Film formation method

Abstract

To provide a film deposition method capable of suppressing a change in film quality on the influence of high energy particles incident from a target while suppressing the growth of a coating film on a target liquid surface.SOLUTION: A film deposition method comprises: supplying nitrogen radicals on a substrate placed in a chamber maintained in a decompressed atmosphere and sputtering a gallium target in a liquid phase state by a magnetron plasma formed by applying a discharge voltage of 200 V or less and a magnetic field to the gallium target to form a gallium nitride film on the substrate.SELECTED DRAWING: Figure 1

Description

[[Technical Field]]

[0001] The present invention relates to a film forming method for forming a gallium nitride film on a substrate by magnetron sputtering. [[Background Art]]

[0002] A reactive sputtering method using a radical gun is known. For example, Patent Document 1 describes a sputtering apparatus that forms a gallium nitride thin film on a substrate by sputtering a target made of metallic gallium with plasma while irradiating nitrogen radicals from an emission port of a radical gun unit onto a substrate disposed in a vacuum chamber. [[Prior Art Literature]] [[Patent Literature]]

[0003] [[Patent Document 1]] International Publication No. 2019 / 167715 [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0004] Films formed by magnetron sputtering are prone to damage caused by the influence of high-energy particles such as recoil ions incident from the target, which causes changes in film properties such as specific resistance. To prevent this, a method of reducing the energy of particles incident on the substrate is employed, for example, by forming high-density plasma by increasing the magnetic field strength or superimposing a high frequency on a sputtering power supply, and lowering the discharge voltage.

[0005] On the other hand, the gallium target melts due to the heat of the plasma, and its liquid surface is sputtered. When performing reactive magnetron sputtering of a metal in a liquid phase (hereinafter also referred to as liquid metal) in this manner, as the plasma density increases, the reactivity between the reactive gas and the target liquid surface increases, and a film, which is the reaction product, accumulates on the liquid surface. Since this film is an insulator, it becomes charged when exposed to the plasma, which may cause phenomena such as abnormal discharge or sudden boiling of the liquid surface.

[0006] In view of the above circumstances, the object of the present invention is to provide a film formation method that can suppress film growth at the target liquid surface while suppressing changes in film quality due to the influence of high-energy particles incident from the target. [Means for solving the problem]

[0007] A film formation method according to one embodiment of the present invention is: Nitrogen radicals are supplied to a substrate placed in a chamber maintained in a reduced-pressure atmosphere. A gallium nitride film is formed on the substrate by sputtering the gallium target using magnetron plasma generated by applying a discharge voltage of 200V or less and a magnetic field to a gallium target in a liquid phase state.

[0008] In the film deposition method of the present invention, a reactive magnetron sputtering method using highly reactive nitrogen radicals is employed to reduce the amount of nitrogen introduced into the chamber, thereby suppressing the growth of the film, which is a reaction product on the liquid surface of the gallium target. Furthermore, by sputtering the gallium target with a discharge voltage of 200V or less, the energy of recoil ions incident from the target toward the substrate is reduced, thereby suppressing changes in film quality due to the influence of these particles.

[0009] The strength of the magnetic field leaking onto the surface of the gallium target may be 0.06 Tesla or greater. By reducing the amount of nitrogen introduced into the chamber, film growth at the target liquid surface is suppressed, allowing for an increase in magnetic field strength and thus improvement of plasma density.

[0010] The reduced pressure atmosphere may be an argon gas atmosphere with a pressure of 0.1 Pa or more and 1 Pa or less. A radical gun may be used as the source of the nitrogen radicals, and the partial pressure ratio of the nitrogen gas supplied to the radical gun to the pressure may be 5% or more and 25% or less.

[0011] The gallium target of the liquid phase material is housed in a metal container, and the gallium target may be sputtered while the magnetic circuit is rotated around a rotation axis eccentric with respect to the center of the container.

[0012] The gallium target of the liquid phase material is housed in a metal container, and the gallium target may be sputtered while the substrate is rotated around a rotation axis eccentric with respect to the center of the container.

[0013] The gallium target may be sputtered while a target material consisting of impurity elements doped onto the gallium nitride film is sputtered.

