Self-boosting method for flash memory cells

• US 7,466,590 B2
• Filed: 12/28/2005
• Issued: 12/16/2008
• Est. Priority Date: 02/06/2004
• Status: Expired due to Term

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Lee, Shih-Chung, Hemink, Gerrit Jan, Nakao, Hironobu
• Primary Examiner(s): TRAN, MICHAEL THANH
• Application Number:
  US11/321,955
• Publication Number:
  US 20060198195A1
• Time in Patent Office: 1,084 Days
• Field of Search: 365/185.18, 365/185.17
• US Class Current: 365/185.18
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/10 Programming or data input circuits
• CPC Class Codes: G11C 16/12 Programming voltage switching 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

Abstract

A low voltage (e.g. of the order of or one to three volts) instead of an intermediate V_PASS voltage (e.g. of the order of five to ten volts) is applied to word line zero immediately adjacent to the source or drain side select gate of a flash device such as a NAND flash device and one or more additional word lines next to such word line to reduce or prevent the shifting of threshold voltage of the memory cells coupled to word line zero during the programming cycles of the different cells of the NAND strings. This may be implemented in any one of a variety of different self boosting schemes including erased areas self boosting and local self boosting schemes. In a modified erased area self boosting scheme, low voltages are applied to two or more word lines on the source side of the selected word line to reduce band-to-band tunneling and to improve the isolation between two boosted channel regions. In a modified local self boosting scheme, zero volt or low voltages are applied to two or more word lines on the source side and to two or more word lines on the drain side of the selected word line to reduce band-to-band tunneling and to improve the isolation of the channel areas coupled to the selected word line. Different intermediate boosting voltage(s) (e.g. of the order of five to ten volts) may be applied to one or more of the word lines adjacent to the selected word line (that is the word line programming the selected transistor), where the boosting voltage(s) applied to the word line(s) adjacent to the selected word line are/is different from that or those applied to other unselected word lines.

Claims

1. A method for programming a memory system, said system comprising strings of charge storage transistors for storing different charge states, each of said strings including two select transistors, each of said strings connected between one of a plurality of bit lines and a source line, said strings controlled by a common set of word lines, wherein a first charge storage transistor is in a first string of the strings and adjacent to one of the two select transistors in the first string, said method comprising:

applying a program voltage level through one of the word lines to a control gate that is capacitively coupled with a second charge storage transistor in a second string of the strings different from the first string to program the second transistor, said second transistor separated from the source line by three or more charge storage transistors in said second string when charge storage transistors closer to the source line than the bit line connected to the second string are programmed or separated from the bit line that is connected to the second string by three or more charge storage transistors in said second string when charge storage transistors closer to such bit line than the source line are programmed, said control gate controlling also to a third charge storage transistor in the first string;

coupling first voltage(s) to the first transistor and to one or more additional charge storage transistors that is or are different from the select transistors in the first string and adjacent to the first transistor;

turning off at least a fourth charge storage transistor in the first string between the source line and the third transistor, in order to electrically isolate the third transistor from transistors in the first string between the source line and the fourth transistor; and

boosting, through some of the word lines, electrical potential(s) of channel regions of the first string of transistors by coupling boosting second voltage(s) to at least some of the remaining transistors in the first string, the second voltage(s) being higher than the first voltage(s), to boost said electrical potential(s) of channel regions of the first string of transistors to a value or values closer to the program voltage, thereby reducing program disturb.

2. The method of claim 1, wherein the first voltage(s) coupled to said one or more additional charge storage transistors is/are higher than the voltage coupled to the first transistor.

3. The method of claim 1, wherein the first voltage(s) coupled to said one or more additional charge storage transistors is/are in a range of about 1 to 6 volts.

4. The method of claim 1, the boosting second voltage(s) being higher than the first voltage(s) by at least 50% of the first voltage(s).

5. The method of claim 1, wherein said first voltage(s) is/are in a range of about 1 to 3 volts.

6. The method of claim 5, wherein the first voltage(s) coupled to said one or more additional charge storage transistors is/are in a range of about 3 to 6 volts.

