U.S. patent application number 14/152848 was filed with the patent office on 2014-12-18 for pseudo block operation mode in 3d nand.
This patent application is currently assigned to SanDisk Technologies Inc.. The applicant listed for this patent is SanDisk Technologies Inc.. Invention is credited to Chris Avila, Xiying Costa, Gautam Dusija, Alexander Kwok-Tung Mak, Man Mui.
Application Number | 20140369122 14/152848 |
Document ID | / |
Family ID | 52015294 |
Filed Date | 2014-12-18 |
United States Patent
Application |
20140369122 |
Kind Code |
A1 |
Costa; Xiying ; et
al. |
December 18, 2014 |
PSEUDO BLOCK OPERATION MODE IN 3D NAND
Abstract
A 3D NAND stacked non-volatile memory device, comprising: a
string comprising a plurality of non-volatile storage elements, the
string comprises a channel and extends vertically through layers of
the 3D stacked non-volatile memory device, and the plurality of
storage elements are subdivided into different groups based on
group assignments, each group of the different groups comprises
multiple adjacent storage elements of the plurality of storage
elements; and a control circuit in communication with the string,
the control circuit, to perform a Pseudo Block Operation Mode.
Inventors: |
Costa; Xiying; (San Jose,
CA) ; Mak; Alexander Kwok-Tung; (Los Altos Hills,
CA) ; Avila; Chris; (Saratoga, CA) ; Dusija;
Gautam; (San Jose, CA) ; Mui; Man; (Santa
Clara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SanDisk Technologies Inc. |
Plano |
TX |
US |
|
|
Assignee: |
SanDisk Technologies Inc.
Plano
TX
|
Family ID: |
52015294 |
Appl. No.: |
14/152848 |
Filed: |
January 10, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61835259 |
Jun 14, 2013 |
|
|
|
Current U.S.
Class: |
365/185.11 |
Current CPC
Class: |
G11C 16/0483 20130101;
G11C 16/14 20130101; G11C 16/107 20130101; G11C 16/0408 20130101;
G11C 16/10 20130101; G11C 16/16 20130101 |
Class at
Publication: |
365/185.11 |
International
Class: |
G11C 16/10 20060101
G11C016/10; G11C 16/16 20060101 G11C016/16; G11C 16/04 20060101
G11C016/04 |
Claims
1. A three-dimensional (3D) NAND stacked non-volatile memory
device, comprising: a plurality of NAND strings in a block, each
NAND string comprising a plurality of non-volatile storage
elements, each NAND string comprising a channel that extends
vertically through layers of the 3D NAND stacked non-volatile
memory device, wherein the block comprises pseudo blocks that each
comprise a portion of each of the NAND strings, wherein each of the
NAND strings is a member of each of the pseudo blocks; and a
control circuit in communication with the NAND strings, the control
circuit is configured to operate in a pseudo block operation mode
that performs operations on the plurality of NAND strings on a
pseudo block level.
2. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit configured to operate in the pseudo
block operation mode comprises the control circuit configured to:
erase all non-volatile storage elements in a first pseudo block in
the block without erasing storage elements in any of the other
pseudo blocks in the block, wherein non-volatile storage elements
in at least one of the other pseudo blocks in the block remain
programmed after erasing all the non-volatile storage elements in
the first pseudo block.
3. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit configured to operate in the pseudo
block operation mode comprises the control circuit configured to:
erase all non-volatile storage elements on a first NAND string of
the plurality of NAND strings that are in a first pseudo block in
the block without erasing any storage elements on the first NAND
string that are in any of the other pseudo blocks in the block.
4. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit configured to operate in the pseudo
block operation mode comprises the control circuit configured to:
program all storage elements in a first pseudo block in the block
prior to programming any storage element in at least one other
pseudo block in the block.
5. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit configured to operate in the pseudo
block operation mode comprises the control circuit configured to:
erase the storage elements using different starting erase voltages
for different pseudo blocks in the block.
6. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit configured to operate in the pseudo
block operation mode comprises the control circuit configured to:
for each of the pseudo blocks in the block, record access times to
read the non-volatile storage elements in the respective pseudo
block; and determine whether the access time for each respective
pseudo block exceeds a limit for the respective pseudo block.
7. The 3D NAND stacked non-volatile memory device of claim 6,
wherein at least two of the pseudo blocks have a different
limit.
8. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit configured to operate in the pseudo
block operation mode comprises the control circuit configured to:
for each of the pseudo blocks in the block, record the number of
program/erase cycles for the respective pseudo block; and determine
whether the number of program/erase cycles for each respective
pseudo block exceeds a limit for the respective pseudo block.
9. The 3D NAND stacked non-volatile memory device of claim 8,
wherein at least two of the pseudo blocks have a different
limit.
10. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit configured to operate in the pseudo
block operation mode comprises the control circuit configured to:
apply a boosting voltage to a control gate of non-volatile storage
elements in the block that are not selected for programming whose
magnitude depends on which pseudo block that the non-volatile
storage element to which the boosting voltage is applied is in.
11. The 3D NAND stacked non-volatile memory device of claim 1,
wherein each of the non-volatile storage elements is characterized
by a memory hole size, the plurality of storage elements are
associated with the pseudo blocks based on memory hole size.
12. The 3D NAND stacked non-volatile memory device of claim 1,
wherein the control circuit is configured to subdivide the
non-volatile storage elements in the block into the pseudo blocks
based on a program speed of the respective non-volatile storage
elements.
13. A method of operating a three-dimensional (3D) NAND stacked
non-volatile memory device comprising a plurality of NAND strings
in a block, each NAND string comprising a plurality of non-volatile
storage elements, each NAND string comprising a channel that
extends vertically through layers of the 3D NAND stacked
non-volatile memory device, wherein the block comprises pseudo
blocks that each comprise a portion of each of the NAND strings,
wherein each of the NAND strings is a member of each of the pseudo
blocks, the method comprising: performing operations associated
with the plurality of NAND strings in the Mock on a pseudo block
level.
14. The method of claim 13, wherein performing operations on a
pseudo block level comprises: erasing non-volatile storage elements
in a first pseudo block in the block without erasing storage
elements in any of the other pseudo blocks in the block, including
erasing all non-volatile storage elements on a first NAND string of
the plurality of NAND strings that are in a first pseudo block in
the block without erasing any storage elements on the first NAND
string that are in any of the other pseudo blocks in the block,
wherein non-volatile storage elements on the first NAND string in
at least one of the other pseudo blocks in the block remain
programmed after erasing all the non-volatile storage elements on
the first NAND string in the first pseudo block.
15. The method of claim 13, wherein the block is associated with a
plurality of bit lines, a group of the plurality of NAND strings
are associated with a first bit line of the bit lines, the group
includes a first NAND string, wherein performing operations on a
pseudo block level comprises: programming the storage elements
associated with the first NAND string that in a first pseudo block
in the block without programming any storage elements on the first
NAND string in the other pseudo blocks in the block; and then
programming the storage elements associated with remaining NAND
strings associated with the first bit line that are in the first
pseudo block without programming any storage elements on the
remaining NAND strings in the other pseudo blocks in the block.
16. The method of claim 13, wherein performing operations on a
pseudo block level comprises: erasing the storage elements using
different starting erase voltages for different pseudo blocks in
the block.
17. The method of claim 13, wherein performing operations on a
pseudo block level comprises: for each of the pseudo blocks in the
block, recording access times to read the non-volatile storage
elements in the respective pseudo block; and determining whether
the access time for each respective pseudo block exceeds a limit
for the respective pseudo block, wherein at least two of the pseudo
blocks have a different limit for the access time.
18. The method of claim 13, wherein performing operations on a
pseudo block level comprises: for each of the pseudo blocks in the
block, recording the number of program/erase cycles for the
respective pseudo block; and determining whether the number of
program/erase cycles for each respective pseudo block exceeds a
limit for the respective pseudo block, wherein at least two of the
pseudo blocks have a different limit for the program/erase
cycles.
19. The method of claim 13, wherein performing operations on a
pseudo block level comprises: applying a boosting voltage to a
control gate of non-volatile storage elements in the block that are
not selected for programming whose magnitude depends on which
pseudo block that the non-volatile storage element to which the
boosting voltage is applied is in.
20. The method of claim 13, further comprising subdividing the
non-volatile storage elements in the block into the pseudo blocks
based on a programming speed of the respective non-volatile storage
elements.
21. A three dimensional (3D) NAND stacked non-volatile memory
device, comprising: a NAND string comprising a plurality of
non-volatile storage elements, the NAND string comprises a channel
that extends vertically through layers of the 3D NAND stacked
non-volatile memory device; and a control circuit in communication
with the NAND string, wherein the control circuit is configured to
perform an erase of a portion of the NAND string while leaving
other non-volatile storage elements on the NAND string
programmed.
22. The 3D NAND stacked non-volatile memory device of claim 21,
wherein the control circuit is configured to apply voltages to
control gates of non-volatile storage elements that are unselected
for erase that generates GIDL in the channel.
23. A three dimensional (3D) NAND stacked non-volatile memory
device, comprising: a NAND string comprising a plurality of
non-volatile storage elements, the NAND string comprises a channel
that extends vertically through layers of the 3D NAND stacked
non-volatile memory device, wherein the NAND string comprises
memory cells that reside in a memory hole in the layers; and a
control circuit in communication with the NAND string, the control
circuit is configured to apply a boosting voltage to a control gate
of an unselected non-volatile storage element on the NAND string
that is not selected for programming, a magnitude of the boosting
voltage depends on location of the unselected non-volatile storage
element on the NAND string, wherein a magnitude of the boosting
voltage for a given memory cell depends on a size of the memory
hole associated with the given memory cell.
24. The 3D NAND stacked non-volatile memory device of claim 23,
wherein the magnitude of the boosting voltage is larger for memory
cells associated with larger memory holes.
25. The 3D NAND stacked non-volatile memory device of claim 23,
wherein the magnitude of the boosting voltage is progressively
smaller as the size of the memory hole is smaller.
26. The three-dimensional stacked non-volatile memory device of
claim 1, wherein the control circuit configured to operate in the
pseudo block operation mode comprises the control circuit
configured to: perform an operation on a first of the pseudo blocks
and not on other pseudo blocks associated with the plurality of
NAND strings in the block.
27. The three-dimensional stacked non-volatile memory device of
claim 1, wherein the control circuit configured to operate in the
pseudo block operation mode comprises the control circuit
configured to: perform an operation with respect to a first
non-volatile storage element of a first of the plurality of NAND
strings that is dependent on the pseudo block with which the first
non-volatile storage element is associated.
28. The method of operating a three-dimensional stacked of claim
13, wherein the performing operations associated with the plurality
of NAND strings in the block on the pseudo block level comprises:
performing an operation on a first of the pseudo blocks and not on
other pseudo blocks associated with the plurality of NAND strings
in the block.
29. The method of operating a three-dimensional stacked of claim
13, wherein the performing operations associated with the plurality
of NAND strings in the block on the pseudo block level comprises:
performing an operation with respect to a first non-volatile
storage element of a first of the plurality of NAND strings that is
dependent on the pseudo block in which the first non-volatile
storage element resides.
Description
PRIORITY
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/835,259, entitled "Pseudo Block Operation Mode
in 3D NAND," by Costa, filed on Jun. 14, 2013, incorporated herein
by reference.
BACKGROUND
[0002] The present disclosure relates to techniques for operating
memory cells in a 3D-non-volatile memory device.
