JP2009188282A - Manufacturing method of high-density silicon oxide film, and silicon substrate and semiconductor device with high-density silicon oxide film manufactured by the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a high-density silicon oxide film, which suppresses the occurrence of a sub-oxide layer as much as possible without damaging a surface of a film formation substrate and improves insulating characteristics of an oxide film furthermore to make the oxide film thin, and a silicon substrate and a semiconductor device which include the high-density silicon oxide film manufactured by the manufacturing method. <P>SOLUTION: A silicon substrate 1 is heated to an arbitrary temperature in a range of 300 to 430°C under one atmosphere oxygen pressure, and an oxide film 3 is formed on a surface of the silicon substrate 1 while irradiating the silicon substrate with ultraviolet rays at ≤222 nm and making oxygen gas flow to the surface of the silicon substrate at ≥100 ml/min in terms of oxygen gas flow rate at 20°C in a flowmeter. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a high-density silicon oxide film formed on a silicon substrate by a photo-oxidation method, a silicon substrate having a high-density silicon oxide film manufactured by the manufacturing method, and a semiconductor device.

In the field of semiconductor devices, the entire miniaturization is attempted at a high speed. The gate insulating film of the element is also required to be thinner, and it is an urgent task to form a thin gate insulating film with a higher withstand voltage with high reliability in a short processing time.
Increasing the dielectric strength while reducing the thickness of the oxide film is difficult to compromise.
As is known, in the thermally oxidized SiO ₂ film, a suboxide [Si _x O _y , except x = 1, y = 2 other than that] is formed in a bonded state in which oxygen is insufficient at the Si-SiO ₂ interface. Is done.

It has been known that a suboxide layer having a thickness of about 1 to 1.5 nm is formed in the thermal oxide film, which has been a cause of deteriorating the insulating characteristics of the silicon oxide film as the film thickness is reduced.
The insulating characteristics of the oxide film depend on the thickness of the oxide film including the suboxide layer (Si _x O _y layer) and its withstand voltage.
The film thickness of the suboxide layer (Si _x O _y layer) is a key factor that determines the effective film thickness of the SiO ₂ gate insulating film, and the surface shape after the treatment of the silicon substrate by the selection of the manufacturing method and the oxide film Depends on the formation method. In other words, the surface roughness of the silicon substrate after processing is suppressed, and homogenization is achieved while increasing the film density of the oxide film.

Attempts to reduce the film thickness of the suboxide layer (Si _x O _y layer) have also been made by a rapid thermal oxidation method, a method using plasma, ozone and light. In particular, Patent Document 1 below describes that the oxide film density is set to a large value of about 2.55 g / cm ^{3 to} 2.60 g / cm ^{3 by} a plasma method.
However, in Patent Document 1, the Dry oxide film 2.55 g ^/ cm 3, Wet oxide 2.60 g / cm ³ and has been, the thermal oxide film described in one same document 2.55-2 Not exceeding 60 g / cm ³ . That is, the matter that the oxide film density is about 2.55 g / cm ^{3 to} 2.60 g / cm ³ in the plasma method described in Patent Document 1 apparently shows a large value. Therefore, it is not improved from the oxide film density by the thermal oxide film method.

The density of a silicon oxide film in the present invention to be described later (2.26-2.32 g / cm ³ at a substrate temperature of 300-430 ° C. under irradiation of 126 nm, 2.28 g / cm ³ at a substrate temperature of 430 ° C. at 222 nm), 222 nm 2.28 g / cm ³⁾ at 390 ° C. under irradiation thermal oxide film density none (2.24 - transcends 2.20 g / cm ^3). That is, the density proposed in Patent Document 1 is numerically inferior to the oxide film density of the present invention, but the oxide film density of Document 1 is based on the density of the thermal oxide film used as a reference in the present invention. Is the same as the thermal oxide film density described in Document 1, the oxide film prepared in Patent Document 1 is lower than the oxide film density of the present invention. The silicon oxide film density in this patent was converted from the X-ray reflectivity to the film density by the Parratt method. The density of Dry and Wet thermal oxidation in this patent is based on Applied Surface Science 172 (2001) P307, and the samples shown for this invention are measured by the same method.

In addition, the growth of the SiO ₂ film by the rapid thermal oxidation method, the plasma method, and the ozone method damages the silicon substrate surface and causes defects at the substrate-silicon oxide film interface. This flaw is inhibited homogenization of film, withstand voltage is lowered, therefore, it is impossible to reduce the thickness in the oxide film to gain on the suboxide layer (Si _x O _y layer). Furthermore, the interface between the suboxide layer (Si _x O _y layer) and the oxide film on the sub oxide layer (Si _x O _y layer) becomes inhomogeneous, and the characteristics of the oxide film also drag the defects of the sub oxide layer (Si _x O _y layer). In a thin film state with an oxide film thickness of 4 nm or less, it is impossible to maintain a high withstand voltage.

