U.S. patent number RE45,732 [Application Number 14/192,277] was granted by the patent office on 2015-10-06 for nonvolatile semiconductor storage device and method for manufacturing same.
This patent grant is currently assigned to KABUSHIKI KAISHA TOSHIBA. The grantee listed for this patent is KABUSHIKI KAISHA TOSHIBA. Invention is credited to Masahiro Kiyotoshi.
United States Patent |
RE45,732 |
Kiyotoshi |
October 6, 2015 |
Nonvolatile semiconductor storage device and method for
manufacturing same
Abstract
A nonvolatile semiconductor storage device includes: a plurality
of stacked units juxtaposed on a major surface of a substrate, each
stacked unit aligning in a first direction parallel to the major
surface of the substrate; and a gate electrode aligning parallel to
the major surface in a second direction non-parallel to the first
direction. Each of the plurality of stacked units includes a
plurality of stacked semiconductor layers via an insulating layer.
The plurality of stacked units are juxtaposed so that the spacings
between adjacent stacked units are alternately a first spacing and
a second spacing larger than the first spacing. The second spacing
is provided at a periodic interval four times a size of a half
pitch F of the bit line. The gate electrode includes a protruding
portion that enters into a gap of the second spacing between the
stacked units. A first insulating film, a charge storage layer, and
a second insulating film are provided between a side face of the
semiconductor layer and the protruding portion.
Inventors: |
Kiyotoshi; Masahiro (Mie-ken,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA |
Tokyo |
N/A |
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
(Tokyo, JP)
|
Family
ID: |
41378695 |
Appl.
No.: |
14/192,277 |
Filed: |
February 27, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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12476799 |
Jun 2, 2009 |
7851849 |
|
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Reissue of: |
12905255 |
Oct 15, 2010 |
8124514 |
Feb 28, 2012 |
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Foreign Application Priority Data
|
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|
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Jun 3, 2008 [JP] |
|
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2008-146188 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
27/11573 (20130101); H01L 27/11578 (20130101); H01L
27/11582 (20130101) |
Current International
Class: |
H01L
21/22 (20060101); H01L 27/115 (20060101) |
Field of
Search: |
;438/591
;257/E27.078 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Korean Office Action issued on Jan. 24, 2011 in corresponding
Korean Application No. 10-2009-48724 (with an English Translation).
cited by applicant.
|
Primary Examiner: Menefee; James
Attorney, Agent or Firm: Oblon, McClelland, Maier &
Neustadt, L.L.P.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is .Iadd.a reissue application of U.S. Pat. No.
8,124,514, which is .Iaddend.a divisional of U.S. application Ser.
No. 12/476,799 filed Jun. 2, 2009 .Iadd.(now U.S. Pat. No.
7,851,849).Iaddend., and is based upon and claims the benefits of
priority from the prior Japanese Patent Application No.
2008-146188, filed on Jun. 3, 2008; the entire contents of each of
which are incorporated herein by reference.
Claims
The invention claimed is:
1. A method for manufacturing a nonvolatile semiconductor storage
device, the device including: a semiconductor layer; a gate
electrode provided facing the semiconductor layer; a charge storage
layer provided between the semiconductor layer and the gate
electrode; a first insulating film provided between the
semiconductor layer and the charge storage layer; and a second
insulating film provided between the charge storage layer and the
gate electrode, the method comprising: stacking to form a
semiconductor layer film and an insulating layer film on a major
surface of a substrate, the semiconductor layer film forming the
semiconductor layer, the insulating layer film forming an
insulating layer; processing the semiconductor layer film and the
insulating layer film to alternately provide a first spacing and a
second spacing larger than the first spacing between stacked units,
by using a side wall as a mask pattern, the side wall being formed
on a side face of a dummy pattern .[.formed at a period four times
a size of a half pitch F of the bit line of the device.]., the side
wall aligning in a first direction parallel to the major surface,
the stacked units including the semiconductor layer and the
insulating layer; forming a film forming the first insulating film,
a film forming the charge storage layer, and a film forming the
second insulating film on a side face of the stacked unit on the
second spacing side, and further forming a gate electrode film
forming the gate electrode on the film forming the second
insulating film and on a major surface of the stacked unit; and
processing the gate electrode film into a band configuration
aligning in a second direction parallel to the major surface and
non-parallel to the first direction; wherein the charge storage
layer film is separated for each side face of the stacked
semiconductor layer on the second spacing side after the processing
the gate electrode; wherein the separating the charge storage layer
film includes selectively oxidizing the charge storage layer film
by oxidizing via the stacked insulating layer.
2. The method according to claim 1, wherein a side face of the
insulating layer on the second spacing side is recessed from a side
face of the adjacent stacked semiconductor layer on the second
spacing side prior to the processing the gate electrode.
3. A method for manufacturing a nonvolatile semiconductor storage
device, the device including: a semiconductor layer; a gate
electrode provided facing the semiconductor layer; a charge storage
layer provided between the semiconductor layer and the gate
electrode; a first insulating film provided between the
semiconductor layer and the charge storage layer; and a second
insulating film provided between the charge storage layer and the
gate electrode, the method comprising: stacking to form a
semiconductor layer film and an insulating layer film on a major
surface of a substrate, the semiconductor layer film forming the
semiconductor layer, the insulating layer film forming an
insulating layer; processing the semiconductor layer film and the
insulating layer film to alternately provide a first spacing and a
second spacing larger than the first spacing between stacked units,
by using a side wall as a mask pattern, the side wall being formed
on a side face of a dummy pattern .[.formed at a period four times
a size of a half pitch F of the bit line of the device.]., the side
wall aligning in a first direction parallel to the major surface,
the stacked units including the semiconductor layer and the
insulating layer; forming a film forming the first insulating film,
a film forming the charge storage layer, and a film forming the
second insulating film on a side face of the stacked unit on the
second spacing side, and further forming a gate electrode film
forming the gate electrode on the film forming the second
insulating film and on a major surface of the stacked unit; and
processing the gate electrode film into a band configuration
aligning in a second direction parallel to the major surface and
non-parallel to the first direction; wherein the stacking to form
the semiconductor layer film and the insulating layer film includes
selectively removing an epitaxially grown silicon-germanium film
and thermally oxidizing at least a portion of an epitaxially grown
silicon film after alternately stacking the epitaxially grown
silicon film and the epitaxially grown silicon-germanium film;
wherein the stacking to form the semiconductor layer film and the
insulating layer film further includes filling an insulating
material between the thermally oxidized epitaxially grown silicon
films, the insulating material having an etching rate higher than
that of the thermally oxidized epitaxially grown silicon film.
.Iadd.4. The method according to claim 1, wherein the dummy pattern
is formed at a period four times a size of a half pitch of the bit
line of the device..Iaddend.
.Iadd.5. The method according to claim 3, wherein the dummy pattern
is formed at a period four times a size of a half pitch of the bit
line of the device..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonvolatile semiconductor
storage device and a method for manufacturing the same.
2. Background Art
Nonvolatile memory typified by NAND flash memory is used widely for
large-capacity data storage in mobile telephones, digital still
cameras, USB memory, silicon audio, and the like. The market
continues to grow due to the reduction of manufacturing costs per
bit enabled by rapid downscaling. NAND flash memory in particular
has a small cell surface area of 4F.sup.2, where F is the minimum
processing dimension (the half pitch); and rapid downscaling due to
the simple structure thereof is progressing. However, bit density
increase of future NAND flash memory will face many issues such as
limitations on the downscaling of lithography, reduced reliability
due to the transistor scaling, increased fluctuation of the
transistor characteristics accompanying the transistor size
downscaling, limitations on operation speed due to negative effects
of downscaling such as short channel effects or narrow channel
effects, and so on.
