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TECHNICAL FIELD
The present invention relates to a semiconductor device in which an oxide film, a nitride film, an oxynitride film, or the like, is formed on a silicon semiconductor, and the fabrication method thereof.
BACKGROUND ART
The gate insulation film of a MIS (metal/insulator/silicon) transistor is required to have various high-performance electric properties and high reliability characteristics, such as low leakage current characteristics, low interface state
density, high breakdown voltage, high resistance against hot carriers, and uniform threshold voltage characteristics.
The thermal oxidation technology using oxygen molecules or water molecules at approximately 800.degree. C. or more has been used conventionally as the formation technology of the gate insulation film that satisfies the above requirements.
A thermal oxidation process has been conducted conventionally after conducting a cleaning process of removing surface contaminants, such as organic materials, metals, and particles, as a preprocessing process. In such a conventional cleaning
process, cleaning using a diluted hydrofluoric acid or hydrogenated water, for example, is performed at last, for terminating the dangling bonds existing on the silicon surface by hydrogen. Thereby, formation of a native oxide film on the silicon
surface is suppressed, and the silicon substrate thus having a clean surface is forwarded to the following thermal oxidation process. In the thermal oxidation process, the terminated hydrogen at the surface undergoes decoupling during the process of
raising the temperature of the silicon substrate in an inert gas atmosphere of argon (Ar), for example at a temperature equal to or more than 600.degree. C., approximately. Then, oxidization of the silicon surface is conducted at approximately
800.degree. C. or more in an atmosphere to which oxygen molecules or water molecules are introduced.
Conventionally, in a case where a silicon oxide film is formed on the silicon surface by using such a thermal oxidization technique, satisfactory oxide film/silicon interface characteristics, high breakdown voltage of the oxide film, leakage
current characteristics, and the like, are achieved only in the case where a silicon surface having the (100) orientation is used. Further, remarkable degradation of leak current occurs in the case where the thickness of the silicon oxide film formed by
the conventional thermal oxidation process is reduced to approximately 2 nm or less. Thus, it has been difficult to realize a high-performance miniaturized transistor that requires decrease of the gate insulation film thickness.
Further, in a crystal silicon having a surface orientation other than the (100) orientation or a polycrystalline silicon generally having a primarily (111)-oriented surface on an insulation film, interface state density at the oxide film/silicon
interface is remarkably high as compared with the silicon oxide film formed on the (100)-oriented silicon even when the silicon oxide film is formed by using the thermal oxidation technology. Thus, a silicon oxide film having a reduced film thickness
possesses poor electric properties in terms of breakdown characteristics, leakage current characteristics, and the like. Hence, there has been a need of increasing the film thickness of the silicon oxide film when using such a silicon oxide film.
Meanwhile, the use of large-diameter silicon wafer substrate or large-area glass substrate is increasing these days for improving the efficiency of semiconductor device production. In order to form transistors on the entire surface of such a
large-size substrate with uniform characteristics and with high throughput, an insulation film forming process conducted at a low temperature so as to decrease the magnitude of the temperature change in heating or cooling and, further, having small
temperature dependence is required. In the conventional thermal oxidation process, there has been a large fluctuation of oxidation reaction rate with respect to temperature fluctuation, and it has been difficult to produce semiconductor devices with
high throughput while using a large-area substrate.
In order to solve these problems associated with the conventional thermal oxidation technology, multitudes of low-temperature film formation processes have been attempted. Among others, the technology disclosed in Japanese Laid-Open Patent
Publication No. 11-279773 or the technology disclosed in Technical Digest of International Electron Devices Meeting, 1999, pp. 249-252, or in 2000 Symposium on VLSI Technology Digest of Technical Papers, pp. 76-177, describes a process in which an
inert gas is introduced into plasma together with gaseous oxygen molecules, thereby effectively causing the inert gas having a large metastable level to conduct the atomization of the oxygen molecules. Hence, relatively good electronic properties are
achieved.
In these technologies, a microwave is irradiated to the mixed gas formed of krypton (Kr) that is an inert gas and an oxygen (O.sub.2) gas, the mixed plasma of Kr and O.sub.2 is generated, and a large amount of atomic state oxygen O* are formed.
