U.S. patent application number 09/852658 was filed with the patent office on 2002-07-25 for semiconductor device including a mis transistor.
This patent application is currently assigned to NEC CORPORATION. Invention is credited to Ikarashi, Nobuyuki, Mogami, Tohru, Ono, Haruhiko, Shiba, Kazutoshi, Tatsumi, Toru, Togo, Mitsuhiro, Watanabe, Koji, Yamamoto, Toyoji.
Application Number | 20020096721 09/852658 |
Document ID | / |
Family ID | 18836003 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020096721 |
Kind Code |
A1 |
Mogami, Tohru ; et
al. |
July 25, 2002 |
Semiconductor device including a MIS transistor
Abstract
A MIS transistor has a gate insulating film made of silicon
oxynitride and having a specific dielectric constant which is
larger than the expected specific dielectric constant calculated
based on a weighted average of the specific dielectric constants
based on the weight ratio of the silicon oxide and the silicon
nitride contained in the silicon oxynitride film. The gate
insulating film having a smaller thickness prevents impurities in
the overlying gate electrode from penetrating through the gate
insulating film to degrade the silicon substrate.
Inventors: |
Mogami, Tohru; (Tokyo,
JP) ; Togo, Mitsuhiro; (Tokyo, JP) ; Watanabe,
Koji; (Tokyo, JP) ; Yamamoto, Toyoji; (Tokyo,
JP) ; Ikarashi, Nobuyuki; (Tokyo, JP) ; Shiba,
Kazutoshi; (Tokyo, JP) ; Tatsumi, Toru;
(Tokyo, JP) ; Ono, Haruhiko; (Tokyo, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN
MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, NW
Washington
DC
20037-3213
US
|
Assignee: |
NEC CORPORATION
|
Family ID: |
18836003 |
Appl. No.: |
09/852658 |
Filed: |
May 11, 2001 |
Current U.S.
Class: |
257/350 ;
257/E21.625 |
Current CPC
Class: |
H01L 29/518 20130101;
H01L 29/513 20130101; H01L 21/823462 20130101; H01L 21/28185
20130101; H01L 2924/0002 20130101; H01L 2924/0002 20130101; H01L
21/28202 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
257/350 |
International
Class: |
H01L 027/01; H01L
027/12; H01L 031/0392 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2000 |
JP |
2000-365180 |
Claims
What is claimed is:
1. A semiconductor device comprising an active element having a
silicon oxynitride film including silicon nitride and silicon oxide
as main components thereof, said silicon oxynitride film having a
first specific dielectric constant which is larger than a second
specific dielectric constant theoretically calculated from a
weighted average of a specific dielectric constant of silicon oxide
and a specific dielectric constant of silicon oxide, the weighted
average being based on a weight ratio between the silicon nitride
and the silicon oxide in said silicon oxynitride film.
2. The semiconductor device as defined in claim 1, wherein said
first specific dielectric constant (.epsilon.) satisfies the
following relationship: .epsilon.>(1-x).times.3.9+x.times.7.5
where the specific dielectric constant of silicon oxide is assumed
at 3.9, the specific dielectric constant of silicon nitride is
assumed at 7.5, and x is the weight ratio of the silicon nitride to
said silicon oxynitride film.
3. The semiconductor device as defined in claim 1, wherein said
first specific dielectric constant is equal to or more than 4.5 and
below 6.0, and said the weight ratio of the silicon nitride to said
silicon oxynitride film is between 0.05 and 0.5.
4. The semiconductor device as defined in claim 1, wherein said
first specific dielectric constant is equal to or more than 6.0 and
below 6.5, and the weight ratio of the silicon nitride to said
silicon oxynitride film is between 0.05 and 0.45.
5. The semiconductor device as defined in claim 1, wherein said
first specific dielectric constant is equal to or more than 6.5 and
below 7.5, and the weight ratio of the silicon nitride to said
silicon oxynitride film is between 0.05 and 0.4.
6. The semiconductor device as defined in claim 1, wherein said
silicon oxynitride film has a physical thickness of 1.5 to 4.5 nm
corresponding to an equivalent oxide-film thickness of 1 to 3
nm.
7. The semiconductor device as defined in claim 1, wherein a
nitrogen distribution in said silicon oxynitride film resides
within 1 nm from a top surface thereof, and a nitrogen peak
position resides on the top surface side as viewed from a center of
said silicon oxynitride film.
8. The semiconductor device as defined in claim 1, wherein said
active element is a MIS transistor having a gate insulating film
including said silicon oxynitride film.
9. The semiconductor device as defined in claim 8, wherein said
gate insulating film further includes another insulating film
between said silicon oxynitride film and a gate electrode of said
MIS transistor.
10. A method for forming a semiconductor device including a MIS
transistor comprising the steps of: forming source/drain/channel
regions of the MIS transistor on a silicon substrate; forming a
silicon oxide film on said silicon substrate in association with
said source/drain/channel regions by using active oxygen; and
nitriding said silicon oxide film by using active nitrogen to form
a silicon oxynitride film as a gate insulating film of the MIS
transistor, said silicon oxide film forming step and said nitriding
step being conducted continuously in a single chamber while
controlling a pressure inside said single chamber and electric
power so that said silicon oxynitride film has a specific
dielectric constant which is larger than an expected specific
dielectric constant theoretically calculated from an amount of said
active nitrogen used.
11. The method as defined in claim 10, wherein said pressure is
between 0.1.times.10.sup.-2 Pa and 10.times.10.sup.-2 Pa, and said
electric power is between 100 watts and 300 watts.
12. A method for forming a semiconductor device including a MIS
transistor comprising the steps of: forming source/drain/channel
regions of the MIS transistor on a silicon substrate; and forming a
silicon oxynitride film as a gate insulating film of the MIS
transistor on said silicon substrate in association with said
source/drain/channel regions by using active oxygen and active
nitrogen, said silicon oxynitride film forming step being conducted
while controlling a pressure inside a chamber and electric power so
that said silicon oxynitride film has a specific dielectric
constant which is larger than an expected specific dielectric
constant theoretically calculated from an amount of said active
nitrogen used.
13. The method as defined in claim 12, wherein said pressure is
between 0.1.times.10.sup.-2 Pa and 10.times.10.sup.-2 Pa, and said
electric power is between 100 watts and 300 watts.
14. A method for forming a semiconductor device including a MIS
transistor comprising the steps of: forming source/drain/channel
regions of the MIS transistor on a silicon substrate; forming a
silicon nitride film on said silicon substrate in association with
said source/drain/channel regions by using active nitrogen; and
oxidizing said silicon nitride film by using active oxygen to form
a silicon oxynitride film as a gate insulating film of the MIS
transistor, said silicon nitride film forming step and said
oxidizing step being conducted continuously in a single chamber
while controlling a pressure inside said single chamber and
electric power so that said silicon oxynitride film has a specific
dielectric constant which is larger than an expected specific
dielectric constant theoretically calculated from an amount of said
active nitrogen used.
15. The method as defined in claim 14, wherein said pressure is
between 0.1.times.10.sup.-2 Pa and 10.times.10.sup.-2 Pa, and said
electric power is between 100 watts and 300 watts.
Description
BACKGROUND OF THE INVENTION
[0001] (a) Field of the Invention
[0002] The present invention relates to a semiconductor device
including a MIS (Metal-Insulator-Semiconductor) transistor, and
more particularly, to the structures of a semiconductor device
including a MIS transistor having a gate insulating film, which is
capable of preventing impurities in an electrode from penetrating
therethrough to degrade the transistor characteristics.
[0003] The present invention also relates to a method for
manufacturing such a semiconductor device.
[0004] (b) Description of the Related Art
[0005] Along with reduction of the dimensions of semiconductor
devices, gate insulating films have been rapidly reduced in
thickness, and there is a need for an ultra thin gate insulating
film having a thickness of 3 nm or less. However, a conventional
silicon oxide film used for a gate insulating film and having such
a small thickness suffers from the following phenomena or problems.
