Method for manufacturing sic semiconductor element and sic semiconductor element

Abstract

A method for manufacturing a SiC semiconductor element includes: a step of etching a surface of a SiC substrate (1) using H2 gas at a temperature of 1200°C or higher; a step of forming a SiO2 film (3, 4) on the SiC substrate under conditions in which the SiC substrate is not oxidized; and a step of heat-treating the SiC substrate on which the SiO2 film is formed in an N2 gas atmosphere at a temperature of 1350°C or higher.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a SiC (silicon carbide) semiconductor device and the SiC semiconductor device itself. Background Technology

[0002] In MOS transistors (SiC MOSFETs) using SiC substrates, when a SiO2 film (gate oxide film) is formed on the surface of the SiC substrate through thermal oxidation, there is a problem of very high defect density at the interface between the SiO2 film and the SiC substrate. If the interface defect density is high, the channel mobility and other characteristics of the SiC MOSFET cannot be fully obtained.

[0003] As a method to reduce the density of interface defects, Patent Document 1 discloses the following method: depositing a Si thin film on the surface of a SiC substrate and then oxidizing the Si thin film to form a SiO2 film, instead of directly forming the SiO2 film on the surface of the SiC substrate by thermal oxidation.

[0004] As another method to reduce the density of interface defects, Non-Patent Document 1 discloses the following method: after forming a SiO2 film on the surface of a SiC substrate by thermal oxidation, heat treatment is performed in a NO (nitric oxide) gas atmosphere to nitrid the interface between the SiO2 film and the SiC substrate (interface nitriding).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Publication No. 11-067757

[0008] Non-patent literature

[0009] Non-patent literature 1: GYChung et al., IEEE Electron Device Lett., vol.22, 176 (2001)

[0010] Non-patent literature 2: K. Kawahara et al., Appl. Phys. Express, vol. 6, 051301 (2013)

[0011] Non-patent literature 3: F. Devynck et al., Phys. Rev. B, vol. 84, 235320 (2011)

[0012] Non-patent literature 4: K. Kawahara et al., J. Appl. Phys. vol. 113, 033705 (2013) Summary of the Invention

[0013] -The technical problem the invention aims to solve-

[0014] While the methods disclosed in the prior art literature can significantly reduce the defect density at the interface between the SiO2 film and the SiC substrate, the interface defect density remains high, which greatly limits the characteristics of SiC MOSFETs. Furthermore, the method of nitriding the interface between the SiO2 film and the SiC substrate through NO thermal treatment not only nitrides the interface but also oxidizes it, thus creating a competing process of "nitriding" and "oxidation," making optimization difficult. Moreover, NO gas is highly toxic and therefore unsuitable for large-scale production.

[0015] The present invention was made to solve the above-mentioned technical problems. Its main objective is to provide a method for manufacturing SiC semiconductor devices, which can significantly reduce the defect density at the interface between the SiO2 film and the SiC substrate.

[0016] - Technical solutions used to solve technical problems -

[0017] The method for manufacturing SiC semiconductor devices according to the present invention includes: etching the surface of a SiC substrate using H2 gas at a temperature of 1200°C or higher; forming a SiO2 film on the SiC substrate without oxidizing the SiC substrate; and heat-treating the SiC substrate on which the SiO2 film is formed in an N2 gas atmosphere at a temperature of 1350°C or higher.

[0018] -The effects of the invention-

[0019] According to the present invention, a method for manufacturing a SiC semiconductor device can be provided, which can significantly reduce the defect density at the interface between the SiO2 film and the SiC substrate. Attached Figure Description

[0020] Figure 1 (A)~ Figure 1 (D) is a diagram illustrating a method for manufacturing a SiC semiconductor device according to an embodiment of the present invention;

[0021] Figure 2 It is a graph showing the defect density at the interface between the SiO2 film and the SiC substrate;

[0022] Figure 3 It is a graph showing the types of defects present on the SiC substrate side;

[0023] Figure 4 It is a graph showing the types of defects present on the SiC substrate side;

[0024] Figure 5 This is a graph showing the C-V characteristics of the SiO2 film / SiC substrate;

[0025] Figure 6 This is a graph showing the I-V characteristics of the SiO2 film;

[0026] Figure 7 This is a graph showing the nitrogen atom density near the interface between the SiO2 film and the SiC substrate;

[0027] Figure 8 This is a graph showing the dependence of interface defect density on H2 gas etching temperature;

[0028] Figure 9 This is a graph showing the dependence of interface defect density on N2 gas heat treatment temperature;

[0029] Figure 10 This is a graph showing the nitrogen atom density near the SiO2 / SiC interface;

[0030] Figure 11 (A)~ Figure 11 (C) is a diagram illustrating a method for manufacturing a SiC semiconductor device according to another embodiment of the present invention;

[0031] Figure 12 This is a graph showing the dependence of interface defect density on H2 gas etching temperature;

[0032] Figure 13 This is a graph showing the dependence of interface defect density on N2 gas heat treatment temperature;

[0033] Figure 14 This is a graph showing the nitrogen atom density near the SiO2 / SiC interface;

[0034] Figure 15 This is a graph showing the effect of high-temperature H2 etching in a Si-rich atmosphere. Detailed Implementation

[0035] Before describing the embodiments of the present invention, the process by which the present invention came to be explained will be described first.

