Method for forming single crystal diamond film

Abstract

To provide a method for forming a large area single crystal diamond film having high-density diamond cores, and excellent in crystal orientation without having a grain boundary in the film.SOLUTION: A method for forming a single crystal diamond film on a single crystal silicon substrate comprises: a first step of preparing the single crystal silicon substrate having a plane direction of (100) or (111); a second step of subjecting the prepared single crystal silicon substrate to RTA treatment in a carbon containing atmosphere to form a 3C-SiC single crystal film on the surface; a third step of transforming the 3C-SiC single crystal film into diamond cores by a microwave plasma CVD method applying bias voltage in the carbon containing atmosphere to form the diamond cores on the single crystal silicon substrate; and a fourth step of growing the single crystal diamond film on the single crystal silicon substrate by the microwave plasma CVD method in the carbon containing atmosphere.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for forming a single-crystal diamond film, and particularly provides a method for forming a large-area single-crystal diamond film having excellent crystal orientation and no grain boundaries in the film.

Background Art

[0002] Diamond has excellent heat resistance, a large bandgap of 5.5 eV, and is usually an insulator but can be made into a semiconductor by doping with impurities. In addition, it also has excellent electrical properties such as a high breakdown voltage, a high saturation drift velocity, and a small dielectric constant. Therefore, diamond is expected to be used as an electronic device and sensor material for high temperature, high frequency, or high electric field applications.

[0003] As methods for vapor-phase synthesizing diamond, a microwave CVD (Chemical Vapor Deposition) method (Non-Patent Documents 1 to 3), a high-frequency plasma CVD method, a thermal filament CVD method, a DC plasma CVD method, a plasma jet method, a combustion method, a thermal CVD method, and the like are known.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] The lattice mismatch rate between diamond and Si is about 34%. Since the lattice constants of diamond and silicon are significantly different in this way, when forming a diamond single crystal film by the MPCVD (Microwave Plasma Chemical Vapor Deposition) method, methods using a SiC substrate (Non-Patent Document 1) or a metal Pt film (Non-Patent Document 3) have been proposed to reduce the lattice mismatch rate with diamond. Furthermore, a method has been adopted in which a bias voltage is applied to a silicon single crystal substrate during the MPCVD process to promote the decomposition of hydrocarbons on the silicon single crystal substrate, form diamond nuclei in the 3C-SiC layer, and then form a single crystal diamond film.

[0007] However, in the above method, it is difficult to form diamond nuclei of 3C-SiC. Therefore, the surface density of diamond nuclei of 3C-SiC is 1×10 8 ~1×10 10 / cm 2It is low to a certain extent, and the growth of single-crystal diamond is difficult. Furthermore, due to the presence of significant grain boundaries, there are also problems with the flatness of the surface. In addition, the film formation rate of a conventional microwave plasma CVD apparatus is about 0.2 μm / hr. In order to form a film thickness of several hundred μm, long-term vapor phase synthesis must be carried out, increasing the manufacturing cost. On the other hand, a large-scale microwave CVD apparatus with an input power of 60 kW or more has a high film formation rate and is suitable for thick film formation, but there has been no study on the technology for synthesizing a diamond film with an oriented crystal plane. Therefore, at present, the technology for vapor phase synthesizing a thick film diamond film with an oriented crystal plane and no grain boundaries over a large area has not been established.

[0008] In addition, when using a silicon single crystal substrate, a method via an Ir buffer layer (Non-Patent Document 2) has also been proposed. In this method, by forming an intermediate layer (buffer layer) of a noble metal single substance or alloy having a crystal plane of (111) or (001), the surface density of diamond nuclei is increased to 1×10 11 / cm 2 and a single-crystal diamond film is vapor phase synthesized by epitaxial growth. However, there is a problem that the diamond film peels off from the substrate, and furthermore, as in Non-Patent Documents 1 and 3, the area without grain boundaries is small.

[0009] Furthermore, Patent Document 1 describes forming a SiC single crystal film on a silicon substrate, then forming an amorphous carbon film while supplying CH4 gas on the SiC single crystal film, and then crystallizing the amorphous carbon film by applying energy from the outside to change it into a single-crystal diamond film, and further epitaxially growing the diamond single crystal film. However, the method of applying high energy to such an amorphous carbon film to change it into a single-crystal diamond film randomly converts from an sp2 structure to an sp3 structure, so the orientation of diamond nuclei also becomes random and does not become a highly oriented single-crystal diamond film.

