U.S. patent application number 12/898914 was filed with the patent office on 2011-02-03 for semiconductor memory.
Invention is credited to Tatsuo Izumi, Takeshi Kamigaichi, Shinya Takahashi.
Application Number | 20110024825 12/898914 |
Document ID | / |
Family ID | 40406082 |
Filed Date | 2011-02-03 |
United States Patent
Application |
20110024825 |
Kind Code |
A1 |
Izumi; Tatsuo ; et
al. |
February 3, 2011 |
SEMICONDUCTOR MEMORY
Abstract
A semiconductor memory according to an example of the invention
includes active areas, and element isolation areas which isolate
the active areas. The active areas and the element isolation areas
are arranged alternately in a first direction. An n-th (n is odd
number) active area from an endmost portion in the first direction
and an (n+1)-th active area are coupled to each other at an endmost
portion in a second direction perpendicular to the first
direction.
Inventors: |
Izumi; Tatsuo;
(Yokohama-shi, JP) ; Kamigaichi; Takeshi;
(Yokohama-shi, JP) ; Takahashi; Shinya;
(Yokohama-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
40406082 |
Appl. No.: |
12/898914 |
Filed: |
October 6, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12191592 |
Aug 14, 2008 |
7825439 |
|
|
12898914 |
|
|
|
|
Current U.S.
Class: |
257/324 ;
257/E29.309 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 27/0207 20130101; H01L 27/11534 20130101; H01L 27/11526
20130101; H01L 2924/0002 20130101; H01L 27/105 20130101; H01L
2924/00 20130101 |
Class at
Publication: |
257/324 ;
257/E29.309 |
International
Class: |
H01L 29/792 20060101
H01L029/792 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2007 |
JP |
2007-222783 |
Claims
1. A semiconductor memory comprising: active areas; and the element
isolation areas which isolate the active areas, wherein the element
isolation areas have an element isolation insulating film, the
element isolation areas have first element isolation areas, a
second element isolation area and a third element isolation area,
the first isolation areas are narrower than the second and third
element isolation areas, the second element isolation area is
adjacent to one of the first element isolation areas via one of the
active areas, the third element isolation area is adjacent to one
of the second element isolation areas via another one of the active
areas, and the third element isolation area has a step.
2. The semiconductor memory according to claim 1 further
comprising: a gate electrode disposed on each of the active areas,
and an inter-electrode insulating film disposed on the element
isolation areas and gate electrode, wherein the inter-electrode
insulating film contacts a top face of the step.
3. The semiconductor memory according to claim 2 further
comprising: a first gate insulating film disposed on first ones of
the active areas located between the first element isolation areas,
and a second gate insulating film disposed on second ones of the
active areas located between the one of first element isolation
area and the second element isolation area, wherein the second gate
insulating film is thicker than the first gate insulating film.
4. The semiconductor memory according to claim 2 wherein, a top
face of the gate electrode is higher than a top face of the element
isolation insulating film in the first and second element isolation
areas, and a top face of the gate electrode is at the same level as
a part of a top face of the element isolation insulating film in
the third element isolation area.
5. The semiconductor memory according to claim 1, wherein a width
of the active areas located between the first element isolation
areas is equal to a width of the active area located between the
one of the first element isolation areas and the second element
isolation area.
6. A semiconductor memory comprising: active areas; and element
isolation areas which isolate the active areas, wherein the element
isolation areas have an element isolation insulating film, the
element isolation areas have first element isolation areas, a
second element isolation area and a third element isolation area,
the first isolation areas are narrower than the second and third
element isolation areas, the second element isolation area is
adjacent to one of the first element isolation areas via an one of
the active areas, the third element isolation area is adjacent to
the second element isolation areas via another one of the active
areas, and the second element isolation area has a step.
7. The semiconductor memory according to claim 6 further
comprising: a gate electrode disposed on each of the active areas,
an inter-electrode insulating film disposed on the element
isolation areas and the gate electrode, wherein the inter-electrode
insulating film contacts a top face of the step.
8. The semiconductor memory according to claim 6 further
comprising: a first gate insulating film disposed on first ones of
the active areas located between the first element isolation areas,
a second gate insulating film disposed on second ones of the active
areas located between the one first element isolation area and the
second element isolation area, wherein the second gate insulating
film is thicker than the first gate insulating film.
9. The semiconductor memory according to claim 7 wherein, a top
face of the gate electrode is higher than a top face of the element
isolation insulating film in the first and second element isolation
areas, and a top face of the gate electrode is at the same level as
part of a top face of the element isolation insulating film in the
third element isolation area.
10. The semiconductor memory according to claim 6, wherein a width
of the active areas located between the first element isolation
areas is equal to a width of the active area located between the
one first element isolation area and the second element isolation
area.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional application of U.S. Ser.
No. 12/191,592, filed Aug. 14, 2008, and based upon and claims the
benefit of priority from prior Japanese Patent Application No.
2007-222783, filed Aug. 29, 2007, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an active area structure of
a memory cell array.
[0004] 2. Description of the Related Art
[0005] In recent years, many electronic devices which use NAND type
flash memories have been commercially available. On the other hand,
a large memory capacity of the NAND type flash memories is required
due to multi-functions of the electronic devices, and thus the
compatibility between shrink of memory cells and improvement in
reliability becomes a problem (for example, see Jpn. Pat. Appln.
KOKAI Publication Nos. 2002-184875, 5-88375 and 8-55920).
