U.S. patent application number 11/190120 was filed with the patent office on 2006-10-26 for semiconductor memory device and method of manufacturing the same.
Invention is credited to Isao Kamioka, Yoshio Ozawa, Junichi Shiozawa.
Application Number | 20060240619 11/190120 |
Document ID | / |
Family ID | 37187484 |
Filed Date | 2006-10-26 |
United States Patent
Application |
20060240619 |
Kind Code |
A1 |
Ozawa; Yoshio ; et
al. |
October 26, 2006 |
Semiconductor memory device and method of manufacturing the
same
Abstract
A semiconductor memory device manufacturing method includes
forming a floating gate electrode above a semiconductor substrate,
forming an interelectrode insulating film above the floating gate
electrode, forming a first radical nitride film on a surface of the
interelectrode insulating film by first radical nitriding, and
forming a control gate electrode on the first radical nitride
film.
Inventors: |
Ozawa; Yoshio;
(Yokohama-shi, JP) ; Kamioka; Isao; (Machida-shi,
JP) ; Shiozawa; Junichi; (Yokkaichi-shi, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
37187484 |
Appl. No.: |
11/190120 |
Filed: |
July 27, 2005 |
Current U.S.
Class: |
438/257 ;
257/E21.682; 257/E27.103 |
Current CPC
Class: |
H01L 27/115 20130101;
H01L 27/11521 20130101 |
Class at
Publication: |
438/257 |
International
Class: |
H01L 21/336 20060101
H01L021/336 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2005 |
JP |
2005-128232 |
Claims
1. A semiconductor memory device comprising: a semiconductor
substrate; a tunnel insulating film formed on the semiconductor
substrate; a floating gate electrode formed on the tunnel
insulating film, and having a first side-surface portion positioned
in an upper portion and a second side-surface portion positioned
below the first side-surface portion; an element isolation trench
formed in the semiconductor substrate to be adjacent to the
floating gate electrode; a first element isolation insulating film
formed along a side surface and bottom surface of the element
isolation trench from the second side-surface portion of the
floating gate electrode; a second element isolation insulating film
formed on the first element isolation insulating film to expose a
side-surface portion in an upper portion of the first element
isolation insulating film; a first radical nitride film formed on
the floating gate electrode and first and second element isolation
insulating films; an interelectrode insulating film formed on the
first radical nitride film; a nitrogen-containing film formed on
the interelectrode insulating film; and a control gate electrode
formed on the nitrogen-containing film, wherein in the second
side-surface portion of the floating gate electrode, a portion of
the first element isolation insulating film exists between the
floating gate electrode and first radical nitride film.
2. The device according to claim 1, wherein in the first
side-surface portion of the floating gate electrode, the first
element isolation insulating film does not exist between the
floating gate electrode and first radical nitride film.
3. The device according to claim 1, wherein a film thickness of the
first radical nitride film in the second side-surface portion of
the floating gate electrode is smaller than a film thickness of the
first radical nitride film in the first side-surface portion of the
floating gate electrode.
4. The device according to claim 1, wherein the first element
isolation insulating film is an oxide film.
5. The device according to claim 1, wherein the nitrogen-containing
film is a second radical nitride film.
6. The device according to claim 1, wherein the interelectrode
insulating film is one of a silicon oxide film, a metal silicate
film, a multilayered film having a silicon oxide film as an upper
layer, and a multilayered film having a metal silicate film as an
upper layer.
7. A semiconductor memory device manufacturing method comprising:
forming a floating gate electrode above a semiconductor substrate;
forming an interelectrode insulating film above the floating gate
electrode; forming a first radical nitride film on a surface of the
interelectrode insulating film by first radical nitriding; and
forming a control gate electrode on the first radical nitride
film.
8. The method according to claim 7, wherein in the formation of the
first radical nitride film, a nitrogen radical is formed by a gas
mixture of nitrogen gas and a rare gas, or a nitrogen radical is
formed by nitrogen gas alone.
9. The method according to claim 7, wherein the interelectrode
insulating film is one of a silicon oxide film, a metal silicate
film, a multilayered film having a silicon oxide film as an upper
layer, and a multilayered film having a metal silicate film as an
upper layer.
10. The method according to claim 7, wherein a nitriding pressure
in the first radical nitriding is not less than 5 Pa and not more
than 1 kPa.
11. The method according to claim 7, wherein a nitriding pressure
in the first radical nitriding is not less than 10 Pa and not more
than 100 Pa.
12. The method according to claim 7, wherein a nitriding pressure
in the first radical nitriding is less than 5 Pa.
13. The method according to claim 7, further comprising forming a
second radical nitride film between the floating gate electrode and
interelectrode insulating film by second radical nitriding.
14. The method according to claim 13, wherein a nitriding pressure
in the second radical nitriding is not less than 5 Pa and not more
than 1 kPa.
15. The method according to claim 13, wherein a nitriding pressure
in the second radical nitriding is not less than 10 Pa and not more
than 100 Pa.
16. The method according to claim 7, further comprising: forming a
tunnel insulating film on the semiconductor substrate; forming the
floating gate electrode on the tunnel insulating film; forming an
element isolation trench by etching the floating gate electrode,
tunnel insulating film, and semiconductor substrate; forming a
first element isolation insulating film on an exposed surface in
the element isolation trench; forming a second element isolation
insulating film on the first element isolation insulating film;
selectively removing the first and second element isolation
insulating films to expose a first side-surface portion in an upper
portion of the floating gate electrode and a side-surface portion
in an upper portion of the first element isolation insulating film;
forming a second radical nitride film on surfaces of the floating
gate electrode and first and second element isolation insulating
films by second radical nitriding; and forming the interelectrode
insulating film on the second radical nitride film.
17. The method according to claim 16, wherein in a second
side-surface portion in a lower portion of the floating gate
electrode, the first element isolation insulating film exists
between the floating gate electrode and second radical nitride
film, and in the first side-surface portion of the floating gate
electrode, the first element isolation insulating film does not
exist between the floating gate electrode and second radical
nitride film.
18. The method according to claim 16, wherein a film thickness of
the second radical nitride film in a second side-surface portion in
a lower portion of the floating gate electrode is smaller than a
film thickness of the second radical nitride film in the first
side-surface portion of the floating gate electrode.
19. The method according to claim 16, wherein the first element
isolation insulating film is an oxide film.
20. The method according to claim 16, wherein a second etching rate
of the second element isolation insulating film is higher than a
first etching rate of the first element isolation insulating film.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2005-128232,
filed Apr. 26, 2005, 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 a semiconductor memory
device including a floating gate electrode and control gate
electrode, and a method of manufacturing the same.
[0004] 2. Description of the Related Art
[0005] Conventionally, a nonvolatile semiconductor memory device
including a floating gate electrode FG and control gate electrode
CG has been proposed as a semiconductor memory.