[0010] The present invention has been made in view of such problems, and an object thereof is to provide a method for forming a large-area single-crystal diamond film in which high-density diamond nuclei are formed, the crystal orientation is excellent, and there are no grain boundaries in the film.

Means for Solving the Problems

[0011] In order to achieve the above object, the present invention is a method for forming a single-crystal diamond film on a single-crystal silicon substrate, a first step of preparing a single-crystal silicon substrate having a plane orientation of (100) or (111) as the single-crystal silicon substrate, a second step of performing RTA treatment on the single-crystal silicon substrate prepared in the first step in a carbon-containing atmosphere to form a 3C-SiC single-crystal film on the surface of the single-crystal substrate, a third step of converting the 3C-SiC single-crystal film into diamond nuclei by microwave plasma CVD method with a bias voltage applied in a carbon-containing atmosphere after the second step to form the diamond nuclei on the single-crystal silicon substrate, a fourth step of growing a single-crystal diamond film on the single-crystal silicon substrate by microwave plasma CVD method in a carbon-containing atmosphere after the third step, and provides a method for forming a single-crystal diamond film, characterized by including the above steps.

[0012] In the method for forming a single-crystal diamond film of the present invention as described above, by preparing and using a single-crystal silicon substrate having a plane orientation of (100) or (111), highly oriented diamond nuclei can be formed. Further, since a 3C-SiC single-crystal film is formed on the surface of the single-crystal silicon substrate by sublimation method by RTA (Rapid Thermal Annealing) treatment, the carbon concentration on the outermost surface of the 3C-SiC single-crystal film can be made higher than the silicon concentration. And by converting such a 3C-SiC single-crystal film into diamond nuclei, it becomes possible to form high-density diamond nuclei uniformly over the entire surface on the single-crystal silicon substrate. In addition, since the high-density diamond nuclei formed on the surface of the 3C-SiC single-crystal film have a high degree of orientation, the conversion from sp3-structured SiC to sp3-structured diamond becomes easy. As a result, it becomes possible to vapor-phase synthesize a thick-film single-crystal diamond film with crystal planes oriented and no grain boundaries over a large area.

[0013] At this time, the 3C-SiC single-crystal film formed in the second step can be a 3C-SiC single-crystal film having a thickness of 0.5 nm or more and 10 nm or less and a carbon concentration on the outermost surface higher than the Si concentration.

[0014] By forming such a thick 3C-SiC single-crystal film, it becomes difficult for Si to sublime from the surface of the single-crystal silicon substrate, and the carbon concentration on the outermost surface of the 3C-SiC single-crystal film can be made more surely higher than the Si concentration. As a result, it becomes possible to more surely form high-density diamond nuclei on the surface of the 3C-SiC single-crystal film.

[0015] In addition, in the second step, the carbon-containing atmosphere can be a CH4-containing atmosphere of 0.5% or more and 10% or less, and the heat treatment temperature can be 1100 °C or more and 1350 °C or less.

[0016] By setting the second step under such conditions, a 3C-SiC single-crystal film can be more surely formed in a uniform planar state, so that the diamond nuclei formed by converting the 3C-SiC single-crystal film can have a high degree of orientation.

[0017] In addition, when performing the third step, the single-crystal silicon substrate having the 3C-SiC single-crystal film formed on its surface can be divided into a size that can be placed in a microwave plasma CVD apparatus.

[0018] Since the formation of diamond nuclei and the growth of the single-crystal diamond film are performed by the microwave plasma method, it is preferable to divide the single-crystal silicon substrate having the 3C-SiC single-crystal film formed on its surface into a size that can be placed in a microwave plasma CVD apparatus.

[0019] Also, the average planar density of the diamond nuclei formed in the third step can be set to 1×10 11 / cm 2 or more.

[0020] In this way, in the subsequent growth process of the single-crystal diamond film, a single-crystal diamond film without grain boundaries can be effectively grown and formed.

[0021] Also, in the third step, the carbon-containing atmosphere can be set to a CH4-containing atmosphere of 0.5% or more and 5.5% or less, the bias voltage can be set to 400 V or more and 800 V or less, and the heat treatment temperature can be set to 900 °C or more and 1200 °C or less.

[0022] By setting the third step under such conditions, the average planar density of the diamond nuclei can be more easily controlled to a high density. For example, it can be controlled to 1×10 11 / cm 2 or more.