[0006] For example, a memory cell array of the NAND type flash
memories has a periodical structure in which active areas and
element isolation areas are arranged alternately with a constant
spacing (for example, feature size). Further, when such a
periodical structure is formed by using a conventional lithography
technique, after an exposure margin (pattern blur) is taken into
consideration, at least an active area at the endmost portion of
the memory cell array is set as a dummy area and the width of the
dummy area is made to be wider than the constant width.
[0007] A dummy cell which does not function as the memory cell but
has a similar structure to that of the memory cell, is formed in
the dummy area. In this case, when a writing potential is applied
to a word line shared by the dummy cell and the memory cell, an
electric field applied to an inter-electrode insulating film (or
block insulating film) of the dummy cell becomes higher than an
electric field applied to an inter-electrode insulating film (or
block insulating film) of the memory cell.
[0008] The inter-electrode insulating film (or block insulating
film) of the dummy cell is, therefore, easily broken by the writing
potential, and this happens, the memory cell which shares the word
line with the broken dummy cell will not function.
[0009] Such a problem arises not only in NAND type flash memories
but also in other semiconductor memories where a large memory
capacity is required.
SUMMARY OF THE INVENTION
[0010] A semiconductor memory according to an aspect of the
invention comprises active areas, and element isolation areas which
isolate the active areas. The active areas and the element
isolation areas are arranged alternately in a first direction. An
n-th (n is odd number) active area from an endmost portion in the
first direction and an (n+1)-th active area are coupled to each
other at an endmost portion in a second direction perpendicular to
the first direction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a diagram showing a positional relationship of a
memory cell array;
[0012] FIGS. 2A and 2B are diagrams showing a basic structure of
the present invention;
[0013] FIG. 3 is a diagram showing a NAND type flash memory;
[0014] FIG. 4 is a plan view showing a layout of a closed-loop
structured active area;
[0015] FIG. 5 is a cross-sectional view taken along line V-V of
FIG. 4;
[0016] FIG. 6 is a cross-sectional view taken along line VI-VI of
FIG. 4;
[0017] FIG. 7 is a plan view showing one step of a manufacturing
method;
[0018] FIG. 8 is a cross-sectional view taken along line VIII-VIII
of FIG. 7;
[0019] FIG. 9 is a plan view showing one step of the manufacturing
method;
[0020] FIG. 10 is a cross-sectional view taken along line X-X of
FIG. 9;
[0021] FIG. 11 is a plan view showing one step of the manufacturing
method;
[0022] FIG. 12 is a cross-sectional view taken along line XII-XII
of FIG. 11;
[0023] FIG. 13 is a plan view showing one step of the manufacturing
method;
[0024] FIG. 14 is a cross-sectional view taken along line XIV-XIV
of FIG. 13;
[0025] FIG. 15 is a plan view showing one step of the manufacturing
method;
[0026] FIG. 16 is a cross-sectional view taken along line XVI-XVI
of FIG. 15;
[0027] FIG. 17 is a plan view showing one step of the manufacturing
method;
[0028] FIG. 18 is a cross-sectional view taken along line
XVIII-XVIII of FIG. 17;
[0029] FIG. 19 is a plan view showing one step of the manufacturing
method;
[0030] FIG. 20 is a cross-sectional view taken along line XX-XX of
FIG. 19;
[0031] FIG. 21 is a diagram showing a coupling capacitance of a
cell;
[0032] FIG. 22 is a plan view showing one step of the manufacturing
method;
[0033] FIG. 23 is a cross-sectional view taken along line
XXIII-XXIII of FIG. 22;
[0034] FIG. 24 is a plan view showing a layout of a closed-loop
structured active area;
[0035] FIG. 25 is a cross-sectional view taken along line XXV-XXV
of FIG. 24;
[0036] FIG. 26 is a cross-sectional view taken along line XXVI-XXVI
of FIG. 24;
[0037] FIG. 27 is a plan view showing one step of the manufacturing
method;
[0038] FIG. 28 is a cross-sectional view taken along line
XXVIII-XXVIII of FIG. 27;
[0039] FIG. 29 is a plan view showing one step of the manufacturing
method;
[0040] FIG. 30 is a cross-sectional view taken along line XXX-XXX
of FIG. 29;
[0041] FIG. 31 is a plan view showing one step of the manufacturing
method;
[0042] FIG. 32 is a cross-sectional view taken along line
XXXII-XXXII of FIG. 31;
[0043] FIG. 33 is a plan view showing one step of the manufacturing
method;
[0044] FIG. 34 is a cross-sectional view taken along line
XXXIV-XXXIV of FIG. 33;
[0045] FIG. 35 is a plan view showing one step of the manufacturing
method;
[0046] FIG. 36 is a cross-sectional view taken along line
XXXVI-XXXVI of FIG. 35;
[0047] FIG. 37 is a plan view showing one step of the manufacturing
method;
[0048] FIG. 38 is a cross-sectional view taken along line
XXXVIII-XXXVIII of FIG. 37;
[0049] FIG. 39 is a plan view showing one step of the manufacturing
method;
[0050] FIG. 40 is a cross-sectional view taken along line XL-XL of
FIG. 39;
[0051] FIG. 41 is a diagram showing a coupling capacitance of the
cell;
[0052] FIG. 42 is a plan view showing one step of the manufacturing
method;
[0053] FIG. 43 is a cross-sectional view taken along line
XLIII-XLIII of FIG. 42;
[0054] FIG. 44 is a cross-sectional view showing a modification
example of a step position;
[0055] FIG. 45 is a cross-sectional view showing a modification
example of the step position;
[0056] FIG. 46 is a diagram showing a MONOS type memory cell as the
modification example;
[0057] FIG. 47 is a diagram showing a system as an application
example;
[0058] FIG. 48 is a diagram showing a layout of a semiconductor
memory as an application example; and
[0059] FIG. 49 is a diagram showing a NAND cell unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0060] A semiconductor memory of an aspect of the present invention
will be described blow in detail with reference to the accompanying
drawings.