Conversely, P-A 2007-266143 (Kokai) discusses technology relating
to a nonvolatile semiconductor storage device in which insulating
films and charge storage layers are provided around a semiconductor
of a columnar configuration, and multiple recording layers are
stacked. However, in the case of this structure, it is necessary to
pattern the gate electrode that selects the charge storage layer
into a striped configuration in a plane parallel to the substrate.
Therefore, the cell surface area thereof is 6F.sup.2 as compared to
the cell surface area of normal NAND flash memory of 4F.sup.2
(where F is the half pitch). Further, in the case of the structure
of P-A 2007-266143 (Kokai), downscaling is constrained by
manufacturing process constraints when forming a silicon plug after
forming the gate electrode. Then, forming the silicon plug after
the gate electrode formation constrains the materials used in the
gate electrode; and it is difficult to reduce the resistance.
Therefore, it is necessary to make the gate electrode thick or
increase the space between adjacent cells to prevent the depletion
of the electrode. Thereby, downscaling is constrained. Moreover, an
operation mechanism is different than that of NAND flash memory.
Therefore, conventional peripheral circuit configurations of NAND
flash memory will be re-designed before diversion
SUMMARY OF THE INVENTION
According to an aspect of the invention, there is provided a
nonvolatile semiconductor storage device including: a substrate; a
plurality of stacked units juxtaposed on a major surface of the
substrate, each stacked unit aligning in a first direction parallel
to the major surface of the substrate; a gate electrode aligning
parallel to the major surface in a second direction non-parallel to
the first direction; a charge storage layer; a first insulating
film; and a second insulating film, each of the plurality of
stacked units including a plurality of stacked semiconductor layers
via an insulating layer, spacings between adjacent stacked units
having a first spacing and a second spacing larger than the first
spacing, the first spacing and the second spacing being alternately
provided, the second spacing being provided at a periodic interval
four times a size of a half pitch F of the bit line of the device,
the gate electrode including a protruding portion entering into a
gap between the stacked units, the gap having the second spacing,
the charge storage layer being provided between a side face of the
semiconductor layer and the protruding portion, the first
insulating film being provided between the side face of the
semiconductor layer and the charge storage layer, and the second
insulating film being provided between the charge storage layer and
the protruding portion.
According to another aspect of the invention, there is provided a
method for manufacturing a nonvolatile semiconductor storage
device, the device including: a semiconductor layer; a gate
electrode provided facing the semiconductor layer; a charge storage
layer provided between the semiconductor layer and the gate
electrode; a first insulating film provided between the
semiconductor layer and the charge storage layer; and a second
insulating film provided between the charge storage layer and the
gate electrode, the method including: stacking to form a
semiconductor layer film and an insulating layer film on a major
surface of a substrate, the semiconductor layer film forming the
semiconductor layer, the insulating layer film forming an
insulating layer; processing the semiconductor layer film and the
insulating layer film to alternately provide a first spacing and a
second spacing larger than the first spacing between stacked units,
by using a side wall as a mask pattern, the side wall being formed
on a side face of a dummy pattern formed at a period four times a
size of a half pitch F of the bit line of the device, the side wall
aligning in a first direction parallel to the major surface, the
stacked units including the semiconductor layer and the insulating
layer; forming a film forming the first insulating film, a film
forming the charge storage layer, and a film forming the second
insulating film on a side face of the stacked unit on the second
spacing side, and further forming a gate electrode film forming the
gate electrode on the film forming the second insulating film and
on a major surface of the stacked unit; and processing the gate
electrode film into a band configuration aligning in a second
direction parallel to the major surface and non-parallel to the
first direction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1C are schematic views illustrating a configuration of
a nonvolatile semiconductor storage device according to a first
embodiment of the present invention;
FIG. 2 is a schematic plan view illustrating the half pitch of the
nonvolatile semiconductor storage device according to the first
embodiment of the present invention;
FIGS. 3A to 3E are schematic cross-sectional views in order of the
steps, illustrating main components of a method for manufacturing
the nonvolatile semiconductor storage device according to the first
embodiment of the present invention;
FIGS. 4A and 4B are schematic cross-sectional views illustrating
structures of main components of the nonvolatile semiconductor
storage device according to the first embodiment of the present
invention;
FIGS. 5A and 5B are schematic cross-sectional views illustrating a
method for manufacturing a nonvolatile semiconductor storage device
according to a first example of the first embodiment of the present
invention;
FIGS. 6A and 6B are schematic cross-sectional views illustrating a
method for manufacturing a nonvolatile semiconductor storage device
according to the first example;
FIGS. 7A to 7C are views schematic cross-sectional views
illustrating a method for manufacturing a nonvolatile semiconductor
storage device according to the first example;
FIGS. 8A and 8B are views schematic cross-sectional views
illustrating a method for manufacturing a nonvolatile semiconductor
storage device according to the first example;
FIGS. 9A and 9B are schematic cross-sectional views illustrating a
method for manufacturing a nonvolatile semiconductor storage device
according to a second example of the first embodiment of the
present invention;
FIGS. 10A and 10B are schematic cross-sectional views illustrating
a method for manufacturing a nonvolatile semiconductor storage
device according to the second example;
FIGS. 11A and 11B are schematic cross-sectional views illustrating
a method for manufacturing a nonvolatile semiconductor storage
device according to the second example;
FIG. 12 is a graph illustrating characteristics of a nonvolatile
semiconductor storage device according to an embodiment of the
present invention;
FIG. 13 is a flowchart illustrating a method for manufacturing a
nonvolatile semiconductor storage device according to a second
embodiment of the present invention; and
FIG. 14 is a flowchart illustrating a method for manufacturing a
nonvolatile semiconductor storage device according to a third
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will now be described in
detail with reference to the drawings.
The drawings are schematic or conceptual; and the relationships
between the thickness and width of portions, the proportions of
sizes among portions, etc., are not necessarily the same as the
actual values thereof. Further, the dimensions and proportions may
be illustrated differently among drawings, even for identical
portions.
In the specification and drawings, components similar to those
described in regard to a drawing thereinabove are marked with like
reference numerals, and a detailed description is omitted as
appropriate.
First Embodiment
FIGS. 1A to 1C are schematic views illustrating a configuration of
a nonvolatile semiconductor storage device according to a first
embodiment of the present invention.
That is, FIG. 1A is a plan view, FIG. 1B is a cross-sectional view
along line A-A' of FIG. 1A, and FIG. 1C is a cross-sectional view
along line B-B' of FIG. 1A.
As illustrated in FIGS. 1A to 1C, a nonvolatile semiconductor
storage device 10 according to the first embodiment of the present
invention includes a semiconductor layer 50, a gate electrode 70
provided to face the semiconductor layer 50, a charge storage layer
60 provided between the semiconductor layer 50 and the gate
electrode 70, a first insulating film (not illustrated) provided
between the semiconductor layer 50 and the charge storage layer 60,
and a second insulating film (not illustrated) provided between the
charge storage layer 60 and the gate electrode 70. In other words,
the nonvolatile semiconductor storage device 10 is a memory
including a MONOS (Metal Oxide Nitride Oxide Semiconductor)
structure.
The semiconductor layer 50 aligns in a first direction (for
example, an X-axis direction) parallel to a major surface 41 of a
substrate 40, is multiply stacked on the substrate 40 via an
insulating layer 55, and is multiply provided in a plane parallel
to the major surface 41 with a spacing therebetween alternately
arranged to be a first spacing 51 and a second spacing 52 larger
than the first spacing 51.