[0014] The gallium target may be sputtered while the substrate is heated at a temperature of 300°C to 900°C.

[0015] The power supply for the discharge voltage may include at least one of a DC power supply and a high-frequency power supply. [Effects of the Invention]

[0016] According to the present invention, it is possible to suppress the growth of the film on the target liquid surface while suppressing changes in film quality due to the influence of high-energy particles incident from the target. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic side view showing a sputtering apparatus according to one embodiment of the present invention. [Figure 2] Another schematic side view showing the sputtering apparatus described above. [Figure 3] It is an enlarged cross-sectional view of essential parts showing a sputtering source of a gallium target in the above sputtering apparatus. [Figure 4] It is a diagram showing one experimental result obtained using the above sputtering apparatus, and is a graph showing the correlation between discharge voltage and specific resistance. [Figure 5] It is a diagram showing one experimental result obtained using the above sputtering apparatus, and is a graph showing the correlation between discharge voltage and specific resistance distribution. MODE FOR CARRYING OUT THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0019] FIG. 1 and FIG. 2 are schematic configuration diagrams showing a sputtering apparatus 100 according to an embodiment of the present invention. In the figures, the X axis, Y axis and Z axis are three mutually orthogonal axes; the X axis and Y axis indicate the horizontal direction, and the Z axis indicates the height direction, respectively. FIG. 1 is a schematic side view of the sputtering apparatus 100 viewed from the Y axis direction, and FIG. 2 is a schematic side view of the sputtering apparatus 100 viewed from the X axis direction.

[0020] The sputtering apparatus 100 of the present embodiment is configured as a film deposition apparatus for epitaxially growing a gallium nitride (GaN) film on a substrate W.

[0021] [Overall Configuration] The sputtering apparatus 100 includes a chamber 10, a substrate support 20, a first sputtering source 30, a reactive gas source 40, a second sputtering source 70, and a control unit 80.

[0022] (Chamber) The chamber 10 is made of metal, and forms a film deposition chamber 11 inside. The chamber 10 is connected to a vacuum pump 12, and is configured to be capable of maintaining the film deposition chamber 11 in a reduced-pressure atmosphere. The chamber 10 is typically connected to ground potential. Although not shown in the drawings, the chamber 10 is provided with a gate valve for carrying the substrate W in and out between the inside and the outside of the chamber 10.

[0023] (Substrate support section) The substrate support section 20 is installed on the top surface of the chamber 10. The substrate support section 20 has a substrate holder 21 which is placed in the film deposition chamber 11. The substrate holder 21 has a cylindrical shape, and an annular claw portion 22 is provided at the open end of its lower end to support the peripheral edge of the substrate W with the film deposition surface of the substrate W facing the bottom of the chamber 10.

[0024] The substrate support section 20 further includes a motor 23 having a rotation axis 23a (second rotation axis) for rotating the substrate holder 21 around the Z axis. The motor 23 is installed outside (in the atmosphere) of the chamber 10, and the rotation axis 23a is connected to the substrate holder 21 via a rotation introduction mechanism 24 with a vacuum seal function. The rotation axis 23a is connected to the center of the upper surface of the substrate holder 21 so that the substrate holder 21 can be rotated around its axis. The substrate holder 61 is connected to ground potential.

[0025] The substrate W is a semiconductor substrate. Examples of semiconductor substrates include gallium nitride substrates, sapphire substrates, and silicon substrates. The size of the substrate W is not particularly limited, and in this embodiment, an 8-inch wafer is used.

[0026] The substrate support portion 20 further includes a heating portion 25 capable of heating the substrate W to a predetermined temperature. The heating portion 25 is, for example, a resistance heater or a lamp heater, and is stationaryly positioned inside the substrate holder 21 (for example, on the top surface of the chamber 10), and is configured to heat the entire back surface of the substrate W. The predetermined temperature is not particularly limited as long as it is the temperature necessary for the epitaxial growth of the gallium nitride film deposited on the substrate W (for example, 300°C to 900°C), and in this embodiment it is 600°C.

[0027] (Reaction gas source) The reaction gas source 40 is a radical gun that generates nitrogen radicals 47. The reaction gas source 40 includes a reaction cylinder 41, a coil 42 provided around the reaction cylinder 41, and a housing section 43 that houses the reaction cylinder 41 and the coil 42.