[0003] Recently, ultra high density storage devices have been
proposed using a 3D stacked memory structure sometimes referred to
as a Bit Cost Scalable (BiCS) architecture. For example, a 3D NAND
stacked memory device can be formed from an array of alternating
conductive and dielectric layers. A memory hole is drilled in the
layers to define many memory layers simultaneously. A NAND string
is then formed by filling the memory hole with appropriate
materials. A straight NAND string extends in one memory hole, while
a pipe- or U-shaped NAND string (P-BiCS) includes a pair of
vertical columns of memory cells which extend in two memory holes
and which are joined by a bottom back gate. Control gates of the
memory cells are provided by the conductive layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Like-numbered elements refer to common components in the
different figures.
[0005] FIG. 1A is a perspective view of a 3D stacked non-volatile
memory device.
[0006] FIG. 1B is a functional block diagram of the 3D stacked
non-volatile memory device of FIG. 1A.
[0007] FIG. 2A depicts a top view of a U-shaped NAND embodiment of
a block.
[0008] FIG. 2B depicts a cross-sectional view of the portion 210 of
the block 200 of FIG. 2A, along line 220.
[0009] FIG. 2C depicts an embodiment of a block that includes
U-shaped NAND strings.
[0010] FIG. 3A depicts a close-up view of the region 236 of the
column C0 of FIG. 2C, showing a drain-side select gate SGD0 and a
memory cell MC303.
[0011] FIG. 3B depicts a cross-sectional view of the column C0 of
FIG. 3A.
[0012] FIG. 4A depicts a top view of a straight NAND string
embodiment of the block BLK0 of FIG. 1A.
[0013] FIG. 4B depicts a cross-sectional view of the portion 488 of
the block 480 of FIG. 4A along line 486.
[0014] FIG. 4C depicts an embodiment of a block which includes
straight NAND strings.
[0015] FIG. 5A depicts an example assignment of storage elements to
pseudo-blocks for U-shaped NAND strings.
[0016] FIG. 5B depicts another example assignment of storage
elements to pseudo-blocks along U-shaped NAND strings.
[0017] FIG. 5C is one example of how straight NAND strings in a 3D
stacked memory device can be divided into pseudo blocks.
[0018] FIG. 5D is a flowchart of one embodiment of a process of
operating a 3D stacked memory device in a pseudo block operation
mode.
[0019] FIG. 6A is a flowchart of an overview of a pseudo block
program and erase operation.
[0020] FIGS. 6B-6D show further details of erasing and
programming
[0021] FIG. 6E depicts a local potential drop in a channel of a
string in an erase operation based on a distance from a driven end
of the string.
[0022] FIG. 6F depicts variations in an erase depth along a string
based on variations in memory hole diameter along the string.
[0023] FIG. 6G depicts example starting erase voltages for
different pseudo blocks.
[0024] FIG. 6H is a flowchart of one embodiment of a process of
performing a pseudo block erase operation.
[0025] FIG. 7A-7C depicts example voltages during the erase portion
of an erase-verify iteration of an erase operation.
[0026] FIGS. 8A-8C depict voltages in the verify portion of an
erase-verify iteration of an erase operation.
[0027] FIG. 9A depicts the movement of holes and electrons in a
U-shaped NAND string during a two-sided erase.
[0028] FIG. 9B depicts the movement of holes and electrons in a
U-shaped NAND string during a one-sided erase.
[0029] FIG. 10A depicts one embodiment of programming a pseudo
block in pseudo block mode.
[0030] FIG. 10B is a flowchart of one embodiment of a process of
grouping word lines into pseudo blocks.
[0031] FIG. 10C is a flowchart of one embodiment of a process of
grouping word lines into pseudo blocks.
[0032] FIG. 10D is a flowchart describing one embodiment of a
programming process 900, which includes one or more verification
steps.
[0033] FIG. 11A shows two curves that represent how read disturb
can be worse for smaller memory holes.
[0034] FIG. 11B shows read access time versus memory hole size for
two different read voltages.
[0035] FIG. 11C is a flowchart of one embodiment of a process of a
read disturb pseudo operation mode.
[0036] FIG. 11D and FIG. 11E are flowcharts depicting two different
embodiments for performing a pseudo block read disturb
operation.
[0037] FIG. 12A shows expected endurance characteristics as a
function of memory hole size.
[0038] FIG. 12B is a flowchart of one embodiment of a process of a
pseudo block endurance mode of operation.
[0039] FIG. 12C is a flowchart for one embodiment of a process of
pseudo block endurance mode of operation.
[0040] FIG. 13A shows a relationship between boosted channel
potential versus memory hole size for one example 3D memory
device.
[0041] FIG. 13B shows V.sub.PASS disturb versus memory hole size
for several values of V.sub.PASS.
[0042] FIG. 13C is a flowchart of one embodiment of a process of a
pseudo block dependent V.sub.PASS operation.
DETAILED DESCRIPTION
[0043] Techniques are provided for operating storage elements in a
3D stacked non-volatile memory device. The memory device is
operated in a pseudo block operation mode in embodiments. The
device includes vertically oriented NAND strings, in embodiments. A
given NAND string is a part of multiple different pseudo blocks, in
accordance with embodiments. Given that the NAND strings may be
vertically oriented in different layers of the 3D stacked memory
device, this results in the pseudo block assignment being based on
which layer a storage element is in. A pseudo block typically
contains (parts of) multiple NAND strings.
[0044] In some 3D stacked non-volatile memory devices the storage
elements are formed in "memory holes" that are drilled in the
stacked structure. The diameter of a memory hole may be narrower at
the lower layers, for at least some fabrication techniques. This
narrowing of the memory holes can have an impact on a variety of
performance factors. In one embodiment, the pseudo blocks are
assigned based on the size of the memory holes. However, note that
even if the memory holes at different layers have the same size,
pseudo block operation can still be used, for at least some
operations.
[0045] Various operations are possible in a pseudo block operation
mode. Those operations include, but are not limited to, pseudo
block program and erase operation, pseudo block read disturb
operation, pseudo block endurance operation, and pseudo block
boosting voltage (V.sub.PASS) operation.
[0046] As noted, a given NAND string may reside in different pseudo
blocks. In one embodiment, storage elements on a NAND string in a
given pseudo block are erased together. However, storage elements
on that NAND string that are in other pseudo blocks are not erased
at that time. If those other storage elements are currently in a
programmed state, they remain in the programmed state when the
other portion of the NAND string is erased. In other words, storage
elements in one pseudo block are erased while storage element in
other pseudo blocks may remain programmed.
[0047] One benefit of such an operation is that the size of the
unit that is erased/programmed can be more convenient for operation
of the memory device, as compared to erasing/programming units that
are based on entire NAND strings. For example, the pseudo blocks
permit smaller units to be erased/programmed, which increases the
flexibility from the perspective of system applications.
[0048] Another benefit of such an operation is that different
program parameters and/or different erase parameters can be used
for different pseudo blocks. For example, pseudo blocks at
different levels of the 3D stacked memory device can have different
program and/or erase conditions. This can reduce erase time. This
can also reduce the maximum erase voltage, which can reduce stress
on the storage elements. The stress reduction can improve storage
element reliability and improve data retention.
[0049] It is possible for read disturb to have a different impact
on storage elements at different levels of the 3D stacked memory
device. As one possibility, storage elements that were formed in
smaller memory holes could suffer more severe read disturb,
compared to those formed in larger memory holes. In one embodiment
of pseudo block read disturb operation, a metric that describes
total read time of storage elements is kept for each pseudo block.
This metric could simply be the number of times the pseudo block
was read. When the metric reaches a threshold, the pseudo block
could be re-programmed, marked as bad, etc. Pseudo blocks at
different levels of the 3D stacked memory device can be allotted
different maximum allowed reads. This allows the impact of read
disturb to be minimized without relaxing the read disturb
specification for all storage elements.
[0050] It is possible for endurance of storage elements at
different levels of the 3D stacked memory device to be different.
Endurance here is referring to the number of erase/program cycles.
One possible explanation for this is that endurance may be a
function of memory hole size, which as noted, may itself be a
function of the layer. For some devices, storage elements that were
formed in a smaller memory hole have better endurance. In one
embodiment of pseudo block endurance operation, the number of
erase/program cycles is kept for each pseudo block. A pseudo block
may be marked as failed when a maximum allowed number of
erase/program cycles is reached. Pseudo blocks at different levels
of the 3D stacked memory device may have different endurance
thresholds. This allows the impact of storage elements having
different endurance to be minimized without relaxing the endurance
specification for all storage elements.
[0051] For some programming operations, the channel potentials of
unselected NAND strings are boosted to prevent program disturb.
This can involve applying a boosting voltage to the control gate of
unselected storage elements. The ability of the boosting voltage to
boost the channel potential may be a function of the layer of the
3D stacked memory device. For some devices, the boosting ability is
worse if the size of the memory hole is larger. On the other hand,
boosting voltage disturb can be worse for smaller memory holes. In
one embodiment, the magnitude of a boosting voltage depends on the
pseudo block. For example, pseudo blocks at lower layers (and
having smaller memory holes) may have a lower boosting voltage.
[0052] FIG. 1A is a perspective view of a 3D stacked non-volatile
memory device. The memory device 100 includes a substrate 101. On
the substrate are example blocks BLK0 and BLK1 of memory cells and
a peripheral area 104 with circuitry for use by the blocks. The
substrate 101 can also carry circuitry under the blocks, along with
one or more lower metal layers which are patterned in conductive
paths to carry signals of the circuitry. The blocks are formed in
an intermediate region 102 of the memory device. In an upper region
103 of the memory device, one or more upper metal layers are
patterned in conductive paths to carry signals of the circuitry.
Each block comprises a stacked area of memory cells, where
alternating levels of the stack represent word lines. In one
possible approach, each block has opposing tiered sides from which
vertical contacts extend upward to an upper metal layer to form
connections to conductive paths. While two blocks are depicted as
an example, additional blocks can be used, extending in the x-
and/or y-directions.
[0053] In one possible approach, the length of the plane, in the
x-direction, represents a direction in which signal paths to word
lines extend in the one or more upper metal layers (a word line or
SGD line direction), and the width of the plane, in the
y-direction, represents a direction in which signal paths to bit
lines extend in the one or more upper metal layers (a bit line
direction). The z-direction represents a height of the memory
device.
[0054] FIG. 1B is a functional block diagram of the 3D stacked
non-volatile memory device 100 of FIG. 1A. The memory device 100
may include one or more memory die 108. The memory die 108 includes
a 3D (three-dimensional) memory array 126 of storage elements,
e.g., including the blocks BLK0 and BLK1, control circuitry 110,
and read/write circuits 128. The memory array 126 is addressable by
word lines via a row decoder 124 and by bit lines via a column
decoder 132. The read/write circuits 128 include multiple sense
blocks 130 (sensing circuitry) and allow a page of storage elements
to be read or programmed in parallel. Typically a controller 122 is
included in the same memory device 100 (e.g., a removable storage
card) as the one or more memory die 108. Commands and data are
transferred between the host and controller 122 via lines 120 and
between the controller and the one or more memory die 108 via lines
118.
[0055] The control circuitry 110 cooperates with the read/write
circuits 128 to perform memory operations on the memory array 126,
and includes a state machine 112, an on-chip address decoder 114,
and a power control module 116. The state machine 112 provides
chip-level control of memory operations. The on-chip address
decoder 114 provides an address interface between that used by the
host or a memory controller to the hardware address used by the
decoders 124 and 132. The power control module 116 controls the
power and voltages supplied to the word lines and bit lines during
memory operations. It can includes drivers for word line layers and
word line layer portions, drain- and source-side select gate
drivers (referring, e.g., to drain- and source-sides or ends of a
string of memory cells such as a NAND string, for instance) and
source lines. The sense blocks 130 can include bit line drivers, in
one approach.