  On the other hand, the photo-oxidation method, in which oxygen is irradiated with ultraviolet rays to generate oxygen active species to oxidize silicon, does not damage the substrate surface. Research and development related to the oxidation method has been performed with the aim of improving the growth rate (see, for example, Non-Patent Document 1 and Patent Document 2). However, when different gases are mixed in oxygen gas, the film deposition rate and film thickness are improved, but the insulation is low (see Non-Patent Document 2). Since insulation and density are related, oxidation using pure oxygen gas at 1 atm also has an advantageous effect on increasing the density of the oxide film.

Applied Surface Science 168 (2000) PP. 288-291 Applied Surface Science 186 (2002) 64. JP 2000-235975 A JP 2003-224117 A

  As described above, in the photo-oxidation method in which the approach is to improve the conventional growth rate, the film formation rate is shortened, and the oxide film is formed without damaging the substrate surface, which is a feature of the manufacturing method. It has been difficult to form a highly reliable silicon oxide film that can meet the demands of improving the insulating characteristics of the film, reducing the thickness of the oxide film, and suppressing the generation of the suboxide layer.

  The present invention has been made in view of this point, and its purpose is not to damage the surface of the film formation substrate, but to improve the insulating properties by increasing the density and to suppress the generation of the suboxide layer as much as possible. A high-density silicon oxide film manufacturing method capable of reducing the thickness of the oxide film by further improving the insulating properties of the oxide film, and a silicon substrate having a high-density silicon oxide film manufactured by the manufacturing method and a semiconductor device There is to do.

In order to achieve the above object, various solutions are employed in combination.
In the photo-oxidation method of the present invention, as a basic treatment, oxygen is irradiated with ultraviolet rays to generate oxygen active species to oxidize silicon. Although this photo-oxidation method does not damage the substrate surface, the film formation speed, which has been a problem in the past, is mainly affected by the heating temperature of the silicon substrate, the wavelength used for ultraviolet rays, and the blowing of oxygen gas. A processing step in which conditions related to the flow rate are selected and improved is taken.

Specifically:
In the present invention, a silicon substrate is heated to an arbitrary temperature within a range of 300 ° C. to 430 ° C. in an atmosphere of 1 atm oxygen, and oxygen gas at a predetermined flow rate is supplied to the substrate surface in a state where ultraviolet rays of 222 nm or less are irradiated to oxygen. An oxide film (SiO ₂ ) is formed while flowing through the substrate.
What is important is that a certain amount of oxygen gas has to flow on the surface of the heated silicon substrate in order to increase the density of the oxide film (SiO ₂ ).

The flow rate of oxygen gas sprayed on the substrate surface is 100 ml / min (seconds) or more in the growth layer of the silicon substrate in terms of oxygen gas flow rate at 20 ° C. in a flow meter. At this time, if the surface temperature of the silicon substrate is lower by 10 ° C., the density is lowered by 0.01 g / cm ³ or more. Therefore, it is necessary to heat and keep the silicon substrate so that the surface temperature of the silicon substrate does not decrease. Film density of the oxide film which has now become a problem, but will be described ^later, a value in the range of ^{2.27g / cm 3 ~2.32g / cm 3} , there is only distance 0.05g / cm ^3, 10 ℃ Therefore, a decrease in density of 0.01 g / cm ³ or more presents an important problem that must be avoided.

In order to increase the density of the oxide film (SiO ₂ ), the substrate temperature and the oxygen flow rate are very important factors.
Further, while the oxygen gas is being blown, the oxygen gas is kept warm or heated so that the surface temperature of the substrate does not drop by flowing the oxygen gas.
If the oxidation is performed in a state in which oxygen gas is confined at 1 atm as in the prior art, the density is lower by 0.1 g / cm ³ or more than the case of oxidation by flowing oxygen as in the present invention. For this reason as well, the control of the surface temperature of the silicon substrate and the flow rate of the oxygen gas spray is an important factor for increasing the density of the oxide film (SiO ₂ ).

In consideration of the above conditions, the flow rate of the oxygen gas can be set to the same value as the arbitrary temperature A within the range of 300 ° C. to 430 ° C. that is the heating temperature of the silicon substrate when the oxygen gas is heated and kept warm. , And can be set to an arbitrary flow rate value of 100 ml / min or more.
Furthermore, when the heating / kneading temperature of the oxygen gas to be blown exceeds 430 ° C., which is the heating temperature of the silicon substrate, the flow rate value is 100 ml / min or more if the temperature difference between the two is within 10 ° C. However, it is also possible to manufacture by restricting the flow rate so that the temperature difference approaches 0 ° C.

When the above method is used, the silicon substrate of the present invention is formed on a substrate having a substrate surface having no damage on a suboxide layer having a film thickness of 0.1 nm or less (Si _x O _y layer, where x = 1, y In other words, a high-density oxide film having a film density of 2.27 g / cm ³ or more is provided.

A silicon substrate manufactured by applying the above manufacturing method is configured by providing an oxide film manufactured by the above-described high-density silicon oxide film manufacturing method on a silicon substrate.
Furthermore, the semiconductor device includes a silicon substrate manufactured by applying the above manufacturing method. This semiconductor device means a single semiconductor element or an IC or LSI in which a large number of them are incorporated.