Then, the oxidation of silicon is conducted at a temperature of about 400.degree. C., and low leakage current characteristics, low interface state density, and high breakdown voltage comparable to those of the conventional thermal oxidation are
achieved. Further, according to this oxidation technology, a high-quality oxide film is obtained also on the silicon surface having a surface orientation other than the (100) surface.
However, in such a conventional silicon oxide film formation technology using the microwave-excited plasma, in spite of the fact that the oxidation is conducted by using atomic state oxygen O*, only a silicon oxide film having electric properties
comparable to those obtained by the conventional thermal oxidation process that uses oxygen molecules or water molecules is obtained. Particularly, it has been impossible to obtain the good low leakage current characteristics in the silicon oxide film
having a thickness of approximately 2 nm or less on the silicon substrate surface. Thus, it has been difficult to realize high-performance, miniaturized transistors that require further decrease of the gate insulation film thickness, similarly to the
case of conventional thermal oxide film formation technology.
Further, there has been a problem that degradation of conductance caused by hot carrier injection into the oxide film of a transistor, or degradation of electric properties with time such as increase of leakage current, in a device that causes
tunneling of electrons through the silicon oxide film as in the case of a flash memory, occur more noticeably than in the case where the silicon oxide film is formed by the conventional thermal processes.
DISCLOSURE OF THE INVENTION
Accordingly, a general object of the present invention is to provide a novel and useful semiconductor device and a fabrication method thereof in which the problems described above are eliminated.
A more specific object of the present invention is to provide a low-temperature plasma oxidation technology as an alternative to the conventional thermal oxidation technology.
Another object of the present invention is to provide high-quality insulation film formation technology at low temperatures that can be applied to silicon surfaces of every orientation.
Still another object of the present invention is to provide reliable, high performance, and miniaturized semiconductor devices using such high-quality insulation film formation technology at low temperatures, particularly, transistor integrated
circuit device, flash memory devices, and three dimensional integrated circuit devices provided with a plurality of transistors or various function elements, and to provide a fabrication method thereof.
A further object of the present invention is to provide a semiconductor device comprising a silicon compound layer formed on a silicon surface,
wherein the silicon compound layer contains at least a predetermined inert gas and has a hydrogen content of 10.sup.11 /cm.sup.2 or less in terms of surface density.
Another object of the present invention is to provide a semiconductor memory device comprising, on a common substrate, a transistor including a polysilicon film formed on a silicon surface via a first silicon compound layer, and a capacitor
including a second silicon compound layer formed on a polysilicon surface,
wherein each of the first and second silicon compound layers contains at least a predetermined inert gas and has a hydrogen content of 10.sup.11 /cm.sup.2 or less in terms of surface density.
Another object of the present invention is to provide a semiconductor device having a polysilicon layer or amorphous silicon layer formed on a substrate as an active layer,
wherein a silicon compound layer containing at least a predetermined inert gas and having a hydrogen content of 10.sup.11 /cm.sup.2 or less in terms of surface density is formed on a surface of the silicon layer, and
the semiconductor device drives a display device formed on the substrate.
Another object of the present invention is to provide a fabrication method of a semiconductor device on a silicon surface, including the steps of:
exposing the silicon surface to a first plasma of a first inert gas so as to remove hydrogen existing on at least a part of the silicon surface in advance; and
generating a second plasma of a mixed gas of a second inert gas and one or a plurality of kinds of gaseous molecules, and forming, on the silicon surface, a silicon compound layer containing at least a part of elements constituting the gaseous
molecules under the second plasma.
Another object of the present invention is to provide a fabrication method of a semiconductor memory device having, on a common substrate, a transistor including a polysilicon film formed on a silicon surface via a first insulation film and a
capacitor including a second insulation film formed on a polysilicon surface, including the steps of:
exposing the silicon surface to a first plasma of a first inert gas so as to remove hydrogen existing on at least a part of the silicon surface in advance; and
generating a second plasma of a mixed gas of a second inert gas and one or a plurality of kinds of gaseous molecules, and forming, on the silicon surface, a silicon compound layer containing at least a part of elements constituting the gaseous
molecules as the first insulation film under the second plasma.