[0056] In some implementations, some of the components can be
combined. In various designs, one or more of the components (alone
or in combination), other than memory array 126, can be thought of
as at least one control circuit. For example, a control circuit may
include any one of, or a combination of, control circuitry 110,
state machine 112, decoders 114/132, power control module 116,
sense blocks 130, read/write circuits 128, and controller 122, and
so forth.
[0057] In another embodiment, a non-volatile memory system uses
dual row/column decoders and read/write circuits. Access to the
memory array 126 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 two row decoders and the
column decoder into two column decoders. Similarly, the read/write
circuits are split into read/write circuits connecting to bit lines
from the bottom and read/write circuits connecting to bit lines
from the top of the memory array 126. In this way, the density of
the read/write modules is reduced by one half.
[0058] Other types of non-volatile memory in addition to NAND flash
memory can also be used.
[0059] FIG. 2A depicts a top view of a U-shaped NAND embodiment of
a block 200, as an example implementation of BLK0 in FIG. 1A. The
view is of a representative layer among the multiple word line
layers in a stack. Referring also to FIG. 2B, the stack includes
alternating insulating (e.g., dielectric) and conductive layers.
The dielectric layers include D0 to D5 and may be made of
SiO.sub.2, for instance. The conductive layers include BG, which is
a back gate layer, WL0 to WL3, which form word line layers, e.g.,
conductive paths to control gates of the memory cells at the layer,
and SG, which forms a select gate layer, e.g., a conductive path to
control gates of select gate transistors of NAND strings. The word
line layer of FIG. 2A may represent any one of WL0 to WL3, for
instance. The conductive layers may include doped polysilicon,
metal silicide, or metal, for instance. An example voltage of 5-10
V may be applied to the back gate to maintain a conductive state
which connects the drain- and source-side columns.
[0060] For each block, the word line layer is divided into two word
line layer portions 202 and 204. Each block includes a slit
pattern. A slit refers, e.g., to a void which extends vertically in
the stack, typically from an etch stop layer at the bottom to at
least a top layer of the stack. The slit can be filled with
insulation to insulate words line layer portions from one another.
A slit 206 is a single continuous slit which extends in a zig-zag
pattern in the block so that the block is divided into 5b, which
are insulated from one another. This approach can provide greater
flexibility in controlling the memory cells since the word line
layer portions can be driven independently.
[0061] Each block includes rows of columnar, e.g., vertical, memory
holes or pillars, represented by circles. Each row represents a
vertical group of columns in the figure. The memory holes extend
vertically in the stack and include memory cells such as in a
vertical NAND string. Example columns of memory cells along a line
220 include C0 to C11. The figure represents a simplification, as
many more rows of memory holes will typically be used, extending to
the right and left in the figure. Also, the figures are not
necessarily to scale. The columns of memory cells can be arranged
in subsets such as sub-blocks.
[0062] The drawings are not to scale and do not show all memory
columns. For example, a more realistic block might have 12 memory
columns in the y direction as shown, but a very large number such
as 32 k memory columns in the x direction, for a total of 384 k
memory columns in a block. With U-shaped NAND strings, 192 k NAND
strings are provided in this example. With straight NAND strings,
384 k NAND strings are provided in this example. Assume there are
four memory cells per column, so there are 384 k.times.4=1,536 k or
1,536,000 total cells in the set.
[0063] A portion 210 of the block 200 is described further below in
connection with FIG. 3A.
[0064] FIG. 2B depicts a cross-sectional view of the portion 210 of
the block 200 of FIG. 2A, along line 220. Columns of memory cells
C0 to C7 are depicted in the multi-layer stack. The stack 230
includes the substrate 101, an insulating film 250 on the
substrate, and a back gate layer BG, which is a conductive layer,
on the insulating film. A trench is provided in portions of the
back gate below pairs of columns of memory cells of a U-shaped NAND
string. Layers of materials which are provided in the columns to
form the memory cells are also provided in the trenches, and the
remaining space in the trenches is filled with a semiconductor
material to provide connecting portions 263 to 266 which connect
the columns. The back gate thus connects the two columns of each
U-shaped NAND string. For example, NS0 (NS=NAND string) includes
columns C0 and C1 and connecting portion 263. NS0 has a drain end
232 and a source end 240. NS1 includes columns C2 and C3 and
connecting portion 264. NS1 has a drain end 244 and a source end
242. NS2 includes columns C4 and C5 and connecting portion 265. NS3
includes columns C6 and C7 and connecting portion 266.
[0065] The source line SL0 is connected to the source ends 240 and
242 of two adjacent memory strings NS0 and NS1. The source line SL0
is also connected to other sets of memory strings which are behind
NS0 and NS1 in the x direction. Recall that additional U-shaped
NAND strings in the stack 230 extend behind the U-shaped NAND
strings depicted in the cross-section, e.g., along the x-axis, in a
SGD line direction. The U-shaped NAND strings NS0 to NS3 are each
in a different SGD line subset, but are in a common BL subset.
[0066] The slit 206 from FIG. 2A is also depicted as an example. In
the cross-section, multiple slit portions are seen, where each slit
portion is between the drain- and source-side columns of a U-shaped
NAND string. A portion of the bit line BL0 is also depicted.
[0067] Short dashed lines depict memory cells and select gate
transistors, as discussed further below. A region 236 of the stack
is shown in greater detail in FIG. 3A.
[0068] FIG. 2C depicts an embodiment of a block that includes
U-shaped NAND strings. The block includes U-shaped NAND strings
arranged in sets (SetA0, SetA1, SetA2, SetA3, . . . , SetAn, where
there are n-1 sets in a block). Each set of NAND strings is
associated with one bit line (BLA0, BLA1, BLA2, BLA3, BLAn). In one
approach, all NAND strings in a block which are associated with one
bit line are in the same set. Each U-shaped NAND string thus has
two columns of memory cells--a drain-side column and a source-side
column. For example, SetA0 includes NAND strings NSA0 (having
drain-side column C0 and source-side column C1), NSA1 (having
drain-side column C3 and source-side column C2), NSA2 (having
drain-side column C4 and source-side column C5), NSA3 (having
drain-side column C7 and source-side column C6), NSA4 (having
drain-side column C8 and source-side column C9) and NSA5 (having
drain-side column C11 and source-side column C10). Source lines
extend transversely to the bit lines and include SLA0, SLA1 and
SLA2. The source lines join the source-side columns of adjacent
NAND string in a set. For example, SLA0 joins C1 and C2, SLA1 joins
C5 and C6 and SLA2 joins C9 and C10. In one approach, the source
lines in a block are joined to one another and driven by one
driver. The bit lines and the source lines are above the memory
cell array in this example. In FIG. 2C there are six NAND strings;
however, there could be more or fewer NAND strings in a block.
[0069] FIG. 3A depicts a close-up view of the region 236 of the
column C0 of FIG. 2B, showing a drain-side select gate transistor
SGD0 and a memory cell MC303. The region shows portions of the
dielectric layers D3 to D5 and the conductive layers WL3 and SG.
Each column includes a number of layers which are deposited along
the sidewalls of the column. These layers can include
oxide-nitride-oxide and polysilicon layers which are deposited,
e.g., using atomic layer deposition. For example, a block oxide
(BOX) can be deposited as layer 296, a nitride such as SiN as a
charge trapping layer (CTL) can be deposited as layer 297, a tunnel
oxide (TNL) can be deposited as layer 298, a polysilicon body or
channel (CH) can be deposited as layer 299, and a core filler
dielectric can be deposited as region 300. Additional memory cells
are similarly formed throughout the columns.
[0070] When a memory cell is programmed, electrons are stored in a
portion of the CTL which is associated with the memory cell. For
example, electrons are represented by "-" symbols in the CTL 297
for MC303. These electrons are drawn into the CTL from the channel,
and through the TNL. The threshold voltage of a memory cell is
increased in proportion to the amount of stored charge.
[0071] During an erase operation, as discussed further below, a
voltage in the channel is raised due to gate-induced drain leakage
(GIDL). The voltage of the one or more selected word line layers is
then driven down to a reduced level such as 0 V to create an
electric field across the TNL which causes holes to be injected
from the memory cell's body to the CTL, resulting in a large
V.sub.TH downshift toward an erase-verify level, Vv_erase. This
process can be repeated in successive iterations until a verify
condition is met, as discussed further below. For unselected word
lines, the word lines remain at an elevated level so that the
electric field across the TNL is relatively small, and no, or very
little, hole tunneling will occur. Memory cells of the unselected
word lines will experience little or no V.sub.TH downshift, and as
a result, they will not be erased.
[0072] FIG. 3B depicts a cross-sectional view of the column C0 of
FIG. 3A. Each layer is ring-shaped in one possible approach, except
the core filler, which is cylindrical.
[0073] FIG. 4A depicts a top view of a straight NAND string
embodiment 480 of the block BLK0 of FIG. 1A. In this configuration,
a NAND string has only one column, and the source-side select gate
is on the bottom of the column instead of on the top, as in a
U-shaped NAND string. Moreover, a given level of a block has one
word line layer which is connected to each of the memory cells of
the layer. These insulation-filled slits are used in the
fabrication process to provide structural support for the stack
when undoped polysilicon layers are removed by a wet etch and a
dielectric is deposited to form the alternating dielectric layers.
A dashed line 486 extends through columns C12 to C17. A
cross-sectional view along line 486 of portion 488 is shown in FIG.
4B.
[0074] FIG. 4B depicts a cross-sectional view of the portion 488 of
the block 480 of FIG. 4A along line 486. Columns of memory cells
corresponding to NAND strings NS8 to NS11 are depicted in the
multi-layer stack. The stack 490 includes a substrate 101, an
insulating film 250 on the substrate, and a portion of a source
line SL0A. Recall that the additional straight NAND strings in a
SGD line subset extend in front of and in back of the NAND strings
depicted in the cross-section, e.g., along the x-axis. NS8 has a
source end 494 and a drain end 492. The slit 482 from FIG. 4A is
also depicted with other slits. A portion of the bit line BL0A is
also depicted. Dashed lines depict memory cells and select gate
transistors. The erase techniques described herein can be used with
a U-shaped or straight NAND.
[0075] FIG. 4C depicts an embodiment of a block which includes
straight NAND strings. The block includes straight NAND strings
arranged in sets (SetB0, SetB1, SetB2, SetB3, . . . , SetBn, where
there are n-1 sets in a block). Each set of NAND strings is
associated with one bit line (BLB0, BLB1, BLB2, BLB3, BLBn). In one
approach, all NAND strings in a block which are associated with one
bit line are in the same set. Each straight NAND string has one
column of memory cells. For example, SetA0 includes NAND strings
NSB0, NSB1, NSB2, NSB3, NSB4 and NSB5. Source lines extend parallel
to the bit line and include SLB0, SLB1, SLB2, SLB3, . . . , SLBn.
In one approach, the source lines in a block are joined to one
another and driven by one driver. The bit lines are above the
memory cell array and the source lines are below the memory cell
array in this example.
[0076] FIG. 5A depicts an example assignment of storage elements to
pseudo-blocks. This example is for U-shaped NAND strings. Four NAND
strings are shown in this example. Each NAND string has a drain
side 531 which extends between a bit line (BL) and a back gate (BG)
and a source side 532 which extends between a source line (SL) and
the BG. The drain side includes an SGD transistor, a dummy
transistor (CGDD) and storage elements that are connected to word
lines (WL24-WL47). The source side includes an SGS transistor, a
dummy transistor (CGSD) and storage elements that are connected to
word lines (WL00-WL23).
