Multilayer substrate, multilayer substrate provided with functional film, and method for manufacturing multilayer substrate

Abstract

Provided is a multilayer substrate which has both a function as a heat sink and a function as a heat spreader and on which it is possible to form a functional layer (for example, a GaN layer or an AlGaN layer) for a device at a low cost. This multilayer substrate has a three-layer structure comprising: a support substrate formed of a material that is other than diamond and that is selected from the group consisting of ceramics and metals; a diamond film provided on the support substrate; and a single crystal film that is provided on the diamond film and that is for use in forming a functional layer.

Description

Multilayer substrate, multilayer substrate with functional film, and method for manufacturing a multilayer substrate

[0001] This disclosure relates to a multilayer substrate, a multilayer substrate with a functional film, and a method for manufacturing a multilayer substrate.

[0002] Diamond is a material with extremely high thermal conductivity (approximately 22 W / cm·K), and is expected to have applications in semiconductor devices such as heat spreaders. For example, direct bonding of semiconductor chips to diamond single-crystal substrates is being investigated (see Non-Patent Literature 1 (Hidekazu Hinoki et al., Applied Physics Vol. 90 No. 3 (2021), pp. 167-171)). Such applications require diamond films or substrates of a certain size.

[0003] Non-patent document 2 (Makoto Kasu, Ryota Takaya, and Seong-Woo Kim, Diamond & Related Materials 126 (2022) 109086) discloses a method for manufacturing diamond substrates in which a 1 μm thick Ir buffer layer is deposited on a sapphire substrate by sputtering, then diamond nuclei are formed on the Ir buffer layer using a bias-enhanced nucleation (BEN) process with a DC plasma CVD apparatus, and a diamond layer is grown on the BEN-treated Ir buffer layer by microwave plasma CVD to create a sapphire / Ir buffer layer / diamond layer laminate. This document also states that a microneedle process is essential to obtain a self-supporting diamond layer from the above laminate without damage.

[0004] Meanwhile, substrates for devices used to epitaxially grow gallium nitride (GaN) and aluminum gallium nitride (AlGaN) have also been developed. For example, Patent Document 1 (Japanese Patent No. 6626607) discloses a substrate comprising: (i) a support structure comprising, in order, a polycrystalline ceramic core that may contain aluminum nitride, a first adhesive layer, a conductive layer, a second adhesive layer, and a barrier layer; (ii) a silicon oxide layer bonded to the support structure; (iii) a substantially single-crystal silicon layer bonded to the silicon oxide layer; and (iv) an epitaxial III-V layer that may be an epitaxial gallium nitride layer bonded to the substantially single-crystal silicon layer.

[0005] Incidentally, a bonded substrate consisting of a support substrate and a group 13 element nitride crystal substrate has been proposed. For example, Patent Document 2 (Japanese Patent No. 7295351) discloses a bonded substrate consisting of a support substrate such as silicon carbide and a group 13 element nitride crystal substrate, and discloses that the support substrate and the group 13 element nitride crystal substrate are directly bonded by irradiating the bonding surface of the group 13 element nitride crystal substrate and the bonding surface of the support substrate with a neutral atomic beam to activate them.

[0006] Japanese Patent Publication No. 6626607, Japanese Patent Publication No. 7295351, Japanese Unexamined Patent Publication No. 2014-086400, WO2022 / 190465

[0007] Hidekazu Hinozawa et al., "Development of GaN-HEMT using diamond heat dissipation substrates," Applied Physics, Vol. 90, No. 3 (2021), pp. 167-171. Makoto Kasu, Ryota Takaya, and Seong-Woo Kim, "Growth of high-quality inch-diameter heteroepitaxial diamond layers on sapphire substrates in comparison to MgO substrates," Diamond & Related Materials 126 (2022) 109086

[0008] Self-supporting diamond single-crystal substrates are difficult to manufacture in large sizes and are very expensive. Similarly, self-supporting polycrystalline diamond substrates are also costly. Therefore, there is a need for inexpensive alternatives to diamond single-crystal and polycrystalline self-supporting substrates. On the other hand, device substrates such as those disclosed in Patent Document 1 contain layers with low thermal conductivity, posing challenges in heat dissipation.

[0009] The present inventors have now discovered that by employing a three-layer structure consisting of a support substrate, a diamond film, and a single crystal film, it is possible to provide a laminated substrate that combines the function of a heat sink due to the support substrate and the function of a heat spreader due to the diamond film, and furthermore, allows for the formation of a functional layer for devices (e.g., a GaN layer or an AlGaN layer) on top of it, at a low cost.

[0010] Therefore, the object of the present invention is to provide a low-cost laminated substrate that combines the functions of a heat sink and a heat spreader, and on which a functional layer for a device (for example, a GaN layer or an AlGaN layer) can be formed.