[0036] When manufacturing semiconductor devices using SiC substrates, the following process is typically performed before or during the manufacturing process: sacrificial oxidation of the SiC substrate surface followed by removal of the oxide film formed on the SiC substrate surface using a solution such as hydrofluoric acid. This removes impurities accidentally adhering to the surface and damage to the SiC crystals near the outermost surface (such as deviations in chemical composition), thereby stabilizing the characteristics of the semiconductor device and improving the yield.

[0037] Indeed, removing the oxide film after sacrificial oxidation is effective in removing impurities attached to the surface of the SiC substrate and damage to the SiC crystals near the outermost surface. However, it is possible that many defects may remain on the surface of the SiC substrate. In fact, it is known that if the SiC crystal is oxidized, a high density of point defects will be generated near the SiC surface (Non-Patent Document 2). Furthermore, theoretical calculations have reported that if the SiC crystal is oxidized, interface defects caused by excess C atoms will be generated at the interface between the oxide film and SiC (Non-Patent Document 3). As described above, it can be considered that even slight oxidation of the SiC crystal will inevitably produce a large number of interface defects and point defects on the SiC side.

[0038] To verify this, the inventors of this application investigated the following: as a pretreatment before forming a SiO2 film on the surface of a SiC substrate, after sacrificial oxidation, etching the surface of the SiC substrate after removing the oxide film using high-temperature H2 gas. Furthermore, to prevent the formation of an oxide film at the interface between the SiO2 film and the SiC substrate during interface nitriding via NO thermal treatment, interface nitriding via N2 thermal treatment was investigated.

[0039] (Preparation of verification samples)

[0040] To verify the pretreatment effect of high-temperature H2 gas etching and the effect of interface nitriding treatment via N2 heat treatment, the following methods were used: Figure 1 (A)~ Figure 1 The method shown in (D) was used to prepare a sample with a SiO2 film formed on the surface of a SiC substrate.

[0041] like Figure 1 As shown in (A), as a pretreatment step, the surface of SiC substrate 1 was etched using high-temperature H2 gas. The etching using H2 gas was carried out under the conditions of H2 flow rate of 1000 sccm, temperature of 1300 °C, pressure of 0.1 MPa, and time of 3 minutes.

[0042] It should be noted that, as SiC substrate 1, a substrate on which a SiC epitaxial layer (not shown) is formed is used. Here, SiC substrate 1 is an n-type 4H-SiC(0001) substrate, and the donor concentration of the SiC epitaxial growth layer is set to 5 × 10⁻⁶. 15 cm -3 In addition, before the pretreatment process, the surface of the SiC epitaxial layer is subjected to sacrificial oxidation, and then the oxide film is removed.

[0043] Next, as Figure 1As shown in (B), a Si thin film 2 was deposited on a SiC substrate 1 by CVD. The deposition of the Si thin film 2 was carried out under the conditions of SiH4 flow rate of 50 sccm, H2 flow rate of 50 sccm, temperature of 630℃, pressure of 173 Pa, and time of 90 s. Thus, a Si thin film 2 with a thickness of approximately 18 nm was formed on the SiC substrate 1.

[0044] Next, as Figure 1 As shown in (C), the Si thin film 2 was thermally oxidized to form a SiO2 film 3. The thermal oxidation of the Si thin film 2 was performed under the conditions of an O2 flow rate of 2000 sccm, a temperature of 750°C, and a time of 24 hours. It should be noted that these conditions are preferably in the temperature range of 750–850°C, ensuring that the SiC substrate 1 is not oxidized. If the temperature exceeds 850°C, the SiC substrate 1 may be oxidized, which is therefore not preferred.

[0045] Next, as Figure 1 As shown in (D), the SiC substrate 1 on which the SiO2 film 3 is formed was heat-treated in an N2 gas atmosphere. The heat treatment was carried out under the conditions of N2 flow rate of 500 sccm, temperature of 1600℃, pressure of 1 atm, and time of 1 minute.

[0046] It should be noted that, for ease of comparison, the following samples were prepared: [samples were not subjected to further testing / treatment]. Figure 1 (A) shows a pretreatment for etching using high-temperature H2 gas, but instead, a heat treatment using NO gas was performed. Figure 1 (D) shows the interface nitriding treatment, which forms a SiO2 film on a SiC substrate.

[0047] (Analysis of interface defect density)

[0048] For use Figure 1 (A)~ Figure 1 The SiO2 film 3 formed by the method shown in (D) is used to fabricate a MOS capacitor, and the defect density at the interface between the SiO2 film 3 and the SiC substrate 1 is determined by the High-Low CV method.