[0077] One or more dummy transistors on each side can be used to
avoid disturbs. One or more dummy transistors can optionally be
provided between groups as well.
[0078] Five pseudo blocks (PB1-PB5) are represented in FIG. 5A.
Pseudo block 1 (PB1) includes storage elements associated with word
lines WL00-WL04, as well as storage elements associated with word
lines WL43-WL47. Pseudo block 2 (PB2) includes storage elements
associated with word lines WL05-WL09, as well as storage elements
associated with word lines WL38-WL42. Pseudo block 3 (PB3) includes
storage elements associated with word lines WL10-WL14, as well as
storage elements associated with word lines WL33-WL37. Pseudo block
4 (PB4) includes storage elements associated with word lines
WL15-WL19, as well as storage elements associated with word lines
WL28-WL32. Pseudo block 5 (PB5) includes storage elements
associated with word lines WL20-WL23, as well as storage elements
associated with word lines WL24-WL27.
[0079] In this example, there are five pseudo blocks, but there
could be more or fewer. In this example, most of the pseudo blocks
are the same size, although one is smaller. That is, PB1-PB4 each
occupy five memory cell layers, whereas PB5 occupies four memory
cell layers. All of the pseudo blocks could be the same size. All
of the pseudo blocks could be different sizes. There is no limit to
the possibilities in how many pseudo blocks are used and their
sizes. However, note that having pseudo blocks that are about the
same size is possibly a benefit from the perspective of simplifying
system performance.
[0080] Note that FIG. 5A depicts the yz plane, but does not depict
how the pseudo blocks extend in the x-direction. Referring back to
FIG. 2C, recall that a block may contain many NAND strings
extending in the x-direction also. For example, NSA0 and many other
NAND strings are associated with source line SLA0. Just as a block
extends in the x-direction, a pseudo block may extend in the
x-direction.
[0081] The four depicted NAND strings in FIG. 5A roughly correspond
to NSA0-NSA3 in FIG. 2C. Thus, it will be understood that each
pseudo block may contain additional NAND strings in the x-direction
that are not depicted in FIG. 5A.
[0082] Referring back to FIG. 2A, a top view of a U-shaped NAND
embodiment of a block 200 is depicted. In effect, this represents
one layer of a block. This, in effect, also represents one layer of
a pseudo block. A pseudo block may have multiple such layers, as
shown in FIG. 5A. Thus, it will be appreciated that the pseudo
blocks also extend in the x-direction, although this is not
depicted in FIG. 5A.
[0083] FIG. 5B depicts another example assignment of storage
elements to pseudo-blocks along a string. The U-shaped NAND strings
are similar to the example of FIG. 5A.
[0084] Ten pseudo blocks (PB1A, PB1B, PB5B) are represented in FIG.
5B. Pseudo block 1A (PB1A) includes storage elements associated
with word lines WL43-WL47. Pseudo block 1B (PB1B) includes storage
elements associated with word lines WL00-WL04. Pseudo block 2A
(PB2A) includes storage elements associated with word lines
WL38-WL42. Pseudo block 2B (PB2B) includes storage elements
associated with word lines WL05-WL09. Pseudo block 3A (PB3A)
includes storage elements associated with word lines WL33-WL37.
Pseudo block 3B (PB3B) includes storage elements associated with
word lines WL10-WL14. Pseudo block 4A (PB4A) includes storage
elements associated with word lines WL28-WL32. Pseudo block 4B
(PB4B) includes storage elements associated with word lines
WL15-WL19. Pseudo block 5A (PB5A) includes storage elements
associated with word lines WL24-WL27. Pseudo block 5B (PB5B)
includes storage elements associated with word lines WL20-WL23.
[0085] Note that FIG. 5B depicts the yz plane, but does not depict
how the pseudo blocks extend in the x-direction. The pseudo blocks
may extend in the x-direction, as discussed with respect to FIG.
5A. Also note that each pseudo block in FIG. 5B that is labeled
with the same reference is part of the same pseudo block. For
example, the several regions labeled "PB1A" are all part of the
same pseudo block. This can be understood by examining the two
portions 202, 204 of the block depicted in FIG. 2A.
[0086] Straight NAND strings, such as depicted in FIGS. 4A-4C may
also be divided into pseudo blocks. FIG. 5C is one example of how
straight NAND strings in a 3D stacked memory device can be divided
into pseudo blocks. FIG. 5C shows eight straight NAND stings. Each
NAND string is associated with word lines WL00-WL23. In this
example, there are five pseudo blocks (PB1-PB5). There could be
more or fewer pseudo blocks. As with the example of FIG. 5C, the
pseudo blocks may extend in the x-direction.
[0087] FIG. 5D is a flowchart of one embodiment of a process of
operating a 3D stacked memory device in a pseudo block operation
mode. The process could be applied to devices having U-shaped NAND
strings or straight NAND strings, but is not limited to the example
3D memory devices provided herein.
[0088] Step 502 assigns storage elements in a NAND string to pseudo
blocks. FIG. 5A-5C show a few examples, but step 502 is not limited
to those examples. Each NAND string comprises a channel and extends
vertically through layers of the 3D stacked non-volatile memory
device, in embodiments. The block is divided into pseudo blocks
that each comprise a portion of the NAND string. The portion of the
NAND string may include multiple contiguous storage elements. Each
NAND string may be a member of each of the pseudo blocks. That is,
some storage elements on the NAND string belong to a first pseudo
block, other storage elements to a second pseudo block, etc. In one
embodiment, the pseudo blocks are assigned based on location along
the NAND string. A state machine or other circuit can store the
group assignment information. Techniques are described below for
how to determine the mapping of storage elements to pseudo
blocks.
[0089] In step 504, control logic operates the 3D NAND in pseudo
block operation mode. In one embodiment, a control circuit performs
operations on a pseudo block level. Performing an operation on a
pseudo block level means that either the operation is performed on
that pseudo block and not on other pseudo blocks in the block or
the operation is somehow dependent on the pseudo block in which it
is performed. Thus, in one embodiment, performing an operation on a
pseudo block level includes performing the operation (e.g., erase,
program) on that pseudo block but not on other pseudo blocks in the
block. Performing operations on a pseudo block level includes
performing operations (e.g., applying starting erase voltage,
applying boosting voltage, checking a read disturb parameter,
checking an endurance parameter) in a way that depends on which
pseudo block the operation is performed in, for one embodiment.
Examples of pseudo block operations include, but are not limited
to, pseudo block program and erase operation, pseudo block read
disturb operation, pseudo block endurance operation, and pseudo
block boosting voltage (Vpass) operation. Pseudo block operation
mode is not limited to these examples.
Pseudo Block Program and Erase Operation
[0090] In 2D NAND memory devices, the p-well substrate is biased at
a high voltage to erase the storage elements (memory cells). Note
that the NAND strings are typically formed in the substrate, such
that by applying the erase voltage to the substrate while, for
example, grounding the control gates, the memory cells can be
erased. In contrast, the NAND strings in a 3D stacked non-volatile
memory device such as BiCS are not formed in a substrate (see, for
example, FIG. 2B or 4B).
[0091] One approach to erasing in a 3D stacked non-volatile memory
device is to generate gate induced drain leakage (GIDL) current to
charge up the channel, raise the channel potential to an erase
voltage, and maintain this channel potential during erase.
Therefore, the erase speed is affected by channel charging up time
and erase performance is impacted by interactions between channel
potential, tunneling speed, and GIDL current. Further, the
tunneling speed is also affected by variations in the diameter of
the memory hole. The erase process thus has a layer dependent
pattern.
[0092] The erase speed and depth can vary along a string due to
factors such as variations in a potential drop in the channel
relative to a driven end of a string of storage elements and
variations in the diameter of the memory hole. A potential drop
occurs when holes are diverted from the channel and tunnel through
a tunneling layer to a charge trapping layer of the storage
elements. The holes recombine with electrons there to reduce the
charge in the charge trapping layer, thereby erasing the storage
elements.
[0093] Storage elements which are further from the driven end,
e.g., the drain side or source side of a NAND string, will
experience a larger potential drop which slows their erase speed.
Regarding the memory hole, its diameter can vary due to process
variations. In some cases, the memory hole is vertical and tapered,
becoming smaller at the bottom. A smaller diameter results in a
stronger field and a faster erase speed.
[0094] FIG. 6A is a flowchart of an overview of a pseudo block
program and erase operation. This is one embodiment, of operating a
3D NAND in a pseudo block operation mode (step 504 of FIG. 5D). In
step 602, a command is received to program data into a given pseudo
block. This program command may be preceded by or accompanied by a
command to erase the pseudo block. Note that the erasing can be
performed well prior to the programming.
[0095] In step 604, the pseudo block is erased while not erasing
other pseudo blocks in the block. This includes erasing a part of a
NAND string, while not erasing other parts of the NAND string, in
one embodiment. Note that other pseudo blocks in the block may
remain programmed after the erase of the pseudo block is competed.
In one embodiment, the entire pseudo block is erased prior to
programming. FIG. 6E provides further details for one embodiment of
erasing a pseudo block.
[0096] In step 606, the pseudo block is programmed. As noted, the
programming can occur well after the erasing. Also, it is not
required that the entire pseudo block be programmed at the same
time. FIG. 10A provides further details for one embodiment of
programming a pseudo block.
[0097] FIGS. 6B-6D show further details of erasing and programming
FIGS. 6B and 6C show example threshold distributions after erase
and program. FIG. 6B depicts threshold voltage distributions of a
deep erased state, a final erased state and higher data states of
one example erase operation. Storage elements can be programmed so
that their threshold voltages are in respective ranges which
represent data states. Initially, an erase operation is performed
which places all of the storage elements in the erased state (E).
Subsequently, some of the storage elements can be programmed to a
higher threshold voltage such as to represent the A, B or C data
states.
[0098] The x-axis indicates a threshold voltage and the y-axis
indicates a number of storage elements. In this example, there are
four data states (each represented by a threshold voltage
distribution): an initial (deep) erased state distribution 601, a
soft programmed erased state (E) distribution 603, an A state
distribution 605, a B state distribution 607 and a C state
distribution 609. Memory devices with additional data states, e.g.,
eight or sixteen data states, can also be used. The distribution
601 is realized after the erase operation when storage elements are
typically over-erased, past the erase state distribution 603. In
the erase operation, one or more erase pulses are applied to the
NAND string at its source and/or drain ends, until the threshold
voltage of the storage elements being erased transitions below an
erase-verify level, Vv_erase which can be 0 V or close to 0 V, in
one approach. Once the erase operation is completed for a pseudo
block, the soft programming operation can be optionally performed,
in which one or more positive voltage pulses are applied to the
control gates of the storage elements, such as via a word line, to
increase the threshold voltages of some or all of the storage
elements in the distribution 601 above a soft programming (SPGM)
verify level, Vv_spgm, to the erased state distribution 603. This
avoids a deep erased state.
[0099] The soft programming operation advantageously results in a
narrow erase state distribution 603. Once the soft programming
operation is complete, programming to higher data states can occur,
such as to states A, B and C using verify levels VvA, VvB and VvC,
respectively. A subsequent read operation can use the levels
VreadA, VreadB and VreadC.
[0100] FIG. 6C depicts threshold voltage distributions of a soft
erased state and higher data states of another example erase
operation. In this approach, the erase state distribution 610 is
reached after a soft erase. In one option, no soft programming is
performed. Vv_erase can be the same or different in FIGS. 6B and
6C.