1. OUTLINE
[0061] An example of the present invention relates to a periodical
structure where active areas and element isolation areas are
arranged alternately in a first direction. This example adopts a
layout where an n-th (n is odd number) active area from the endmost
portion in the first direction and an (n+1)-th active area are
coupled to each other at the endmost portion in a second direction
perpendicular to the first direction.
[0062] The active areas having such a layout are called closed-loop
structured active areas. The term closed loop refers to a perfect
loop, and the opposite to a closed-loop structure is an open-loop
structure. An open loop means that part of the loop is cut
(open).
[0063] The closed-loop structure is realized by a side wall
patterning technique.
[0064] The side wall patterning technique is a technique for
forming patterns which are finer than a limit of resolution by
means of photolithography.
[0065] When the side wall patterning technique is used, an exposure
margin (pattern blur) of the photolithography does not have to be
taken into consideration. For this reason, widths of all the active
areas (including dummy area) in the memory cell array area in the
first direction can be set to a constant width which is narrower
than the limit of resolution by means of photolithography.
[0066] When a first active area from the endmost portion in the
first direction is set as the dummy area, an electric field to be
applied to an inter-electrode insulating film (or block insulating
film) of the dummy cell in the dummy area is the same as an
electric field to be applied to an inter-electrode insulating film
(or block insulating film) of the memory cell. For this reason,
breakage of the dummy cell can be prevented.
[0067] As a result, miniaturization and improvement in reliability
of the memory cell can be realized at the same time.
2. BASIC STRUCTURE
[0068] A basic structure of the active areas in the semiconductor
memory according to the present invention will be described.
[0069] As shown in FIG. 1, as to a positional relationship of a
memory cell array area 1, a row decoder (word line driver) 2 is
arranged at an endmost portion of the memory cell array area 1 in
the first direction, and a column decoder (sense amplifier) 3 is
arranged at an endmost portion of the memory cell array area 1 in a
second direction perpendicular to the first direction.
[0070] As shown in FIGS. 2A and 2B, a periodical structure is such
that active areas AA1, AA2, . . . and element isolation areas
(areas other than the active areas AA1, AA2, . . . ) are arranged
alternately in the first direction. Further, a closed-loop
structure is formed in such a manner that an n-th (n is an odd
number) active area AAn from the endmost portion in the first
direction and an (n+1)-th active area AAn+1 are coupled to each
other at the endmost portion in the second direction.
[0071] In FIG. 2A, the closed-loop structured active areas AA1,
AA2, . . . are formed only in the memory cell array area 1. A MOS
transistor constituting a row decoder is formed in the active areas
AA (peripheral) in a peripheral circuit area.
[0072] When the first active area AA1 from the endmost portion in
the first direction is set as a dummy area where a dummy cell which
does not function as a memory cell is formed, a width of the dummy
area in the first direction is the same as a width of the active
areas AA2, AA3, . . . where the memory cells are formed in the
first direction.
[0073] That is, since the memory cell and the dummy cell have the
same structure and the same size, a problem such that the dummy
cell is broken by an electric potential applied to the word line
and thus the reliability is deteriorated can be prevented.
[0074] The closed-loop structured active areas (including dummy
area) AA1, AA2, . . . are formed by a side wall patterning
technique. According to the side wall patterning technique, the
widths of all the active areas AA1, AA2, . . . in the first
direction can be uniform widths which are narrower than a limit of
resolution by means of photolithography.
[0075] In FIG. 2B, m-th (m=2) and subsequent active areas AA2, AA3,
. . . from the endmost portion of the closed-loop structured active
areas (including dummy area) AA1, AA2, . . . in the first direction
are formed in the memory cell array area 1, and the active area AA1
before m-th one from the endmost portion in the first direction is
formed in the peripheral circuit area.
[0076] m=2, but m may be any number which is not less than 2.
[0077] In this case, a first gate insulating film of the MOS
transistor is formed on the active areas AA2, AA3, . . . , and a
second gate insulating film of the MOS transistor is formed on the
active area AA1. The second gate insulating film is thicker than
the first gate insulating film.
[0078] Similarly to the active area AA1, the second gate insulating
film is formed on the active area AA (peripheral) in the peripheral
circuit area.
[0079] That is, since the second gate insulating film, which is
thicker than the first gate insulating film, is formed on the first
active area AA1 from the endmost portion in the first direction,
the MOS transistor having the second gate insulating film is hardly
broken. As a result, the reliability is improved.
[0080] The active areas (including the dummy area) AA1, AA2, . . .
having the closed-loop structure are formed by the side wall
patterning technique. According to the side wall patterning
technique, the widths of all the active areas AA1, AA2, . . . in
the first direction can be a uniform width which is narrower than
the limit of resolution by means of photolithography.
[0081] Since the active area AA1 is formed in the peripheral
circuit area, the width of the element isolation area between the
active areas AA1 and AA2 in the first direction is wider than the
width of the active areas AA1, AA2, . . . in the first
direction.
[0082] The widths of the other element isolation areas constituting
the periodical structure in the first direction are the same as the
widths of the active areas AA1, AA2, . . . , for example.
[0083] That is, the widths of the element isolation areas on the
end of the periodical structure are wider than that on the center
of the periodical structure.
3. EXAMPLES
[0084] Examples of the present invention will be described by
exemplifying a NAND type flash memory.
(1) First Example
A. Structure
[0085] FIG. 3 shows a main section of the NAND type flash
memory.