Specifically, impurity (boron) ions in a gate electrode are
thermally diffused and penetrate through the silicon oxide film to
a silicon substrate. Meanwhile, a so-called direct tunneling effect
is also caused by which electrons penetrate through the insulating
film based on the quantum mechanical theory. These phenomena
increase gate leakage current.
[0006] As a method of solving the problem of penetration through
the silicon oxide film by the boron from the gate electrode, the
use of a silicon oxynitride film produced by introducing nitrogen
into a silicon oxide film has been proposed. As a conventional
method of forming a silicon oxynitride film, formation of a gate
insulating film by directly thermally oxynitriding a surface of a
semiconductor substrate is known.
[0007] Many proposed methods use a rapid thermal treatment system
to form an oxynitride film for a MOS (Metal Oxide Semiconductor)
transistor. For example, an oxide film produced on a silicon
substrate is thermally nitrided to form a silicon oxynitride film.
Alternatively, a nitride film produced on a silicon substrate is
thermally oxidized to form a silicon oxynitride film.
[0008] In the above processes, when oxidation is conducted to a
nitride film directly formed on a silicon substrate, the nitrogen
peak is generally positioned at the interface between the
oxynitride film and the silicon substrate, and the interface state
increases as a result. Therefore, normally, an oxide film is first
formed and the film is then nitrided. For example, according to a
method described by Japanese Patent Laid-Open Publication No. Hei.
2-256274, a thermally oxidized film is formed on a surface of a
silicon substrate by pyrogenic oxidation, or dry oxygen oxidation,
and then the film is allowed to thermally react with nitrogen in a
nitriding gas ambient. Thus, nitrogen is introduced into the
silicon oxide film. Herein, the nitriding gas may be a nitrogen
gas, an ammonia gas, a nitrous oxide gas or a nitrogen monoxide
gas.
[0009] For example, Japanese Patent Laid-Open Publication No. Hei.
6-140392 describes nitriding using plasma in order to solve the
disadvantage associated with the above described thermal nitriding.
In the plasma treatment, a semiconductor wafer having a silicon
oxide film as thick as 4.0 nm formed on a silicon substrate is
transported into a vacuum chamber, and heated up to temperatures in
the range from 700.degree. C. to 900.degree. C. using a rapid
thermal treatment system. Then, an ammonia gas is introduced as a
nitriding gas, and a vacuum ultraviolet beam by Ar plasma generated
at a vacuum ultraviolet plasma light emitting disc lamp is
irradiated upon the wafer surface.
[0010] In the above plasma treatment, the ammonia gas is
photodisintegrated by photo excitation, and the silicon oxide film
is directly nitrided using resulting much reactive, high energy
active nitrogen, so that a silicon oxynitride film is formed. In
recent years, control of the nitrogen profile in a silicon
oxynitride film has been recognized as an indispensable technique
in forming a gate oxide film with superior electric
characteristics.
[0011] Journal of Applied Physics, Vol. 84, page 2980 (J. Appl.
Phys. 84 (1998) 2980) describes a method of controlling the
nitrogen position in such a gate oxynitride film by using a rapid
thermal treatment system. More specifically, treatments with
nitrogen monoxide, oxygen, and nitrogen monoxide are sequentially
performed in this order, resulting in that the nitrogen in the gate
oxynitride film is localized at both the interface and the surface
of the gate oxynitride film having a thickness of 4.0 nm. In this
structure, hot carrier resistance in the MOS transistor is improved
by the nitrogen positioned at the interface. In addition, boron
ions in the gate electrode can be prevented from penetrating to the
silicon substrate by the nitrogen positioned at the surface.
[0012] In Material Research Society 1999, page 84 (MRS 1999 Spring
Meeting Abstract 84), for improvement of the charge mobility in the
channel region of a MOS transistor, the nitrogen position in the
gate insulating film is allowed to reside at the center of the gate
insulating film.
[0013] Meanwhile, it is reported that, in view of the function of
controlling the gate leakage phenomenon, a silicon oxynitride film
is equivalent to a silicon oxide film. In contrast, it has been
reported that the use of high dielectric constant film other than a
silicon oxide film, such as a high dielectric constant metal oxide
film including an aluminum oxide film having a dielectric constant
of 7 to 9 and a zirconium oxide film having a dielectric constant
of 10, can reduce the gate leakage current more than the use of a
silicon oxide film.
[0014] However, a high dielectric constant film other than the
silicon oxide film suffers from significant disadvantages such as
mismatching between a gate electrode and a polysilicon material,
and degradation in thermal stability and heat resistance, which
lead to degradation in the transistor characteristics. These are
significant disadvantages in using the film in practice. Therefore,
it is extremely important in next-generation fine transistors to
reduce gate leakage current in a silicon-oxide-based insulating
film having a thickness of 3.0 nm or less.
[0015] The silicon oxynitride film as described above is
encountered with the following problems. Firstly, the dielectric
constant of an ultra thin, silicon oxynitride film as thin as 3.0
nm or less cannot be higher than that of a silicon oxide film. A
silicon oxynitride film, if it is possible for the silicon
oxynitride film to have a higher dielectric constant, may have a
physical film thickness as large as possible for a fixed, smaller
electrical film thickness similarly to other high dielectric
constant films. In this case, gate leakage current can be reduced
without degrading the transistor characteristics in such an 1.5
ultra thin film having a thickness of 3.0 nm or less.
[0016] Secondly, the nitrogen profile in the silicon oxynitride
film can hardly be controlled. For example, when the control method
described in Journal of Applied Physics, vol. 84, page 2980 is
employed, the use of a nitrogen monoxide gas prevents the nitrogen
amount from being reduced at the silicon substrate interface.
According to the nitriding method described in Material Research
Society 1999, Spring Meeting, Abstract, page 84, a high pressure
gas ambient at 25 atm., for example, is necessary to set the
nitrogen position in the center of the film, which is not suitable
for mass production type devices however. A high temperature,
thermal nitriding reaction using a nitrogen monoxide gas and an
oxygen gas is encountered, and therefore the control of the
nitrogen position should be even harder.
[0017] Thirdly, the nitriding reaction is caused in the interface,
which increases the roughness of the interface as the nitrogen
amount increases. According to the method described in Japanese
Patent Laid-Open Publication No. Hei. 6-140392 in particular, the
roughness of the interface increases in the process of nitriding
the oxide film.
SUMMARY OF THE INVENTION
[0018] The present invention is directed to a solution to the above
described disadvantages. More specifically, it is an object of the
present invention to provide a semiconductor device and a MIS
device, which is capable of preventing impurity ions in an
electrode adjacent to the insulating film from penetrating through
the insulating film to degrade the opposite electrode or substrate,
and thereby providing a semiconductor device or a MIS device having
excellent transistor characteristics.
[0019] It is another object of the present invention to provide a
method of manufacturing such a semiconductor device of a MIS
device.
[0020] The present invention provides, in a first aspect thereof, a
semiconductor device including an active element having a silicon
oxynitride film including silicon nitride and silicon oxide as main
components thereof, the silicon oxynitride film having a first
specific dielectric constant which is larger than a second specific
dielectric constant theoretically calculated from a weighted
average of a specific dielectric constant of silicon oxide and a
specific dielectric constant of silicon oxide, the weighted average
being based on a weight ratio between the silicon nitride and the
silicon oxide in the silicon oxynitride film.
[0021] The present invention provides, in a second aspect thereof,
a method for forming a semiconductor device having a MIS transistor
including the steps of:
[0022] forming source/drain/channel regions of the MIS transistor
on a silicon substrate;
[0023] forming a silicon oxide film on the silicon substrate in
association with the source/drain/channel regions by using active
oxygen; and
[0024] nitriding the silicon oxide film by using active nitrogen to
form a silicon oxynitride film as a gate insulating film of the MIS
transistor,
[0025] the silicon oxide film forming step and the nitriding step
being conducted continuously in a single chamber while controlling
a pressure inside the single chamber and electric power so that the
silicon oxynitride film has a specific dielectric constant which is
larger than an expected specific dielectric constant theoretically
calculated from an amount of the active nitrogen used.
