JP2005175023A - High-density silicon oxide film and its forming method, and semiconductor device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a super-thin (5 nm or less) silicon oxide film having a high insulation breakdown voltage reduced in leakage current. <P>SOLUTION: On a silicon substrate 1, the high-density silicon oxide film 3 having a density of 2.25 g/cm<SP>3</SP>or higher is formed via a sub-oxide layer 2 formed of not more than several atomic layers. The oxide film 3 is formed by a method wherein, while heating the silicon substrate to 100-500°C in an oxygen atmosphere at 1 atmospheric pressure, vacuum ultraviolet rays are irradiated on the silicon substrate to oxidize it. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-density silicon oxide film formed on a silicon substrate by a photo-oxidation method and having a higher density than a silicon oxide film by a thermal oxidation method, a manufacturing method, and a semiconductor device using the same.

The semiconductor integrated circuit is being miniaturized according to the semiconductor technology roadmap, and the technology node is going from the current 130nm to 90nm, and it is scheduled to move to 65nm process in several years. In semiconductor devices, the entire structure is miniaturized according to Moore's law (so-called scaling law), and it is necessary to further reduce the thickness of the gate insulating film. It is an urgent issue to form an ultra-thin gate insulating film with high insulating properties with high reliability.
However, as is well known, the thermally oxidized SiO ₂ film has a sub-oxide [Si _x O _y , except x = 1, y = 2 except for the oxygen-deficient bonding state at the Si-SiO ₂ interface. A layer is formed. Typically, a thermal oxide film has a sub-oxide layer having a thickness of 1 to 1.5 nm, which causes deterioration of the insulating characteristics of the silicon oxide film. The insulating limit is dependent on the thickness of the Si _x O _y layer, the thickness of the Si _x O _y layer is the key factor that determines the effective thickness of the SiO ₂ gate insulating film.

Attempts to reduce the thickness of the Si _x O _y layer have also been made by a rapid thermal oxidation method, a method using plasma, ozone and light. However, the growth of the SiO ₂ film by the rapid thermal oxidation method, the plasma method, and the ozone method damages the surface of the silicon substrate and causes defects at the substrate-silicon oxide film interface. 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 conducted with the aim of improving the growth rate (see, for example, Non-Patent Document I and Patent Document 1). In addition, the insulating properties of the silicon film formed by the conventional photo-oxidation method were the same as those of the thermal oxidation method with an oxide film thickness of 5 nm or more.
Applied Surface Science 168 (2000) pp.288-291 JP 2003-224117 A

The present inventor has developed a silicon oxide film having a high withstand voltage and a thin suboxide layer (ideally not present) that can meet the demand for thinning the gate insulating film in accordance with the trend of miniaturization of semiconductor integrated circuits. In the process, the conventional photo-oxidation approach that aims to increase the growth rate is to form a highly reliable silicon oxide film that can meet the above requirements. I found that I can not.
The present invention has been made in view of this point, and an object of the present invention is to firstly provide a silicon oxide film that exhibits a high withstand voltage even if it is ultra-thin. In addition, a high breakdown voltage silicon oxide film can be formed without damaging the silicon substrate.

In order to achieve the above object, according to the present invention, a high-density silicon oxide film having a density of 2.25 g / cm ³ or more formed on a silicon substrate is provided.

Further, in order to achieve the above object, according to the present invention, in a gas containing oxygen, heating to 100 to 500 ° C. while irradiating ultraviolet rays having a wavelength of maximum intensity of less than 172 nm on the silicon substrate. A high-density silicon oxide film manufacturing method for forming a silicon oxide film is provided.
For example, an excimer lamp having a nominal value of 126 nm has a large number of emission signals other than 126 nm from the visible region to the infrared region. The above-mentioned “ultraviolet light having a wavelength showing the maximum intensity of less than 172 nm” means that any light having a wavelength that does not show the maximum intensity may be included.