[0028] The containment section 43 is installed at the bottom of the chamber 10, and the inside of the containment section 43 is in communication with the film deposition chamber 11. Outside the containment section 43 (out of the atmosphere), a gas source 44 for supplying reaction gas to the reaction cylinder 41 and a high-frequency power supply 45 for inputting high-frequency power to the coil 42 are installed. When the high-frequency power supply is input, the coil 42 forms an alternating magnetic field inside the reaction cylinder 41, activating the reaction gas supplied into the reaction cylinder 41 and generating nitrogen radicals 47. The amount of reaction gas supplied is not particularly limited, for example, 10 sccm. The frequency and power of the high-frequency power supply 45 are not particularly limited, for example, the frequency is 13.56 MHz and the power is 400 W.

[0029] The generated nitrogen radicals 47 are released into the deposition chamber 11 through the opening 46 of the containment section 43. The opening of the reaction cylinder 41 is positioned toward the substrate support section 20, and the nitrogen radicals 47 released from the reaction cylinder 41 are irradiated onto the deposition surface of the substrate W supported by the substrate support section 20.

[0030] The reaction gas is typically nitrogen (N2). Other nitrogen-containing gases, such as hydrogen nitride and nitrogen oxide, can also be used.

[0031] (First sputtering source) The first sputtering source 30 is a sputtering source that supplies gallium sputtered particles 38 onto a substrate W and forms a gallium nitride film on the substrate W through a reaction with nitrogen radicals 47. The first sputtering source 30 is installed at the bottom of the chamber 10. The first sputtering source 30 has a container 31 with a recess 32 formed on its surface that can accommodate a target material T1, a magnetic circuit 50 positioned opposite the back of the container 31, and a rotation mechanism 60 that can rotate the magnetic circuit 50 around an axis parallel to the Z-axis direction.

[0032] The container 31 is formed in a disc shape from a metal material having a higher melting point than the target material T1. The container 31 is preferably formed of a material having relatively good wettability with the target material T1. In the present embodiment, the target material T1 is metallic gallium (Ga), and the container 31 is formed of stainless steel. The container 31 is installed inside a cylindrical adhesion-preventing plate 33 provided at the bottom of the chamber 10.

[0033] FIG. 3 is an enlarged cross-sectional view of a main part of the first sputtering source 30. The recess 32 is typically a circular recess, and is formed on a surface 31a of the container 31 facing the film formation chamber 11. The center of the recess 32 coincides with the center (central axis C1) of the container 31. The container 31 has a bottom wall portion 311 and a peripheral wall portion 312, and the recess 32 is formed between the bottom wall portion 311 and the peripheral wall portion 312. The size of the recess 32 is not particularly limited, and for example, the diameter is about 100 mm (4 inches) and the depth is about 5 mm.

[0034] The container 31 is disposed at a position facing the substrate support portion 20 in the Z-axis direction. As shown in FIG. 1, the center (central axis C1) of the container 31 is disposed at a position eccentric from the rotation axis 23a (central axis C3) of the substrate holder 21. In the present embodiment, when the horizontal distance between the central axis C1 and the central axis C3 is X and the radius of the substrate W is a, the central axes C1 and C3 are set so as to satisfy the relationship X < a. This allows the substrate W to be placed on the central axis C1.

[0035] As shown in FIG. 3, the boundary portion between the bottom wall portion 311 and the peripheral wall portion 312 corresponding to the corner portion 321 of the recess 32 is preferably formed as a curved surface so that no sharp corner is formed. This makes it difficult for a void to be formed between the corner portion 321 of the recess 32 and the liquid-phase target material T1 (liquid metal), and suppresses the occurrence of abnormal discharge (splash) caused by the void. In FIG. 3, the radius of curvature of the corner portion 321 is, for example, 2 mm or more. Further, the angle formed between the bottom wall portion 311 and the peripheral wall portion 312 is preferably an obtuse angle of 120° or more. This improves the wettability between the corner portion 321 of the recess 32 and the liquid metal, thereby suppressing the formation of the void.