Another object of the present invention to provide a fabrication method of a semiconductor device having a polysilicon layer or amorphous silicon layer on a substrate as an active layer, including the steps of:
forming, on said substrate, a silicon layer formed by said polysilicon layer or amorphous layer;
exposing a surface of said silicon layer to a plasma of a first inert gas so as to remove hydrogen existing on at least a part of said surface of said silicon layer; and
generating a plasma of a mixed gas of a second inert gas and one or a plurality of kinds of gaseous molecules and forming, on said surface of said silicon layer, a silicon compound layer including at least a part of elements constituting said
gaseous molecules.
According to the present invention, it becomes possible to completely remove surface-terminating hydrogen even at low temperature of about 400.degree. C. or less in continuous processing without breaking vacuum and without degrading the
planarity of a silicon surface. Hence, it is possible to form a silicon oxide film, silicon nitride film, and silicon oxynitride film, having characteristics and reliability superior to those of a silicon oxide film formed by a conventional thermal
oxidation process or microwave plasma processing, on a silicon of any surface orientation at low temperature of about 500.degree. C. or less. Consequently, it becomes possible to realize a miniaturized transistor integrated circuit having high
reliability and high performance.
Also, according to the present invention, it becomes possible to form a thin and high-quality silicon oxide film, silicon nitride film, and silicon oxynitride film having good characteristics such as leakage current and breakdown voltage even on
a silicon surface of a corner part of a device isolation sidewall of, for example, a shallow-trench isolation or on a silicon surface having a surface form with projections and depressions. Consequently, it becomes possible to achieve high-density
device integration with a narrowed device isolation width and high-density device integration having a three-dimensional structure.
In addition, by using the gate insulation film of the present invention, it was possible to realize a flash memory device and the like capable of significantly increasing the number of times of rewriting.
Further, according to the present invention, it becomes possible to form a high-quality silicon gate oxide film and silicon gate nitride film even on a polysilicon formed on an insulation film and having a predominantly (111)-oriented surface.
As a result, it becomes possible to realize a display apparatus that uses a polysilicon transistor having high driving ability, and further, a three-dimensional integrated circuit device in which a plurality of transistors or functional devices are
stacked.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG.1 is a conceptual diagram of a plasma apparatus that uses a radial line slot antenna;
FIG. 2 is a characteristic diagram showing the dependence of the bond formed between the surface-terminating hydrogen at a silicon surface and silicon on the exposure to Kr plasma as measured by an infrared spectroscopy;
FIG. 3 is a characteristic diagram showing the dependence of silicon oxide film thickness on the gas pressure of the processing chamber;
FIG. 4 is a characteristic diagram showing the depth distribution profile of the Kr density in the silicon oxide film;
FIG. 5 is a characteristic diagram showing the current versus voltage characteristic of the silicon oxide film;
FIG. 6 is a diagram showing the relationship between the leakage current characteristics of the silicon oxide film and the silicon oxynitride film, and the film thickness;
FIG. 7 is a characteristic diagram showing the dependence of the silicon nitride film thickness on the gas pressure of the processing chamber;
FIG. 8 is a characteristic diagram showing the photoemission intensity of atomic state oxygen and atomic state hydrogen at the time of formation of the silicon oxynitride film;
FIG. 9 is a characteristic diagram showing the elemental distribution in the silicon oxynitride film;
FIG. 10 is a characteristic diagram showing the current versus voltage characteristic of the silicon oxynitride film;
FIGS. 11A-11C are conceptual cross-sectional views of the shallow trench isolation;
FIG. 12 is a cross-sectional view of a three-dimensional transistor formed on a silicon surface having projections and depressions;
FIG. 13 is a schematic diagram of a cross-sectional structure of a flash memory device;
FIG. 14 is a schematic cross-sectional view for explaining the fabrication method of the flash memory device of the present invention step by step;
FIG. 15 is a schematic cross-sectional view for explaining the fabrication method of the flash memory device of the present invention step by step;
FIG. 16 is a schematic cross-sectional view for explaining the fabrication method of the flash memory device of the present invention step by step;
FIG. 17 is a schematic cross-sectional view for explaining the fabrication method of the flash memory device of the present invention step by step;
FIG. 18 is a schematic diagram of a cross-sectional structure of a MOS transistor formed on a metal substrate SOI;
FIG. 19 is a conceptual diagram of a plasma apparatus accommodated to a glass substrate or plastic substrate;
FIG. 20 is a schematic diagram of a cross-sectional structure of a polysilicon transistor on an insulation film; and
FIG. 21 is a conceptual diagram of a cross-sectional structure of a three-dimensional LSI.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, various preferable embodiments in which the present invention is applied will be explained in detail with reference to the drawings.