In a silicon oxide film formed by a conventional thermal oxidation method or a conventional photo-oxidation method, a suboxide layer containing Si ₂ O, SiO, Si ₂ O ₃ is formed to a thickness of several nm, and the composition of SiO ₂ silicon oxide film is also conceivable density is lower (2.20 g / cm ³ by wet oxidation, 2.24 g / cm ³ order by dry oxidation) and. In such a silicon oxide film, the leakage (leakage) current takes a large value, and the withstand voltage is also lowered.
The silicon oxide film is thought to be formed by multiple types of rings with Si-O bonds connected in a ring, but it contains a large amount of low-density rings, in other words, rings that occupy a large space. Therefore, the conventional silicon oxide film cannot be densified.

On the other hand, the manufacturing method of the high-density silicon oxide film of the present invention and the silicon substrate and semiconductor device having the high-density silicon oxide film manufactured by the manufacturing method are 2.27 g / cm ³ or more, more preferably 2.30 g / cm. ^The silicon oxide film has a higher density than ³ and is packed with a smaller ring (Si _x O _y layer, except x = 1 and y = 2). Also, the thickness of the suboxide layer formed at the interface is reduced to several atomic layers or less. According to this silicon oxide film, high withstand voltage and low leakage current characteristics can be realized. A silicon substrate using this silicon oxide film mainly has an appropriate wavelength of ultraviolet light to be irradiated. Since the heating temperature and the heating / retaining temperature of the sprayed oxygen gas are controlled to be the same, the surface of the substrate is prevented from becoming rough and the formation of the suboxide layer is suppressed, and the oxide film on the layer is suppressed. The density can be increased.

  As a result, the method for producing a high-density silicon oxide film of the present invention and the silicon substrate and semiconductor device having the high-density silicon oxide film produced by the production method do not damage the surface of the film-forming substrate, and a suboxide layer is generated. As much as possible, the insulating properties of the oxide film can be further improved, and the oxide film can be made thinner.

The present invention has the following features.
The method for producing a silicon substrate of the present invention basically includes:
(A) A silicon substrate is placed in a 1 atmosphere oxygen atmosphere,
(B) heating the substrate to any temperature within the range of 300 ° C. to 430 ° C .;
(C) Irradiating the substrate with ultraviolet rays of 222 nm or less,
(D) A step of flowing oxygen gas through the growth layer on the substrate surface at a rate of 100 ml / min or more in terms of an oxygen gas flow rate at 20 ° C. using a flow meter.

By the above manufacturing method, without damaging the substrate surface, the film density of the oxide film is improved to improve the insulating characteristics, the oxide film is thinned, and the silicon oxide film that can suppress the generation of the suboxide layer is formed. be able to.
During production, unless oxygen is allowed to flow to some extent, SiO ₂ is not densified. At that time, if the surface temperature of the substrate is lower by 10 ° C., the density becomes lower than 0.01 g / cm ^3, so that the substrate surface temperature is kept constant. If the oxidation is performed in a state where oxygen gas is contained at 1 atm as in the prior art, the film density is lower by 0.1 g / cm ³ or more than the case where the oxidation is performed by flowing oxygen as in the present invention.

further,
(E) A step of heating and keeping the oxygen gas sprayed on the substrate surface in the step (d) is added.
By adding this step, the growth of the oxide film can be satisfactorily maintained so that the surface temperature of the substrate does not decrease even when oxygen gas is allowed to flow.
Unless oxygen gas for spraying is flowed to some extent, the oxide film (SiO ₂ ) is not densified.

The flow rate of oxygen gas can be set to any flow rate of 100 ml / min or more when the heating / retaining temperature of the oxygen gas to be sprayed can be set to be the same as any temperature within the range of 300 ° C. to 430 ° C. which is the heating temperature of the silicon substrate. Can be set to a value.
Furthermore, when the heating / keeping temperature of the oxygen gas to be blown exceeds 430 ° C. which is the heating temperature of the silicon substrate, an arbitrary flow rate value of 100 ml / min or more is possible if the temperature difference between the two is within 10 ° C. It is also possible to set to.

At the time of manufacture, when a silicon substrate is heated from 300 ° C. to 430 ° C. in 1 atmosphere oxygen under ultraviolet irradiation of 222 nm or less, the knowledge that the density of the thermally oxidized SiO ₂ film is higher than 2.24 to 2.26 g / cm ³ is Already got.
The current maximum oxide film density was 2.31 g / cm ³ when irradiated with ultraviolet rays of 126 nm and a substrate heating temperature of 385 ° C.

In order to increase the density of the oxide film (SiO ₂ ), the heating temperature of the silicon substrate, the irradiation wavelength of the ultraviolet rays, and the flow rate of the oxygen gas to be blown are very important factors.
The oxygen gas needs to flow through the growth layer on the surface of the silicon substrate at a rate of 100 ml / min or more in terms of the oxygen gas flow rate at 20 ° C. in a flow meter. However, the oxygen substrate does not lower the surface temperature of the substrate. As such, oxygen gas is heated and kept warm.
At the time of spraying, if the surface temperature of the silicon substrate is 10 ° C. lower than the predetermined temperature, the film density becomes 0.01 g / cm ³ or more, so the substrate surface temperature needs to be constant.
When oxidation is performed in a state where oxygen gas is contained at 1 atm as in the prior art, the density is lower by 0.1 g / cm ³ or more than when oxidation is performed by flowing oxygen as in the present invention.