[0086] j (j is any number not less than 2) blocks BK1, BK2, . . . ,
BKj are arranged in the second direction in the memory cell array
1. The blocks BK1, BK2, . . . , BKj have a plurality of NAND cell
units which are arranged in the first direction, respectively.
[0087] The row decoder (word line driver) 2 selects selected one
word line in selected one block BKi (i is one of 1 to j) at the
time of reading/writing.
[0088] A data latch circuit 4 has a function for temporarily
latching data at the time of reading/writing. The data latch
circuit 4 is arranged on both endmost portions of the memory cell
array area 1 in the second direction. This layout is effective for
an ABL (all bit line) sense principle.
[0089] FIG. 4 is a plan view showing the memory cell array area.
FIG. 5 is a cross-sectional view taken along line V-V of FIG. 4,
and FIG. 6 is a cross-sectional view taken along line VI-VI of FIG.
4.
[0090] A double well region which includes an n-type well region
(n-well) 11b and a p-type well region (p-well) 11c is formed in a
p-type semiconductor substrate (p sub) 11a.
[0091] The active areas AA and the element isolation areas (the
areas other than the active areas AA) are arranged alternately in
the first direction in the memory cell array area 1 so as to form
the periodical structure. The n-th (n is an odd number) active area
AA from the endmost portion in the first direction and the (n+1)-th
active area AA are coupled with each other at the endmost portion
in the second direction so as to form the closed-loop
structure.
[0092] A floating gate electrode 14 (FG) and lower gate electrodes
14 (SGS) and 14 (SGD) are formed on the active areas AA via a gate
insulating film (tunnel insulating film) 13A. The floating gate
electrode 14 (FG) and the lower gate electrodes 14 (SGS) and
14(SGD) are composed of, for example, a conductive polysilicon
layer.
[0093] A conductive layer 14 which is composed of the same material
as that of the floating gate electrode 14 (FG) is formed on the
active area AA in the peripheral circuit area via a gate insulating
film 13B thicker than the gate insulating film 13A.
[0094] An element isolation insulating film 12 having an STI
(shallow trench isolation) structure is formed in the element
isolation areas. An upper surface of the element isolation
insulating film 12 is flat, and has a step S on the outside of the
periodical structure of the active area AA and the element
isolation area, for example, between the memory cell array area 1
and the peripheral circuit area.
[0095] As to the step S, the upper surface of the element isolation
insulating film 12 in the portion of the periodical structure is
low.
[0096] As a result, a part of the side surface of the floating gate
14 (FG) on the active area AA in the memory cell array area 1 is
exposed from the element isolation insulating film 12.
[0097] A control gate electrode (word line) 16 (WL) is formed on
the element isolation insulating film 12, the floating gate
electrode 14 (FG) and the conductive layer 14 via an
inter-electrode insulating film (for example, an ONO film, a
high-dielectric-constant material or the like) 15.
[0098] The control gate electrode 16 (WL) extends in the first
direction across the memory cell array area 1 and the peripheral
circuit area. The control gate electrode 16 (WL) can have a
multi-layered structure, for example, a laminated structure of a
conductive polysilicon layer and a silicide layer.
[0099] Upper gate electrodes (select gate lines) 16 (SGS) and 16
(SGD) are formed on the lower gate electrodes 14 (SGS) and 14 (SGD)
via the inter-electrode insulating film 15. The lower gate
electrodes 14 (SGS) and 14 (SGD) and the upper gate electrodes 16
(SGS) and 16 (SGD) are electrically connected.
[0100] Such a closed-loop structured active area AA is formed by
the side wall patterning technique as mentioned later. According to
the side wall patterning technique, the widths of all the active
areas AA in the memory cell array area 1 in the first direction can
be made uniform and narrower than the limit of resolution, by means
of photolithography.
[0101] When the first active area AA from the endmost portion of
the memory cell array area 1 in the first direction is set as the
dummy area where the dummy cell which does not function as the
memory cell is formed, a width (Wend) of the dummy area in the
first direction is the same as a width (Wmiddle) in the first
direction of the active areas AA where the memory cells are
formed.
[0102] When the side wall patterning technique is used, the widths
of all the active areas AA constituting the periodical structure
basically become uniform. However, the uniform width includes a
change in the widths due to patterning dispersion (margin of
error).
[0103] For this reason, the memory cell and the dummy cell have the
same structure and the same size. Specifically, a coupling ratio of
the memory cell and a coupling ratio of the dummy cell are equal to
each other.
[0104] Therefore, even if a writing potential is applied to the
word line shared by the memory cell and the dummy cell, an electric
field to be applied to the inter-electrode insulating film (or
block insulating film) of the dummy cell is equal to an electric
field to be applied to the inter-electrode insulating film (or
block insulating film) of the memory cell. As a result, a problem
such that the dummy cell is broken and the reliability is
deteriorated can be prevented.
B. Manufacturing Method
[0105] The method for manufacturing the NAND type flash memory
shown in FIGS. 4 to 6 will be described.
[0106] As shown in FIGS. 7 and 8, a double well region which
includes the n-type well region 11b and the p type well region 11c
is formed in the p-type semiconductor substrate 11a.
[0107] The gate insulating film 13A is formed on the p type well
region 11c in the memory cell array area 1 by the thermal oxidation
method. The gate insulating film 13B, which is thicker than the
gate insulating film 13A, is formed on the p-type well region 11c
in the peripheral circuit area.
[0108] Thereafter, a first mask layer 17 and a second mask layer 18
are sequentially formed on the gate insulating films 13A and 13B by
the CVD method. The first mask layer 17 and the second mask layer
18 are made of different materials which have different etching
rate.