7. The method of claim 1, wherein said second voltage(s) is/are in a range of about 5 to 10 volts.

8. The method of claim 1, further comprising coupling a third voltage to the one select transistor in the first string to cause it to be electrically non-conducting, and wherein the first voltage(s) is/are higher than the third voltage.

9. The method of claim 1, further comprising turning off at least a fifth charge storage transistor in the first string between the bit line connected to the first string and the third transistor, in order to electrically isolate the third transistor from transistors in the first string between such bit line and the fifth transistor.

10. The method of claim 9, wherein each of the fourth and fifth transistors are separated from the third transistor by at least one transistor in the first string.

11. The method of claim 1, wherein the fourth transistor is separated from the third transistor by at least one transistor in the first string.

12. A method for programming a memory system, said system comprising strings of charge storage transistors for storing different charge states, said strings including a first string, each of the strings including two select transistors, said strings controlled by a common set of word lines and connected between a plurality of bit lines and a source line, wherein a first charge storage transistor is in the first string and adjacent to one of the two select transistors, said method comprising:

applying a program voltage level to a first control gate that is capacitively coupled with a second charge storage transistor, and subsequently applying a program voltage level to a second control gate that is capacitively coupled with a third charge storage transistor different from the second transistor to program the second and third transistors, each of said second and third transistors being in a second string of the strings different from the first string and being separated from the source line by one or more charge storage transistors in said second string when charge storage transistors closer to the source line than the bit line connected to the second string are programmed or separated from the bit line that is connected to the second string by one or more charge storage transistors in said second string when charge storage transistors closer to such bit line than the source line are programmed; and

coupling first voltage level(s) to the first transistor and to one or more additional charge storage transistors in the first string and adjacent to the first transistor; and

boosting, through some of the word lines, electrical potential(s) of channel regions of the first string of transistors by coupling boosting second voltage level(s) to at least some of the remaining transistors in the first string, the second voltage level(s) being higher than the first voltage level(s), when a program voltage level is applied to the control gates coupled to the second and third transistors, to boost said electrical potential(s) of channel regions of the first string of transistors to a value or values closer to the program voltage, thereby reducing program disturb, wherein the first or second control gate also controls a fourth charge storage transistor in the first string, said method further comprising turning off at least a fifth charge storage transistor in the first string between the source line and the fourth transistor, in order to electrically isolate the fourth transistor from charge storage transistors in the first string between the source line and the fifth transistor.

13. The method of claim 12, said method comprising a plurality of programming time intervals to program all the transistors in the second string, wherein during each of said plurality of programming time intervals a program voltage level is coupled to one of the transistors in said second string to program such transistor, and the first voltage level(s) coupled to the first transistor and the one or more additional transistors is/are different from that/those coupled to other transistors in the first string during two or more of said plurality of programming time intervals.

14. The method of claim 12, wherein the electrical potential(s) of the channel regions of the first string are/is boosted so that breakdown at the drain or source side of the one select transistor is reduced to such an extent that it does not result in a change of the first transistor charge storage state to a different charge state.

15. The method of claim 14, wherein the electrical potential(s) of the channel regions of the first string are/is boosted so that said band-to-band tunneling at the drain or source side of the one select transistor is suppressed.

16. The method of claim 14, wherein the desired charge storage state is one of more than two possible charge storage states of the first transistor, and said coupling the first voltage level(s) to the first transistor and to the one or more additional transistors does not result in a change of the first transistor'"'"'s desired charge storage state to a different one of the more than two possible charge states.

17. The method of claim 12, the second voltage level(s) being higher than the first voltage level(s) by at least 50% of the first voltage level(s).

18. The method of claim 17, wherein said first voltage level(s) is/are in a range of about 1 to 3 volts.

19. The method of claim 18, wherein the first voltage(s) coupled to said one or more additional charge storage transistors is/are in a range of about 3 to 6 volts.

20. The method of claim 17, wherein said second voltage level(s) are/is in a range of about 5 to 10 volts.

21. The method of claim 12, further comprising coupling a third voltage level to the one select transistor in the first string to cause it to be electrically non-conducting, and wherein the first voltage level(s) is/are higher than the third voltage.