[0049] Figure 2 This is a graph showing the results, with the horizontal axis representing the conduction band bottom (E). C ) and energy position (E T The difference between the two curves is shown on the left and right sides, with the vertical axis representing the interface defect density. Curve A represents the case where etching with high-temperature H2 gas was performed as a pretreatment, followed by heat treatment with N2 gas as an interface nitriding treatment. Curve B represents the case where etching with high-temperature H2 gas was not performed as a pretreatment, but heat treatment with NO gas was performed as an interface nitriding treatment.

[0050] like Figure 2 As shown, the sample pretreated with H2 gas etching (curve A) exhibits an interface defect state density (hereinafter referred to as "interface defect density") of 3 × 10⁻⁶ over a wide energy range. 10 cm -2 eV -1 Below, compared to the sample that did not undergo H2 gas etching as a pretreatment (curve B), the interface defect density is significantly reduced.

[0051] Especially at the bottom of the conduction band (E C Near the energy location 0.3 eV lower, the interface defect density is 3 × 10⁻⁶. 10 cm -2 eV -1 The energy region is close to the Fermi level when an n-channel MOSFET is turned on (energized), therefore the defect density in this energy range is low, which means that the channel resistance in SiC MOSFETs can be significantly reduced.

[0052] Based on the above analysis results, it can be seen that there are many defects remaining on the surface of SiC substrate 1 after the oxide film is removed by sacrificial oxidation. In order to efficiently remove these defects, etching the surface of SiC substrate 1 with high-temperature H2 gas is effective.

[0053] (Analysis of defects on the SiC substrate side)

[0054] For use Figure 1 (A)~ Figure 1 The SiO2 film 3 formed by the method shown in (D) was used to fabricate a MOS capacitor, and the defects on the SiC substrate side were analyzed. Specifically, deep level transient spectroscopy (DLTS) was used to analyze the types of defects present on the SiC substrate side.

[0055] Figure 3 The graphs show the results, with the horizontal axis representing temperature and the vertical axis representing the DLTS signal. Curve C represents the case where etching with high-temperature H2 gas was performed as a pretreatment, followed by heat treatment with N2 gas as an interface nitriding treatment. Curve D represents the case where etching with high-temperature H2 gas was not performed as a pretreatment, but heat treatment with NO gas was performed as an interface nitriding treatment.

[0056] like Figure 3As shown, in the sample that underwent heat treatment with N2 gas (curve C), only the peak indicated by arrow Z was observed. On the other hand, in the sample that underwent heat treatment with NO gas (curve D), in addition to the peak indicated by arrow Z, three other peaks indicated by arrows N1, N2, and N3 were observed. Here, it is known that the peak indicated by arrow Z is caused by crystal defects (carbon vacancies) present in the SiC substrate 1, which are introduced during the crystal growth of SiC. On the other hand, the peaks indicated by arrows N1, N2, and N3 are consistent with defects generated near the SiC surface due to the thermal oxidation of the SiC substrate 1 (Non-Patent Document 2).

[0057] Based on the above results, it was found that when the interface nitriding treatment was performed using NO gas, the surface of SiC substrate 1 was slightly oxidized during the interface nitriding process. On the other hand, it was found that when the interface nitriding treatment was performed using N2 gas, the surface of SiC substrate 1 was not oxidized.

[0058] The above results also indicate that even if the SiC surface is cleaned and of high quality through high-temperature H2 gas etching, a sufficiently low interface defect density cannot be obtained if the SiC is slightly oxidized in subsequent processes.

[0059] For example, the above description describes a sample that underwent interface nitriding treatment using NO gas. However, the same DLTS peaks N1, N2, and N3 were also observed in the following sample: a sample obtained by etching with high-temperature H2 gas, depositing a Si thin film 2, forming a SiO2 film 3 by oxidation at high temperature (950°C), and then performing nitriding treatment with N2 gas at high temperature (1600°C). This means that the surface of the SiC substrate 1 was oxidized during the oxidation treatment at 950°C.

[0060] In other words, even if high-temperature H2 gas etching is performed to remove defects near the surface of the SiC substrate, when the Si thin film 2 is oxidized to form the SiO2 film 3, if the oxidation is performed at a high temperature (950°C), the surface of the SiC substrate 1 will also be slightly oxidized. Therefore, even if nitriding with N2 gas is performed at a high temperature (1600°C) afterwards, a sufficiently low interface defect density cannot be obtained.

[0061] It should be noted that the samples used in the verification were fabricated using an n-type SiC substrate, while those using a p-type SiC substrate and utilizing [other materials / materials]... Figure 1 (A)~ Figure 1 The sample prepared by the same method shown in (D) was also analyzed by DLTS method to determine the types of defects present on the SiC substrate side.

[0062] Figure 4 The graphs show the results. Curve C represents the case where H2 gas etching was performed as a pretreatment, followed by heat treatment using N2 gas as an interface nitriding treatment. Curve D represents the case where H2 gas etching was not performed as a pretreatment, but heat treatment using NO gas was performed as an interface nitriding treatment.