[0036] The back surface (back) 31b of the container 32, opposite to the front surface 31a, is formed as a flat surface. The back surface 31b of the container 32 may be joined to a backing plate 34 (see Figure 1) made of copper or other material with excellent thermal conductivity. The joining method is not particularly limited, and appropriate methods such as brazing or welding can be used. The backing plate 34 may be provided with cooling passages through which a cooling medium circulates. Alternatively, cooling passages through which a cooling medium circulates may be provided inside the bottom wall 311 and peripheral wall 312 of the container 31. This protects the container 31 from the heat of the plasma P1 (Figure 1) formed during sputter deposition.

[0037] The container 31 is further connected to the sputtering power supply 35 to form a sputtering electrode (sputtering cathode). The sputtering power supply 35 is a high-frequency power supply (RF), but a direct current (DC) power supply may be used instead or in addition to it. The container 31 may be connected to the sputtering power supply 35 via a backing plate 34. The frequency and power of the high-frequency power supply are not particularly limited; for example, the frequency is 13.56 MHz and the power is 80 W.

[0038] As shown in Figure 1, the sputtering source 30 has a gas source 37 that supplies sputtering gas to the deposition chamber 11. The sputtering gas is a gas used to form plasma and is typically argon (Ar). The location where the sputtering gas is introduced is not particularly limited; for example, the sputtering gas is introduced between the protective plate 33 and the container 31. The amount of sputtering gas introduced is not particularly limited, for example, 60 sccm.

[0039] Furthermore, a shielding plate 36 connected to ground potential is placed around the recess 32 of the container 31. The shielding plate 36 is an annular metal plate having an inner diameter slightly smaller than the opening diameter of the recess 32. As shown in Figure 2, the shielding plate 36 is positioned opposite the surface 31a of the recess 31 with a gap G between them.

[0040] The shield plate 36 functions as a shield to protect the surface 31a of the container 31 from sputtering by the plasma P1 by shielding the surface 31a from the plasma P1. Furthermore, since the shield plate 36 has an inner diameter smaller than the opening diameter of the recess 32, abnormal discharge at the interface between the container 31 and the target material T1 can be suppressed. The size of the gap G is not particularly limited as long as it is large enough to avoid electrical contact with the container 31, and in this embodiment, it was confirmed that the number of abnormal discharge occurrences could be significantly reduced in the range of 1 mm to 1.5 mm.

[0041] The magnetic circuit 50 is positioned opposite the back surface 31b of the container 31. The magnetic circuit 50 is positioned outside the chamber 10 (in the atmosphere). The magnetic circuit 50 includes a magnet section 51 that forms a magnetic field B1 on the surface 31a side of the container 31, a yoke 52 that supports the magnet section 51, and a housing section 53 that houses the magnet section 51 and the yoke 52.

[0042] The magnetic section 51 is composed of multiple magnets. For example, as shown in Figure 2, the magnetic section 51 has a first magnetic section 51a and a plurality of second magnetic sections 51b arranged in a ring around the first magnetic section 51a. The magnetic poles of the first magnetic section 51a and the second magnetic sections 51b are different from each other, facing the back surface 31b of the container 31. For example, the first magnet 51a is the north pole and the second magnetic sections 51b are the south poles. The magnetic field lines formed between these first magnets 51a and second magnets 51b leak to the surface 31a side of the container 31, and this leaked magnetic field B1 forms a magnetic field component that generates a magnetron discharge (plasma P1) directly above the recess 32.

[0043] The yoke 52 has a disc shape and is made of a magnetic material with high magnetic permeability. The diameter of the yoke 52 is not particularly limited, for example, 3 inches. The first magnet 51a is positioned at the center of the yoke 52, and the second magnet 51b is positioned concentrically with the center of the yoke 52. The center of the yoke 52 coincides with the center of the magnetic circuit 50 (central axis C2) and is positioned eccentrically with respect to the center of the container 51 (central axis C1).

[0044] The rotating mechanism 60 includes a turntable 61 that supports the magnetic circuit 50, and a drive motor 62 that has a rotating shaft 62a (first rotating shaft) connected to the turntable 61 and rotates the magnetic circuit 50 around this rotating shaft 62a. The drive motor 62 is positioned outside the housing portion 53 of the magnetic circuit 50, and the rotating shaft 62a passes through the bottom of the housing portion 53 and is connected to the turntable 61.

