Method for stacking hexagonal boron nitride layers

Abstract

Provided is a method for stacking hexagonal boron nitride layers, with which it is possible to further improve flatness and uniformity of hexagonal boron nitride layers to be stacked. This method for stacking hexagonal boron nitride layers comprises: disposing pyrolytic boron nitride as a target inside a chamber; disposing a substrate inside the chamber; heating the substrate to a temperature that is higher than 1000°C but not higher than 1600°C, and is lower than the melting point of the material that constitutes the substrate; and sputtering the target so as to stack a plurality of hexagonal boron nitride layers on the substrate.

Description

Hexagonal boron nitride layer stacking method

[0001] The present disclosure relates to a method for depositing hexagonal boron nitride layers.

[0002] A method for depositing multiple hexagonal boron nitride layers on a substrate is known. "Developing a Synthesis Process for Large-Scale h-BN Nanosheets Using Magnetron Sputtering and Heat Annealing" (Hirata et al., Advanced Engineering Materials: Volume 25, Issue 23, December 2023, hereinafter referred to as Non-Patent Document 1) describes a technique for depositing multiple hexagonal boron nitride layers on a copper (Cu) substrate by sputtering a pyrolytic boron nitride target while heating the copper (Cu) substrate to 500°C, 900°C, or 1000°C. The technique described in Non-Patent Document 1 allows for the rapid deposition of multiple flat and uniform hexagonal boron nitride layers by sputtering while heating the copper substrate.

[0003] The crystallinity of the hexagonal boron nitride layer deposited on the substrate is determined by the E of the six-membered ring of boron nitride. ２ｇ The model shows 1364.6 cm －１It is known that the Raman peak of a film formed by sputtering at 900°C for 30 minutes can be used to evaluate the Raman peak (hereinafter referred to as the "Raman peak"). Non-Patent Document 1 describes that the above-mentioned Raman peak was observed when the film was formed by sputtering at 900°C for 30 minutes (Figure 7), but was not observed when the film was formed by sputtering at 1000°C for 30 minutes. TEM observations showed that hexagonal boron nitride (hBN) grew in the region close to the interface with the Cu substrate, while amorphous boron nitride (aBN) grew in the region away from the interface. It is presumed that the reason the above-mentioned Raman peak was not observed under the sputtering condition of 1000°C for 30 minutes is that there are aBN regions where hexagonal boron nitride grows in various directions. That is, the hexagonal boron nitride lamination method described in Non-Patent Document 1 can quickly laminate a plurality of flat and uniform hexagonal boron nitride layers, but it is desired to further improve the crystallinity and uniformity of the laminated hexagonal boron nitride layers.

[0004] The present disclosure is intended to solve these problems and to provide a method for laminating a hexagonal boron nitride layer that can further improve the crystallinity and uniformity of the hexagonal boron nitride layer to be laminated.

[0005] A method for depositing hexagonal boron nitride layers according to the present disclosure includes placing pyrolytic boron nitride as a target inside a chamber, placing a substrate inside the chamber, heating the substrate to a temperature greater than 1000°C and not greater than 1600°C but lower than the melting point of a material forming the substrate, and sputtering the target to deposit multiple hexagonal boron nitride layers on the substrate.

[0006] Furthermore, in the hexagonal boron nitride layer stacking method according to the present disclosure, the substrate is preferably made of any of nickel (Ni), platinum (Pt), and cobalt (Co).

[0007] Furthermore, in the hexagonal boron nitride layer stacking method according to the present disclosure, the substrate is preferably made of nickel (Ni).

[0008] The method for depositing a hexagonal boron nitride layer according to the present disclosure can further improve the crystallinity and uniformity of the deposited hexagonal boron nitride layer.

[0009] 1 is a diagram showing a sputtering apparatus used in a hexagonal boron nitride layer deposition method according to an embodiment; FIG. 2 is a flowchart showing a hexagonal boron nitride layer deposition method according to an embodiment; FIG. 3 is a TEM image of a cross section of an hBN film according to Example 1, FIG. 4 is a diagram showing an electron diffraction image of an hBN film according to Example 1, and FIG. 5 is a diagram showing a Raman spectrum of an hBN film according to Example 1; FIG. 4 is a TEM image of a cross section of an hBN film according to Comparative Example 1, and FIG. 5 is a diagram showing a Raman spectrum of an hBN film according to Comparative Example 1; FIG. 5 is a TEM image of a cross section of an hBN film according to Comparative Example 2, and FIG. 6 is an enlarged view of the region indicated by a in FIG. 1; and FIG. 7 is an enlarged view of the region indicated by b in FIG.