[0006] In this nonvolatile semiconductor memory device, as shown in
FIG. 19, a floating gate electrode FG, interelectrode insulating
film 208, and control gate electrode CG are processed into a
desired shape, and a gate sidewall oxide film 212 is formed on the
entire surface by oxidation after that. When the gate sidewall
oxide film 212 is formed, however, thick so-called "bird's beak"
oxidation regions 213 are formed in the upper and lower end
portions of the interelectrode insulating film 208. This decreases
the electric capacitance of the interelectrode insulating film 208,
and causes faulty operations of the memory device. These problems
caused by the formation of the bird's beak oxidation regions 213
similarly arise when a high-k oxide film such as a hafnium oxide
film, zirconium oxide film, or tantalum oxide film, or an
insulating film obtained by adding an impurity to a high-k oxide
film like this is used as the interelectrode insulating film
208.
[0007] As a method of avoiding the formation of the bird's beaks
described above, a technique as shown in FIGS. 20A and 20B is
disclosed (e.g., Jpn. Pat. Appln. KOKAI Publication No. 7-249697,
8-153814, or 9-219459). In this technique, silicon nitride layers
231 and 232 are formed in the upper and lower interfaces of an
interelectrode insulating film 208, thereby preventing the
penetration of an oxidation species during gate sidewall oxidation
and preventing the formation of bird's beaks.
[0008] This technique discloses CVD (Chemical Vapor Deposition) or
thermal nitriding as the method of forming the silicon nitride
layers 231 and 232. However, the following problems arise if the
silicon nitride layers 231 and 232 are formed by using CVD or
thermal nitriding.
[0009] When CVD is used, the total physical film thickness of an
interelectrode insulating film 208a between a control gate
electrode CG and floating gate electrode FG increases. Therefore,
as shown in FIG. 20B, the depth P of the control gate electrode CG
buried between adjacent cells decreases. This decreases the
capacitance of the interelectrode insulating film 208 and increases
the parasitic capacitance between the adjacent cells, so the memory
device suffers faulty operation. In addition, the width Q of the
control gate electrode CG buried between the adjacent cells
decreases. Since this depletes the buried portion of the control
gate electrode CG, the memory device suffers faulty operation.
Furthermore, if the depth P is well increased, the distance R
between a substrate 201 and the interelectrode insulating film 208
shortens. This decreases the breakdown voltage between the
substrate 201 and control gate electrode CG, so the memory device
suffers faulty operation. These problems are significant when the
depth P, width Q, and distance R are approximately 100 nm or
less.
[0010] On the other hand, when thermal nitriding is used, a
high-temperature, long-time thermal budget is necessary to form
silicon nitride layers 231 and 232 having a thickness which
prevents the formation of bird's beaks. Since this degrades the
quality of a tunnel oxide film 202, the reliability of the memory
device is also degraded.
BRIEF SUMMARY OF THE INVENTION
[0011] A semiconductor memory device according to a first aspect of
the present invention comprises a semiconductor substrate, a tunnel
insulating film formed on the semiconductor substrate, a floating
gate electrode formed on the tunnel insulating film, and having a
first side-surface portion positioned in an upper portion and a
second side-surface portion positioned below the first side-surface
portion, an element isolation trench formed in the semiconductor
substrate to be adjacent to the floating gate electrode, a first
element isolation insulating film formed along a side surface and
bottom surface of the element isolation trench from the second
side-surface portion of the floating gate electrode, a second
element isolation insulating film formed on the first element
isolation insulating film to expose a side-surface portion in an
upper portion of the first element isolation insulating film, a
first radical nitride film formed on the floating gate electrode
and first and second element isolation insulating films, an
interelectrode insulating film formed on the first radical nitride
film, a nitrogen-containing film formed on the interelectrode
insulating film, and a control gate electrode formed on the
nitrogen-containing film, wherein in the second side-surface
portion of the floating gate electrode, a portion of the first
element isolation insulating film exists between the floating gate
electrode and first radical nitride film.
[0012] A semiconductor memory device manufacturing method according
to a second aspect of the present invention comprises forming a
floating gate electrode above a semiconductor substrate, forming an
interelectrode insulating film above the floating gate electrode,
forming a first radical nitride film on a surface of the
interelectrode insulating film by first radical nitriding, and
forming a control gate electrode on the first radical nitride
film.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0013] FIGS. 1A and 1B are sectional views in the bit line
direction and word line direction, respectively, showing a
nonvolatile semiconductor memory device according to a first
embodiment of the present invention;
[0014] FIGS. 2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7A and 7B are
sectional views in the bit line direction and word line direction,
respectively, showing the manufacturing steps of the nonvolatile
semiconductor memory device according to the first embodiment of
the present invention;
[0015] FIGS. 8A and 8B are sectional views in the bit line
direction and word line direction, respectively, showing a
nonvolatile semiconductor memory device according to a second
embodiment of the present invention;
[0016] FIGS. 9A, 9B, 10A, 10B, 11A, 11B, 12A and 12B are sectional
views in the bit line direction and word line direction,
respectively, showing the manufacturing steps of the nonvolatile
semiconductor memory device according to the second embodiment of
the present invention;
[0017] FIG. 13 is a sectional view showing a nonvolatile
semiconductor memory device according to a third embodiment of the
present invention;
[0018] FIG. 14 is a sectional view showing a nonvolatile
semiconductor memory device of means example 1 according to the
third embodiment of the present invention;
[0019] FIGS. 15, 16 and 17 are sectional views showing the
manufacturing steps of the nonvolatile semiconductor memory device
of means example 1 according to the third embodiment of the present
invention;
[0020] FIG. 18 is a graph showing the film thinning ratio and
radical nitriding rate as functions of the radical nitriding
pressure according to means example 2 of the third embodiment of
the present invention;
[0021] FIG. 19 is a sectional view showing a nonvolatile
semiconductor memory device according to prior art; and
[0022] FIGS. 20A and 20B are sectional views in the bit line
direction and word line direction, respectively, showing another
nonvolatile semiconductor memory device according to prior art.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Embodiments of the present invention will be described below
with reference to the accompanying drawing. In the following
explanation, the same reference numerals denote the same parts
throughout the drawing.
First Embodiment
[0024] In a first embodiment, a radical nitride film is formed by
radical nitriding on an interelectrode insulating film between a
floating gate electrode and control gate electrode.
[0025] FIGS. 1A and 1B are sectional views of a nonvolatile
semiconductor memory device according to the first embodiment of
the present invention. FIG. 1A is a sectional view in the bit line
direction (channel length direction), and FIG. 1B is a sectional
view in the word line direction (channel width direction). This
nonvolatile semiconductor memory device according to the first
embodiment will be explained below.
[0026] As shown in FIGS. 1A and 1B, floating gate electrodes FG are
formed on a silicon substrate 101 via a tunnel oxide film 102, and
an interelectrode insulating film 108 is formed on the floating
gate electrodes FG. A radical nitride film 109 is formed on the
interelectrode insulating film 108 by radical nitriding. A control
gate electrode CG is formed on the radical nitride film 109, and a
mask material 111 is formed on the control gate electrode CG. A
gate sidewall oxide film 112 is formed on the side surfaces of the
mask material 111, control gate electrode CG, radical nitride film
109, interelectrode insulating film 108, floating gate electrodes
FG, and tunnel oxide film 102. An interlayer dielectric film 115 is
formed on the gate sidewall oxide film 112. Source/drain diffusion
layers 114 are formed in the silicon substrate 101 to form memory
cell transistors Tr.