The second spacing 52 is arranged at a period L4 which is four
times the size of a half pitch F of the bit line of the nonvolatile
semiconductor storage device 10. The half pitch F is described
below.
Such a configuration can be formed by using a mask pattern formed
by a side wall formed on a side face of a dummy pattern formed at a
period four times the size of the half pitch F, and alternately
arranging the spacing of the semiconductor layers 50 to be the
first spacing 51 and the second spacing 52 which is larger than the
first spacing 51. A method for forming the first spacing 51 and the
second spacing 52 by utilizing the side wall is described
below.
On the other hand, the gate electrode 70 is provided parallel to
the major surface 41 to align in a second direction (for example, a
Y-axis direction) non-parallel to the first direction (the X-axis
direction). A portion of the gate electrode 70 enters into the
second spacing 52 side of the semiconductor layers 50 to form a
protruding portion 78. The gate electrode 70 has a single period,
that is, is arranged at a distance L7. For example, the distance L7
may be twice the size of the half pitch F. In other words, the gate
electrode 70 is arranged, for example, at a period twice the size
of the half pitch F of the word line of the device.
The charge storage layer 60 is provided between a face 50v of the
semiconductor layer 50 on the second spacing 52 side perpendicular
to the major surface 41 and a face 70v of the gate electrode 70
perpendicular to the major surface 41.
As illustrated in FIGS. 1A to 1C, a Z axis is a direction
perpendicular to the major surface 41 of the substrate 40; the X
axis is an axis perpendicular to the Z axis; and the Y axis is a
direction perpendicular to the Z axis and the X axis.
In this example, the stacked semiconductor layers 50 are multiply
provided in the X-axis direction and align in band configurations.
The spacing between the semiconductor layers 50 includes a narrow
portion (the first spacing 51) and a wide portion (the second
spacing 52). The gate electrode 70 aligns to intersect with the
semiconductor layer 50. The first insulating film, the charge
storage layer 60, and the second insulating film are provided
between a side face of the semiconductor layer 50 and a side face
of the gate electrode 70. In other words, the charge storage layers
60 which form memory cells (also referred to as "cells") of the
nonvolatile semiconductor storage device 10 are stacked in both the
direction perpendicular to the major surface 41 of the substrate 40
and directions parallel thereto.
In other words, the nonvolatile semiconductor storage device 10
according to this embodiment includes the substrate 40, the
multiple stacked units 58 juxtaposed on the major surface 41 of the
substrate 40 such that each aligns in the first direction parallel
to the major surface 41 of the substrate 40, and the gate electrode
70 aligning parallel to the major surface 41 in the second
direction non-parallel to the first direction. Each of the multiple
stacked units 58 includes multiple semiconductor layers 50 stacked
via the insulating layer 55. The multiple stacked units 58 are
juxtaposed such that the spacing between adjacent stacked units 58
is alternately provided as the first spacing 51 and the second
spacing 52 which is larger than the first spacing 51. The second
spacing 52 is provided at a periodic interval four times the size
of the half pitch F of the bit line of the nonvolatile
semiconductor storage device 10. The gate electrode 70 includes the
protruding portion 78 which enters into the gap of the second
spacing 52 between the stacked units 58. The nonvolatile
semiconductor storage device 10 further includes the charge storage
layer 60 provided between the side face 50v of the semiconductor
layer 50 and the protruding portion 78, the first insulating film
(not illustrated) provided between the side face 50v of the
semiconductor layer 50 and the charge storage layer 60, and the
second insulating film (not illustrated) provided between the
charge storage layer 60 and the protruding portion 78.
The nonvolatile semiconductor storage device 10 according to this
embodiment having such a structure provides a nonvolatile
semiconductor storage device that has few constraints on
miniaturization, is easy to manufacture, and does not require
drastic modifications to the peripheral circuit.
Although six semiconductor layers 50 and six gate electrodes 70 are
illustrated in FIGS. 1A to 1C, the present invention is not limited
thereto. The numbers of semiconductor layers 50 and gate electrodes
70 are arbitrary. Although two of the semiconductor layers 50 are
stacked via the insulating layer 55, the number of stack layers of
the semiconductor layer 50 is arbitrary.
In the configuration recited above, the semiconductor layer 50 is
periodically arranged with the wide spacing and the narrow spacing.
Therefore, it is easy to provide the first insulating film, the
charge storage layer 60, and the second insulating film on the wide
spacing (the second spacing) side.
For example, in the case where the semiconductor layers 50 are
provided with a single spacing, it is difficult to set aside space
to provide the first insulating film, the charge storage layer 60,
and the second insulating film recited above in the spacing
therebetween and miniaturize the element. For example, generally,
the half pitch F of a nonvolatile semiconductor storage device is
established based on the minimum processing dimension of the
semiconductor manufacturing step. F is reduced when attempting to
increase the bit density of the nonvolatile semiconductor storage
device. In such a case, the first insulating film, the charge
storage layer 60, the second insulating film, the protruding
portion 78, the second insulating film, the charge storage layer
60, and the first insulating film must be provided in a gap having
a width of the half pitch F. Therefore, it is extremely difficult
to reduce F.
Conversely, in the nonvolatile semiconductor storage device 10
according to this embodiment, the spacing between the semiconductor
layers 50 is provided to be the narrow first spacing 51 and the
wide second spacing 52. Thereby, the second spacing 52 can be made
larger than the half pitch F of the bit line; and it is relatively
easy to set aside space to provide the first insulating film, the
charge storage layer 60, the second insulating film, the protruding
portion 78, the second insulating film, the charge storage layer
60, and the first insulating film recited above in the gap on the
wide second spacing side.
As illustrated in FIGS. 1A to 1C, the spacing between the
semiconductor layers 50 includes a portion of a distance t1 (the
first spacing 51) and a portion of a distance t2 (the second
spacing 52) larger than the distance t1 alternately arranged. The
width of the semiconductor layer 50, that is, a width t3, may
essentially be constant.
The semiconductor layer 50 is arranged at a narrow pitch L1 and a
wide pitch L2. Here, narrow pitch L1=distance t1+width t3, and wide
pitch L2=distance t2+width t3. A semiconductor layer pair 53 is
arranged at a pair pitch L3, where the semiconductor layer pair 53
is a pair of semiconductor layers 50 arranged with a spacing of the
distance t1. At this time, pair pitch L3=narrow pitch L1+wide pitch
L2=distance t1+distance t2+2.times.width t3. Then, a pitch L4 of
the second spacing 52 is equal to the pair pitch L3.
The first spacing (the distance t1) may be provided at a period of
4F, where F is the half pitch of the bit line of the nonvolatile
semiconductor storage device 10. The second spacing (the distance
t2) also may be provided at a period (L4) of 4F. The semiconductor
layer pair 53 also may be provided at a period of 4F. In other
words, the pair pitch L3 recited above may be 4F.
At this time, the spacing, i.e., the distance t1, between the pair
of the semiconductor layers 50 forming the semiconductor layer pair
53 may be set smaller than F. The spacing, i.e., the distance t2,
between adjacent semiconductor layer pairs 53 may be set larger
than F.
However, the nonvolatile semiconductor storage device 10 according
to this embodiment is not limited thereto. It is sufficient that
the semiconductor layer pair 53 is provided at the period of 4F,
the distance t2 is set larger than F, and the charge storage layer
60 is disposed between the semiconductor layer pairs 53 having a
spacing of the distance t2, that is, on the second spacing 52
side.
Similar to the semiconductor layer 50, the gate electrode 70
aligning in the second direction also may be formed such that the
spacing between the gate electrodes 70 has a narrow portion and a
wide portion. Thereby, a connection portion between the gate
electrode 70 and the peripheral circuit, for example, may be
provided on the wide spacing side to easily provide a connection.