(First Embodiment)
First, a description will be given of an oxide film formation at low temperatures by using plasma.
FIG. 1 is a cross-sectional view showing an example of a plasma processing apparatus used in the present invention and using a radial line slot antenna.
In this embodiment, in order to remove the hydrogen terminating the dangling bonds at a silicon surface, Kr, which is used as the plasma excitation gas in the subsequent oxide film formation process, is used, and the removal process of the
surface-terminating hydrogen and the oxidation process are conducted in the same processing chamber continuously.
First, a vacuum vessel (processing chamber) 101 is evacuated and an Ar gas is introduced first from a shower plate 102. Then, the gas is changed to the Kr gas. Further, the pressure inside the processing chamber 101 is set to about 13.3 Pa (1
Torr).
Next, a silicon substrate 103 is placed on a stage 104 having a heating mechanism, and the temperature of a specimen is set to about 400.degree. C. As long as the temperature of the silicon substrate 103 is in the range of 200-500.degree. C.,
almost the same results explained as below are obtained. It should be noted that the silicon substrate 103 is cleaned by a diluted hydrofluoric acid in the preprocessing step immediately before and, as a result, the silicon dangling bonds on the surface
are terminated by hydrogen.
Next, a microwave having the frequency of 2.45 GHz is supplied to a radial line slot antenna 106 from a coaxial waveguide 105, wherein the microwave is introduced into the processing chamber 101 from the radial line slot antenna 106 through a
dielectric plate 107 provided on a part of the wall of the processing chamber 101. The introduced microwave causes excitation of the Kr gas that is introduced into the processing chamber 101 from the shower plate 102 and, consequently, there is induced
high-density Kr plasma right underneath the shower plate 102. As long as the frequency of the microwave to be supplied is in the range of about 900 MHz or more but not exceeding about 100 GHz, almost the same results explained as below are obtained.
In the construction of FIG. 1, the interval between the shower plate 102 and the substrate 103 is set to 6 cm in this embodiment. The narrower the interval, the faster the film formation becomes. This embodiment shows the example of film
formation by using the plasma apparatus that uses the radial line slot antenna, however, it should be noted that the plasma may be induced by introducing the microwave into the processing chamber by other methods.
By exposing the silicon substrate 103 to the plasma thus excited by the Kr gas, the surface of the silicon substrate 103 is subjected to irradiation of low energy Kr ions, and the surface-terminating hydrogen are removed.
FIG. 2 shows the result of analysis of the silicon-hydrogen bond on the surface of the silicon substrate 103 by means of infrared spectrometer and shows the effect of removal of the surface-terminating hydrogen at the silicon surface by the Kr
plasma induced by introducing the microwave into the processing chamber 101 under the pressure of 13.3 Pa (1 Torr) with the power of 1.2 W/cm.sup.2.
Referring to FIG. 2, it can be seen that the optical absorption at about 2100 cm.sup.-, which is characteristic to the silicon-hydrogen bond, is more or less vanished after the Kr plasma irradiation conducted for only about 1 second, and is
almost completely vanished after irradiation for approximately thirty seconds. That is, the surface-terminating hydrogen on the silicon surface can be removed by the Kr plasma irradiation conducted for approximately 30 seconds. In this embodiment, the
surface-terminating hydrogen is completely removed by conducting the Kr plasma irradiation for 1 minute.
Next, a Kr/O.sub.2 mixed gas is introduced from the shower plate 102 with a partial pressure ratio of 97/3. On this occasion, the pressure of the processing chamber is maintained at about 13.3 Pa (1 Torr). In the high-density excitation plasma
in which the Kr gas and the O.sub.2 gas are mixed, Kr* in the intermediate excitation state and the O.sub.2 molecules cause collision, and it is possible to efficiently form atomic state oxygen O* in large amount.