From the above, it can be seen that the control of the substrate surface temperature, the wavelength of ultraviolet rays, and the flow rate of oxygen gas are important factors for increasing the density of the oxide film (SiO ₂ ) and reducing the suboxide layer.
When the flow rate of the blowing oxygen gas is 100 ml / min in terms of the oxygen gas flow rate at 20 ° C. in a flow meter, the oxide film (SiO ₂ ) density at 126 nm ultraviolet irradiation is 2.30 g / cm at a substrate temperature of 350 ° C. ^3, 390 2.29 g ^/ cm 3, 430 ° C. at ° C., at 2.27 g ^/ cm 3, 450 ° C., was 2.24 g / cm ^3.
To explain a conventional example for comparison, the density of conventional thermal oxidation SiO _2, 2.24g / cm ³ in dry oxidation, a 2.20 g / cm ³ by wet oxidation.

Next, data for increasing the density of the oxide film (SiO ₂ ) will improve the insulating characteristics.
In 2000, J.M. W. McPherson and R.M. B. Comparing the current-voltage characteristics of the thermally oxidized SiO ₂ thin film measured by Khamankar and the oxide film (SiO ₂ ) (density 2.27 g / cm ³ , 2.31 g / cm ³ ) according to the present invention The results shown in Table 1 below are obtained.

A graph showing the characteristics of Table 1 is shown in FIG.
The measuring means for obtaining the data of Table 1 is shown in FIG.
3 indicates the characteristics of the current density (A / cm ² ) film density of 2.27 g / cm ³ (film thickness of 3.0 nm) in Table 1, and the code B in FIG. 3 indicates the current density of Table 1. The characteristics of (A / cm ² ) film density 2.31 g / cm ³ (film thickness 3.5 nm) are shown. The horizontal axis in FIG. 3 is the electric field (MV / cm), and the vertical axis is the current density (A / cm ² ). The other characteristics in FIG. 3 are shown for reference.
The measuring means is configured as shown in FIG.
The wafer is, for example, P-type Si (plane orientation 001), a wafer thickness of 5.23 × 10 ⁵ nm, and a resistivity of 14 to 22 Ωcm.
A photo-oxidized SiO ₂ film is formed on one side of the wafer by a photo-oxidation method,
Al (aluminum) electrodes are provided on both sides,
Apply a variable voltage between both electrodes,
The current value at that time is measured with an ammeter.
A wafer provided with an oxide film with different conditions is measured as appropriate.

In Table 1,
In the range where the electric field strength is low (about 1 MV / cm when the film thickness is 3 to 4 nm), the conventional example (conventional film density 2.24 g / cm ³ ) and the example of the present invention (film density 2.27 g / cm ³ , film density) 2.31 g / cm ³ ), the difference in the electric field strength does not appear remarkably, but in the region of 2 MV / cm or more, the difference is clear (the unit of current / voltage is normalized, but the current density is Replaced by leakage current).

The dielectric breakdown of the thermally oxidized oxide film (SiO ₂ ) is 11 to 13 MV / cm when the oxide film (SiO ₂ ) thickness is 2.6 to 4.8 nm, and the leakage current at that time is 1 A / cm ² . On the other hand, in the oxide film (SiO ₂ ) having a film density of 2.27 g / cm ³ , when the electric field intensity is 15 MV / cm at a film thickness of 3.0 nm, the leakage current is 10 ⁻⁵ A / cm ² .
With an oxide film (SiO ₂ ) having a film density of 2.31 g / cm ³ , the electric field intensity is 40 MV / cm at a film thickness of 3.5 nm, the leakage current is 10 ⁻⁵ A / cm, and the leakage current is reduced. It was.

Although oxide film of the thermal oxide (SiO ₂₎ and the oxide film produced by UV irradiation (SiO ₂₎ is amorphous, in nature you include the 2.5~2.65g / cm ³ even amorphous (Rusha triethanolamine Light) When crystallized, a highly densified oxide film (SiO ₂ ) such as 4.34 g / cm ³ (stishovite) exists, so that a further densified oxide film (SiO ₂ ) is formed on the semiconductor silicon wafer. By further laminating, further improvement of the insulation characteristics can be expected.

FIG. 3 shows current-voltage characteristics at a film density of 2.27 g / cm ³ and 2.31 g / cm ³ .
In the low electric field range, 2.27 g / cm ³ has a lower leakage current, but it can be seen that the higher the oxide film (SiO ₂ ) density, the higher the dielectric breakdown. As a result, it is considered that the density does not necessarily have to be high when used for a low-voltage operation device in a low electric field. However, when the thermal oxidation oxide film (SiO ₂ ) is 3 nm or less in thickness, the leakage current suddenly increases even at a low voltage. Will increase. This is because a low-insulating oxidation state called SiO, Si ₂ O ₃ , or Si ₂ O (suboxide) exists between the thermally oxidized SiO ₂ film and the substrate at a thickness of about 1 nm. Since the oxide film formed by ultraviolet irradiation has almost no suboxide layer, the insulating property can be maintained even in an ultra-thin state.