[0023] Also, in the fourth step, the carbon-containing atmosphere can be set to a CH4-containing atmosphere of 0.5% or more and 5.5% or less, and the heat treatment temperature can be set to 850 °C or more and 1150 °C or less.

[0024] By setting the fourth step under such conditions, a high-quality, thick-film single-crystal diamond film without grain boundaries can be more reliably formed.

[0025] Also, the thickness of the single-crystal diamond film grown in the fourth step can be set to 50 μm or more.

[0026] According to the diamond formation method of the present invention, a thick single-crystal diamond film with a thickness of 50 μm or more and without grain boundaries and with high orientation can be formed.

Advantages of the Invention

[0027] In the method for forming a single-crystal diamond film of the present invention, diamond nuclei can be formed uniformly and at a high density on a single-crystal silicon substrate. As a result, a thick film with no grain boundaries and excellent crystal orientation can be obtained, and a large-area single-crystal diamond film can be obtained.

Brief Description of Drawings

[0028]

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Modes for Carrying Out the Invention

[0029] As a result of intensive studies on the above-described problems, the present inventors formed a 3C-SiC single-crystal film on the surface of a single-crystal silicon substrate, and in order to form a thick single-crystal diamond film with no grain boundaries, high orientation, and a thick film from this film, it was conceived that it is necessary to increase the areal density of diamond nuclei when forming diamond nuclei from the 3C-SiC single-crystal film. By forming a 3C-SiC single-crystalline film on a sublimation method by RTA treatment, the carbon concentration on the outermost surface of the 3C-SiC single-crystalline film is made higher than the Si concentration, and high-density diamond nuclei can be formed by the MPCVD method with an applied bias voltage. Furthermore, the present invention has been completed by finding that a single-crystalline diamond film with no grain boundaries, high orientation, and a thick film can be formed by growing a single-crystalline diamond film by the MPCVD method.

[0030] Hereinafter, the present invention will be described in more detail, but the present invention is not limited thereto. FIG. 1 shows the flow of a method for forming a single-crystalline diamond film of the present invention. It is roughly divided into four steps, namely, <First step> a step of preparing a single-crystalline silicon substrate, <Second step> a step of forming a 3C-SiC single-crystalline film, <Third step> a step of forming diamond nuclei, and <Fourth step> a step of growing a single-crystalline diamond film. Each step will be described in detail below.

[0031] <First step> A step of preparing a single-crystalline silicon substrate Prepare a single-crystalline silicon substrate with a plane orientation of (100) or (111). Orientation growth depends on the fastest growth orientation of diamond. Since the plane orientations (100) and (111) are planes with fast growth, high-orientation diamond nuclei can be formed by using a single-crystalline silicon substrate with such a plane orientation.

[0032] <Second step> A step of forming a 3C-SiC single-crystalline film By performing RTA treatment, a 3C-SiC single-crystalline film is formed on the surface of the single-crystalline silicon substrate by the sublimation method. The apparatus used for the RTA treatment is not particularly limited, and for example, a conventionally used RTA apparatus can be used. Place the single-crystalline silicon substrate in the quartz chamber of the RTA apparatus, set it to a carbon-containing atmosphere using Ar gas, hydrogen gas, etc. as carrier gases, and perform RTA treatment.

[0033] At this time, the film thickness of the 3C-SiC single crystal film to be formed is not particularly limited, but for example, it can be 0.5 nm or more and 10 nm or less. If the film thickness of the 3C-SiC single crystal is 0.5 nm or more, the carbon concentration on the outermost surface of the 3C-SiC single crystal film can be more surely made higher than the Si concentration. And the formation of high-density diamond nuclei in the next third step becomes more certain. In the formation of a 3C-SiC single crystal film by the sublimation method using RTA treatment, as the thickness of the 3C-SiC single crystal film increases, the amount of Si supplied from the single crystal silicon substrate interface to the reaction surface decreases, and the carbon concentration on the outermost surface of the 3C-SiC single crystal film increases. From the viewpoint of this carbon concentration, 10 nm is sufficient, and when it becomes thicker than 10 nm, the growth of the 3C-SiC single crystal film stops.