In this embodiment, with the atomic state oxygen O* thus formed, the surface of the silicon substrate 103 is oxidized. In the conventional thermal oxidation methods conducted on a silicon surface, the oxidation is caused by O.sub.2 molecules or
H.sub.2 O molecules and a very high processing temperature of 800.degree. C. or more has been needed. In the oxidation processing of the present invention conducted by the atomic state oxygen, the oxidation is possible at a very low temperature of
about 400.degree. C. In order to facilitate the collision of Kr* and O.sub.2, it is preferable that the processing chamber pressure be high. However, under excessively high pressure, the O* thus formed collide mutually and return to O.sub.2 molecules.
Obviously, there exists an optimum gas pressure.
FIG. 3 shows the relationship between the thicknesses of the oxide film formed and the internal pressure of the processing chamber of the case where the gas pressure inside the processing chamber 101 is changed while maintaining the Kr/O.sub.2
pressure ratio inside the processing chamber to 97/3. In FIG. 3, the temperature of the silicon substrate 103 is set to 400.degree. C. and 10 minutes oxidation processing is conducted.
Referring to FIG. 3, it can be seen that the oxidation rate becomes maximum when the pressure inside the processing chamber 101 is approximately 13.3 Pa (1 Torr) and that this pressure or the pressure condition near this is optimum. This optimum
pressure is not limited to the case where the silicon substrate 103 has the (100) surface orientation but is the same also in other cases where the silicon surface has any other surface orientations.
After the silicon oxide film of the desired film thickness is formed, the introduction of the microwave power is shutdown and the plasma excitation is terminated. Further, the Kr/O.sub.2 mixed gas is replaced with the Ar gas, and the oxidation
processing is terminated. It should be noted that the use of the Ar gas before and after this step is intended to use a gas cheaper than Kr for the purging gas. The Kr gas used in the this step is recovered and reused.
Following the above oxide film formation, a semiconductor integrated circuit device including transistors and capacitors is completed after conducting electrode formation processing, passivation film formation processing, hydrogen sintering
processing, and the like.
The result of measurement of hydrogen content in the silicon oxide film formed according to the foregoing processing indicates that the hydrogen content is about 10.sup.12 /cm.sup.2 or less in terms of surface density in the case where the
silicon oxide film has a thickness of 3 nm, wherein it should be noted that the foregoing measurement was conducted by measuring the hydrogen release caused with temperature rise. Particularly, it was confirmed that the oxide film characterized by a
small leakage current shows that the hydrogen content in the silicon oxide film is about 10.sup.11 /cm.sup.2 or less in terms of surface density. On the other hand, the oxide film not exposed to the Kr plasma before the oxide film formation contained
hydrogen with the surface density exceeding 10.sup.12 /cm.sup.2.
Further, the comparison was made between the roughness of the silicon surface after the oxide film formed according to the foregoing processing was removed and the roughness of the silicon surface before the oxide film formation wherein the
measurement of the surface roughness was made by using an atomic force microscope. It was confirmed that there is caused no change of surface roughness. That is, there is caused no roughening of silicon surface even when the oxidation processing is
conducted after the removal of the surface-terminating hydrogen.
FIG. 4 shows the depth profile of Kr density in the silicon oxide film formed according to the foregoing processing as measured by the total reflection X-ray fluorescent spectrometer. It should be noted that FIG. 4 shows the result for the
silicon (100) surface, however, this result is not limited to the (100) surface and a similar result is obtained also in other surface orientations.
In the experiment of FIG. 4, the partial pressure of oxygen in Kr is set to 3% and the pressure of the processing chamber is set to 13.3 Pa (133 Torr). Further, the plasma oxidation processing is conducted at the substrate temperature of
400.degree. C.
Referring to FIG. 4, the Kr density in the silicon oxide film increases with increasing distance from the underlying silicon surface and reaches the value of about 2.times.10.sup.11 /cm.sup.2 at the surface of the silicon oxide film. This
indicates that the silicon oxide film obtained according to the foregoing processing is a film in which the Kr concentration is constant in the film in the region where the distance to the underlying silicon surface is 4 nm or more and in which the Kr
concentration decreases toward the silicon/silicon oxide interface in the region within the distance of 4 nm from the silicon surface.