[0063] like Figure 4 As shown, in the sample that underwent heat treatment with N2 gas (curve C), no peaks due to point defects in the SiC crystal were observed. However, in the sample that underwent heat treatment with NO gas (curve D), the peak indicated by arrow P1 was observed. Here, it is known that the peak indicated by P1 is caused by defects generated near the SiC surface due to the thermal oxidation of the SiC substrate 1 (Non-Patent Document 4).

[0064] Based on the above results, it can be seen that when using a p-type SiC substrate, the surface of SiC substrate 1 is oxidized when the interface is nitrided by heat treatment with NO gas, but the surface of SiC substrate 1 is not oxidized when the interface is nitrided by heat treatment with N2 gas.

[0065] Table 1 shows the energy location and defect density of the defects indicated by arrows N1-N3 and P1: OX-N1, OX-N2, OX-N3, and OX-P1. Here, E... C Ev is the energy level at the bottom of the conduction band, and Ev is the energy level at the top of the valence band. It should be noted that the energy positions were determined by analyzing the temperature dependence of the time constant for carrier release, which can be obtained through DLTS measurements. Furthermore, the defect density was determined based on the peak intensities observed in the DLTS measurements.

[0066] [Table 1]

[0067] Types of defects Energy position Defect density OX-N1 Ec - 0.8eV <![CDATA[~1×10 13 cm -3 ]]> OX-N2 Ec - 1.0 eV <![CDATA[~5×10 12 cm -3 ]]> OX-N3 Ec - 1.6 eV <![CDATA[~2×10 13 cm -3 ]]> OX-P1 Ev+0.7eV <![CDATA[~1×10 13 cm -3 ]]>

[0068] Based on Table 1, it can be inferred that after H2 heat treatment as a pretreatment and N2 gas heat treatment as an interface nitriding treatment, the point defect density at energy positions 1.0 eV lower than the conduction band bottom is 5 × 10⁻⁶ for the following reasons. 11 cm -3 Furthermore, it can be inferred that the point defect density at the energy position with a valence band height of 0.7 eV is 5 × 10⁻⁶. 11 cm -3 the following.

[0069] In other words, such as Figure 3 , Figure 4 As shown, in samples that underwent H2 heat treatment as a pretreatment and N2 gas heat treatment as an interface nitriding treatment, no DLTS peaks corresponding to these point defects were observed. Using the properties of defects observed in other samples, DLTS peaks were simulated numerically, and the results showed that if the point defect density is at least 5 × 10⁻⁶, the DLTS peaks would not be present. 11 cm -3 The above indicates that a DLTS peak was observed under these measurement conditions. Therefore, it can be inferred that the point defect density on the SiC substrate side in the unoxidized sample is approximately 5 × 10⁻⁶. 11 cm -3 the following.

[0070] (Evaluation of the properties of SiO2 films)

[0071] For use Figure 1 (A)~ Figure 1 The SiO2 film 3 formed by the method shown in (D) was evaluated for its properties by the following method in order to confirm the etching effect of using high-temperature H2 gas and the effect of interface nitriding treatment by N2 heat treatment.

[0072] (A) Evaluation of C-V characteristic shift caused by voltage stress

[0073] For use Figure 1 (A)~ Figure 1 The SiO2 film 3 formed by the method shown in (D) was used to fabricate a MOS capacitor, and the characteristic shift caused by voltage stress was evaluated. Specifically, after applying a voltage of 10V to the MOS capacitor for 300 seconds, the bias voltage was scanned from positive to negative. Then, after applying a voltage of -10V for 300 seconds, the bias voltage was scanned from negative to positive, and the C-V characteristic shift was measured.

[0074] Figure 5 This is a graph showing the results, with the horizontal axis representing voltage and the vertical axis representing capacitance. For example... Figure 5 As shown, even when a high electric field (3.3 MV / cm) is applied to the SiO2 film, no shift in the C-V characteristics is observed. This result indicates that there are very few traps in the SiO2 film.

[0075] (B) Evaluation of the insulating properties of SiO2 films

[0076] For use Figure 1 (A)~ Figure 1 The SiO2 film 3 formed by the method shown in (D) was used to fabricate a MOS capacitor, and the insulation characteristics of the SiO2 film 3 were evaluated. Specifically, a positive voltage was applied to the MOS capacitor, and the I-V characteristics were measured.

[0077] Figure 6 This is a graph showing the results, with the horizontal axis representing the electric field (V / t). ox , t ox (This represents the thickness of the SiO2 film 3), with the vertical axis representing the current density. For example... Figure 6 As shown, SiO2 film 3 at 10MVcm -1 Insulation breakdown occurs in the electric field, demonstrating good insulation properties. Furthermore, in the range of 6–9 MVcm... -1 Within a certain range, Fowler-Nordheim tunneling currents were observed. This indicates that there are few defects in the SiO2 film.

[0078] (C) Nitrogen atom density at the SiO2 / SiC interface

[0079] To verify the effectiveness of the interface nitriding treatment using N2 gas, the nitrogen atom density at the interface between the SiO2 film 3 and the SiC substrate 1 was measured using SIMS (secondary ion mass spectrometry).