Hereinbelow, the description assumes the case where the gate
electrode 70 is provided at a single period, that is, an example in
which the distance L7 is 2F.
The half pitch F of the nonvolatile semiconductor storage device 10
according to this embodiment will now be described. Hereinbelow,
the description assumes that the same half pitch is used in both
the X-axis direction and the Y-axis direction.
FIG. 2 is a schematic plan view illustrating the half pitch of the
nonvolatile semiconductor storage device according to the first
embodiment of the present invention.
In the nonvolatile semiconductor storage device 10 according to
this embodiment illustrated in FIG. 2, the semiconductor layer pair
53 formed by two semiconductor layers 50 juxtaposed with the narrow
first spacing 51 (t1) is juxtaposed at the pair pitch L3, that is,
the period of 4F. On the other hand, the gate electrode 70 is
juxtaposed at a period of 2F.
As illustrated in FIG. 2, one memory cell 80a corresponds to a
portion where one semiconductor layer 50a and one gate electrode
70a intersect. Now focusing on the memory cell 80a including a
charge storage layer provided between a side face of the
semiconductor layer 50a and a side face of the gate electrode 70a,
an occupied surface 81a occupied by the memory cell 80a has a
surface area S. In this case, as illustrated in FIG. 2, four memory
cells 80b, 80c, 80d, and 80e are adjacent to the memory cell 80a.
The memory cells 80b, 80c, 80d, and 80e also have occupied surfaces
81b, 81c, 81d, and 81e that have the same surface area S as the
memory cell 80a.
In this case, boundaries between the occupied surface 81a of the
memory cell 80a and occupied surfaces of memory cells adjacent in
the X-axis direction may be, for example, a center line between the
semiconductor layer 50a of the memory cell 80a and an adjacent
semiconductor layer 50b, and a center line between the
semiconductor layer 50a and another adjacent semiconductor layer
50c.
Similarly, boundaries between the occupied surface 81a of the
memory cell 80a and occupied surfaces of adjacent memory cells
adjacent in the Y-axis direction may be, for example, a center line
between the gate electrode 70a of the memory cell 80a and an
adjacent gate electrode 70b, and a center line between the gate
electrode 70a and another adjacent gate electrode 70c.
The region enclosed by these four center lines may define the
occupied surface 81a of the memory cell 80a.
In the nonvolatile semiconductor storage device 10, the period of
the semiconductor layer pair 53 is 4F, and the period of the gate
electrode 70 is 2F, and therefore the relationship
2.times.S=2F.times.4F, that is, S=4F.sup.2 holds.
A method for constructing the semiconductor layer 50 and the
insulating layer 55 thus including a spacing therebetween having a
narrow portion and a wide portion will now be schematically
described. To simplify the description, the case is illustrated
where only two layers of each of the semiconductor layer 50 and the
insulating layer 55 are used. Hereinbelow, the case is described
where F is 25 nm.
FIGS. 3A to 3E are schematic cross-sectional views in order of the
steps, illustrating main components of a method for manufacturing
the nonvolatile semiconductor storage device according to the first
embodiment of the present invention.
First, as illustrated in FIG. 3A, two alternating layers of a
semiconductor layer film 59 which forms the semiconductor layers 50
and an insulating layer film 55a which forms the insulating layers
55, a CVD-deposited silicon oxide film 85 which forms a hard mask,
and a silicon nitride film 86 are formed on the substrate 40.
Lithography is used to form a resist mask pattern 87 having a pitch
of L3 (equal to 4F, that is, 100 nm) and a width W1 of, for
example, 30 nm.
As illustrated in FIG. 3B, hot phosphoric acid processing is
performed on the silicon nitride film 86, and the silicon nitride
film 86 is slimmed (its profile is reduced) and patterned to a
width W2 of, for example, 15 nm.
Then, as illustrated in FIG. 3C, an amorphous silicon film 89 is
formed on the entire surface of the substrate 40 with a thickness
of 20 nm. Reactive ion etching is then performed and the amorphous
silicon film 89 is left only on side walls of the silicon nitride
film 86.
Continuing as illustrated in FIG. 3D, hot phosphoric acid
processing is performed to remove the silicon nitride film 86. The
CVD-deposited silicon oxide film 85 is patterned using the
amorphous silicon film 89 as a mask.
Then, as illustrated in FIG. 3E, the amorphous silicon film 89 is
removed and a hard mask is formed by the CVD-deposited silicon
oxide film 85. The hard mask is used to pattern the two layers of
the semiconductor layer film 59 and the insulating layer film
55a.
Such a method, i.e., double patterning technology, can form the
semiconductor layer 50 and the insulating layer 55 such that a
spacing therebetween has a narrow portion and a wide portion. The
semiconductor layer pair 53 of the semiconductor layers 50 is
arranged at a period of 4F, i.e., the pair pitch L3. The spacing
(the distance t1) between the pair of the semiconductor layers 50
forming the semiconductor layer pair 53 is smaller than F (15 nm
ignoring the conversion difference of the patterning). The width of
the semiconductor layer 50 (the width t3) is 20 nm ignoring the
conversion difference of the patterning. The spacing (the distance
t2) between adjacent semiconductor layer pairs 53 is larger than F
(45 nm ignoring the conversion difference of the patterning).
Although a structure is illustrated where the semiconductor layer
film 59 which forms the semiconductor layers 50 and the insulating
layer film 55a which forms the insulating layers 55 are alternately
stacked in two layers to simplify the above description, the number
of stacks of the semiconductor layer 50 and the insulating layer 55
is arbitrary. The gate electrode 70 also may have a structure in
which a narrow spacing and a wide spacing are periodically arranged
using the double patterning technology recited above as
necessary.
FIGS. 4A and 4B are schematic cross-sectional views illustrating
structures of main components of the nonvolatile semiconductor
storage device according to the first embodiment of the present
invention.
In other words, FIGS. 4A and 4B illustrate two examples of
structures of the first insulating film, the charge storage layer,
and the second insulating film of the nonvolatile semiconductor
storage device according to this embodiment. FIGS. 4A and 4B
illustrate enlarged cross sections corresponding to line A-A' of
FIG. 1A.
As illustrated in FIG. 4A, the charge storage layers 60 are
provided between the faces 50v (the side faces) of the
semiconductor layers 50 perpendicular to the major surface 41 on
the second spacing 52 side and the face 70v (the side face) of the
gate electrode 70 perpendicular to the major surface 41. A first
insulating film 61 is provided between the semiconductor layers 50
and the charge storage layers 60. A second insulating film 62 is
provided between the charge storage layers 60 and the gate
electrode 70. The first insulating film 61 may function as a tunnel
insulating film. The second insulating film 62 may function as a
block insulating film. An insulating film 56 is provided between
the semiconductor layers 50 on the first spacing 51 side.
In the nonvolatile semiconductor storage device 10 illustrated in
FIG. 4A, the charge storage layers 60 are provided separately for
each face 50v (side face) of the stacked semiconductor layers 50
perpendicular to the major surface 41 on the second spacing 52
side. Thereby, the deterioration of charge retention
characteristics due to a lateral diffusion of the charge in the
charge storage layer 60, from a cell to which the charge is
written, to an unwritten cell or into other films can be
inhibited.