FIG. 5 shows the dependence of the leakage current on the applied electric field for the silicon oxide film obtained according to the foregoing process. It should be noted that the result of FIG. 5 is for the case where the thickness of the
silicon oxide film is 4.4 nm. For the purpose of comparison, FIG. 5 also shows the leakage current characteristic of the oxide film of the same thickness in the case where no exposure to the Kr plasma was conducted before the formation of the oxide
film.
Referring to FIG. 5, the leakage current characteristic of the silicon oxide film not exposed to the Kr plasma is equivalent to the leakage current characteristic of the conventional thermal oxide film. This means that the Kr/O.sub.2 microwave
plasma oxidation processing does not improve the leakage current characteristics of the oxide film thus obtained very much. On the other hand, in the oxide film formed according to this embodiment where the oxidation processing is conducted by
introducing the Kr/O.sub.2 gas after removing the terminated hydrogen by the Kr plasma irradiation, it can be seen that the leakage current is improved by the order of 2 or 3 as compared with the leakage current of the silicon oxide film formed by the
conventional microwave plasma oxidation processing when measured at the same electric field, indicating that the silicon oxide film formed by this embodiment has excellent low leakage characteristics. It is further confirmed that a similar improvement
of leakage current characteristic is achieved also in the silicon oxide film having a much thinner film thickness of up to about 1.7 nm.
FIG. 6 shows the result of measurement of the leakage current characteristics of the silicon oxide film of this embodiment for the case where the thickness of the silicon oxide film is varied. In FIG. 6, .DELTA. shows the leakage current
characteristic of a conventional thermal oxide film, .largecircle. shows the leakage current characteristic of the silicon oxide film formed by conducting the oxidation by the Kr/O.sub.2 plasma while omitting the exposure process to the Kr plasma, and
.circle-solid. shows the leakage current characteristic of the silicon oxide film of this embodiment in which the oxidation is conducted by the Kr/O.sub.2 plasma after exposure to the Kr plasma. In FIG. 6, it should be noted that the data represented
by .box-solid. show the leakage current characteristic of an oxynitride film to be explained later.
Referring to FIG. 6, it can be seen that the leakage current characteristic of the silicon oxide film represented by .largecircle. and formed by the plasma oxidation processing while omitting the exposure process to the Kr plasma coincides with
the leakage current characteristic of the thermal oxide film represented by .DELTA., while it can be seen also that the leakage current characteristics of the silicon oxide film of this embodiment and represented by .circle-solid. is reduced with
respect to the leakage current characteristics represented by .largecircle. by the order of 2-3. Further, it can be seen that a leakage current of 1.times.10.sup.-2 A/cm.sup.2, which is comparable to the leakage current of the thermal oxide film having
the thickness of 2 nm, is achieved in the silicon oxide film of this embodiment even when the thickness thereof is approximately 1.5 nm.
Further, the measurement of the surface orientation dependence conducted on the silicon/silicon oxide interface state density for the silicon oxide film obtained by this embodiment has revealed the fact that a very low interface state density of
approximately 1.times.10.sup.10 cm.sup.-2 eV.sup.-1 is obtained for any silicon surface of any surface orientation.
Further, the oxide film formed by this embodiment shows equivalent or superior characteristics as compared with the conventional thermal oxide film with regard to electric and reliability characteristics, such as breakdown voltage
characteristics, hot carrier resistance, electric charges QBD (Charge-to-Breakdown) up to the failure of the silicon oxide film when a stress current is applied.
As described above, it is possible to form a silicon oxide film on a silicon of any surface orientation even at a low temperature of 400.degree. C. by conducting the silicon oxidation processing by the Kr/O.sub.2 high-density plasma after
removal of the surface-terminating hydrogen. It is thought that such an effect is achieved because of the reduced hydrogen content in the oxide film caused as a result of the removal of the terminating hydrogen and because of the fact that the oxide
film contains Kr. Because of the reduced amount of hydrogen in the oxide film, it is believed that weak element bonding is reduced in the silicon oxide film. Further, because of the incorporation of Kr in the film, the stress inside the film and
particularly at the Si/SiO.sub.2 interface is relaxed, and there is caused a reduction of electric charges in the film or interface state density. Consequently, the electric properties of the silicon oxide film is significantly improved.