[0027] As described above, the radical nitride film 109 is formed
on the interelectrode insulating film 108. Therefore, when
oxidation is performed to form the gate sidewall oxide film 112 or
the interlayer dielectric film 115, the radical nitride film 109
prevents the penetration of an oxidation species to the lower
surface of the control gate electrode CG through the interelectrode
insulating film 108, so no bird's beak oxidation region exists on
the interelectrode insulating film 108. However, bird's beak
oxidation regions 113 exist below the two ends of the
interelectrode insulating film 108. The bird's beak oxidation
regions 113 are thick regions formed when portions of the
interelectrode insulating film 108 are oxidized by the oxidation
described above.
[0028] Note that the interelectrode insulating film 108 is
desirably a silicon oxide film, a metal silicate film, a stacked
film having a silicon oxide film as an upper layer, or a stacked
film having a metal silicate film as an upper layer. Examples of
the metal of the metal silicate film are aluminum (Al), zirconium
(Zr), lanthanum (La), hafnium (Hf), and tantalum (Ta). In this
embodiment, the interelectrode insulating film 108 is an ONO
(Oxide/Nitride/Oxide) film as an example.
[0029] The radical nitride film 109 is a nitrogen-containing film
formed by performing radical nitriding on the surface of the
interelectrode insulating film 108. Therefore, the radical nitride
film 109 is, e.g., a silicon oxynitride film or silicon nitride
film in accordance with the material of the interelectrode
insulating film 108 as an undercoat. In this embodiment, the
radical nitride film 109 is a silicon oxynitride film as an
example.
[0030] FIGS. 2A and 2B to FIGS. 7A and 7B are sectional views of
the manufacturing steps of the nonvolatile semiconductor memory
device according to the first embodiment of the present invention.
Each figure with a suffix A is a sectional view in the bit line
direction (channel length direction), and each figure with a suffix
B is a sectional view in the word line direction (channel width
direction). A method of manufacturing the nonvolatile semiconductor
memory device according to the first embodiment will be explained
below.
[0031] First, as shown in FIGS. 2A and 2B, a tunnel oxide film 102
having a thickness of, e.g., 10 nm is formed by thermal oxidation
on the surface of a silicon substrate 101 in which a desired
impurity is doped. Then, CVD (Chemical Vapor Deposition) is used to
sequentially deposit a phosphorus-doped polysilicon layer 103
having a thickness of, e.g., 100 nm and serving as floating gate
electrodes FG, and a mask material (e.g., a silicon nitride film)
104 for element isolation processing. After that, the mask material
104, polysilicon layer 103, and tunnel oxide film 102 are
sequentially etched by RIE (Reactive Ion Etching) using a resist
mask (not shown). In addition, an exposed region of the silicon
substrate 101 is etched to form an element isolation trench 105
having a depth D of, e.g., 100 nm in the silicon substrate 101.
[0032] As shown in FIGS. 3A and 3B, an element isolation insulating
film 106 made of, e.g., a silicon oxide film is deposited in the
element isolation trench 105 and on the mask material 104, and the
element isolation trench 105 is completely filled with the element
isolation insulating film 106. After that, the upper surface of the
element isolation insulating film 106 is planarized by CMP
(Chemical Mechanical Polish) until the mask material is
exposed.
[0033] As shown in FIGS. 4A and 4B, the exposed mask material 104
is selectively etched away. Then, the element isolation insulating
film 106 is partially etched away by using a dilute hydrofluoric
acid solution until the upper surface of the element isolation
insulating film 106 is positioned below the upper surface of the
polysilicon layer 103. Consequently, a trench 107 is formed between
the polysilicon layers 103 (floating gate electrodes FG) adjacent
to each other, and sidewall surfaces 107a of the polysilicon layers
103 are exposed. A height H of the sidewall surfaces 107a is, e.g.,
50 nm. Also, an interval (the width of the element isolation
insulating film 106) W of the adjacent polysilicon layers 103
(floating gate electrodes FG) is, e.g., 50 nm.
[0034] As shown in FIGS. 5A and 5B, an interelectrode insulating
film 108 is formed in the trench 107 and on the polysilicon layers
103. The interelectrode insulating film 108 is an ONO film having a
thickness of, e.g., 15 nm. This ONO film is made up of three films,
i.e., a silicon oxide film/silicon nitride film/silicon oxide film
each having a thickness of, e.g., 5 nm.
[0035] Then, the silicon substrate 101 is placed in a radical
nitriding chamber (not shown) and heated to 400.degree. C., the
pressure is adjusted to 6 Pa while a gas mixture of 40 sccm of
nitrogen (N.sub.2) gas and 1,000 sccm of argon (Ar) gas is supplied
into the chamber, and microwaves having a power of 1 kW are
supplied to generate radical nitrogen, thereby performing radical
nitriding on the surface of the interelectrode insulating film 108
for 90 sec. As a consequence, a radical nitride film 109 is formed
on the surface of the interelectrode insulating film 108. The
radical nitride film 109 is formed because the radical nitrogen
diffuses in the surface portion of the interelectrode insulating
film 108. In this embodiment, a silicon oxynitride film having a
thickness of, e.g., 2 nm is formed as the radical nitride film 109
in the surface portion of the silicon oxide film in the upper
portion of the ONO film as the interelectrode insulating film
108.
[0036] Radical nitrogen means a nitrogen atom in an excited state,
a nitrogen atom in a ground state, and a nitrogen molecule in an
excited state, and includes both electrically neutral and charged
atoms or molecules. The above effect is obtained when one or more
types of the radical nitrogen are contained. Radical nitriding is
also a nitriding method which uses one or more types of the radical
nitrogen described above as nitriding species. A typical radical
nitrogen density is 1E12 cm.sup.-3 or more. To shorten the process
time of the nitriding step by increasing the nitriding efficiency,
a radical nitrogen density of 1E15 cm.sup.-3 or more is
desired.
[0037] In this embodiment, radical nitriding of the surface of the
interelectrode insulating film 108 is performed in the gas mixture
ambient containing nitrogen gas and argon gas. However, the radical
nitriding method is not limited to this method, and it is also
possible to use another nitrogen-containing gas or a gas mixture of
another nitrogen-containing gas and a rare gas. However, to prevent
an element, such as hydrogen or chlorine, other than nitrogen from
mixing in the radical nitride film 109 and degrading the insulation
properties, it is desirable to generate nitrogen radicals by using
nitrogen gas alone or a gas mixture of nitrogen gas and a rare gas.
Furthermore, a rare gas is preferably mixed in order to increase
the radical nitrogen generation efficiency.
[0038] Also, in this embodiment, the radical nitride film 109 is
formed by generating radical nitrogen by supplying microwaves into
the reaction chamber in which the silicon substrate 101 is placed.