[0101] FIG. 6D depicts a series of erase pulses and verify pulses
in an erase operation, where Verase is stepped up in successive
erase-verify iterations. A waveform 620 represents a number of
erase-verify iterations EV0, EV1, EV2, . . . . Each erase-verify
iteration includes an erase pulse 611-615 (erase portion) followed
by a verify pulse such as 619 of magnitude Vv_erase (erase-verify
portion). The erase pulses have two levels. A first level is Vgidl
and a second level is Verase. In this example, Verase is stepped up
in each iteration by a step size Verase_step so that erase pulse
611 has a peak amplitude of Verase0, erase pulse 612 has a peak
amplitude of Verase1, erase pulse 613 has a peak amplitude of
Verase2, erase pulse 614 has a peak amplitude of Verase3, and erase
pulse 615 has a peak amplitude of Verase max, a maximum allowed
level of Verase. In this case, the maximum erase voltage is reached
relatively soon in the erase operation. Vgidl can be fixed in each
erase pulse or varying, such as by stepping up with each erase
pulse.
[0102] FIG. 6E depicts a local potential drop in a channel of a
string in an erase operation based on a distance from a driven end
of the string. The channel potential can decrease further from a
driven end during an erase operation. The x-axis depicts a distance
from a driven end and the y-axis depicts the channel voltage, Vch,
relative to Verase. For two-sided erase, the storage elements
associated with WL24 and WL23 in FIG. 5A are furthest from the bit
line and source line, respectively. For one-sided erase, the
storage element associated with WL00 in FIG. 5B is furthest from
the bit line. As mentioned, a larger potential drop makes it harder
to erase the storage elements.
[0103] However, another factor in erase speed is the memory hole
diameter. FIG. 6F depicts variations in an erase depth along a
string based on variations in memory hole diameter along the
string. As mentioned, it is easier to erase a storage element which
is at a part of the memory hole which has a smaller diameter. The
x-axis depicts a memory hole diameter and the y-axis depicts a
threshold voltage Vth level of the storage elements. The Vth level
indicates an erase depth such that a lower Vth indicates a deeper
erase or an easier to erase storage element. This data was obtained
by applying an erase pulse of fixed amplitude and duration to each
storage element and measuring the resulting Vth of the storage
element.
[0104] FIG. 6G depicts example starting erase voltages for
different pseudo blocks. For some devices, storage elements that
have smaller memory holes will erase faster relative to those with
larger memory holes. The starting erase voltage may be smaller for
storage elements having smaller memory holes to take advantage of
this. In some devices, the memory holes are smaller at lower
layers. This leads to using a smaller starting erase voltages for
pseudo blocks that are at lower layers, in one embodiment.
[0105] In the example of FIG. 6G, five pseudo blocks PB1-PB5 are
depicted. The x-axis depicts a pseudo block number and the y-axis
depicts an initial erase voltage (Verase_min). Five levels,
Vera1-Vera5, are depicted.
[0106] In one approach, the levels of Verase_min are a function
layer of the 3D stacked memory. Note that the memory hole size
decreases at the lower layers of some 3D stacked memory device, for
at least some fabrication techniques. Thus, the levels of
Verase_min are a function of memory hole size, in one approach. In
one approach, the levels of Verase are a function of distance from
a driven end of a NAND string although other variations are
possible. An optimal assignment of Verase_min level for a group can
be determined from tests.
[0107] A reasonable approximation is to have Verase_min be smaller
for smaller memory holes, and larger for larger memory holes.
Another factor in Verase_min may, optionally, be the distance of
the pseudo block from where the erase voltage is applied. The value
for Verase_min may be different for a one-sided erase than for a
two sided erase.
[0108] FIG. 6H is a flowchart of one embodiment of a process of
performing a pseudo block erase operation. This is one embodiment
of erasing a pseudo block while not erasing other pseudo blocks
(step 604 of FIG. 6A). The process will describe erasing storage
elements of a selected NAND string that are in the pseudo block
that is selected for erase. For example, referring to FIG. 5A, the
selected NAND string may be string 1 and the selected pseudo block
may be PB1. Referring to FIG. 5B, the selected NAND string may be
string 1 and the selected pseudo block may be PB1A. Note that in
this case only the drain side of the NAND string is being erased.
As one alternative, the selected NAND string may be string 1 and
the selected pseudo block may be PB1B. In this case, the source
side of the NAND string is being erased. The process may also be
applied to straight NAND strings. Referring to FIG. 5C, the
selected NAND string may be string 1 and the selected pseudo block
may be PB1. Note that (portions of) many NAND strings could be
erased at the same time. As one example, referring back to FIG. 2C,
the pseudo block could include (portions of) a set of NAND strings
in the x-direction. A "portion of" of NAND string is being referred
to because typically a pseudo block is a few layers of the 3D
memory device, as shown in FIG. 5A, for example.
[0109] Portions of other NAND strings in the selected pseudo block
could also be selected for erase. For example, referring to FIG.
5A, some of the storage elements associated with NAND strings 2-4
may be unselected or selected, as desired. Also, portions of other
NAND strings in the x-direction could be selected for erase. For
example, referring to FIG. 2C, portions of NAND strings associated
with source line SLA0 could be selected for erase. The portions
being referred to could be portions associated with word lines
WL00-WL04 and WL43-WL47. Thus, the erase process of FIG. 6E can be
applied to more than one NAND string in a pseudo block. For purpose
of discussion, an example of erasing a portion of one NAND string
will be discussed.
[0110] Generally, an erase operation can include multiple
erase-verify iterations which are performed until a verify
condition is met, at which point the erase operation ends (see FIG.
6D). In one approach, the memory device includes NAND strings which
have a drain-side select gate (SGD) transistor on one end and a
source-side select gate (SGS) transistor on the other end. The
erase may be a "one-sided erase" or a "two-sided erase." When an
erase voltage is applied to the bit line in a one-sided erase, or
to the bit line and source line in a two-sided erase, the select
gate transistors generate a sufficient amount of gate-induced drain
leakage (GIDL) current to charge up the floating body (channel) of
the NAND string. GIDL increases in proportion to the drain-to-gate
voltage (Vdg) of the select gate transistors.
[0111] Step 616 sets status=erase for the storage elements on the
selected NAND string that are in the selected pseudo block. For
example, referring to FIG. 5A, storage elements in NAND string 1
that are associated with word lines WL00-WL04 and WL43-WL47 are
selected for erase. As noted, the portions of other NAND strings
that are in the selected pseudo block may (but are not required to)
have their status set to erase.
[0112] Step 618 sets status=inhibit for the storage elements in the
unselected pseudo blocks. For example, pseudo blocks PB2-PB5 (see
FIG. 5A) are unselected for erase. Note this also applies to
storage elements on the selected NAND string that are not in the
selected pseudo block. For example, referring to FIG. 5A, storage
elements in NAND string 1 that associated with word lines WL05-WL42
have their status set to inhibit.
[0113] Step 621 initializes an erase voltage (Verase) to a starting
value. This starting value depends on which pseudo block is being
erased, in one embodiment. FIG. 6G depicts one example of how
Verase_min might decrease in magnitude for pseudo blocks having
smaller memory holes.
[0114] Step 622 charges the channel of the string by applying the
erase voltage to the string, setting a control gate voltage for
each storage element with status=erase to encourage erasing, and
floating or setting a control gate voltage for each storage element
with status=inhibit to discourage erasing. For example, a control
gate voltage at a relatively high level (e.g., 10-15 V) discourages
erasing by creating a small potential difference across the
tunneling layer.
[0115] A control gate voltage at or slightly above, for example, 0
V encourages erasing by creating a large potential difference
across the tunneling layer, between the channel and the control
gate, which encourages further tunneling. See, e.g., FIGS. 7C and
8C for further details.
[0116] Step 623 performs an erase-verify test for storage elements
in the selected pseudo block (for the selected NAND string). The
erase-verify test can be performed concurrently for the different
storage elements on the string in the pseudo block. For example,
this can involve applying a common erase-verify control gate
voltage (Vv_erase) to the control gate of each storage element in
the selected pseudo block while detecting the current through the
string.
[0117] Note that storage elements on the selected NAND string that
are in other (unselected) pseudo blocks could be in the programmed
state or in the erased state. For example, these storage elements
could be in any of states E, A, B C, for a four state example (see
FIG. 6C). In one embodiment, a suitable voltage is applied to the
control gate of those storage elements depending on their state.
For example, if the storage element is known to be erased, then the
erase-verify control gate voltage (Vv_erase) can be applied.
However, if the storage element might be in a programmed state,
then another voltage may be applied to the control gate. Note that
it is not required to know which of the programmed states that the
storage element is in. As one possibility, a voltage is applied of
sufficient magnitude to cause the memory cell to turn on regardless
of its state. For example, a voltage V.sub.READ could be applied to
the control gate. V.sub.READ is a voltage that should cause the
storage element to conduct regardless of its threshold voltage (or
state).
[0118] If the current of the string is above a reference current,
indicating the string is conductive, the erase-verify test is
passed.
[0119] Step 624 sets status=inhibit if the selected NAND string
passed the erase-verify test. Note this refers to the portion of
the selected NAND string that is in the selected pseudo block. The
erase status=erase is continued if the selected NAND string does
not pass the erase-verify test.
[0120] Decision step 626 determines if Verase is at Verase max. If
the answer is "yes," the erase operation ends unsuccessfully at
step 627. If the answer is "no," Verase is stepped up at step 628
and another iteration is performed at step 622.
[0121] The erase operation of FIG. 6E may be a one-side erase
operation or a two-sided erase operation. In an example two-sided
erase, pseudo block includes storage elements on the drain and
source sides. However, other approaches are possible. The pseudo
block may be mostly equal in size although this can vary as well.
The pseudo block assignments can be made by grouping storage
elements which are expected to have a common erase speed.
Typically, a pseudo block will include multiple adjacent storage
elements.
[0122] In one embodiment of a one-sided erase, the bit line is the
driven end. This is a factor in the source side storage elements
having a slower erase speed since they are furthest from the driven
end. However, the memory hole size may also be a factor in the
erase speed.
[0123] In the examples of FIGS. 5A and 5B, the strings are
U-shaped. The groups extend from one end of the string (e.g., the
BL or SL end) to the back gate of the string. The groups also
extend from one end of the string (e.g., the BL or SL end) to the
opposing end of the string (e.g., the SL or BL end). Optionally,
the string is straight and the groups extend from one end of the
string (BL or SL end) to an opposing end of the string (SL or BL
end).
[0124] FIG. 7A-7C depicts example voltages during the erase portion
of an erase-verify iteration of an erase operation. An erase
operation can include multiple erase-verify iterations, e.g., EV0,
EV1, EV2, . . . such as discussed in connection with FIG. 6D. Each
erase-verify iteration can include an erase portion followed by an
erase-verify portion. Moreover, each erase portion can have
multiple levels. In one approach, a two-level erase portion is
used. Further, an erase operation can involve a one-sided or a
two-sided erase. A one-sided erase applies a voltage to the
drain-end of a NAND string via a bit line to charge up the NAND
string, while the source-end of the NAND string is not used to
charge up the NAND string. A two-sided erase charges up a NAND
string via both the drain-end and the source-end. Two-sided erase
is faster than one-sided erase but requires a larger chip size.
Generally, the erase portion can include a pre-charge phase (t1-t3)
and an erase phase (t3-t6).
[0125] FIGS. 7A-7C have a common time axis but the time increments
are not necessarily equally spaced and the figures are not
necessarily to scale. Moreover, the voltage levels provided are
examples only as other values can be used.