[0011] The following embodiments are provided according to this disclosure: [Embodiment 1] A three-layer laminated substrate comprising: a support substrate made of a material other than diamond, selected from the group consisting of ceramics and metals; a diamond film provided on the support substrate; and a single crystal film provided on the diamond film for use in forming a functional layer. [Embodiment 2] The laminated substrate according to Embodiment 1, wherein the single crystal film is at least one selected from the group consisting of (i) a Si film, (ii) a SiC film, (iii) a laminated film consisting of a Si film and a SiC film thereon, and (iv) a laminated film consisting of a SiC film and a Si film thereon. [Embodiment 3] The laminated substrate according to Embodiment 1 or 2, wherein the diamond film is made of oriented polycrystalline diamond and is (100)-plane oriented. [Embodiment 4] The laminated substrate according to Embodiment 3, wherein the oriented polycrystalline diamond is biaxially oriented. [Embodiment 5] The laminated substrate according to any one of Embodiments 1 to 4, wherein there are no dissimilar metallic elements other than carbon and the constituent elements of the support substrate at the interface between the diamond film and the support substrate. [Aspect 6] A laminated substrate according to any one of aspects 1 to 5, wherein carbon and other dissimilar metallic elements other than the constituent elements of the single crystal film are not present at the interface between the diamond film and the single crystal film. [Aspect 7] The support substrate is made of ceramics, and the ceramics are AlN, SiC, and Si ３ N ４ A laminated substrate according to any one of embodiments 1 to 6, wherein the laminated substrate is at least one selected from the group consisting of the following: [Embodiment 8] A laminated substrate according to any one of embodiments 1 to 7, wherein the support substrate is made of metal, and the metal is Cu or a Cu-containing alloy: [Embodiment 9] The single crystal film is 19.625 cm² ２ A laminated substrate according to any one of embodiments 1 to 8, having the above area. [Embodiment 10] A laminated substrate according to any one of embodiments 1 to 9, wherein the single crystal film has a thickness of 0.5 to 100 μm. [Embodiment 11] The thickness T of the diamond film. １ The thickness T of the support substrate relative to the thickness T ２ Ratio T ２ / T １is 3 to 4000, the multilayer substrate according to any one of Aspects 1 to 10. [Aspect 12] The multilayer substrate according to any one of Aspects 3 to 11, wherein the oriented polycrystalline diamond comprises a plurality of diamond single crystal particles each having a single crystal structure substantially in a normal direction to the surface of the diamond film. [Aspect 13] The multilayer substrate according to any one of Aspects 12 to 12, wherein the diamond single crystal particles exposed on the surface of the diamond film on the support substrate side communicate with the surface of the diamond film on the single crystal film side without interposing grain boundaries. [Aspect 14] An aspect ratio defined as a ratio of a thickness T Ｔ of the diamond film to a cross-sectional average diameter D １ of the diamond single crystal particles at the outermost surface exposed on the surface of the diamond film on the support substrate side, wherein T １ / D Ｔ is 1.0 or more, the multilayer substrate according to any one of Aspects 12 to 13. [Aspect 15] A ratio D Ｂ of a cross-sectional average diameter D Ｔ of the diamond single crystal particles at the outermost surface exposed on the surface of the diamond film on the support substrate side to a cross-sectional average diameter D Ｔ / D Ｂ of the diamond single crystal particles at the outermost surface on the single crystal film side of the diamond film, wherein the ratio D is greater than 1.0, the multilayer substrate according to any one of Aspects 12 to 14. [Aspect 16] A multilayer substrate with a functional film, comprising: the multilayer substrate according to any one of Aspects 1 to 15; and a functional film formed on the single crystal film, the functional film being composed of GaN and / or a GaN-containing solid solution. [Aspect 17] A method for producing a multilayer substrate, comprising the steps of: forming a diamond film on a single crystal Si substrate or a single crystal SiC substrate; polishing and planarizing a surface of the diamond film; directly bonding the polished diamond film to a support substrate made of a material other than diamond, the material being selected from the group consisting of ceramics and metals; and after the direct bonding, polishing the single crystal Si substrate or the single crystal SiC substrate to form a thinned single crystal film.

[0012] This is a conceptual cross-sectional view of the laminated substrate of this disclosure. This is a diagram of ICDD card #01-071-3649. The average cross-sectional diameter D is on the surface of the diamond film on the supporting substrate side and on the surface of the single crystal film side. Ｔ and D Ｂ This is a schematic cross-sectional view illustrating the calculation of [the value]. This is a schematic cross-sectional view showing an example of the manufacturing process of the laminated substrate of this disclosure. This is a schematic cross-sectional view showing the HEMT element fabricated in Example 1.

[0013] Figure 1 conceptually shows the laminated substrate 10 of this disclosure. The laminated substrate 10 is a three-layer substrate consisting of a support substrate 12, a diamond film 14 provided on the support substrate 12, and a single crystal film 16 provided on the diamond film 14. The support substrate 12 is made of a material other than diamond, and this material is selected from the group consisting of ceramics and metals. The single crystal film 16 is a film used for forming a functional layer. By adopting this three-layer structure consisting of a support substrate 12, a diamond film 14, and a single crystal film 16, it is possible to provide a laminated substrate (i.e., a device substrate with a heat spreader) at low cost that combines the function of a heat sink by the support substrate 12 and the function of a heat spreader by the diamond film 14, and also allows for the formation of a functional layer for a device (e.g., a GaN layer or an AlGaN layer) on the single crystal film 16.