[0045] The rotating shaft 62a is positioned on the central axis C1 of the container 31. In other words, the rotating shaft 62a is positioned eccentrically with respect to the central axis C2 of the magnetic circuit 50. Therefore, the magnetic circuit 50 rotates eccentrically with respect to the center of the container 51 by the drive motor 62. The amount of eccentricity ΔC between the central axis C1 and the central axis C2 is not particularly limited, and is, for example, 10 mm or more and 20 mm or less.

[0046] As described above, the magnetic circuit 50 is configured to rotate eccentrically with respect to the container 31. This rotation of the magnetic circuit 50 causes the magnetic field formed on the surface (liquid surface) of the target material T1 to move relative to the liquid surface, periodically changing the distribution of the plasma P1. This expands the erosion region of the target material T1, thereby suppressing the growth of reaction products (films) on the liquid surface.

[0047] [Second sputtering source] The second sputtering source 70 is a sputtering source that supplies sputtered particles 79 of impurity elements to the substrate W to dope the gallium nitride film. As shown in Figure 2, the second sputtering source 70 is installed at the bottom of the chamber 10, similar to the first sputtering source 30. The second sputtering source 70 has a backing plate 72 that supports the target material T2 and a magnetic circuit 73 that is positioned opposite the back surface of the backing plate 72.

[0048] The target material T2 is a metal or semimetallic material that constitutes the impurity elements added to the gallium nitride film, and is formed in the shape of a disc or rectangular plate. The type of target material T2 can be arbitrarily set according to the conductivity type of the gallium nitride film to be formed (n-type in this embodiment), and in this embodiment it is silicon (Si). In addition to this, germanium (Ge) or magnesium (Mg) may also be used as materials. The target material T2 is placed inside the cylindrical anti-deposition plate 71 provided at the bottom of the chamber 10.

[0049] The backing plate 72 is made of a metal material with excellent thermal conductivity, such as copper, and is placed at the bottom of the chamber 10. The backing plate 72 is joined to the back surface of the target material T2 using appropriate methods such as brazing or welding. The backing plate 72 may be provided with cooling passages through which a cooling medium circulates.

[0050] The magnetic circuit 73 is positioned opposite the back surface of the backing plate 72. The magnetic circuit 73 is located outside the chamber 10 (in the atmosphere). The magnetic circuit 73 includes a magnet section 74 that forms a magnetic field B2 on the surface of the target material T2, a yoke 75 that supports the magnet section 74, and a housing section 76 that houses the magnet section 74 and the yoke 75.

[0051] The magnet section 74 is configured similarly to the magnet section 51 in the first sputtering source 30. The magnetic field lines formed by the magnet section 74 leak to the surface side of the target material T2, and the leaked magnetic field B2 forms a magnetic field component that generates a magnetron discharge (plasma P2) directly above the target material T2.

[0052] The magnetic circuit 73 may be fixedly positioned relative to the back surface of the backing plate 72, or it may be rotatably positioned similarly to the first sputtering source 30, or it may be oscillating along the Y-axis, for example.

[0053] The backing plate 72 is further connected to the sputtering power supply 77 to form a sputtering electrode (sputtering cathode). The sputtering power supply 77 is a high-frequency power supply (RF), but is not limited to this, and may also be a direct current (DC) power supply.

[0054] The second sputtering source 70 has a gas source 78 that supplies sputtering gas to the deposition chamber 11. The sputtering gas is a gas for forming plasma and is typically argon (Ar). The location where the sputtering gas is introduced is not particularly limited; for example, the sputtering gas is introduced between the protective plate 71 and the target material T2.

[0055] (Control Unit) The control unit 80 is installed outside the chamber 10 (in the atmosphere) and comprehensively controls the operation of the sputtering apparatus 100, including the substrate support unit 20, the first sputtering source 30, the reaction gas source 40, and the second sputtering source 70. The control unit 80 can be implemented using hardware elements used in computers, such as a CPU (Central Processing Unit), RAM (Random Access Memory), and ROM (Read Only Memory), as well as the necessary software. Instead of or in addition to the CPU, a PLD (Programmable Logic Device) such as an FPGA (Field Programmable Gate Array), or a DSP (Digital Signal Processor), or other ASIC (Application Specific Integrated Circuit) may be used.