[0010] Hereinafter, a method for laminating a hexagonal boron nitride (hereinafter also referred to as "hBN") layer according to the present disclosure will be described with reference to the drawings. However, it should be noted that the technical scope of the present disclosure is not limited to the embodiments, but extends to the inventions described in the claims and their equivalents.

[0011] FIG. 1 is a diagram showing a sputtering apparatus used in the hBN layer stacking method according to the embodiment.

[0012] The sputtering apparatus 1 includes a vacuum chamber 10, a substrate holder 11, a sputtering cathode 20, an exhaust device 13, a gas supply device 14, a bias power supply 15, and a heating device 16. The sputtering apparatus 1 heats a substrate 100 held by the substrate holder 11 while sputtering a target 101 placed on the sputtering cathode 20 via a backing plate 23, thereby depositing multiple hBN layers on the surface of the substrate 100 by SAHP (Sputtering-Annealing Hybrid Process). When performing SAHP, the sputtering apparatus 1 generates a magnetic field around the target 101, thereby performing magnetron sputtering.

[0013] The vacuum chamber 10 is formed from a material with high thermal conductivity and a high melting point, such as stainless steel, steel, permalloy, pure titanium, or a heat-resistant alloy such as Inconel, molybdenum, or tungsten, and accommodates the substrate 100 and the target 101. The vacuum chamber 10 is preferably formed from stainless steel from a cost perspective. However, when the vacuum chamber 10 is formed from stainless steel, a water-cooled flange is disposed around the vacuum chamber for water cooling. The substrate holder 11 is formed from a material with high thermal conductivity and a high melting point, such as ceramics or heat-resistant alloys such as Inconel, molybdenum, or tungsten, and holds the substrate 100 on the surface facing the sputtering cathode 20. The substrate holder 11 may be formed from stainless steel, but when formed from stainless steel, the substrate holder 11 is preferably cooled by water cooling via a chiller as necessary. The sputter cathode 20 is formed of a material with high thermal conductivity, electrical conductivity, and a high melting point, and includes a plurality of permanent magnets 21 and a cooling device (not shown) disposed inside the sputter cathode 20, as well as a high-frequency power supply 22 and a matching box (not shown) disposed outside. The sputter cathode 20 converts a sputtering gas, such as Ar gas, introduced into the vacuum chamber 10 into plasma using a high-frequency voltage applied from the high-frequency power supply 22, and causes the generated ions to collide with the target 101. The sputter cathode 20 has a plurality of permanent magnets 21 disposed inside it, which generates a magnetic field that encloses electrons, thereby generating a high-density plasma region near the target 101, and efficiently causes ions to collide with the target 101. The sputter cathode 20 deposits sputtered particles generated by the ions colliding with the target 101 onto the substrate 100.

[0014] The exhaust device 13 includes a vacuum pump 31 and exhausts the gas filling the vacuum chamber 10. By improving the degree of vacuum within the vacuum chamber 10, the mean free path of atoms ejected from the target can be lengthened. The gas supply device 14 supplies a sputtering gas, such as Ar gas, into the vacuum chamber. The bias power supply 15 supplies high-frequency power to the vicinity of the substrate holder 11 in response to the high-frequency power supplied from the high-frequency power supply 22, so that the vicinity of the target 101 is at a negative potential. The heating device 16 includes a heater 40 disposed in the center of the rear surface of the substrate holder 11, a temperature sensor 41 that detects the temperature of the heater 40, and a DC current source 42 that supplies DC current to the heater 40. The heater 40 is an electric heating wire made of boron nitride and tantalum (Ta), and the temperature sensor 41 is a JIS standard C thermocouple. The heating device 16 heats the substrate 100 via the substrate holder 11 using the heater 40, thereby heating the substrate 100 to a temperature in the range of more than 1000°C and not more than 1600°C.