However, the radical nitriding method is not limited to this
method. That is, to alleviate deterioration of the tunnel oxide
film 102 and interelectrode insulating film 108 caused by so-called
plasma damage, the radical nitride film 109 may also be formed by
transferring, to the reaction chamber, radical nitrogen generated
in a chamber different from the reaction chamber. Also, to form a
thicker radical nitride film 109, it is possible to directly place
the silicon substrate 101 in a plasma containing radical nitrogen,
or apply a bias to the silicon substrate 101 and draw excited
nitrogen ions.
[0039] As shown in FIGS. 6A and 6B, a conductive layer 110 having a
thickness of, e.g., 100 nm and serving as a control gate electrode
CG is deposited on the radical nitride film 109 by CVD. The
conductive layer 110 has a two-layered structure including, e.g., a
polysilicon layer/tungsten silicide layer. In addition, an RIE mask
material 111 is deposited on the conductive layer 110. After that,
the mask material 111, conductive layer 110, radical nitride film
109, interelectrode insulating film 108, polysilicon layer 103, and
tunnel oxide film 102 are sequentially etched by RIE using a resist
mask (not shown). By this etching, slits SL are formed in the word
line direction. In this manner, floating gate electrodes FG and a
control gate electrode CG each having a desired shape are
formed.
[0040] As shown in FIGS. 7A and 7B, thermal oxidation and CVD are
combined to form a gate sidewall oxide film 112 made of, e.g., a
silicon oxide film on the silicon substrate 101, on the side
surfaces of the tunnel oxide film 102, floating gate electrodes FG,
interelectrode insulating film 108, radical nitride film 109,
control gate electrode CG, and mask material 111, and on the mask
material 111. Bird's beak oxidation regions 113 are also formed
below the end portions of the interelectrode insulating film
108.
[0041] Then, as shown in FIGS. 1A and 1B, ion implantation is used
to form source/drain diffusion layers 114 in the silicon substrate
101. In addition, an interlayer dielectric film 115 made of, e.g.,
a TEOS film is formed on the gate sidewall oxide film 112. After
that, a nonvolatile semiconductor memory device is completed by
forming interconnecting layers and the like by using the well-known
method.
[0042] In the first embodiment described above, the radical nitride
film 109 is formed on the interelectrode insulating film 108 by
radical nitriding. Since, therefore, the upper surface of the
interelectrode insulating film 108 is protected by the radical
nitride film 109 when the gate sidewall oxide film 112 and the
interlayer dielectric film 115 are formed, no bird's beak oxidation
region is formed on the upper surface of the interelectrode
insulating film 108 even after the oxidation step. In addition, the
radical nitride film 109 is formed when radical nitrogen diffuses
into the surface portion of the interelectrode insulating film 108,
so an increase in physical film thickness of the interelectrode
insulating film 108 caused by the formation of the radical nitride
film 109 is almost negligible. Instead, the physical film thickness
slightly decreases in the embodiment case. From the foregoing, it
is possible to suppress the formation of bird's beaks on the
interelectrode insulating film 108 in the oxidation step of forming
the gate sidewall oxide film 112 and the interlayer dielectric film
115, with almost no increase in total physical film thickness of
the interelectrode insulating film 108a between the floating gate
electrode FG and control gate electrode CG.
[0043] Also, as described above, the total physical film thickness
of an interelectrode insulating film 108a hardly increases. This
makes it possible to avoid malfunctions of the memory device caused
by an increase in parasitic capacitance between adjacent cells,
depletion of the control gate electrode CG, and a breakdown voltage
drop between the silicon substrate 101 and control gate electrode
CG. Note that if the interval (the width of the element isolation
insulating film 106) W between the floating gate electrodes FG
adjacent to each other is 100 nm or less, the effect of preventing
the increase in physical film thickness of the interelectrode
insulating film 108 becomes notable.
[0044] Furthermore, as described above, the radical nitride film
109 is formed by radical nitriding. Since this increases the
dielectric constant of the surface portion (e.g., in the case of
the ONO film, the surface portion of the upper silicon oxide film)
forming the interelectrode insulating film 108, the electric
capacitance of the interelectrode insulating film 108 also
increases. An example of the effects is an increase in memory cell
coupling ratio. In addition, when radical nitriding is performed by
using a gas not containing hydrogen as in the above embodiment,
insulation properties of the radical nitride film 109 improve, and
an electric charge leak flowing through the interelectrode
insulating film 108 is suppressed. An example of the effects is an
improvement in memory cell charge retention characteristics. In
addition, it is possible to avoid the thermal degradation of the
tunnel oxide film by using low-temperature radical nitridation
process.
Second Embodiment
[0045] In the first embodiment, the radical nitride film is formed
only on the interelectrode insulating film. In a second embodiment,
however, radical nitride films are formed above and below an
interelectrode insulating film.
[0046] FIGS. 8A and 8B are sectional views of a nonvolatile
semiconductor memory device according to the second embodiment of
the present invention. FIG. 8A is a sectional view in the bit line
direction (channel length direction), and FIG. 8B is a sectional
view in the word line direction (channel width direction). This
nonvolatile semiconductor memory device according to the second
embodiment will be explained below.
[0047] As shown in FIGS. 8A and 8B, the second embodiment differs
from the first embodiment in that a radical nitride film 109 is
formed above an interelectrode insulating film 108, and a radical
nitride film 120 is formed below the interelectrode insulating film
108. When oxidation is performed to form a gate sidewall oxide film
112, therefore, the upper surface of the floating gate electrode FG
and lower surface of the control gate electrode CG are protected by
the radical nitride films 120 and 109, respectively, so no bird's
beak oxidation region is formed above and below the interelectrode
insulating film 108.
[0048] As shown in FIG. 8B, the radical nitride film 120 below the
interelectrode insulating film 108 is formed by performing radical
nitriding on the exposed surfaces of floating gate electrodes FG
and an element isolation insulating film 106. Therefore, in
accordance with the material as an undercoat, a silicon nitride
film 120a is formed on the floating gate electrodes FG made of,
e.g., polysilicon layers 103, and a silicon oxynitride film 120b is
formed on the element isolation insulating film 106 made of, e.g.,
a silicon oxide film. As described above, the radical nitride film
120 (120a and 120b) below the interelectrode insulating film 108
changes in accordance with the materials of the floating gate
electrodes FG and element isolation insulating film 106 as
undercoats.
[0049] As in the first embodiment, the interelectrode insulating
film 108 is desirably a silicon oxide film, a metal silicate film,
a stacked film having a silicon oxide film as the upper layer, or a
stacked film having a metal silicate film as an upper layer. In
this embodiment, an aluminum silicate film is used as the
interelectrode insulating film 108 as an example.
[0050] FIGS. 9A and 9B to FIGS. 12A and 12B are sectional views of
the manufacturing steps of the nonvolatile semiconductor memory
device according to the second embodiment of the present invention.
Each figure with a suffix A is a sectional view in the bit line
direction (channel length direction), and each figure with a suffix
B is a sectional view in the word line direction (channel width
direction). A method of manufacturing the nonvolatile semiconductor
memory device according to the second embodiment will be explained
below.