The thickness of the suboxide can be estimated by X-ray photoelectron spectroscopy. In the sample with a density of 2.27 g / cm ³ , the suboxide was 0.15 nm with respect to the oxide film (SiO ₂ ) film thickness of 3 nm. That is, since the above method does not produce suboxide, the insulation can be maintained even when the SiO ₂ film is thinned, and the high insulation state can be maintained by the high density.

In FIG. 4, the vertical axis represents the SiO ₂ density (g / cm ³ ), the horizontal axis represents the substrate temperature (° C.), and the density change (SiO ₂ Density) of the ultraviolet irradiation oxide film SiO ₂ with respect to the substrate temperature (° C.) according to the present invention. (g / cm ³ )) (density is converted from measurement of oblique incidence X-ray reflectivity).
FIG. 4 shows the characteristics of the following symbols. In FIG. 4, the black ● shape indicates a wavelength of 126 nm, the black square indicates a wavelength of 126 nm VUV SR at BL20A, the white rhombus indicates a wavelength of 222 nm Excimer Lamp, the white hexagon indicates a wavelength of 172 nm, and an Excimer Lamp.
The characteristics in FIG. 4 take the values in Table 2 below.

The wavelengths of ultraviolet rays used were 126 nm, 172 nm, and 222 nm, and the light source was an excimer lamp and radiation light. The data when irradiating ultraviolet rays with wavelengths of 172 nm and 222 nm is one point at a time, but the optimum conditions at each wavelength may be different in substrate temperature (the maximum density and optimum substrate temperature are different at each wavelength). In the case of the present invention, it can be seen that even if SiO ₂ is formed while irradiating ultraviolet rays having wavelengths of 172 nm and 222 nm, a higher density state can be produced than conventional thermally oxidized SiO ₂ .

FIG. 7 shows the spectrum obtained by X-ray photoelectron spectroscopy. The ratio of each element in the spectrum of FIG.

Next, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a cross-sectional view showing an embodiment of the present invention.
As shown in FIG. 1, a high-density silicon oxide film 3 is formed on a silicon substrate 1 via an ultrathin suboxide layer 2 having a composition of Si _x O _y (except for x = 1 and y = 2). Is formed. The conductivity type of the silicon substrate may be either n-type or p-type, or may be intrinsic. Also, the plane orientation is not particularly limited. The high-density silicon oxide film 3 according to the present invention has a density of 2.27 g / cm ³ or more. More preferably has a 2.30 g / cm ³ or more density, more preferably it has a 2.32 g / cm ³ or more density.
The high-density silicon oxide film 3 is a film having a dense SiO ₂ composition, and has high breakdown voltage and low leakage characteristics that cannot be expected from a low-density silicon oxide film such as a thermal oxide film. Further, it can be expected that a dense silicon oxide film has a high oxygen atom diffusion blocking ability.

  The suboxide layer 2 formed below the high-density silicon oxide film 3 is a film that is inevitably formed when the silicon oxide film 3 is formed by photo-oxidation. According to the manufacturing method, the film can be formed extremely thin, and ideally, it is a film having a thickness of not more than 1 atomic layer, not more than several molecular atomic layers, that is, a film thickness of 0.1 nm or less.

  The high-density silicon oxide film according to the present invention is formed on a silicon substrate at an average growth rate of 5 nm / h or less by heating to 300 to 430 ° C. while irradiating ultraviolet rays in a gas containing oxygen. More preferred is an average growth rate of 2 nm / h or less, and even more preferred is an average growth rate of 1.2 nm / h or less. Oxidation at a slow rate enables formation of a high-density silicon oxide film and suppresses the growth of the suboxide layer.

Preferably, the high-density silicon oxide film according to the present invention is photooxidized by heating the silicon substrate to 300 to 430 ° C. while irradiating ultraviolet rays having a wavelength of less than 222 nm (for example, 126 nm) in a gas containing oxygen. It is formed by doing. By performing photo-oxidation using ultraviolet rays having a short wavelength, it becomes possible to form a silicon oxide film having a higher density of 2.30 g / cm ³ or more.

  The photo-oxidation of the present invention is performed in an oxygen atmosphere of 1 atm (1 atm), for example. However, the same effect can be obtained by performing oxidation in an oxygen atmosphere of 1 Pa to 1.5 MPa even if the pressure is not 1 atm. Moreover, it can replace with the method of performing photo-oxidation in a pure oxygen atmosphere, and can oxidize in the gas atmosphere to which inert gas, such as argon, was added. Also in this case, the partial pressure of oxygen is desirably 1 Pa to 1.5 MPa.

  FIG. 2 is a cross-sectional view of a MOS transistor which is an example of a device employing a high-density silicon oxide film according to the present invention. A gate electrode 4 is formed on the silicon substrate via a high-density silicon oxide film 3, and source / drain regions 5 are formed on both sides of the gate electrode 4 in the surface region of the silicon substrate 1. According to the MOS transistor using the high-density silicon oxide film as the gate insulating film, it is possible to manufacture a high quality transistor having a high gate withstand voltage and a low leakage current.