[0026] The present invention provides, in a third aspect thereof, a
method for forming a semiconductor device having a MIS transistor
including the steps of:
[0027] forming source/drain/channel regions of the MIS transistor
on a silicon substrate; and
[0028] forming a silicon oxynitride film on the silicon substrate
in association with the source/drain/channel regions by using
active oxygen and active nitrogen;
[0029] the silicon oxynitride film forming step being conducted
while controlling a pressure inside a chamber and electric power so
that the silicon oxynitride film has a specific dielectric constant
which is larger than an expected specific dielectric constant
theoretically calculated from an amount of the active nitrogen
used.
[0030] The present invention provides, in a fourth aspect thereof,
a method for forming a semiconductor device having a MIS transistor
including the steps of:
[0031] forming source/drain/channel regions of the MIS transistor
on a silicon substrate;
[0032] forming a silicon nitride film on the silicon substrate in
association with the source/drain/channel regions by using active
nitrogen; and
[0033] oxidizing the silicon nitride film by using active oxygen to
form a silicon oxynitride film as a gate insulating film of the MIS
transistor;
[0034] the silicon nitride film forming step and the oxidizing step
being conducted continuously in a single chamber while controlling
a pressure inside the single chamber and electric power so that the
silicon oxynitride film has a specific dielectric constant which is
larger than an expected specific dielectric constant theoretically
calculated from an amount of the active nitrogen used.
[0035] In accordance with the semiconductor device of the present
invention and the semiconductor device manufactured by the method
of the present invention, the silicon oxynitride film has a larger
specific dielectric constant, which is larger than the expected
specific dielectric constant theoretically calculated from the
composition of the oxynitride film. Thus, the silicon oxynitride
film has a smaller physical thickness compared to the equivalent
oxide-film thickness, and has a larger function for preventing
impurities in the overlying layer or gate electrode of the MIS
transistor from penetrating therethrough toward the underlying
layer or the silicon substrate.
[0036] The term "equivalent oxide-film thickness" of a subject
insulating film as used herein means the thickness of a specific
silicon oxide film having an electric property (or capacitance
characteristic) equivalent to the electric property of the subject
insulating film. The electric property of a film is typically
measured by a capacitance it affords when it is used as a capacitor
insulator film. On the other hand, the physical thickness of the
insulating film can be measured by an electron microscope or
ellipsometer. The physical thickness is generally referred to as
simply "thickness".
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a schematic sectional view showing a MOS
transistor according to a first embodiment of the present
invention;
[0038] FIG. 2 is a schematic sectional view showing a MOS
transistor according to a second embodiment of the present
invention;
[0039] FIG. 3 is a schematic sectional view showing a MOS
transistor according to a third embodiment of the present
invention;
[0040] FIG. 4 is a schematic sectional view showing a MOS
transistor according to a fourth embodiment of the present
invention;
[0041] FIG. 5 is a schematic sectional view showing a MOS
transistor according to a fifth embodiment of the present
invention;
[0042] FIG. 6 is a schematic view showing the general structure of
a UHV-oxynitride filming unit used in a method of manufacturing a
semiconductor device according to the present invention;
[0043] FIGS. 7A to 7C are sectional views showing a method of
manufacturing a semiconductor device according to the present
invention, illustrating the process step by step;
[0044] FIGS. 8A to 8C are sectional views showing another
manufacturing method according to the present invention,
illustrating the process step by step;
[0045] FIGS. 9A and 9B are sectional views showing yet another
method of manufacturing a semiconductor device according to the
present invention, illustrating the process step by step;
[0046] FIG. 10 is a graph representing nitrogen profiles in silicon
oxynitride films formed using active oxygen and active
nitrogen;
[0047] FIG. 11 is a graph representing the relation between the
gate current and the drain current in MOS transistors using silicon
oxynitride films according to the present invention and a
conventional silicon oxynitride film;
[0048] FIG. 12 is a graph representing the relation between the
drain current, the gate voltage, and the gate current as the
substrate voltage changes in a MOS transistor;
[0049] FIG. 13 is a graph representing the substrate voltage and
the threshold voltage in a MOS transistor for obtaining an
electrical thickness;
[0050] FIG. 14 is a graph representing a result of comparison
between an equivalent oxide-film thickness produced based on MOS
transistor characteristics and an equivalent oxide film thickness
produced by physical measurement or optical measurement;
[0051] FIG. 15 is a graph representing the specific dielectric
constant for a silicon oxynitride film as thick as 1.5 nm;
[0052] FIG. 16 is a graph representing in comparison the electron
mobility obtained from MOS transistors having silicon oxynitride
films formed as gate insulating films;
[0053] FIG. 17 is a graph representing in comparison the drain
current in MOS transistors having silicon oxynitride films formed
as gate insulating films; and
[0054] FIG. 18 is a graph showing in comparison the insulation
breakdown reliability of silicon oxynitride films in MOS
transistors having the silicon nitride films as gate insulating
films.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0055] Now, the present invention will be described with reference
to accompanying drawings in connection with MIS devices according
to preferred embodiments of the present invention. The present
invention is devised based on the experiment results of evaluating
MIS type FET transistors each having a gate insulating film made of
silicon oxynitride and formed by a nitriding technique using active
nitrogen.
[0056] FIG. 10 is a graph representing SIMS profiles in silicon
oxynitride films produced by various processes. The graph
represents the nitrogen profiles of silicon oxynitride films formed
by first to third radical processes. In the first radical process,
active oxygen O* is first used followed by the use of active
nitrogen N* (i.e., O*.fwdarw.N*). In the second radical process,
active oxygen O* and active nitrogen N* are used at a time (i.e.,
N*.fwdarw.O*). In the third radical process, active nitrogen N* is
first used followed by the use of active oxygen O* (i.e.,
N*.fwdarw.O*). The abscissa represents the depth is as viewed in
the thickness direction of a Si substrate, with the surface thereof
in contact with the oxynitride film being the origin. The ordinate
represents the nitrogen concentration. The broken line near the
area at a depth of 2.4 nm represents the interface between the
silicon oxynitride film and the silicon substrate. In the
oxynitride films produced by these processes, the nitrogen
concentrations obtained by XPS measurement are given in the
graph.
[0057] In the first radical process, a ultra-high vacuum chamber is
used, the sample temperature is set to 620.degree. C., and radical
nitriding is performed after radical oxidation. An oxygen gas as
much a pressure as about 5.times.10.sup.-1 Pa is introduced into
the chamber, and a microwave is applied at 150 W for 240 seconds
for treatment. Thus, an underlying oxide film can be formed to have
a thickness of about 2.0 nm. Then, a nitrogen gas as much a
pressure as about 5.times.10.sup.-1 Pa is introduced, and a
microwave is applied at 150 W for 600 seconds for treatment. Thus,
a nitriding reaction is produced from the side of the oxide film
surface, and an oxynitride film having nitrogen localized on the
oxide film surface side can be formed.
[0058] In the second radical process, the sample temperature in the
ultra-high vacuum chamber is set to 620.degree. C., and an oxygen
gas and a nitrogen gas are simultaneously introduced into the
chamber in the ratio of 1:1 and each about in a partial pressure of
1.times.10.sup.31 1 Pa. A microwave is applied at 150 W for 300
seconds for treatment. In these conditions, the silicon oxynitride
film about as thick as 2.0 nm may be formed. Thus, an oxynitride
film having nitrogen localized in the vicinity of the center can be
formed.
[0059] In the third radical process, the sample temperature in the
ultra-high vacuum chamber is set to 620.degree. C., a nitrogen gas
as much a pressure as about 1.times.10.sup.-1 Pa is introduced, and
a microwave is applied at 150 W for 300 seconds for treatment.
Thus, a silicon nitride film to be an underlying layer can be
formed to have a thickness of about 2.0 nm. Then, an oxygen gas as
much a pressure as about 1.times.10.sup.-1 Pa is introduced, and a
microwave is applied at 150 W for 600 seconds. Thus, an oxidizing
reaction is produced on the side of the nitride film, and an
oxynitride film having nitrogen localized on the substrate
interface side can be formed.
