Systems And Methods For Read Disturb Management In Non-Volatile Memory

• US 20150071008A1
• Filed: 09/06/2013
• Published: 03/12/2015
• Est. Priority Date: 09/06/2013
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: Sandisk Technologies Incorporated (Western Digital Corporation)
• Inventors: Yang, Nian Niles, Avila, Chris, Sprouse, Steven, Bauche, Alexandra
• Granted Patent: US 9,245,637 B2
• Time in Patent Office: Days
• US Class Current: 365/185.23
• CPC Class Codes: G11C 11/5628 Programming or writing circuits; Data input circuits
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/26 Sensing or reading circuits; Data output circuits
• 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 16/3459 Circuits or methods to verify correct programming of nonvolatile memory cells

Abstract

Non-volatile memory and methods of reading non-volatile memory are provided for managing and reducing read related disturb. Techniques are introduced to reduce read disturb using state-dependent read pass voltages for particular word lines during a read operation. Because of their proximity to a selected word line, adjacent word lines can be biased using state-dependent pass voltages while other unselected word lines are biased using a standard or second set of pass voltages. Generally, each state-dependent pass voltage applied to a word line adjacent to the selected word line is larger than the second set of pass voltages applied to other unselected word lines, although this is not required. Other word lines, may also be biased using state-dependent pass voltages. System-level techniques are provided with or independently of state-dependent pass voltages to further reduce and manage read disturb. Techniques may account for data validity and memory write and erase cycles.

Claims

1. A method of reading non-volatile storage, comprising:

applying a set of read voltages to a selected word line as part of reading data from a set of non-volatile storage elements coupled to the selected word line, the set of read voltages including a first read voltage, a second read voltage, and a third read voltage;

applying to a first unselected word line that is adjacent to the selected word line a first set of state-dependent pass voltages, the first set of state-dependent pass voltages including a first pass voltage that is applied while applying the first read voltage, a second pass voltage that is applied while applying the second read voltage, and a third pass voltage that is applied while applying the third read voltage, the first pass voltage is greater than the second pass voltage and the second pass voltage is greater than the third pass voltage; and

applying to a second unselected word line a fourth pass voltage corresponding to the set of read voltages, the fourth pass voltage being less than the first pass voltage and the second pass voltage, the fourth pass voltage being less than or equal to the third pass voltage.

2. A method according to claim 1, wherein the second unselected word line is adjacent to the first unselected word line.

3. A method according to claim 1, further comprising:

applying to a third unselected word line that is adjacent to the first unselected word line the first set of state-dependent pass voltages while applying the first read voltage, the second read voltage, and the third read voltage.

4. A method according to claim 1, further comprising:

applying to a third unselected word line that is adjacent to the first unselected word line a second set of state-dependent pass voltages.

7. A method according to claim 1, wherein the selected word line is a first selected word line that is part of a memory array block including a source line, the first selected word line is closer to the source line than a second selected word line of the memory array block, the method further comprising:

applying the set of three or more read voltages to the second selected word line as part of reading data from a set of non-volatile storage elements coupled to the second selected word line;

applying to a third unselected word line a second set of state-dependent pass voltages corresponding to the set of read voltages, the second set including a fifth pass voltage that is applied while applying the first read voltage, a sixth pass voltage that is applied while applying the second read voltage, and a seventh pass voltage that is applied while applying the third read voltage, the first pass voltage is less than the fifth pass voltage, the second pass voltage is less than the sixth pass voltage, the third pass voltage is less than the seventh pass voltage.

8. A method according to claim 1, wherein said applying the set of read voltages is part of a first read process, the method further comprising:

determining whether the data of the second word line is valid;

if the data of the second word line is not valid, performing a second read process by applying to the first unselected word line the fourth pass voltage while applying to the second unselected word line the fourth pass voltage; and

if the data of the second word line is valid, performing the second read process by applying to the first unselected word line the first set of state-dependent pass voltages while applying to the second unselected word line the fourth pass voltage.

9. A method according to claim 1, wherein said applying the set of read voltages is part of a first read process, the method further comprising:

determining whether the data of the second word line is valid;

if the data of the second word line is not valid, performing a second read process by applying to the first unselected word line the first pass voltage while applying to the second unselected word line the fourth pass voltage; and

if the data of the second word line is valid, performing the second read process by applying to the first unselected word line the first set of state-dependent pass voltages while applying to the second unselected word line the fourth pass voltage.

10. A method according to claim 1, wherein said applying the set of read voltages is part of a first read process, the method further comprising:

determining a number of cycles of a non-volatile memory including the set of non-volatile storage elements;

if the number of cycles is greater than or equal to a threshold number of cycles of the non-volatile memory, performing a second read process, where performing the second read process includes,

using a level for said first pass voltage that is less than a level for said first pass voltage during the first read,

using a level for said second pass voltage that is less than a level for said second pass voltage during the first read, and

using a level for said third pass voltage that is less than a level for said third pass voltage during the first read.