[0126] FIG. 7A depicts example voltages for a bit line and source
line in a two-sided erase. A selected bit line or source line is in
communication with one or more NAND strings having storage elements
which are to be erased in the current erase iteration. Waveform 700
depicts Vbl and Vsl (two-sided erase) initially at 0 V (Vss), then
being raised to Vgidl (a pre-charge voltage) (e.g., 8 V) from
t1-t2, then being raised to Verase (e.g., 20 V) from t3-t4, then
returning to 0 V from t6-t7. For one-sided erase, Vsl is raised to
a reduced level such as 14 V, for instance, from t5-t6, and
provided at 2 V at t2-t3
[0127] When Vbl is at Vgidl, holes are generated at the SGD drain
edge so that the channel is charged from the bit line end of the
NAND string. Similarly, when Vsl is at Vgidl, holes are generated
at the SGS drain edge so that the channel is also charged from the
source line end of the NAND string. Vgidl is a voltage which is at
or above a minimum voltage for creating GIDL. When Vbl is raised to
Verase, charging of the channel continues from the bit line end.
Similarly, when Vsl is raised to Verase, charging of the channel
continues from the source line end.
[0128] FIG. 7B depicts example voltages for SGD and SGS
transistors. From t2-t3, with Vbl at 8 V and Vsgd and/or Vsgs at 0
V, the junction of the SGD and/or SGS transistor is reversed biased
at Vgidl (the potential difference between Vbl and/or Vsl and
Vsgd). As a result, electron-hole pairs will be generated near the
bit line and/or source line junctions. Electrons will be swept away
by the electrical field and collected at the bit line and/or source
line terminals; while holes will drift to the channel and help to
charge up the channel. That is, the electrons will drift toward a
high potential (e.g., at the BL or SL), while the holes will drift
toward a low potential. Given a sufficiently long pre-charge time,
the channel will be boosted close to Vgidl.
[0129] From t4-t6 (waveform 702), Vsgd and/or Vsgs (selected
string) is raised to Verase-Vgidl (e.g., 20-8=12 V). Since Vbl
and/or Vsl is raised to Verase at this time, the junction of the
SGD and/or SGS transistor is still reversed biased at Vgidl (since
Verase-(Verase-Vgid1))=Vgidl. As a result, electron-hole pairs will
continue to be generated and the channel will be charged to a
higher potential.
[0130] For an unselected string, Vsgd and/or Vsgs is raised to
Verase-Vrev (e.g., 20-2=18 V). Vrev is a reverse bias such as 2 V
which is maintained on the select gate. For one-sided erase of an
unselected string, Vsgd is raised to Verase-Vrev (e.g., 20-2=18
V).
[0131] FIG. 7C depicts example control gate voltages for
uninhibited and inhibited of storage elements. In this example, the
control gate voltages are the same for the different groups of
uninhibited storage elements which are being erased (and have
status=erase) in a given iteration. The control gate voltages can
be set to 0 V, for instance, from t4-t6 (waveform 705) to encourage
further erasing of the associated storage elements.
[0132] For the inhibited groups (status=inhibit), the control gate
voltages can be set to an inhibit level (Vinhibit) or floated
(waveform 706) to discourage erasing of the associated storage
elements. Specifically, for an unselected word line, the control
gate can be floated or driven at a higher bias such as 12 V. If the
channel potential reaches 20 V during erase, the WL is biased at 12
V. Therefore, the gated junction of a data WL sees 8 V, which is
large enough to generate GIDL to help expedite charging up the
channel to speed up the erasing of the remaining groups of storage
elements with status=erase. During the pre-charge step, an
unselected WL can be biased at Vgidl, which is the same as Vbl.
[0133] The control gate voltages from t2-t3 are configured at a
level which discourages erase of the associated storage elements,
while the control gate voltages from t3-t6 are configured at a
level which encourages erase of the associated storage elements. To
discourage erase of a storage element, the control gate voltage
should be sufficiently high to discourage tunneling of holes into a
charge trapping layer of the storage element. To encourage erase of
a storage element, the channel voltage should be sufficiently
higher than the control gate voltage to encourage tunneling.
[0134] In either case, the control gate voltages can be at a level
which encourages charging of the channel through GIDL throughout
the erase operation. For example, the control gate voltages from
t2-t3 can be floated or driven at a level such as 8 V which is to
minimize reverse bias stress on a memory cells' junction.
Specifically, when Vbl is biased at 8 V to pre-charge the channel,
the maximum channel potential is 8 V. If Vwl/Vcg is biased at 8 V,
memory cell's junction sees 0 V. Because the maximum channel
potential is 8 V, the channel potential is not high enough to erase
the memory cell. Therefore, we bias Vcg at 8V at that time to
minimize the reverse bias stress time on a memory cell's junction.
The control gate voltages can be driven at Vss from t6-t7.
[0135] FIGS. 8A-8C depict voltages in the verify portion of an
erase-verify iteration of an erase operation. FIG. 8A depicts a bit
line voltage 800. FIG. 9B depicts a SGS transistor and SGD
transistor voltage 802. FIG. 8C depicts an unselected word line
voltage 804 and a selected word line voltage 806. During the verify
portion, the selected sets of NAND strings are sensed using sensing
circuitry to determine whether the threshold voltages of the
selected memory cells have passed the verify test. During the
sensing, Vbl is set to a sense voltage, Vsense. Vsgs and Vsgd are
set to a level which renders them conductive. Vsgs and Vsgd can be
set to the same or different levels. Vcg_unselected is a pass
voltage which is set to a sufficiently high level, e.g., 8 V, to
render the unselected memory cells in a conductive state. This
could be referred to a Vread. Vcg_selected is set to Vv_erase.
Sensing circuitry connected to a NAND string senses whether the
NAND string is in a conductive state, which indicates the selected
memory cells of the NAND string have been erased and therefore pass
the verify test.
[0136] In one approach, all of the storage elements on the selected
NAND string and in the selected pseudo block are verified at the
same time. There is no need to verify storage elements in the
unselected pseudo blocks, as they are not undergoing erase.
[0137] It may be possible to verify each storage element
separately. For example, the verify voltage could be applied to a
single word line, while all other word lines have the pass voltage
(e.g., Vread) applied.
[0138] The erase operation may be a one-side erase operation or a
two-sided erase operation. FIG. 9A depicts the movement of holes
and electrons in a U-shaped NAND string during a two-sided erase.
Consistent with the previous discussions, an example U-shaped NAND
string 1000 includes a drain side 1001 and a source side 1051. The
drain side includes a channel layer 1002 connected to a bit line
1096 which continues as a channel layer 1052 on the source side
connected to a source line 1097. A charge trapping layer (CTL)
1090, a tunnel layer (TNL) 1091 and a block oxide (BOX) 1092 are
ring shaped layers which extend around the memory hole of the
string. Different regions of the channel layers represent channel
regions which are associated with respective storage elements or
select gate transistors. These channel regions are at a same height
and stack level in the stacked memory device as the control gates
of the storage elements or select gate transistors.
[0139] The drain side includes a SGD transistor 1005 with a control
gate 1006 and a channel region 1007. The drain side also includes
storage elements 1010, 1015, 1020, 1025 and 1030, control gates
1011, 1016, 1021, 1026 and 1031, CTL regions 1013, 1018, 1023, 1028
and 1033, and channel regions 1012, 1017, 1022, 1027 and 1032,
respectively.
[0140] The source side includes a SGS transistor 1055 with a
control gate 1056 and a channel region 1057. The source side also
includes storage elements 1060, 1065, 1070, 1075 and 1080, control
gates 1061, 1066, 1071, 1076 and 1081, CTL regions 1063, 1068,
1073, 1078 and 1083, and channel regions 1062, 1067, 1072, 1077 and
1082, respectively.
[0141] A back gate 1035 has a control gate 1036 and a channel
region 1037.
[0142] Representative holes are depicted in the channel layers as
circles with a "+" sign and representative electrons are depicted
in the channel layers as circles with a "-" sign. As discussed
previously, electron-hole pairs are generated by a GIDL process.
Initially, during an erase operation, the electron-hole pairs are
generated at the SGD and SGS transistors. The holes move away from
the driven ends, thereby charging the channel. The electrons move
toward the bit line due to the positive charge there. Subsequently,
during the erase period of each storage element, additional holes
are generated by GIDL at junctions which are formed in the channel
for the storage element. However, holes are also removed from the
channel as they tunnel to the CTL regions.
[0143] In this example, storage elements 1060, 1065, 1070, 1075 on
the source side and storage elements 1010, 1015, 1020, 1025 on the
drain side are being erased. This is represented by the arrows
showing charge moving towards the CTL regions. However, storage
elements 1080 and 1030 are not being erased, as they are part of a
different pseudo block.
[0144] The voltage on the control gates of unselected storage
elements (e.g., 1030, 1080) is high enough to generate GIDL, in one
embodiment. However, the voltage is not so high that these
unselected storage elements are erased. Thus, storage elements in
unselected pseudo blocks are not erased. However, these storage
elements may help to generated GIDL. Note that although only two
unselected storage elements are depicted in FIG. 9A, up to all of
the unselected storage elements can have a control gate voltage
that is high enough to generate GIDL, in one embodiment.
[0145] Electrons are also generated by the GIDL process. Initially,
during the erase operation, the electrons are generated at the SGD
and SGS transistors and move toward the driven ends. Subsequently,
during the erase period of each storage element, additional
electrons are generated by GIDL at junctions which are formed in
the channel for the storage element.
[0146] In the drain side, example electrons 1040 and 1041 move
toward the bit line. Electron 1040 is generated at the SGD
transistor and electron 1041 is generated at a junction of the
storage element 1015 in the channel region 1017. Also, in the drain
side, example holes including a hole 1042 move away from the bit
line as indicated by arrows. The hole 1042 is generated at a
junction of the storage element 1015 in the channel region 1017 and
can tunnel into the CTL region 1018 as indicated by arrow 1043.
[0147] In the source side, example electrons 1045 and 1049 move
toward the source line. Electron 1045 is generated at the SGS
transistor and electron 1049 is generated at a junction of the
storage element 1065 in the channel region 1067. Also, in the
source side, example holes including a hole 1047 move away from the
source line as indicated by the arrow. The hole 1047 is generated
at a junction of the storage element 1065 in the channel region
1067 and can tunnel into the CTL region 1068 as indicated by arrow
1048.
[0148] FIG. 9B depicts the movement of holes and electrons in a
U-shaped NAND string during a one-sided erase. The diagram of FIG.
9A is modified to show that the holes on the source side move in
the opposite direction of FIG. 9A, away from the bit line end and
toward the source line end. Similarly, the electrons on the source
side move in the opposite direction of FIG. 9A, toward the bit line
end and away from the source line end. As with FIG. 9A, storage
elements 1060, 1065, 1070, 1075 on the source side and storage
elements 1010, 1015, 1020, 1025 on the drain side are being erased.
This is represented by the arrows showing charge moving towards the
CTL regions. However, storage elements 1080 and 1030 are not being
erased, as they are part of a different pseudo block. Also, the
voltage on the control gates of unselected storage elements (e.g.,
1030, 1080) is high enough to generate GIDL, in one embodiment.
However, the voltage is not so high that these unselected storage
elements are erased.
[0149] FIG. 10A depicts one embodiment of programming a pseudo
block in pseudo block mode. This is one embodiment, of step 606 of
FIG. 6A. It is also one embodiment of step 504 of FIG. 5D. This
process describes programming a number of NAND strings in a given
pseudo block. Up to all of the NAND strings in the pseudo block may
be programmed Note that this refers to programming the portion of
the NAND string that is in the selected pseudo block.