[0051] First, as shown in FIGS. 9A and 9B, following the same
procedure (FIGS. 2A and 2B to FIGS. 4A and 4B) as in the first
embodiment, a trench 107 is formed between polysilicon layers 103
(floating gate electrodes FG) adjacent to each other, and sidewall
surfaces 107a of the polysilicon layers 103 are exposed. A height H
of the sidewall surfaces 107a is, e.g., 50 nm. Also, an interval
(the width of an element isolation insulating film 106) W of the
adjacent floating gate electrodes FG is, e.g., 50 nm.
[0052] Then, as shown in FIGS. 10A and 10B, a silicon substrate 101
is placed in a radical nitriding chamber (not shown) and heated to
400.degree. C., the pressure is adjusted to 6 Pa while a gas
mixture of 40 sccm of nitrogen gas and 1,000 sccm of argon gas is
supplied into the chamber, and a microwave having a power of 1 kW
is supplied to generate radical nitrogen, thereby performing
radical nitriding on the surfaces of the polysilicon layers 103 and
element isolation insulating film 106 for 90 sec. As a consequence,
a radical nitride film 120 is formed. In this embodiment, a silicon
nitride film 120a having a thickness of, e.g., 1 nm is formed on
the exposed surfaces of the polysilicon layers 103, and a silicon
oxynitride film 120b having a thickness of, e.g., 2 nm is formed on
the exposed surface of the element isolation insulating film 106
made of a silicon oxide film.
[0053] Then, an interelectrode insulating film 108 is formed on the
radical nitride film 120. In this embodiment, a 15 nm-thick
aluminum silicate film is deposited as the interelectrode
insulating film 108 by, e.g., ALD (Atomic Layer Deposition).
[0054] The silicon substrate 101 is placed in the radical nitriding
chamber (not shown) again and heated to 400.degree. C., the
pressure is adjusted to 6 Pa while a gas mixture of 40 sccm of
nitrogen gas and 1,000 sccm of argon gas is supplied into the
chamber, and a microwave having a power of 1 kW is supplied to
generate radical nitrogen, thereby performing radical nitriding on
the surface of the interelectrode insulating film 108 for 90 sec.
As a consequence, a radical nitride film 109 is formed on the
surface of the interelectrode insulating film 108. The radical
nitride film 109 is formed because the nitrogen radicals diffuse in
the surface portion of the interelectrode insulating film 108. In
this embodiment, a silicon oxynitride film having a thickness of,
e.g., 2 nm is formed as the radical nitride film 109.
[0055] Radical nitrogen means a nitrogen atom in an excited state,
a nitrogen atom in a ground state, and a nitrogen molecule in an
excited state, and includes both electrically neutral and charged
atoms or molecules. The above effect is obtained when one or more
types of the radical nitrogen are contained. Radical nitriding is
also a nitriding method which uses one or more types of the radical
nitrogen described above as nitriding species. A typical radical
nitrogen density is 1E12 cm.sup.-3 or more. To shorten the process
time of the nitriding step by increasing the nitriding efficiency,
a radical nitrogen density of 1E15 cm.sup.-3 or more is
desired.
[0056] In this embodiment, radical nitriding of the surface of the
interelectrode insulating film 108 is performed in the gas mixture
ambient containing nitrogen gas and argon gas. However, the radical
nitriding method is not limited to this method, and it is also
possible to use another nitrogen-containing gas or a gas mixture of
another nitrogen-containing gas and a rare gas. However, it is
desirable to generate nitrogen radicals by using nitrogen gas alone
or a gas mixture of nitrogen gas and a rare gas. This is so because
it is possible to well reduce the hydrogen content in the silicon
oxynitride film 120b formed on the exposed surface of the element
isolation insulating film 106, and avoid faulty operations of the
memory device caused by charge transfer between the floating gate
electrodes FG adjacent to each other.
[0057] As shown in FIGS. 11A and 11B, a conductive layer 110 having
a thickness of, e.g., 100 nm and serving as a control gate
electrode CG is deposited on the radical nitride film 109 by CVD.
The conductive layer 110 has a two-layered structure including,
e.g., a polysilicon layer/tungsten silicide layer. In addition, an
RIE mask material 111 is deposited on the conductive layer 110.
After that, the mask material 111, conductive layer 110, radical
nitride film 109, interelectrode insulating film 108, silicon
nitride film 120a, polysilicon layers 103, and tunnel oxide film
102 are sequentially etched by RIE using a resist mask (not shown).
By this etching, slits SL are formed in the word line direction. In
this manner, floating gate electrodes FG and a control gate
electrode CG each having a desired shape are formed.
[0058] As shown in FIGS. 12A and 12B, thermal oxidation and CVD are
combined to form a gate sidewall oxide film 112 made of, e.g., a
silicon oxide film on the silicon substrate 101, on the side
surfaces of the tunnel oxide film 102, floating gate electrodes FG,
silicon nitride film 120a, interelectrode insulating film 108,
radical nitride film 109, control gate electrode CG, and mask
material 111, and on the mask material 111. In this step, no bird's
beak oxidation regions are formed below and above the end portions
of the interelectrode insulating film 108.
[0059] Then, as shown in FIGS. 8A and 8B, ion implantation is used
to form source/drain diffusion layers 114 in the silicon substrate
101. In addition, an interlayer dielectric film 115 is formed on
the gate sidewall oxide film 112. After that, a nonvolatile
semiconductor memory device is completed by forming interconnecting
layers and the like by using the well-known method.
[0060] In the second embodiment described above, the same effects
as in the first embodiment can be obtained. Additionally, in the
second embodiment, the radical nitride film 120 (silicon nitride
film 120a) is formed below the interelectrode insulating film 108
between the floating gate electrodes FG and control gate electrode
CG by radical nitriding. Since, therefore, the upper surface of the
floating gate electrodes FG is protected by the radical nitride
film 120 (silicon nitride film 120a) when the gate sidewall oxide
film 112 is formed, no bird's beak oxidation region is formed below
the interelectrode insulating film 108 even after the oxidation
step. Accordingly, it is possible to suppress the formation of
bird's beaks much better than in the first embodiment, and
effectively prevent the decrease in capacitance of the
interelectrode insulating film 108.
[0061] Also, radical nitriding improves the quality of the silicon
oxynitride film 120b formed on the surface of the element isolation
insulating film 106. This makes it possible to further prevent
faulty operations of the device caused by charge transfer between
the floating gate electrodes FG adjacent to each other.
[0062] Furthermore, an increase in total physical film thickness of
an interelectrode insulating film 108a between the control gate
electrode CG and floating gate electrodes FG is only about 1 nm
which is the thickness of the silicon nitride film 120a.
Accordingly, the total physical film thickness of the
interelectrode insulating film 108a does not greatly increase in
this embodiment as well. This makes it possible to avoid
malfunctions of the memory device because an increase in parasitic
capacitance between adjacent cells, depletion of the control gate
electrode CG, and a breakdown voltage drop between the silicon
substrate 101 and control gate electrode CG are almost no
problem.