[0060] As can be seen from FIG. 10, the nitrogen position is
localized in the vicinity of the surface in the first radical
process in which the radical oxidizing is followed by the radical
nitriding. In the third radical process in which the radical
nitriding is followed by the radical oxidizing, the nitrogen
position is localized in the vicinity of the interface denoted by
the broken line. These exhibit that oxidizing and nitriding
reactions using radicals are mainly produced on the surface
side.
[0061] The nitrogen position control according to the conventional
method takes advantage of reactions caused at the interface by
oxidizing and nitriding species diffusing within the film. The
nitrogen position control according to the present invention
utilizes reactions at the surface, and thus, a nitrogen profile can
readily be controlled in a shallow region at a depth of 3.0 nm or
less. The structure having nitrogen localized especially on the
surface side can be formed without causing a nitriding reaction at
the interface. As a result, superior interface electric
characteristics of the oxide film can be maintained.
[0062] Then, a combination with a High-k film (high dielectric
constant film) will be now described. A silicon oxynitride film
produced by the manufacturing method according to an embodiment of
the present invention is used as an underlying film for a High-k
film (a metal oxide such as oxide of Zr, HF, La, Ti, Ta, Y, La, or
Al). A silicon oxide film is used as an underlying film according
to a conventional method, wherein the dielectric constant of such a
silicon oxide film is as low as 3.9 which has been a great
hindrance in reducing film thickness.
[0063] In the silicon oxynitride film produced by the manufacturing
method according to an embodiment of the present invention, a
dielectric constant of 5 to 7 results. A combination with a high
dielectric constant, High-k film, the film thickness can be
reduced. When a FET is produced using a silicon oxynitride film
according to an embodiment of the present invention and a High-k
films interface electric characteristics equivalent to the
conventional oxide film result. The nitrogen localized on the
oxynitride film surface side prevents the metal used as the High-k
material from diffusing to the silicon substrate side, and
therefore a reaction between the metal and oxygen at the
High-k/silicon oxynitride film interface can be restrained.
[0064] In order to obtain superior electric characteristics for a
transistor, the nitrogen peak should be separated from both the
surfaces of the silicon oxynitride film and the silicon substrate
(Si-Sub). The nitrogen peak in the silicon oxynitride film formed
by the first radical process (O*.fwdarw.N*) is on the surface side
of the silicon oxide film as described above. Thus, the optimum
state for the gate insulating film results. More specifically, the
silicon oxide film is formed using active oxygen O* and then the
silicon oxide film is nitrided using active nitrogen N*, whereby
the nitrogen can be distributed at the surface of the silicon
oxynitride film and in the vicinity thereof. This is because the
nitriding using active nitrogen proceeds on the surface of the
silicon oxide film.
[0065] The nitrogen peak in the oxynitride film formed by the
second radical process (O*+N*) is positioned near the interface
between the silicon oxynitride film and the silicon substrate as
described above. This position is denoted by the broken line in
FIG. 10. More specifically, in the silicon oxynitride film formed
using active oxygen O* and active nitrogen N* simultaneously, the
nitrogen is positioned substantially in the center of the
oxynitride film, which would hardly be optimum for obtaining
superior transistor characteristics.
[0066] The nitrogen peak in the oxynitride film formed by the third
radical process (N*.fwdarw.O*) is positioned near the interface
between the silicon oxynitride film and the silicon substrate as
described above. More specifically, after a silicon nitride film is
formed using active nitrogen N*, a silicon oxynitride film is
formed using active oxygen O*, and therefore a distribution having
much nitrogen at the silicon substrate interface results, which
again would hardly be optimum for obtaining superior transistor
characteristics.
[0067] A process of forming a silicon oxynitride film according to
the present invention will be described hereinafter in detail. A
cleaned and dried silicon substrate is introduced into a filming
unit for forming an UHV-oxynitride film (ultra-high vacuum chamber:
see FIG. 6). By using the filming unit, in the maintained
atmosphere of 1.times.10.sup.-6 Pa (1.times.10.sup.-8 Torr) or
less, the silicon substrate is thermally treated at 920.degree. C.
for five minutes for cleaning the substrate surface.
[0068] Then, a silicon oxide film is formed on the silicon
substrate in an atmosphere of an oxygen gas, a mixed atmosphere of
an oxygen gas and a hydrogen gas or an atmosphere of active oxygen.
After the silicon oxide film is formed, the filming temperature is
set to 750.degree. C. (620.degree. C. at the substrate). The
nitrogen gas flow rate is set to 0.25 sccm, the pressure in the
treatment chamber (the pressure of active species) in forming the
film is set to 5.times.10.sup.-1 Pa, and the microwave power of an
ECR device supplying active nitrogen is set to 150 watts. In these
conditions, the silicon oxide film is nitrided for 5 to 30 minutes
to form a silicon oxynitride film. The step of forming the silicon
oxide film and the step of forming the silicon oxynitride film are
performed sequentially in the same ultra-high vacuum chamber. Then,
an ordinary process of forming a gate electrode and an ordinary
process of forming source/drain/channel regions are performed to
complete the MOS device as one of the MIS devices.
[0069] A MOS transistor formed by the above method and having a
gate insulating film as thick as 1.5 nm was examined for the
transistor characteristics. FIG. 11 is a graph representing drain
current and gate leakage current in the conventional silicon
oxynitride film and the silicon oxynitride films formed by the
above nitriding methods. The abscissa represents gate voltage
V.sub.G-V.sub.TH normalized by a threshold voltage, and the
ordinate represents drain current I.sub.D and gate current
I.sub.G.
[0070] In the graph, the solid line represents the characteristic
of the silicon oxynitride film formed by the first radical process
(O*.fwdarw.N*), and the broken line represents the characteristic
of the silicon oxide film formed using active oxygen O*. As can be
seen from the graph, the silicon oxynitride film has the same drain
current as that of the silicon oxide film, while its gate leakage
current is smaller by one to two orders of magnitude. More
specifically, in the manufacture of the silicon oxynitride film by
the first radical process, a silicon oxide film is formed on a
silicon substrate and then the oxide film is nitrided with active
nitrogen N* and formed into the silicon oxynitride film. As a
result, the gate leakage current is significantly reduced as
compared with the conventional silicon oxide film.
[0071] Then, the specific dielectric constant of the silicon
oxynitride film thus formed was measured. Physical film thickness
is measured using an electron microscope. Electrical thickness
could not be measured by the general MOS capacitance method because
the leakage current was too large. Therefore, the electrical
thickness was measured based on the relation between the threshold
voltage of the MOS transistor and the substrate voltage, wherein
the measurement was unaffected by the leakage current.
[0072] The measuring method has been proposed as a method of
evaluating a gate insulating film by the applicant (Japanese Patent
Application No. Hei. 11-364206). The method addresses the inability
of calculating physical film thickness for a thin oxide film
relative to the equivalent in the thickness of the oxide film
(referred to as equivalent oxide-film thickness) because of large
gate leakage current of the thin oxide film. According to the
evaluation method, a field effect transistor having a thick silicon
oxide film as a gate insulating film and a field effect transistor
having a gate insulating film to be evaluated are formed at the
same time. Thus, the field effect transistor having the thick
silicon oxide film in known film quality and known film thickness
is used as a gate insulating film. By using this technique,
impediment factors to calculation of the thickness of the gate
insulating film (parasitic film thickness) or the channel
concentration resulting from depletion in the impurities in the
gate electrode can stably be obtained. In this technique, even if
the gate insulating film to be evaluated has a complicated
structure or even if the thickness is not known, the electrical and
physical gate insulating film thicknesses can be obtained.
