshold voltage level for a storage element and a target threshold voltage level.

26. A method according to claim 24, wherein:

said steps of applying a first phase, applying a second phase and changing are repeated with said intermediate level being lower with each repetition.

27. A method according to claim 22, further comprising:

performing coarse mode programming for said NAND strings; and

performing fine mode programming for said NAND strings, said changing said selected bit lines is performed as part of said fine mode programming.

28. A method for programming NAND flash memory, comprising:

applying a set of boosting phases to one or more unselected word lines for a set of NAND strings;

applying a programming level to selected bit lines of said NAND strings while applying a first phase of said boosting signal;

applying an inhibit level to unselected bit lines of said NAND strings while applying said first phase of said boosting signal;

changing selected bit lines by applying said inhibit level to said selected bit lines after different boosting phases based on a target programming level; and

applying a program voltage signal to a selected word line in order to program storage elements connected to said selected word line and on selected bit lines.

29. A non-volatile memory system, comprising:

a first non-volatile storage element; and

a control circuit, said control circuit applies a source of boosting to said first non-volatile storage element, discourages boosting of said first non-volatile storage element during a first time period while applying said source of boosting, allows boosting of said first non-volatile storage element during a second time period while applying said source of boosting so that said first non-volatile storage element experiences at least some boosting, and causes said first non-volatile storage element to be programmed while said first non-volatile storage element is at least partially boosted.

30. A non-volatile memory system according to claim 29, wherein:

said control circuit includes a controller, a state machine, decoders, and sense amplifiers.

31. A non-volatile memory system according to claim 29, further comprising:

a bit line associated with said first non-volatile storage element;

said control circuit applies a programming voltage to said bit line to discourage boosting; and

said control circuit applies an inhibit voltage to said bit line to allow boosting.

32. A non-volatile memory system according to claim 29, wherein:

said first non-volatile storage element is part of a NAND string;

said applying a source of boosting includes raising a word line signal to a first level and subsequently raising said word line signal to a second level, said word line signal is associated with another non-volatile storage element on said NAND string; and

said allowing boosting starts after said signal is at said first level and prior to raising said signal to said second level.

33. A non-volatile memory system according to claim 29, wherein:

said programming includes said control circuit causing a pulse to be applied to a control gate for said first non-volatile storage element; and

said discouraging boosting and allowing boosting are performed prior to said pulse.

34. A non-volatile memory system according to claim 29, wherein:

said control circuit performs coarse mode programming for said first non-volatile storage element and performs fine mode programming for said first non-volatile storage element, said step of allowing boosting is performed as part of said fine mode programming.

35. A non-volatile memory system according to claim 34, wherein:

said performing fine mode programming for said first non-volatile storage element includes repeating said applying, discouraging, allowing and programming with said first time period being shortened.

36. A non-volatile memory system according to claim 29, wherein:

said control circuit repeats said steps of applying, discouraging, allowing and programming with said first time period being shortened in successive iterations of said applying, discouraging, allowing and programming.

37. A non-volatile memory system according to claim 29, wherein:

a length of said first time period being based on a differential between an existing threshold voltage of said first non-volatile storage element and target threshold voltage of said first non-volatile storage element.

38. A non-volatile memory system according to claim 29, wherein:

said first non-volatile storage element includes a NAND flash memory device.

39. A non-volatile memory system according to claim 29, wherein:

said first non-volatile storage element includes a multi-state NAND flash memory device.

40. A non-volatile memory system, comprising:

a set of NAND strings, each NAND string having multiple storage elements; and

a control circuit, said control circuit asserts a first bit line condition on a first bit line such that said first bit line condition discourages boosting of a first NAND string selected for programming while applying a first boosting phase to said set of NAND strings, said control circuit changes said first bit line from said first bit line condition that discourages boosting to a second bit line condition that encourages boosting and applies a second boosting phase to said set of NAND strings after changing said first bit line, and said control circuit programs a storage element on said first NAND string while said first NAND string is at least partially boosted from said second boosting phase.

41. A non-volatile memory system according to claim 40, wherein:

said control circuit applies said first boosting phase by raising one or more word lines for said NAND strings to an intermediate boosting voltage; and

said control circuit applies said second boosting phase by raising said one or more word lines for said NAND strings to a higher boosting voltage.

42. A non-volatile memory system according to claim 40, wherein:

said control circuit causes coarse mode programming for said NAND strings and fine mode programming for said NAND strings, said changing is performed as part of said fine mode programming.

43. A non-volatile memory system according to claim 42, wherein:

said fine mode programming includes repeating said applying of said first boosting voltage, asserting of said first bit line condition, changing said first bit line and programming, with said changing being performed earlier in time relative to previous performance of said changing.

44. A non-volatile memory system according to claim 40, wherein:

repeating said applying of said first boosting voltage, asserting of said first bit line condition, changing said first bit line and programming, with said changing being performed earlier in time in successive iterations relative to a previous iteration of said changing.

45. A non-volatile memory system according to claim 40, wherein:

for a set of storage elements on said NAND strings, individual timing for commencing said changing with respect to commencement of said applying said first boosting voltage is based on a differential between existing threshold voltages and target threshold voltages.

46. A non-volatile memory system, comprising:

a set of NAND strings, each NAND string having multiple storage elements;

a set of bit lines connected to said NAND strings, a subset of said bit lines are selected for programming;

a set of word lines, each word line connects to storage elements on multiple NAND strings, one of said word lines is selected for programming; and

a control circuit, said control circuit applying a first phase of a boosting signal to one or more unselected word lines, applies a programming level to selected bit lines while applying said first phase of said boosting signal, applies an inhibit level to unselected bit lines while applying said first phase of said boosting signal, applies a second phase of said boosting signal to said one or more unselected word lines, changes said selected bit lines by applying said inhibit level to said selected bit lines so that NAND strings associated with said selected bit lines will be at least partially boosted by said second phase of said boosting signal, and applies a program voltage signal to said selected word line in order to program storage elements connected to said selected word line and on selected bit lines.

47. A non-volatile memory system according to claim 46, wherein:

said changing said selected bit lines is performed prior to said applying said second phase of said boosting signal.

48. A non-volatile memory system according to claim 46, wherein:

said first phase of said boosting signal includes raising voltages on said unselected word lines to an intermediate level; and

said second phase of said boosting signal includes raising voltages on said unselected word lines to a higher level.

49. A non-volatile memory system according to claim 46, further comprising:

performing coarse mode programming for said NAND strings; and

performing fine mode programming for said NAND strings, said changing said selected bit lines is performed as part of said fine mode programming.