In an embodiment of the present invention, an Ar ₂ excimer lamp (wavelength: λ = 126 nm), an Xe ₂ excimer lamp (λ = 172 nm), and a KrCl excimer lamp (λ = 222 nm) capable of emitting discrete energy photons. Was used as the light source. A 4 inch (10.16 cm), boron-doped, p-type Si (001) wafer having a specific resistance of 14-22 Ωcm was prepared and subjected to chemical treatment to clean the surface. Prior to loading into the growth chamber, the natural oxide film was removed with diluted hydrofluoric acid. After carrying into the growth chamber and evacuating the growth chamber to 0.1 Pa, pure oxygen gas was supplied and the flow rate of the gas was controlled to maintain the inside of the growth chamber at 1 atm. The silicon substrate was controlled to maintain a growth temperature of 300-430 ° C. Oxygen gas was heated and kept in accordance with the substrate temperature so that the substrate temperature was not lowered by the blown oxygen gas.

The photo-oxidation process was performed under the following conditions.
1. Irradiation ultraviolet ray: λ = 126 nm, substrate temperature: 390 ° C., growth time: 7.5 hours.
2. Irradiation ultraviolet ray: λ = 172 nm, substrate temperature: 430 ° C., growth time: 5 hours.
3. Irradiation ultraviolet ray: λ = 222 nm, substrate temperature: 390 ° C., growth time: 5 hours.
The manufactured ultra-thin SiO ₂ film was analyzed by X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), X-ray reflection and ellipsometry (ellipsometry). The growth rate was converted from the X-ray reflectivity calibrated from the growth conditions. The TEM technique was used for evaluating the sharpness of the interface, and XPS was used for calculating the thickness of the transition layer (region where the density changes) using the average silicon value.

FIG. 5 shows the depth profile of the oxide film (SiO ₂ ) density obtained from X-ray reflection measurements for samples grown using each wavelength. In FIG. 5, the vertical axis represents density (g / cm ³ ), and the horizontal axis represents depth (nm) from the surface.
The characteristics of FIG. 5 are as follows. When forming a SiO ₂ film in which silicon Si is aligned in the plane orientation (001), the X-ray wavelength λ to be irradiated is 126 nm (characteristic of reference C in the figure) and 172 nm (in the figure). The characteristic of the symbol D) and 222 nm (characteristic of the symbol E in the figure).
Grazing incidence x-ray reflectometry (GIXR) performed over a wide range of angles and intensities contains information about density and roughness. Assuming that the SiO ₂ layer has a gradient density along the vertical direction of the surface, a square parameterized density distribution is obtained that is compatible with X-ray reflection based on the Parratt method. Therefore, the technology developed by them for the photo-oxidized SiO ₂ density and the determined depth profile were used here.

The average density of the photo-oxidized SiO ₂ films at λ = 172 nm and 222 nm is 2.27 g / cm ³ , and at λ = 126 nm, the average density is 2.32 g / cm ³ . FIG. 5 shows that all the photo-oxidized SiO ₂ films exhibit a higher average density than that of the conventional thermal oxide film (2.20-2.24 g / cm ³ ). A photo-oxide film with λ = 126 nm has a particularly high value. The flat part on the right side of each curve indicates a silicon substrate (density = 2.33 g / cm ³ ).
The density change is observed as an inclination change at the interface between the photooxidized SiO ₂ films of λ = 172 nm and 222 nm. However, in the photo-oxide film with λ = 126 nm, since the density difference is small compared to Si (2.33 g / cm ³ ), almost no change in inclination is observed.

The density profile shows a significant wavelength dependence between λ = 126 nm and 172 nm. The difference in average density is slight between the photo-oxidized films (SiO ₂ ) of the ultraviolet wavelengths λ = 172 nm and 222 nm. The density change in the transition layer is considerably different between the photo-oxidized SiO ₂ films of λ = 172 nm and 222 nm. The change in density of the transition layer in SiO ₂ photooxidized at λ = 222 nm is small compared to that at λ = 172 nm.

As described above, GIXR measurement shows that an oxide film formed by higher energy VUV (vacuum ultraviolet) photons has a higher density and a sharper interface. Therefore, it is considered that both the densification mechanism and the interface formation mechanism are related to the excited state of oxygen atoms formed by VUV photon irradiation. The photo-oxidized SiO ₂ according to the present invention is presumed to have a high-density polymorphic crystal structure such as cristobalite having the same density as silicon. In amorphous SiO ₂ , the Si—O bond structure has a plurality of types of cyclic structures having different numbers of atoms. That is, it is composed of rings having different numbers of atoms. In the high-density SiO ₂ film, it is presumed that smaller rings are assembled.