[0014] As mentioned above, self-supporting diamond single-crystal substrates are difficult to manufacture in large sizes and are very expensive. Even self-supporting polycrystalline diamond substrates are expensive. To address these problems, by adopting the form of a film (which does not need to be in the form of a self-supporting substrate) as the form of the diamond product, as described above, and combining that film with the support substrate 12 (for example, by direct bonding), a laminated substrate 10 equipped with a diamond film 14 can be provided at low cost. Therefore, the laminated substrate 10 can have both the heat sink function provided by the support substrate 12 and the heat spreader function provided by the diamond film 14. Moreover, as shown in Figure 4, by thinning the single-crystal substrate 8 used for depositing the diamond film 14 (instead of removing it) and leaving it as a single-crystal film 16, the laminated substrate 10 can be provided as a device base substrate on which a functional layer for devices (for example, a GaN layer or an AlGaN layer) can be formed on the single-crystal film 16. Therefore, a device substrate with a heat spreader can be provided, in which the diamond film 14 functions as a heat spreader, the support substrate 12 functions as a heat sink, and the single crystal film 16 functions as a base substrate for forming a functional layer for the device. Thus, the laminated substrate 10 may be in the form of a laminated substrate with a functional film. Such a laminated substrate with a functional film may comprise the laminated substrate 10 and a functional film formed on the single crystal film 16, composed of GaN and / or a GaN-containing solid solution (e.g., AlGaN).

[0015] The laminated substrate 10 is a three-layer substrate consisting of a support substrate 12, a diamond film 14, and a single crystal film 16. Therefore, it is possible that there are no dissimilar metal elements other than carbon and the constituent elements of the support substrate at the interface between the diamond film 14 and the support substrate 12. Similarly, it is possible that there are no dissimilar metal elements other than carbon and the constituent elements of the single crystal film at the interface between the diamond film 14 and the single crystal film 16. In other words, this three-layer laminated substrate 10 is formed by direct bonding between the diamond film 14 and the support substrate 12 without using bonding materials such as solder. Therefore, improved heat dissipation can be achieved by effectively utilizing the heat spreader function of the diamond film 14 and the heat sink function of the support substrate 12 without hindering thermal conductivity by other intervening layers.

[0016] The support substrate 12 is a substrate made of a material other than diamond, and this material is selected from the group consisting of ceramics and metals. If the support substrate 12 is made of ceramics, this ceramic is AlN, SiC, and Si ３ N ４ It is preferable that the support substrate 12 is selected from the group consisting of the following. If the support substrate 12 is made of metal, it is preferable that the metal is Cu or a Cu-containing alloy. A support substrate 12 made of such ceramics or metal can exhibit excellent thermal conductivity, and thus can impart good performance as a heat sink to the laminated substrate 10. From this viewpoint, the thickness of the support substrate 12 is preferably 300 μm to 2 mm, and more preferably 500 μm to 1.5 mm.

[0017] The diamond film 14 is preferably composed of oriented polycrystalline diamond. That is, the oriented polycrystalline diamond film 14 can be manufactured inexpensively on a substrate such as a silicon substrate, and it has the advantage that the diamond film 14 can be easily enlarged by using a large Si substrate or the like as the substrate. In other words, by adopting the oriented polycrystalline diamond film 14, the laminated substrate 10 can be provided at a low cost and in a configuration suitable for large-scale production. Therefore, the diamond film 14 can be used as an inexpensive alternative to diamond single-crystal substrates and diamond polycrystalline freestanding substrates. Moreover, because the diamond film 14 is an oriented polycrystalline diamond film, it also has the advantage of improved thermal conductivity and polishability compared to random polycrystalline diamond films. Therefore, the diamond film 14 is particularly advantageous for applications that diffuse heat in the direction of the film surface (especially heat spreaders for semiconductor devices).

[0018] The oriented polycrystalline diamond constituting the diamond film 14 preferably contains a plurality of diamond single-crystal particles 14a, and these diamond single-crystal particles 14a preferably have a single-crystal structure in a direction substantially normal to the surfaces 14b and 14c of the diamond film 14. In this case, the diamond film 14 is composed of a plurality of diamond single-crystal particles 14a (typically columnar particles) linked two-dimensionally in the horizontal plane, and therefore has a single-crystal structure in a direction substantially normal to the surface. Thus, although the diamond film 14 as a whole is not a single crystal, it has a single-crystal structure at the local domain level, and can therefore have sufficient crystallinity to ensure device characteristics. Nevertheless, the diamond film 14 of this disclosure is not a diamond single-crystal film, much less a diamond single-crystal self-supporting substrate.

[0019] As described above, when an oriented polycrystalline diamond contains multiple diamond single crystal particles 14a having a single crystal structure in a direction approximately normal to the surfaces 14b and 14c, the diamond single crystal particles 14a exposed (or reaching) the surface 14c of the diamond film 14 on the support substrate 12 side are typically in communication with the surface 14b of the diamond film 14 on the single crystal film 16 side without grain boundaries. The presence of grain boundaries is a factor that reduces the thermal conductivity in the thickness direction, but with this configuration, such a reduction in thermal conductivity can be avoided. The diamond single crystal particles 14a are typically columnar particles, and more typically prismatic particles. The top view shape or cross-sectional shape of the prismatic particles is typically a quadrilateral, or a polygon formed by the fusion of multiple quadrilaterals.

[0020] The diamond film 14, composed of oriented polycrystalline diamond, is preferably (100) plane oriented. Although diamond is known as a hard material, the (100) plane of diamond is relatively easy to polish compared to other planes such as the (110) plane and the (111) plane, which is advantageous when polishing the surface 14b or 14c of the diamond film 14 to form a flat surface. The (100) plane orientation can be confirmed by analyzing the cross-section of the diamond film 14, or the surface 14b or 14c of the diamond film 14, with an electron backscatter diffraction (EBSD) device to obtain an inverse pole figure orientation mapping. Since diamond is an isotropic crystal, the (100) plane is equivalent to the (001) plane and the (010) plane. Therefore, the evaluation of the (001) plane orientation or the (010) plane orientation of the diamond film 14 may be substituted for the evaluation of the (100) plane orientation by evaluating the (001) plane orientation or the (010) plane orientation with EBSD.