Third Embodiment
[0063] If a structure (to be referred to as an NONON structure
hereinafter) having radical nitride films 109 and 120 above and
below an interelectrode insulating film 108 made of an ONO film is
formed when a trench 107 between adjacent floating gates FG1 and
FG2 has a high aspect ratio and a narrow opening, a film thickness
Tside of the radical nitride film 120 (120a) on the sidewall
surfaces of the trench 107 sometimes decreases in a portion X (a
side-surface portion in the lower portion of the trench 107) in the
lower portion of the floating gate electrode FG1 (FG2) (FIG. 13).
This is presumably because radical nitriding hardly reaches the
bottom of the trench 107 to make the supply amount of radical
nitriding insufficient. If this is the case, an electric charge
leak between the floating gate electrode FG and a control gate
electrode CG in the portion X may increase to worsen the charge
retention characteristics.
[0064] To solve the above problem, a third embodiment uses means
(1) and (2) below as examples.
[0065] (1) The portion X in FIG. 13 is given an ONONON structure (a
structure in which an oxide film is formed as the lowermost layer
of the NONON structure) (means example 1).
[0066] (2) A radical nitride film as the lowermost layer of the
NONON structure is formed by radical nitriding at a nitriding
pressure of 5 Pa or more (desirably, 10 Pa or more) (means example
2).
[0067] Note that the above problem becomes significant when the
aspect ratio of the trench 107 is 1 or more and an opening width W
is 100 nm or less, and becomes more significant when the aspect
ratio of the trench 107 is 2 or more and the opening width W is 50
nm or less.
[0068] The details of means examples 1 and 2 will be explained
below.
(1) MEANS EXAMPLE 1
[0069] FIG. 14 is a sectional view of a nonvolatile semiconductor
memory device of means example 1 according to the third embodiment
of the present invention. FIG. 14 is a sectional view in the word
line direction (channel width direction). The nonvolatile
semiconductor memory device of means example 1 according to the
third embodiment will be described below.
[0070] As shown in FIG. 14, an element isolation trench 105 is
formed in a silicon substrate 101, and an element isolation
insulating film 106 is formed in the element isolation trench 105.
This separates adjacent floating gate electrodes FG1 and FG2.
[0071] Since the floating gate electrodes FG1 and FG2 project
upward from the element isolation insulating film 106, a trench 107
is formed on the element isolation insulating film 106 between the
adjacent floating gate electrodes FG1 and FG2.
[0072] A radical nitride film 120 is formed on the exposed surfaces
of the floating gate electrodes FG1 and FG2 and on the exposed
surfaces (the side surfaces and bottom surface of the trench 107)
of the element isolation insulating film 106. The radical nitride
film 120 is formed by performing radical nitriding on the exposed
surfaces of the floating gate electrodes FG and element isolation
insulating film 106. Therefore, in accordance with the material as
an undercoat, a silicon nitride film 120a is formed on the floating
gate electrodes FG made of, e.g., polysilicon layers 103, and a
silicon oxynitride film 120b is formed on the element isolation
insulating film 106 made of, e.g., a silicon oxide film. That is,
the radical nitride film 120 (120a and 120b) changes in accordance
with the materials of the floating gate electrodes FG and element
isolation insulating film 106 as undercoats.
[0073] An interelectrode insulating film 108 is formed on the
radical nitride film 120, and a radical nitride film 109 is formed
on the interelectrode insulating film 108. The radical nitride film
109 is formed by performing radical nitriding on the exposed
surface of the interelectrode insulating film 108. A control gate
electrode CG is formed on the radical nitride film 109.
[0074] The element isolation insulating film 106 is made up of a
first element isolation insulating film 106a and second element
isolation insulating film 106b. The first element isolation
insulating film 106a is formed along the side surfaces and bottom
surface of the element isolation trench 105 from the side-surface
portions in the lower portions of the floating gate electrodes FG.
The second element isolation insulating film 106b is formed on the
first element isolation insulating film 106a.
[0075] The upper surface of the second element isolation insulating
film 106b is positioned below the uppermost surface of the first
element isolation insulating film 106a, so the side surfaces in the
upper portion of the first element isolation insulating film 106a
are exposed. Accordingly, the first element isolation insulating
film 106a projects upward from the second element isolation
insulating film 106b. In the side-surface portions in the lower
portions of the floating gate electrodes FG1 and FG2, therefore,
portions of the first element isolation insulating film 106a exist
between the floating gate electrodes FG1 and FG2 and the radical
nitride film 120 (120b). Hence, in the portion X, i.e., in the
side-surface portion in the lower portion of the floating gate
electrodes FG1 and FG2 (on the side surface in the lower portion of
the trench 107), a stacked structure of the first element isolation
insulating film (oxide film) 106a/radical nitride film 120
(120a)/interelectrode insulating film (ONO film) 108/radical
nitride film 109, i.e., an ONONON structure, is formed.
[0076] The uppermost surface of the first element isolation
insulating film 106a is positioned below the upper surfaces of the
floating gate electrodes FG1 and FG2, so the side-surface portions
in the upper portions of the floating gate electrodes FG1 and FG2
are exposed. Therefore, in the side-surface portions in the upper
portions of the floating gate electrodes FG1 and FG2, the first
element isolation insulating film 106a does not exist between the
floating gate electrodes FG1 and FG2 and the radical nitride film
120 (120a). In other words, the radical nitride film 120 (120a) is
in contact with the side-surface portions in the upper portions of
the floating gate electrodes FG1 and FG2. Accordingly, in a portion
Y, i.e., in the side-surface portion in the upper portion of the
floating gate electrodes FG1 and FG2 (on the side surface in the
upper portion of the trench 107), a stacked structure of the
radical nitride film 120 (120a)/interelectrode insulating film (ONO
film) 108/radical nitride film 109, i.e., an NONON structure, is
formed.
[0077] As described above, on the side surface of the floating gate
electrodes FG1 and FG2 (on the side surface of the trench 107), the
ONONON structure is formed in the portion X, and the NONON
structure is formed in the portion Y, i.e., the portions X and Y
have different structures. This is so because it is desirable to
form the first element isolation insulating film 106a in the
portion X, and not to form the first element isolation insulating
film 106a in the portion Y. The reasons are as follows.
[0078] First, if the first element isolation insulating film 106a
is formed in the portion Y, the electric capacitance of the
interelectrode insulating film 108a decreases. To avoid this
problem, a height A of the floating gate electrodes FG1 and FG2
must be increased. This increases the difficulty of cell shape
processing. In addition, faulty operations occur owing to an
increase in parasitic capacitance between the floating gate
electrodes FG1 and FG2 adjacent to each other in the direction
perpendicular to the paper.
[0079] Second, if the first element isolation insulating film 106a
is formed in the portion Y, a distance W1 between the adjacent
floating gate electrodes FG1 and FG2 shortens. This causes faulty
operation by depletion of the control gate electrode CG.
[0080] The first element isolation insulating film 106a is
desirably an oxide film such as an HTO (High Temperature Oxide)
film or silicon oxide film. This is so because an oxide film has a
high potential barrier to electric charge, and this increases the
effect of suppressing the electric charge leak between the floating
gate electrode FG and control gate electrode CG.
[0081] Note that the first element isolation insulating film 106a
is not limited to an oxide film and may also be another insulating
film, e.g., a silicon oxynitride film, silicon nitride film, or
high-k insulating film, because even these insulating films have
the effect of increasing the distance between the floating gate
electrode FG and control gate electrode CG.
[0082] To increase the effect of suppressing the electric charge
leak between the floating gate electrode FG and control gate
electrode CG, however, the material of the first element isolation
insulating film 106a desirably has a high potential barrier to
electric charge. Also, to avoid faulty operations of the memory
caused by interference between adjacent cells when the electric
capacitance between the adjacent floating gate electrodes FG1 and
FG2 decreases, the material of the first element isolation
insulating film 106a desirably has a low dielectric constant. From
these viewpoints, the material of the first element isolation
insulating film 106a is most desirably a silicon oxide film.
[0083] The second element insulating film 106b desirably has an
etching rate higher than that of the first element isolation
insulating film 106a. For example, the second element isolation
insulating film 106a is made of a polysilazane film or HDP (High
Density Plasma) oxide film.
[0084] The radical nitride film 120 below the interelectrode
insulating film 108 has different film thicknesses in the portions
X and Y, because radical nitriding is used. Accordingly, the
thickness of the radical nitride film 120 (120b) in the ONONON
structure in the portion X is smaller than that of the radical
nitride film 120 (120a) in the NONON structure in the portion
Y.
[0085] Likewise, the radical nitride film 109 above the
interelectrode insulating film 108 also has different film
thicknesses in the portions X and Y, because radical nitriding is
used. Accordingly, the thickness of the radical nitride film 109 in
the ONONON structure in the portion X is smaller than that of the
radical nitride film 109 in the NONON structure in the portion
Y.
[0086] Referring to FIG. 14, it is desirable to optimize the height
A of the floating gate electrodes FG1 and FG2, a distance B from
the upper surfaces of the floating gate electrodes FG1 and FG2 to
the upper surface of the first element isolation insulating film
106a, a distance C from the bottom surfaces of the floating gate
electrodes FG1 and FG2 to the upper surface of the first element
isolation insulating film 106a, and a distance F from the upper
surface of the second element isolation insulating film 106b to the
upper surface of the first element isolation insulating film 106a,
on the basis of the tradeoff relationship between "the suppression
of the electric charge leak between the floating gate electrode FG
and control gate electrode CG" and "the avoidance of the decrease
in electric capacitance of the interelectrode insulating film
108a".
[0087] Referring to FIG. 14, it is important to ensure a distance E
between the silicon substrate 101 and control gate electrode CG.
This is so because high electric fields are applied to these two
components, so dielectric breakdown may occur. Accordingly, the
distance E between the silicon substrate 101 and control gate
electrode CG is desirably a distance at which no dielectric
breakdown occurs between the silicon substrate 101 and control gate
electrode CG.
[0088] FIGS. 15 to 17 are sectional views showing the manufacturing
steps of the nonvolatile semiconductor memory device of means
example 1 according to the third embodiment of the present
invention. Each figure is a sectional view in the word line
direction (channel width direction). A method of manufacturing the
nonvolatile semiconductor memory device of means example 1
according to the third embodiment will be explained below.
[0089] First, an element isolation trench 105 is formed in a
silicon substrate 101 in the same manner as in the first embodiment
(FIGS. 2A and 2B).
[0090] Then, as shown in FIG. 15, an element isolation insulating
film 106 is deposited on the exposed surfaces in the element
isolation trench 105 and on a mask material 104, and the element
isolation trench 105 is completely filled with the element
isolation insulating film 106. The element isolation insulating
film 106 is made up of a first element isolation insulating film
106a and second element isolation insulating film 106b. The first
element isolation insulating film 106a has a low etching rate, and
is made of an HTO film or the like. The second element isolation
insulating film 106b is formed on the first element isolation
insulating film 106a, has an etching rate higher than that of the
first element isolation insulating film 106a, and is made of a
polysilazane film, HDP oxide film, or the like. After that, the
upper surface of the element isolation insulating film 106 is
planarized by CMP until the mask material 104 is exposed.
[0091] As shown in FIG. 16, the mask material 104 is selectively
etched away by hot phosphoric acid. Then, the element isolation
insulating film 106 is partially etched by using a dilute
hydrofluoric acid solution. Since the first and second element
isolation insulating films 106a and 106b have an etching rate
difference, the upper surface of the first element isolation
insulating film 106a becomes lower than the upper surfaces of
polysilicon layers 103, so sidewall surfaces 107a of the
polysilicon layers 103 are exposed. In addition, the upper surface
of the second element isolation insulating film 106b becomes lower
than the upper surface of the first element isolation insulating
film 106a, so sidewall surfaces 107b of the first element isolation
insulating film 106a are exposed. Consequently, a trench 107 having
a step is formed between the polysilicon layers 103 (floating gate
electrodes FG) adjacent to each other.
[0092] As shown in FIG. 17, the silicon substrate 101 is placed in
a radical nitriding chamber (not shown) and heated to 400.degree.
C., the pressure is adjusted to 6 Pa while 40 sccm of nitrogen gas
and 1,000 sccm of argon gas are supplied into the chamber, and
microwaves having a power of 1 kW are supplied to generate radical
nitrogen, thereby performing radical nitriding on the surfaces of
the polysilicon layers 103 and element isolation insulating film
106 for 90 sec. Consequently, a silicon nitride film 120a is formed
on the exposed surfaces of the polysilicon layers 103, and a
silicon oxynitride film 120b is formed on the exposed surface of
the element isolation insulating film 106.
[0093] Then, an interelectrode insulating film 108 is formed on the
silicon nitride film 120a and silicon oxynitride film 120b. In this
embodiment, an ONO film having a thickness of, e.g., 15 nm is
formed as the interelectrode insulating film 108 by CVD. After
that, a radical nitride film 109 is formed by radical nitriding in
the same manner as in the first embodiment.
[0094] As shown in FIG. 14, a conductive layer 110 and mask
material 111 are sequentially deposited on the radical nitride film
109 and etched. As a consequence, floating gate electrodes FG and a
control gate electrode CG each having a desired shape are formed. A
gate sidewall oxide film 112 is then formed by thermal oxidation.
This thermal oxidation forms no bird's beak oxidation region below
and above the end portions of the interelectrode insulating film
108. In addition, source/drain diffusion layers 114 and an
interlayer dielectric film 115 are formed. After that, a
nonvolatile semiconductor memory device is completed by forming
interconnecting layers and the like by the well-known method.
[0095] In means example 1 of the third embodiment described above,
even when the trench 107 has a high aspect ratio and a narrow
opening, the first element isolation insulating film 106a exists in
the portion X in the lower portion of the floating gate electrode
FG1 (FG2). Since this increases the total film thickness of the
insulating films (the first element isolation insulating film 106a,
radical nitride film 120, interelectrode insulating film 108, and
radical nitride film 109) in the portion X, the distance between
the floating gate electrode FG and control gate electrode CG can be
increased. This makes it possible to suppress the electric charge
leak between the floating gate electrode FG and control gate
electrode CG, and ensure the charge retention characteristics.
[0096] Also, when the first element isolation insulating film 106a
is formed by an oxide film which is a material having a high
potential barrier to electric charge, it is possible to further
suppress the electric charge leak between the floating gate
electrode FG and control gate electrode CG, and ensure the charge
retention characteristics.
(2) MEANS EXAMPLE 2
[0097] FIG. 18 shows the film thinning ratio and radical nitriding
rate as functions of the radical nitriding pressure according to
means example 2 of the third embodiment of the present invention.
Letting Ttop be the film thickness of the radical nitride film 120
(120a) on the upper surfaces of the floating gate electrodes FG1
and FG2 and Tside be the film thickness of the radical nitride film
120 (120a) on the side surfaces of the floating gate electrodes FG1
and FG2, a film thinning ratio Tside/Ttop shown in FIG. 18
indicates the film thinning ratio of the radical nitride film 120
near the bottom of the trench 107. The film thickness Tside of the
radical nitride film 120 (120a) is a film thickness in a position
50 nm below the upper surfaces of the floating gate electrodes FG1
and FG2.
[0098] As shown in FIG. 18, the radical nitriding rate at high
pressures decreases as the electron temperature lowers. On the
other hand, the radical nitriding rate at low pressures increases
as the electron temperature rises. In a pressure region (a region
of less than 5 Pa) in which the radical density decreases, the mean
free path of radical nitrogen as a nitriding agent prolongs, and
the amount of radical nitrogen supplied to the side walls near the
bottom of the trench 107 having a high aspect ratio and a narrow
opening reduces. As a consequence, thinning of the radical nitride
film 120 occurs. This thinning can be avoided if the radical
nitride film 120 is formed at a pressure higher than the pressure
(5 Pa) at which the radical nitriding rate peaks.
[0099] From the foregoing, radical nitriding for forming the
radical nitride film 120 as the lowermost layer of the NONON
structure is desirably performed at a nitriding pressure of 5 Pa or
more (preferably, 10 Pa or more). Also, the upper limit of the
nitriding pressure is preferably 1 kPa or less (more preferably,
100 Pa or less), in order to ensure the film formation rate of
radical nitriding (avoid an increase in process time of the
nitriding step).
[0100] In radical nitriding for forming the radical nitride film
109 as the uppermost layer of the NONON film, the radical nitriding
pressure is preferably selected as follows in accordance with which
of "the prevention of bird's beak formation" or "the prevention of
depletion of the control gate electrode CG" is regarded as
important.
[0101] When the prevention of bird's beak formation is regarded as
important, radical nitriding for forming the radical nitride film
109 is preferably performed at a nitriding pressure of 5 Pa or more
(desirably, 10 Pa or more). In this case, it is possible to
suppress thinning of the radical nitride film 109 near the bottom
of the trench 107 on the side walls of the floating gate electrodes
FG1 and FG2. This makes it possible to prevent bird's beak
formation in the interface between the control gate electrode CG
and interelectrode insulating film 108 during gate sidewall
oxidation. As a consequence, a decrease in electric capacitance of
the interelectrode insulating film 108 can be prevented. Note that
in this case, the upper limit of the nitriding pressure is
preferably 1 kPa or less (more preferably, 100 Pa or less).
[0102] When the prevention of depletion of the control gate
electrode CG is regarded as important, radical nitriding for
forming the radical nitride film 109 is preferably performed at a
nitriding pressure of less than 5 Pa. Since a thin radical nitride
film 109 can be formed, it is possible to suppress a decrease in
distance W between the floating gate electrodes FG1 and FG2
adjacent to each other. This makes it possible to prevent depletion
of the control gate electrode CG.
[0103] Note that the above-mentioned critical pressure was 5 Pa
when radical nitriding was performed at Ar/N.sub.2 flow rates of
1,000 sccm/40 sccm and a microwave power of 1,000 W.
[0104] In means example 2 of the third embodiment described above,
the radical nitriding pressure is set at 5 Pa or more (desirably,
10 Pa or more) when the radical nitride film 120 is to be formed in
the trench 107 having a distance W of, e.g., 100 nm or less between
the floating gate electrodes FG1 and FG2 adjacent to each other. In
this manner, thinning of the radical nitride film 120 near the
bottom of the trench 107 can be avoided. Therefore, it is possible
to suppress the electric charge leak between the control gate
electrode CG and floating gate electrode FG, and ensure the charge
retention characteristics.
[0105] Note that this embodiment is explained by taking the NONON
structure as an example, but the present invention is not limited
to the NONON structure. That is, the present invention is
applicable to any structure in which the lowermost and uppermost
layers of the interelectrode insulating film 108 are formed by
radical nitriding.
[0106] It is, of course, also possible to combine means examples 1
and 2 of this embodiment.
[0107] Furthermore, the present invention is not limited to the
above embodiments, and, when practiced, can be variously modified
as follows without departing from the spirit and scope of the
invention.
[0108] (1) In the manufacturing method of each embodiment, the
element isolation insulating film 106 is formed after the tunnel
oxide film 102 is formed. However, it is also possible to form the
tunnel oxide film 102 after the element isolation insulating film
106 is formed.
[0109] (2) A radical nitride film may also be formed only between
the interelectrode insulating film 108 and floating gate electrode
FG, without forming any radical nitride film between the
interelectrode insulating film 108 and control gate electrode
CG.
[0110] (3) The material of the radical nitride film in each
embodiment is different from that of a nitride film formed by CVD
or thermal nitriding. For example, the radical nitride film
contains almost no impurity such as hydrogen or chlorine. This
characteristic of the radical nitride film can be analyzed by
various methods such as SIMS (Secondary Ion Mass Spectroscopy) or
XPS (X-ray Photoelectron Spectroscopy).
[0111] (4) In each embodiment, a nitrogen-containing film is formed
at least above or below the interelectrode insulating film 108.
This nitrogen-containing film can be either a nitrogen-containing
film or nitrogen-containing conductive film. Therefore, although
the radical nitride film as a nitrogen-containing insulating film
is taken as an example of the nitrogen-containing film in each
embodiment, a nitrogen-containing conductive film such as a
titanium nitride film may also be used. Even when this
nitrogen-containing conductive film is used, it is possible to
suppress the formation of bird's beaks to the interelectrode
insulating film 108, while an increase in physical film thickness
of the interelectrode insulating film 108 is prevented.
[0112] (5) A semiconductor substrate is used in each embodiment,
but the substrate is not limited to a semiconductor substrate. For
example, it is also possible to use an SOI (Silicon On Insulator)
substrate having a semiconductor substrate, a semiconductor layer,
and a buried insulating film formed between the semiconductor
substrate and semiconductor layer.
[0113] 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 may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
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