22. The method of claim 12, wherein the fifth transistor is separated from the fourth transistor by at least one transistor in the first string.

23. The method of claim 12, further comprising turning off at least a sixth charge storage transistor in the first string between the bit line connected to the first string and the fourth transistor, in order to electrically isolate the fourth transistor from charge storage transistors in the first string between such bit line and the sixth transistor.

24. The method of claim 23, wherein each of the fifth and sixth transistors are separated from the fourth transistor by at least one charge storage transistor in the first string.

25. The method of claim 12, wherein the voltage(s) coupled to said one or more additional charge storage transistors is/are higher than the voltage coupled to the first transistor.

26. The method of claim 13, wherein the first voltage(s) coupled to said one or more additional charge storage transistors is/are in a range of about 1 to 6 volts.

27. A method for programming a memory system, said system comprising strings of charge storage transistors for storing different charge states, said strings including a first and a second string each connected between one of a plurality of bit lines and a source line and controlled by common word lines, each of said strings including a source select transistor connected to the source line, said method comprising:

applying a program voltage to a selected word line coupled to a selected charge storage transistor in the first string to program the selected transistor;

coupling by means of said common word lines first or second boosting voltages to at least some of the transistors in the second string to boost electrical potential(s) of channel regions of transistors in the second string to a value or values closer to the program voltage to reduce program disturb, wherein the first boosting voltage is coupled to a first charge storage transistor in the second string adjacent to the selected word line, and the second boosting voltage(s) is/are coupled to the remaining transistors in the second string except for the transistors coupled to the selected word line, the first boosting voltage being different from the second boosting voltage(s).

28. The method of claim 27, wherein the first boosting voltage is lower than the second boosting voltage(s).

29. The method of claim 27, wherein the applying applies the program voltage sequentially to a plurality of said common word lines to successively program selected transistors in the first string in sequential programming cycles, and wherein the first boosting voltage is lower than the second boosting voltage(s) during at least some of said programming cycles.

30. The method of claim 27, wherein the applying applies the program voltage to a first word line, and said first boosting voltage is applied to the common word line adjacent to the first word line and between the first word line and the bit line connected to the second string.

32. The method of claim 27, further comprising coupling, through one or more of said common word lines, positive voltage(s) to one or more transistors in the first string between the selected transistor and a first one of the bit lines and to one or more transistors in the second string between the selected word line and a second one of the bit lines, so that a voltage on the first bit line is transferred to the channel area of the selected transistor, and so that boosting of channel voltage of a transistor in the second string coupled to the selected word line caused by the applying is reduced, to reduce or prevent breakdown at the drain side of a transistor in the second string between the selected word line and the source line.

33. The method of claim 32, wherein said positive voltage is in a range between about 0 volts and the first or second boosting voltages.

34. The method of claim 27, wherein said coupling is performed when said program voltage level is applied.

31. A method for programming a memory system, said system comprising strings of charge storage transistors for storing different charge states, said strings including a first and a second string each connected between one of a plurality of bit lines and a source line and controlled by common word lines, each of said strings including a source select transistor connected to the source line, said method comprising:

applying a program voltage to a selected word line coupled to a selected charge storage transistor in the first string to program the selected transistor;

coupling by means of said common word lines first or second boosting voltages to at least some of the transistors in the second string to boost electrical potential(s) of channel regions of transistors in the second string to a value or values closer to the program voltage to reduce program disturb, wherein different first boosting voltages are coupled to two charge storage transistors, one on each side and adjacent to the selected word line, and the second boosting voltage(s) is/are coupled to at least some of the remaining transistors in the second string, the first boosting voltage being different from the second boosting voltage(s).

35. The method of claim 31, wherein said coupling couples the second boosting voltage(s) to all of the remaining transistors in the second string between said selected word line and the bit line.

36. The method of claim 31, wherein the first boosting voltages are lower than the second boosting voltage(s).

37. The method of claim 31, wherein said coupling is performed when said program voltage level is applied.