[0080] Figure 7 This is a graph showing the results. The horizontal axis represents the position in the film thickness direction, zero represents the interface between SiO2 film 3 and SiC substrate 1, the positive side represents the position within the SiC substrate, and the negative side represents the position within the SiO2 film. The vertical axis represents the nitrogen atom density.

[0081] like Figure 7 As shown, the interface between the SiO2 film 3 and the SiC substrate 1 has a spacing of approximately 2 × 10⁻⁶. 21 cm -3 Nitrogen atoms are present at a density of approximately 1 × 10⁻⁶ in the SiO₂ film. 21 cm -3 The density distribution contains nitrogen atoms.

[0082] Based on the above results, it can be concluded that by using N2 gas for heat treatment, nitrogen atoms are introduced at a sufficiently high density into the interface between the SiO2 film and the SiC substrate, as well as into the SiO2 film. Therefore, it can be considered that the defect density at the interface between the SiO2 film and the SiC substrate has been sufficiently reduced.

[0083] Based on the above results, it can be seen that by performing sacrificial oxidation on the surface and then etching the surface of the SiC substrate 1, after removing the oxide film, using high-temperature H2 gas, the defects remaining near the surface of the SiC substrate 1 can be significantly reduced. Furthermore, by forming the SiO2 film 3 on the surface of the SiC substrate 1 without oxidizing it, and then heat-treating the SiC substrate 1 in an N2 gas atmosphere, oxidation of the surface of the SiC substrate 1 can be prevented. Thus, the defect density at the interface between the SiO2 film and the SiC substrate is significantly reduced, resulting in a high-quality SiO2 film with stable properties.

[0084] (Dependence of interface defect density on H2 gas etching temperature)

[0085] Figure 8 This is a graph showing the results of etching the surface of SiC substrate 1 using high-temperature H2 gas at temperatures of 1100℃, 1200℃, and 1300℃, respectively, and measuring the interface defect density at these temperatures. It should be noted that the measurements were performed using the High-Low CV method described above. Furthermore, the dashed curve shown as B in the figure represents etching without the use of high-temperature H2 gas.

[0086] like Figure 8 As shown, the interface defect density decreases significantly at temperatures above 1200°C. On the other hand, at 1100°C, almost no effect of H2 gas etching is observed. This can be attributed to the fact that the SiC substrate is barely etched at 1100°C. It should be noted that at temperatures above 1400°C, the melting point of Si is exceeded, making it difficult to maintain the chemical composition of the SiC substrate surface at normal values. Therefore, to achieve the effect of reducing interface defect density, etching using H2 gas is preferably performed within the temperature range of 1200°C to 1300°C.

[0087] (Dependence of interface defect density on N2 gas heat treatment temperature)

[0088] Figure 9 This graph shows the results of measuring the interface defect density after heat treatment (interface nitriding) of the SiC substrate 1 in an N2 atmosphere at temperatures of 1350℃, 1400℃, and 1600℃, following the formation of a SiO2 film 3 on the surface of the SiC substrate 1. It should be noted that the measurements were performed using the High-Low CV method described above. Furthermore, the dashed curve shown in B of the graph indicates that etching using high-temperature H2 gas was not performed (interface nitriding was performed using NO gas).

[0089] like Figure 9As shown, the interface defect density decreases significantly at temperatures above 1350°C. On the other hand, the effect of N2 gas heat treatment is almost unobservable at temperatures below 1350°C. This can be attributed to the fact that nitrogen atoms are not introduced into the interface between the SiO2 film 3 and the SiC substrate 1 at sufficiently high densities below 1350°C. It should be noted that at temperatures above 1700°C, the surface of the SiO2 film begins to thermally decompose, making it difficult to maintain the film quality. Therefore, to achieve a reduction in interface defect density, heat treatment (interface nitriding) in an N2 gas atmosphere is preferably performed within a temperature range of 1350°C to 1600°C.

[0090] Figure 10 This is a graph showing the results obtained by measuring the nitrogen atom density at the SiO2 / SiC interface after heat treatment (interface nitriding) was performed at temperatures of 1350℃ and 1600℃ in an N2 gas atmosphere. It should be noted that the measurements were performed using the aforementioned SIMS (secondary ion mass spectrometry) method.

[0091] like Figure 10 As shown, when the interface nitriding treatment is performed at a temperature above 1350℃, the interface between the SiC substrate and the SiO2 film, as well as within the SiO2 film, exhibits a 2×10⁻⁶ nitriding rate. 19 cm -3 The above densities contain nitrogen atoms.