In another nonvolatile semiconductor storage device 11 of this
embodiment illustrated in FIG. 4B, the charge storage layer 60 is
continuously provided along the faces 50v (the side faces) of the
stacked semiconductor layers 50 perpendicular to the major surface
41 on the second spacing 52 side and faces 55v (side faces) of the
stacked insulating layers 55 perpendicular to the major surface 41
on the second spacing 52 side. The faces 55v (the side faces) of
the stacked insulating layers 55 perpendicular to the major surface
41 on the second spacing 52 side are provided recessed toward the
first spacing 51 side as viewed from the second spacing 52 than are
the faces 50v (the side faces) of the stacked semiconductor layers
50 perpendicular to the major surface 41 on the second spacing 52
side. In other words, the charge storage layer 60 is continuously
provided from the side faces of the stacked semiconductor layers 50
to the side faces of the adjacent insulating layers 55; and the
side faces of the insulating layers 55 are recessed from the side
faces of the semiconductor layers 50.
In other words, the charge storage layer 60 is formed such that the
length of the charge storage layer 60 aligning along the side faces
of the insulating layers 55 between adjacent semiconductor layers
50 (the length aligning in the direction perpendicular to the major
surface 41 of the substrate 40) is longer in comparison with the
shortest distance between the memory cells stacked in the direction
perpendicular to the major surface 41.
Thereby, the deterioration of charge retention characteristics due
to a lateral diffusion of the charge in the charge storage layer
60, from a cell to which the charge is written, to an unwritten
cell or into other films can be inhibited.
Thus, the nonvolatile semiconductor storage devices 10 and 11
provide good retention characteristics.
In the nonvolatile semiconductor storage devices 10 and according
to this embodiment described above, the bit density can be
increased by stacking memory cells in both the direction
perpendicular and the direction parallel to the substrate. In other
words, the integration can be improved without performing
miniaturization.
Further, a transistor of each memory cell has an SOI (Silicon on
Insulator) structure including a silicon channel on an insulating
film. The gate electrode has a strong controllability over the
channel and therefore provides resistance to short channel effects
and easily enables multi-level storage. In this embodiment in
particular, a UT-SOI (Ultra-Thin Silicon On Insulator) structure is
used, and therefore a transistor highly resilient to short channel
effects can be realized. Therefore, the dominance over the channel
is particularly strong, providing the advantage that multi-level
storage such as two bits (that is, four values) per cell or three
bits (that is, eight values) per cell can easily be realized. In
other words, the memory cell including the charge storage layer 60
can retain multi-bit information.
The semiconductor layer pair 53 is formed at the period of 4F.
Thereby, the space necessary to form the stacked films of the MONOS
structure is ensured.
Also, the charge storage layer 60 is shielded by the semiconductor
layer 50 on the first spacing 51 side and by the gate electrode 70
on the second spacing 52 side. Therefore, an advantage is provided
that inter-cell interference causing the threshold voltage shift of
the memory cell according to the writing and erasing operations of
an adjacent cell can be inhibited.
Moreover, downscaling is easier than by the technology of, for
example, JP-A 2007-266143 (Kokai); and the number of stacking
layers can be reduced. Different than the technology of JP-A
2007-266143 (Kokai), NAND chains are formed in the direction
parallel to the major surface of the substrate 40. Therefore, the
structure of the peripheral circuit can be similar to that of
conventional NAND flash memory.
Thus, the nonvolatile semiconductor storage device and the method
for manufacturing the same according to this embodiment provide a
new structure that has few constraints on downscaling, is easy to
manufacture, and does not require drastic modifications to the
peripheral circuit.
First and second examples according to the first embodiment are
described as follows.
First Example
A nonvolatile semiconductor storage device 20 according to a first
example of the present invention is a flash memory using two
stacked layers of memory cells having a 22 nm half pitch to realize
a cell surface area of 968 nm.sup.2 corresponding to a nonvolatile
semiconductor storage device of a conventional planar cell
structure having a 15 nm half pitch.
An insulating layer that separates the substrate 40 and the first
semiconductor layer 50 is shared as a gate insulation film of a
peripheral high-voltage circuit. Restated, the gate insulation film
of a transistor of the peripheral high-voltage circuit includes a
layer which forms an insulating layer between the substrate 40 and
the semiconductor layer 50 most proximal to the substrate 40. Then,
polycrystalline silicon is used as the silicon channel of the
semiconductor layer 50. The charge storage layer 60 is formed in
the structure illustrated in FIG. 3A. In other words, the charge
storage layer 60 is provided separately for each face 50v of the
stacked semiconductor layers 50 perpendicular to the major surface
41 on the second spacing 52 side. The description assumes the case
where the semiconductor layer 50 is a bit line and the gate
electrode 70 is a word line.
A method for manufacturing the nonvolatile semiconductor storage
device 20 according to this example will now be described.
FIGS. 5A and 5B are schematic cross-sectional views illustrating
the method for manufacturing the nonvolatile semiconductor storage
device according to the first example of the first embodiment of
the present invention.
FIG. 5A is a view of the first step. FIG. 5B is a view continuing
from FIG. 5A.
FIGS. 6A and 6B are views continuing from FIG. 5B. FIGS. 7A to 7C
are views continuing from FIG. 6B. FIGS. 8A and 8B are views
continuing from FIG. 7C.
In FIGS. 5A to 7A and FIGS. 8A and 8B, each view on the left side
is a cross-sectional view in the word line (the gate electrode)
direction, that is, a cross-sectional view cut along a plane
perpendicular to the extension direction of the word line. Each
view on the right is a cross-sectional view in the bit line (the
semiconductor layer) direction, that is, a view cut along a plane
perpendicular to the extension direction of the bit line. FIGS. 7B
and 7C are enlarged cross-sectional views of main components in the
bit line direction. Each view on the left side of FIGS. 5B to 7A
and FIGS. 8A and 8B is a cross-sectional view along line A-A' of
the view on the right side.
As illustrated in FIG. 5A, first, a cell portion and a peripheral
high-voltage circuit portion on a semiconductor substrate 101 are
recessed a distance of 30 nm by lithography and reactive ion
etching. This step resolves the difference in levels resulting from
the gate oxide film thicknesses of the high-voltage circuit portion
and a low-voltage circuit portion, and is a step performed for
normal flash memory as well.
Continuing, a silicon thermal oxide film, which forms a gate oxide
film of the high-voltage circuit portion and an inter-layer
dielectric film of the cell, is formed with a thickness of 35 nm. A
gate oxide film 102 which forms peripheral low-voltage circuit
portion transistors is then formed with a thickness of 8 nm.
Thereby, the silicon thermal oxide film 103 which forms the gate
oxide layer of the high-voltage circuit portion and the inter-layer
dielectric film of the cells has a film thickness of 40 nm.
Then, an n-type polycrystalline silicon film 104 which forms a
portion of the gate electrode of the peripheral circuit and a first
semiconductor layer is formed with a thickness of 30 nm. A silicon
oxide film 105 which forms an insulating layer between the first
and second semiconductor layers is formed with a thickness of 40
nm. An n-type polycrystalline silicon film 106 which forms a second
semiconductor layer is formed with a thickness of 30 nm. A silicon
oxide film 107 is formed with a thickness of 20 nm. A silicon
nitride film 108 which forms a CMP (Chemical Mechanical Polishing)
stopper is formed with a thickness of 50 nm.
Continuing as illustrated in FIG. 5B, the semiconductor layers are
patterned by lithography and reactive ion etching. Here, during the
lithography of the cell portion, a semiconductor layer pair 109 is
repeatedly formed at a period of 4F=88 nm using side-wall transfer
technology.
The width of each of the semiconductor layers is 15 nm. The width
of a gap 110 (the first spacing 51) between adjacent semiconductor
layers is 15 nm. The width of a gap 111 (the second spacing 52)
between semiconductor layer pairs is 43 nm. The collective
patterning of the memory cells (also referred to as "cells") also
makes an isolation trench which forms STI (Shallow Trench
Isolation) of the peripheral circuit portion.