11. A method according to claim 10, further comprising, if the number of cycles is less than a threshold number of cycles of the non-volatile memory, performing the second read process, where performing the second read process includes,

using a same level for said first pass voltage as used during the first read process;

using a same level for said second pass voltage as used during the first read process; and

using a same level for said third pass voltage as used during the first read process.

12. A method according to claim 1, wherein:

the first word line, the second word line, the third word line, and the set of non-volatile storage elements are part of a plurality of strings of serially-connected non-volatile storage elements.

13. A method according to claim 1, wherein:

the fourth pass voltage is a default pass voltage;

the first read voltage is less than the second read voltage; and

the second read voltage is less than the third read voltage.

15. A non-volatile storage device according to claim 12, wherein the second unselected word line is adjacent to the first unselected word line.

16. A non-volatile storage device according to claim 12, wherein:

the controller applies to a third unselected word line that is adjacent to the first unselected word line the first set of state-dependent pass voltages while applying the first read voltage, the second read voltage, and the third read voltage.

17. A non-volatile storage device according to claim 12, wherein:

the controller applies to a third unselected word line that is adjacent to the first unselected word line a second set of state-dependent pass voltages;

the controller applies a fifth pass voltage of the second set to the third unselected word line while applying the first pass voltage to the second unselected word line, the fifth pass voltage being less than the first pass voltage;

the controller applies a sixth pass voltage of the second set to the third unselected word line while applying the second pass voltage to the second unselected word line, the sixth pass voltage being less than the second pass voltage; and

the controller applies a seventh pass voltage of the second set to the third unselected word line while applying the third pass voltage to the second unselected word line, the seventh pass voltage being less than or equal to the third pass voltage.

18. A non-volatile storage device according to claim 12, wherein:

the controller applies the set of read voltages as part of a first read process;

the controller determines a number of cycles of the non-volatile storage device;

if the number of cycles is greater than or equal to a threshold number of cycles of the non-volatile memory, the controller performs a second read process using a level for said first pass voltage that is less than a level for said first pass voltage during the first read, a level for said second pass voltage that is less than a level for said second pass voltage during the first read, and a level for said third pass voltage that is less than a level for said third pass voltage during the first read.

20. A method according to claim 18, wherein the second unselected word line is adjacent to the first unselected word line.

21. A method according to claim 18, further comprising:

applying to a third unselected word line that is adjacent to the first unselected word line the first pass voltage while applying the first read voltage, the second pass voltage while applying the second read voltage, and the third pass voltage while applying the third read voltage.

5. A method according to claim 5, wherein applying the second set of state-dependent pass voltages comprises:

applying a fifth pass voltage to the third unselected word line while applying the first pass voltage to the second unselected word line, the fifth pass voltage being less than the first pass voltage;

applying a sixth pass voltage to the third unselected word line while applying the second pass voltage to the second unselected word line, the sixth pass voltage being less than the second pass voltage; and

applying a seventh pass voltage to the third unselected word line while applying the third pass voltage to the second unselected word line, the seventh pass voltage being less than or equal to the third pass voltage.

6. A method according to claim 6, further comprising:

applying an eighth pass voltage to a fourth unselected word line that is adjacent to the third unselected word line while applying the second set of one or more pass voltages, the eighth pass voltage being less than or equal to the third pass voltage.

14. A non-volatile storage device, comprising:

a plurality of strings of serially-connected non-volatile storage elements;

a plurality of word lines, each word line is coupled to one non-volatile storage element of each of the plurality of strings; and

a controller in communication with the plurality of strings and the plurality of word lines, the controller biases a selected word line during reading using a first read voltage, a second read voltage, and a third read voltage, the controller biases a first unselected word line that is adjacent to the selected word line using a first pass voltage that is applied while applying the first read voltage, a second pass voltage that is applied while applying the second read voltage, and a third pass voltage that is applied while applying the third read voltage, the controller biases a second unselected word line using a fourth pass voltage that is applied while applying the first read voltage, the second read voltage, and the third read voltage, the fourth pass voltage is less than the first pass voltage and the second pass voltage, the fourth pass voltage is less than or equal to the third pass voltage.

19. A method of operating non-volatile storage, the non-volatile storage including a plurality of strings of serially-connected non-volatile storage elements, the method comprising:

applying a first read voltage and a second read voltage to a selected word line, the selected word line is coupled to a selected non-volatile storage element on a first of the plurality of strings, the second read voltage is greater than the first read voltage;

applying a first pass voltage to a first unselected word line adjacent to the selected word line while applying the first read voltage to the selected word line and applying a second pass voltage to the first unselected word line while applying the second read voltage to the selected word line, the first unselected word line is coupled to a first unselected non-volatile storage element on the first string, the second pass voltage is less than the first pass voltage; and

applying a third pass voltage to a second unselected word line adjacent to the first unselected word line while applying the first pass voltage to the first unselected word line and the second read compare voltage to the selected word line, the second unselected word line is coupled to a second unselected non-volatile storage element on the first string, the third pass voltage is less than the first pass voltage and is less than or equal to the second pass voltage.

22. A method according to claim 19, wherein:

the third pass voltage is a default pass voltage.