[0109] Thereafter, a photoresist film 19 is formed on the second
mask layer 18.
[0110] The photoresist film 19 is processed into a predetermined
pattern by the photolithography process. For example, the
photoresist film 19 is formed by a line and space pattern in the
memory cell array area 1.
[0111] The pitch of the lines and spaces of the photoresist film 19
is set to a limit (feature size) of resolution of photolithography,
for example, 120 nm (line width x=60 nm and space width x=60
nm).
[0112] The second mask layer 18 is etched by using the photoresist
film 19 as a mask according to RIE, and then the photoresist film
19 is removed.
[0113] As a result, as shown in FIGS. 9 and 10, the pattern of the
photoresist film 19 in FIGS. 7 and 8 is transferred onto the second
mask layer 18.
[0114] In the memory cell array area 1, the width of the second
mask layer 18 is made to be smaller than the limit of resolution of
photolithography by a slimming technique. A dotted line in the
cross-sectional view of FIG. 10 shows the pattern of the second
mask layer 18 before slimming, and a solid line shows the pattern
of the second mask layer 18 after slimming.
[0115] A line width (a) of the second mask layer 18 is set to 30
nm, and a space width (b) thereof is set to 90 nm by the slimming
technique.
[0116] As shown in FIGS. 11 and 12, the second mask layer 18 in the
peripheral circuit area is solidified. The etching selectivity of
the solidified second mask layer 18 is sufficiently smaller than
that of the unsolidified second mask layer 18 under the same
etching condition. The solidified portion is shown in gray.
[0117] A third mask layer 20 is formed on the first mask layer 17
and the second mask layer 18 according to the CVD method, and the
third mask layer 20 is made to remain only on a side wall portion
of the second mask layer 18 by RIE.
[0118] The third mask layer 20 has a closed-loop shape which
surrounds the second mask layer 18.
[0119] Both the width of the second mask layer 18 and the width of
the third mask layer 20 are 30 nm. The width between the second and
third mask layers 18 and 20 is, for example, 30 nm.
[0120] Thereafter, when only the unsolidified second mask layer 18
is removed, as shown in FIGS. 13 and 14, the line and space
structure including the third mask layer (line width=30 nm) 20 and
the space (space width=30 nm) is formed in the memory cell array
area 1.
[0121] The first mask layer 17 is etched by using the solidified
second mask layer 18 and the third mask layer 20 as a mask
according to RIE. As a result, the patterns of the solidified
second mask layer 18 and the third mask layer 20 are transferred
onto the first mask layer 17.
[0122] As a result, a fine pattern of the first mask layer 17 is
formed by the side wall patterning technique.
[0123] This pattern (line=30 nm, space=30 nm) is finer than the
limit (feature size) of resolution of photolithography.
[0124] Thereafter, the solidified second mask layer 18 and the
third mask layer 20 are removed.
[0125] As shown in FIGS. 15 and 16, the conductive layer 14, the
gate insulating films 13A and 13B, and the semiconductor substrate
(including the n-type well region and the p-type well region) 11a
are etched by using the first mask layer 17 as a mask according to
RIE, so that a trench is formed.
[0126] As shown in FIGS. 17 and 18, the trench is filled with the
element isolation insulating film 12 according to the CVD method.
The upper surface of the element isolation insulating film 12 is
flattened by CMP (chemical mechanical polishing).
[0127] The upper surface of the element isolation insulating film
12 approximately matches the upper surface of the first mask layer
17.
[0128] As shown in FIGS. 19 and 20, the element isolation
insulating film 12 in the memory cell array area 1 is selectively
etched, and a position of the upper surface of the element
isolation insulating film 12 is lowered so that the side surface of
the conductive layer 14 is partially exposed.
[0129] The side surface of the conductive layer 14 on all the
active areas (including the dummy area) in the memory cell array
area 1 is exposed.
[0130] As a result, when the first active area AA from the endmost
portion of the memory cell array area 1 in the first direction is
set as the dummy area, the structure of the dummy cell is exactly
the same as that of the memory cell (the size is also the same) as
shown in FIG. 21. For this reason, the coupling ratio of the dummy
cell and that of the memory cell are equal to each other.
[0131] The breakage of the dummy cell is, therefore, prevented.
[0132] In this case, the step S is formed on the element isolation
insulating film 12 outside the periodical structure in the memory
cell array area 1, for example, between the memory cell array area
1 and the peripheral circuit area. The position of the step S can
be anywhere as long as it is outside the periodical structure.
[0133] Finally, as shown in FIGS. 22 and 23, the inter-electrode
insulating film (for example, the ONO film, the
high-dielectric-constant material or the like) 15 and the
conductive layer 16 are formed.
[0134] When the conductive layers 14 and 16, and the
inter-electrode insulating film 15 are patterned, the floating gate
electrode 14 (FG) and the control gate electrode (word line) 16
(CG) of the memory cells (including the dummy cell) are formed.
Further, the select gate lines 16 (SGS) and 16 (SGD) of the select
gate transistor are formed.
[0135] As a result, the NAND type flash memory shown in FIGS. 4 and
6 is completed.
[0136] The above manufacturing method adopts the solidifying
process for the second mask layer 18, but even if the solidified
process is not used, one-time PEP (photo engraving process) is
performed so that the second mask layer 18 remains in the
peripheral circuit area.
C. Conclusion
[0137] According to the first example, the side wall patterning
technique is adopted so that the closed-loop structured active area
having uniform width can be formed. As a result, the
miniaturization and the improvement in reliability of the memory
cell can be simultaneously realized.