[0150] In step 632, a counter ("n") of the NAND string number is
initialized. In step 634, memory cells on string "n" within the
selected pseudo block are programmed. There is no required order in
which the memory cells should be programmed. As one example, the
memory cells are programmed from the source end of the NAND string
downward for this pseudo block. Then, the memory cells are
programmed from the drain end of the NAND string downward for this
pseudo block. Using FIG. 5A as one example, the memory cells of
NAND string 1 might be programmed first. The order can be WL00,
WL01, WL02, WL03, WL47, WL46, WL45, WL44, for pseudo block PB
1.
[0151] As another example, programming alternates between the
source side and the drain side of the NAND string, working
downward. Using FIG. 5A as one example, the order can be WL00,
WL47, WL01, WL46, WL02, WL45, WL03, WL44, for pseudo block PB1.
[0152] Note that when programming NAND string 1, there can be other
NAND strings in the pseudo block that are associated with other bit
lines that are also undergoing programming. For example, referring
to FIG. 2C, other NAND strings associated with source line SLA0 can
be programmed at the same time as NAND string NAS0. Specifically,
storage elements associated with the selected word line can be
programmed together. Note this refers to programming the portions
of these NAND strings that are in the selected pseudo block.
Suitable voltages can be applied to the different bit lines to
control whether the NAND string undergoes programming or not.
Programming may continue until the storage element associated with
each of the NAND strings in the pseudo block is programmed.
[0153] In step 636, a determination is made whether there are more
NAND strings in the pseudo block to program. Note that these other
NAND strings may be associated with the same bit line as string 1.
Referring to FIG. 5A, as one example, the next NAND string to
program could be string 2, which is associated with the same bit
line as string 1. If step 636 indicates more strings are to be
programmed, the string count is incremented and control passes to
step 634 to program the next string.
[0154] FIG. 10B is a flowchart of one embodiment of a process of
grouping word lines into pseudo blocks. This process is based on
layer dependent programming speed of the storage elements. The
programming speed may depend on the memory hole size. Therefore,
this is one way of forming pseudo blocks based on memory hole size,
without directly measuring memory hole size. The storage elements
may be erased prior to this process to establish their threshold
voltages. However, the storage elements could be programmed to some
other threshold voltage.
[0155] This process will test memory cells on one word line at a
time. In step 842, the word line number "n" is initialed to 0. In
step 844, a program pulse is applied to memory cells on WLn. A
single pulse may be used. However, using multiple pulses is not
precluded.
[0156] In step 846, the threshold voltage of storage elements
associated with WLn is recorded. In one technique, the storage
elements are sensed at various levels to determine the threshold
voltage within a suitable level of granularity. Step 846 could
record one or more values that is representative of the threshold
voltages. For example, an average Vt (median, mode, etc.) could be
recorded. Any other statistical technique could be used to record a
value representative of the Vts.
[0157] In step 848, the word line count (n) is incremented.
Assuming that there are more word lines to process, control passes
to step 844, to apply a program pulse to the next word line.
Eventually, threshold voltages that are representative of each word
line are recorded.
[0158] In step 852, the word lines are grouped into pseudo blocks
based on the recorded Vts.
[0159] FIG. 10C is a flowchart of one embodiment of a process of
grouping word lines into pseudo blocks. Like the process of FIG.
10B, this process is also based on layer dependent programming
speed of the storage elements. However, rather than applying a
pulse and testing the Vt, this process tests how many pulses it
takes to reach a target Vt. The storage elements may be erased
prior to this process to establish their threshold voltages.
However, the storage elements could be programmed to some other
threshold voltage.
[0160] In step 842, the word line number "n" is initialed to 0. In
step 862 a pulse count is initialized to "1". In step 864, a
program pulse is applied to memory cells on WLn. A single pulse may
be used. However, using multiple pulses is not precluded.
[0161] In step 866, the storage elements associated with WLn are
sensed to determine whether their Vt is at a target Vt. In step
868, the pulse number is recorded for memory cells that have
reached the target Vt. Step 870 determines whether there are more
cells to test. If so, the pulse count is incremented in step 872.
Control then passes to step 864 to apply the next program pulse.
Eventually, when enough memory on this word line reach the target
Vt (not all have to pass), control passes to step 874, to determine
whether there are more word lines to test. If so, then control
passes to step 842 to increment the word line count. Eventually all
of the word lines are tested and control passes to step 876.
[0162] In step 876, the word lines are grouped into pseudo blocks
based on how many pulses it took memory cells on each word line to
reach the target Vt. Any statistical technique can be used to
characterize the pulse count for a given word line (e.g., median,
mean, etc.).
[0163] FIGS. 10B and 10C discussed two ways to form pseudo blocks
based on programming speed. As noted, programming speed may
correlate to memory hole size. Programming speed may correlate to
other factors as well. Thus, the grouping into pseudo blocks based
on programming speed may be effective at forming pseudo blocks
based on other factors.
[0164] The techniques of FIGS. 10B and 10C could be combined. For
example, the word lines can be grouped into pseudo blocks based on
analysis of both the Vt from the process of FIG. 10B and the count
of the process of FIG. 10C.
[0165] FIG. 10D is a flowchart describing one embodiment of a
programming process 900, which includes one or more verification
steps. This process can be used to program memory cells associated
with a selected word line. In step 910, the program voltage (Vpgm)
is set to an initial value. This initial value depends on which
pseudo block is being programmed, in one embodiment. That is, it
depends on which pseudo block the selected word line is in. Also,
in step 910, a program counter (PC) is initialized to zero.
[0166] In step 920, program conditions are applied. These program
conditions may include, but are not limited to, a program voltage
applied to the selected word line. Other program conditions such as
voltages to selected bit lines, unselected bit lines, unselected
word lines, and select gates may be applied in step 920. For
example, a program enable voltage (e.g., 0V) can be applied to
selected bit lines, whereas a program inhibit voltage (e.g., Vdd)
may be applied to unselected bit lines. In one embodiment, boosting
voltages are applied to unselected word lines. The magnitude of the
boosting voltages depends on the pseudo block in which the boosting
voltage is applied, in one embodiment. Further details are
described below.
[0167] In step 922, a verification process is performed. Note that
when a particular memory cell has been verified as being programmed
to its intended state, it may be locked out from further
programming. The memory cell can be inhibited from further
programming while boosting the NAND string channel, in one
embodiment.
[0168] In step 924, it is determined whether memory cells have
verified that their threshold voltages are at the final target
voltage for that memory cell. Note that it is not required that
every memory cell for a given state is verified to be at the
appropriate threshold voltage. Error correction is able to correct
for some memory cells being above their target threshold voltage.
Error correction is able to correct for some memory cells being
over-programmed to a Vt that is lower than the target Vt
distribution. Step 924 is referring to all states having completed
programming.
[0169] If verification passes, the programming process is completed
successfully (status=pass) in step 926. If all of the memory cells
are not all verified, then it is determined whether the program
counter (PC) is less than a maximum value such as 20. If the
program counter (PC) is not less than max (step 928), then the
program process has failed (step 930). If the program counter (PC)
is less than a maximum value (e.g., 20), then the program counter
(PC) is incremented by 1 and the program voltage is stepped up to
the next pulse in step 932. Subsequent to step 932, the process
loops back to step 920 and the next set of program conditions are
applied to the memory cells.
Pseudo Block Read Disturb Operation
[0170] One operation that is performed in a pseudo block operation
mode is a pseudo block read disturb operation. The read disturb
characteristics may vary between storage elements in a 3D memory
device. As one possibility, the size of the memory hole affects
read disturb characteristics. In one embodiment, storage elements
are grouped into pseudo blocks based on read disturb
characteristics. For example, those with similar read disturb
characteristics may be grouped together. This grouping can minimize
the negative impact of read disturb without relaxing the entire
specification. Some pseudo blocks will be able to withstand a
greater number or reads (or read time) than others. Therefore, a
higher endurance pseudo block can be operated longer than a lower
endurance pseudo block.
[0171] For some 3D memory devices, read disturb is worse for
storage elements having smaller memory holes. FIG. 11A shows two
curves that represent how read disturb can be worse for smaller
memory holes. The curves 1102, 1104 show erase state upper tail
after a test number of reads versus memory hole size. For example,
memory cells are read a large number of times to determine the
effect on the upper tail of the erase state distribution. A greater
voltage corresponds to more read disturb. The two curves are for
two different read voltages applied to the control gate of the
memory cells. Both curves 1102, 1104 show that there is more read
disturb (higher Vt) for memory cells having smaller memory holes.
Curve 1102 is for a greater Vread than curve 1104, showing more
disturb.
[0172] FIG. 11B shows read access time versus memory hole size for
two different read voltages. Curve 1112 is for a higher Vread than
curve 1114. The target read time may be based on a target Vt
window. The curves should reach or exceed the target read time
(longer read time corresponds to better endurance). Each curve
1112, 1114 shows a larger memory hole is helpful to reach the
target read time. Conversely, if the memory hole is too small, then
the target read time may not be met. For example, curve 1114 fails
to reach the target read time for memory holes smaller than "A"
units. Curve 1112 fails to reach the target read time for memory
holes smaller than "B" units.
[0173] FIG. 11C is a flowchart of one embodiment of a process of a
read disturb pseudo operation mode. This is one embodiment of step
504 of FIG. 5D. In step 1122, a string read access time is recorded
for each pseudo block. It is not necessary to measure the actual
time of access. Rather, the number of times that the pseudo block
is read could be recorded. The access time can be estimated based
on the number of reads.
[0174] In one embodiment, reads are typically performed one word
line at a time. The read count may be of how many times a word line
was read in the pseudo block. It is not required that a separate
count be made of each of the word lines. Rather, the count can be
cumulative for all of the word lines in the pseudo block. Other
techniques can be used to accumulate a count of reads in the pseudo
block.
[0175] In step 1124, a determination is made whether the read
access time for given pseudo block is greater than a threshold for
that pseudo block. This threshold could be expressed as a number of
reads. Each pseudo block may have its own threshold. For example,
referring to FIG. 11B, a pseudo block having smaller memory holes
might have a lower threshold (e.g., smaller number of reads). As
noted, pseudo blocks having smaller memory holes can be more
susceptible to read disturb. Therefore, the lower threshold allows
the pseudo block to meet the target. On the other hand, pseudo
blocks having larger memory holes can have a higher threshold. Such
pseudo blocks can be operated longer without failing.
[0176] Step 1126 is to take an action for the pseudo block in the
event that the threshold is exceeded. One option is to mark the
pseudo block as a bad block such that it is no longer used. Its
data can be transferred to another block. Another option is to
re-program the pseudo block.
[0177] FIG. 11D and FIG. 11E are flowcharts depicting two different
embodiments for performing a pseudo block read disturb operation.
These flows show greater detail for the process of FIG. 11C. FIG.
11D describes one embodiment in which the pseudo block is
re-programmed when the read time crosses a threshold. In step 1132,
the pseudo block is read. This could be a read of any size unit in
the pseudo block. Thus, it will be understood that it is not
required that the entire pseudo block be read in step 1132. In one
embodiment, step 1132 is for a read of a word line. However, this
is just one possibility.
[0178] In step 1134, the total access time for the pseudo block is
incremented. This is achieved by incrementing a counter of the
number of read accesses, in one embodiment. This counter may be
re-set whenever the pseudo block is re-programmed.
[0179] In step 1136, the total read time is compared to a total
read time threshold for this pseudo block. The total read time
threshold is expressed as a maximum allowed number of read accesses
in one embodiment. If the total read time exceeds the threshold,
then the pseudo block is re-programmed in step 1138. Otherwise, the
process ends.