[0034] Also, in this second step, an RTA treatment in a carbon-containing atmosphere may be used, and the conditions are not particularly limited. For example, the carbon-containing atmosphere can be a CH4-containing atmosphere of 0.5% or more and 10% or less, and the heat treatment temperature can be 1100 °C or more and 1350 °C or less. In a carbon-containing atmosphere, if CH4 is 0.5% or more, it is possible to effectively prevent the formation of an "island-like" discontinuous 3C-SiC single crystal film instead of growing a uniform 3C-SiC single crystal film on the surface of the single crystal silicon substrate. In other words, a 3C-SiC single crystal film with a more uniform surface state can be formed, and the formation of highly oriented diamond nuclei in the next third step can be carried out more surely. On the other hand, if CH4 is 10% or less, there is no particular problem in the formation of the 3C-SiC single crystal film, and it is possible to suppress the surface of the quartz chamber of the RTA apparatus from being easily contaminated. Note that the introduction gas for forming the carbon-containing atmosphere is not limited to CH4, and C2H6, C3H8, etc. may also be used. This is the same not only in the second step but also in the third and fourth steps described later.

[0035] Furthermore, by setting the heat treatment temperature to 1100 °C or higher, the film formation rate is good, and a thick 3C-SiC single crystal film can be efficiently formed. On the other hand, if the heat treatment temperature is 1350 °C or lower, the decomposition of carbon due to the too-high heat treatment temperature can be suppressed, and the quartz chamber of the RTA apparatus can be prevented from being contaminated. Also, the heating rate, the cooling rate, the holding time at the above heat treatment temperature, etc. are not particularly limited. For example, the heating rate and the cooling rate can be 10 to 50 °C / sec, and the holding time can be 1 to 100 sec. It can be appropriately determined according to the film thickness to be formed, etc.

[0036] <Third step> Diamond nucleus formation step By the MPCVD method with a bias voltage applied in a carbon-containing atmosphere, the 3C-SiC single crystal film formed in the second step is converted into diamond nuclei, and diamond nuclei are formed on the single crystal silicon substrate. Note that the apparatus used in the treatment by the MPCVD method is not particularly limited, and for example, a conventionally used MPCVD apparatus can be used. The carbon atoms of the diamond nuclei are supplied not only from the carbon atoms of the 3C-SiC single crystal film but also from the carbon-containing atmosphere. Also, the Si atoms in the 3C-SiC single crystal film are discharged out of the apparatus together with the carrier gas. Here, first, due to the relationship between the MPCVD apparatus and the size of the object to be treated, if necessary, the single crystal silicon substrate with a 3C-SiC single crystal film formed on the surface in the second step can be divided. For example, it can be divided into about 20 mm□ and placed in the MPCVD apparatus, but the divided size can be appropriately determined. Of course, if possible, the entire substrate can be placed in the MPCVD apparatus without division for treatment, and finally, a single crystal diamond film with a larger area can be obtained.

[0037] Incidentally, diamond nuclei have a three-dimensional sp3 structure. Since 3C-SiC also has the same sp3 structure, diamond nuclei can be formed with low energy by any method of converting 3C-SiC into diamond nuclei. At this time, place the substrate after the second step (or the divided substrate) in the chamber of the MPCVD apparatus, create a carbon-containing atmosphere using Ar gas, hydrogen gas, etc. as carrier gases, and while applying a bias voltage to the substrate having a 3C-SiC single crystal film with a carbon concentration on the surface higher than the Si concentration obtained in the second step, convert the 3C-SiC single crystal film into diamond nuclei by the MPCVD method. Then, the conversion into diamond nuclei becomes more efficient, and it becomes possible to form high-density diamond nuclei more efficiently.

[0038] The conditions of MPCVD can be appropriately determined according to the film thickness of the 3C-SiC single crystal film formed in the second step, etc. For example, the carbon-containing atmosphere can be a CH4-containing atmosphere of 0.5% or more and 5.5% or less, the bias voltage can be 400 V or more and 800 V or less, and the heat treatment temperature can be 900 °C or more and 1200 °C or less. At this time, especially when CH4 is 0.5% or more in the carbon-containing atmosphere, the bias voltage is 400 V or more, and the heat treatment temperature is 900 °C or more, diamond nuclei can be more surely formed at high density. On the other hand, when CH4 is 5.5% or less, the bias voltage is 800 V or less, and the heat treatment temperature is 1200 °C or less, diamond nuclei can be more surely controlled to have highly oriented crystallinity. Under such conditions, the average surface density of diamond nuclei formed by conversion from a 3C-SiC single crystal film can be more easily controlled to a high density, especially to 1×10 11 / cm 2 or more. If it is 1×10 11 / cm 2 or more, a single crystal diamond film without grain boundaries can be more surely grown in the next fourth step. Note that the upper limit value of the average surface density of diamond nuclei is not limited, and the higher the better.