  Further, in order to achieve the above object, according to the present invention, silicon is heated at a temperature of 100 to 500 ° C. while irradiating ultraviolet rays in an oxygen-containing gas at an average growth rate of 5 nm / h or less on a silicon substrate. A high-density silicon oxide film manufacturing method for forming an oxide film is provided.

In order to achieve the above object, according to the present invention, there is provided a semiconductor device having a high-density silicon oxide film formed on a silicon substrate and having a density of 2.25 g / cm ³ or more.
Preferably, the thickness of the suboxide layer (Si _x O _y layer, except for x = 1 and y = 2) interposed between the silicon substrate and the high-density silicon oxide film is 0.1 nm or less. .

The silicon oxide film formed by a thermal oxidation method or the conventional optical oxidation, Si ₂ O, SiO, suboxide layer containing Si ₂ O ₃ is formed to a thickness of a few nm, also considered the composition of SiO ₂ silicon oxide film is also low density (2.20 g / cm ³ by wet ^oxidation, 2.24 g / cm ³ order by dry oxidation), which is has. Such a silicon oxide film has a large leakage current and a low breakdown voltage. A silicon oxide film is thought to be formed by multiple types of rings in which Si-O bonds are connected in a ring, but it contains a large amount of low-density rings, in other words, rings that occupy a large space. Therefore, it is presumed that the conventional silicon oxide film could not be densified. On the other hand, the silicon oxide film according to the present invention, 2.25 g / cm ³ or more, more preferably are densified to 2.30 g / cm ³ or more, it is estimated that more silicon oxide film stuffed a small annulus . 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 breakdown voltage and low leakage current characteristics can be realized, and a high quality characteristic can be expected in a device using this silicon oxide film.

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 through an ultrathin suboxide layer 2 having a composition of Si _x O _y (except x = 1, 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 2.25 g / cm ³ or more. More preferably, it has a density of 2.26 g / cm ³ or more, and more preferably 2.30 g / cm ³ or more.
The high-density silicon oxide film 3 is a film having a dense SiO ₂ composition and has a 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 one 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 100 to 500 ° C. while irradiating ultraviolet rays in a gas containing oxygen. More preferably, the growth rate is 2 nm / h or less on average, and still more preferably the growth rate is 1.2 nm / h or less on average. 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 to 100 to 500 ° C. while irradiating ultraviolet rays having a wavelength of less than 172 nm (for example, 126 nm) in a gas containing oxygen. Formed by. By performing photo-oxidation using ultraviolet rays having a short wavelength, it is 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 provide a high-quality transistor having a high gate withstand voltage and a low leakage current.

FIG. 3 is a cross-sectional view showing another example of a device employing the high-density silicon oxide film according to the present invention. The MOS transistor of this embodiment uses a high dielectric constant insulating film (high-k film) as a gate insulating film. As shown in FIG. 3, a high dielectric constant film 6 is formed on a high-density silicon oxide film 3 formed on a silicon substrate 1, and a gate electrode 4 is formed thereon.
The high-k gate insulating film is formed by depositing a high dielectric constant insulating film on the silicon oxide film by sputtering or the like and then performing a heat treatment in an oxygen atmosphere for improving the film quality. In this heat treatment, oxygen is more easily bonded to silicon than a high dielectric constant insulating film material having a low binding energy, and the oxygen supplied by the heat treatment diffuses into the silicon substrate, thereby increasing the thickness of the insulating film layer. The silicon oxide film formed at this time is considered to be a layer containing suboxide. Further, it has been known that the high dielectric constant insulating film also reduces the mobility when the SiO ₂ layer is not included between the high dielectric constant insulating film and the silicon substrate. Since the high-density silicon oxide film has high hardness, an effect of reducing damage to the substrate can be expected even by a film forming method using high energy particles such as a sputtering method. In addition, since the method leading to the present invention can form a high quality oxide film, oxygen can be supplied to the high dielectric constant insulating film having a low oxygen concentration at a low temperature by the photo-oxidation method. By adopting this method in a process for improving film quality, a gate insulating film made of a high dielectric constant insulating film can be manufactured by a low temperature process.