Continuing, the isolation trench of the peripheral circuit portion,
the gap 110 between the semiconductor layers, and the gap 111
between the semiconductor layer pairs are filled by an insulating
film 112 formed by a TEOS (tetraethoxysilane)/O.sub.3 film or an
SOG (Spin On Glass) film, etc.
Then, the insulating film 112 is flattened by CMP.
As illustrated in FIG. 6A, the silicon nitride film 108, the
silicon oxide film 107, the n-type polycrystalline silicon film
106, and the silicon oxide film 105 of the peripheral circuit
portion and a select gate portion of the first semiconductor layer
are then removed by lithography and reactive ion etching. The
silicon nitride film 108 and the silicon oxide film 107 of the
select gate portion of the second layer are removed by lithography
and reactive ion etching.
Continuing, a silicon oxide film 113 which forms the gate oxide
film of the select gate and an n-type polycrystalline silicon film
114 which forms the gate electrode of the select gate are
formed.
Through-holes are then made through the n-type polycrystalline
silicon film 114 and the silicon oxide film 113 of the peripheral
circuit portion to the n-type polycrystalline silicon film 104 by
lithography and reactive ion etching. An n-type polycrystalline
silicon film 115 is then formed.
Thereby, a structure can be formed in which the n-type
polycrystalline silicon film 115 and the n-type polycrystalline
silicon film 104 are connected in the peripheral circuit
portion.
Continuing as illustrated in FIG. 6B, known transistor formation
technology is used to pattern the n-type polycrystalline silicon
film 115, the silicon oxide film 113, and the n-type
polycrystalline silicon film 104 of the select gates and the
peripheral circuit portion to form gate electrodes 116 of the
peripheral circuit, and to form a side wall oxide film 117 and a
diffusion layer (not illustrated). An inter-layer dielectric film
118 is filled and flattened.
Then, as illustrated in FIG. 7A, the insulating film 112 in the gap
111 between the adjacent semiconductor layer pairs is etched back
by lithography and reactive ion etching.
Side walls of the n-type polycrystalline silicon films 104 and 106
are then cleaned by HF/NH.sub.3 gas. MONOS cells are formed by a
stacked insulating film 119, which includes a silicon thermal
oxidation film (having a thickness of 4 nm), a charge storage layer
(having a thickness of 5 nm), and a charge block film (having a
thickness of 11 nm). Thus, the width of the n-type polycrystalline
silicon film which forms the semiconductor layer is 13 nm; and the
gap between charge block films is 7 nm. The charge storage layer
includes, for example, a silicon nitride film. The charge block
film includes, for example, an alumina film.
Continuing, a CVD-deposited WSi film 120 which forms the gate
electrode is formed on the entire substrate surface.
The CVD-deposited WSi film 120 is then patterned by lithography and
reactive ion etching to form the gate electrodes of the cells.
At this time, the stacked insulating film 119 is collectively
patterned to separately provide the charge storage layers in each
of the stacked cells. Restated, the silicon nitride film, i.e., the
charge storage layer in the stacked insulating film 119, is
oxidized via the insulating film 112, the silicon thermal oxide
film 103, the silicon oxide film 105, and the silicon oxide film
107 by a high-temperature rapid thermal oxidation using radicals
from a hydrogen/oxygen gas mixture. The charge storage layers are
thereby provided separately for the stacked cells.
In other words, prior to the oxidation as illustrated in FIG. 7B,
the charge storage layer 121 and the charge block film 122 exist
continuously on the side walls of the silicon thermal oxidation
film 103, the silicon oxide film 105, and the silicon oxide film
107.
However, as illustrated in FIG. 7C, only the charge storage layer
121 is selectively oxidized by radical oxidation. A silicon thermal
oxynitride film 123 is formed, and the charge storage layers 121
can be provided separately for each stacked cell.
Thus, as illustrated in FIG. 8A, a stacked UT-SOI MONOS cell is
formed.
Then, an inter-layer dielectric film 124 is formed on the entire
substrate surface. Contact plugs 125 are formed to draw out from
the peripheral circuit and the stacked semiconductor layers. The
elements are connected by leads 126.
Hereafter, a circuit of the flash memory is formed by multilevel
interconnection; but a description thereof is omitted.
Thereby, the nonvolatile semiconductor storage device 20 according
to this example illustrated in FIG. 8B is manufactured.
In FIG. 8B, inter-layer dielectric films 127, 128, 129, and 130 are
stacked, and leads 134, 135, and 136 are connected by contact plugs
131, 132, and 133.
In the structure of the nonvolatile semiconductor storage device 20
of this example, the transistor of each cell has a UTSOI structure
that is resistant to short channel effects and has a strong
dominance over the channel. Therefore, an advantage is provided
that multi-level storage such as two bits (that is, four values)
per cell or three bits (that is, eight values) per cell can easily
be realized. Additionally, the charge storage layers of the
semiconductor layers forming the pair are shielded by the
semiconductor layers themselves, and the charge storage layers of
adjacent semiconductor layer pairs are shielded by the control gate
electrodes. Therefore, an advantage is provided that inter-cell
interference causing the threshold of the cell to vary according to
the writing and erasing operations of an adjacent cell can be
inhibited.
Moreover, the nonvolatile semiconductor storage device 20 of this
example can achieve a storage density twice that of a normal
single-layer memory. Although the semiconductor layers are stacked
on the substrate in the nonvolatile semiconductor storage device
20, the patterning of the semiconductor layers (the bit lines) and
the gate electrodes (the word lines) can be performed by the same
patterning as a normal non-stacked memory, that is, by one
lithography step each.
Thus, the nonvolatile semiconductor storage device 20 of this
example enables improvement of the integration of cells without
miniaturization; and even an advantage is provided that
leading-edge lithography steps such as EUV (Extreme Ultraviolet),
ArF immersion, and the like are not necessary.
The projected surface area of the cell of the nonvolatile
semiconductor storage device 20 of this example is 1,936 nm.sup.2,
and is, for example, about 26% of the surface area in comparison to
a cell manufactured by the technology of JP-A 2007-266143
(Kokai).
Further, while the two-layer stacked structure provides an
effective cell surface area of 1936/2=968 nm.sup.2, it is necessary
to use an eight-layer stacked structure to realize the same
effective cell surface area by the technology of JP-A 2007-266143
(Kokai), resulting in four times the number of stacking layers than
that of the nonvolatile semiconductor storage device of this
example.
Generally, the yield of a stacked structure is the product of the
yield of each layer. Therefore, it is clear that the nonvolatile
semiconductor storage device 20 of this example may achieve a
higher yield in comparison to that of conventional art.
Although a polycrystalline silicon film and a silicon oxide film
are stacked and collectively patterned in the nonvolatile
semiconductor storage device 20 of this example, this configuration
can be replaced by stacking and collectively patterning an
epitaxial silicon film and a silicon oxide film.
Methods for forming the epitaxial silicon film may include a method
that uses a portion of the substrate as a seed and forms an
epitaxial silicon film at a high temperature. Also, a method may be
used that forms an amorphous silicon film and then implements
lateral epitaxial growth using a portion of the substrate as a
seed. Formation is also possible by forming a stacked film of an
epitaxial silicon-germanium film/epitaxial silicon film,
selectively etching the epitaxial silicon-germanium film, and
performing thermal oxidation of the openings thereof.