(2) Second Example
[0138] In contrast to the first example, a second example relates
to the closed-loop structured active area where at least the first
active area from the endmost portion in the first direction is
arranged in the peripheral circuit area.
[0139] A. Structure
[0140] FIG. 24 is a plan view showing the memory cell array area.
FIG. 25 is a cross-sectional view taken along line XXV-XXV of FIG.
24, and FIG. 26 is a cross-sectional view taken along line
XXVI-XXVI of FIG. 24.
[0141] The double well region, which includes the n-type well
region (n-well) 11b and the p-type well region (p well) 11c, is
formed in the p-type semiconductor substrate (p-sub) 11a.
[0142] In the memory cell array area 1, the active areas AA and the
element isolation areas (areas other than the active areas AA) are
arranged alternately in the first direction so as to form the
periodical structure. The n-th (n is odd number) active area AA
from the endmost portion in the first direction and the (n+1)-th
active area AA are coupled to each other at the endmost portion in
the second direction so as to form the closed-loop structure.
[0143] The floating gate electrode 14 (FG) and the lower gate
electrodes 14 (SGS) and 14 (SGD) are formed on the active areas AA
via the gate insulating film (tunnel insulating film) 13A. The
floating gate electrode 14 (FG) and the lower gate electrodes 14
(SGS) and 14 (SGD) are made of, for example, a conductive
polysilicon layer.
[0144] The conductive layer 14 which is made of the same material
as that of the floating gate electrode 14 (FG) is formed on the
active area AA in the peripheral circuit area via the gate
insulating film 13B thicker than the gate insulating film 13A.
[0145] The element isolation insulating film 12 having the STI
structure is formed in the element isolation area. The upper
surface of the element isolation insulating film 12 is flat, and
has the step S on the outside of the periodical structure between
the active area AA and the element isolation area, for example,
between the 1-th active area AA in the memory cell array area 1 and
the active area AA in the peripheral area.
[0146] As to the step S, the upper surface of the element isolation
insulating film 12 in the portion of the periodical structure is
low.
[0147] As a result, a part of the side surface of the floating gate
14 (FG) on the active areas AA in the memory cell array area 1 is
exposed from the element isolation insulating film 12.
[0148] The control gate electrode (word line) 16 (WL) is formed on
the element isolation insulating film 12, the floating gate
electrode 14 (FG) and the conductive layer 14 via the
inter-electrode insulating film (for example, ONO film,
high-dielectric-constant material or the like) 15.
[0149] The control gate electrode 16 (WL) extends in the first
direction across the memory cell array area 1 and the peripheral
circuit area. The control gate electrode 16 (WL) can have a
multi-layered structure, for example, a laminated structure
composed of a conductive polysilicon layer and a silicide
layer.
[0150] The upper gate electrodes (select gate lines) 16 (SGS) and
16 (SGD) are formed on the lower gate electrodes 14 (SGS) and 14
(SGD) via the inter-electrode insulating film 15. The lower gate
electrodes 14 (SGS) and 14 (SGD), and the upper gate electrodes 16
(SGS) and 16 (SGD) are electrically connected.
[0151] The closed-loop structured active areas AA are formed by the
side wall patterning technique as described later. According to the
side wall patterning technique, the widths of all the active areas
AA in the memory cell array area 1 in the first direction can be
made uniform and narrower than the limit of resolution of
photolithography.
[0152] m-th (m=2) and subsequent active areas AA from the endmost
portion of the closed-loop structured active areas AA in the first
direction are formed in the memory cell array area 1. The active
areas AA before the m-th one from the endmost portion in the first
direction are formed in the peripheral circuit area.
[0153] m=2, but m may be any number not less than 2.
[0154] In this case, the gate insulating film 13A is formed on the
active areas AA in the memory cell array area 1 of the closed-loop
structured active areas AA. The gate insulating film 13B thicker
than the gate insulating film 13A is formed on the active areas AA
in the peripheral circuit area.
[0155] That is, since the gate insulating film 13B is formed on the
first active area AA from the endmost portion in the first
direction, the MOS transistor having the gate insulating film 13B
is highly resistant to breakage caused by a writing potential, and
thus the reliability is improved.
[0156] The second active area AA from the endmost portion in the
first direction in the memory cell array area 1 is set as the dummy
area where the dummy cell which does not function as the memory
cell is formed. In this case, the width (Wend) of the dummy area in
the first direction becomes the same as the width (Wmiddle) of the
active areas AA in the first direction where the memory cell is
formed.
[0157] For this reason, the memory cell and the dummy cell have the
same structure and the same size. Specifically, the coupling ratio
of the memory cell and that of the dummy cell are equal to each
other.
[0158] Therefore, even if a writing potential is applied to the
word line shared by the memory cell and the dummy cell, an electric
field to be applied to the inter-electrode insulating film (or
block insulating film) of the dummy cell is equal to an electric
field to be applied to the inter-electrode insulating film (or
block insulating film) of the memory cell. As a result, the dummy
cell is highly resistant to breakage, and thus the reliability is
improved.
[0159] B. Manufacturing Method
[0160] The method for manufacturing the NAND type flash memory
shown in FIGS. 24 to 26 will be described.
[0161] As shown in FIGS. 27 and 28, the double well region
including the n-type well region 11b and the p type well region 11c
is formed in the p-type semiconductor substrate 11a.
[0162] The gate insulating film 13A is formed on the p type well
region 11c in the memory cell array area 1, and the gate insulating
film 13B thicker than the gate insulating film 13A is formed on the
p-type well region 11c in the peripheral circuit area according to
the thermal oxidation method.