[0092] As described above, the method for manufacturing a SiC semiconductor device in this embodiment includes: etching the surface of a SiC substrate 1 with H2 gas at a temperature of 1200°C or higher; depositing a Si thin film 2 on the SiC substrate 1 by CVD; thermally oxidizing the Si thin film 2 at a temperature that does not oxidize the SiC substrate 1 to form a SiO2 film 3; and heat-treating the SiC substrate 1 with the SiO2 film 3 formed thereon in an N2 gas atmosphere at a temperature of 1350°C or higher. This significantly reduces the defect density at the interface between the SiO2 film 3 and the SiC substrate 1, resulting in a high-quality SiO2 film 3 with stable properties.

[0093] (Other implementation methods)

[0094] In the above embodiment, the SiO2 film 3 is formed by thermally oxidizing the Si thin film 2 on the SiC substrate 1 at a temperature that does not oxidize the SiC substrate 1. Therefore, the surface of the SiC substrate 1 is not oxidized. Furthermore, after the SiO2 film 3 is formed, an interface nitriding treatment is performed by heat treatment in a high-temperature N2 gas atmosphere, thereby maintaining the surface of the SiC substrate 1 in a state where it is not oxidized.

[0095] In other words, if a SiO2 film 3 is formed on the SiC substrate 1 after etching the surface of the SiC substrate 1 with high-temperature H2 gas without oxidizing the SiC substrate 1, then the defect density at the interface between the SiO2 film 3 and the SiC substrate 1 can be significantly reduced by heat-treating the SiC substrate 1 with the SiO2 film 3 formed in a high-temperature N2 gas atmosphere.

[0096] Figure 11 (A)~ Figure 11 (C) is a diagram illustrating other methods for forming a SiO2 film on a SiC substrate without oxidizing the SiC substrate.

[0097] like Figure 11 As shown in (A), as a pretreatment step, the surface of the SiC substrate 1 is etched using high-temperature H2 gas. For example, etching using H2 gas can be performed under conditions of H2 flow rate of 1000 sccm, temperature of 1300°C, pressure of 0.1 MPa, and time of 3 minutes.

[0098] It should be noted that the SiC substrate 1 can be a substrate on which a SiC epitaxial layer (not shown) is formed. Furthermore, it is preferable to remove the oxide film after sacrificial oxidation of the surface of the SiC epitaxial layer before the pretreatment process. It should also be noted that, for reasons described later, etching using H2 gas is preferably performed in a Si-rich atmosphere. For example, SiH4 gas with a flow rate of approximately 0.01 to 0.1 sccm can be added to the H2 gas.

[0099] Next, as Figure 11 As shown in (B), a SiO2 film 4 is deposited on a SiC substrate 1 by plasma CVD. The SiO2 film 4 can be deposited under conditions that prevent the SiC substrate 1 from being oxidized, such as a TEOS (tetraethoxysilane) flow rate of 0.3 sccm, an O2 flow rate of 450 sccm, a temperature of 400℃, a pressure of 43 Pa, a high-frequency power of 100 W, and a time of 30 minutes.

[0100] It should be noted that thermal CVD can also be used to deposit the SiO2 film 4. In this case, the SiO2 film 4 can be deposited under conditions where the SiC substrate 1 is not oxidized, such as SiH4 flow rate of 5 sccm, N2O flow rate of 300 sccm, N2 flow rate of 3000 sccm, temperature of 720℃, pressure of 15 kPa, and time of 4 minutes.

[0101] Even under the conditions described above, when depositing the SiO2 film 4, the surface of the SiC substrate 1 may sometimes be slightly oxidized in the initial stage of deposition due to the presence of O2 or N2O gas in the reaction gas. However, even under such conditions, by performing the deposition in a Si-rich atmosphere... Figure 11 (A) shows that an extremely thin Si layer of about one to three layers is formed on the surface of SiC substrate 1 by etching with H2 gas. Therefore, only the extremely thin Si layer is oxidized, while the surface of SiC substrate 1 is not oxidized.

[0102] Next, as Figure 11 As shown in (C), the SiC substrate 1 on which the SiO2 film 4 is formed is heat-treated in an N2 gas atmosphere. For example, heat treatment can be performed under conditions such as N2 flow rate of 500 sccm, temperature of 1600℃, pressure of 1 atm, and time of 1 minute.

[0103] (Dependence of interface defect density on H2 gas etching temperature)

[0104] Figure 12 It shows the use of Figure 11 (A)~ Figure 11 (C) shows a curve illustrating the dependence of the defect density at the interface between SiO2 film 4 and SiC substrate 1 on the H2 gas etching temperature when SiO2 film 4 is formed using the method described above. It should be noted that the measurements were performed using the High-Low CV method described above. Furthermore, the dashed curve shown in B of the figure represents the case where etching with high-temperature H2 gas was not performed (interface nitriding was performed using NO gas).

[0105] like Figure 12 As shown, the interface defect density decreases significantly at temperatures above 1200°C. On the other hand, at 1100°C, almost no effect of H2 gas etching is observed. This can be attributed to the fact that the SiC substrate is barely etched at 1100°C. It should be noted that at temperatures above 1400°C, the melting point of Si is exceeded, making it difficult to maintain the chemical composition of the SiC substrate surface at normal values. Therefore, to achieve the effect of reducing interface defect density, etching using H2 gas is preferably performed within the temperature range of 1200°C to 1300°C.