[0039] <Fourth Step> Growth step of single crystal diamond film Next, grow a single crystal diamond film on a single crystal silicon substrate by the MPCVD method in a carbon-containing atmosphere. The same MPCVD apparatus as in the third step can be used. As the conditions for MPCVD at this time, for example, the carbon-containing atmosphere can be a CH4-containing atmosphere of 0.5% or more and 5.5% or less, and the heat treatment temperature can be 850°C or more and 1150°C or less. When CH4 is 0.5% or more in the carbon-containing atmosphere and the heat treatment temperature is 850°C or more, the growth rate of the single crystal diamond film is high, which is effective for forming a thick single crystal diamond film. On the other hand, if CH4 is 5.5% or less and the heat treatment temperature is 1150°C or less, the film growth will not be too fast, and it is also possible to sufficiently control the highly oriented crystallinity.

[0040] By forming under the above conditions, a single crystal diamond film with no grain boundaries and high orientation can be formed more reliably. Moreover, it is also possible to sufficiently obtain a thick and large-area single crystal diamond film. In particular, a thick single crystal diamond film with a thickness of 50 μm or more can be formed. In addition, a thickness of 100 μm is also sufficient. It is possible to obtain a product in which a 3C-SiC single crystal film with a slight thickness remains on the surface of the single crystal silicon substrate without being converted, and a thick single crystal diamond film is formed thereon.

Example

[0041] Hereinafter, the present invention will be described more specifically by showing examples and comparative examples, but the present invention is not limited to these examples. (Example 1) The diamond film was formed by the method for forming a single crystal diamond film of the present invention shown in FIG. 1 as follows. <First step> A single crystal silicon substrate with a diameter of 200 mm and a plane orientation of (100) was prepared. <Second step> Using an RTA apparatus (apparatus name: AST2800 manufactured by Mattson Technology), a 3C-SiC single crystal film with a film thickness of 3 nm was formed by RTA treatment under the following conditions. Carbon-containing atmosphere: Ar + H2 atmosphere containing 1.4% CH4 Heat treatment temperature: 1200°C / 10 sec, heating rate: 25°C / sec · cooling rate: 33°C / sec <Third Process> After the second process, the single-crystalline silicon substrate with a 3C-SiC single-crystalline film was cleaved and divided into 20 mm squares. After that, using an MPCVD apparatus (apparatus name: microwave diamond film deposition apparatus manufactured by Nippon RF Kogyo Co., Ltd.), the conversion formation from a 3C-SiC single-crystalline film to diamond nuclei was performed by the MPCVD method while applying a bias under the conditions shown in Table 1.

[0042] [Table 1]

[0043] [Evaluation of Diamond Nuclei] Here, the average surface density of the diamond nuclei formed in the third process was measured. The measurement was performed using an SEM (Scanning Electron Microscope) and calculation software (WinROOF 2018). As a result, the average surface density was about 2×10 12 / cm 2 (the upper limit value of the measurement). Figure 2 shows an observation image of the substrate surface on which diamond nuclei were formed by SEM. In addition, Figure 3 shows a cross-section of the substrate surface layer on which diamond nuclei were formed by TEM (Transmission Electron Microscope). It was confirmed that the morphology of the diamond crystal nuclei at this time was pyramid-shaped. In addition, the diamond nuclei generated under the above conditions were evaluated by Raman spectroscopy. The results are shown in Figure 4. A peak was observed at the position where the Raman Shift was 1333 cm -1 , and it was confirmed that they were diamond nuclei.

[0044] <Fourth Process> The growth of a single-crystalline diamond film with a size of 20 mm square was performed by the MPCVD method under the conditions shown in Table 2.

[0045] [Table 2]

[0046] [Evaluation of Single-Crystalline Diamond Film] Fig. 5 shows a single crystal diamond film with a size of 20 mm□ formed thereon. Also, as shown in the evaluation results by Raman spectroscopy in Fig. 6, a peak was observed at the position of Raman Shift of 1333 cm -1 , and a high-purity single crystal diamond film was confirmed. Also, as shown in the observation diagram of the surface of the single crystal diamond film by SEM in Fig. 7, abnormal growth was not confirmed, and no significant grain boundaries were observed. Also, the grown single crystal diamond film was cleaved, and the results of SEM observation are shown in Fig. 8. As can be seen from Fig. 8, it was confirmed that the film thickness was 104 μm. It can be seen that a single crystal diamond film was formed on an extremely thin 3C-SiC single crystal film that remained without being converted into diamond nuclei from the 3C-SiC single crystal film in the third step.