[0073] Therefore, the electrical thickness can be produced even if
the thickness of the gate insulating film is not known or even if
the gate insulating film has large gate leakage current. Thus, the
interface state, the thickness of the gate insulating film and the
mobility at the region including the interface between the gate
insulating film and the silicon substrate can be produced according
to the following expressions (1) and (2):
S=(kT/q)ln{10(l+(C.sub.D+C.sub.it)/C.sub.ox)} (1)
[0074] where S is the subthreshold coefficient, k is the Boltzmann
constant, T is the temperature, q is the elementary electric
charge, C.sub.D is the depletion layer capacitance, C.sub.it is the
equivalent capacitance in the interface state, and C.sub.OX is the
silicon oxide film capacitance, and
.mu..sub.eff=(dI.sub.D/dV.sub.D)(L/W)/(C.sub.OX(V.sub.G-V.sub.th))
(2)
[0075] where .mu..sub.eff is the effective mobility, I.sub.D is the
drain current, V.sub.D is the drain voltage, L is the gate
electrode length, W is the gate electrode width, C.sub.OX is the
silicon oxide film capacitance, V.sub.G is the gate voltage, and
V.sub.TH is the threshold voltage. Thus, the equivalent oxide-film
thickness can be introduced based on the substrate voltage
dependence of the threshold voltage of the MOS transistor.
[0076] FIG. 12 is a graph representing the dependence of the drain
current I.sub.D and the gate current I.sub.G on the gate voltage
V.sub.G as the substrate voltage V.sub.B is changed. The abscissa
represents the gate voltage V.sub.G, while the ordinate represents
the drain current I.sub.D and the gate current I.sub.G. In the
graph, the drain current is larger than the gate current by two
orders of magnitude in the vicinity of the threshold voltage
V.sub.TH and therefore the process of obtaining the threshold
voltage used for calculation of the equivalent oxide-film thickness
is unaffected by the gate current.
[0077] More specifically, as can be understood from the graph, a
value corresponding to the range of the gate voltage V.sub.G from
0.2V to 0.5V is used as the threshold voltage V.sub.TH, and the
gate current I.sub.G in the range is smaller than the drain current
I.sub.D by about two orders of magnitude, and therefore the
threshold voltage V.sub.TH is little affected. In general, the
dependence of the threshold voltage V.sub.TH on the substrate
voltage V.sub.B can be represented by the following expression
(3):
V.sub.TH=((2.epsilon..sub.s.epsilon..sub.0qN.sub.ch).sup.1/2/C.sub.OX)(V.s-
ub.B+2.phi..sub.F).sup.1/2+V.sub.FB+2.phi..sub.F (3)
[0078] where N.sub.ch is the impurity concentration of the channel,
C.sub.OX is the capacitance value of the gate insulating film
including the electrical thickness T.sub.OX-ele, .epsilon..sub.s
.epsilon..sub.0 is the specific dielectric constant of silicon, q
is the elementary electric charge, .phi..sub.F is the Fermi level,
and V.sub.FB is the flat-band voltage.
[0079] FIG. 13 is a graph representing the dependence of the
transistor threshold voltage V.sub.TH on the substrate voltage
V.sub.B when the gate insulating film is an oxide film. The
abscissa represents (substrate voltage +2.times.Fermi
level).sup.1/2, and the ordinate represents the threshold voltage.
In the graph, the channel concentration N.sub.CH and the equivalent
oxide-film thickness (oxide film capacitance C.sub.OX ) can be
obtained based on the inclination of the straight line from the
expression (3).
[0080] In the graph, in the thick gate oxide film formed by the
same process as the thin gate oxide film, the physical film
thickness T.sub.OX-phy is the same as the equivalent oxide-film
thickness T.sub.OX-eq. Therefore, the parasitic thickness
T.sub.OX-para by depletion of the gate electrode or quantization of
the channel can be obtained. More specifically, the following
expression is established:
T.sub.OX-ele=T.sub.OX-eq+T.sub.OX-para.
[0081] Finally, the equivalent oxide-film thickness of the thin
oxide film is obtained from the following expression:
T.sub.OX-cq=T.sub.OX-clc-T.sub.OX-para.
[0082] FIG. 14 is a graph showing the relation between the
equivalent oxide-film thickness T.sub.OX-cq (on the ordinate)
obtained from the substrate voltage dependence of the threshold
voltage (from the expression (3)) in the transistor and the
physical film thickness T.sub.OX-phy (on the abscissa) obtained by
the thickness measuring device (section TEM and an
ellipsometer).
[0083] When the inclination of the straight line in the graph
changes by 10%, the obtained equivalent oxide-film thickness
changes by 3%. If the cannel concentration changes by 10%, the
equivalent oxide-film thickness changes by 5%. When the inclination
of the straight line and the channel concentration both change by
10%, the equivalent oxide-film thickness changes by 8%. The error
is within 0.11 nm for a thickness of 1.5 nm. Thus, the method of
producing the equivalent oxide-film thickness from the electric
characteristics of the transistor is highly precise in a thin
region. The error for the equivalent oxide-film thickness may
result from the extraction of the inclination of the straight line
or the channel concentration. The thickness obtained from the
expression (3) is substantially the same as that obtained by the
thickness measuring device obtained from the expression (3).
Therefore, the thickness measuring method using the expression (3)
is clearly suitable.
[0084] The results of the physical film thickness and the
equivalent oxide-film thickness obtained by the measuring method
are given in the Table 1.
1TABLE 1 Nitrogen Physical film concentration in thickness/
Specific oxide film Equivalent oxide- dielectric Filming process
(XPS) film thickness constant O*.fwdarw.N* 7% 2.5/1.5 nm 6.5 O* +
N* 4% 2.5/1.6 nm 6.1 N*.fwdarw.O* 12% 2.5/1.9 nm 5.1 O* 0% 1.5/1.5
nm 3.9
[0085] As shown in the Table 1, the silicon oxynitride film formed
by the first radical process (O*.fwdarw.N*) has a nitrogen
concentration (XPS) of 7% in the film, the physical film thickness
and the equivalent oxide-film thickness relative to the physical
film thickness are 2.5 nm and 1.5 nm, respectively. The specific
dielectric constant is 6.5. The silicon oxynitride film formed by
the second radical process (O*+N*) has a nitrogen concentration of
4% in the film, and the physical film thickness and the equivalent
oxide thickness relative to the physical film thickness are 2.5 nm
and 1.6 nm, respectively. The specific dielectric constant is
6.1.
[0086] The silicon oxynitride film formed by the third radical
process (N*.fwdarw.O*) has a nitrogen concentration of 12% in the
film, and the physical film thickness and the equivalent oxide-film
thickness are 2.5 nm and 1.9 nm, respectively. The specific
dielectric constant is 5.1. The silicon oxide film formed using
only active oxygen O* has a nitrogen concentration of 0% in the
film, and the physical film thickness and the equivalent oxide-film
thickness are both 1.5 nm. The specific dielectric constant is
3.9.
[0087] As can be seen from the Table 1, at least the specific
dielectric constants of the silicon oxynitride films produced by
the first and second radical processes are not less than 6.0, which
is higher than that of the general silicon oxynitride film expected
from the amount of nitrogen introduced. In the silicon oxynitride
film having a nitrogen concentration (XPS) in the oxide film in the
range from 5% to 10%, the value is close to that of a general
silicon nitride film. The physical film thickness is about 1.5
times as large as the equivalent oxide-film thickness. Therefore,
as can be understood, if the physical film thickness relative to
the equivalent of 1 nm to 3 nm in the thickness of the silicon
oxide film for example is increased in the range from 1.5 to 4.5
nm, the gate leakage current can effectively be reduced, while
superior transistor characteristics are maintained.
[0088] Meanwhile, there is an article about the dielectric constant
of thick silicon oxynitride films in Journal of Electrochemical
Society (J. Electrochem. Soc.), 1968, vol. 115, pp.311-317.
[0089] According to the article, at a substrate temperature in the
range from 750.degree. C. to 1100.degree. C., using a silane gas
and an ammonia gas, a silicon nitride film is formed to have a
thickness of 150 nm. At the same substrate temperature, using a
silane gas, an ammonia gas and a nitrogen monoxide gas, a silicon
oxynitride film is also formed to have a thickness of 150 nm. The
specific dielectric constant of the manufactured silicon oxynitride
film is obtained based on the MOS capacitance method and the
physical film thickness. According to the description, the specific
dielectric constant is produced by a monotonous primary linear
interpolation between the specific dielectric constant of the
silicon oxide film and the specific dielectric constant of the
silicon nitride film.