Particularly, it is believed that reducing the hydrogen concentration to the level of 10.sup.12 /cm.sup.2 or less, preferably to the level of 10.sup.11 cm.sup.2 or less, and incorporation of Kr with a concentration of about 5.times.10.sup.11
/cm.sup.2 or less in terms of the surface concentration are thought to contribute to the improvement of electric properties and reliability characteristics of the silicon oxide film.
In order to realize the oxide film of the present invention, in addition to the apparatus of FIG. 1, it is also possible to use a plasma processing apparatus capable of conducting the oxide film formation at low temperatures by using plasma. For
example, it is also possible to use a two-stage shower plate-type plasma processing apparatus that is provided with a first gas release structure releasing the Kr gas for plasma excitation by a microwave and a second gas release structure that is
different from the first gas release structure and releases the oxygen gas.
In this embodiment, it should be noted that the oxidation processing is terminated such that the feeding of the microwave power is shutdown and plasma excitation is finished upon formation of the silicon oxide film to a desired film thickness,
followed by the process of replacing the Kr/O.sub.2 mixed gas with the Ar gas. However, it is also possible to introduce a Kr/NH.sub.3 mixed gas from the shower plate 102 with the partial pressure ratio of 98/2 before shutting down the microwave power
while maintaining the pressure at about 13.3 Pa (1 Torr), and terminate the processing when a silicon nitride film of approximately 0.7 nm is formed on the surface of the silicon oxide film. According to such a method, a silicon oxynitride film in which
a silicon nitride film is formed on the surface thereof is obtained, and thus it becomes possible to form an insulation film having a higher specific dielectric constant.
(Second Embodiment)
Next, a description will be given of nitride film formation at low temperatures by using plasma. An apparatus similar to that shown in FIG. 1 is used for the nitride film formation.
In this embodiment, it is preferable to use Ar or Kr for the plasma excitation gas for removing the terminating hydrogen and for the nitride film formation, in order to form a high-quality nitride film.
Hereinafter, an example of using Ar will be represented.
First, the interior of the vacuum vessel (processing chamber) 101 is evacuated to vacuum and an Ar gas is introduced from the shower plate 102 such that the pressure inside the processing chamber is set to about 13.3 Pa (100 mTorr).
Next, the silicon substrate 103, subjected to hydrogenated water cleaning and the silicon dangling bonds at the surface are terminated by hydrogen in the preprocessing step immediately before, is introduced into the processing chamber 101 and is
placed on the stage 104 having the heating mechanism. Further, the temperature of the specimen is set to 500.degree. C. As long as the temperature is in the range of 300-550.degree. C., results almost the same as the one described below are obtained.
Next, a microwave of 2.45 GHz is supplied into the processing chamber from the coaxial waveguide 105 via the radial line slot antenna 106 and the dielectric plate 107 and a high-density plasma of Ar is generated in the processing chamber. As
long as the frequency of the supplied microwave is in the range of about 900 MHz or more but not exceeding about 10 GHz, results almost the same as the one described below are obtained. The interval between the shower plate 102 and the substrate 103 is
set to 6 cm in this embodiment. With decreasing interval, faster deposition rate becomes possible. While this embodiment shows the example of film formation by a plasma apparatus that uses the radial line slot antenna, it is also possible to introduce
the microwave into the processing chamber by other methods.
The silicon surface thus exposed to the plasma excited based on an Ar gas is subjected to bombardment of low energy Ar ions, and the surface-terminating hydrogen are removed. In this embodiment, the Ar plasma exposure is conducted for 1 minute.
Next, an NH.sub.3 gas is introduced and mixed to the Ar gas from the shower plate 102 with a partial pressure ratio of 2%. On this occasion, the pressure of the processing chamber is held at about 13.3 Pa (100 mTorr). In excited high-density
plasma in which the Ar gas and the NH.sub.3 gas are mixed, there are caused collision of Ar* in the intermediate excited state and the NH.sub.3 molecules, and NH* radicals are formed efficiently. The NH* radicals cause nitridation of the silicon
substrate surface.
Upon formation of the silicon nitride film with a desired thickness, the introduction of the microwave power is shutdown and the excitation of the plasma is terminated. Further, the Ar/NH.sub.3 mixed gas is replaced with the Ar gas and the
nitridation processing is terminated.