[0015] The substrate 100 is made of any one of nickel (Ni), platinum (Pt), and cobalt (Co), and is disposed so that its back surface is in contact with the substrate holder 11 and its front surface faces the target 101. The melting point of nickel is 1455°C, that of platinum is 1768°C, and that of cobalt is 1495°C. When the substrate 100 is a nickel substrate, the substrate 100 is formed so that the (111) plane of nickel is arranged on the surface facing the target 101. When the substrate 100 is a platinum substrate, the substrate 100 is formed so that the (111) plane of platinum is arranged on the surface facing the target 101. When the substrate 100 is a cobalt substrate, the substrate 100 is formed so that the (0001) plane of cobalt is arranged on the surface facing the target 101.

[0016] The target 101 is made of boron nitride, and more preferably made of pyrolytic boron nitride (PBN). The target 101 is disposed on the surface of the cathode 20 so as to face the substrate 100. The target 101 may be formed by fastening it to a backing plate 23 made of copper with fastening members such as bolts.

[0017] FIG. 2 is a flowchart showing a method for stacking a hexagonal boron nitride layer according to an embodiment.

[0018] First, in a target placement step, the target 101 is placed on the sputtering cathode 20 (S101). Next, in a substrate placement step, the substrate 100 is held by the substrate holder 11 (S102). Next, in a vacuum pumping step, the exhaust device 13 exhausts the atmosphere in the vacuum chamber 10 and reduces the pressure in the vacuum chamber 10 to a predetermined pressure (S103). The exhaust device 13 is configured to exhaust, for example, 8.0×10 －４ The internal pressure of the vacuum chamber 10 is reduced to 1 Pa. Next, in a sputtering gas introducing step, the gas supply device 14 introduces argon (Ar) as a sputtering gas into the vacuum chamber 10 (S104).

[0019] Next, in the substrate heating step, the heating device 16 heats the substrate 100 at a predetermined temperature for a predetermined time (S105). The heating device 16 heats the substrate 100, for example, at 1200°C for 60 minutes. The heating temperature of the substrate 100 is preferably greater than 1000°C and less than 1600°C, and less than the melting point of the material forming the substrate, more preferably 1100°C or greater, and even more preferably 1200°C or greater. If the heating temperature of the substrate 100 is less than 1000°C, the crystallinity and uniformity of the deposited hBN layer will be reduced. If the heating temperature of the substrate 100 is greater than 1600°C, malfunctions may occur in the components of the sputtering apparatus 1, including the heating device 16. When the substrate 100 is a nickel substrate, the heating temperature of the substrate 100 is preferably less than 1455°C. When the substrate 100 is a platinum substrate, the heating temperature of the substrate 100 is preferably less than 1600°C. When the substrate 100 is a cobalt substrate, the heating temperature of the substrate 100 is preferably less than 1495°C. The heating time of the substrate 100 is preferably 30 minutes or more and 60 minutes or less. If the heating time of the substrate 100 is less than 30 minutes, the temperature of the substrate 100 may not rise sufficiently, which may result in a deterioration in the film quality of the deposited hBN layer. If the heating time of the substrate 100 exceeds 60 minutes, excessive power is consumed by heating, unnecessarily increasing the manufacturing cost of the hBN layer. The heating device 16 continues to heat the substrate 100 during the sputtering process.

[0020] Next, in the sputtering step, a sputtering process is performed (S106). The internal pressure of the vacuum chamber 10 during sputtering is preferably 0.1 Pa or more and 1.0 Pa or less. The high-frequency power supply 22 applies a predetermined AC power at a predetermined wavelength. The high-frequency power supply 22 applies, for example, 300 W AC power having a frequency of 13.56 MHz. The frequency of the AC power applied during the sputtering process is preferably 1 MHz or more and 100 MHz or less. If the frequency of the AC power applied during the sputtering process is less than 1 MHz, the plasma density decreases and becomes unstable, resulting in a decrease in the sputtering rate and a loss of film uniformity. If the frequency of the AC power applied during the sputtering process exceeds 100 MHz, a more advanced RF power supply and matching network are required, which complicates the device and increases costs. The power of the AC power applied during the sputtering process is preferably 50 W or more and 500 W or less. If the AC power applied during the sputtering process is less than 50 W, the incident energy to the substrate 100 will decrease, resulting in a deterioration in the film quality of the deposited hBN layer. If the AC power applied during the sputtering process is more than 500 W, the AC power supplied will be excessive, unnecessarily increasing the manufacturing cost of the hBN layer. In the sputtering process, multiple hBN layers are deposited on the surface of the substrate 100 facing the target 101.