[0180] FIG. 11E describes one embodiment in which the pseudo block
is marked as bad when the read time crosses a threshold. The
process is similar to the one of FIG. 11D. Similar numbered steps
may be similar in the two processes. A difference is that, in step
1148, the pseudo block is marked as bad if the total read time
exceeds the threshold.
Pseudo Block Endurance Operation
[0181] One operation that is performed in a pseudo block operation
mode is a pseudo block endurance operation. Endurance refers to how
many erase/program cycles that a storage element can endure. The
tunnel oxide is one component that may be damaged. For some 3D
memory device, a storage element having a smaller memory hole has
better endurance. For some 3D memory device, erase damages the
storage element more than program.
[0182] For some 3D memory devices, a larger erase voltage is needed
to successfully erase storage elements having larger memory holes.
Recall that FIG. 6F shows that storage elements having a smaller
memory hole can be easier to erase. This means that such storage
elements might be erased with a smaller erase voltage. Thus, such
storage elements might be damaged less during a given erase process
relative to ones with larger memory holes.
[0183] FIG. 12A shows expected endurance characteristics as a
function of memory hole size. The y-axis is the number of
program/erase cycles. The x-axis is memory hole size. The curve
shows that for a larger memory hole size, endurance is expected to
be worse.
[0184] FIG. 12B is a flowchart of one embodiment of a process of a
pseudo block endurance mode of operation. The process is one
embodiment of step 504 of FIG. 5D. In step 1202, program/erase
cycles are recorded for each pseudo block.
[0185] In step 1204, a determination is made whether the cycle
number for a given pseudo block is greater than a threshold for
that pseudo block. Each pseudo block may have its own endurance
threshold. For example, referring to FIG. 12A, a pseudo block
having larger memory holes might have a lower threshold (e.g.,
fewer cycles). As noted, pseudo blocks having larger memory holes
might have lower endurance. Therefore, the lower endurance
threshold allows the pseudo block to operate without failure. On
the other hand, pseudo blocks having smaller memory holes can have
a higher threshold. Such pseudo blocks can be operated longer
without failing. Note that if the same endurance threshold were
applied to all storage elements, this hypothetical endurance
threshold might need to small enough such that storage elements
having the largest memory holes could meet the target endurance
(see FIG. 12A). However, by having a pseudo block dependent
endurance threshold, pseudo blocks having smaller memory holes can
successfully undergo greater numbers of cycles. Therefore, overall
the number of cycles can be improved.
[0186] Step 1206 is to make the pseudo block as failed, in response
to the endurance threshold for the pseudo block having been
exceeded.
[0187] FIG. 12C is a flowchart for one embodiment of a process of
pseudo block endurance mode of operation. This process depicts
specific operation for a certain pseudo block, thereby providing
more details for the process of FIG. 12B. In step 1212, a request
to erase a pseudo block is received. In step 1214, the
erase/program cycle count for this pseudo block is incremented. In
step 1216, a cycle limit for this pseudo block is accessed. As
noted, each pseudo block may have its own endurance threshold (or
cycle limit). Step 1218 determines whether erasing this pseudo
block would exceed the cycle limit. If so, then this pseudo block
is marked as bad, in step 1220. In step 1222, a failure message is
sent to the controller. The controller may then take a suitable
action, such as requesting that a different pseudo block be
allocated for erase/program. If step 1218 determines that the cycle
is not yet reached, the process of FIG. 6H may be used to erase the
pseudo block.
Pseudo Block Boosting Operation
[0188] One operation that is performed in a pseudo block operation
mode is a pseudo block boosting operation. Boosting refers to
boosting the channel potential of strings that are inhibited from
programming during a programming operation. Boosting the channel
potential helps to reduce or prevent program disturb. Boosting
involves applying a boosting voltage to word lines that are not
selected for programming. There are many different boosting
schemes.
[0189] For some 3D memory devices boosting is less effective for
storage elements having larger memory holes. Note that for an
architecture such as depicted in FIG. 5A, a program voltage is
applied to a selected word line, whereas boosting voltages may be
applied to unselected words. For example, the selected word line
might be WL45, which is in pseudo block PB1, in that example. Thus,
boosting voltages could be applied to word lines in any (or all) of
the pseudo blocks. In one embodiment, the boosting voltage that is
applied to the control gate depends on which pseudo block the
storage element is in. However, also note that the boosting voltage
may depend on the relative location of the unselected word line to
the selected word line. This will be further discussed below.
[0190] Since for some 3D memory devices, a larger memory hole may
have worse boosting, the boosting voltage depends on the memory
hole size, in one embodiment. This may be achieved by having the
boosting voltage depend on the pseudo block.
[0191] FIG. 13A shows a relationship between boosted channel
potential versus memory hole size for one example 3D memory device.
The curve shows the channel potential for some arbitrary boosting
voltage V.sub.PASS applied to the unselected word line(s). The
curve shows that channel potential is expected to be lower for
larger memory holes.
[0192] A larger boosting voltage (V.sub.PASS) can be used to
increase the channel potential. However, using a larger V.sub.PASS
may result in greater V.sub.PASS disturb. V.sub.PASS disturb refers
to the impact of V.sub.PASS on storage elements that are already
programmed. Storage elements having a smaller memory hole may
suffer more from V.sub.PASS disturb than those with larger memory
holes. This effect may be similar to how read disturb may be worse
for storage elements having a smaller memory hole.
[0193] FIG. 13B shows V.sub.PASS disturb versus memory hole size
for several values of V.sub.PASS. Each curve represents a different
value of V.sub.PASS. The arrow shows that the higher curves are
those for higher V.sub.PASS. Lower y-axis values correspond to less
V.sub.PASS disturb. The fail line represents a maximum amount of
read disturb that is tolerable to meet an arbitrary specification.
FIG. 13B shows that for smaller memory holes, the tolerance level
is reached when using a smaller V.sub.PASS. This suggests that
smaller memory holes should have a smaller V.sub.PASS to meet a
read disturb specification.
[0194] Moreover, by using a smaller V.sub.PASS for smaller memory
holes, the read disturb specification can be met for all memory
cells without relaxing the read disturb specification. In other
words, a larger V.sub.PASS can be used for larger memory holes
while still meeting the read disturb specification.
[0195] FIG. 13B also shows one possible way of dividing the layers
of storage elements into pseudo blocks (PB1-PB5). Referring again
to FIG. 5A, PB1 has the largest memory holes (higher layers) and
PB5 has the smallest memory holes (lowest layer), for that example.
Thus, PB1 (having largest memory holes) might use higher values of
V.sub.PASS, whereas PB5 (having smallest memory holes) might use
lower values of V.sub.PASS. Pseudo blocks PB2-PB4 can use
progressively lower values for V.sub.PASS, according to their
memory hole sizes.
[0196] FIG. 13C is a flowchart of one embodiment of a process of a
pseudo block dependent V.sub.PASS operation. The process is one
embodiment of step 504 of FIG. 5D. In step 1302 a request is
received to program a pseudo block. In step 1304, boosting voltages
(V.sub.PASS) are accessed for various pseudo blocks. Note that the
boosting voltages will be applied to unselected word lines in other
pseudo blocks (and possibly in the pseudo block having the selected
word line).
[0197] In step 1306, the pseudo block is programmed using pseudo
block dependent boosting voltages. Step 1306 includes applying a
boosting voltage to a control gate of an unselected non-volatile
storage element on the NAND string that is not selected for
programming. That is, the boosting voltage is applied to an
unselected word line. The magnitude of the boosting voltage depends
on location of the unselected non-volatile storage element (and
hence the unselected word line) on the NAND string.
[0198] For purposes of illustration, the following is one example
boosting scheme. It will be understood that other boosting schemes
can benefit from a pseudo block dependent V.sub.PASS operation.
TABLE-US-00001 TABLE I Voltage WL Example Voltages Vpgm WLn Vpgm
(varies) Vpass3 WLn - 1, WLn + 1 6.5 V-12.5 V Vpass2 all other WLs
6.5 V-11.5 V Vpass1 WLn - 7, WLn + 7 6.5 V-8.5 V VGP1/2 WLn - 6,
WLn - 4, WLn + 4, WLn + 6 4.5 V VISO WLn - 5, WLn + 5 0.5 V
[0199] Note that it is not required that all of the boosting
voltages have their value depend on the pseudo block. As one
example, the boosting voltages Vpass2 and Vpass3 have their values
depend on the pseudo block in which the boosting voltage is
applied. However, Vpass1, VGP1/2 and VISO are not dependent on the
pseudo block. However, any of Vpass1, VGP1/2 and VISO could depend
on the pseudo block in which they are applied.
[0200] The following table shows one possible scheme for a pseudo
block dependent Vpass3. In this example, Vpass3 is a nominal value.
The pseudo blocks refer to an example such as in FIG. 5A, in which
PB1 to PB5 have progressively smaller memory holes.
TABLE-US-00002 TABLE II Pseudo Block Voltage PB5 Vpass3 PB4 Vpass3
+ Vpass3_shift1 PB3 Vpass3 + Vpass3_shift2 PB2 Vpass3 +
Vpass3_shift3 PB1 Vpass3 + Vpass3_shift4
[0201] The following table shows one possible scheme for a pseudo
block dependent Vpass2. In this example, Vpass2 is a nominal value.
The pseudo blocks refer to an example such as in FIG. 5A, in which
PB1 to PB5 have progressively smaller memory holes.
TABLE-US-00003 TABLE III Pseudo Block Voltage PB5 Vpass2 PB4 Vpass2
+ Vpass2_shift1 PB3 Vpass2 + Vpass2_shift2 PB2 Vpass2 +
Vpass2_shift3 PB1 Vpass2 + Vpass2_shift4
[0202] One embodiment disclosed herein includes a 3D NAND stacked
non-volatile memory device, comprising a plurality of NAND strings
and a control circuit in communication with the NAND strings. Each
NAND string comprises a plurality of non-volatile storage elements.
Each NAND string comprises a channel that extends vertically
through layers of the 3D NAND stacked non-volatile memory device.
The block is divided into pseudo blocks that each comprise a
portion of each of the NAND strings. Each of the NAND strings is a
member of each of the pseudo blocks. The control circuit operates
in a pseudo block operation mode that performs operations on a
pseudo block level.
[0203] One embodiment disclosed herein includes a method of
operating a 3D NAND stacked non-volatile memory device comprising a
plurality of NAND strings in a block. Each NAND string comprises a
plurality of non-volatile storage elements. Each NAND string
comprises a channel that extends vertically through layers of the
3D NAND stacked non-volatile memory device. The block is divided
into pseudo blocks that each comprise a portion of each of the NAND
strings. Each of the NAND strings is a member of each of the pseudo
blocks. The method comprises performing operations on a pseudo
block level.
[0204] One embodiment disclosed herein includes a 3D NAND stacked
non-volatile memory device, comprising a plurality of NAND strings
and a control circuit in communication with the NAND strings. The
NAND string comprises a channel that extends vertically through
layers of the 3D NAND stacked non-volatile memory device. The
control circuit is configured to perform an erase of a portion of
the NAND string while leaving other non-volatile storage elements
on the NAND string programmed.
[0205] One embodiment disclosed herein includes a 3D NAND stacked
non-volatile memory device, comprising a plurality of NAND strings
and a control circuit in communication with the NAND strings. The
control circuit is configured to apply a boosting voltage to a
control gate of an unselected non-volatile storage element on the
NAND string that is not selected for programming A magnitude of the
boosting voltage depends on location of the unselected non-volatile
storage element on the NAND string.
[0206] The foregoing detailed description of embodiments 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.
* * * * *