Further, following the above nitride film formation, electrode formation processes, passivation film formation processes, hydrogen sintering processes, and the like are conducted, and a semiconductor integrated device that includes transistors
and capacitors is completed.
While this embodiment showed the example in which the nitride film is formed by the plasma apparatus that uses the radial line slot antenna, it is also possible to introduce the microwave into the processing chamber by other methods. In
addition, while this embodiment uses Ar for the plasma excitation gas, similar results are obtained also when Kr is used. Further, while this embodiment uses NH.sub.3 for the plasma process gas, it is also possible to use a mixed gas of N.sub.2 and
H.sub.2 for this purpose.
In the silicon nitride film formation process of the present invention, it is one of important requirements that there remains hydrogen in the plasma even after the surface-terminating hydrogen are removed. As a result of existence of hydrogen
in the plasma, the dangling bonds inside the silicon nitride film as well as the dangling bond at the interface are terminated by forming Si--H bonds or N--H bond. Consequently, electron traps are eliminated from the silicon nitride film and the
interface.
The existence of the Si--H bond and the N--H bond in the nitride film of the present invention is confirmed respectively by infrared absorption spectroscopy and by X-ray photoelectron spectroscopy. As a result of existence of hydrogen, the
hysteresis in the CV characteristics is eliminated and the interface state density at the silicon/silicon nitride film is suppressed to 2.times.10.sup.10 cm.sup.-2. In the case of forming the silicon nitride film by using a rare gas (Ar or Kr) and an
N.sub.2 /H.sub.2 mixed gas, it is possible to suppress the traps of electrons or holes in the film drastically by setting the partial pressure of the hydrogen gas to 0.5% or more.
FIG. 7 shows the pressure dependence of the silicon nitride film thickness formed according to the process described above. In the experiment of FIG. 7, it should be noted that the Ar/NH.sub.3 partial pressure ratio was set to 98/2 and the
deposition time was 30 minutes.
Referring to FIG. 7, it can be seen that there occurs an increase of deposition rate of the nitride film by reducing the pressure in the processing chamber and thus by increasing the energy given to NH.sub.3 (or N.sub.2 /H.sub.2) by the rare gas
(Ar or Kr) From the viewpoint of efficiency of nitride film formation, it is preferable that the gas pressure be in the range of 6.65-13.3 Pa (50-100 mTorr). However, from the viewpoint of productivity, it is preferable to use a unified pressure
suitable to the oxidation, for example, 133 Pa (1 Torr), also for nitridation, in the process where oxidation and nitridation are continuously conducted, as will be explained in other embodiments. Additionally, it is preferable that the partial pressure
of NH.sub.3 (or N.sub.2 /H.sub.2) in the rare gas be in the range of 1-10%, more preferably, in the range of 2-6%.
It should be noted that the silicon nitride film obtained by this embodiment showed the specific dielectric constant of 7.9, which value is about twice as large as the specific dielectric constant of a silicon oxide film.
Measurement of the current versus voltage characteristics of the silicon nitride film obtained by this embodiment has revealed the fact that a leakage current characteristic smaller by the order of 5-6 than that of a thermal oxide film having the
thickness of 1.5 nm is obtained in the case where the film thickness is 3.0 nm (equivalent to the oxide film thickness of 1.5 nm), under the condition that a voltage of 1V is applied. This means that it is possible to break through the limitation of
miniaturization that appears in the transistors using a silicon oxide film for the gate insulation film, by using the silicon nitride film of this embodiment.
It should be noted that the film formation condition of the nitride film described above as well as the physical and electrical properties are not limited on the (100) oriented silicon surface but are valid in the same way on the silicon of any
surface orientation including the (111) surface.
It is believed that the preferable results achieved by this embodiment are not only attained by the removal of the terminating hydrogen, but also by the existence of Ar or Kr in the nitride film. In other words, in the nitride film of this
embodiment, it is believed that Ar or Kr existing in the nitride film relaxes the stress inside the nitride film or at the silicon/nitride film interface, and as a result, the fixed electric charges in the silicon nitride film or the interface state
density is reduced, thereby significantly improving the electric properties and | | |