[0106] (Dependence of interface defect density on N2 gas heat treatment temperature)

[0107] Figure 13This graph shows the results of measuring the interface defect density after heat treatment (interface nitriding) of the SiC substrate 1 in an N2 atmosphere at temperatures of 1300℃, 1400℃, 1450℃, and 1600℃, following the formation of the SiO2 film 4 on the surface of the SiC substrate 1. It should be noted that the measurements were performed using the High-Low CV method described above. Furthermore, the dashed curve shown in B of the graph represents the case where etching using high-temperature H2 gas was not performed (interface nitriding was performed using NO gas).

[0108] like Figure 13 As shown, the interface defect density decreases significantly at temperatures above 1400°C. On the other hand, almost no effect of N2 gas heat treatment was observed at 1300°C. This can be attributed to the fact that at 1300°C, nitrogen atoms were not introduced into the interface between the SiO2 film 3 and the SiC substrate 1 at a sufficiently high density. It should be noted that at temperatures above 1700°C, the surface of the SiO2 film begins to thermally decompose, making it difficult to maintain the film quality. Therefore, to achieve the effect of reducing the interface defect density, heat treatment (interface nitriding) is preferably performed in an N2 gas atmosphere within a temperature range of 1350°C to 1600°C.

[0109] (Nitrogen atom density at the SiO2 / SiC interface)

[0110] Figure 14 This is a graph showing the results of measuring the nitrogen atom density at the interface between SiO2 film 4 and SiC substrate 1. The measurements were performed using the aforementioned SIMS (Secondary Ion Mass Spectrometry). The horizontal axis represents the position along the film thickness direction; zero represents the interface between SiO2 film 4 and SiC substrate 1, positive sides represent positions within the SiC substrate, and negative sides represent positions within the SiO2 film. The vertical axis represents the nitrogen atom density. Curve G in the graph represents the case where H2 gas etching at 1300°C was performed as a pretreatment, followed by heat treatment using N2 gas at 1600°C as an interface nitriding treatment. Curve H represents the case where H2 gas etching was not performed as a pretreatment, but heat treatment using NO gas was performed as an interface nitriding treatment.

[0111] like Figure 14 As shown, when etched using high-temperature H2 gas, the interface between the SiO2 film 4 and the SiC substrate 1 has a spacing of approximately 2 × 10⁻⁶. 21 cm -3 Nitrogen atoms are present at a density of approximately 1 × 10⁻⁶ in the SiO₂ film. 20 cm -3The density distribution above contains nitrogen atoms. It should be noted that, although in Figure 14 Not shown in the image, but related to Figure 10 As shown, when heat treatment with N2 gas was performed at 1350°C, a concentration of approximately 2 × 10⁻⁶ was also observed in the SiO₂ film. 19 cm -3 The above density distribution contains nitrogen atoms.

[0112] On the other hand, when NO gas is used to nitrid the interface, the interface between the SiO2 film 4 and the SiC substrate 1 also has a density of approximately 2 × 10⁻⁶. 21 cm -3 Nitrogen atoms are present in the density of the film, but almost none are distributed in the SiO2 film.

[0113] (High-temperature H2 etching in a Si-rich atmosphere)

[0114] As described above, even when a SiO2 film 4 is deposited on the SiC substrate 1 under conditions where the SiC substrate 1 is not oxidized, the surface of the SiC substrate 1 may be slightly oxidized in the initial stage of deposition due to the presence of O2 or N2O gas in the reaction gas. However, even under such conditions, by performing etching with H2 gas as a pretreatment in a Si-rich atmosphere, an extremely thin Si layer of about one to three layers is formed on the surface of the SiC substrate 1. Therefore, only this extremely thin Si layer is oxidized, and the surface of the SiC substrate 1 is not oxidized.

[0115] Figure 15 The graph shows the difference in defect density at the interface between the SiO2 film 4 and the SiC substrate 1 when etching was performed in a Si-rich atmosphere using high-temperature H2 gas versus when etching was performed in a Si-rich atmosphere without using high-temperature H2 gas. Curve J represents the case where etching was performed in a Si-rich atmosphere, and curve K represents the case where etching was not performed in a Si-rich atmosphere.

[0116] Here, etching using high-temperature H2 gas was performed under the conditions of H2 flow rate of 1000 sccm, temperature of 1300℃, pressure of 0.1 MPa, and time of 3 minutes. Furthermore, in the case of etching using high-temperature H2 gas in a Si-rich atmosphere, SiH4 gas with a flow rate of 0.05 sccm was added. Heat treatment in an N2 gas atmosphere was performed under the conditions of N2 flow rate of 500 sccm, temperature of 1450℃, pressure of 1 atm, and time of 1 minute.