[0021] It is preferable that the diamond single crystal particles 14a have a crystal orientation that is generally aligned in the in-plane direction of the diamond film 14. In this case, the diamond film 14 can be said to be a biaxially oriented film having a crystal orientation that is generally aligned not only in the approximate normal direction but also in the in-plane direction, particularly in the rotational direction (hereinafter referred to as the twist direction) centered on a normal oriented approximately perpendicular to the film surface of the diamond film 14. That is, it is preferable that the oriented polycrystalline diamond is biaxially oriented. Specifically, this can be confirmed by obtaining the crystal orientation distribution in the film thickness direction and the in-plane direction when performing EBSD inverse pole figure mapping on the cross-section of the diamond film 14.

[0022] Thickness T of the diamond film 14 １ The thickness of the diamond film 14 is preferably 0.5 μm or more, more preferably 1.0 μm or more, even more preferably 5 μm or more, and particularly preferably 10 μm or more. １ While there shouldn't be an upper limit, from a manufacturing cost perspective, anything below 100 μm is realistic.

[0023] The average cross-sectional diameter D at the outermost surface (i.e., surface 14b) of the diamond single crystal grains 14a on the surface 14b of the diamond film 14 facing the single crystal film 16. Ｂ The average cross-sectional diameter D at the outermost surface (i.e., surface 14c) of the diamond single crystal grains 14a that are exposed (or reach) the surface 14c of the diamond film 14 on the support substrate 12 side. Ｔ Ratio D Ｔ / D Ｂ It is preferable that it is greater than 1.0, more preferably 5.0 or higher, even more preferably 10 or higher, and particularly preferably 20 or higher. Within this range, there is the advantage that the thermal conductivity in the film thickness direction improves. On the other hand, in terms of thermal conductivity in the in-plane direction of the film, D Ｔ / D Ｂ A smaller value is preferable, preferably 100 or less, more preferably 80 or less, even more preferably 60 or less, and particularly preferably 20 or less. D that balances thermal conductivity in the film surface and thermal conductivity in the film thickness direction. Ｔ / D Ｂ The preferred range is 1.0 to 100, and more preferably 5 to 20. Average cross-sectional diameter DＴ and the average cross-sectional diameter D Ｂ This can be determined using a band contrast diagram based on the inverse pole figure orientation mapping of the cross-section of the diamond film 14, as illustrated in Figure 3. That is, a straight line S parallel to the support substrate 12 or bonding surface 14c is located at a predetermined distance (e.g., 3 μm) below the surface 14c (outermost surface) of the diamond film 14, as shown in Figure 3. Ｔ Draw a line S. Ｔ Let n be the number of intersection points X between the diamond film 14 and the grain boundaries, and let L be the distance between intersection points X that contain at least 10 intersection points X. Ｔ Defined as, D Ｔ = L Ｔ The value calculated by the formula / (n-1) represents the average cross-sectional diameter D of the diamond single crystal grains 14a on the surface 14b of the diamond film 14. Ｔ This can then be determined. Similarly, a straight line S parallel to the single crystal film 16 or the bonding surface 14b is positioned at a predetermined distance (e.g., 3 μm) above the surface 14b of the diamond film 14 (e.g., the interface between the single crystal film 16 and the diamond film 14), as shown in Figure 3. Ｂ Draw a line S. Ｂ Let n be the number of intersection points X between the diamond film 14 and the grain boundaries, and let L be the distance between intersection points X that contain at least 10 intersection points X. Ｂ Defined as, D Ｂ = L Ｂ The value calculated by the formula / (n-1) is used to determine the average cross-sectional diameter of the diamond single crystal grains 14a at the bonding surface 14c of the diamond film 14 as D Ｂ That should be the decision.

[0024] The average cross-sectional diameter D at the outermost surface (i.e., surface 14c) of the diamond single crystal grains 14a exposed to (or reaching) the surface 14c of the diamond film 14 on the support substrate 12 side. Ｔ The thickness T of the diamond film 14 relative to this. １ The aspect ratio T is defined as the ratio of １ / D Ｔ The average cross-sectional diameter D is preferably 1 or more, more preferably 10 or more, and even more preferably 30 or more. Ｔ The method for determining this is as described above. On the other hand, aspect ratio T / D ＴIf the ratio is too large, the thermal conductivity within the film surface may decrease. Therefore, T / D Ｔ It is preferably 50 or less, more preferably 10 or less, and even more preferably 5 or less. From these viewpoints, T / D Ｔ A preferred range is 1 to 50, and more preferably 2 to 20.