Second Example
A nonvolatile semiconductor storage device 21 according to a second
example of the present invention is a five-layer stacked memory
corresponding to a 10-nm generation half pitch. The nonvolatile
semiconductor storage device 21 is an example in which each
monocrystalline silicon film forming the semiconductor layers 50 is
formed by a stacked film of a monocrystalline silicon film/oxide
film formed by sequentially growing stacks of epitaxial silicon and
epitaxial silicon-germanium, removing the silicon-germanium film by
selective etching, and performing thermal oxidation of the
resulting openings.
Although the charge storage layer of the first example is separated
by reactive ion etching during the patterning of the gate electrode
and thermal oxidation of the charge storage layer, the movement of
charge is inhibited in this example by making the length of the
charge storage layer 60 between cells in the stacking direction
longer than the distance between the stacked cells by forming the
insulating films between the stacked semiconductor layers 50 in a
three-layer structure. Restated, the nonvolatile semiconductor
storage device 21 of this example has the structure schematically
illustrated in FIG. 4B.
A method for manufacturing the nonvolatile semiconductor storage
device 21 according to this example will now be described.
The method for forming the peripheral circuit and the select gate
portion is basically the same even as the number of stacking layers
of the semiconductor layers 50 stacked in the direction
perpendicular to the major surface 41 of the substrate 40
increases. Therefore, to avoid complexity, only the cell portion is
described below. The description assumes that the semiconductor
layer 50 is a bit line and the gate electrode 70 is a word
line.
FIGS. 9A and 9B are schematic cross-sectional views illustrating
the method for manufacturing the nonvolatile semiconductor storage
device according to the second example of the first embodiment of
the present invention.
FIG. 9A is a view of the first step. FIG. 9B is a view continuing
from FIG. 9A.
FIGS. 10A and 10B are views continuing from FIG. 9B. FIGS. 11A and
11B are views continuing from FIG. 10B.
These views are cross-sectional views in the bit line (the
semiconductor layer) direction, that is, views cut along planes
perpendicular to the extension direction of the bit line.
First, as illustrated in FIG. 9A, an epitaxial silicon-germanium
film 202 is formed with a thickness of 30 nm on the cell portion of
a semiconductor substrate 201. An epitaxial silicon film 203 with a
thickness of 40 nm and an epitaxial silicon-germanium film 204 with
a thickness of 30 nm are then formed in ten alternately stacked
layers.
Then, a silicon nitride film 205 is formed with a thickness of 100
nm. Lithography and reactive ion etching are used to collectively
pattern the silicon nitride film and the multi-layer epitaxial film
of the cell region. A CVD silicon oxide film 206 which forms a
support of each layer is then formed on the end face of the exposed
multi-layer epitaxial film with a thickness of 200 nm.
Continuing as illustrated in FIG. 9B, a slit-shaped through-hole
(not illustrated) is made by collectively patterning the silicon
nitride film and the multi-layer epitaxial film. The epitaxial
silicon-germanium films are removed by selective wet etching from
the through-hole.
Then, as illustrated in FIG. 10A, the epitaxial silicon films 203
are oxidized by thermal oxidation to form silicon thermal oxidation
films 207 with a thickness of 10 nm. SOG is applied over the entire
substrate surface, and an SOG film 208 is filled into the openings
between the silicon thermal oxidation films 207.
As illustrated in FIG. 10B, the semiconductor layers are then
patterned by lithography and reactive ion etching similarly to the
first example.
Here, semiconductor layer pairs 209 are repeatedly formed at a
period of 4F=88 nm by lithography of the cell portion using
side-wall transfer technology. The width of each semiconductor
layer in the direction parallel to the substrate is 15 nm. The gap
210 (the first spacing 51) between the semiconductor layers is 15
nm. The gap 211 (the second spacing 52) between adjacent
semiconductor layer pairs is 43 nm. An isolation trench (not
illustrated) which forms an STI of the peripheral circuit portion
also is formed by the collective patterning of the cells.
Continuing, an insulating film 212 formed by a TEOS/O.sub.3 film or
an SOG film, etc., is filled into the gap 210 between the
semiconductor layers and the gap 211 between the semiconductor
layer pairs. The insulating film 212 is then flattened by CMP. The
insulating film 212 of the gap 211 between adjacent semiconductor
layer pairs is then removed by lithography and reactive ion
etching.
As illustrated in FIG. 11A, HF wet etching is then used to clean
the side walls of the epitaxial silicon film 203 and etch the SOG
film 208 into a recessed configuration. At this time, the silicon
thermal oxidation film 207 is finer and has a lower wet etching
speed in comparison to the SOG film 208. Therefore, the insulating
films on the (second spacing 52) side between the stacked
semiconductor layer pairs have a tapered configuration as
illustrated in FIG. 11A.
Continuing as illustrated in FIG. 11B, MONOS cells are formed by a
stacked insulating film 213, which includes a silicon thermal
oxidation film (having a thickness of 4 nm), a charge storage layer
(having a thickness of 5 nm), and a charge block film (having a
thickness of 11 nm). Thus, the width of the n-type polycrystalline
silicon film which forms the semiconductor layer is 13 nm; and the
gap between charge block films is 7 nm. A CVD-deposited TiN film
214 which forms the gate electrode is then formed over the entire
substrate surface.
Then, the CVD-deposited TiN film 214 is patterning to form gate
electrodes of the cells by lithography and reactive ion etching.
Simultaneously, the stacked insulating film 213 is collectively
patterned to separately provide the charge storage layers for
adjacent cells.
Thus, stacked MONOS cells are formed.
Hereafter, an inter-layer insulating film is formed to fill between
the gate electrodes, and multiple layer leads are formed; but a
description thereof is omitted.
In the structure of the nonvolatile semiconductor storage device 21
according to this example as well, the transistor of each cell has
a UTSOI structure that is resistant to short channel effects and
has a strong controllability over the channel. Therefore, an
advantage is provided that multi-level storage such as two bits per
cell or three bits per cell can easily be realized.
The nonvolatile semiconductor storage device 21 according to this
example differs from the first example in that the charge storage
layers are not provided separately for each of the stacked
semiconductor layers; but the length of the charge storage layer in
the stacking direction is longer than the distance between the
stacked semiconductor layers. The movement of charge between
adjacent cells is thereby inhibited. Therefore, an advantage is
provided similarly to the first example that the retention
characteristics of the stored charge are excellent.
While the five stacked layers of the nonvolatile semiconductor
storage device 21 according to this example provide an effective
cell surface area of 1936/5=387 nm.sup.2, it is necessary to use
nineteen stacked layers to realize a similar effective cell surface
area by the technology of, for example, JP-A 2007-266143
(Kokai).
The yield of a stacked structure is the product of the yield of
each layer. Therefore, it is clear that the nonvolatile
semiconductor storage device 21 according to this example may
achieve a high yield in comparison to that of conventional art.
In each of the examples recited above, the method for forming the
silicon film which forms the semiconductor layer, the MONOS film
structure, the method for processing the MONOS film structure, and
the like are not limited to combinations of the structures and the
methods described in the first and second examples. Any combination
is possible as long as the purport of the present invention is not
forfeited.
Further, a polycrystalline silicon film and a monocrystalline
silicon film crystallized by laser annealing or a Ni catalyst
method, for example, may be used as the channel silicon.
In addition to Al.sub.2O.sub.3, a metal oxide film such as
HfO.sub.2, La.sub.2O.sub.3, Pr.sub.2O.sub.3, Y.sub.2O.sub.3,
ZrO.sub.2, and the like, and films of multiple combinations thereof
can be used as the charge block film (the second insulating film)
in the MONOS structure.