[0090] FIG. 15 is a graph representing the specific dielectric
constant for a silicon oxynitride film as thick as 1.5 nm. The
abscissa represents the ratio of a silicon nitride film in a mixed
film including a silicon oxide film and the silicon nitride film,
in other words, the composition ratio x of the silicon nitride in
the silicon oxynitride film. The ordinate represents the specific
dielectric constant.
[0091] In the graph, the silicon oxide film formed using only
active oxygen O* has the same specific dielectric constant as that
of the conventional oxide film. On the other hand, the silicon
oxynitride films formed by the first radical process (O*.fwdarw.N*)
and the second radical process (O*+N*) have values extremely higher
than what is expected from the general silicon oxynitride film
(denoted by the broken line).
[0092] In the graph in FIG. 15, the silicon oxynitride film having
silicon nitride in the composition ratio x of 0.05 to 0.5, and a
specific dielectric constant of at least 4.5 and less than 6.0 is
formed by the third radical process (N*.fwdarw.O*). The dielectric
constant of the silicon oxynitride film is a value expected from a
general silicon oxide film. The silicon oxynitride film allows the
general transistor characteristics and the gate leakage current
characteristic to be provided. Meanwhile, the reliability of the
silicon oxynitride film by nitrogen in the vicinity of the
interface is slightly lower.
[0093] A silicon oxynitride film having silicon nitride in the
composition ratio x of 0.05 to 0.45 and a specific dielectric
constant of at least 6.0 and less than 6.5 is formed by the second
radical process (O*+N*). The specific dielectric constant of the
silicon oxynitride film is a value extremely higher than what is
expected from the general silicon oxide film. Using the silicon
oxynitride film, general transistor characteristics and general
gate leakage current characteristic result, and the reliability of
the silicon oxynitride film by the nitrogen in the vicinity of the
interface is slightly lower.
[0094] A silicon oxynitride film having silicon nitride in the
composition ratio x of 0.05 to 0.4 and a specific dielectric
constant of at least 6.5 and less than 7.5 is formed by the first
radical process (O* .fwdarw.N*). In this case, an insulating film
of a silicon oxynitride film having a reduced thickness and a high
specific dielectric constant can be provided, wherein the nitrogen
does not reach the interface between the insulating film and the
adjacent substrate. As a result, the film is preferable in
improving the transistor characteristics. The gate leakage current
can be reduced and high reliability as a silicon oxynitride film
results. The specific dielectric constant of the silicon oxynitride
film is set to less than 7.5, because in levels beyond 7.5, the
entire region in the depth-wise direction in the insulating film is
nitrided, the gate insulating film on the silicon substrate side is
not oxidized and the transistor characteristics can be
degraded.
[0095] FIG. 16 is a graph showing in comparison the electron
mobility produced from MOS transistors having silicon oxynitride
films formed by various methods as gate insulating films. More
specifically, the effective mobility in the silicon oxynitride
films formed by the first to third radical processes and the
silicon oxide film formed using only active oxygen O* are shown.
The abscissa represents an effective electric field (MV/cm) and the
ordinate represents the effective mobility (cm.sup.2/Vs).
[0096] As can be observed from the graph, the electron mobility is
substantially equal between in the silicon oxide film and in the
silicon oxynitride film formed in the nitriding process following
the oxidizing process. The drain current of the produced NMOS
transistor is substantially equal between for the silicon oxide
film and for the silicon oxynitride films except for the silicon
oxynitride film formed in the oxidizing process after the nitriding
(see FIG. 15).
[0097] The oxynitride film formed by the third radical process
(N*.fwdarw.O*) has its nitrogen peak positioned at the interface
between the silicon oxynitride film and the silicon substrate, and
therefore the effective mobility is small. Meanwhile, the silicon
oxynitride films formed by the first and second radical processes
(O*.fwdarw.N*, O*+N*) have their nitrogen peaks positioned away
from the interface between the silicon oxynitride film and the
silicon substrate. Therefore, the effective mobility is larger than
that of the silicon oxynitride film formed by the third radical
process (N*.fwdarw.O*). It is to be noted, however, that since
there is some nitrogen at the interface between the silicon
oxynitride film and the silicon substrate, the maximum value is
smaller than the effective mobility of the oxide film (O*) free
from nitrogen. The power supply voltage at an effective electric
field of 1.5 MV/cm is used in practice, and the effective mobility
of each of the silicon oxynitride films formed in the first and
second radical processes (O*.fwdarw.N*, O*+N*) is similar to that
of the oxide film (O*).
[0098] Therefore, the drain current is also substantially equal.
FIG. 17 is a graph showing in comparison the drain current in the
MOS transistors having gate insulating films of silicon oxynitride
films formed by various methods. More specifically, the drain
current for each of the silicon oxynitride films formed by the
first to third radical processes and the drain current for the
silicon oxide film formed using only active oxygen O* are shown.
The abscissa represents the gate electrode length (microm), while
the ordinate represents the drain current I.sub.D (mA/microm) and
the equivalent oxide-film thickness T.sub.OX-cq(nm).
[0099] The silicon oxynitride film formed by the third radical
process (N*.fwdarw.O*) has a nitrogen peak at the interface between
the oxynitride film and the silicon substrate, and the drain
current is small. On the other hand, the silicon oxynitride films
formed by the first and second radical processes (O*.fwdarw.N*,
O*+N*) have their nitrogen peaks away from the interface between
the oxynitride film and the silicon substrate. The drain current
for the oxide film O* free from nitrogen is substantially
equal.
[0100] As can be understood from the graph in FIG. 17 and the Table
1, the silicon oxynitride film formed by the first radical process
(O*.fwdarw.N*) according to which the silicon substrate surface is
oxidized 20 followed by nitriding has an increased nitrogen content
in the film. The gate leakage current is reduced for the equivalent
oxide-film thickness in the Table 1 at a fixed value. It has been
confirmed that the MOS transistor having the silicon oxynitride
film operates normally.
[0101] FIG. 18 shows in comparison the insulation breakdown
reliability of the silicon oxynitride films obtained from the MOS
transistors having the silicon nitride films formed by various
methods as gate insulating films. More specifically, FIG. 18 shows
the insulation breakdown reliability in the silicon oxynitride
films formed by the first and second radical processes, the silicon
oxide film formed using only active oxygen O*, and the oxynitride
film formed by the conventional method (NO.fwdarw.Dry.fwdarw.O) The
abscissa represents time (sec.) before the insulation breakdown,
and the ordinate represents the cumulative frequency distribution
(Weibull plot) of the samples that are broken down in the
insulation.
[0102] The silicon oxynitride film formed by the first radical
process (O*.fwdarw.N*) has particularly high reliability, and is
not broken down after 1000 seconds or longer. As can be observed
from FIG. 18, the silicon oxynitride film formed by the first
radical process (O*.fwdarw.N*) produced in the process of oxidizing
the silicon substrate followed by nitriding has insulation
breakdown reliability 100 times or more as high as that of the
conventional oxide film.
[0103] Thus, the silicon oxynitride films formed by the
manufacturing method according to the present invention have a
higher dielectric constant than that of the conventional oxide film
produced by thermal nitriding for a fixed equivalent oxide-film
thickness. Therefore, it will be understood that the gate leakage
current can be reduced effectively.
[0104] According to the present invention, the gate insulating film
includes a silicon oxynitride film with a specific dielectric
constant larger than a specific dielectric constant produced by
simply averaging the specific dielectric constants of silicon oxide
and silicon nitride by the composition ratios. Specifically,
according to the present invention, a value greater than the value
represented by the following expression (4):
(1-x).times.3.9+x.times.7.5 (4)
[0105] results for the specific dielectric constant .epsilon.,
wherein the specific dielectric constant of the silicon oxynitride
film is .epsilon. .epsilon., the specific dielectric constant of
silicon oxide is 3.9, and the specific dielectric constant of
silicon nitride is 7.5, and the composition ratio of the silicon
nitride in the silicon oxynitride film is x. The use of the silicon
oxynitride film having the above characteristics, the penetration
of impurity ions from an adjacent gate electrode through the ultra
thin gate insulating film to affect a silicon substrate on the
opposite side can surely be restrained.