[0025] As mentioned above, by employing the oriented polycrystalline diamond film 14, the laminated substrate 10 can be provided at a low cost and in a configuration suitable for large-scale applications. The area of ​​the diamond film 14 is 19.625 cm². ２ Preferably, it is 78.5 cm² or larger (corresponding to a circular area with a diameter of 5 cm (approximately 2 inches)), and more preferably 78.5 cm². ２ (corresponding to a circular area of ​​10 cm (approximately 4 inches) or larger in diameter), more preferably 176.625 cm². ２ This corresponds to a circular area of ​​15 cm (approximately 6 inches) or more in diameter. Therefore, when the plan view shape of the diamond film 14 is circular or includes a circular portion, the diameter of the diamond film 14 is preferably 5 cm or more, more preferably 10 cm or more, and even more preferably 15 cm or more. However, the plan view shape of the diamond film 14 does not have to be circular or include a circular portion. In any case, the larger the area of ​​the diamond film 14, the more useful it is as a heat spreader, so there should be no upper limit on its area, but from the viewpoint of manufacturing cost, the area of ​​the diamond film 14 is 706.5 cm². ２ The following (corresponding to a circular area with a diameter of 30 cm (approximately 12 inches) or less) is realistic.

[0026] The diamond single crystal particles 14a may be doped with an n-type dopant or a p-type dopant. In this case, the diamond film 14 can be used as a component or layer such as a p-type electrode, an n-type electrode, a p-type layer, or an n-type layer.

[0027] Thickness T of the diamond film 14 １ The thickness T of the support substrate 12 relative to the support substrate 12 ２ Ratio T ２ / T １ It is preferable that the value is 3 to 4000, and more preferably 10 to 1000.

[0028] The single crystal film 16 is a film used for forming a functional layer. Therefore, it is not particularly limited as long as it is a single crystal film on which a functional film composed of GaN and / or a GaN-containing solid solution (e.g., AlGaN) can be formed by epitaxial growth or the like. Preferred examples of the single crystal film 16 include (i) a Si film, (ii) a SiC film, (iii) a laminated film consisting of a Si film and a SiC film thereon, (iv) a laminated film consisting of a SiC film and a Si film thereon, and any combination of (i) to (iv).

[0029] The area of ​​the single crystal film 16 is 19.625 cm², the same as the area of ​​the diamond film 14. ２ Preferably, it is 78.5 cm² or larger (corresponding to a circular area with a diameter of 5 cm (approximately 2 inches)), and more preferably 78.5 cm². ２ (corresponding to a circular area of ​​10 cm (approximately 4 inches) or larger in diameter), more preferably 176.625 cm². ２ This corresponds to a circular area of ​​15 cm (approximately 6 inches) or larger in diameter. Therefore, when the planar shape of the single crystal film 16 is circular or includes a circular portion, the diameter of the single crystal film 16 is preferably 5 cm or larger, more preferably 10 cm or larger, and even more preferably 15 cm or larger. However, the planar shape of the single crystal film 16 does not have to be circular or include a circular portion. In any case, the larger the area of ​​the single crystal film 16, the larger the functional layer that can be formed on it, so there should be no upper limit on its area. However, from the viewpoint of manufacturing cost, the area of ​​the single crystal film 16 should be 706.5 cm². ２ The following (corresponding to a circular area with a diameter of 30 cm (approximately 12 inches) or less) is realistic.

[0030] The thickness of the single crystal film 16 is preferably 0.5 μm or more, and more preferably 1 μm or more.

[0031] Manufacturing Method The laminated substrate 10 of this disclosure can preferably be manufactured by following the steps of (a) forming a diamond film 14 on a single crystal substrate 8, (b) polishing the diamond film 14, (c) directly bonding the diamond film 14 to the support substrate 12, and (d) thinning the single crystal substrate 8, as shown in Figure 4. Each of steps (a) to (d) will be described below.

[0032] (a) Formation of the diamond film First, a diamond film 14 is formed on a single crystal substrate 8, which is a single crystal Si substrate or a single crystal SiC substrate. The diamond film 14 can be formed by first performing a bias-enhanced nucleation (BEN) process (hereinafter referred to as bias treatment) as a pretreatment for forming diamond nuclei on the single crystal substrate 8, and then depositing the diamond film 14 by known deposition methods such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). It is particularly preferable to use a (100)-oriented, double-sided polished Si single crystal substrate 8 with no off-angle. The pretreatment for forming diamond nuclei can preferably be carried out by setting the single crystal substrate 8 in a bias treatment device, evacuating it, heating the single crystal substrate 8 to a temperature of 800 to 1100°C, and performing bias treatment at a pressure of 50 to 200 Torr in the presence of hydrogen-diluted methane gas. The diamond film 14 can preferably be formed by heteroepitaxial growth of the diamond film 14 on the bias-treated side of the single crystal substrate 8 at 800 to 1100°C for 2 to 40 hours using microwave plasma CVD.

[0033] (b) Polishing the diamond film Next, the exposed surface of the diamond film 14 (the bonding surface 14c that will be bonded to the support substrate 12) is polished to make it smooth. At this time, it is preferable to polish the bonding surface 14c of the diamond film 14 until the arithmetic mean roughness Ra is 0.6 nm or less. On the other hand, as the support substrate 12, a substrate made of a material other than diamond, selected from the group consisting of ceramics and metals, is prepared. It is preferable to polish the bonding surface 12a of this support substrate 12 (that will be bonded to the diamond film 14) to make it smooth. At this time, it is preferable to polish the bonding surface 12a of the support substrate 12 until the arithmetic mean roughness Ra is 0.6 nm or less. However, if the bonding surface 12a of the support substrate 12 already has the desired smoothness (for example, the arithmetic mean roughness Ra mentioned above), the polishing step is unnecessary.

[0034] (c) Direct bonding of the diamond film and the support substrate The diamond film 14 and the support substrate 12 after polishing are directly bonded so that their bonding surfaces 14c and 12a are in contact. Direct bonding of the diamond film 14 and the support substrate 12 can be performed by known methods, but it is preferable to increase the bonding strength by performing surface activation of each bonding surface of the diamond film 14 and the support substrate 12 by irradiation with a neutral atomic beam such as an Ar beam (see, for example, Patent Document 2 (Japanese Patent No. 7295351)). Surface activation with a neutral atomic beam is preferably performed by generating a neutral atomic beam using a known apparatus such as the one disclosed in Patent Document 3 (Japanese Patent Application Publication No. 2014-086400) and irradiating each bonding surface. That is, a saddle field type high-speed atomic beam source is used as the beam source. Then, an inert gas is introduced into the chamber and a high voltage is applied to the electrodes from a DC power supply. As a result, electrons move due to the saddle field type electric field generated between the electrode (positive electrode) and the housing (negative electrode), and a beam of atoms and ions from the inert gas is generated. Of the beam that reaches the grid, the ion beam is neutralized at the grid, so a beam of neutral atoms is emitted from the high-speed atomic beam source. The atomic species constituting the beam are preferably inert gases (argon, nitrogen, etc.). The voltage during activation by beam irradiation is preferably 0.5 to 2.0 kV, and the current is preferably 50 to 200 mA. Next, the diamond film 14 and the support substrate 12 are joined by bringing the activated bonding surfaces into contact with each other in a vacuum atmosphere. The bonding temperature can be room temperature, but specifically, it is preferably 40°C or lower, more preferably 30°C or lower, and even more preferably 20 to 25°C. The pressing load during bonding is preferably 100 to 20000 N. Furthermore, the flatter the bonding surfaces 14c and 12a of the diamond film 14 and the support substrate 12, the higher the bonding strength can be, which is preferable. Alternatively, the bonding strength may be increased by surface activation of each bonding surface by irradiation with a plasma such as nitrogen gas plasma (plasma activation method), as proposed in Patent Document 4 (WO2022 / 190465).

[0035] (d) After thinning and direct bonding of the single crystal substrate, the single crystal substrate 8 (i.e., single crystal Si substrate or single crystal SiC substrate) is polished to thin it, thereby forming a single crystal film 16. The polishing method is not particularly limited as long as it can achieve the desired thickness and smoothness, but it is preferable to use a combination of diamond lapping and CMP (chemical mechanical polishing). The exposed surface of the single crystal film 16 is preferably smoothed by polishing until the arithmetic mean roughness Ra is 0.6 nm or less. In this way, a laminated substrate 10 with a three-layer structure of single crystal film 16 / diamond film 14 / support substrate 12 is obtained.

[0036] The present invention will be further described in detail by the following examples. However, the present invention is not limited to the following examples.

[0037] Example 1 (1) Fabrication of a diamond film A double-sided polished Si single crystal substrate with no off-angle, oriented at (100) planes, and with a diameter of 50.8 mm and a thickness of 1 mm was prepared as a single crystal substrate 8 for diamond film deposition. Pretreatment such as bias treatment was performed on this Si single crystal substrate 8 to form diamond nuclei. Specifically, the Si single crystal substrate 8 was set on the negative voltage application electrode (cathode) of a bias treatment device and evacuated. Next, the Si single crystal substrate 8 was heated to 850°C, and then 3 volume% hydrogen-diluted methane gas was introduced to set the pressure to 140 Torr and perform bias treatment. This bias treatment was performed by applying a DC voltage between the two electrodes and passing a predetermined DC current. Finally, a diamond film 14 was obtained by heteroepitaxial growth of a diamond film 14 on the bias-treated side of the Si single crystal substrate 8 at 1100°C for 8 hours using microwave plasma CVD.

[0038] (2) Fabrication of the support substrate and polishing of the diamond film and support substrate A support substrate 12 with a thickness of 0.5 mm was prepared, consisting of an aluminum nitride sintered body. Next, the bonding surface 14c of the diamond film 14 and the bonding surface 12a of the support substrate 12 were polished, and both surfaces were smoothed so that the arithmetic mean roughness Ra was 0.5 nm (see Figure 4(b)). The thickness of the support substrate 12 after polishing was 0.45 mm.

[0039] (3) Bonding of the diamond film and the support substrate Next, the bonding surface 14c of the diamond film 14 and the bonding surface 12a of the support substrate 12 were irradiated with a neutralizing beam to activate each bonding surface and directly bond them (see Figures 4(b) and (c)). Specifically, after cleaning each of the bonding surfaces 12a and 14c to remove dirt, the diamond film 14 and the support substrate 12 were set in a vacuum chamber. The inside of the vacuum chamber was 10 －６ After evacuating to a Pa-level vacuum, each bonding surface was irradiated with a high-speed atomic beam (acceleration voltage: 1 kV, Ar flow rate: 27 sccm) for 120 seconds. Then, the bonding surface 14c of the diamond film 14 and the bonding surface 12a of the support substrate 12 were brought into contact, and the diamond film 14 and the support substrate 12 were bonded by applying pressure of 10,000 N for 2 minutes. In this way, a laminated substrate with a three-layer structure of Si single crystal substrate 8 / diamond film 14 / support substrate 12 was obtained (see Figure 4(c)).

[0040] (4) Polishing of Si single crystal substrate The surface of the Si single crystal substrate 8 was polished until the thickness of the substrate 8 was 5 μm and the arithmetic mean roughness Ra of the surface was 0.5 nm, thereby creating a Si single crystal film 16 that was in a state suitable for epitaxial growth of GaN films, etc. This polishing was performed by combining diamond lapping and CMP (chemical mechanical polishing). In this way, a laminated substrate with a three-layer structure of Si single crystal film 16 / diamond film 14 / support substrate 12 was obtained (see Figure 4(d)).

[0041] (5) Analysis of the diamond film The diamond film 14 that was prepared was analyzed as follows.

[0042] (Cross-sectional EBSD) The laminated substrate 10 containing the diamond film 14 was cut to expose the surface perpendicular to the plate surface, and polished using a CP polishing machine (JEOL Ltd., IB-09010CP). The cross-section of the diamond film 14 was then observed using a SEM (Hitachi High-Technologies Corporation, SU-5000) equipped with an electron backscatter diffractometer (EBSD) (Oxford Instruments, Nordlys Nano), and inverse pole figure orientation mapping was performed in a field of view of 797 μm × 1061 μm. The conditions for this EBSD measurement were as follows: <EBSD Measurement Conditions> - Acceleration voltage: 15 kV - Spot intensity: 70 - Working distance: 22.5 mm - Step size: 0.5 μm - Sample tilt angle: 70° - Measurement program: Aztec (version 3.3)

[0043] From the obtained inverse pole figure mapping and band contrast diagram, it was found that (i) the shape of the diamond single crystal particles 14a is columnar, (ii) the diamond film 14 is composed of a plurality of diamond single crystal particles 14a having a single crystal structure in the approximately normal direction, and (iii) these diamond single crystal particles 14a are biaxially oriented particles, oriented in the (100) axis in the approximately normal direction to the film plane, as well as in the in-plane direction of the film.

[0044] Furthermore, in the band contrast diagram, a straight line S parallel to the support substrate 12 is shown in Figure 3, at a position 3 μm below the interface between the diamond film 14 and the support substrate 12, and at a position 3 μm above the interface between the single crystal film 16 and the diamond film 14. Ｔ , S Ｂ I drew a line. Straight line S Ｔ , S Ｂ For each of these, the straight line S Ｔ , S Ｂ Let n be the number of intersection points X between the diamond film 14 and the grain boundaries, and let L be the distance between the intersection points X that contain at least 10 intersection points X. Ｔ , L Ｂ This was defined as follows. And, as shown in Figure 3, D Ｔ = L Ｔthe value calculated by the formula / (n−1) is defined as the average cross-sectional diameter D of the diamond single crystal particles 14a at the interface between the diamond film 14 and the supporting substrate 12 (that is, the surface 14c) Ｔ , while D Ｂ = L Ｂ the value calculated by the formula / (n−1) is determined as the average cross-sectional diameter D of the diamond single crystal particles 14a at the interface between the single crystal film 16 and the diamond film 14 (that is, the bonding surface 14b) Ｂ . As a result, in this example, the average cross-sectional diameter D Ｔ is 7.4 μm, and D Ｂ is 1.0 μm, and the ratio of D Ｔ / D Ｂ was 7.4. The thickness T of the diamond film 14 １ is 20 μm, and the aspect ratio T defined as the ratio of the thickness T of the diamond film 14 to the average cross-sectional diameter D Ｔ １ １ / D Ｔ was 2.7. The ratio T of the thickness T of the supporting substrate 12 to the thickness T of the diamond film 14 １ ２ ２ / T １ was 22.5.

[0045] (6) Analysis of interface between diamond film and supporting substrate In order to analyze the diamond film, the obtained Si single crystal film 16 of the laminated substrate 10 was removed by etching with hydrofluoric acid to obtain an analysis sample. After polishing and partially removing the diamond film 14 of the analysis sample to a thickness of about 2 μm, depth analysis for positive ions was performed on a region having a thickness of about 5 μm including the interface between the diamond film 14 and the supporting substrate 12 using D-SIMS (dynamic SIMS) under the following measurement conditions. <D-SIMS measurement for positive ions> ・Measurement device: SIMS4550 manufactured by FEI Company ・Primary ion species: O ２ ＋ ・Primary ion acceleration energy: 3 keV ・Secondary ion polarity: Positive

[0046] As a result, the only metal elements detected from the diamond film 14, the supporting substrate 12 and the interface therebetween were Al and Y contained in the supporting substrate, and other metal elements were below the detection limit.​​​​

[0047] (7) Evaluation of heat dissipation during HEMT element driving A HEMT element 20 having a HEMT structure 22 as shown in FIG. 5 was produced on the exposed surface of the Si single crystal film 16 of the multilayer substrate 10 by MOCVD. The HEMT element 20 comprises, stacked in this order on the single crystal film 16 of the multilayer substrate 10: a GaN buffer layer 24 (thickness: 500 nm), a GaN channel layer 26 (thickness: 150 nm), and an Al ０．２ Ga ０．８ N barrier layer 28 (thickness: 20 nm), and on the surface of the Al ０．２ Ga ０．８ N barrier layer 28, a source electrode 30, a gate electrode 32, and a drain electrode 34 are formed spaced apart from each other. A temperature change ΔT when this HEMT element 20 was operated under direct current with the conditions of drain voltage: 20 V, gate voltage: 4 V, and drain current: 0.5 A was measured by the thermoreflectance method. As a result, ΔT was about 60°C, demonstrating that the multilayer substrate 10 has high heat dissipation performance when driving the HEMT.

[0048] Example 2 (Comparative) Production and evaluation of a HEMT element not having a diamond film 14 were carried out as follows. First, in the same manner as in Example 1, a support substrate 12 made of an aluminum nitride sintered body having a thickness of 0.5 mm, and a (111)-plane oriented Si single crystal substrate 8 were prepared. After the Si single crystal substrate 8 and the support substrate 12 were bonded by direct bonding, the surface of the Si single crystal substrate 8 was ground until the thickness of the substrate 8 reached 5 μm and the arithmetic average roughness Ra of the surface reached 0.5 nm, to obtain a Si single crystal film 16 in a state allowing epitaxial growth of a GaN film or the like. A HEMT structure 22 was produced on the Si single crystal film 16 in the same manner as in Example 1 to obtain a HEMT element not having the diamond film 14. In the same manner as in Example 1, the temperature change ΔT when the HEMT element was operated under direct current was about 250°C, which confirmed that the heat dissipation performance during driving was insufficient.

[0049] 8 Single crystal substrate 10 Multilayer substrate 12 Support substrate 12a Bonding surface 14 Diamond film 14a Diamond single crystal grains 14b Single crystal film side surface 14c Support substrate side surface 16 Single crystal film 20 HEMT element 22 HEMT structure 24 GaN buffer layer 26 GaN channel layer 28 Al ０．２ Ga ０．８ N barrier layer 30 Source electrode 32 Guard electrode 34 Drain electrode L Ｔ , L Ｂ Length S Ｔ , S Ｂ Straight line X Intersection T １ , T ２ thickness

Claims

1. A three-layer laminated substrate comprising: a support substrate made of a material other than diamond, selected from the group consisting of ceramics and metals; a diamond film provided on the support substrate; and a single crystal film provided on the diamond film for use in forming a functional layer.

2. The laminated substrate according to claim 1, wherein the single crystal film is at least one selected from the group consisting of (i) a Si film, (ii) a SiC film, (iii) a laminated film comprising a Si film and a SiC film thereon, and (iv) a laminated film comprising a SiC film and a Si film thereon.

3. The laminated substrate according to claim 1 or 2, wherein the diamond film is composed of oriented polycrystalline diamond and is (100) plane oriented.

4. The laminated substrate according to claim 3, wherein the oriented polycrystalline diamond is biaxially oriented.

5. The laminated substrate according to claim 1 or 2, wherein no dissimilar metallic elements other than carbon and the constituent elements of the support substrate are present at the interface between the diamond film and the support substrate.

6. The laminated substrate according to claim 1 or 2, wherein no dissimilar metallic elements other than carbon and the constituent elements of the single crystal film are present at the interface between the diamond film and the single crystal film.

7. The support substrate is made of ceramics, and the ceramics are AlN, SiC, and Si ３ N ４ The laminated substrate according to claim 1 or 2, which is at least one selected from the group consisting of the following.

8. The laminated substrate according to claim 1 or 2, wherein the support substrate is made of metal, and the metal is Cu or a Cu-containing alloy.

9. The single crystal film is 19.625 cm². ２ A laminated substrate according to claim 1 or 2, having the above area.

10. The laminated substrate according to claim 1 or 2, wherein the single crystal film has a thickness of 0.5 to 100 μm.

11. Thickness T of the diamond film １ The thickness T of the support substrate relative to the thickness T ２ Ratio T ２ / T １ A laminated substrate according to claim 1 or 2, wherein the value is 3 to 4000.

12. The laminated substrate according to claim 3, wherein the oriented polycrystalline diamond comprises a plurality of diamond single crystal particles having a single crystal structure in a direction substantially normal to the surface of the diamond film.

13. The laminated substrate according to claim 12, wherein the diamond single crystal particles exposed on the surface of the diamond film on the support substrate side are in communication with the surface of the diamond film on the single crystal film side without grain boundaries.

14. A cross-sectional average diameter D of the outermost surface of diamond single crystal particles exposed on the surface of said diamond film on the support substrate side Ｔ , the thickness T of said diamond film relative to １ , the aspect ratio T defined as a ratio of １ / D Ｔ is 1.0 or more, the laminated substrate according to claim 12.

15. The average cross-sectional diameter D of the diamond single crystal grains at the outermost surface of the diamond film on the single crystal film side of the diamond film. Ｂ The average cross-sectional diameter D at the outermost surface of the diamond single crystal particles exposed on the surface of the diamond film on the support substrate side. Ｔ Ratio D Ｔ / D Ｂ The laminated substrate according to claim 12, wherein is greater than 1.

0.

16. A laminated substrate with a functional film, comprising: a laminated substrate according to claim 1 or 2; and a functional film formed on the single crystal film, the functional film being composed of GaN and / or a GaN-containing solid solution.

17. A method for manufacturing a laminated substrate, comprising: forming a diamond film on a single-crystal Si substrate or a single-crystal SiC substrate; polishing the surface of the diamond film to flatten it; directly bonding the polished diamond film to a support substrate made of a material other than diamond, selected from the group consisting of ceramics and metals; and polishing the single-crystal Si substrate or the single-crystal SiC substrate after the direct bonding to form a thinned single-crystal film.

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