[0021] Then, in a substrate unloading step, the substrate 100 having a plurality of hBN layers stacked on the surface facing the target 101 is unloaded from the vacuum chamber 10 (S107).

[0022] In the hBN layer stacking method according to the embodiment, by heating the substrate 100 to a temperature exceeding 1000°C, it is possible to quickly stack multiple hBN layers and improve the crystallinity and uniformity of the stacked hBN layers.

[0023] Furthermore, the hBN layer lamination method according to the embodiment uses a substrate formed of any of nickel, platinum, and cobalt as the substrate 100, so that even if the heating temperature of the substrate 100 exceeds 1000° C., there is little risk of the crystallinity of the atoms forming the substrate 100 collapsing, as long as the heating temperature is below the melting point of the metal forming the substrate. The hBN layer lamination method according to the embodiment uses a substrate formed of a metal that does not collapse the crystallinity of the atoms forming the substrate 100, even if the heating temperature exceeds 1000° C., thereby making it possible to improve the crystallinity and uniformity of the hBN layer laminated on the substrate.

[0024] Furthermore, the hBN layer stacking method according to the embodiment uses a substrate made of nickel as the substrate 100, thereby enabling the manufacturing costs of the hBN layer to be lower than when platinum and cobalt are used as the substrate 100.

[0025] A method for producing an hBN layer according to Example 1 will be described. The hBN layer according to Example 1 was produced by the method described with reference to FIG. 2. A SEC-400MK3 manufactured by Horiba Estec Co., Ltd. was used as the gas supply device 14. A RP-5000R manufactured by Pearl Industrial Co., Ltd. was used as the high-frequency power source 22. A BHS-4-Ta-P-O manufactured by ThermoCera Japan Co., Ltd. was used as the heater 40 of the heating device 16, and a PAT40-200T manufactured by Kikusui Chemical Industry Co., Ltd. was used as the DC current source 42 of the heating device 16.

[0026] In a target placement step indicated by S101, the target 101 placed on the sputtering cathode 20 was made of pyrolytic boron nitride. In a substrate placement step indicated by S102, a nickel substrate was held by the substrate holder 11. In a vacuum evacuation step indicated by S103, the internal pressure of the vacuum chamber 10 was increased to 8.0×10 －４ In the sputtering gas introduction step shown in S104, the flow rate of argon when introduced into the vacuum chamber 10 was 10 cm ３ / min.

[0027] In the substrate heating step indicated by S105, the substrate 100 was heated at 1200°C for 60 minutes. In the sputtering step indicated by S106, the high-frequency power supply 22 applied 300 W of AC power having a frequency of 13.56 MHz for 15 minutes. In the sputtering step, the substrate 100 was heated at 1200°C.

[0028] The methods for producing hBN layers according to Comparative Examples 1 and 2 will now be described. The methods for producing hBN layers according to Comparative Examples 1 and 2 differ from the method for producing an hBN layer according to Example 1 in that a copper (Cu) substrate, rather than a nickel substrate, is held by the substrate holder 11 in the substrate placement step indicated by S102. Furthermore, the methods for producing hBN layers according to Comparative Examples 1 and 2 further differ from the method for producing an hBN layer according to Example 1 in the temperatures to which the substrate is heated in the substrate heating step indicated by S105 and the sputtering step indicated by S106. In the method for producing an hBN layer according to Comparative Example 1, the temperature to which the substrate is heated in the substrate heating step indicated by S105 and the sputtering step indicated by S106 is 900°C. On the other hand, in the method for producing an hBN layer according to Comparative Example 2, the temperature to which the substrate is heated in the substrate heating step indicated by S105 and the sputtering step indicated by S106 is 1000°C.

[0029] FIG. 3(a) is a TEM image of a cross section of the hBN film according to Example 1, FIG. 3(b) is a diagram showing an electron diffraction image of the hBN film according to Example 1, and FIG. 3(c) is a diagram showing the Raman spectrum of the hBN film according to Example 1. FIGS. 3(a) and 3(b) were taken after an hBN film formed on a nickel substrate was transferred onto a silicon (Si) substrate. The TEM image shown in FIG. 3(a) and the electron diffraction image shown in FIG. 3(b) were taken using a JEM-2100 manufactured by JEOL Ltd. The acceleration voltage was 200 kV. The Raman spectrum shown in FIG. 3(c) was measured using an inVia Raman microscope manufactured by Renishaw Inc. In FIG. 3(c), the horizontal axis represents the Raman shift, and the vertical axis represents the intensity in arbitrary units.