[0117] like Figure 15 As shown, when etching was performed in a Si-rich atmosphere using high-temperature H2 gas (curve J), ​​the interface defect density was significantly reduced to 3 × 10⁻⁶. 10 cm-2 eV -1 On the other hand, when etching with high-temperature H2 gas was not performed in a Si-rich atmosphere, even after depositing a SiO2 film under what was considered optimal conditions and performing a high-temperature N2 treatment, the interface defect density was not sufficiently reduced, and a high-quality interface was not obtained. This can be attributed to the fact that an extremely thin Si film was not formed on the surface of the SiC substrate, and therefore the surface of the SiC substrate was oxidized in the initial stage of SiO2 film deposition.

[0118] For samples etched in a Si-rich atmosphere using high-temperature H2 gas, the types of defects present on the SiC substrate side were analyzed using the DLTS method. No defects caused by SiC crystal oxidation were observed. Figure 3 and Figure 4 (Defects indicated by arrows N1-N3 and P1). On the other hand, for samples that were not etched using high-temperature H2 gas in a Si-rich atmosphere, defects caused by SiC crystal oxidation were observed on the SiC substrate side. This means that even when a SiO2 film 4 was deposited on the SiC substrate 1 under conditions where it was assumed that the SiC substrate 1 would not be oxidized, the surface of the SiC substrate 1 was still oxidized.

[0119] As described above, the method for manufacturing a SiC semiconductor device in this embodiment includes: etching the surface of a SiC substrate 1 using H2 gas at a temperature of 1200°C or higher in a Si-rich atmosphere; forming a SiO2 film 4 on the SiC substrate 1 by CVD; and heat-treating the SiC substrate 1 on which the SiO2 film 4 is formed in an N2 gas atmosphere at a temperature of 1350°C or higher. This significantly reduces the defect density at the interface between the SiO2 film 4 and the SiC substrate 1, resulting in a high-quality SiO2 film 3 with stable properties.

[0120] (SiC semiconductor device)

[0121] The SiO2 film formed using the manufacturing method of this embodiment can be used as a gate insulating film to construct a SiC semiconductor device (SiC MOSFET). In the above-described SiC semiconductor device, the interface between the SiC substrate and the SiO2 film, and the SiO2 film at a density of 2 × 10⁻⁶... 19 cm -3 The above densities contain nitrogen atoms.

[0122] The interfacial defect density between the SiC substrate and the SiO2 film near an energy position 0.3 eV below the conduction band bottom is 3 × 10⁻⁶. 10 cm -2 eV -1 the following.

[0123] Among the point defects present on the SiC substrate side, the point defect density at the energy position 1.0 eV lower than the conduction band bottom and the point defect density at the energy position 0.7 eV higher than the valence band top are respectively 5 × 10⁻⁶. 11 cm -3 the following.

[0124] The present invention has been described above through preferred embodiments; however, the above description is not limiting and various modifications are possible. For example, in the above embodiments, a SiC epitaxial layer is formed on the surface of a SiC substrate, and a SiO2 film is formed on the SiC epitaxial layer, but the SiO2 film can also be formed directly on the SiC substrate.

[0125] Furthermore, in the above embodiments, a SiC substrate in which the oxide film was removed after sacrificial oxidation of the surface was used, but the manufacturing method of the present invention can also be applied to SiC substrates in which sacrificial oxidation was not performed.

[0126] - Symbol Explanation -

[0127] 1 SiC substrate

[0128] 2 Si thin film

[0129] 3, 4 SiO2 film.

Claims

1. A method for manufacturing a SiC semiconductor device, characterized in that: The method for manufacturing the SiC semiconductor device includes: Step (A) involves etching the surface of a SiC substrate using H2 gas in a Si-rich atmosphere at a temperature above 1200°C. Step (B) involves forming a SiO2 film on the SiC substrate using CVD without oxidizing the SiC substrate; and Step (C) involves heat-treating the SiC substrate on which the SiO2 film is formed in an N2 gas atmosphere at a temperature above 1350°C. In step (A), one to three Si layers are formed on the surface of the SiC substrate.

2. The method for manufacturing a SiC semiconductor device according to claim 1, characterized in that: The method for manufacturing the SiC semiconductor device further includes: prior to step (A), after sacrificial oxidation of the SiC substrate, removing the oxide film formed on the surface of the SiC substrate by etching.

3. A method for manufacturing a SiC semiconductor device, characterized in that: The method for manufacturing the SiC semiconductor device includes: Step (A) involves etching the surface of a SiC substrate using H2 gas at temperatures above 1200°C. Step (B) involves forming a SiO2 film on the SiC substrate without oxidizing it; and Step (C) involves heat-treating the SiC substrate on which the SiO2 film is formed in an N2 gas atmosphere at a temperature above 1350°C. Following the heat treatment in step (C), the interface defect density between the SiC substrate and the SiO2 film near an energy position 0.3 eV lower than the conduction band bottom is 3 × 10⁻⁶. 10 cm -2 eV -1 the following.

4. The method for manufacturing a SiC semiconductor device according to claim 1, characterized in that: The SiC substrate includes a SiC substrate on which a SiC epitaxial layer is formed on its surface.

Images