In the embodiment of the present invention, Ar ₂ excimer lamp (wavelength: λ = 126 nm), Xe ₂ excimer lamp (λ = 172 nm), KrCl excimer lamp (λ = 222 nm) capable of emitting photons of discrete energy Was used as the light source. A 4-inch (10.16 cm), boron-doped, p-type Si (001) wafer with a specific resistance of 14-22 Ωcm was prepared, and the surface was cleaned by chemical treatment. Prior to loading into the growth chamber, the natural oxide film was removed with diluted hydrofluoric acid. After carrying into the growth chamber and exhausting the growth chamber to 0.1 Pa, pure oxygen gas was supplied and the gas flow rate was controlled to maintain the inside of the growth chamber at 1 atm. The silicon substrate was controlled to maintain a growth temperature below 500 ° C.

Photooxidation was performed as follows.
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 ultrathin 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 to evaluate the sharpness of the interface, and XPS was used to calculate the thickness of the transition layer (area where the density changes) using the average silicon value.

FIG. 4 shows the SiO ₂ density depth profile obtained from X-ray reflection measurements for samples grown using each wavelength. Grazing incidence x-ray reflectometry (GIXR) performed over a wide range of angles and intensities contains information about density and roughness. Assuming the SiO ₂ layer has a gradient density along the vertical direction of the surface, a square parameterized density distribution is obtained that fits the X-ray reflection based on the Parratt method. Therefore, the technology developed by them on 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. 1 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 between the photo-oxidized SiO ₂ films of λ = 172 nm and 222 nm is slight. In this experiment, the wavelength used was discrete, so it was not possible to determine the optimum wavelength for silicon oxidation. However, at least to obtain a higher-density photo-oxidized SiO ₂ film, such as λ = 126 nm It can be seen that it is desirable to use ultraviolet light of less than 172 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.
In summary, GIXR measurements show that higher energy VUV (vacuum ultraviolet) photon-formed oxides have higher densities and sharper interfaces. Therefore, both the densification and interface formation mechanisms are thought to be 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 like 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. 5 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. 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 is slower in film formation than 172 nm photo-oxidation, and that the density change at the interface is small because diffusion is slow.

FIG. 6 is a TEM cross-sectional image of a SiO ₂ film sample photooxidized at λ = 126 nm on a Si (001) substrate. The formed SiO ₂ film is covered with an aluminum vapor deposition film used as an electrode for (current density-voltage characteristic) measurement. The sample was produced by the ion milling method. The TEM image was obtained by H-9000NAR (manufactured by Hitachi, Ltd.) having an electron acceleration voltage of 200 kV. An electron beam along <110> was used for high resolution measurements. The film thickness is 3.5-3.7 nm. This was obtained after calibration using a lattice spacing along <111>. The roughness of the interface is the same as the original surface roughness of silicon. From this, it can be concluded that the growth of the photo-oxidized SiO ₂ does not roughen the surface.

In SiO ₂ , the silicon value is Si ⁴⁺ and silicon atoms are tetrahedrally bonded to oxygen atoms. On the other hand, in ^suboxides , Si has a lower valence such as Si ³⁺ , Si ²⁺ , Si ¹⁺ depending on the lack of oxygen coordination. Bulk silicon is constant Si ⁰ neutral. The chemical shift of photoelectron spectroscopy can be used to evaluate the average chemical composition within the probe region. XPS data is obtained at a 45 ° extraction angle. The X-ray source is a monochrome AlΚα ray (1486.6 eV), and the energy resolution is 0.52 eV. The beam diameter is 100 μmφ and the exploration depth is about 5 μm when the extraction angle is 45 °. The background was determined in consideration of energy loss due to inelastic scattering. The XPS system was a PHI Quantum 2000 manufactured by ULVAC-PHI Inc.
FIG. 7 shows the XPS Si2p _1/2 , 3/2 signal profile. In FIG. 7, the entire profile is shown on the lower side, and the enlarged view is shown on the upper side. A small amount of suboxide contribution (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 the maximum error was estimated to be less than 5%. 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.
Fit to the curve results are summarized in Table 1. Table 1 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 in Equation 1 from the Si2p signal intensity of XPS data.