[0106] Referring to the accompanying drawings, embodiments of the
present invention will be now described in detail. FIG. 1 is a
sectional view of the general structure of a MOS transistor
according to a first embodiment of the present invention.
[0107] Adjacent MOS transistors isolated from each other by an
element isolation region 20 each includes, on a silicon substrate
10, an element region, a gate electrode 51 (52), and a gate
sidewall insulating film 60. The element region includes a source
region 70 and a drain region 80 adjacent to each other with a
channel therebetween. The gate electrode 51 (52) is formed on the
channel region with a gate insulating film 30 therebetween. The
gate sidewall insulating film 60 covers each sidewall of the gate
electrode 51 (52).
[0108] The MOS transistor has the gate insulating film 30 on the
silicon substrate 10. The gate insulating film 30 is a silicon
oxynitride film formed on the silicon substrate 10 by the following
process. Specifically, a silicon oxide film having a thickness in
the range from 1 nm to 3 nm is formed on the silicon substrate 10.
The silicon oxide film is then formed into a silicon oxynitride
film having a thickness from 1 nm to 3 nm by nitriding using active
nitrogen at a temperature in the range from 300.degree. C. to
900.degree. C. The silicon oxynitride film is formed by the first
radical process (O*.fwdarw.N*), and the nitrogen concentration
distribution in the film has a peak on the side of the oxynitride
film surface as described in conjunction with FIG. 10. Using the
surface reaction with the active nitrogen species as described
above, the concentration distribution of the nitrogen in the
oxynitride film is controlled.
[0109] It is to be noted that the thickness of the oxynitride film
should be in the range from 1 nm to 3 nm. This is because nitriding
with active nitrogen cannot be effectively controlled within the
oxynitride film for a thickness not more than 1 nm, whereby the
silicon substrate could be nitrided, which increases the roughness
of the interface between the oxynitride film and the silicon
substrate and would impair the transistor characteristics. On the
other hand, for a thickness greater than 3 nm, the resulting film
is no longer advantageous over the conventional silicon oxide film
in connection with the impurity penetration or tunneling leakage
current.
[0110] A MOS transistor according to a second embodiment of the
present invention will be now described in conjunction with FIG. 2.
The MOS transistor includes a silicon oxynitride film as an
insulating film 31. The silicon oxynitride film is formed by the
following process. A silicon oxide film having a thickness in the
range from 1 nm to 3 nm is formed on the substrate 10. The silicon
oxide film is then nitrided at a temperature in the range from
300.degree. C. to 900.degree. C. using active nitrogen, so that the
silicon oxynitride film having a nitrogen concentration from 3% to
10% results. The silicon oxynitride film is therefore formed by the
first radical process (O*.fwdarw.N*). The other members and
structures in this embodiment are similar to those of the MOS
transistor shown in FIG. 1.
[0111] It is to be noted that the nitrogen concentration should be
in the range from 3% to 10%. This is because the gate leakage
current cannot be effectively reduced, as shown in FIG. 11, for a
nitrogen concentration less than 3%. For a nitrogen concentration
greater than 10%, the nitrogen in the oxynitride film reaches the
silicon substrate, whereby the roughness of the interface between
the oxynitride film and the silicon substrate increases, and
accordingly the carrier mobility in the silicon substrate is
reduced. This may deteriorate the transistor characteristics.
[0112] A MOS transistor according to a third embodiment of the
present invention will be now described in conjunction with FIG. 3.
The MOS transistor includes a silicon oxynitride film formed by the
following process as a gate insulating film 32. A silicon oxide
film having a thickness in the range from 1 nm to 3 nm is formed on
a silicon substrate 10. The silicon oxide film is then nitrided at
a temperature in the range from 300.degree. C. to 900.degree. C.
using active nitrogen. More specifically, the silicon oxynitride
film is also formed by the first process (O*.fwdarw.N*), and the
nitrogen peak in the film is within 1 nm from the surface and
positioned more on the surface side than the central part of the
film. The other members and structures of the embodiment are
similar to those of the MOS transistor in FIG. 1.
[0113] It is to be noted that the nitrogen distribution in the film
should be within 1 nm from the surface, and that the maximum
nitrogen concentration position is more on the surface side than
the central part of the film. This is because, if the thickness of
the oxynitride film is in the range from 1 nm to 3 nm, nitrogen is
not introduced into the silicon substrate at least in the nitriding
process of the silicon oxide film using active nitrogen, and a
corresponding nitrogen profile results. The nitrogen, if introduced
into the silicon substrate, degrades the carrier mobility, which
degrades the transistor characteristics.
[0114] A MOS transistor according to a fourth embodiment of the
present invention will be now described in conjunction with FIG. 4.
The MOS transistor includes a silicon oxynitride film formed by the
following process as a gate insulating film 33 A silicon oxide film
having a thickness of 1 nm to 3 nm is formed on a silicon substrate
10. Then, the silicon oxide film is nitrided using active nitrogen
at a temperature in the range from 300.degree. C. to 900.degree. C.
The resulting silicon oxynitride film has a specific dielectric
constant of at least 6.0.
[0115] It is to be noted that the specific dielectric constant is
at least 6.0. This is because the physical film thickness is about
1.5 times as large as that of the general silicon oxide film (see
Table 1) for a silicon oxynitride film thickness from 1 nm to 3 nm.
This reduces the gate leakage current at least by one order of
magnitude.
[0116] A MOS transistor according to a fifth embodiment of the
present invention will be now described in conjunction with FIG. 5.
The MOS transistor according to the embodiment has a structure
substantially the same as that of the MOS transistor according to
the first embodiment. In the fifth embodiment, the MOS transistor
includes a gate insulating film 54 formed of a general oxide film
having a composition different from the gate insulating film 30 on
the gate insulating film 30 formed of the silicon oxynitride film.
In the MOS transistor including such a layered structure having the
gate insulating films 30 and 54, impurity ions in the upper layer
gate insulating film 54 can be prevented from diffusing/reacting
to/with the lower layer gate insulating film 30. Superior
transistor characteristics by the high quality gate insulating film
30 result and reduction in the gate leakage current can be
achieved.
[0117] Application of the silicon oxynitride films (30 to 33)
according to the first to fifth embodiments described above to the
gate insulating film of a MOS transistor prevents impurity (boron)
ions in the gate electrodes 51 and 52 from penetrating through the
silicon oxynitride film (30 to 33), whereby the silicon substrate
can be effectively restrained from being degraded.
[0118] A method of manufacturing a MOS transistor according to an
embodiment of the present invention will be now described. A
UHV-oxynitride filming unit (ultra high-vacuum chamber) used
according to the method will be specifically described. FIG. 6 is a
schematic view of the general structure of filming unit for forming
the oxynitride film.
[0119] The UHV-oxynitride filming unit includes an exchange chamber
101, a sample treatment chamber 102, and a heater 103. The exchange
chamber 101 can accommodate a plurality of wafers 107. A gate valve
104 is provided between the sample treatment chamber 102 and the
exchange chamber 101. Each chamber is exhausted by exhaust systems
151, 152 and 153 including a plurality of pumps.
[0120] The exhaust system 153 is provided with a pressure control
system, and adjusts the internal pressure of the sample treatment
chamber 102. A wafer transport mechanism 106 to move a wafer
between the sample treatment chamber 102 and the exchange chamber
101 is provided adjacent to the exchange chamber 101. The wafer
transport mechanism 106 allows the wafer to be exchanged or moved
without exposing the sample treatment chamber 102 to the air in the
UHV-oxynitride filming unit. The sample treatment chamber 102 is
provided with a heater 105 to heat the wafer, an ECR plasma source
108 to produce active oxygen O* and active nitrogen N*, and a gas
supply system.
[0121] The heater 105 can heat the substrate to a temperature of
1200.degree. C. The gas supply system includes an oxygen gas
container 123, a nitrogen gas container 127, a disilane gas
container 131, stop valves 120, 122, 124, 126, 128 and 130, and
mass flow controllers 121, 125, and 129. The gas supply systems 120
to 131 allow an oxygen gas and a nitrogen gas to be introduced
through the ECR plasma source 108 into the sample treatment chamber
102. The introduced oxygen gas and nitrogen gas can be adjusted by
the mass flow controllers 121, 125 and 129 in the range from
1.times.10.sup.-2 Pa to 50 Pa.