[0030] In the hBN film according to Example 1, it is visually recognized that approximately 20 layers of the hBN film are uniformly and flatly stacked. Furthermore, in the electron diffraction image of the hBN film according to Example 1, a diffraction pattern corresponding to the

[0002] plane is clearly visible. Furthermore, the hBN film according to Example 1 has a 1364.6 cm vibration, which corresponds to the in-plane stretching vibration of the six-membered ring structure of the hBN film. －１ The hBN film according to Example 1 maintains high crystallinity and flatness even after being transferred onto a silicon (Si) substrate.

[0031] The hBN film according to Example 1 was deposited in about 20 layers during a 15-minute sputtering time, with the deposition time per layer being about 45 seconds, which is extremely fast compared to the deposition time per layer in deposition processes involving chemical reactions, such as CVD or ALD.

[0032] Fig. 4(a) is a TEM image of a cross section of the hBN film according to Comparative Example 1, and Fig. 4(b) is a diagram showing the Raman spectrum of the hBN film according to Comparative Example 1. Fig. 4 is a TEM image of a cross section of the hBN film according to Comparative Example 1. The image was taken on a copper substrate on which the hBN film was formed. The Raman spectrum shown in Fig. 4(b) was measured under the same conditions as the Raman spectrum shown in Fig. 3(c).

[0033] In the hBN film according to Comparative Example 1, it is visually recognized that about 20 layers of hBN films are uniformly and flatly stacked, similar to the hBN film according to Example 1. In the hBN film according to Comparative Example 1, similar to the hBN film according to Example 1, the hBN film according to Comparative Example 1 has a 1364.6 cm vibration frequency corresponding to the in-plane stretching vibration of the six-membered ring structure of the hBN film. －１ However, although the hBN film according to Comparative Example 1 was transferred onto a silicon (Si) substrate in the same manner as the hBN film according to Example 1, an electron diffraction image showing a diffraction pattern corresponding to the

[0002] plane could not be taken. Since an electron diffraction image could not be taken of the hBN film according to Comparative Example 1, it is presumed that the crystallinity is inferior to that of the hBN film according to Example 1.

[0034] Fig. 5(a) is a TEM image of the cross section of the hBN film according to Comparative Example 2, Fig. 5(b) is an enlarged view of the region indicated by a in Fig. 5(a), and Fig. 5(c) is an enlarged view of the region indicated by b in Fig. 5(a). Fig. 5(a) was photographed on a copper substrate on which an hBN film had been formed, similar to Fig. 4.

[0035] It is visually recognized that the hBN film according to Comparative Example 2 has fewer laminated layers than the hBN film according to Example 1 and the hBN film according to Comparative Example 1. Furthermore, the hBN film according to Comparative Example 2 includes a film extending parallel to the surface of the copper substrate, a film extending in a direction perpendicular to the surface of the copper substrate, and a film extending while curving, and it is visually recognized that the hBN film according to Comparative Example 2 has lower uniformity than the hBN film according to Comparative Example 1. Furthermore, since the hBN film according to Comparative Example 2 does not have a continuous crystal structure, it differs from the hBN films according to Example 1 and Comparative Example 1 in that it has a peak at 1364.6 cm in the Raman spectrum. －１ No peaks were measured.

Claims

1. A method for depositing hexagonal boron nitride layers, comprising: placing pyrolytic boron nitride as a target inside a chamber; placing a substrate inside the chamber; heating the substrate to a temperature higher than 1000°C and lower than 1600°C, but lower than the melting point of a material forming the substrate; and sputtering the target to deposit multiple hexagonal boron nitride layers on the substrate.

2. The method for depositing a hexagonal boron nitride layer according to claim 1, wherein the substrate is made of any one of nickel (Ni), platinum (Pt), and cobalt (Co).

3. The method for depositing a hexagonal boron nitride layer according to claim 1, wherein the substrate is made of nickel (Ni).

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