[0047] (Comparative Example 1) Single crystal diamond film growth was carried out under the same conditions as in Example 1 except that the formation of the 3C-SiC single crystal film in the second step was performed by CVD method. The CVD conditions at this time were 5 min at 900 to 1100 °C in a CH4 atmosphere. Thereby, a SiC layer of about 100 nm was formed on the single crystal silicon substrate. Then, when diamond nuclei were formed under the same conditions as in the third step of Example 1, the average surface density was about 1×10 8 / cm 2 , and diamond nuclei could not be formed at high density. Also, the single crystal diamond film grown under the same conditions as in the fourth step of Example 1 was island-shaped with grain boundaries, and a uniform single crystal diamond film could not be formed.

[0048] (Comparative Example 2) Single crystal diamond film growth was carried out under the same conditions as in Example 1 except that the diamond nucleus formation step in the third step was performed without applying a bias voltage. As a result, the average surface density was less than 1×10 4 / cm 2 , and diamond nuclei could not be formed at high density. In addition, the single-crystal diamond film grown under the same conditions as in the fourth step of Example 1 was island-shaped with grain boundaries and a uniform single-crystal diamond film could not be formed.

[0049] Note that the present invention is not limited to the above-described embodiments. The above-described embodiments are examples, and any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits the same operational effects is included in the technical scope of the present invention.

Claims

1. A method for forming a single-crystal diamond film on a single-crystal silicon substrate, comprising: a first step of preparing a single-crystal silicon substrate having a plane orientation of (100) or (111) as the single-crystal silicon substrate; a second step of performing RTA treatment on the single-crystal silicon substrate prepared in the first step in a carbon-containing atmosphere to form a 3C-SiC single-crystal film having a thickness of 0.5 nm or more and 10 nm or less on the surface of the single-crystal silicon substrate, and having a carbon concentration on the outermost surface higher than the Si concentration; a third step of converting the 3C-SiC single-crystal film into diamond nuclei by a microwave plasma CVD method with a bias voltage applied in a carbon-containing atmosphere after the second step, and forming the diamond nuclei on the single-crystal silicon substrate; a fourth step of growing a single-crystal diamond film on the single-crystal silicon substrate by a microwave plasma CVD method in a carbon-containing atmosphere after the third step. A method for forming a single-crystal diamond film, characterized by including the above steps.

2. In the second step, the carbon-containing atmosphere is a CH-containing atmosphere of 0.5% or more and 10% or less, and the heat treatment temperature is 1100°C or more and 1350°C or less. The method for forming a single crystal diamond film according to claim 1, characterized in that. ４ The method for forming a single crystal diamond film according to claim 1, characterized in that.

3. The method for forming a single-crystal diamond film according to claim 1 or claim 2, characterized in that when performing the third step, the single-crystal silicon substrate having the 3C-SiC single-crystal film formed on its surface is divided into a size that can be placed in a microwave plasma CVD apparatus and the step is performed.

4. The average surface density of the diamond nuclei formed in the third step is 1×10 １１ / cm ２ or more, and the method for forming a single crystal diamond film according to any one of claims 1 to 3 is characterized in that.

5. In the third step, the carbon-containing atmosphere is a CH-containing atmosphere of 0.5% or more and 5.5% or less, the bias voltage is 400 V or more and 800 V or less, and the heat treatment temperature is 900 °C or more and 1200 °C or less. The method for forming a single crystal diamond film according to any one of claims 1 to 4, characterized in that. ４ The method for forming a single crystal diamond film according to any one of claims 1 to 4, characterized in that.

6. In the fourth step, the carbon-containing atmosphere is set to a CH-containing atmosphere of 0.5% or more and 5.5% or less, and the heat treatment temperature is set to 850°C or more and 1150°C or less. The method for forming a single crystal diamond film according to any one of claims 1 to 5, characterized in that. ４ The method for forming a single crystal diamond film according to any one of claims 1 to 5, characterized in that in the fourth step, the carbon-containing atmosphere is a CH-containing atmosphere of 0.5% or more and 5.5% or less, and the heat treatment temperature is 850°C or more and 1150°C or less.

7. The method for forming a single-crystal diamond film according to any one of claims 1 to 6, characterized in that the thickness of the single-crystal diamond film grown in the fourth step is 50 μm or more.

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