FIG. 6 shows the relationship between the film thickness of the SiO ₂ film photooxidized at λ = 126 nm and 172 nm and the light irradiation time, particularly 1, 3, and 5 hours, respectively.
In FIG. 6, the vertical axis represents the film thickness (nm) and the horizontal axis represents the oxidation time (hours). In FIG. 6, white circles ◯ indicate photooxidation characteristics with a wavelength λ of 172 nm, and black circles ● indicate photooxidation characteristics with a wavelength λ of 126 nm.
The substrate temperatures are 460 ° C. (λ = 126 nm) and 470 ° C. (λ = 172 nm). The film thickness of the SiO ₂ film shows saturation when the irradiation time is prolonged. Also, depending on the substrate temperature, the lower the photon energy, the higher the growth rate. In the case of photooxidation, it is considered that the diffusion rate is slow, making it difficult to produce suboxides. It is presumed that 126 nm photo-oxidation has a slower film formation rate than 172 nm photo-oxidation, and that the density change at the interface is small because diffusion is slow.

FIG. 7 shows the XPS Si2p _1/2 , 3/2 signal profile. In FIG. 7, the vertical axis represents intensity (arbitrary unit), and the horizontal axis represents binding energy (ev).
A small amount of suboxide contributions (Si ¹⁺ and Si ²⁺ ) is observed between the bulk Si (Si ⁰ ) and silicon dioxide (Si ⁴⁺ ) signals. The signal was fitted to a Gaussian function and was estimated to have a maximum error of 5% or less.
From FIG. 7, it can be seen that the suboxide in the SiO ₂ film is very small or the suboxide layer at the interface is very thin.

Table 3 shows sampling values of the characteristics shown in FIG. Table 2 shows the chemical shift, FWHM (full width half maximum value) and signal intensity of the Si 2p signal in each chemical state. The results show that the suggestion of suboxide thickness in the photo-oxidized SiO ₂ film obtained from the XPS results is significantly smaller.
The SiO ₂ film thickness (d _SiO2 ) is calculated as shown in Equation 1 from the Si2p signal intensity of the XPS data.

ここで、ｌ_ＳｉＯ２は、ＳｉＯ_２膜中における光電子の平均自由工程、αは光電子の放出角度である。従来のＳｉＯ_２膜のｌ_ＳｉＯ２は、ＡｌΚαに対し２．６×１０^−７と決定される。Ｉ_Ｓｉ ^４＋とＩ_Ｓｉ ^０は、ＳｉＯ_２とバルクシリコンの光電子強度の積分値である。実験的な強度比Ｉ_∞／Ｉ_０は、ＡｌΚαに対し０．８２と決定される。サブ酸化状態におけるシリコン原子の数は下記の数２で与えられる。

Here, l _SiO2 is the mean free path of photoelectrons in the SiO ₂ film, and α is the photoelectron emission angle. L _SiO2 of the conventional SiO ₂ film is determined to be 2.6 × 10 ⁻⁷ with respect to AlΚα. I _Si ⁴⁺ and I _Si ⁰ are integral values of the photoelectron intensities of SiO ₂ and bulk silicon. The experimental intensity ratio I _∞ / I ₀ is determined to be 0.82 with respect to AlΚα. The number of silicon atoms in the sub-oxidation state is given by the following formula 2.

ここで、ｎ_Ｓｉ ^０は、シリコンの原子密度で、５．０×１０^２２ｃｍ^−３、ｌ_Ｓｉ ^０は、Ｓｉにおける光電子の平均フリーパスで、ＡｌΚαに対し１．６×１０^−７ｃｍと決定される。σ_Ｓｉ ^ｘ＋とσ_Ｓｉ ^０は光電離断面（ｐｈｏｔｏｉｏｎｉｚａｔｉｏｎ ｃｒｏｓｓ ｓｅｃｔｉｏｎ）であり、ＸＰＳにおける高フォトンエネルギー状態であるＳｉ_ｘＯ_ｙとＳｉと推定される。データからの計算結果は、ｄ_ＳｉＯ２＝３．０ｎｍ、ΣＮ_Ｓｉ ^ｘ＋＝４．０×１０^１４ａｔｏｍｓ／ｃｍ^２である。

Here, n _Si ⁰ is the atomic density of silicon, 5.0 × 10 ²² cm ⁻³ , and l _Si ⁰ is the average free path of photoelectrons in Si, which is 1.6 × 10 ⁻⁷ cm with respect to AlΚα. It is determined. σ _Si ^{x +} and σ _Si ⁰ are photoionization sections, and are estimated to be Si _x O _y and Si, which are high photon energy states in XPS. The calculation results from the data are d _SiO2 = 3.0 nm and ΣN _Si ^{x +} = 4.0 × 10 ¹⁴ atoms / cm ² .

FIG. 8 is a diagram showing the voltage-current density characteristics of the oxide film by the photo-oxidation method and the thermal oxidation method.
In FIG. 8, the vertical axis represents current density (A / cm ² ), and the horizontal axis represents voltage (v).
The characteristic indicated by F in FIG. 8 indicates the characteristics of a high-density SiO ₂ film having a film density of 2.32 g / cm ³ and a film thickness of 3.5 nm according to the present invention.
The characteristics indicated by reference sign G in FIG. 8 indicate the characteristics of a high-density SiO ₂ film having a film density of 2.27 g / cm ³ and a film thickness of 3.0 nm according to the present invention.
The photo-oxidation method of the present invention shows voltage-current density characteristic curves of photo-oxidized films having ultraviolet wavelengths λ = 126 nm and 172 nm. This characteristic is a leakage current characteristic.

