U.S. patent application number 11/745082 was filed with the patent office on 2008-11-13 for boosting for non-volatile storage using channel isolation switching.
Invention is credited to Yingda Dong, Gerrit Jan Hemink, Shih-Chung Lee, Jeffrey W. Lutze, Ken Oowada.
Application Number | 20080279007 11/745082 |
Document ID | / |
Family ID | 39969370 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080279007 |
Kind Code |
A1 |
Dong; Yingda ; et
al. |
November 13, 2008 |
BOOSTING FOR NON-VOLATILE STORAGE USING CHANNEL ISOLATION
SWITCHING
Abstract
Program disturb is reduced in non-volatile storage by preventing
source side boosting in selected NAND strings. A self-boosting mode
which includes an isolation word line is used. A channel area of an
inhibited NAND string is boosted on a source side of the isolation
word line before the channel is boosted on a drain side of the
isolation word line. Further, storage elements near the isolation
word line are kept in a conducting state during the source side
boosting so that the source side channel is connected to the drain
side channel. In this way, in selected NAND strings, source side
boosting can not occur and thus program disturb due to source side
boosting can be prevented. After the source side boosting, the
source side channel is isolated from the drain side channel, and
drain side boosting is performed.
Inventors: |
Dong; Yingda; (Sunnyvale,
CA) ; Lutze; Jeffrey W.; (San Jose, CA) ; Lee;
Shih-Chung; (Yokohama City, JP) ; Hemink; Gerrit
Jan; (Yokohama, JP) ; Oowada; Ken; (Kanagawa,
JP) |
Correspondence
Address: |
VIERRA MAGEN/SANDISK CORPORATION
575 MARKET STREET, SUITE 2500
SAN FRANCISCO
CA
94105
US
|
Family ID: |
39969370 |
Appl. No.: |
11/745082 |
Filed: |
May 7, 2007 |
Current U.S.
Class: |
365/185.17 |
Current CPC
Class: |
G11C 16/0483 20130101;
G11C 16/12 20130101 |
Class at
Publication: |
365/185.17 |
International
Class: |
G11C 11/34 20060101
G11C011/34 |
Claims
1. A method for operating non-volatile storage, comprising:
performing first boosting of at least one NAND string on a source
side of a first word line before boosting the at least one NAND
string on a drain side of a second word line, the second word line
is on a drain side of the first word line, a plurality of word
lines including the first and second word lines are associated with
the at least one NAND string, and the at least one NAND string has
a plurality of non-volatile storage elements; during the first
boosting, applying a voltage to the first word line for providing a
first non-volatile storage element of the plurality of non-volatile
storage elements which is associated with the first word line in a
conducting state, and applying a voltage to the second word line
for providing a second non-volatile storage element of the
plurality of non-volatile storage elements which is associated with
the second word line in a conducting state; and performing second
boosting of the at least one NAND string on the drain side of the
second word line, after the first boosting, while applying a
voltage to the first word line for providing the first non-volatile
storage element in a non-conducting state, and while applying a
program voltage to the second word line.
2. The method of claim 1, wherein: during the second boosting, a
voltage at a first level is applied to word lines of the plurality
of word lines on the drain side of the second word line, the
voltage applied to the first word line is at a second level which
is less than the first level, and a voltage at a level greater than
the second level is applied to at least one in between word line
which is between the first word line and the second word line.
3. The method of claim 2, wherein: during the second boosting,
applying a voltage at a level which is between the first and second
levels to a word line of the plurality of word lines which is
between the at least one in between word line and the first word
line.
4. The method of claim 2, wherein: during the second boosting,
applying a voltage at a level which is between the first and second
levels to a word line of the plurality of word lines which is on
the source side of the first word line, adjacent to the first word
line.
5. The method of claim 1, further comprising: during the first
boosting, applying a voltage to at least one word line of the
plurality of word lines which is between the first and second word
lines for providing at least one non-volatile storage element of
the plurality of non-volatile storage elements which is between the
first and second word lines in a conducting state, so that every
non-volatile storage element in the at least one NAND string
between the first and second non-volatile storage elements is
provided in a conducting state.
6. The method of claim 1, further comprising: during the first
boosting, applying a voltage to at least one word line of the
plurality of word lines which is on the drain side of the second
word line, adjacent to the second word line, for providing at least
one non-volatile storage element of the plurality of non-volatile
storage elements which is on the drain side of the second word
line, adjacent to the second word line in a conducting state.
7. The method of claim 1, further comprising: during the first
boosting, applying a voltage to a set of word lines of the
plurality of word lines which is on the drain side of the second
word line for avoiding boosting of the at least one NAND string on
the drain side of the second word line.
8. The method of claim 1, wherein: the voltage applied to the
second word line for providing the second non-volatile storage
element in a conducting state is at a first level; and the first
boosting is performed by applying a voltage at a second level,
greater than the first level, to a set of word lines of the
plurality of word lines which is on the source side of the first
word line.
9. The method of claim 1, wherein: providing the first and second
non-volatile storage elements in a conducting state allows charge
transfer in the at least one NAND string between the source side of
the first word line and the drain side of the second word line; and
providing the first storage element in a non-conducting state
discourages charge transfer in the at least one NAND string between
the source side of the first word line and the drain side of the
second word line.
10. The method of claim 1, further comprising: during the first
boosting, applying a voltage to at least one word line of the
plurality of word lines which is on the drain side of the second
word line for providing at least a third non-volatile storage
element of the plurality of non-volatile storage elements which is
on the drain side of the second word line in a conducting state,
and applying a voltage to a set of word lines of the plurality of
word lines which is on a drain side of the at least a third
non-volatile storage element, for avoiding boosting of the at least
one NAND string on a drain side of the at least a third
non-volatile storage element.
11. The method of claim 10, wherein: the at least a third
non-volatile storage element is adjacent to the second word
line.
12. The method of claim 1, wherein: the voltage applied to the
first word line for providing the first non-volatile storage
element in a conducting state is at a first level; and the first
boosting is performed by applying voltages at a second level,
greater than the first level, to a set of non-volatile storage
elements in the at least one NAND string which is on the source
side of the first non-volatile storage element.
13. The method of claim 12, further comprising: applying voltages
at a third level, less than the first level, to a set of
non-volatile storage elements of the plurality of non-volatile
storage elements which is on a drain side of the second
non-volatile storage element, while performing the first
boosting.
14. The method of claim 13, further comprising: applying voltages
at the second level to the set of non-volatile storage elements in
the at least one NAND string which is on the drain side of the
second non-volatile storage element, while performing the second
boosting.
15. The method of claim 1, wherein: the second non-volatile storage
element is programmed by the program voltage.
16. The method of claim 1, further comprising: during the second
boosting, applying a voltage to at least one of the word lines of
the plurality of word lines which is between the first word line
and the second word line for boosting the at least one NAND string
between the first word line and the second word line.
17. The method of claim 1, further comprising, during the second
boosting: applying a voltage to an additional word line for
providing an additional non-volatile storage element in a
non-conducting state, the additional word line is on a drain side
of the second word line, the second boosting is performed on a
portion of the at least one NAND string which is between the second
word line and the additional word line; and performing third
boosting of the at least one NAND string on a drain side of the
additional word line.
18. A method for operating non-volatile storage, comprising:
performing first boosting of at least one NAND string on a side of
a first non-volatile storage element in the at least one NAND
string which is before the first non-volatile storage element in a
programming sequence; during the first boosting, providing the
first non-volatile storage element and a second non-volatile
storage element in the at least one NAND string which is on a side
of the first non-volatile storage element which is after the first
non-volatile storage element in the programming sequence in a
conducting state; and performing second boosting of the at least
one NAND string, after the first boosting, on a side of the second
non-volatile storage element which is after the second non-volatile
storage element in the programming sequence while providing the
first storage element in a non-conducting state.
19. The method of claim 18, further comprising: during the second
boosting, applying a program voltage to a word line associated with
the second non-volatile storage element.
20. The method of claim 18, wherein: the programming sequence
starts at a source side of the at least one NAND string and ends at
a drain side of the at least one NAND string.
21. The method of claim 18, wherein: the side of the first
non-volatile storage element which is before the first non-volatile
storage element in the programming sequence includes non-volatile
storage elements in the at least one NAND string which are fully
programmed; and the side of the second non-volatile storage element
which is after the second non-volatile storage element in the
programming sequence includes non-volatile storage elements in the
at least one NAND string which are unprogrammed and/or partially
programmed.
22. The method of claim 18, wherein: the side of the first
non-volatile storage element which is before the first non-volatile
storage element in the programming sequence has undergone
programming operations, since a last erase operation, needed to
fully program a non-volatile storage element; and the side of the
second non-volatile storage element which is after the second
non-volatile storage element in the programming sequence has not
undergone programming operations, since the last erase operation,
needed to fully program a non-volatile storage element.
23. The method of claim 18, further comprising: during the first
boosting, providing at least one non-volatile storage element which
is between the first and second non-volatile storage elements in a
conducting state, so that every non-volatile storage element in the
at least one NAND string between the first and second non-volatile
storage element is provided in a conducting state.
24. The method of claim 18, further comprising: during the first
boosting, providing at least one non-volatile storage element which
is on the side of the second non-volatile storage element which is
after the second non-volatile storage element in the programming
sequence in a conducting state, and adjacent to the second
non-volatile storage element.
25. The method of claim 18, further comprising: during the first
boosting, avoiding boosting of a set of non-volatile storage
elements which is on the side of the second non-volatile storage
element which is after the second non-volatile storage element in
the programming sequence.
26. A method for operating non-volatile storage, comprising: (a) in
a first time period: (i) applying voltages to a first set of word
lines on a source side of a particular word line in a set of word
lines for boosting a first channel region of at least one NAND
string; (ii) applying voltages to a second set of word lines which
includes the particular word line, the second set of word lines is
on a drain side of the first set of word lines, to provide
non-volatile storage elements in the at least one NAND string which
are associated with the second set of word lines in a conducting
state; and (iii) applying voltages to a third set of word lines on
a drain side of the second set of word lines, to avoiding boosting
of a second channel region of the at least one NAND string; and (b)
in a second time period which follows the first time period: (i)
applying voltages to the third set of word lines for boosting the
second channel region of the at least one NAND string; (ii)
applying a program voltage to a word line in the second set of word
lines; and (iii) applying a voltage to the particular word line to
isolate the first channel region from the second channel
region.
27. The method of claim 26, wherein: the voltages in steps (a)(i)
and (b)(i) are the same.
28. The method of claim 26, wherein: the voltages in step (a)(ii)
are lower than the voltages in step (a)(i) but higher than the
voltages in step (a)(iii).
29. The method of claim 26, wherein: the program voltage is greater
than the voltages in step (a)(i).
30. The method of claim 26, further comprising: during the second
boosting, applying a voltage to at least one of the word lines of
the plurality of word lines which is between the particular word
line and the word line to which the program voltage is applied, for
boosting the at least one NAND string between the particular word
line and the word line to which the program voltage is applied.
31. The method of claim 26, further comprising: during the second
time period, applying voltages to a fourth set of word lines on a
drain side of the third set of word lines for boosting a third
channel region of the at least one NAND string on a drain side of
the second channel region, and applying a voltage to a word line
between the second and third channel regions for isolating the
second and third channel regions from each other.
32. The method of claim 26, further comprising: in step (a)(ii) the
voltages applied to the second set of word lines are set based on
programmed states of the non-volatile storage elements in the at
least one NAND string which are associated with the second set of
word lines.