[0163] Thereafter, the conductive layer 14, the first mask layer 17
and the second mask layer 18 are sequentially formed on the gate
insulating films 13A and 13B according to the CVD method. The first
mask layer 17 and the second mask layer 18 are made of different
materials which have different etching selectivities.
[0164] Thereafter, the photoresist film 19 is formed on the second
mask layer 18.
[0165] The photoresist film 19 is processed into a predetermined
pattern by the photolithography process. For example, the
photoresist film 19 is formed by the line and space pattern in the
memory cell array area 1.
[0166] The pitches of the lines and spaces of the photoresist film
19 are set to the limit of resolution of photolithography (feature
size), for example, 120 nm (line width x=60 nm, space width x=60
nm).
[0167] The line width of the photoresist film 19 is set to, for
example, 120 nm on a boundary portion between the memory cell array
area 1 and the peripheral circuit area.
[0168] After the second mask layer 18 is etched by using the
photoresist film 19 as a mask according to RIE, the photoresist
film 19 is removed.
[0169] As a result, as shown in FIGS. 29 and 30, the pattern of the
photoresist film 19 shown in FIGS. 27 and 28 is transferred onto
the second mask layer 18.
[0170] Thereafter, the width of the second mask layer 18 is made to
be smaller by a slimming technique. The width of the second mask
layer 18 becomes smaller than the limit of resolution of
photolithography in the memory cell array area 1.
[0171] In the cross-sectional view of FIG. 30, a dotted line shows
the pattern of the second mask layer 18 before slimming, and a
solid line shows the pattern of the second mask layer 18 after
slimming.
[0172] In the memory cell array area 1, the line width (a) of the
second mask layer 18 is set to 30 nm, and the space width (b)
thereof is set to 90 nm by the slimming technique.
[0173] As shown in FIGS. 31 and 32, the second mask layer 18 in the
peripheral circuit area is solidified. The etching selectivity of
the solidified second mask layer 18 becomes sufficiently smaller
than that of the unsolidified second mask layer 18 under the same
etching condition. The solidified portion is shown in gray.
[0174] The third mask layer 20 is formed on the first mask layer 17
and the second mask layer 18 by the CVD method, and is left only on
the side wall portion of the second mask layer 18 by RIE.
[0175] The third mask layer 20 has the closed-loop shape which
surrounds the second mask layer 18.
[0176] In the memory cell array area 1, both the width of the
second mask layer 18 and the width of the third mask layer 20 are,
for example, 30 nm. The space width between the second mask layer
18 and the third mask layer 20 is also 30 nm, for example.
[0177] Thereafter, when only the unsolidified second mask layer 18
is removed, as shown in FIGS. 33 and 34, the line and space
structure which is composed of the third mask layer (line width=30
nm) 20 and the space (space width=30 nm) is formed in the memory
cell array area 1.
[0178] When the first mask layer 17 is etched by using the
solidified second mask layer 18 and the third mask layer 20 as the
mask according to RIE, the patterns of the solidified second mask
layer 18 and the third mask layer 20 are transferred onto the first
mask layer 17.
[0179] As a result, the fine pattern of the first mask layer 17 is
formed by the side wall patterning technique.
[0180] This pattern (line=30 nm, space=30 nm) is finer than the
limit (feature size) of resolution of photolithography.
[0181] Thereafter, the solidified second mask layer 18 and the
third mask layer 20 are removed.
[0182] As shown in FIGS. 35 and 36, the conductive layer 14, the
gate insulating films 13A and 13B and the semiconductor substrate
(including the n-type well region and the p-type well region) 11a
are etched by using the first mask layer 17 as the mask according
to RIE, so that the trench is formed.
[0183] As shown in FIGS. 37 and 38, the trench is filled with the
element isolation insulating film 12 according to the CVD method.
The upper surface of the element isolation insulating film 12 is
flattened by CMP, for example.
[0184] The upper surface of the element isolation insulating film
12 approximately matches the upper surface of the first mask layer
17.
[0185] As shown in FIGS. 39 and 40, the element isolation
insulating film 12 in the memory cell array area 1 is selectively
etched. The position of the upper surface thereof is lowered, so
that the side surface of the conductive layer 14 is partially
exposed.
[0186] The side surface of the conductive layer 14 on all the
active areas (including the dummy area) AA constituting the
periodical structure in the memory cell array area 1 and the
peripheral circuit area is exposed.
[0187] As a result, when the second active area AA from the endmost
portion in the first direction in the memory cell array area 1 is
set as the dummy area, the structure of the dummy cell is exactly
the same as the memory cell (the size is also the same) as shown in
FIG. 41. For this reason, the coupling ratio of the dummy cell and
that of the memory are equal to each other.
[0188] The breakage of the dummy cell is, therefore, prevented.
[0189] In this case, the step S is formed on the element isolation
insulating film 12 on the outside of the periodical structure in
the memory cell array area 1 and the peripheral circuit area. The
position of the step S can be anywhere as long as it is on the
outside of the periodical structure.
[0190] Finally as shown in FIGS. 42 and 43, the inter-electrode
insulating film (for example, ONO film, high-dielectric-constant
material or the like) 15 and the conductive layer 16 are
formed.
[0191] When the conductive layers 14 and 16 and the inter-electrode
insulating film 15 are patterned, the floating gate electrode 14
(FG) and the control gate electrode (word line) 16 (CG) of the
memory cell (including the dummy cell) are formed. At the same
time, the select gate lines 16 (SGS) and 16 (SGD) of the select
gate transistor are formed.
[0192] The NAND type flash memory shown in FIGS. 24 to 26 is thus
completed.