[0122] Using the UHV-oxynitride filming unit, the active oxygen O*
and the active nitrogen N* necessary for forming the gate
insulating film may be introduced into the sample treatment chamber
102 to achieve the filming conditions according to the present
invention. More specifically, the active oxygen O* and active
nitrogen N* are supplied by introducing the oxygen gas and the
nitrogen gas through the ECR plasma source 108. The flow ratio of
the oxygen gas and the nitrogen gas is controlled using the mass
flow controllers 121, 125 and 129, and the pressure in the sample
treatment chamber 102 is set in the range from 0.1 to 1 Pa.
[0123] In an experiment using the UHV-oxynitride filming unit
described above, a sample of p-Si 100 having a diameter of 200 mm
and a sample having an element isolation region on a silicon
substrate where .rho.=0.02 .OMEGA.cm are used. In the element
isolation, a thermally oxidized film is formed on the silicon
substrate and then a silicon nitride film to serve as a mask at the
time of selectively oxidizing the element isolation region is
coated thereon. Then, patterning is performed to leave the silicon
nitride film only in an element region, and impurity ions of the
same type as that of the conductivity type of the silicon substrate
are introduced in the element isolation region followed by
formation of a thick oxide film. The sample is subjected to a wet
cleaning process (APM cleaning, pure water cleaning, HF cleaning,
pure water cleaning and IPA drying in this order), and then
immediately transported to the UHV-oxynitride filming unit.
[0124] The APM cleaning was conducted for five minutes in a
chemical mixture solution of NH.sub.4OH: H.sub.2O.sub.2: H.sub.2O
in the ratio of 1:6:20 heated up to 65.degree. C. Then, pure water
cleaning by rapid damping was conducted twice, and a treatment in a
chemical solution of HF: H.sub.2O in the ratio of 1:50 was
conducted for 45 seconds. Then, over flow pure water cleaning was
conducted for two minutes, and finally the IPA drying was conducted
to remove water droplets from the wafer surface. After the wet
cleaning, the wafer was immediately transported to the exchange
chamber 101 in the UHV-oxynitride filming unit.
[0125] The vacuum degree in the exchange chamber 101 was set to
1.33.times.10.sup.-5 Pa (1.0.times.10.sup.31 7 Torr) or less, and
the vacuum degree in the sample treatment chamber 102 was set to
1.33.times.10.sup.-7 Pa (1.0.times.10.sup.-9 Torr) or less. After
sufficiently exhausted in this exchange chamber 101, the sample was
transported to the sample treatment chamber 102. The transported
sample was annealed for five minutes at a temperature of
920.degree. C. from the bottom surface using the heater 105. As a
result, a native oxide film formed after the cleaning desorbed from
the Si surface to expose the Si cleaned surface. Thus, a flat
surface in the structure of atoms was formed on the Si cleaned
surface.
[0126] Then, while the sample temperature was kept at 620.degree.
C., a gate oxynitride film was formed by the manufacturing method
according to the present invention. Then, while the sample was kept
at 650.degree. C., disilane was allowed to flow at a flow rate of
1.0 sccm, and gate electrode polysilicon was deposited. In this
example, the sample was later taken out into the atmosphere, and a
MOS-FET was manufactured.
[0127] A method of manufacturing a semiconductor device according
an embodiment of to the present invention (the third radical
process (N*.fwdarw.O*)) will be now described in conjunction with
FIGS. 7A to 7C. FIGS. 7A to 7C are sectional views showing the
process step by step. As shown in FIG. 7A, a silicon oxide film 40
having a thickness of 1 nm to 3 nm is formed on a silicon substrate
10 by general thermal oxidation. Then, the silicon oxide film 40 is
nitrided at a temperature in the range from 300.degree. C. to
900.degree. C. using active nitrogen formed by plasma excitation of
a nitrogen gas. Thus, as shown in FIG. 7B, a silicon oxynitride
film 30 having a thickness from 1 nm to 3 nm is formed. An ordinary
process of forming gate electrodes 51 and 52 and an ordinary
process of forming a source region 70 and a drain region 80 are
conducted as shown in FIG. 7C to complete the MOS transistor.
[0128] Another method of manufacturing a semiconductor device
according to the present invention (the first radical process
(O*.fwdarw.N*)) will be now described in conjunction with FIGS. 8A
to 8C. FIGS. 8A to 8C are sectional views showing the process step
by step. As shown in FIG. 8A, a silicon oxide film 41 having a
thickness in the range from 1 nm to 3 nm is formed on a silicon
substrate 10 by an oxidizing method using active oxygen. Then, the
silicon oxide film 41 is nitrided at a temperature in the range
from 300.degree. C. to 900.degree. C. using active nitrogen formed
by plasma excitation of a nitrogen gas. Thus, as shown in FIG. 8B,
a silicon oxynitride film 31 having a thickness in the range from 1
nm to 3 nm is formed. Then, a general step of forming electrodes 51
and 52, and a general step of forming a source region 70 and a
drain region 80 are performed to complete the MOS transistor as
shown in FIG. 8C.
[0129] Still another method of manufacturing a semiconductor device
(by the second radical process (O*+N*)) according to the present
invention will be described in conjunction with FIGS. 9A and 9B.
FIGS. 9A and 9B are sectional views showing the process step by
step. As shown in FIG. 9A, a silicon oxynitride film 42 having a
thickness in the range from 1 nm to 3 nm is formed on a silicon
substrate 10 in a single process by oxynitriding at a temperature
in the range from 300.degree. C. to 900.degree. C. using active
oxygen and active nitrogen. Thereafter, a general step of forming
gate electrodes 51 and 52 and a step of forming a source region 70
and a drain region 80 are performed to complete the MOS
transistor.
[0130] According to the above described manufacturing methods, the
penetration of impurity ions in the gate electrode to the silicon
substrate surface through the gate insulating film caused in the
manufacturing step of the MOS transistor having a thickness in the
range from 1 nm to 3 nm can be completely solved. The roughness of
the interface between the silicon oxynitride film and the silicon
substrate can be significantly reduced. Furthermore, using the
silicon oxynitride film for the gate insulating film in the MOS
transistor, the mobility of electrons or holes can be maintained in
the same level as a general silicon oxide film. Meanwhile, the gate
leakage current in the gate oxynitride film having a thickness in
the range from 1 nm to 3 nm can be reduced by one to two orders of
magnitude.
[0131] According to these methods, active species of oxygen or
nitrogen are formed by ECR plasma excitation of an oxygen gas or a
nitrogen gas, while the active species may be produced by
excitation using ICP, RF plasma or helicon wave plasma.
Alternatively, photo excitation of an oxygen gas or a nitrogen gas
may be employed.
[0132] An oxynitride film may be formed using only neutral radicals
of oxygen or nitrogen as the active species. For example, among the
active species formed in a plasma excitation chamber, neutral
radicals having a relatively long lifetime may be taken out by a
down flow method, and the taken neutral radicals are directed upon
the surface of the silicon substrate. Thus, the oxynitride film is
formed by a reaction between the neutral radicals and the surface
of the silicon substrate. According to the method, the active
species can be controlled to be one kind, and therefore the quality
of the resultant oxynitride film can be even more improved.
[0133] According to the above manufacturing methods, the treatment
temperature is set in the range from 300.degree. C. to 900.degree.
C. This is because an oxynitriding reaction with a highly reactive
active species Is employed rather than a thermal reaction, and
according to the present invention, an oxynitride film may be
formed at a temperature lower than the conventional method. It is
to be noted that the silicon oxynitride film in the present
invention may be applied to a capacitor insulator film in a stacked
capacitor for a DRAM or the like.
[0134] Since the above embodiments are described only for examples,
the present invention is not limited to the above embodiments and
various modifications or alterations can be easily made therefrom
by those skilled in the art without departing from the scope of the
present invention.
* * * * *