33. The method of claim 26, further comprising: in step (a)(ii) the
voltages applied to the second set of word lines include a higher
voltage which is applied to a non-volatile storage element with a
higher programmed state, and a lower voltage which is applied to a
non-volatile storage element with a lower programmed state.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to co-pending, commonly assigned
U.S. patent application Ser. No. ______, filed herewith, titled
"Non-Volatile Storage With Boosting Using Channel Isolation
Switching" (docket no. SAND-1229US1), incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to non-volatile memory.
[0004] 2. Description of the Related Art
[0005] Semiconductor memory has become increasingly popular for use
in various electronic devices. For example, non-volatile
semiconductor memory is used in cellular telephones, digital
cameras, personal digital assistants, mobile computing devices,
non-mobile computing devices and other devices. Electrically
Erasable Programmable Read Only Memory (EEPROM) and flash memory
are among the most popular non-volatile semiconductor memories.
With flash memory, also a type of EEPROM, the contents of the whole
memory array, or of a portion of the memory, can be erased in one
step, in contrast to the traditional, full-featured EEPROM.
[0006] Both the traditional EEPROM and the flash memory utilize a
floating gate that is positioned above and insulated from a channel
region in a semiconductor substrate. The floating gate is
positioned between the source and drain regions. A control gate is
provided over and insulated from the floating gate. The threshold
voltage (V.sub.TH) of the transistor thus formed is controlled by
the amount of charge that is retained on the floating gate. That
is, the minimum amount of voltage that must be applied to the
control gate before the transistor is turned on to permit
conduction between its source and drain is controlled by the level
of charge on the floating gate.
[0007] Some EEPROM and flash memory devices have a floating gate
that is used to store two ranges of charges and, therefore, the
memory element can be programmed/erased between two states, e.g.,
an erased state and a programmed state. Such a flash memory device
is sometimes referred to as a binary flash memory device because
each memory element can store one bit of data.
[0008] A multi-state (also called multi-level) flash memory device
is implemented by identifying multiple distinct allowed/valid
programmed threshold voltage ranges. Each distinct threshold
voltage range corresponds to a predetermined value for the set of
data bits encoded in the memory device. For example, each memory
element can store two bits of data when the element can be placed
in one of four discrete charge bands corresponding to four distinct
threshold voltage ranges.
[0009] Typically, a program voltage V.sub.PGM applied to the
control gate during a program operation is applied as a series of
pulses that increase in magnitude over time. In one possible
approach, the magnitude of the pulses is increased with each
successive pulse by a predetermined step size, e.g., 0.2-0.4 V.
V.sub.PGM can be applied to the control gates of flash memory
elements. In the periods between the program pulses, verify
operations are carried out. That is, the programming level of each
element of a group of elements being programmed in parallel is read
between successive programming pulses to determine whether it is
equal to or greater than a verify level to which the element is
being programmed. For arrays of multi-state flash memory elements,
a verification step may be performed for each state of an element
to determine whether the element has reached its data-associated
verify level. For example, a multi-state memory element capable of
storing data in four states may need to perform verify operations
for three compare points.
[0010] Moreover, when programming an EEPROM or flash memory device,
such as a NAND flash memory device in a NAND string, typically
V.sub.PGM is applied to the control gate and the bit line is
grounded, causing electrons from the channel of a cell or memory
element, e.g., storage element, to be injected into the floating
gate. When electrons accumulate in the floating gate, the floating
gate becomes negatively charged and the threshold voltage of the
memory element is raised so that the memory element is considered
to be in a programmed state. More information about such
programming can be found in U.S. Pat. No. 6,859,397, titled "Source
Side Self Boosting Technique For Non-Volatile Memory," and in U.S.
Patent Application Publication 2005/0024939, titled "Detecting Over
Programmed Memory," published Feb. 3, 2005; both of which are
incorporated herein by reference in their entirety.
[0011] However, one issue which continues to be problematic is
program disturb. Program disturb can occur at inhibited NAND
strings during programming of other NAND strings, and sometimes at
the programmed NAND string itself. Program disturb occurs when the
threshold voltage of an unselected non-volatile storage element is
shifted due to programming of other non-volatile storage elements.
Program disturb can occur on previously programmed storage elements
as well as erased storage elements that have not yet been
programmed.
SUMMARY OF THE INVENTION
[0012] The present invention addresses the above and other issues
by providing a method for reducing program disturb in non-volatile
storage.
[0013] In one embodiment, a method for operating non-volatile
storage includes performing first boosting of at least one NAND
string on a source side of a first word line before boosting the at
least one NAND string on a drain side of a second word line, where
the second word line is on a drain side of the first word line. A
number of word lines including the first and second word lines are
associated with the at least one NAND string, and the at least one
NAND string has a number of non-volatile storage elements. The
method further includes, during the first boosting, applying a
voltage to the first word line for providing a first non-volatile
storage element which is associated with the first word line in a
conducting state, and applying a voltage to the second word line
for providing a second non-volatile storage element which is
associated with the second word line in a conducting state. The
method further includes performing second boosting of the at least
one NAND string on the drain side of the second word line, after
the first boosting, while applying a voltage to the first word line
for providing the first non-volatile storage element in a
non-conducting state, and while applying a program voltage to the
second word line. Thus, source side boosting occurs before applying
the program pulse.
[0014] In another embodiment, a method for operating non-volatile
storage includes performing first boosting of at least one NAND
string on a side of a first non-volatile storage element in the at
least one NAND string which is before the first non-volatile
storage element in a programming sequence. The method further
includes, during the first boosting, providing the first
non-volatile storage element and a second non-volatile storage
element in the at least one NAND string which is on a side of the
first non-volatile storage element which is after the first
non-volatile storage element in the programming sequence in a
conducting state. The method further includes performing second
boosting of the at least one NAND string, after the first boosting,
on a side of the second non-volatile storage element which is after
the second non-volatile storage element in the programming sequence
while providing the first storage element in a non-conducting
state.
[0015] In another embodiment, a method for operating non-volatile
storage includes (a) in a first time period: (i) applying voltages
to a first set of word lines on a source side of a particular word
line in a set of word lines for boosting a first channel region of
at least one NAND string, (ii) applying voltages to a second set of
word lines which includes the particular word line, the second set
of word lines is on a drain side of the first set of word lines, to
provide non-volatile storage elements in the at least one NAND
string which are associated with the second set of word lines in a
conducting state, and (iii) applying voltages to a third set of
word lines on a drain side of the second set of word lines, to
avoiding boosting of a second channel region of the at least one
NAND string. The method further includes (b) in a second time
period which follows the first time period: (i) applying voltages
to the third set of word lines for boosting the second channel
region of the at least one NAND string, (ii) applying a program
voltage to a word line in the second set of word lines, and (iii)
applying a voltage to the particular word line to isolate the first
channel region from the second channel region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a top view of a NAND string.
[0017] FIG. 2 is an equivalent circuit diagram of the NAND string
of FIG. 1.
[0018] FIG. 3 is a block diagram of an array of NAND flash storage
elements.
[0019] FIG. 4 depicts a cross-sectional view of a NAND string
showing a program disturb mechanism.
[0020] FIGS. 5a-h depict different examples of self-boosting
modes.
[0021] FIG. 6 depicts a time line of word line and other voltages,
based on the self-boosting mode of FIG. 5a.
[0022] FIG. 7 depicts a time line of word line and other voltages,
based on the self-boosting mode of FIG. 5b.
[0023] FIG. 8 depicts a time line of word line and other voltages,
based on the self-boosting mode of FIG. 5c
[0024] FIG. 9 depicts a time line of word line and other voltages,
as an alternative to the time line of FIG. 8.
[0025] FIG. 10 depicts a programming process in which a source side
of a NAND string is boosted before a drain side of the NAND
string.
[0026] FIG. 11 is a block diagram of an array of NAND flash storage
elements.
[0027] FIG. 12 is a block diagram of a non-volatile memory system
using single row/column decoders and read/write circuits.
[0028] FIG. 13 is a block diagram of a non-volatile memory system
using dual row/column decoders and read/write circuits.
[0029] FIG. 14 is a block diagram depicting one embodiment of a
sense block.
[0030] FIG. 15 illustrates an example of an organization of a
memory array into blocks for an all bit line memory architecture or
for an odd-even memory architecture.
[0031] FIG. 16 depicts an example set of threshold voltage
distributions and one-pass programming.
[0032] FIG. 17 depicts an example set of threshold voltage
distributions and two-pass programming.
[0033] FIGS. 18a-c show various threshold voltage distributions and
describe a process for programming non-volatile memory.
[0034] FIG. 19 is a flow chart describing one embodiment of a
process for programming non-volatile memory.
[0035] FIG. 20 depicts an example pulse train applied to the
control gates of non-volatile storage elements during
programming.
DETAILED DESCRIPTION
[0036] The present invention provides a method for reducing program
disturb in non-volatile storage.
[0037] One example of a memory system suitable for implementing the
present invention uses the NAND flash memory structure, which
includes arranging multiple transistors in series between two
select gates. The transistors in series and the select gates are
referred to as a NAND string. FIG. 1 is a top view showing one NAND
string. FIG. 2 is an equivalent circuit thereof. The NAND string
depicted in FIGS. 1 and 2 includes four transistors, 100, 102, 104
and 106, in series and sandwiched between a first select gate 120
and a second select gate 122. Select gate 120 gates the NAND string
connection to bit line 126. Select gate 122 gates the NAND string
connection to source line 128. Select gate 120 is controlled by
applying the appropriate voltages to control gate 120CG. Select
gate 122 is controlled by applying the appropriate voltages to
control gate 122CG. Each of the transistors 100, 102, 104 and 106
has a control gate and a floating gate. Transistor 100 has control
gate 100CG and floating gate 100FG. Transistor 102 includes control
gate 102CG and floating gate 102FG. Transistor 104 includes control
gate 104CG and floating gate 104FG. Transistor 106 includes a
control gate 106CG and floating gate 106FG. Control gate 100CG is
connected to (or is) word line WL3, control gate 102CG is connected
to word line WL2, control gate 104CG is connected to word line WL1,
and control gate 106CG is connected to word line WL0. In one
embodiment, transistors 100, 102, 104 and 106 are each storage
elements, also referred to as memory cells. In other embodiments,
the storage elements may include multiple transistors or may be
different than that depicted in FIGS. 1 and 2. Select gate 120 is
connected to select line SGD. Select gate 122 is connected to
select line SGS.
[0038] FIG. 3 is a circuit diagram depicting three NAND strings. A
typical architecture for a flash memory system using a NAND
structure will include several NAND strings. For example, three
NAND strings 320, 340 and 360 are shown in a memory array having
many more NAND strings. Each of the NAND strings includes two
select gates and four storage elements. While four storage elements
are illustrated for simplicity, modern NAND strings can have up to
thirty-two or sixty-four storage elements, for instance.
[0039] For example, NAND string 320 includes select gates 322 and
327, and storage elements 323-326, NAND string 340 includes select
gates 342 and 347, and storage elements 343-346, NAND string 360
includes select gates 362 and 367, and storage elements 363-366.
Each NAND string is connected to the source line by its select
gates (e.g., select gates 327, 347 or 367). A selection line SGS is
used to control the source side select gates. The various NAND
strings 320, 340 and 360 are connected to respective bit lines 321,
341 and 361, by select transistors in the select gates 322, 342,
362, etc. These select transistors are controlled by a drain select
line SGD. In other embodiments, the select lines do not necessarily
need to be in common among the NAND strings; that is, different
select lines can be provided for different NAND strings. Word line
WL3 is connected to the control gates for storage elements 323, 343
and 363. Word line WL2 is connected to the control gates for
storage elements 324, 344 and 364. Word line WL1 is connected to
the control gates for storage elements 325, 345 and 365. Word line
WL0 is connected to the control gates for storage elements 326, 346
and 366. As can be seen, each bit line and the respective NAND
string comprise the columns of the array or set of storage
elements. The word lines (WL3, WL2, WL1 and WL0) comprise the rows
of the array or set. Each word line connects the control gates of
each storage element in the row. Or, the control gates may be
provided by the word lines themselves. For example, word line WL2
provides the control gates for storage elements 324, 344 and 364.