[0193] The above manufacturing method adopts the solidifying
process for the second mask layer 18, but similarly to the first
example, even if the solidifying process is not used, one-time PEP
is performed so that the second mask layer 18 is left in the
peripheral circuit area.
[0194] C. Conclusion
[0195] According to the second example, employing the side wall
patterning technique enables the closed-loop structured active
areas having a uniform width to be formed. As a result, the
miniaturization and the improvement in reliability of the memory
cell can be simultaneously realized.
[0196] The first active area AA from the endmost portion in the
first direction may be arranged in the peripheral circuit area.
Then, the gate insulating film 13B on the active areas AA is
thicker than that of the memory cell. As a result, the improvement
in reliability of the memory cell can be simultaneously realized
effectually.
4. MODIFICATION EXAMPLE
[0197] Some modification examples of the present invention will be
described.
[0198] The position of the step of the element isolation insulating
film can be set between the (m-1)-th active area from the endmost
portion in the first direction and the m-th active area in the
second example. However, m is any number not less than 2.
[0199] For example as shown in FIGS. 44 and 45, the step may be set
between the first active area AA from the endmost portion in the
first direction and the second active area AA in the closed-loop
structured active areas AA constituting the periodical
structure.
[0200] FIGS. 44 and 45 correspond to FIGS. 40 and 43 in the above
manufacturing method.
[0201] In this case, as is clear from FIG. 45, an opposed area
between the conductive layer 14 and the control gate electrode 16
(WL) is small on the first active area AA from the endmost portion
in the first direction. As a result, the capacitance of the
capacitor (corresponding to C2 in FIG. 41) to be formed thereon
becomes small, and thus the breakage of the inter-electrode
insulating film is concerned.
[0202] However, since the first active area AA from the endmost
portion in the first direction is arranged in the peripheral
circuit area, the gate insulating film 13B on the active areas AA
is thicker than that of the memory cell. As a result, the
capacitance of the capacitor (corresponding to C1 in FIG. 41) to be
formed thereon becomes small.
[0203] Therefore, in the case of the second example, even when the
position of the step of the element isolation insulating film is
set inside the periodical structure, the reliability of the
semiconductor memory is sufficiently maintained.
[0204] As a matter of course, as described in the second example,
when the position of the step of the element isolation insulating
film is set on the outside of the periodical structure, the
reliability can be further improved.
[0205] In the first and second examples, the structure of the
memory cell is not limited to a stack gate type which has the
floating gate electrode and the control gate electrode.
[0206] FIG. 46 shows the memory cell structure as a modification
example.
[0207] This memory cell is of a so-called MONOS type.
[0208] The above example refers to the stack gate type memory cell,
but the present invention can be a MONOS type memory cell
(including a dummy cell).
[0209] The MONOS type memory cell is a nonvolatile semiconductor
memory cell in which a charge accumulation layer is composed of an
insulating film.
[0210] A source/drain diffusion layer 22 is arranged in the
semiconductor substrate (active area) 21. A gate insulating film
(tunnel insulating film) 23, a charge accumulation layer 24, a
block insulating film 25 and a control gate electrode (word line)
26 are arranged on a channel area between the source/drain
diffusion layers 22.
[0211] The block insulating film 25 is composed of, for example, an
ONO (oxide/nitride/oxide) film or a high-dielectric-constant
(high-k) material.
5. APPLICATION EXAMPLE
[0212] An example of a system to which the semiconductor memory of
the present invention is applied will be described.
[0213] FIG. 47 shows one example of the memory system.
[0214] This system is, for example, a memory card or a USB
memory.
[0215] A circuit substrate 32, and a plurality of semiconductor
chips 33, 34 and 35 are arranged in a package 31. The circuit
substrate 32 and the semiconductor chips 33, 34 and 35 are
electrically connected by a bonding wire 36. One of the
semiconductor chips 33, 34 and 35 is the semiconductor memory of
the present invention.
[0216] FIG. 48 shows a chip layout of the semiconductor memory as
the application example.
[0217] Memory cell arrays 41A and 41B are arranged on a
semiconductor chip 40. The memory cell arrays 41A and 41B each have
blocks BK0, BK1, . . . BKn-1 which are arranged in the second
direction. The blocks BK0, BK1, . . . BKn-1 have a plurality of
cell units CU which are arranged in the first direction,
respectively.
[0218] As shown in FIG. 49, the cell unit CU is a NAND string which
is composed of a plurality of memory cells MC connected in series
in the second direction, and two select gate transistors ST
connected to both ends of the memory cell MC.
[0219] Bit lines BL which extend in the second direction are
arranged on the memory cell arrays 41A and 41B. A page buffer (PB)
43 is arranged on both ends of the memory cell arrays 41A and 41B
in the second direction. The page buffer 43 has a function to
temporarily store reading data/writing data therein at the time of
reading/writing. The page buffer 43 functions as a sense amplifier
during reading or verifying of a writing/erasing operation.
[0220] A row decoder (RDC) 44 is arranged at one end (the end
portion opposite to the end portion on the edge side of the
semiconductor chip 40) of the memory cell arrays 41A and 41B in the
first direction. A pad area 42 is arranged along the edge of the
semiconductor chip 40 on one end of the memory cell arrays 41A and
41B in the second direction. A peripheral circuit 45 is arranged
between the page buffer 43 and the pad area 42.
6. CONCLUSION
[0221] According to the present invention, the miniaturization of
the memory cell is compatible with the improvement in the
reliability due to the new active area structure of the memory cell
array.
[0222] According to the present invention, the widths of the
closed-loop structured active areas become uniform, but this
uniformity includes a slight difference in the widths due to
patterning dispersion.
[0223] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications can be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *