U.S. patent application number 10/463952 was filed with the patent office on 2003-11-20 for semiconductor device and method for manufacturing the same.
Invention is credited to Kimizuka, Naohiko, Yasuda, Yuri.
Application Number | 20030214001 10/463952 |
Document ID | / |
Family ID | 19017991 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030214001 |
Kind Code |
A1 |
Yasuda, Yuri ; et
al. |
November 20, 2003 |
Semiconductor device and method for manufacturing the same
Abstract
A silicon oxide film with a film thickness of 5 to 7 nm is
formed on a first region, a silicon oxynitride film with a film
thickness of 2 to 3 nm, and a nitrogen concentration of 1 to 3 atom
% is formed on a second region, and a silicon oxynitride film with
a film thickness of 1 to 2 nm, and a nitrogen concentration of 3 to
5 atom % is formed on a third region on a silicon substrate. Then,
radical nitriding is applied to the silicon oxide film, and the
silicon oxynitride films.
Inventors: |
Yasuda, Yuri; (Tokyo,
JP) ; Kimizuka, Naohiko; (Tokyo, JP) |
Correspondence
Address: |
KATTEN MUCHIN ZAVIS ROSENMAN
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Family ID: |
19017991 |
Appl. No.: |
10/463952 |
Filed: |
June 18, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10463952 |
Jun 18, 2003 |
|
|
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10166189 |
Jun 10, 2002 |
|
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Current U.S.
Class: |
257/392 ;
257/E21.268; 257/E21.269; 257/E21.625 |
Current CPC
Class: |
H01L 29/513 20130101;
H01L 21/823462 20130101; H01L 21/28185 20130101; H01L 21/28202
20130101; H01L 21/28176 20130101; H01L 21/3144 20130101; H01L
29/518 20130101; H01L 21/3145 20130101 |
Class at
Publication: |
257/392 |
International
Class: |
H01L 029/76 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2001 |
JP |
2001-177095 |
Claims
What is claimed is:
1. A semiconductor device comprising: a semiconductor substrate;
and a plurality of types of transistors provided with gate
dielectric films different in film thickness and nitrogen
concentration from one another, wherein said gate dielectric film
having a higher film thickness has a higher nitrogen
concentration.
2. The semiconductor device according to claim 1, wherein said gate
dielectric films are formed by forming a plurality of types of
dielectric films different in film thickness and nitrogen
concentration from one another, and applying radical nitriding to
the dielectric films.
3. The semiconductor device according to claim 1, wherein the types
of the gate dielectric films are three, a first gate dielectric
film has a film thickness of 5 to 7 nm, and a nitrogen
concentration of 10 to 15 atom %, a second gate dielectric film has
a film thickness of 2 to 3 nm, and a nitrogen concentration of 8 to
12 atom %, and a third gate dielectric film has a film thickness of
1 to 2 nm, and a nitrogen concentration of 6 to 10 atom %.
4. A semiconductor device comprising: a semiconductor substrate;
and a plurality of types of transistors provided with gate
dielectric films different in film thickness and nitrogen
concentration from one another, wherein said gate dielectric films
are formed by forming a plurality of types of dielectric films
different in film thickness and nitrogen concentration from one
another, and applying radical nitriding to the dielectric films
such that nitrogen does not reach an interface between the
dielectric films and said semiconductor substrate, and non-radical
nitrided layers are provided on a side in contact with said
semiconductor substrate.
5. The semiconductor device according to claim 1, wherein said gate
dielectric films consist of silicon oxynitride, hafnium nitride, or
alumina nitride.
6. A method for manufacturing a semiconductor device, comprising
the steps of: forming a plurality of types of dielectric films
different in film thickness and nitrogen concentration from one
another on a semiconductor substrate; and applying radical
nitriding to said dielectric films to form a plurality of types of
gate dielectric films.
7. The method for manufacturing a semiconductor device according to
claim 6, wherein said dielectric films are formed such that the
dielectric film with a lower film thickness has a higher nitrogen
concentration.
8. The method for manufacturing a semiconductor device according to
claim 7, wherein said gate dielectric films are formed such that
the gate dielectric film with a higher film thickness has a higher
nitrogen concentration.
9. The method for manufacturing a semiconductor device according to
claim 7, wherein said step of forming the dielectric films
comprises the steps of: dividing a surface of said semiconductor
substrate into a plurality of regions, and forming a first
dielectric film so as to cover the individual regions; and
selectively removing the first dielectric film formed on a second
region, and forming a second dielectric film with a lower film
thickness and a higher nitrogen concentration than said first
dielectric film on said second region.
10. The method for manufacturing a semiconductor device according
to claim 9, wherein the dielectric film formed on an nth ((n) is a
natural number equal to or more than 3) region is removed, and a
dielectric film with a lower film thickness and a higher nitrogen
concentration than a dielectric film ford on an (n-1)th region is
formed on the nth region after said second dielectric film is
formed.
11. The method for manufacturing a semiconductor device according
to claim 7, wherein said step for forming the dielectric films
comprises the steps of: dividing a surface of said semiconductor
substrate into a plurality of regions, and forming a first
dielectric film so as to cover the individual regions; and
selectively removing the first dielectric film formed on a second
region, and forming a second dielectric film with a higher film
thickness and a lower nitrogen concentration than said first
dielectric film on said second region.
12. The method for manufacturing a semiconductor device according
to claim 11, wherein the dielectric film formed on an nth ((n) is a
natural number equal to 3 or more) region is removed, and a
dielectric film with a higher film thickness and a lower nitrogen
concentration than a dielectric film formed on an (n-1)th region is
formed on the nth region after said second dielectric film is
formed.
13. The method for manufacturing a semiconductor device according
to claim 12, wherein a protection film is formed on the dielectric
films formed on the first to (n-1)th regions to prevent the
dielectric film formed on the nth region from being foamed on these
regions when the dielectric film is formed on the nth region.
14. The method for manufacturing a semiconductor device according
to claim 6, wherein said semiconductor substrate is a silicon
substrate, and a surface layer of said silicon substrate is
oxidized or oxynitrided to form a silicon oxide or oxynitride film
serving as said dielectric film.
15. The method for manufacturing a semiconductor device according
to claim 6, wherein said semiconductor substrate is a silicon
substrate, the types of said dielectric films are three, and
forming said three types of dielectric films comprises the steps
of: applying oxidizing or oxynitriding to a surface of said silicon
substrate to form a first silicon oxide or oxynitride film so as to
cover first to third regions on the surface of said silicon
substrate; forming a first resist covering said first and third
regions, and having an opening on said second region; etching said
first silicon oxide or oxynitride film using said first resist as a
mask to remove said first silicon oxide or oxynitride film formed
on said second region; removing said first resist; applying
oxynitriding to the surface of said silicon substrate to form a
second silicon oxynitride film with a lower film thickness and a
higher nitrogen concentration than said first silicon oxide or
oxynitride film on said second region; forming a second resist
covering said first and second regions, and having an opening on
said third region; etching said first silicon oxide or oxynitride
film using said second resist as a mask to remove said first
silicon oxide or oxynitride film formed on said third region;
removing said second resist; and applying oxynitriding to the
surface of said silicon substrate to form a third silicon
oxynitride film with a lower film thickness and a higher nitrogen
concentration than said second silicon oxynitride film on said
third region.
16. The method for manufacturing a semiconductor device according
to claim 15, wherein said first silicon oxide or oxynitride film is
a silicon oxide film having a film thickness of 5 to 7 nm, said
second silicon oxynitride film has a film thickness of 2 to 3 nm,
and a nitrogen concentration of 1 to 3 atom %, and said third
silicon oxynitride film has a film thickness of 1 to 2 nm, and a
nitrogen concentration of 3 to 5 atom %.
17. The method for manufacturing a semiconductor device according
to claim 6, wherein said semiconductor substrate is a silicon
substrate, the types of said dielectric films are three, and
forming said three types of dielectric films comprises the steps
of: applying oxynitriding to a surface of said silicon substrate to
form a first silicon oxynitride film so as to cover first to third
regions on the surface of said silicon substrate; forming a first
silicon nitride film on said first silicon oxynitride film; forming
a first resist covering said first and third regions, and having an
opening on said second region; etching said first silicon nitride
film and said first silicon oxynitride film using said first resist
as a mask to remove said first silicon nitride film and said first
silicon oxynitride film formed on said second region; removing said
first resist; applying oxynitriding to the surface of said silicon
substrate to form a second silicon oxynitride film with a higher
film thickness and a lower nitrogen concentration than said first
silicon oxynitride film on said second region; removing said first
silicon nitride film; forming a second silicon nitride film on said
first and second silicon oxynitride films; forming a second resist
covering said first and second regions, and having an opening on
said third region; etching said second silicon nitride film and
said first silicon oxynitride film using said second resist as a
mask to remove said second silicon nitride film and said first
silicon oxynitride film formed on said third region; removing said
second resist; applying oxidizing or oxynitriding to the surface of
said silicon substrate to form a third silicon oxide or oxynitride
film with a higher film thickness and a lower nitrogen
concentration than said second silicon oxynitride film on said
third region; and removing said second silicon nitride film.
18. The method for manufacturing a semiconductor device according
to claim 17, wherein said first silicon oxynitride film has a film
thickness of 1 to 2 nm, and a nitrogen concentration of 3 to 5 atom
%, said second silicon oxynitride film has a film thickness of 2 to
3 nm, and a nitrogen concentration of 1 to 3 atom %, and said third
silicon oxide or oxynitride film has a film thickness of 5 to 7
nm.
19. A method for manufacturing a semiconductor device comprising
the steps of: forming a plurality of types of dielectric films
different in film thickness from one another on a semiconductor
substrate; and applying radical nitriding to said dielectric films
such that nitrogen does not reach an interface between said
dielectric film with the lowest film thickness, and said
semiconductor substrate to form a plurality of types of gate
dielectric film.
20. The method for manufacturing a semiconductor device according
to claim 19, wherein said semiconductor substrate is a silicon
substrate, and a surface layer of said silicon substrate is
oxidized to form a silicon oxide film serving as said dielectric
film.
21. The method for manufacturing a semiconductor device according
to claim 20, wherein said semiconductor substrate is a silicon
substrate, the types of said dielectric films are three, and
forming said three types of dielectric films comprises the steps
of: applying oxidizing to a surface of said silicon substrate to
form a first silicon oxide film so as to cover first to third
regions on the surface of said silicon substrate; forming a first
resist covering said first and third regions, and having an opening
on said second region; etching said first silicon oxide film using
said first resist as a mask to remove said first silicon oxide film
formed on said second region; removing said first resist; applying
oxidizing to the surface of said silicon substrate to form a second
silicon oxide film with a lower film thickness than said first
silicon oxide film on said second region; forming a second resist
covering said first and second regions, and having an opening on
said third region; etching said first silicon oxide film using said
second resist as a mask to remove said first silicon oxide film
formed on said third region; removing said second resist; and
applying oxidizing to the surface of said silicon substrate to form
a third silicon oxide film with a lower film thickness than said
second silicon oxide film on said third region.
22. The method for manufacturing a semiconductor device according
to claim 21, wherein said first silicon oxide film has a film
thickness of 5 to 7 nm, said second silicon oxide film has a film
thickness of 2 to 3 nm, and said third silicon oxide film has a
film thickness of 1 to 2 nm.
23. The method for manufacturing a semiconductor device according
to claim 61 wherein said dielectric films consist of alumina or
hafnium.
24. The method for manufacturing a semiconductor device according
to claim 6, wherein said radical nitriding comprises the steps of:
forming nitrogen radical in a first chamber; introducing said
nitrogen radical into a second chamber connected with said first
chamber, and storing said semiconductor substrate; and bringing
said nitrogen radical in contact with said dielectric film formed
on said semiconductor substrate in said second chamber.
25. The method for manufacturing a semiconductor device according
to claim 24, wherein mixed gas of helium and nitrogen, or ammonia
is converted into plasma to form said nitrogen radical.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor device
including a plurality of types of transistors different in required
characteristics, and a method for manufacturing this semiconductor
device, and in particular, it relates to a semiconductor device
having a plurality of types of gate dielectric film different in
film thickness and nitrogen concentration, and a method for
efficiently manufacturing this semiconductor device.
[0003] 2. Description of the Related Art
[0004] A few types of transistors are selectively produced
according to required performance in a semiconductor device. When a
gate dielectric film of a transistor is made thin, an on-current of
the transistor increases, and high speed performance increases.
However, when the gate dielectric film in thin, a tunnel current
flows between a gate electrode and a substrate, a gate leak current
increases, and a power consumption of the transistor increases. To
the contrary, when the gate dielectric film is made thick, though
the gate leak current decreases, the on-current decreases, and the
high speed performance decreases. Thus, when the high speed
performance is required for a transistor, its gate dielectric film
is manufactured as thin. When it is necessary to reduce the power
consumption of a transistor by restraining the gate leak current,
its gate dielectric film is manufactured as thick. A silicon oxide
or oxynitride film is generally used as the gate dielectric
film.
[0005] A high performance transistor (an HP transistor) is used in
a core unit of a conventional semiconductor device. The core unit
is a part where a circuit for executing high speed arithmetic and
logic processing is provided. The film thickness and a threshold
voltage of the gate dielectric film of the HP transistor are set to
lower than those of transistors provided in other parts. The HP
transistor has a structure for giving priority to securing the
on-current which determines the high speed capability of the
transistor over restraining the gate leak current which increases
as the film thickness of the gate dielectric film becomes lower,
and restraining the off-current which increases as the threshold
voltage decreases. The off-current is also referred to as a
sub-threshold current in general, and is a leak current which flows
between the source and the drain when the gate electric potential
and the source electric potential is equal in a transistor, namely
when the transistor is tuned off.
[0006] A transistor (an I/O transistor), whose gate withstand
voltage is prioritized, is used in an I/O unit. The I/O unit is a
part where a circuit for providing data for and receiving data from
other semiconductor devices is provided. The film thickness of the
gate dielectric film of the I/O transistor is set to be higher than
that of the transistor in other parts, and its threshold voltage is
set to be higher than that of the transistors in the core.
[0007] A lower power transistor (an LP transistor) is used in a low
power unit. The low power unit is a part where a circuit whose leak
current is restrained as low as possible is provided to control
power consumption in a standby state. The film thickness of the
gate dielectric film of the LP transistor is set to a value between
the film thickness of the gate dielectric film in the core unit,
and the film thickness of the gate dielectric film in the I/O unit.
With this constitution, the gate leak current is restrained.
[0008] Further, a middle performance transistor (an MP transistor)
whose characteristics are between those of the HP transistor and
the LP transistor is ford on the same chip in acme cases.
Generally, the film thickness of the gate dielectric film of the MP
transistor is set to equal to the film thickness of the gate
dielectric film of the HP transistor. The off-current of the MP
transistor is set to be lower than the one of the HP transistor by
setting the threshold voltage thereof. The MP transistor is used in
a core unit of a conventional semiconductor device.
[0009] As described above, a common gate dielectric film is
generally used both for the HP transistor and the MP transistor.
And, the film thickness of the gate dielectric film of the LP
transistor is set to be higher than the film thickness of the gate
dielectric film of the HP (MP) transistor, the film thickness of
the gate dielectric film of the I/O transistor is set to be higher
than the film thickness of the gate dielectric film of the LP
transistor. Namely, three types of transistors, which are the core
transistor (the HP transistor and the MP transistor), the LP
transistor, and the I/O transistor, are used for a semiconductor
device. The off-current of the LP transistor is about 1 to 50
pA/.mu.m, and the LP transistor is used for a circuit for which low
power consumption is required. It is preferable to scale the gate
dielectric film of the LP transistor, and to make it common with
the gate dielectric film of the core transistor for simplifying a
manufacturing step of the semiconductor device. However, when the
gate dielectric films are made common, the gate leak current
exceeds the off-current in a circuit where a low power consumption
is prioritized, and the gate leak current determines the power
consumption of the transistor. Because of the foregoing, the film
thickness of the gate dielectric film of the LP transistor is not
scaled, and is set to a film thickness different from that of the
gate dielectric film of the core transistor (the HP transistor and
the MP transistor). In this way, the film thicknesses of the gate
dielectric films of the core transistor and the LP transistor are
reduced almost to their limits in terms of the gate leak
current.
[0010] A technique of introducing nitrogen (N) into the gate
dielectric film that consists silicon oxide, and increasing the
dielectric constant has been applied for simultaneously increasing
the high speed performance and restraining the gate leak current of
a transistor. Increasing the dielectric constant of the gate
dielectric film allows decreasing an electrical film thickness of
the gate dielectric film. As a result, the on-current of the
transistor increases, and the speed of the transistor increases.
Alternatively, the thickness of the gate dielectric film can be
increased by an amount corresponding to the increase of the
dielectric constant, and the gate leak current can be reduced.
[0011] As a method for introducing nitrogen into the gate
dielectric film, heat treatment is applied to a silicon substrate
in an NO atmosphere, for example. As another method, silane gas,
O.sub.2 gas, and N.sub.2 gas are simultaneously supplied when the
dielectric film is formed on the silicon substrate. Also, as
another method, a silicon oxide film is annealed in an ammonia
atmosphere. As yet another method, nitrogen is directly implanted
into the silicon oxide film. However, the amount of nitrogen
introduced into the silicon oxide film is about 2 to 3 atom %, and
there is such a problem as the dielectric constant is not
sufficiently increased in these methods.
[0012] Japanese Patent Publication Laid-Open No. Hei. 6-140392
discloses a method for radical-nitriding a silicon oxide film With
the method disclosed in Japanese Patent Publication Laid-Open No.
Hei. 6-140392, a wafer on which a silicon oxide film is formed is
loaded in a chamber, and is heated to 700 to 900.degree. C. Then,
NH.sub.3 gas is introduced into the chamber, VUV plasma light
emitting disc lamp is used to form Ar plasma, and nitrogen radical
is generated. The generated nitrogen radical is used to directly
nitride the silicon oxide film, and a silicon oxynitride film is
formed. As a result, the silicon oxynitride film with a nitrogen
concentration exceeding 10 atom % is formed. Nitrogen radical is
nitrogen having one unpaired electron, and has larger energy and
higher reactivity compared with non-radical nitrogen. Radical
nitriding is also called as remote plasma nitriding.
[0013] However, the prior art has the following problems. Namely,
when radical nitriding is simultaneously applied to a plurality of
types of silicon oxide films having a film thickness different from
one another, a larger amount of nitrogen is introduced into a
silicon oxide film with a lower film thickness compared with a
silicon oxide film with a higher film thickness.
[0014] Thus, in the thinnest silicon oxide film, the nitrogen
reaches an interface between this silicon oxide film and the
silicon substrate earliest When a large amount of nitrogen reaches
the interface between the silicon oxide film and the silicon
substrate, a silicon nitride film is formed at this interface, and
a physical film thickness of the dielectric film increases. When
the film thickness increases excessively, the increase of the
dielectric constant of the dielectric film does not compensate the
increase of the film thickness, and the electrical film thickness
increases as a result. Also, a large number of defects are
generated at the interface, and the mobility of carriers decrease.
As a result, the performance of the transistor decreases.
[0015] To increase the performance of the transistor, it is
preferable to introduce as much nitrogen as possible into a thicker
dielectric film, and to reduce an equivalent film thickness (the
electrical film thickness). However, when excessive radical
nitriding is applied in the conventional manufacturing method of a
semiconductor, a large amount of nitrogen reaches the interface
between the thinnest dielectric film and the semiconductor
substrate, and the performance of a transistor decreases. In this
way, when dielectric films with a different film thickness are
simultaneously radical-nitrided in the prior art, nitrogen
concentration introduced into the dielectric film with a lower film
thickness increases more, and the nitriding may reach as far as the
interface between the dielectric film and the semiconductor
substrate.
SUMMARY OF THE INVENTION
[0016] An object of the present invention is to provide a
semiconductor device which has a plurality of types of transistors
whose gate dielectric films have a film thickness different from
one another, and nitrogen concentration of these individual gate
dielectric film is optimized. Another object of the present
invention is to provide a method for manufacturing this
semiconductor device.
[0017] A semiconductor device according to the present invention
includes a semiconductor substrate, and a plurality of types of
transistors provided with gate dielectric films different in film
thickness and nitrogen concentration from one another. To form the
gate dielectric films, a plurality of types of dielectric films
different in film thickness and nitrogen concentration from one
another are formed, and radical nitriding is applied to these
dielectric films.
[0018] It is preferable that the plurality of types of gate
dielectric films be formed such that the gate dielectric film with
a higher film thickness has a higher nitrogen concentration. With
this constitution, in the gate dielectric film with a lower film
thickness, the nitrogen is prevented from reaching an interface
between the gate dielectric film and the semiconductor substrate.
Simultaneously, in the gate dielectric film with a higher film
thickness, the nitrogen concentration is increased, the dielectric
constant is increased, and the high speed performance is increased
in a transistor provided with this gate dielectric film.
[0019] An alternative semiconductor device according to the present
invention includes a semiconductor substrate, and a plurality of
types of transistors provided with gate dielectric films different
in film thickness and nitrogen concentration from one another. A
non-radical nitrided layer is provided on the side in contact with
the semiconductor substrate in the gate dielectric films. The
non-radical nitrided layer means an area that the radical nitrogen
does not reach in the radical nitriding.
[0020] In a method for manufacturing a semiconductor device
according to the present invention, a plurality of types of
dielectric films different in film thickness and nitrogen
concentration from one another are formed on a semiconductor
substrates. Then, radical nitriding is applied to these dielectric
films to form a plurality of types of gate dielectric films.
[0021] In the present invention, since the dielectric films
containing nitrogen are formed on the semiconductor substrate, the
contained nitrogen blocks nitrogen introduced in the radical
nitriding to arbitrarily control the amounts of the nitrogen
introduced into the gate dielectric films by the radical nitriding.
Therefore, by making the nitrogen concentrations in the plurality
of types of dielectric films different from one another, the
nitrogen concentrations in the plurality of types of gate
dielectric films are individually and optimally controlled.
[0022] The plurality of types of dielectric films can be formed
such that the dielectric film with a lower film thickness has a
higher nitrogen concentration. Consequently, the introduction of
the nitrogen is more effectively blocked in the dielectric film
with a lower film thickness in the radical nitriding. As a result,
in the dielectric film with a lower film thickness, the nitrogen is
prevented from reaching an interface between the dielectric film
and the semiconductor substrate. Simultaneously, in the dielectric
film with a higher film thickness, a larger amount of nitrogen is
introduced in the radical nitriding.
[0023] In the step of forming the plurality of types of dielectric
films, it is possible to form a second and the following dielectric
films as follows. That is, the surface of the semiconductor
substrate is divided into a plurality of regions, and a first
dielectric film is formed so as to cover the individual regions.
Then, the first dielectric film formed on a second region is
selectively removed, and a second dielectric film with a lower film
thickness and a higher nitrogen concentration than the first
dielectric film is formed on the second region After the second
dielectric film is formed, the dielectric film formed on an nth
((n) is a natural number equal to or more than 3) region is
removed, and a dielectric film with a lower film thickness and a
higher nitrogen concentration than a dielectric film formed on an
(n-1)th region is formed on the nth region. Consequently, since the
second dielectric film whose film thickness is lower is formed
after the formation of the first dielectric film whose film
thickness is higher, the second dielectric film is not damaged when
the first dielectric film is formed.
[0024] In the step of forming the plurality of types of dielectric
films, it is also possible to form a second and the following
dielectric films as follows. That is, the surface of the
semiconductor substrate is divided into a plurality of regions, and
a first dielectric film is formed so as to cover the individual
regions. Then, the first dielectric film formed on a second region
is selectively removed, and a second dielectric film with a higher
film thickness and a lower nitrogen concentration than the first
dielectric film is formed on the second region. After the second
dielectric film is formed, the dielectric film formed on an nth
((n) is a natural number equal to 3 or more) region is removed, and
a dielectric film with a higher film thickness and a lower nitrogen
concentration than a dielectric film formed on an (n-1)th region is
formed on the nth region. A protection film nay be formed on the
first to (n-1)th regions for preventing the dielectric film formed
on the nth region from being formed on these regions when the
dielectric film is formed on the nth region. With this
constitution, since the second dielectric film is formed while the
protection film is formed on the first dielectric film, it is
possible to prevent the step for forming the second dielectric film
from affecting the film thickness and the nitrogen concentration of
the first dielectric film. The protection film may be a silicon
nitride film, for example.
[0025] It is preferable that the semiconductor substrate be a
silicon substrate, and the step of forming the dielectric film be a
step of oxidizing or oxynitriding a surface layer of the silicon
substrate to form a silicon oxide or oxynitride film. Consequently,
since the surface layer of the silicon substrate is nitrided or
oxynitrided, the dielectric films are easily formed.
[0026] It is preferable that in the radical nitriding, nitrogen
radical be formed in a first chamber, the nitrogen radical be
introduced into a second chamber, which is connected with the first
chamber, and stores the semiconductor substrate, and the nitrogen
radical come in contact with the dielectric film formed on the
semiconductor substrate in the second chamber.
[0027] Consequently, plasma is formed to form the nitrogen radical
outside the second chamber where the semiconductor substrate is
placed. As a result, it is possible to prevent the plasma from
damaging the dielectric film on the semiconductor substrate.
[0028] In an alternative method of manufacturing a semiconductor
device according to the present invention, a plurality of types of
dielectric films different in film thickness from one another are
formed on a semiconductor substrate. Then, radical nitriding is
applied to the dielectric films such that nitrogen does not reach
an interface between the dielectric film with the lowest film
thickness, and the semiconductor substrate. As a result, a
plurality of types of gate dielectric films are formed.
[0029] In this way, with the present invention, when a
semiconductor device includes a plurality of types of transistors,
and the film thickness of the gate dielectric film of these
transistors is different from one another, the nitrogen
concentration is optimized in the individual gate dielectric films.
Consequently, the characteristics of the individual transistors,
namely the high speed performance and the gate leak current, are
optimized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIGS. 1A to 1D are sectional views showing a method for
manufacturing a semiconductor device using radical nitriding
according to a first embodiment of the present invention while
these sectional views are arranged in the sequence;
[0031] FIGS. 2A to 2C are sectional views showing the method for
manufacturing a semiconductor device using radical nitriding
according to the present embodiment while these sectional views are
arranged in the sequence, and these sectional views indicate steps
following those shown in FIG. 1A to FIG. 1D;
[0032] FIG. 3 is a graph showing a difference in radical nitriding
behavior between when an oxide film is used as a dielectric film
and when an oxynitride film is used as the dielectric film while a
radical nitriding time is assigned to the horizontal axis, and the
film thickness of an equivalent oxide film of a gate dielectric
film, and a gate leak current of this gate dielectric film are
assigned to the vertical axes;
[0033] FIG. 4 is a graph showing ranges of a permissible film
thickness of a gate dielectric film of an HP transistor and an LP
transistor while the film thickness of the gate dielectric film is
assigned to the horizontal axis, and the on-current and the gate
leak current of the transistors are assigned to the vertical
axes;
[0034] FIGS. 5A to 5D are sectional views showing a method for
manufacturing a semiconductor device using radical nitriding
according to a second embodiment of the present invention while
these sectional views are arranged in the sequence;
[0035] FIGS. 6A to 6D are sectional view showing the method for
manufacturing a semiconductor device using radical nitriding
according to the present embodiment while these sectional views are
arranged in the sequence, and these sectional views indicate steps
following those shown in FIG. 5A to FIG. 5D;
[0036] FIGS. 7A to 7D are sectional views showing a method for
manufacturing a semiconductor device using radical nitriding
according to a third embodiment of the present invention while
these sectional views are arranged in the sequence; and
[0037] FIGS. 8A to 8C are sectional views showing the method for
manufacturing a semiconductor device using radical nitriding
according to the present embodiment while these sectional views are
arranged in the sequence, and these sectional views indicate steps
following those shown in FIG. 7A to FIG. 7D.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] The following specifically describes embodiments of the
present invention while referring to the attached drawings. First,
a first embodiment of the present invention will be described.
FIGS. 1A to 1D and FIGS. 2A to 2C are sectional views showing a
method for manufacturing a semiconductor device using radical
nitriding according to the present embodiment while these sectional
views are arranged in the process sequence. First, a silicon
substrate 1 is placed in a chamber (not shown) as shown in FIG. 1A.
Device isolation trenches 2 are formed on the silicon substrate 1,
and divide the surface of the silicon substrate 1 into a region 3
to a region 5. Then, when heat treatment is applied to the silicon
substrate 1, the surface of the silicon substrate 1 is oxidized,
and a silicon-oxide film 6 with a film thickness of 5 to 7 nm is
formed on the regions 3 to 5 as shown in FIG. 1B. As the conditions
of the heat treatment, atmospheric gas is mixed gas with a pressure
of 1 atm produced by mixing H.sub.2 gas and O.sub.2 gas at a ratio
of 1:1, the temperature is about 750.degree. C., and the treatment
time is about 20 minutes.
[0039] Then, a resist 7 having an opening on the region 4 is formed
on the silicon oxide film 6 as shown in FIG. 1C. After this step,
the silicon oxide film 6 is etched with this resist 7 as a mask,
and the silicon oxide film 6 is removed on the region 4. Then, the
resist 7 is removed, and oxynitriding is applied as shown in FIG.
1D. This oxynitriding is applied in an NO atmosphere at a pressure
of 1.3 to 6.7 kPa and at a temperature of 500 to 700.degree. C. for
10 to 30 seconds. Then, the oxynitriding continues to be applied in
an O.sub.2 atmosphere at a pressure of 6.7 to 20 kPa and at a
temperature of 900 to 1050.degree. C. for 50 to 100 seconds.
Consequently, a silicon oxynitride film 8 with a film thickness of
2 to 3 nm, and a nitrogen concentration of 1 to 3 atom % is formed
on the region 4. The silicon oxide film 6 is more or lets
oxynitrided on the regions 3 and 5.
[0040] Then, a resist 9 having an opening on the region 5 is formed
as shown FIG. 2A. The silicon oxide film 6 (see FIG. 1D) on the
region 5 is etched with this resist 9 as a mask, and the silicon
oxide film 6 is removed. Then, the resist 9 is removed, and
oxynitriding is applied in an NO atmosphere at a pressure of 1.3 to
6.7 kPa and at a temperature of 500 to 700.degree. C. for 10 to 20
seconds as shown in FIG. 2B. Consequently, a silicon oxynitride
film 10 with a film thickness of 1 to 2 nm, and a nitrogen
concentration of 3 to 5 atom % is formed on the region 5. The
silicon oxide film 6 and the silicon oxynitride film 8 are more or
less oxynitrided in this step.
[0041] Then, radical nitriding is applied to the silicon oxide film
6, and the silicon oxynitride FIG. 8 and 10 as shown in FIG. 2C.
N.sup.- indicates nitrogen radical in FIG. 2C. The nitrogen radical
is formed in another chamber connected with the chamber where the
silicon substrate 1 is placed. As the conditions for forming the
nitrogen radical, mixed gas produced by mixing He and N.sub.2 at a
ratio of 1:1 is filled in the another chamber so as to present a
pressure of 370 to 400 Pa, and is heated to 500 to 600.degree. C.
Microwave with the output of 3000 W is impressed on this heated
mixed gas. Consequently, plasma is fouled to form the nitrogen
radical. Ammonia may be used as a source material for generating
the nitrogen radical in place of the mixed gas. The nitrogen
radical is introduced into the chamber where the silicon substrate
1 is placed, and come into contact with the silicon oxide film 6,
and the silicon oxynitride films 8 and 10 formed on the silicon
substrate 1. Consequently, the silicon oxide film 6 and the silicon
oxynitride films 8 and 10 are radical-nitrided. The time for the
radical nitriding is 80 to 150 seconds. The nitrogen radical (N*)
is mixture of N.sup.+ radical, N radical, and N.sub.2 radical in
the present embodiment.
[0042] In this step, the silicon oxide film 6 and the silicon
oxynitride films 8 and 10 are nitrided from the surface side.
Because the silicon oxide film 6 includes little nitrogen, the
nitrogen introduced in the radical nitriding is not blocked, and
the nitrogen concentration increases consequently. As a result, the
silicon oxide film 6 is radical-nitrided such that a silicon
oxynitride film 11 with a nitrogen concentration of 10 to 15 atom %
is formed.
[0043] Nitrogen of 2 to 3 atom % has been introduced into the
silicon oxynitride film 8. Thus, this nitrogen serves as a block,
and the amount of the nitrogen introduced into the silicon
oxynitride film 8 in the radical nitriding is smaller than the
amount of the nitrogen introduced into the silicon oxide film 6. As
a result, the silicon oxynitride film 8 is radical-nitrided such
that a silicon oxynitride film 12 with a nitrogen concentration of
8 to 12 atom % is formed.
[0044] Nitrogen of 3 to 5 atom % has been introduced into the
silicon oxynitride film 10. The nitrogen serves as a block, and the
amount of the nitrogen introduced into the silicon oxynitride film
10 in the radical nitriding is smaller than the amount of the
nitrogen introduced into the silicon oxynitride film 8. Thus the
silicon oxynitride film 10 is radical-nitrided, and a silicon
oxynitride film 13 with a nitrogen concentration of 6 to 10 atom %
is formed. The silicon oxynitride films 11 to 13 respectively
function as a gate dielectric film of a transistor. After this
step, gate poly is grown on the silicon oxynitride films 11 and 13
to form gate electrodes (not shown). Consequently, a semiconductor
device having a plurality of types of transistors is
manufactured.
[0045] As shown in FIG. 2C, in the semiconductor device of the
present embodiment, the silicon substrate 1 is provided. The device
isolation trenches 2 divide the surface of the silicon substrate 1
into the regions 3 to 5. The silicon oxynitride films 11 to 13
serving as a gate dielectric film of a transistor are respectively
formed on the regions 3 to 5. The silicon oxynitride film 11 has a
film thickness of 5 to 7 nm, and a nitrogen concentration of 10 to
15 atom %. The silicon oxynitride film 12 has a film thickness of 2
to 3 nm, and a nitrogen concentration of 8 to 12 atom %. The
silicon oxynitride film 13 has a film thickness of 1 to 2 nm, and a
nitrogen concentration of 6 to 10 atom %. The nitrogen
concentration is distributed such that the concentration is highest
at the surface, and monotonically and continuously decreases from
the surface to the interface to the silicon substrate 1 in the film
thickness direction in the silicon oxynitride films 11 to 13, and
little nitrogen is introduced at the interface to the silicon
substrate 1. Thus, the silicon oxynitride films 11 to 13
respectively comprise radical nitrided layers 11a to 13a that are
formed at an upper layer, and the radical nitrogen has reached in
the radical nitriding, and non-radical nitrided layers 11b to 13b
which are formed at a lower layer, and the radical nitrogen has not
reached in the radical nitriding.
[0046] Since the silicon oxynitride film is used as the gate
dielectric film in the present embodiment, the dielectric constant
can be increased compared with a case where a silicon oxide film is
used. Thus, since an effective film thickness of the gate
dielectric film can be scaled, and the physical film thickness
increases simultaneously, the gate leak current can be
decreased.
[0047] Since the radical nitriding is used to introduce nitrogen
into the silicon oxynitride film, the nitrogen can be introduced
from the surf ace of the silicon oxynitride film. Thus, the
non-radical nitrided layers 11b to 13b are formed near the
interface to the silicon substrate 1 in the silicon oxynitride
films 11 to 13. As a result, a formation of a nitride layer and a
generation of a defect are restrained at the interface between the
silicon substrate and the silicon oxynitride film. The present
embodiment is an example of forming the non-radical nitrided layers
11b to 13b respectively in the plurality of gate dielectric films
(the silicon oxynitride films 11 to 13) with the film thickness
different from one another. However, it is only necessary not to
introduce a large amount of nitrogen at the interface between the
gate dielectric films and the semiconductor substrate in any of the
gate dielectric films when the radical nitriding is applied
simultaneously to the plurality of gate dielectric films with the
film thickness different from one another in the present
invention.
[0048] The silicon oxide film 6 and the silicon oxynitride film 8
and 10 whose film thickness and nitrogen concentration are
different from one another are formed on the silicon substrate 1,
and then the radical nitriding is applied in the present
embodiment. As a result, the nitrogen in the silicon oxynitride
films 8 and 10 blocks the nitrogen introduced into the silicon
oxynitride films 8 and 10 during the radical nitriding in this
step, the radical nitriding is applied while the silicon oxynitride
film 10 with the lowest film thickness contains a large amount of
nitrogen, and the silicon oxide film 6 with the highest film
thickness does not contain nitrogen. Consequently, a sufficient
amount of nitrogen is introduced into the silicon oxide film 6 to
increase the dielectric constant, and to decrease the effective
film thickness while the nitrogen is restrained from reaching the
interface between the silicon oxynitride film 10 and the silicon
substrate 1. Thus, for example, when the silicon oxynitride film 11
is used as a gate dielectric film for an I/O transistor, the
silicon oxynitride film 12 is used as a gate dielectric film for an
LP transistor, and the silicon oxynitride film 13 is used as a gate
dielectric film for a core transistor, the performance of the
individual transistors is optimized. In this case, it is preferable
that the silicon oxynitride film 13 used as the gate dielectric
film for the core transistor have a film thickness of 1 nm or more
for restraining the gate leak current. Also, it is preferable that
the silicon oxynitride film 11 used as the gate dielectric film for
the I/O transistor has a film thickness of 5 nm or more for
securing voltage-withstanding capability.
[0049] Next, the following details a difference in radical
nitriding behavior between when an oxide film is used as a
dielectric film, and when an oxynitride film is used as the
dielectric film before radical nitriding in the step of applying
the radical nitriding to the dielectric films to form the gate
dielectric films. FIG. 3 is a graph showing a difference in radical
nitriding behavior between when an oxide film is used as the
dielectric film and when an oxynitride film is used as the
dielectric film while a radical nitriding time is assigned to the
horizontal axis, and the film thickness of the equivalent oxide
film of the gate dielectric film, and the gate leak current of this
gate dielectric film are assigned to the vertical axes. The film
thickness of the equivalent oxide film of the dielectric film is
obtained by converting the physical film thickness of the
dielectric film into the film thickness of the equivalent oxide
film while taking account of the change of the dielectric constant
as a result of the nitrogen introduction. The equivalent film
thickness is a so-called electric film thickness. A line 32
indicates the gate leak current when the dielectric film is an
oxynitride film, and a line 33 indicates the gate leak current when
the dielectric film is an oxide film. A line 34 indicates the film
thickness of an equivalent oxide film when the dielectric film is
an oxynitride film, and a line 35 indicates the film thickness of
an equivalent oxide film when the dielectric film is an oxide
film.
[0050] When the dielectric film is an oxide film as shown in FIG.
3, if the time for the radical nitriding is long, the nitrogen
reaches the interface between the oxide film and the substrate, and
the physical film thickness increases. Consequently, the gate leak
current decreases as the line 33 shows, simultaneously the film
thickness of the equivalent oxide film increases as the line 35
shows, and the high speed performance of the transistor decreases
accordingly. Thus, the time for the radical nitriding should not be
long, and should be set to short.
[0051] On the other hand, when the dielectric film is an oxynitride
film, there is such an effect as the nitrogen that has been
introduced into this oxynitride film blocks the nitrogen to be
introduced by the radical nitriding. Consequently, as the line 34
shows, the increase of both the electric film thickness and the
physical film thickness becomes more gradual than the line 35 with
respect to the radical nitriding time. Thus, the time for the
radical nitriding can be longer. Thus, when an oxynitride film is
formed as a dielectric film with a low film thickness, an oxide
film is formed as a dielectric film with a high film thickness, and
then the radical nitriding is applied, a sufficient amount of
nitrogen is introduced into the dielectric film with the high film
thickness. With this constitution, only with a single application
of the radical nitriding, optimal nitrogen concentration is
realized in the individual dielectric films.
[0052] Generally, a silicon oxynitride film is damaged in steps of
applying a resist on the silicon oxynitride film, and removing the
resist from the silicon oxynitride film. However, the thinnest
silicon oxynitride film 10 is formed last in the present
embodiment. Thus, a resist is not formed on the silicon oxynitride
film 10 in the manufacturing step of the semiconductor device, and
the silicon oxynitride film 10 is not damaged. Therefore, the
reliability of the semiconductor device increases.
[0053] When the gate dielectric film of the LP transistor is
scaled, the gate dielectric film of the LP transistor and the gate
dielectric film of the HP (MP) transistor (the core transistor) can
have the same film thickness in the present invention. FIG. 4 is a
graph showing ranges of a permissible film thickness of the gate
dielectric film for the HP transistor and the LP transistor. The
film thickness of the gate dielectric film is assigned to the
horizontal axis, and the on-current and the gate leak current of
the transistors are assigned to the vertical axes. In FIG. 4, a
broken line 14 shows a relationship between the film thickness of
the gate dielectric film and the on-current of a conventional
transistor whose gate dielectric film is a silicon oxide film. A
broken line 16 in the same drawing shows a relationship between the
film thickness of the gate dielectric film and the gate leak
current in this conventional transistor. A solid line 15 shows a
relationship between the film thickness of the gate dielectric film
and the on-current of a transistor whose gate dielectric film is a
silicon oxynitride film of the present invention. A solid line 17
shows a relationship between the film thickness of the gate
dielectric film and the gate leak current in this transistor.
[0054] Since the on-current (the broken line 14) of the HP (MP)
transistor should be I.sub.ON, MIN (HP) or more as shown in FIG. 4,
the film thickness of the gate dielectric film of the HP (MP)
transistor should be within a range 18 for the conventional
transistor structure. Since the gate leak current (the broken line
16) of the LP transistor should be I.sub.g, MAX (LP) or less, the
film thickness of the gate dielectric film of the LP transistor
should be within a range 19 for the conventional transistor
structure. Since the range 18 and the range 19 do not overlap each
other, it is necessary that the film thickness of the gate
dielectric film of the HP (HP) transistor, and the film thickness
the gate dielectric film of the LP transistor are set independently
to each other. Thus, the HP (MP) transistor and the LP transistor
do not have the common gate dielectric film.
[0055] On the other hand, since the on-current of the HP (MP)
transistor (the solid line 15) of the present invention is larger
than the on-current of the conventional HP (MP) transistor, it is
possible to set the film thickness of the gate dielectric film of
the HP (MP) transistor of the present invention within a range 20.
Also, since the gate leak current (the solid line 17) of the LP
transistor of the present invention is smaller than the gate leak
current of the conventional LP transistor, it is possible to set
the film thickness of the gate dielectric film of the LP transistor
of the present invention within a range 21. Since the range 20 and
the range 21 overlap each other as shown in FIG. 4, when a gate
dielectric film is produced so as to have its film thickness within
a rage where the range 20 and the range 21 overlap, this gate
dielectric film can be applied to both the HP (MP) transistor and
the LP transistor. Namely, a film thickness that simultaneously
satisfies securing the on-current, and reducing the gate leak
current exists in the present invention. Thus, the gate dielectric
film can be commonly used both for the up (MP) transistor and the
LP transistor.
[0056] The following describes a second embodiment of the present
invention. FIGS. 5A to 5D and FIGS. 6A to 6D are sectional views
showing a method for manufacturing a semiconductor device using
radical nitriding according to the present embodiment while these
sectional views are arranged in the process sequence. First, a
silicon substrate 1 is placed in a chamber (not shown) as shown in
FIG. 5A. Device isolation trenches 2 are formed on the silicon
substrate 1, and divide the surface of the silicon substrate 1 into
a region 3 to a region 5. Then, when heat treatment is applied to
the silicon substrate 1, the surface of the silicon substrate 1 is
oxidized, and a silicon oxynitride film 10 with a film thickness of
1 to 2 nm, and a nitrogen concentration of 3 to 5 atom % is formed
on the regions 3 to 5 as shown in FIG. 5B. The silicon oxynitride
film 10 is formed by applying the heat treatment for 10 to 20
seconds in an NO atmosphere which has a pressure of 1.3 to 6.7 kPa,
and a temperature of 500 to 700.degree. C.
[0057] Then, a silicon nitride film 36 is formed on the silicon
oxynitride film 10, and a resist 30 having an opening on the region
4 is formed on the silicon nitride film 36 as shown in FIG. 5C. The
silicon nitride film 36, and the silicon oxynitride film 10 are
etched with this resist 30 as a mask, and the silicon nitride film
36, and the silicon oxynitride film 10 are removed on the region 4.
Then, the resist 30 is removed, and oxynitriding is applied as
shown in FIG. 5D. This oxynitriding is applied in an NO atmosphere
at a pressure of 1.3 to 6.7 kPa and at a temperature of 500 to
700.degree. C. for 10 to 30 seconds, and then, the oxynitriding
continues to be applied in an O.sub.2 atmosphere at a pressure of
6.7 to 20 kPa and at a temperature of 900 to 1050.degree. C. for 50
to 100 seconds. Consequently, a silicon oxynitride film 8 with a
film thickness of 2 to 3 nm, and a nitrogen concentration of 1 to 3
atom % is formed on the region 4 while the film thickness and the
nitrogen concentration of the silicon oxynitride film 10 on the
regions 3 and 5 are maintained.
[0058] Then, the silicon nitride film 36 (see FIG. 5D) is removed,
a silicon nitride film 37 is newly formed, and a resist 31 having
an opening on the region 3 is formed as shown FIG. 6A. After this
step, the silicon nitride film 37 and the silicon oxynitride film
10 are etched with this resist 31 as a mask, and the silicon
nitride film 37 and the silicon oxynitride film 10 are removed on
the region 3. Then, the resist 31 is removed, and heat treatment is
applied as shown in FIG. 6B. As the conditions for this heat
treatment, atmospheric gas is mixed gas with a pressure of 1 atm,
the mixed gas is produced by mixing H.sub.2 gas and O.sub.2 gas at
a ratio of 1:1, the temperature is about 750.degree. C., and the
treatment is applied for about 20 minutes. After this treatment,
the silicon nitride film 37 is removed. Consequently, a silicon
oxide film 6 with a film thickness of 5 to 7 nm is formed on the
region 3 while the film thickness and the nitrogen concentration of
the silicon oxynitride films 8 and 10 are maintained.
[0059] Then, radical nitriding is applied to the silicon oxide film
6, and the silicon oxynitride films 8 and 10 as shown in FIG. 6D.
N* indicates nitrogen radical in FIG. 6D. The radical nitriding in
the present embodiment is the same as the radical nitriding shown
in the first embodiment described before.
[0060] As a result, as in the first embodiment, the silicon oxide
film 6 is radical-nitrided to form a silicon oxynitride film 11
with a nitrogen concentration of 10 to 15 atom %. The silicon
oxynitride film 8 is radical-nitrided to form a silicon oxynitride
film 12 with a nitrogen concentration of 8 to 12 atom %. The
silicon oxynitride film 10 is radical-nitrided to form a silicon
oxynitride film 13 with a nitrogen concentration of 6 to 10 atom %.
The silicon oxynitride films 11 to 13 respectively function as a
gate dielectric film of transistors. After this step, gate poly is
grown on the silicon oxynitride films 11 and 13 to form gate
electrodes (not shown). Consequently, a semiconductor device having
a plurality of types of transistors is manufactured. The
semiconductor device according to the present embodiment, which is
manufactured in this way, has the same constitution as the
semiconductor device of the first embodiment.
[0061] In the present embodiment, the resist 30 which has the
opening only on the region 4 is formed in the step shown in FIG.
5C, and the silicon oxynitride film 8 is ford only on the region 4
in the step shown in FIG. 5D. However, a resist which has openings
on the regions 3 and 4 may be formed in the step shown in FIG. 5C,
and the silicon oxynitride film 8 may be formed on the regions 3
and 4 in the step shown in FIG. 5D.
[0062] Since the silicon oxynitride film is used as the gate
dielectric film in the present embodiment as in the first
embodiment, the dielectric constant can be increased compared with
a case where a silicon oxide film is used. Thus, since the
effective film thickness of the gate dielectric film can be scaled,
and the physical film thickness increases simultaneously, the gate
leak current can be decreased.
[0063] The nitrogen in the silicon oxynitride films 8 and 10 blocks
the nitrogen introduced into the silicon oxynitride films 8 and 10
during the radical nitriding. Therefore, a sufficient amount of
nitrogen is introduced into the silicon oxide film 6 to increase
the dielectric constant, and to decrease the effective film
thickness while the nitrogen is restrained from reaching the
interface between the silicon oxynitride film 10 and the silicon
substrate 1. Consequently, the performances of the individual
transistors are optimized.
[0064] The manufacture of a semiconductor device according to the
present embodiment has the following effects compared with the
first embodiment. The silicon oxynitride film 10 formed on the
region 5 in the step shown in FIG. 5B is protected by the silicon
nitride film 36 in the oxynitriding in the step shown in FIG. 5D.
The silicon oxynitride film 10 is protected by the silicon nitride
film 37 in the oxynitriding in the step shown in FIG. 6B. Thus, the
silicon oxynitride film 10 is not nitrided in these oxynitriding
steps. In the same way, the silicon oxynitride film 8 formed on the
region 4 in the step shown in FIG. 5D is protected by the silicon
nitride film 37 and the resist 31 in the oxynitriding in the step
shown in FIG. 6B. Thus, the silicon oxynitride film 8 is not
nitrided in this oxynitriding. Consequently, the nitrogen
concentrations in the silicon oxynitride films 8 and 10 are
precisely controlled in the radical nitriding. As a result, the
nitrogen concentrations in the silicon oxynitride films 11 and 13
ford in the radical nitriding are precisely controlled.
[0065] However, since the silicon nitride films 36 and 37 are
formed on, and removed from the thin silicon oxynitride film 10,
the silicon oxynitride film 10 tends to be damaged in the method of
the present embodiment compared with the first embodiment. Thus,
the reliability of the semiconductor device decreases more or less.
Therefore, it is preferable to use the method of the first
embodiment for manufacturing semiconductor devices when the
reliability of the semiconductor devices is prioritized over the
precision of the nitrogen concentrations in the silicon oxynitride
films 11 to 13. On the other hand, it is preferable to use the
method of the second embodiment for manufacturing semiconductor
devices when the precision of the nitrogen concentrations in the
silicon oxynitride films 11 to 13 is prioritized over the
reliability of the semiconductor devices.
[0066] The following describes a third embodiment of the present
invention. FIGS. 7A to 7D and FIGS. 8A to 8C are sectional views
showing a method for manufacturing a semiconductor device using
radical nitriding according to the present embodiment while these
sectional views are arranged in the process sequence. First, a
silicon substrate 51 is provided as shown in FIG. 7A. Device
isolation trenches 52 divide the surface of the silicon substrate
51 into a region 53 to a region 55. Then, a silicon oxide film 56
with a film thickness of 5 to 7 nm is formed by thermal oxidation
on the regions 53 to 55 of the silicon substrate 51 as shown in
FIG. 7B. As the conditions of the thermal oxidation, atmospheric
gas is mixed gas with a pressure of 1 atm, the mixed gas is
produced by mixing H.sub.2 gas and O.sub.2 gas at a ratio of 1:1,
the temperature is about 750.degree. C., and the oxidation time is
about 20 minutes.
[0067] Then, a resist 57 having an opening on the region 54 is
formed on the silicon oxide film 56 as shown in FIG. 7C. The
silicon oxide film 56 is etched with this resist 57 as a mask, and
the silicon oxide film 56 is removed on the region 54. Then, the
resist 57 is removed, and a silicon oxide film is formed as shown
in FIG. 7D. This silicon oxide film is formed by heat treatment in
an O.sub.2 atmosphere at a pressure of 6.7 to 20 kPa and at a
temperature of 900 to 1050.degree. C. for 50 to 100 seconds, or by
processing in an atmosphere of mixed gas which is produced by
mixing H.sub.2 gas and O.sub.2 gas at a ratio of 1:1, and has a
pressure of 1 atm and at a temperature of about 750.degree. C. for
about ten minutes. Consequently, a silicon oxide film 58 with a
film thickness of 2 to 3 nm is formed on the region 54.
[0068] Then, a resist 59 having an opening on the region 55 is ford
as shown in FIG. 8A, the silicon oxide film 56 is etched with this
resist 59 as a mask, and the silicon oxide film 56 (see FIG. 7D) is
removed on the region 55. Then, the resist 59 is removed as shown
in FIG. 8B, and heat treatment is applied in an O.sub.2 atmosphere
at a pressure of 1.3 to 6.7 kPa and at a temperature of 500 to
700.degree. C. for 10 to 20 seconds. Consequently, a silicon oxide
film 60 with a film thickness of 1 to 2 nm is formed.
[0069] Then, radical nitriding is applied to the silicon oxide
films 56, 58, and 60 (see FIG. 8B) as shown in FIG. 8C. N*
indicates nitrogen radical in FIG. 8C. As the conditions for
forming the nitrogen radical, atmospheric gas is mimed gas produced
by mixing He and N.sub.2 at a ratio of 1:1, the temperature is 500
to 600.degree. C., and the pressure is 370 to 400 Pa. Microwave
with the output of 3000 W is impressed on this mixed gas.
Consequently, the nitrogen radical is formed, and the silicon oxide
films 56, 51, and 60 are radical-nitrided. The time for the radical
nitriding is 80 to 150 seconds. Consequently, the silicon oxide
films 56, 58, and 60 are nitrided from the surface side, and are
respectively changed into silicon oxynitride films 61, 62, and 63.
Thus, the silicon oxynitride films 61 to 63 respectively comprise
radical nitrided layers 61a to 63a that are formed at an upper
layer, and non-radical nitrided layers 61b to 63b that are formed
at a lower layer.
[0070] As a result, the silicon oxynitride film 61 with a film
thickness of 5 to 7 nm, and a nitrogen concentration of 4 to 8 atom
% is formed on the region 53 of the silicon substrate 51. The
silicon oxynitride film 62 with a film thickness of 2 to 3 nm, and
a nitrogen concentration of 6 to 10 atom % is formed on the region
54 of the silicon substrate 51. The silicon oxynitride film 63 with
a film thickness of 1 to 2 nm, and a nitrogen concentration of 8 to
12 atom % is formed on the region 55 of the silicon substrate 51.
The silicon oxynitride films 61 to 63 function as a gate dielectric
film of a transistor. After this step, gate poly is grown on the
silicon oxynitride films 61 and 63, and gate electrodes are
formed.
[0071] When the radical nitriding is applied to the silicon oxide
films 56, 58, and 60 (see FIG. 8B), the nitriding is conducted such
that the non-radical nitrided layer 63b remains in the thinnest
silicon oxynitride film 63 in the present embodiment. The
processing time until the nitrogen reaches the interface between
the silicon oxide film and the silicon substrate gets shorter as
the film thickness of the silicon oxide film gets thinner.
Therefore, when the non-radical nitrided layer 63b remains in the
thinnest silicon oxynitride film 63, the non-radical nitrided
layers also remain in the silicon oxynitride films 61 and 62 which
have the thicker film thickness than the silicon oxynitride film
63. As a result, such a problem as a nitride film is formed at the
interface between the silicon oxide film and the silicon substrate
does not occur. On the other hand, since the radical nitriding
introduces nitrogen into the radical nitrided layers 61a and 62a in
the silicon oxynitride films 61 and 62, the dielectric constant
increases, and the speed of the transistors can be increased.
[0072] Since the nitrogen is sufficiently introduced into the
thinnest dielectric film (the silicon oxynitride film 63) in the
present embodiment, it is possible to solve such a problem as
impurity elements in the gate electrode diffuse in a channel region
of a transistor as a result of annealing after the source and the
drain are formed. The following details this effect. A MOS
transistor is generally formed as described below. A gate electrode
having a predetermined shape is formed using polycrystalline
silicon or the like after a gate dielectric film of the transistor
is formed. After this step, boron (B) or arsenic (As) as impurity
for forming source and drain regions is ion-implanted into active
regions of a semiconductor substrate with this gate electrode as a
mask. In this step, the impurity is also implanted into the gate
electrode. After this step, heat treatment (annealing) is applied
to activate the impurity ion-implanted in the source and drain
regions. The impurity, which is ion-implanted into the gate
electrode, diffuses through the gate dielectric film, and reaches
the channel region of the transistor in the semiconductor substrate
in this annealing. This diffusion of the impurity becomes more
remarkable as the gate dielectric film becomes thinner. If the
impurity in the gate electrode reaches the channel region, the
impurity causes a leak current and a fluctuation of the threshold
voltage of the transistor, and characteristics of the transistor
fluctuate. However, since the gate dielectric film contains
nitrogen in the present embodiment, the diffusion of the impurity
is restrained during the annealing, and the fluctuation of the
characteristics of the transistor is restrained. As the film
thickness of the gate dielectric film becomes higher, the nitrogen
concentration decreases in the present embodiment. However, the
decrease of the nitrogen concentration does not cause a problem,
because the diffusion of the impurity is restrained more as the
film thickness of the gate dielectric film increases.
[0073] While the first to third embodiments are examples of using a
silicon oxynitride film as the gate dielectric film, the gate
dielectric film is not limited to the silicon oxynitride film in
the present invention, and may be formed using different dielectric
materials such as hafnium and alumina. The phenomenon that applying
radical nitriding to a dielectric film increases the dielectric
constant of this dielectric film is not limited to the silicon
oxide film and the silicon oxynitride film. This phenomenon is also
observed in several so-called high dielectric constant materials.
Especially hafnium presents a higher dielectric constant when it is
nitrided.
* * * * *