In practice, there can be thousands of storage elements on a word
line.
[0040] Each storage element can store data. For example, when
storing one bit of digital data, the range of possible threshold
voltages (V.sub.TH) of the storage element is divided into two
ranges which are assigned logical data "1" and "0." In one example
of a NAND type flash memory, the V.sub.TH is negative after the
storage element is erased, and defined as logic "1." The V.sub.TH
after a program operation is positive and defined as logic "0."
When the V.sub.TH is negative and a read is attempted, the storage
element will turn on to indicate logic "1" is being stored. When
the V.sub.TH is positive and a read operation is attempted, the
storage element will not turn on, which indicates that logic "0" is
stored. A storage element can also store multiple levels of
information, for example, multiple bits of digital data. In this
case, the range of V.sub.TH value is divided into the number of
levels of data. For example, if four levels of information are
stored, there will be four V.sub.TH ranges assigned to the data
values "11", "10", "01", and "00." In one example of a NAND type
memory, the V.sub.TH after an erase operation is negative and
defined as "11". Positive V.sub.TH values are used for the states
of "10", "01", and "00." The specific relationship between the data
programmed into the storage element and the threshold voltage
ranges of the element depends upon the data encoding scheme adopted
for the storage elements. For example, U.S. Pat. No. 6,222,762 and
U.S. Patent Application Pub. 2004/0255090, both of which are
incorporated herein by reference in their entirety, describe
various data encoding schemes for multi-state flash storage
elements.
[0041] Relevant examples of NAND type flash memories and their
operation are provided in U.S. Pat. Nos. 5,386,422, 5,522,580,
5,570,315, 5,774,397, 6,046,935, 6,456,528 and 6,522,580, each of
which is incorporated herein by reference.
[0042] When programming a flash storage element, a program voltage
is applied to the control gate of the storage element, and the bit
line associated with the storage element is grounded. Electrons
from the channel are injected into the floating gate. When
electrons accumulate in the floating gate, the floating gate
becomes negatively charged and the V.sub.TH of the storage element
is raised. To apply the program voltage to the control gate of the
storage element being programmed, that program voltage is applied
on the appropriate word line. As discussed above, one storage
element in each of the NAND strings share the same word line. For
example, when programming storage element 324 of FIG. 3, the
program voltage will also be applied to the control gates of
storage elements 344 and 364.
[0043] However, program disturb can occur at inhibited NAND strings
during programming of other NAND strings, and sometimes at the
programmed NAND string itself. Program disturb occurs when the
threshold voltage of an unselected non-volatile storage element is
shifted due to programming of other non-volatile storage elements.
Program disturb can occur on previously programmed storage elements
as well as erased storage elements that have not yet been
programmed. Various program disturb mechanisms can limit the
available operating window for non-volatile storage devices such as
NAND flash memory.
[0044] For example, if NAND string 320 is inhibited (e.g., it is an
unselected NAND string which does not contain a storage element
which is currently being programmed) and NAND string 340 is being
programmed (e.g., it is a selected NAND string which contains a
storage element which is currently being programmed), program
disturb can occur at NAND string 320. For example, if a pass
voltage, V.sub.PASS, is low, the channel of the inhibited NAND
string is not well boosted, and a selected word line of the
unselected NAND string can be unintentionally programmed. In
another possible scenario, the boosted voltage can be lowered by
Gate Induced Drain Leakage (GIDL) or other leakage mechanisms,
resulting in the same problem. Other effects, such as shifts in the
V.sub.TH of a charge storage element due to capacitive coupling
with other neighboring storage elements that are programmed later,
can also contribute to program disturb.
[0045] FIG. 4 depicts a cross-sectional view of a NAND string
showing a program disturb mechanism. Here, a revised erased area
self-boosting (REASB) mode, such as depicted in FIG. 5c, is used.
The view is simplified and not to scale. The NAND string 400
includes a source-side select gate 406, a drain-side select gate
424, and eight storage elements 408, 410, 412, 414, 416, 418, 420
and 422, formed on a substrate 490. The components can be formed on
a p-well region which itself is formed in an n-well region of the
substrate. The n-well can in turn be formed in a p-substrate. A
source supply line 404 with a potential of V.sub.SOURCE is provided
in addition to a bit line 426 with a potential of V.sub.BL. During
programming, V.sub.PGM is provided on a selected word line, in this
case, WL5, which is associated with a selected storage element 418.
Further, recall that the control gate of a storage element may be
provided as a portion of the word line. For example, WL0, WL1, WL2,
WL3, WL4, WL5, WL6 and WL7 can extend via the control gates of
storage elements 408, 410, 412, 414, 416, 418, 420 and 422,
respectively.
[0046] In one example boosting mode, when storage element 418 is
the selected storage element, a relatively low voltage, V.sub.LOW,
e.g., 2-6 V, is applied to a neighboring source-side word line
(WL3), while an isolation voltage, V.sub.ISO, e.g., 0-4 V, is
applied to another source-side word line (WL2), referred to as an
isolation word line and V.sub.PASS is applied to the remaining word
lines associated with NAND string 400 (i.e., WL0, WL1, WL4, WL6 and
WL7). While the absolute values of V.sub.ISO and V.sub.LOW may vary
over a relatively large and partly overlapping range, V.sub.ISO is
always lower in value than V.sub.LOW, in one possible
implementation. V.sub.SGS is applied to the select gate 406 and
V.sub.SGD is applied to the select gate 424. The source side of a
word line or non-volatile storage element refers to the side which
faces the source end of the NAND string, e.g., at source supply
line 404, while the drain side of a word line or non-volatile
storage element refers to the side which faces the drain end of the
NAND string, e.g., at bit line 426.
[0047] FIGS. 5a-h depict different examples of self-boosting modes.
Note that the voltages depicted indicate the voltages used during
the drain side boosting which occurs after source side boosting.
See also FIGS. 6-9. Various other approaches can be used as well.
Generally, various types of boosting modes have been developed to
combat program disturb. During programming of storage elements on a
selected word line, the boosting modes can be implemented by
applying a set of voltages to unselected word lines which are in
communication with storage elements which are not currently being
programmed. The storage elements which are being programmed are
associated with selected NAND strings while other storage elements
are associated with unselected NAND strings.
[0048] In the examples provided, the word lines are WL0 through
WLi, the selected word line is WLn, the source-side select gate
control line is SGS and the drain-side select gate control line is
SGD. A set of voltages which is applied to the control lines is
also depicted. Programming can proceed in a programming sequence
one word line at a time, from the source side to the drain side of
a NAND string. However, other programming sequences can be used as
well. For example, in a two-step programming technique, the storage
elements of a NAND string may be partially programmed in a first
pass which proceeds one word line at a time from the source side to
the drain side of a NAND string. The programming is then completed
in a second pass which also proceeds one word line at a time from
the source side to the drain side of a NAND string. In another
option, the storage elements are programmed in a two up, one down
process, e.g., in the sequence: WL0 (partial programming), WL1
(partial programming), WL0 (completion of programming), WL2
(partial programming), WL1 (completion programming), WL3 (partial
programming), and so forth.
[0049] In the example shown in FIG. 5a, the voltages which are
applied include V.sub.SGS, which is applied to the source-side
select gate control line SGS, a pass voltage, V.sub.PASS, which is
applied to each of the unselected word lines, WL0 through WLn-2 and
WLn+1 through WLi, a program voltage, V.sub.PGM, which is applied
to the selected word line WLn, an isolation voltage V.sub.ISO which
is applied to WLn-1, the word line which is adjacent to the
selected word line on the source side, and V.sub.SGD, which is
applied via the drain-side select gate control line SGD. Typically,
V.sub.SGS is 0 V so that the source-side select gate is off, an
additional source bias voltage V.sub.SOURCE in a range of 0.5-1.5 V
may be applied to further improve the cut-off behavior of the
source-side select gate. V.sub.SGD is about 1.5-3 V so that the
drain-side select gate is on for the selected NAND strings, due to
application of a corresponding low bit line voltage V.sub.BL such
as 0-1 V. The drain-side select gate is off for the
unselected/inhibited NAND strings, due to application of a
corresponding higher V.sub.BL such as 1.5-3 V. A low isolation
voltage V.sub.ISO, in a typical range of 0-4 V, is applied to the
word line which is adjacent to the selected word line on the source
side, in the example of FIG. 5a.
[0050] Additionally, V.sub.PASS can be about 7-10 V and V.sub.PGM
can vary from about 12-25 V. In one programming scheme, a pulse
train of program voltages is applied to the selected word line. See
FIG. 20. The amplitude of each successive program pulse in the
pulse train increases in a staircase manner, typically by about
0.3-0.5 V per pulse. Further, verify pulses can be applied between
program pulses to verify whether the selected storage elements have
reached a target programming condition. Note also that each
individual program pulse can have a fixed amplitude, or can have a
varying amplitude. For example, some programming schemes apply a
pulse with an amplitude which varies like a ramp or staircase. Any
type of program pulse can be used.
[0051] With WLn as the word line being programmed, and programming
proceeding from the source side to the drain side of each NAND
string, the storage elements associated with WL0 through WLn-1 will
have already been at least partially programmed, since the last
erase operation, and the storage elements associated with WLn+1
through WLi will be erased or at least not yet fully programmed
when the storage elements on WLn are being programmed. The pass
voltages on the unselected word lines couple to the channels
associated with the unselected NAND strings, causing a voltage to
exist in the channels of the unselected NAND strings which tends to
reduce program disturb by lowering the voltage across the tunnel
oxide of the storage elements.
[0052] FIG. 5b depicts a revised erased area self-boosting mode. In
this case, an isolation voltage, V.sub.ISO, is applied to WLn-2,
and a low voltage, V.sub.LOW, which is between V.sub.ISO and
V.sub.PASS, is applied to WLn-1. V.sub.LOW can also be considered
to be an isolation voltage, however, V.sub.LOW is always higher
than V.sub.ISO and lower than V.sub.PASS, in one possible
implementation. In this approach, V.sub.LOW serves as an
intermediate voltage so that there are less abrupt voltage changes
in the channel between the selected word line (WLn) and the
adjacent source side word lines (WLn-1 and WLn-2). For example,
V.sub.LOW may be, e.g., 2-6 V and V.sub.ISO may be, e.g., 0-4 V.
The less abrupt change in channel voltage results in a lower
electric field in the channel region and a lower channel potential,
especially at the storage elements associated with the V.sub.ISO
word line. A high channel voltage at the drain or source side of
the storage elements associated with the V.sub.ISO word line (as in
FIG. 5a) may cause charge carriers (electrons and holes) to be
generated by Gate Induced Drain Leakage (GIDL). The electrons that
are generated by GIDL may subsequently be accelerated in the strong
electric field in the area in between the selected word line and
the V.sub.ISO word line and may subsequently be injected (via hot
electron injection) in some of the storage elements associated with
the selected word line and thus causing program disturb. This
program disturb mechanism can be avoided or reduced by reducing the
electric field, such as by adding one (or more) word lines that are
biased with an intermediate voltage in between the voltage of the
selected word line and V.sub.ISO.
[0053] The remaining unselected word lines receive V.sub.PASS.
Specifically, V.sub.PASS is applied to a first group of storage
elements associated with WL0 through WLn-3, where the first group
is adjacent to the source side select gate, and on a source side of
the isolation word line WLn-2. Also, V.sub.PASS is applied to a
second group of storage elements associated with WLn+1 through WLi,
where the second group is adjacent to the drain side select gate,
and on a drain side of the selected word line WLn.
[0054] FIG. 5c depicts another revised erased area self-boosting
mode. In this case, the source-side word line (WLn-1) adjacent to
the selected word line (WLn) receives V.sub.PASS, the next word
line (WLn-2) receives V.sub.LOW and the next word line after that
(WLn-3) receives V.sub.ISO. The remaining unselected word lines
receive V.sub.PASS. This boosting mode is also discussed in
connection with FIG. 4. Specifically, V.sub.PASS is applied to a
first group of storage elements associated with WL0 through WLn-4,
where the first group is adjacent to the source side select gate,
and on a source side of the isolation word line WLn-3. Also,
V.sub.PASS is applied to a second group of storage elements
associated with WLn+1 through WLi, where the second group is
adjacent to the drain side select gate, and on a drain side of the
selected word line WLn. An advantage of this approach is that the
selected word line, which is most sensitive to program disturb
because of the high program voltage V.sub.PGM that is applied to
that word line, is further away from the V.sub.ISO and V.sub.LOW
word lines. Storage elements associated with the selected word line
are less likely to be disturbed by hot electron injection as the
electric field that is responsible for creating the hot carriers is
located further away from the selected word line.
[0055] FIG. 5d depicts another revised erased area self-boosting
mode. In this case, the source-side word line (WLn-1) adjacent to
the selected word line (WLn) receives V.sub.PASS, the next word
line (WLn-2) receives V.sub.LOW, the next word line (WLn-3)
receives V.sub.ISO, and the next word line receives V.sub.LOW. The
remaining unselected word lines receive V.sub.PASS. Specifically,
V.sub.PASS is applied to a first group of storage elements
associated with WL0 through WLn-5, where the first group is
adjacent to the source side select gate, and on a source side of
the isolation word line WLn-3. Also, V.sub.PASS is applied to a
second group of storage elements associated with WLn+1 through WLi,
where the second group is adjacent to the drain side select gate,
and on a drain side of the selected word line WLn. Providing
V.sub.LOW at both sides of the isolation word line can reduce the
probability that GIDL occurs at the isolation word line due to a
highly boosted source side, e.g., at a portion of the channel which
is associated with WL0 through WL5.
[0056] FIG. 5e depicts another revised erased area self-boosting
mode. In this case, the source-side word line (WLn-1) adjacent to
the selected word line (WLn) receives V.sub.PASS-HIGH, the next
word line (WLn-2) receives V.sub.PASS-MEDIUM, the next word line
(WLn-3) receives V.sub.PASS-LOW, the next word line (WLn-4)
receives V.sub.LOW, the next word line (WLn-5) receives V.sub.ISO
and the next word line (WLn-6) receives V.sub.LOW. The remaining
unselected word lines receive V.sub.PASS. Specifically, V.sub.PASS
is applied to a first group of storage elements associated with WL0
through WLn-7, where the first group is adjacent to the source side
select gate, and on a source side of the isolation word line WLn-5.
Also, V.sub.PASS is applied to a second group of storage elements
associated with WLn+1 through WLi, where the second group is
adjacent to the drain side select gate, and on a drain side of the
selected word line WLn.
[0057] Thus, multiple V.sub.PASS voltages can be used at the same
time. For example, different V.sub.PASS values can be used for the
drain and source sides of the NAND string. Further, multiple
V.sub.PASS voltages can be used at both the drain and source sides.
For instance, a higher V.sub.PASS, V.sub.PASS-HIGH, can be used
next to the selected word line for programming, as depicted. For
the word lines in between the selected word line and the isolation
word line, we can have multiple word lines that are biased to
different V.sub.PASS values, e.g., V.sub.PASS-LOW,
V.sub.PASS-MEDIUM and V.sub.PASS-HIGH. In one implementation,
V.sub.PGM>V.sub.PASS-HIGH>V.sub.PASS-MEDIUM>V.sub.PASS-LOW>V.-
sub.LOW>V.sub.ISO. Note that multiple values of V.sub.LOW and
V.sub.ISO are also possible. Generally, all V.sub.ISO voltages are
less than all V.sub.LOW voltages, which in turn are less than all
V.sub.PASS voltages. By increasing the number of word lines in
between the selected word line and the V.sub.ISO word line, and by
gradually reducing the bias voltage on those word lines, the
electric field in between the selected word line and the V.sub.ISO
word line can be reduced and thus program disturb can be
reduced.
[0058] FIG. 5f depicts another revised erased area self-boosting
mode. In this case, the source-side word line (WLn-1) adjacent to
the selected word line (WLn) receives V.sub.PASS-HIGH, the next
word line (WLn-2) receives V.sub.PASS-MEDIUM, the next word line
(WLn-3) receives V.sub.PASS-LOW, the next word line (WLn-4)
receives V.sub.LOW, the next word line (WLn-5) receives V.sub.ISO,
the next word line (WLn-6) receives V.sub.LOW, and the next word
line (WLn-7) receives V.sub.PASS-LOW. The remaining unselected word
lines receive V.sub.PASS. Specifically, V.sub.PASS is applied to a
first group of storage elements associated with WL0 through WLn-8,
where the first group is adjacent to the source side select gate,
and on a source side of the isolation word line WLn-5. Also,
V.sub.PASS is applied to a second group of storage elements
associated with WLn+1 through WLi, where the second group is
adjacent to the drain side select gate, and on a drain side of the
selected word line WLn.
[0059] FIG. 5g depicts another revised erased area self-boosting
mode. This case differs from that of FIG. 5f in that the drain-side
word line (WLn+1) adjacent to the selected word line (WLn) receives
V.sub.PASS-HIGH instead of V.sub.PASS.
[0060] FIG. 5h depicts another revised erased area self-boosting
mode. In this case, an additional isolation word line is provided
on the drain side of the programmed word line. For example,
compared to the boosting mode of FIG. 5c, WLn+1 receives
V.sub.PASS-HIGH and WLn+3 receives V.sub.ISO, in one possible
implementation. WLn+2 receives V.sub.PASS, where
V.sub.PASS-HIGH>V.sub.PASS. As a result of applying the boosting
voltages and the two isolation voltages, three boosted channel
areas are provided in the NAND string. For example, a first boosted
channel area is in the region of WL0 through WLn-4, a second
boosted channel area is in the region of WLn-1 through WLn+2, and a
third boosted channel area is in the region of WLn+4 through WLi.
The use of V.sub.PASS-HIGH removes the data dependency on WLn+1,
such as when WLn+1 may be partially programmed with lower page data
(see, e.g., the B' state of FIG. 18b). The boosting modes of FIGS.
5d-g can be modified similarly.
[0061] Various other implementations are possible. For example, the
different boosted channel areas can be boosted to different levels.
Also, the number of word lines between the selected word line and
the additional drain side isolation word line can vary, as can the
voltages applied to the unselected word lines in the different
boosted channel areas. Implementations with more than two isolation
voltages and three boosted channel areas can also be provided. For
further details, refer to U.S. patent application Ser. No.
11/535,628, filed Sep. 27, 2006, entitled "Reducing Program Disturb
In Non-Volatile Storage," docket no. SAND-1120/SDK-0868,
incorporated herein by reference.
[0062] Regarding timing of the boosting of the different channel
regions, various implementations are possible. Consider a first
channel region between WL0 and WLn-4, a second channel region
between WLn-1 and WLn+2 and a third channel region between WLn+4
and WLi. In one approach, the first and third channel regions are
boosted together, after which the second channel region is boosted.
In one approach, the first channel region is boosted, after which
the second and third channel regions are boosted together. In one
approach, the first channel region is boosted, after which the
third channel region is boosted, after which the second channel
region is boosted. Generally, the second channel region should
preferably not be boosted before the third channel region because
electrons from the third channel region would be attracted to the
boosted second channel region, thus lowering the boosted channel
potential in the second channel region while slightly boosting the
third channel region. This is an undesired effect as the reduced
boosting may cause program disturb.
[0063] Note that all the above examples serve as illustrations
only, as other bias conditions and different combinations of bias
conditions are possible.
[0064] Referring again to FIG. 4, assuming programming of storage
elements along the NAND string 400 progresses in a programming
sequence from storage element 408 to storage element 422, storage
elements 408-416 will already have been at least partially
programmed, and storage elements 420 and 422 will not yet have been
fully programmed. Thus, all or some of storage elements 408-416
will have electrons programmed into and stored in their respective
floating gates, and storage elements 420 and 422 can be erased or
partially programmed, depending on the programming mode. For
example, the storage elements 420 and 422 may be partially
programmed when they have been previously programmed in the first
step of a two-step programming technique.
[0065] With the EASB or REASB boosting modes, V.sub.ISO is applied
to one or more source-side neighbors of the selected word line at
some point after boosting is initiated, and is sufficiently low to
isolate programmed and erased channel areas in the substrate. That
is, a channel area of the substrate 490 on a source-side of the
isolation word line 412 is isolated from a channel area of the
substrate on a drain-side of the isolation word line 412. The
source side can also be considered to be a programmed side since
most or all of the associated storage elements have been
programmed, while the drain side can also be considered to be an
unprogrammed side since the associated storage elements have not
yet been programmed. Further, the channel area on the source side
is a first boosted region of the substrate 490 which is boosted by
the application of V.sub.PASS on WL0 and WL1, while the channel
area on the drain side is a second boosted region of the substrate
490 which is boosted mainly by the application of V.sub.PGM on WL5
and V.sub.PASS on WL4, WL6 and WL7.
[0066] The programmed area is in general boosted less because the
channel potential under a programmed storage element can only start
to increase (e.g. be boosted) after V.sub.PASS reaches a
sufficiently high level to turn on the programmed storage element.
On the other hand, the channel potential of storage elements in the
erased condition will start to increase (almost) immediately after
V.sub.PASS is applied as most (if not all) of the erased storage
elements will be in a turned on state even when the V.sub.PASS
voltage that is applied to their corresponding word lines is still
very low (during the ramping up of the V.sub.PASS voltage). Thus,
the channel area on the drain side of the isolation word line will
be boosted to a higher potential than the channel area at the
source side of the isolation word line as both areas are isolated
from one another. In some embodiments, the programming voltage
V.sub.PGM that is applied to the selected word line will be applied
after both channel areas are sufficiently boosted.
[0067] While the above embodiments can reduce certain program
disturb mechanisms, other program disturb mechanisms do exist. One
other program disturb fail mode tends to happen on higher word
lines when V.sub.PASS is relatively high. This fail mode occurs on
the NAND strings that are being programmed (e.g., selected NAND
strings), and is cause by hot carrier injection from the drain side
in the selected NAND string channels. This hot carrier injection is
induced by a high boosting potential in the source side channel
when V.sub.PASS reaches a certain level. In particular, with EASB
and REASB, as discussed, the NAND string is separated into a source
side and a drain side, by applying the isolation voltage V.sub.ISO
on a word line below the selected word line. In the selected NAND
string, the drain side channel potential will stay at 0-1 V, for
instance, during boosting. But, on the source side, because the
storage element which receives V.sub.ISO is cut off, e.g., provided
in a non-conductive state, assuming V.sub.ISO<V.sub.TH, where
V.sub.TH is the threshold voltage of the storage element, the
channel is still boosted up. When the source side boosting
potential becomes high and the drain side channel potential remains
at 0-1 V, a large lateral electric field is created which can
induce hot carrier injection to the storage elements on the source
side and cause program disturb fails. This is depicted in FIG. 4,
where the arrows depict electrons moving across the channel under
the isolation storage element 412 and into the floating gate of
storage element 410, raising the threshold voltage of the storage
element.
[0068] To prevent this kind of program disturb in a selected NAND
string, it is better not to isolate the source side channel from
the drain side channel during boosting. However, without isolation,
in the inhibited NAND string channels, the drain side boosting will
be significantly lowered by the source side programmed storage
elements. In particular, when high word lines are being programmed
and the source side and drain side channel capacitance ratio
becomes large, the reduction in the drain side boosting efficiency
can become severe. To overcome this dilemma, a channel isolation
switching method is proposed based on a source side early boosting
scheme. With this approach, the isolation word line stays at a
relatively high voltage, V.sub.COND, such as 4 V, which is
sufficient to turn the isolation storage element on even if it is
at the highest programmed state, thereby connecting the source and
drain side channels during the source side boosting. To further
guarantee the connection of source and drain side channels in the
selected NAND string, V.sub.COND can also be applied to word lines
on the drain side of the isolation storage element up until the
selected word line to open the associated storage elements, e.g.,
so they are in a conductive state or turned on. Further, if a
programming technique is used in which storage elements on the
drain side of the selected storage element may be at least
partially programmed, V.sub.COND can be applied to these storage
elements as well to keep them turned on during the source side
boosting.
[0069] Since the source and drain side channels are connected, in
the selected NAND strings, the channel potential will stay at 0-1 V
and the source side will not be boosted up. As a result, the
transfer of hot electrons from the drain side to the source side of
the channel and the drain side injection type of disturb will be
eliminated or reduced. In order to guarantee that the source side
channel is connected with the drain side channel when the source
side boosts up, V.sub.COND should be applied no later than
V.sub.PASS. To provide a safety margin, V.sub.COND can be applied
shortly before V.sub.PASS starts to ramp up on the source side.
[0070] After the source side boosting finishes, the isolation word
line voltage should be lowered to V.sub.ISO before the drain side
boosting starts. In this way, the inhibited channel's drain side
boosting (in unselected NAND strings) remains isolated from the
source side. Additionally, the inhibited channel's boosting
efficiency is improved since, during the source side boosting, many
electrons in the drain side channel will flow to the source side,
effectively causing some boosting of the drain side channel before
V.sub.PASS is applied to the drain side word lines. On the other
hand, in the selected NAND string, the channel potentials on source
and drain sides still remain at 0-1 V, and again the drain side
injection type of disturb is prevented or reduced.
[0071] FIG. 6 depicts a time line of word line and other voltages,
based on the self-boosting mode of FIG. 5a. The time period shown
depicts a single cycle of boosting and programming using a single
programming pulse. This cycle is typically followed by a sequence
of verify pulses to determine if the storage elements have reached
a desired programming state. The cycle of boosting and programming
is then repeated using another programming pulse, typically at a
stepped-up amplitude. See FIG. 20. Note also that the time period
shown may be preceded by an optional pre-charge period in which the
drain side channel is partially charged up (pre-charged) by a bit
line voltage of, e.g., 1.5-3 V which is transferred to the channel
by opening (providing in a conducting state) the drain select gate.
Typically, 0 V is applied to the word lines during pre-charging.
Moreover, the bit line voltage of the selected NAND string does not
always have to be 0 V. For example, V.sub.BL for the selected NAND
string can be, e.g., 0-1 V. For the inhibited NAND string, in case
the channel is pre-charged, V.sub.CH-DRAIN can be higher than 0 V
even before boosting starts, but not necessarily equal to 1.5-3 V,
as the amount of pre-charging depends on the erased V.sub.TH of the
storage elements. If the storage elements are very deeply erased,
pre-charging could actually reach the 1.5-3 V level. A typical
pre-charge level is in the range of 1-2 V.
[0072] Waveform 800 depicts, in a simplified representation, the
bit line voltage, V.sub.BL, for the inhibited (unselected) NAND
strings, the drain select gate voltage, V.sub.SGD, which is common
the a set of NAND strings, and the source voltage, V.sub.SOURCE,
which is common to a set of NAND strings. In practice, V.sub.SOURCE
need not be equal to V.sub.SGD and V.sub.BL, and there may also be
timing differences between these waveforms. Waveform 805 depicts
the bit line voltage, V.sub.BL, for the selected NAND strings and
the source select gate voltage, V.sub.SGS, which is common to a set
of NAND strings. In one alternative, V.sub.BL of the selected bit
line can have more than one level. For example, in a quick pass
write embodiment, typically two levels are used, such as 0 V and a
higher level, typically 0.3-1 V. 0 V is used first to allow faster
programming, while the higher level is used next to provide finer
control of the threshold voltage of the storage elements being
programmed that have almost reached their target threshold
voltage.
[0073] Waveform 810 depicts the voltage applied to the word lines
on the drain side of the selected word line. WLi denotes the ith or
highest word line and WLn+1 denotes the word line adjacent to the
selected word line (WLn) on the drain side. Waveform 815 depicts
voltages applied to the selected word line (WLn). Waveform 820
depicts the voltage applied to the isolation word line (WLn-1),
which is adjacent to the selected word line on the source side.
Waveform 825 depicts the voltage which is applied to the word lines
(WL0 through WLn-2) which are on the source side of the isolation
word line WLn-1. Waveforms 830 and 835 depict the channel potential
(V.sub.CH-SOURCE) which exists in the channel of the substrate on
the source side of the isolation word line, for the inhibited and
selected NAND strings, respectively. Waveforms 840 and 845 depict
the channel potential (V.sub.CH-DRAIN) which exists in the channel
of the substrate on the drain side of the isolation word line, for
the inhibited and selected NAND strings, respectively. Note how
V.sub.CH-DRAIN (waveform 840) tracks the drain side boosting
voltage (waveform 810) and the program voltage (waveform 815). The
extent to which the program voltage contributes to the drain side
boosting depends on the number of storage elements at the drain
side. With fewer storage elements at the drain side, the influence
of the program voltage on the drain side boosting is greater.
[0074] Further, note that V.sub.CH-DRAIN (waveform 840) increases
slightly at t1, during the source side boosting, since electrons in
the drain side channel flow to the source side, effectively causing
some boosting of the drain side channel before V.sub.PASS is
applied to the drain side word lines, as discussed previously.
[0075] Along the bottom of the time line are time points t0-t9. In
particular, at t0, as indicated by waveform 800, V.sub.BL for the
inhibited (unselected) NAND strings and V.sub.SGD are increased
from 0 V to e.g., 1.5-3 V. Also, V.sub.SOURCE increases from, e.g.,
0.5-1.5 V. With V.sub.SGS at 0 V (waveform 805), this ensures that
the source select gate for all NAND strings remains closed. For the
selected NAND strings, V.sub.BL=0 (or a little higher for quick
pass write embodiments) so that, with V.sub.SGD=1.5-3 V, the drain
select gate is open to allow programming to occur. While the
example provided corresponds to the boosting mode of FIG. 5a,
essentially any type of boosting scheme which uses one or more
isolation word lines on the source side of the selected word line
may be used. For example, the example can be used in combination
with local self-boosting (LSB) and/or revised LSB (RLSB) boosting
modes. In LSB like modes, there may be one or more isolation word
lines on the drain side as well so that the word lines neighboring
the selected word line are at 0 V or other isolation voltage and
the remaining unselected word lines are supplied V.sub.PASS or
other voltages as described herein. RLSB is similar to REASB. The
immediate neighboring drain and source side word lines of the
isolation word line are supplied an intermediate voltage V.sub.LOW,
while the remaining unselected word lines are supplied V.sub.PASS
or other voltages as described herein.
[0076] At t1, V.sub.COND is applied to WLn and WLn-1 so that the
associated storage elements are turned on (e.g., provided in a
conductive state). This allows charge transfer in the NAND string
between the source side of the isolation word line (WLn-1) and the
drain side of the selected word line (WLn).
[0077] At t2, boosting of the source side channel is initiated by
applying V.sub.PASS to WL0 through WLn-2 (waveform 825). V.sub.PASS
can be delayed relative to V.sub.COND as depicted to guarantee that
the source side channel is connected with the drain side channel
when the source side boosts up. The pass voltage boosts the channel
of the NAND string on the source side of the isolation word line.
Note the corresponding increase in V.sub.CH-SOURCE (waveform 830).
In the channel region associated with WLn+1 through WLi, on the
drain side of the selected word line, which is after the selected
word line in the programming sequence, boosting is avoided due to a
voltage such as 0 V which is applied. Although, some boosting may
already occur due to electrons flowing from the drain side to the
boosted source side. Between t2 and t3, boosting of the source side
channel occurs. After t3, V.sub.ISO is applied to close the
associated storage element of the isolation word line (WLn-1),
thereby discouraging charge transfer in the NAND string between the
source side of the isolation word line (WLn-1) and the drain side
of the selected word line (WLn).
[0078] After a delay, which is needed to make sure that WLn-1 has
reached the V.sub.ISO level, and starting at t4, boosting of the
drain side channel is initiated by applying V.sub.PASS (waveform
810). Note the corresponding increase in V.sub.CH-DRAIN (waveform
840). Boosting of the source and drain side channels continues
until t8. Further, at t5, V.sub.PGM1 is applied to WLn and, at t6,
V.sub.PGM2 is applied to WLn. Thus, the program voltage can be
applied initially at a first level and subsequently at a higher
second level. This approach avoids abrupt changes in V.sub.CH-DRAIN
which may be caused by abrupt changes in V.sub.PGM. However, a
single stepped V.sub.PGM pulse may alternatively be used. Note,
furthermore, that in some embodiments, V.sub.PGM1 may be equal to
V.sub.PASS and that in some cases the time between t4 and t5 may be
equal to zero, so that V.sub.PGM1 and V.sub.PASS are essentially
ramped up at the same time. At t7, the program voltage is removed,
at t8, the boosting voltages are removed and, at t9, the boosting
and programming cycle ends. Thus, source side boosting occurs
between t1 and t8 and drain side boosting occurs between t4 and
t8.
[0079] Due to the source side boosting and the application of
voltages for opening the storage elements associated with WLn and
WLn-1 between t1 and t3, charge transfer can occur between the
source side and drain side channels during this time period. For
example, many electrons in the drain side channel will flow to the
source side, effectively causing some boosting of the drain side
channel before V.sub.PASS is applied to the drain side word lines.
Further, removal of V.sub.COND at t3, before the drain side
boosting starts, serves to isolate the inhibited channel's
subsequent drain side boosting from the source side.
[0080] FIG. 7 depicts a time line of word line and other voltages,
based on the self-boosting mode of FIG. 5b. The time lines of FIG.
7 vary from those of FIG. 6 in that WLn+1, the word line on the
drain side of the selected word line, WLn, and adjacent to the
selected word line, receives V.sub.COND instead of 0 V between t1
and t3 (waveform 812). This approach may be used, e.g., when the
non-volatile storage elements associated with WLn+1 may be
partially programmed. Additionally, the word line WLn-1 which is
between the selected word line WLn and the isolation word line
WLn-2 receives V.sub.LOW between t4 and t8, where
V.sub.LOW>V.sub.ISO (waveform 817). This provides a gradual
transition from V.sub.PGM2 to V.sub.ISO over one or more
intermediate word lines. Waveform 810 is then applied to WLn+2
through WLi, waveform 820 is applied to WLn-2 and waveform 825 is
applied to WL0 through WLn-3.
[0081] It is also possible for the level of V.sub.COND to vary for
the different word lines to which it is applied. For example,
V.sub.COND can be set based on the programming state of the
corresponding non-volatile storage elements. V.sub.COND can be
higher when the associated non-volatile storage element has a
higher programmed state, and lower when the associated non-volatile
storage element has a lower programmed state. V.sub.COND need only
be high enough to create a conducting path between the source side
and the drain side channel area. Providing different levels of
V.sub.COND allows a flexibility to address data pattern
dependencies. Depending on the back pattern, e.g., the data
pattern, as an example, WLn+1 could be in a lower middle state B'
(FIG. 18a), while WLn and word lines below WLn could be at state C
(FIG. 18c), the highest programmed state. In this case,
V.sub.COND-LOW can be applied to WLn+1 and V.sub.COND-HIGH can be
applied to WLn-2 through WLn, where
V.sub.COND-HIGH>V.sub.COND-LOW.
[0082] FIG. 8 depicts a time line of word line and other voltages,
based on the self-boosting mode of FIG. 5c. The time lines of FIG.
8 vary from those of FIG. 7 in that WLn-1, the word line on the
source side of the selected word line, WLn, and adjacent to the
selected word line, receives V.sub.PASS instead of V.sub.LOW
between t4 and t8 (waveform 816). Waveform 817 is then applied to
WLn-2, waveform 820 is applied to WLn-3 and waveform 825 is applied
to WL0 through WLn-4. This provides an even more gradual transition
from V.sub.PGM2 to V.sub.ISO over one or more intermediate word
lines.
[0083] As a further alternative which can be used, e.g., when the
non-volatile storage elements associated with WLn+1 are not
programmed, 0 V can be applied to WLn+1 between t1 and t3 instead
of V.sub.COND.
[0084] FIG. 9 depicts a time line of word line and other voltages,
as an alternative to the time line of FIG. 8. The time lines of
FIG. 9 vary from those of FIG. 8 in that a gradual transition in
voltage is made from V.sub.COND to the subsequent voltage, e.g.,
from V.sub.COND to V.sub.PASS on WLn+1 (waveform 912) and WLn-1
(waveform 916), from V.sub.COND to V.sub.PGM1 on WLn (waveform 915)
and/or from V.sub.COND to V.sub.LOW on WLn (waveform 917). The
voltages thus can ramp up or down to V.sub.PASS or V.sub.LOW
directly from V.sub.COND between the source and drain side boosting
transition, in the time period between t3 and t4.
[0085] An advantage of this approach is that GIDL at the V.sub.ISO
and/or V.sub.LOW word lines can be prevented or reduced. In the
above example of FIGS. 7 and 8, the V.sub.LOW word line is pulled
down to 0 V before the voltage V.sub.LOW is applied. Especially in
combination with some of the boosting modes, this can cause an
increase in GIDL. The purpose of applying V.sub.LOW is to reduce
the electric fields during boosting. However, when the voltage on
the V.sub.LOW word line is lowered from V.sub.COND to 0 V, the
electric field in the neighborhood of that word line is increased
due to the boosted source side and GIDL may occur. This increase in
the electric field can be prevented by ramping the signal on the
V.sub.LOW word line directly from V.sub.COND to V.sub.LOW.
[0086] Further, if V.sub.LOW>V.sub.COND, it can be advantageous
to apply V.sub.LOW to the word line instead of V.sub.COND, for
instance, with the boosting scheme of FIG. 5d, in which V.sub.LOW
is applied on WLn-4 and WLn-2 and V.sub.ISO is applied on WLn-3. In
this case, to reduce the probability of GIDL occurring on WLn-3
(when the word line voltage transitions from V.sub.COND to
V.sub.ISO) or on WLn-4 (due to V.sub.COND), it may be preferred to
keep WLn-4 biased to V.sub.LOW from the start.
[0087] The remaining boosting modes of FIGS. 5a-5h, as well as
other boosting modes, can similarly be implemented using similar
time lines as discussed herein. For example, with the boosting mode
of FIG. 5h, as discussed, three or more different channel regions
can be boosted. For the case where the first and third channel
regions are boosted together, after which the second channel region
is boosted, the first and third channel regions can be boosted in
what is referred to as the source side boosting in FIGS. 6-9, while
the second channel region can be boosted in what is referred to as
the drain side boosting. For the case where the first channel
region is boosted, after which the second and third channel regions
are boosted together, the first channel region can be boosted in
what is referred to as the source side boosting, while the second
and third channel regions can be boosted in what is referred to as
the drain side boosting. For the case where the first channel
region is boosted, after which the third channel region is boosted,
after which the second channel region is boosted, the first channel
region can be boosted in what is referred to as the source side
boosting, the third channel region can be boosted in time period
after what is referred to as the source side boosting and prior to
what is referred to as the drain side boosting, and the second
channel region can be boosted in what is referred to as the drain
side boosting.
[0088] FIG. 10 depicts a programming process in which a source side
of a NAND string is boosted before a drain side of the NAND string.
The process is illustrated in connection with the boosting scheme
of FIG. 8, although many variations are possible. Programming
begins at step 1000, and a word line is selected for programming at
step 1005. Source side boosting begins at step 1010. At step 1015,
V.sub.COND is set on the isolation word line (WLn-3) through the
furthest word line on the drain side of the isolation word line
which has been used for programming (WLn+1). At step 1020,
V.sub.PASS is set on the word lines on the source side of the
isolation word line. At step 1025, 0 V is set on the remaining
drain side word lines, e.g., WLn+2 through WLi and, at step 1030,
the source side boosting ends. That is, generally, the boosted
source side level is maintained but not boosted further. At step
1035, drain side boosting along with programming begins. The drain
side boosting may be initiated before the programming, as
illustrated previously. At step 1040, voltages are applied to the
unselected word lines in accordance with the selected boosting
mode. At step 1045, a programming pulse is applied to the selected
word line. The drain side boosting and the programming pulse end at
step 1050.
[0089] A verify operation is performed at step 1055 to determine
whether a selected storage element has been programmed to a desired
target threshold voltage level, e.g., Vva, Vvb or Vvc (FIG. 16). At
decision block 1060, if programming for the current word line is
not complete, an additional cycle of source side boosting followed
by drain side boosting and programming is repeated, starting at
step 1010. If programming for the current word line is complete but
programming for all word lines is not complete, at decision step
1065, the next word line is selected for programming at step 1075.
If programming for the current word line and all word lines is
complete, programming ends at step 1070.
[0090] Note that, in an alternative implementation, a word line
dependency may be used in which a boosting scheme which does not
use source side boosting followed by drain side boosting is used
for lower word lines, such as WL0-WL22 in a 32 word line NAND
string. A boosting scheme which does use source side boosting
followed by drain side boosting can then be used for higher word
lines, such as WL23-WL3 1, where the type of program disturb which
is addressed is more problematic.
[0091] FIG. 11 illustrates an example of an array 1100 of NAND
storage elements, such as those shown in FIGS. 1 and 2. Along each
column, a bit line 1106 is coupled to the drain terminal 1126 of
the drain select gate for the NAND string 1150. Along each row of
NAND strings, a source line 1104 may connect all the source
terminals 1128 of the source select gates of the NAND strings. An
example of a NAND architecture array and its operation as part of a
memory system is found in U.S. Pat. Nos. 5,570,315; 5,774,397; and
6,046,935.
[0092] The array of storage elements is divided into a large number
of blocks of storage elements. As is common for flash EEPROM
systems, the block is the unit of erase. That is, each block
contains the minimum number of storage elements that are erased
together. Each block is typically divided into a number of pages. A
page is a unit of programming. In one embodiment, the individual
pages may be divided into segments and the segments may contain the
fewest number of storage elements that are written at one time as a
basic programming operation. One or more pages of data are
typically stored in one row of storage elements. A page can store
one or more sectors. A sector includes user data and overhead data.
Overhead data typically includes an Error Correction Code (ECC)
that has been calculated from the user data of the sector. A
portion of the controller (described below) calculates the ECC when
data is being programmed into the array, and also checks it when
data is being read from the array. Alternatively, the ECCs and/or
other overhead data are stored in different pages, or even
different blocks, than the user data to which they pertain.
[0093] A sector of user data is typically 512 bytes, corresponding
to the size of a sector in magnetic disk drives. Overhead data is
typically an additional 16-20 bytes. A large number of pages form a
block, anywhere from 8 pages, for example, up to 32, 64, 128 or
more pages. In some embodiments, a row of NAND strings comprises a
block.
[0094] Memory storage elements are erased in one embodiment by
raising the p-well to an erase voltage (e.g., 14-22 V) for a
sufficient period of time and grounding the word lines of a
selected block while the source and bit lines are floating. Due to
capacitive coupling, the unselected word lines, bit lines, select
lines, and c-source are also raised to a significant fraction of
the erase voltage. A strong electric field is thus applied to the
tunnel oxide layers of selected storage elements and the data of
the selected storage elements are erased as electrons of the
floating gates are emitted to the substrate side, typically by
Fowler-Nordheim tunneling mechanism. As electrons are transferred
from the floating gate to the p-well region, the threshold voltage
of a selected storage element is lowered. Erasing can be performed
on the entire memory array, separate blocks, or another unit of
storage elements.
[0095] FIG. 12 is a block diagram of a non-volatile memory system
using single row/column decoders and read/write circuits. The
diagram illustrates a memory device 1296 having read/write circuits
for reading and programming a page of storage elements in parallel,
according to one embodiment of the present invention. Memory device
1296 may include one or more memory die 1298. Memory die 1298
includes a two-dimensional array of storage elements 1100, control
circuitry 1210, and read/write circuits 1265. In some embodiments,
the array of storage elements can be three dimensional. The memory
array 1100 is addressable by word lines via a row decoder 1230 and
by bit lines via a column decoder 1260. The read/write circuits
1265 include multiple sense blocks 1200 and allow a page of storage
elements to be read or programmed in parallel. Typically a
controller 1250 is included in the same memory device 1296 (e.g., a
removable storage card) as the one or more memory die 1298.
Commands and Data are transferred between the host and controller
1250 via lines 1220 and between the controller and the one or more
memory die 1298 via lines 1218.
[0096] The control circuitry 1210 cooperates with the read/write
circuits 1265 to perform memory operations on the memory array
1100. The control circuitry 1210 includes a state machine 1212, an
on-chip address decoder 1214, a boost control 1215 and a power
control module 1216. The state machine 1212 provides chip-level
control of memory operations. The on-chip address decoder 1214
provides an address interface between that used by the host or a
memory controller to the hardware address used by the decoders 1230
and 1260. The boost control 1215 can be used for setting a boost
mode, including determining a timing for initiating source side and
drain side boosting, as discussed herein. The power control module
1216 controls the power and voltages supplied to the word lines and
bit lines during memory operations.
[0097] In some implementations, some of the components of FIG. 12
can be combined. In various designs, one or more of the components
(alone or in combination), other than storage element array 1100,
can be thought of as a managing circuit. For example, one or more
managing circuits may include any one of or a combination of
control circuitry 1210, state machine 1212, decoders 1214/1260,
power control 1216, sense blocks 1200, read/write circuits 1265,
controller 1250, etc.
[0098] FIG. 13 is a block diagram of a non-volatile memory system
using dual row/column decoders and read/write circuits. Here,
another arrangement of the memory device 1296 shown in FIG. 12 is
provided. Access to the memory array 1100 by the various peripheral
circuits is implemented in a symmetric fashion, on opposite sides
of the array, so that the densities of access lines and circuitry
on each side are reduced by half. Thus, the row decoder is split
into row decoders 1230A and 1230B and the column decoder into
column decoders 1260A and 1260B. Similarly, the read/write circuits
are split into read/write circuits 1265A connecting to bit lines
from the bottom and read/write circuits 1265B connecting to bit
lines from the top of the array 1100. In this way, the density of
the read/write modules is essentially reduced by one half. The
device of FIG. 13 can also include a controller, as described above
for the device of FIG. 12.
[0099] FIG. 14 is a block diagram depicting one embodiment of a
sense block. An individual sense block 1200 is partitioned into a
core portion, referred to as a sense module 1280, and a common
portion 1290. In one embodiment, there will be a separate sense
module 1280 for each bit line and one common portion 1290 for a set
of multiple sense modules 1280. In one example, a sense block will
include one common portion 1290 and eight sense modules 1280. Each
of the sense modules in a group will communicate with the
associated common portion via a data bus 1272. For further details
refer to U.S. Patent Application Pub No. 2006/0140007, titled
"Non-Volatile Memory and Method with Shared Processing for an
Aggregate of Sense Amplifiers" published Jun. 29, 2006, and
incorporated herein by reference in its entirety.
[0100] Sense module 1280 comprises sense circuitry 1270 that
determines whether a conduction current in a connected bit line is
above or below a predetermined threshold level. Sense module 1280
also includes a bit line latch 1282 that is used to set a voltage
condition on the connected bit line. For example, a predetermined
state latched in bit line latch 1282 will result in the connected
bit line being pulled to a state designating program inhibit (e.g.,
1.5-3 V).
[0101] Common portion 1290 comprises a processor 1292, a set of
data latches 1294 and an I/O Interface 1296 coupled between the set
of data latches 1294 and data bus 1220. Processor 1292 performs
computations. For example, one of its functions is to determine the
data stored in the sensed storage element and store the determined
data in the set of data latches. The set of data latches 1294 is
used to store data bits determined by processor 1292 during a read
operation. It is also used to store data bits imported from the
data bus 1220 during a program operation. The imported data bits
represent write data meant to be programmed into the memory. I/O
interface 1296 provides an interface between data latches 1294 and
the data bus 1220.
[0102] During read or sensing, the operation of the system is under
the control of state machine 1212 that controls the supply of
different control gate voltages to the addressed storage element.
As it steps through the various predefined control gate voltages
corresponding to the various memory states supported by the memory,
the sense module 1280 may trip at one of these voltages and an
output will be provided from sense module 1280 to processor 1292
via bus 1272. At that point, processor 1292 determines the
resultant memory state by consideration of the tripping event(s) of
the sense module and the information about the applied control gate
voltage from the state machine via input lines 1293. It then
computes a binary encoding for the memory state and stores the
resultant data bits into data latches 1294. In another embodiment
of the core portion, bit line latch 1282 serves double duty, both
as a latch for latching the output of the sense module 1280 and
also as a bit line latch as described above.
[0103] It is anticipated that some implementations will include
multiple processors 1292. In one embodiment, each processor 1292
will include an output line (not depicted) such that each of the
output lines is wired-OR'd together. In some embodiments, the
output lines are inverted prior to being connected to the wired-OR
line. This configuration enables a quick determination during the
program verification process of when the programming process has
completed because the state machine receiving the wired-OR can
determine when all bits being programmed have reached the desired
level. For example, when each bit has reached its desired level, a
logic zero for that bit will be sent to the wired-OR line (or a
data one is inverted). When all bits output a data 0 (or a data one
inverted), then the state machine knows to terminate the
programming process. Because each processor communicates with eight
sense modules, the state machine needs to read the wired-OR line
eight times, or logic is added to processor 1292 to accumulate the
results of the associated bit lines such that the state machine
need only read the wired-OR line one time. Similarly, by choosing
the logic levels correctly, the global state machine can detect
when the first bit changes its state and change the algorithms
accordingly.
[0104] During program or verify, the data to be programmed is
stored in the set of data latches 1294 from the data bus 1220. The
program operation, under the control of the state machine,
comprises a series of programming voltage pulses applied to the
control gates of the addressed storage elements. Each programming
pulse is followed by a read back (verify) to determine if the
storage element has been programmed to the desired memory state.
Processor 1292 monitors the read back memory state relative to the
desired memory state. When the two are in agreement, the processor
1292 sets the bit line latch 1282 so as to cause the bit line to be
pulled to a state designating program inhibit. This inhibits the
storage element coupled to the bit line from further programming
even if programming pulses appear on its control gate. In other
embodiments the processor initially loads the bit line latch 1282
and the sense circuitry sets it to an inhibit value during the
verify process.
[0105] Data latch stack 1294 contains a stack of data latches
corresponding to the sense module. In one embodiment, there are
three data latches per sense module 1280. In some implementations
(but not required), the data latches are implemented as a shift
register so that the parallel data stored therein is converted to
serial data for data bus 1220, and vice versa. In the preferred
embodiment, all the data latches corresponding to the read/write
block of m storage elements can be linked together to form a block
shift register so that a block of data can be input or output by
serial transfer. In particular, the bank of r read/write modules is
adapted so that each of its set of data latches will shift data in
to or out of the data bus in sequence as if they are part of a
shift register for the entire read/write block.
[0106] Additional information about the structure and/or operations
of various embodiments of non-volatile storage devices can be found
in (1) U.S. Pat. No. 7,196,931, titled, "Non-Volatile Memory And
Method With Reduced Source Line Bias Errors," issued Mar. 27, 2007;
(2) U.S. Pat. No. 7,023,736, title "Non-Volatile Memory And Method
with Improved Sensing," issued Apr. 4, 2006; (3) U.S. Pat. No.
7,046,568, titled "Improved Memory Sensing Circuit And Method For
Low Voltage Operation," issued May 16, 2006; (4) U.S. Patent
Application Publication No. 2006/0221692, titled "Compensating for
Coupling During Read Operations of Non-Volatile Memory," published
Aug. 5, 2006; and (5) U.S. Patent Application Publication No.
20060158947, titled "Reference Sense Amplifier For Non-Volatile
Memory, published Jul. 20, 2006. All five of the immediately
above-listed patent documents are incorporated herein by reference
in their entirety.
[0107] FIG. 15 illustrates an example of an organization of a
memory array into blocks for an all bit line memory architecture or
for an odd-even memory architecture. Exemplary structures of memory
array 1100 are described. As one example, a NAND flash EEPROM is
described that is partitioned into 1,024 blocks. The data stored in
each block can be simultaneously erased. In one embodiment, the
block is the minimum unit of storage elements that are
simultaneously erased. In each block, in this example, there are
8,512 columns corresponding to bit lines BL0, BL1, . . . BL8511. In
one embodiment referred to as an all bit line (ABL) architecture
(architecture 1510), all the bit lines of a block can be
simultaneously selected during read and program operations. Storage
elements along a common word line and connected to any bit line can
be programmed at the same time.
[0108] In the example provided, four storage elements are connected
in series to form a NAND string. Although four storage elements are
shown to be included in each NAND string, more or less than four
can be used (e.g., 16, 32, 64 or another number). One terminal of
the NAND string is connected to a corresponding bit line via a
drain select gate (connected to select gate drain lines SGD), and
another terminal is connected to c-source via a source select gate
(connected to select gate source line SGS).
[0109] In another embodiment, referred to as an odd-even
architecture (architecture 1500), the bit lines are divided into
even bit lines (BLe) and odd bit lines (BLo). In the odd/even bit
line architecture, storage elements along a common word line and
connected to the odd bit lines are programmed at one time, while
storage elements along a common word line and connected to even bit
lines are programmed at another time. In each block, in this
example, there are 8,512 columns that are divided into even columns
and odd columns. In this example, four storage elements are shown
connected in series to form a NAND string. Although four storage
elements are shown to be included in each NAND string, more or
fewer than four storage elements can be used.
[0110] During one configuration of read and programming operations,
4,256 storage elements are simultaneously selected. The storage
elements selected have the same word line and the same kind of bit
line (e.g., even or odd). Therefore, 532 bytes of data, which form
a logical page, can be read or programmed simultaneously, and one
block of the memory can store at least eight logical pages (four
word lines, each with odd and even pages). For multi-state storage
elements, when each storage element stores two bits of data, where
each of these two bits are stored in a different page, one block
stores sixteen logical pages. Other sized blocks and pages can also
be used.
[0111] For either the ABL or the odd-even architecture, storage
elements can be erased by raising the p-well to an erase voltage
(e.g., 20 V) and grounding the word lines of a selected block. The
source and bit lines are floating. Erasing can be performed on the
entire memory array, separate blocks, or another unit of the
storage elements which is a portion of the memory device. Electrons
are transferred from the floating gates of the storage elements to
the p-well region so that the V.sub.TH of the storage elements
becomes negative.
[0112] In the read and verify operations, the select gates (SGD and
SGS) are connected to a voltage in a range of 2.5-4.5 V and the
unselected word lines (e.g., WL0, WL1 and WL3, when WL2 is the
selected word line) are raised to a read pass voltage, V.sub.READ,
(typically a voltage in the range of 4.5 to 6 V) to make the
transistors operate as pass gates. The selected word line WL2 is
connected to a voltage, a level of which is specified for each read
and verify operation in order to determine whether a V.sub.TH of
the concerned storage element is above or below such level. For
example, in a read operation for a two-level storage element, the
selected word line WL2 may be grounded, so that it is detected
whether the V.sub.TH is higher than 0 V. In a verify operation for
a two level storage element, the selected word line WL2 is
connected to 0.8 V, for example, so that it is verified whether or
not the V.sub.TH has reached at least 0.8 V. The source and p-well
are at 0 V. The selected bit lines, assumed to be the even bit
lines (BLe), are pre-charged to a level of, for example, 0.7 V. If
the V.sub.TH is higher than the read or verify level on the word
line, the potential level of the bit line (BLe) associated with the
storage element of interest maintains the high level because of the
non-conductive storage element. On the other hand, if the V.sub.TH
is lower than the read or verify level, the potential level of the
concerned bit line (BLe) decreases to a low level, for example,
less than 0.5 V, because the conductive storage element discharges
the bit line. The state of the storage element can thereby be
detected by a voltage comparator sense amplifier that is connected
to the bit line.
[0113] The erase, read and verify operations described above are
performed according to techniques known in the art. Thus, many of
the details explained can be varied by one skilled in the art.
Other erase, read and verify techniques known in the art can also
be used.
[0114] FIG. 16 depicts an example set of threshold voltage
distributions and one-pass programming. Example V.sub.TH
distributions for the storage element array are provided for a case
where each storage element stores two bits of data. A first
threshold voltage distribution E is provided for erased storage
elements. Three threshold voltage distributions, A, B and C for
programmed storage elements, are also depicted. In one embodiment,
the threshold voltages in the E distribution are negative and the
threshold voltages in the A, B and C distributions are
positive.
[0115] Each distinct threshold voltage range corresponds to
predetermined values for the set of data bits. The specific
relationship between the data programmed into the storage element
and the threshold voltage levels of the storage element depends
upon the data encoding scheme adopted for the storage elements. For
example, U.S. Pat. No. 6,222,762 and U.S. Patent Application
Publication No. 2004/0255090, published Dec. 16, 2004, both of
which are incorporated herein by reference in their entirety,
describe various data encoding schemes for multi-state flash
storage elements. In one embodiment, data values are assigned to
the threshold voltage ranges using a Gray code assignment so that
if the threshold voltage of a floating gate erroneously shifts to
its neighboring physical state, only one bit will be affected. One
example assigns "11" to threshold voltage range E (state E), "10"
to threshold voltage range A (state A), "00" to threshold voltage
range B (state B) and "01" to threshold voltage range C (state C).
However, in other embodiments, Gray code is not used. Although four
states are shown, the present invention can also be used with other
multi-state structures including those that include more or less
than four states.
[0116] Three read reference voltages, Vra, Vrb and Vrc, are also
provided for reading data from storage elements. By testing whether
the threshold voltage of a given storage element is above or below
Vra, Vrb and Vrc, the system can determine the state, e.g.,
programming condition, the storage element is in.
[0117] Further, three verify reference voltages, Vva, Vvb and Vvc,
are provided. When programming storage elements to state A, the
system will test whether those storage elements have a threshold
voltage greater than or equal to Vva. When programming storage
elements to state B, the system will test whether the storage
elements have threshold voltages greater than or equal to Vvb. When
programming storage elements to state C, the system will determine
whether storage elements have their threshold voltage greater than
or equal to Vvc.
[0118] In one embodiment, known as full sequence programming,
storage elements can be programmed from the erase state E directly
to any of the programmed states A, B or C. For example, a
population of storage elements to be programmed may first be erased
so that all storage elements in the population are in erased state
E. A series of programming pulses such as depicted by the control
gate voltage sequence of FIG. 20 will then be used to program
storage elements directly into states A, B or C. While some storage
elements are being programmed from state E to state A, other
storage elements are being programmed from state E to state B
and/or from state E to state C. When programming from state E to
state C on WLn, the amount of parasitic coupling to the adjacent
floating gate under WLn-1 reaches a maximum since the change in
amount of charge on the floating gate under WLn is the largest as
compared to the change in charge when programming from state E to
state A or state E to state B. When programming from state E to
state B the amount of coupling to the adjacent floating gate is
less. When programming from state E to state A the amount of
coupling is reduced even further.
[0119] FIG. 17 illustrates an example of a two-pass technique of
programming a multi-state storage element that stores data for two
different pages: a lower page and an upper page. Four states are
depicted: state E (11), state A (10), state B (00) and state C
(01). For state E, both pages store a "1." For state A, the lower
page stores a "0" and the upper page stores a "1." For state B,
both pages store "0." For state C, the lower page stores "1" and
the upper page stores "0." Note that although specific bit patterns
have been assigned to each of the states, different bit patterns
may also be assigned.
[0120] In a first programming pass, the storage element's threshold
voltage level is set according to the bit to be programmed into the
lower logical page. If that bit is a logic "1," the threshold
voltage is not changed since it is in the appropriate state as a
result of having been earlier erased. However, if the bit to be
programmed is a logic "0," the threshold level of the storage
element is increased to be state A, as shown by arrow 1700. That
concludes the first programming pass.
[0121] In a second programming pass, the storage element's
threshold voltage level is set according to the bit being
programmed into the upper logical page. If the upper logical page
bit is to store a logic "1," then no programming occurs since the
storage element is in one of the states E or A, depending upon the
programming of the lower page bit, both of which carry an upper
page bit of "1." If the upper page bit is to be a logic "0," then
the threshold voltage is shifted. If the first pass resulted in the
storage element remaining in the erased state E, then in the second
phase the storage element is programmed so that the threshold
voltage is increased to be within state C, as depicted by arrow
1720. If the storage element had been programmed into state A as a
result of the first programming pass, then the storage element is
further programmed in the second pass so that the threshold voltage
is increased to be within state B, as depicted by arrow 1710. The
result of the second pass is to program the storage element into
the state designated to store a logic "0" for the upper page
without changing the data for the lower page. In both FIG. 16 and
FIG. 17, the amount of coupling to the floating gate on the
adjacent word line depends on the final state.
[0122] In one embodiment, a system can be set up to perform full
sequence writing if enough data is written to fill up an entire
page. If not enough data is written for a full page, then the
programming process can program the lower page programming with the
data received. When subsequent data is received, the system will
then program the upper page. In yet another embodiment, the system
can start writing in the mode that programs the lower page and
convert to full sequence programming mode if enough data is
subsequently received to fill up an entire (or most of a) word
line's storage elements. More details of such an embodiment are
disclosed in U.S. Patent Application Pub. No. 2006/0126390, titled
"Pipelined Programming of Non-Volatile Memories Using Early Data,"
published Jun. 15, 2006, incorporated herein by reference in its
entirety.
[0123] FIGS. 18a-c disclose another process for programming
non-volatile memory that reduces the effect of floating gate to
floating gate coupling by, for any particular storage element,
writing to that particular storage element with respect to a
particular page subsequent to writing to adjacent storage elements
for previous pages. In one example implementation, the non-volatile
storage elements store two bits of data per storage element, using
four data states. For example, assume that state E is the erased
state and states A, B and C are the programmed states. State E
stores data 11. State A stores data 01. State B stores data 10.
State C stores data 00. This is an example of non-Gray coding
because both bits change between adjacent states A and B. Other
encodings of data to physical data states can also be used. Each
storage element stores two pages of data. For reference purposes,
these pages of data will be called upper page and lower page;
however, they can be given other labels. With reference to state A,
the upper page stores bit 0 and the lower page stores bit 1. With
reference to state B, the upper page stores bit 1 and the lower
page stores bit 0. With reference to state C, both pages store bit
data 0.
[0124] The programming process is a two-step process. In the first
step, the lower page is programmed. If the lower page is to remain
data 1, then the storage element state remains at state E. If the
data is to be programmed to 0, then the threshold of voltage of the
storage element is raised such that the storage element is
programmed to state B'. FIG. 18a therefore shows the programming of
storage elements from state E to state B'. State B' is an interim
state B; therefore, the verify point is depicted as Vvb', which is
lower than Vvb.
[0125] In one embodiment, after a storage element is programmed
from state E to state B', its neighbor storage element (WLn+1) in
the NAND string will then be programmed with respect to its lower
page. For example, looking back at FIG. 2, after the lower page for
storage element 106 is programmed, the lower page for storage
element 104 would be programmed. After programming storage element
104, the floating gate to floating gate coupling effect will raise
the apparent threshold voltage of storage element 106 if storage
element 104 had a threshold voltage raised from state E to state
B'. This will have the effect of widening the threshold voltage
distribution for state B' to that depicted as threshold voltage
distribution 1850 of FIG. 18b. This apparent widening of the
threshold voltage distribution will be remedied when programming
the upper page.
[0126] FIG. 18c depicts the process of programming the upper page.
If the storage element is in erased state E and the upper page is
to remain at 1, then the storage element will remain in state E. If
the storage element is in state E and its upper page data is to be
programmed to 0, then the threshold voltage of the storage element
will be raised so that the storage element is in state A. If the
storage element was in intermediate threshold voltage distribution
1850 and the upper page data is to remain at 1, then the storage
element will be programmed to final state B. If the storage element
is in intermediate threshold voltage distribution 1850 and the
upper page data is to become data 0, then the threshold voltage of
the storage element will be raised so that the storage element is
in state C. The process depicted by FIGS. 18a-c reduces the effect
of floating gate to floating gate coupling because only the upper
page programming of neighbor storage elements will have an effect
on the apparent threshold voltage of a given storage element. An
example of an alternate state coding is to move from distribution
1850 to state C when the upper page data is a 1, and to move to
state B when the upper page data is a 0.
[0127] Although FIGS. 18a-c provide an example with respect to four
data states and two pages of data, the concepts taught can be
applied to other implementations with more or fewer than four
states and more or less than two pages.
[0128] FIG. 19 is a flow chart describing one embodiment of a
method for programming non-volatile memory. In one implementation,
storage elements are erased (in blocks or other units) prior to
programming. In step 1900, a "data load" command is issued by the
controller and input received by control circuitry 1210. In step
1905, address data designating the page address is input to decoder
1214 from the controller or host. In step 1910, a page of program
data for the addressed page is input to a data buffer for
programming. That data is latched in the appropriate set of
latches. In step 1915, a "program" command is issued by the
controller to state machine 1212.
[0129] Triggered by the "program" command, the data latched in step
1910 will be programmed into the selected storage elements
controlled by state machine 1212 using the stepped program pulses
of the pulse train 2000 of FIG. 20 applied to the appropriate
selected word line. In step 1920, the program voltage, V.sub.PGM,
is initialized to the starting pulse (e.g., 12 V or other value)
and a program counter (PC) maintained by state machine 1212 is
initialized at zero. In step 1925, source boosting is applied, as
discussed previously. At step 1930, the first V.sub.PGM pulse is
applied to the selected word line to begin programming storage
elements associated with the selected word line, and drain side
boosting occurs, as discussed previously. If logic "0" is stored in
a particular data latch indicating that the corresponding storage
element should be programmed, then the corresponding bit line is
grounded. On the other hand, if logic "1" is stored in the
particular latch indicating that the corresponding storage element
should remain in its current data state, then the corresponding bit
line is connected to 1.5-3 V to inhibit programming.
[0130] In step 1935, the states of the selected storage elements
are verified. If it is detected that the target threshold voltage
of a selected storage element has reached the appropriate level,
then the data stored in the corresponding data latch is changed to
a logic "1." If it is detected that the threshold voltage has not
reached the appropriate level, the data stored in the corresponding
data latch is not changed. In this manner, a bit line having a
logic "1" stored in its corresponding data latch does not need to
be programmed. When all of the data latches are storing logic "1,"
the state machine (via the wired-OR type mechanism described above)
knows that all selected storage elements have been programmed. In
step 1940, a check is made as to whether all of the data latches
are storing logic "1." If all of the data latches are storing logic
"1," the programming process is complete and successful because all
selected storage elements were programmed and verified. A status of
"PASS" is reported in step 1945. In some embodiments, the
programming process is considered complete and successful even if
not all selected storage elements were verified as being
programmed. In such a case, errors during subsequent read
operations can occur due to insufficient programmed storage
elements. However, these errors can be corrected by ECC.
[0131] If, in step 1940, it is determined that not all of the data
latches are storing logic "1," then the programming process
continues. In some embodiments, the program process stops even if
not all of the data latches are storing logic "1". In step 1950,
the program counter PC is checked against a program limit value
PCmax. One example of a program limit value is twenty; however,
other numbers can also be used. If the program counter PC is not
less than PCmax, then the program process has failed and a status
of "FAIL" is reported in step 1955. If the program counter PC is
less than PCmax, then V.sub.PGM is increased by the step size and
the program counter PC is incremented in step 1960. The process
then loops back to step 1930 to apply the next V.sub.PGM pulse.
[0132] FIG. 20 depicts an example pulse train 2000 applied to the
control gates of non-volatile storage elements during programming,
and a switch in boost mode which occurs during a pulse train. The
pulse train 2000 includes a series of program pulses 2005, 2010,
2015, 2020, 2025, 2030, 2035, 2040, 2045, 2050, . . . , that are
applied to a word line selected for programming. In one embodiment,
the programming pulses have a voltage, V.sub.PGM, which starts at
12 V and increases by increments, e.g., 0.5 V, for each successive
programming pulse until a maximum of, e.g., 20-25 V is reached. In
between the program pulses are verify pulses. For example, verify
pulse set 2006 includes three verify pulses. In some embodiments,
there can be a verify pulse for each state that data is being
programmed into, e.g., state A, B and C. In other embodiments,
there can be more or fewer verify pulses. The verify pulses in each
set can have amplitudes of Vva, Vvb and Vvc (FIG. 17) or Vvb' (FIG.
18a), for instance.
[0133] As mentioned, the voltages which are applied to word lines
to implement a boost mode are applied when programming occurs,
e.g., prior to and during a program pulse. On the other hand,
during the verify process, for instance, which occurs between
program pulses, the boost voltages are not applied. Instead, read
voltages, which are typically less than the boost voltages, are
applied to the unselected word lines. The read voltages have an
amplitude which is sufficient to open the previously programmed
storage elements in a NAND string when the threshold voltage of a
currently-programmed storage element is being compared to a verify
level.
[0134] The foregoing detailed description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed. Many modifications and variations are possible in
light of the above teaching. The described embodiments were chosen
in order to best explain the principles of the invention and its
practical application, to thereby enable others skilled in the art
to best utilize the invention in various embodiments and with
various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be
defined by the claims appended hereto.
* * * * *