Here, l _SiO2 is the average 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 for AlΚα. The number of silicon atoms in the sub-oxidation state is given by the following formula 2.

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, and is determined to be 1.6 × 10 ⁻⁷ cm with respect to AlΚα. σ _Si ^{x +} and σ _Si ⁰ are photoionization cross 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 shows JV (current density-voltage) characteristic curves of the photooxide films of λ = 126 nm and 172 nm. That is, the leakage current characteristic is shown. In FIG. 8, data of thermal oxide films having film thicknesses of 2.0 nm, 3.0 nm, and 4.0 nm are also shown for reference. Data on these thermal oxide films are based on 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 at λ = 126 nm and 172 nm has d _SiO2 = 3.5 and 3.0 nm film thickness, and breakdown voltages of 15.5V and 5.5V. Note that the 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 SiO ₂ film and the breakdown electric field strength. The estimated electric field strengths of the photo-oxidized SiO ₂ films with 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. Accordingly, 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 photo-oxide film at 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.

Sectional drawing of the silicon substrate for demonstrating embodiment of this invention. Sectional drawing of the MOS transistor for describing embodiment of this invention (the 1). Sectional drawing of the MOS transistor for describing embodiment of this invention (the 2). The graph which shows the density profile of the silicon oxide film formed by the Example of this invention. The graph which shows the relationship between the film thickness of the silicon oxide film formed by the Example of this invention, and oxidation time. The transmission electron micrograph which shows the cross section of the silicon oxide film formed by the Example of this invention. The binding energy profile obtained by the X ray photoelectron spectroscopy (XPS) of the silicon oxide film formed by the Example of this invention. 3 is a graph showing leakage characteristics of a silicon oxide film formed according to an example of the present invention. The graph which shows the relationship between the density of the silicon oxide film formed by the Example of this invention, and electric field strength.

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 (12)

A high-density silicon oxide film having a density of 2.25 g / cm ³ or more formed on a silicon substrate. High-density silicon oxide with a density of 2.25 g / cm ³ or more formed on a silicon substrate via a suboxide layer (Si _x O _y layer, except x = 1, y = 2) with a thickness of 0.1 nm or less film.   The high-density silicon oxide film according to claim 1 or 2, wherein the film thickness is 5 nm or less.   A method for producing a high-density silicon oxide film in which a silicon oxide film is formed on a silicon substrate by heating to 100 to 500 ° C. while irradiating an ultraviolet ray having a maximum intensity of less than 172 nm in a gas containing oxygen.   A method for producing a high-density silicon oxide film in which a silicon oxide film is formed on a silicon substrate by heating to 100 to 500 ° C. while irradiating an ultraviolet ray having a maximum intensity of 126 nm in a gas containing oxygen.   A method for producing a high-density silicon oxide film in which a silicon oxide film is formed on a silicon substrate at an average growth rate of 5 nm / h or less by heating to 100 to 500 ° C. while irradiating ultraviolet rays in a gas containing oxygen.   The method for producing a high-density silicon oxide film according to any one of claims 4 to 6, wherein oxidation is performed under a pressure of 1 Pa to 1.5 MPa.   The method for producing a high-density silicon oxide film according to any one of claims 4 to 6, wherein oxidation is performed in a gas having a partial pressure of oxygen of 1 Pa to 1.5 MPa. A semiconductor device having a high-density silicon oxide film formed on a silicon substrate and having a density of 2.25 g / cm ³ or more. The thickness of a suboxide layer (Si _x O _y layer, except x = 1 and y = 2) interposed between a silicon substrate and the high-density silicon oxide film is 0.1 nm or less. Item 10. The semiconductor device according to Item 9.   11. The semiconductor device according to claim 9, wherein a high dielectric constant film having a dielectric constant higher than that of silicon dioxide is formed on the high-density silicon oxide film. The semiconductor device according to claim 9, wherein the high-density silicon oxide film constitutes at least a part of a gate insulating film of a MOS field effect transistor.