In addition to those described in the first and second examples, at
least one selected from the group consisting of TaN, W, WSi, TiN,
CoSi, NiSi, NiPtSi, PtSi, Pt, Ru, and RuO.sub.2 may be used as the
electrode film (a conductive film of at least a portion of the
protruding portion 78 of the gate electrode 70 facing the charge
storage layer 60) in the MONOS structure.
FIG. 12 is a graph illustrating characteristics of a nonvolatile
semiconductor storage device according to an embodiment of the
present invention.
FIG. 12 illustrates a miniaturization roadmap to the half pitch F=6
nm generation.
In other words, FIG. 12 illustrates the number of stacked layers
for the nonvolatile semiconductor storage device of this embodiment
to realize the cell surface area of memory cells of conventional
art as the half pitch of a flash memory of conventional art
changes. Results for the nonvolatile semiconductor storage device
of this embodiment with half pitch F of 22 nm and 43 nm are
illustrated. The horizontal axis of FIG. 12 represents the half
pitch F, and the vertical axis represents the number of stacked
layers.
The limitation of downscaling of the nonvolatile semiconductor
storage device according to this embodiment is considered to be a
half pitch of about 22 nm for the semiconductor layer 50 in the
direction parallel to the major surface 41 of the substrate 40,
basically due to constraints of the film thickness of the MONOS
film.
Focusing on the case of FIG. 12 where the half pitch F is 22 nm, it
is clear that a nonvolatile semiconductor storage device according
to this embodiment having not more than ten stacked layers realizes
an effective cell surface area corresponding to a conventional
single-layer NAND flash memory of the 8-nm generation.
Second Embodiment
A method for manufacturing a nonvolatile semiconductor storage
device according to a second embodiment of the present invention is
a method for manufacturing a device including the semiconductor
layer 50, the gate electrode 70 provided to face the semiconductor
layer 50, the charge storage layer 60 provided between the
semiconductor layer 50 and the gate electrode 70, the first
insulating film 61 provided between the semiconductor layer 50 and
the charge storage layer 60, and the second insulating film 62
provided between the charge storage layer 60 and the gate electrode
70. The method for forming the semiconductor layer 50, the gate
electrode 70, and the charge storage layer 60 recited above is
distinctive and therefore described in detail.
FIG. 13 is a flowchart illustrating the method for manufacturing
the nonvolatile semiconductor storage device according to the
second embodiment of the present invention.
In the method for manufacturing the nonvolatile semiconductor
storage device according to this embodiment as illustrated in FIG.
13, first, the semiconductor layer film 59 which forms the
semiconductor layers 50 and the insulating layer film 55a which
forms the insulating layers 55 are formed by stacking on the major
surface 41 of the substrate 40 (step S110). The semiconductor layer
film 59 may include, for example, the n-type polycrystalline
silicon film or the epitaxial silicon film described in the first
and second examples. The insulating layer film 55a may include, for
example, the silicon oxide film or the stacked film of the silicon
thermal oxidation film and the SOG film described in the first and
second examples.
The semiconductor layer film 59 and the insulating layer film 55a
are patterned (processed) to alternately provide a first spacing 51
and a second spacing 52 larger than the first spacing 51 between
stacked units including the semiconductor layers 50 and the
insulating layers 55 by using a mask pattern formed by a side wall
formed on a side face of a dummy pattern formed parallel to the
major surface 41 of the substrate 40 at a period four times the
size of the half pitch F of the device to align in a first
direction (step S120). The method described in FIGS. 3A to 3E may
be used for this step.
A film which forms the first insulating film 61, a charge storage
layer film which forms the charge storage layer 60, and a film
which forms the second insulating film 62 are formed on a side face
of the stacked unit including the semiconductor layers 50 and the
insulating layers 55 on the second spacing 52 side; and a gate
electrode film which forms the gate electrode 70 is formed on the
film which forms the second insulating film and on a major surface
of the stacked unit (step S130). The various materials described in
the first and second embodiments may be used in this step.
The gate electrode film is patterned (processed) into a band
configuration parallel to the major surface 41 of the substrate 40
to align in the second direction (for example, the Y-axis
direction) non-parallel to the first direction (step S140).
Thereby, a method for manufacturing a nonvolatile semiconductor
storage device having a new configuration can be provided with few
constraints on miniaturization, is easy to manufacture, and does
not require drastic modifications to the peripheral circuit.
A step that fills an insulating film into the gap on the first
spacing 51 side of the patterned (processed) and formed
semiconductor layers 50 and the insulating layers 55 may be
provided between the step S120 and the step S130 recited above.
Third Embodiment
FIG. 14 is a flowchart illustrating a method for manufacturing a
nonvolatile semiconductor storage device according to a third
embodiment of the present invention.
In the method for manufacturing the nonvolatile semiconductor
storage device according to the third embodiment of the present
invention illustrated in FIG. 14, after step S140 described above
in FIG. 13, the charge storage layer film is provided separately
for each side face (the face 50v perpendicular to the major surface
41) of the stacked semiconductor layers 50 on the second spacing 52
side (step S150). Alternatively, a side face (the face 55v
perpendicular to the major surface 41) of the insulating layer 55
on the second spacing 52 side is recessed (recessed toward the
first spacing 51 side as viewed from the second spacing 52) from
side faces (the faces 50v perpendicular to the major surface 41) of
the adjacent stacked semiconductor layers 50 on the second spacing
52 side prior to the step S130 (step S125). It is sufficient that
either step S125 or step S150 recited above is implemented.
In other words, the charge storage layer 60 is provided separately
for each stacked memory cell after the step forming the gate
electrode 70; or the insulating layer 55 between the semiconductor
layers 50 is recessed to make the length of the charge storage
layer aligning along the side faces of the insulating layers 55 in
the stacking direction larger than the distance between the stacked
memory cells.
The methods described in the first example or the second example
can be used for these steps. For example, the step that separately
provides the charge storage layer film 69 for each side face (each
face 50v perpendicular to the major surface 41 on the second
spacing 52 side) of the stacked semiconductor layers 50 may include
the method of selectively oxidizing the charge storage layer film
69 by oxidizing via the stacked insulating layers 55 as described
in the first example.
Thereby, the charge storage layer 60 can be essentially divided for
each memory cell; and the deterioration of charge retention
characteristics due to lateral diffusion of the charge in the
charge storage layer 60, from a cell to which the charge is
written, to an unwritten cell or into other films can be
inhibited.
It is clear that the embodiments of the present invention described
above are applicable to a stacked memory having more stacked layers
than the examples recited above. Use of embodiments of the present
invention will hereafter enable further continuous improvement of
integration. Therefore, various fields of application are expected
to expand.
Hereinabove, embodiments of the present invention are described
with reference to specific examples. However, the present invention
is not limited to these specific examples. For example, one skilled
in the art may appropriately select specific configurations of
components of the nonvolatile semiconductor storage device and the
method for manufacturing the same from known art and similarly
practice the present invention. Such practice is included in the
scope of the present invention to the extent that similar effects
thereto are obtained.
Further, any two or more components of the specific examples may be
combined within the extent of technical feasibility; and are
included in the scope of the present invention to the extent that
the purport of the present invention is included.
Moreover, all nonvolatile semiconductor storage devices and methods
for manufacturing the same that can be obtained by an appropriate
design modification by one skilled in the art based on the
nonvolatile semiconductor storage devices and the methods for
manufacturing the same described above as embodiments of the
present invention also are within the scope of the present
invention to the extent that the purport of the present invention
is included.
Furthermore, various modifications and alterations within the
spirit of the present invention will be readily apparent to those
skilled in the art. All such modifications and alterations should
therefore be seen as within the scope of the present invention.
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