FIG. 8 shows the characteristics of the thermally oxidized SiO ₂ film for reference.
The characteristic indicated by the symbol H in FIG. 8 indicates the characteristic of a thermally oxidized SiO ₂ film having a film thickness of 4.0 nm at an arbitrary value within the film density of 2.20-2.24 g / cm ³ .
The characteristic indicated by symbol I in FIG. 8 indicates the characteristic of a thermally oxidized SiO ₂ film having a film thickness of 3.0 nm at an arbitrary value within the film density of 2.20-2.24 g / cm ³ .
The characteristic of the symbol J in FIG. 8 shows the characteristic of a thermally oxidized SiO ₂ film having a film thickness of 2.0 nm at an arbitrary value within the film density of 2.20-2.24 g / cm ³ .

The data relating to these thermal oxide films can be found in Microelectronics Journal vol. 34, p. 363 (2003).
The sample was formed in a MIS structure having an Al film on the photo-oxidized SiO ₂ film and the lower surface of the silicon substrate, and measured by the two-terminal method. The SiO ₂ film formed by photo-oxidation of λ = 126 nm and 172 nm has a film thickness of d _SiO2 = 3.5 and 3.0 nm, and breakdown voltages of 15.5 V and 5.5 V. This breakdown voltage is higher for a photo-oxide film of λ = 126 nm.

FIG. 9 is a graph showing the relationship between the density of the oxide film (SiO ₂ ) and the breakdown field strength. In FIG. 9, the vertical axis represents the electric field strength (MV / cm), and the horizontal axis represents the SiO ₂ film density (g / cm ³ ).
The estimated electric field strengths of the photo-oxidized SiO ₂ films with wavelengths λ = 126 nm and 172 nm are 45 MV / cm and 18 MV / cm. There is a large difference in electric field strength between λ = 126 nm and 172 nm. Therefore, in order to form a photo-oxidized SiO ₂ film having a high dielectric breakdown electric field, it is desirable to use VUV less than λ = 172 nm. The leakage current of the photooxide film of λ = 126 nm and 172 nm is 10 ⁻⁴ −10 ⁻¹⁰ A / cm ² from 0 to the breakdown voltage. This result reveals the superior insulating properties of the photo-oxidized and high-density ultra-thin SiO ₂ film compared to the reported ultra-thin SiO ₂ film of the previous oxide.

It is sectional drawing of the silicon substrate which is an Example of this invention. It is sectional drawing of the MOS transistor which is the other Example of this invention. It is a graph which shows the characteristic of Table 1. Change in density (SiO ² Density (g / cm ³ )) of the ultraviolet-irradiated oxide film SiO ₂ with respect to the substrate temperature (Substrate Temperature (° C.)) according to the present invention (density is converted from measurement of oblique incidence X-ray reflectivity) It is a graph. It is a graph of the film thickness-film density characteristic of the oxide film of this invention. It is a graph of the film thickness-oxidation time characteristic of the oxide film of this invention. It is a bond energy-strength characteristic view obtained by X-ray photoelectron spectroscopy (XPS) of the oxide film of the present invention. It is a graph which shows the voltage-current density characteristic of the oxide film of this invention. It is a graph which shows the density-electric field strength characteristic of the oxide film of this invention. The measurement means for obtaining the data of Table 1 is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Suboxide layer 3 High-density silicon oxide film 4 Gate electrode 5 Source / drain region 6 High dielectric constant film

Claims (6)

In a 1 atmosphere oxygen atmosphere,
Heating the silicon substrate to any temperature within the range of 300 ° C. to 430 ° C .;
In a state where the silicon substrate is irradiated with ultraviolet rays of 222 nm or less,
A method for producing a high-density silicon oxide film comprising the step of forming an oxide film on the surface of a silicon substrate while flowing oxygen gas at a rate of 100 ml / min or more in terms of oxygen gas flow rate at 20 ° C. in a flow meter on the substrate surface . 2. The method for producing a high-density silicon oxide film according to claim 1, wherein the oxygen gas is heated so that the temperature of the silicon substrate is equal to the temperature of the oxygen gas. The oxygen gas flow rate is set so that the silicon substrate spraying temperature of the oxygen gas becomes an arbitrary heating temperature within a range of 300 ° C. to 430 ° C. of the silicon substrate. Manufacturing method of high-density silicon oxide film. The flow rate of the oxygen gas can be set to any value of 100 ml / min or more when the heating / retaining temperature of the oxygen gas to be sprayed can be set to be the same as any temperature within the range of 300 ° C. to 430 ° C. which is the heating temperature of the silicon substrate. 4. The method for producing a high-density silicon oxide film according to claim 1, wherein the flow rate value is set. A silicon substrate having a high-density silicon oxide film, wherein an oxide film manufactured by the method for manufacturing a high-density silicon oxide film according to any one of claims 1 to 4 is provided on a silicon substrate. A semiconductor device comprising the silicon substrate according to claim 5.

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