[0056] [Film forming method] Next, a typical operation of the sputtering apparatus 100 of this embodiment, configured as described above, will be explained.

[0057] After the substrate W is supported by the substrate holder 21 of the substrate support section 20, the deposition chamber 11 is evacuated to a predetermined pressure (e.g., 0.3 Pa) by the vacuum pump 12. When the deposition chamber 11 reaches the predetermined pressure, the heating section 25 heats the substrate W to a predetermined temperature while driving the motor 23, rotating the substrate holder 21 and the substrate W at a predetermined rotational speed. The rotational speed of the substrate W is arbitrarily adjusted to obtain the desired in-plane film thickness distribution.

[0058] In the reaction gas source 40, reaction gas (nitrogen gas) is supplied to the reaction cylinder 41 and a high-frequency power supply is input to the coil 42, generating nitrogen radicals 47, which are then irradiated onto the substrate W.

[0059] In the first sputtering source 30, sputtering gas is supplied from the gas source 37 to the deposition chamber 11, and high-frequency power is applied to the container 31, thereby forming a sputtering gas (Ar) plasma P1 between the container 31 and the substrate holder 21. The density of the plasma P1 is maximum in the region where the magnetic field B1 of the magnetic circuit 50 leaking to the surface 31a side of the container 31 is perpendicular to the electric field, and the target material T1 contained in the recess 32 of the container 31 is sputtered mainly in this plasma region of maximum density (magnetron sputtering).

[0060] The target material T1, which is metallic gallium, is melted by the heat of the plasma P1, and the surface (liquid surface) of the target material T is sputtered by Ar ions generated by the plasma P1. The sputtered target particles 38 (Figure 1) are scattered toward the substrate W and react with nitrogen radicals 47 on the film deposition surface (bottom surface) of the substrate W. The gallium nitride particles, which are the reaction products of the target particles 38 and nitrogen radicals 47, are epitaxially grown by the heat of the substrate W, thereby forming a gallium nitride film on the substrate W.

[0061] Meanwhile, in the second sputtering source 70, sputtering gas is supplied from the gas source 78 to the deposition chamber 11, and high-frequency power is applied to the backing plate 72, thereby forming a plasma P2 of sputtering gas (Ar). The density of the plasma P2 is maximum in the region where the magnetic field B2 of the magnetic circuit 73 leaking to the surface side of the target material T2 is perpendicular to the electric field, and the target material T2 is sputtered mainly in this plasma region of maximum density (magnetron sputtering). The sputtered target particles 79 (Figure 2) are scattered toward the substrate W and added to the gallium nitride film formed on the deposition surface (bottom surface) of the substrate W. The sputtering rate of the second target material T2 is optimized so that the resistivity of the gallium nitride film reaches the target value.

[0062] In this case, films formed by magnetron sputtering are susceptible to damage from high-energy particles such as recoil ions incident from the target, which causes changes in film quality such as resistivity. In this embodiment, since the first sputtering source 30 is positioned opposite the center of the substrate W at an offset position, more high-energy particles from the first sputtering source 30 are incident on the peripheral side of the substrate W than on the central side. For this reason, changes in film quality (e.g., an increase in resistivity) are more likely to occur at the peripheral side of the substrate than at the center.

[0063] To prevent film damage caused by high-energy particles, methods are known to reduce the energy of particles incident on the substrate by forming a high-density plasma, for example, by increasing the magnetic field strength or superimposing high-frequency waves on the sputtering power supply, and lowering the discharge voltage. However, in this embodiment, the gallium target melts due to the heat of the plasma, and its liquid surface is sputtered. When performing reactive magnetron sputtering of a metal in a liquid phase (hereinafter also referred to as liquid metal) in this manner, as the plasma density increases, the reactivity between the reactive gas and the target liquid surface increases, and a film, which is the reaction product, accumulates on the liquid surface. Since this film is an insulator, it becomes charged when exposed to the plasma, which may cause phenomena such as abnormal discharge or sudden boiling of the liquid surface.

[0064] To solve these problems, the film deposition method of this embodiment involves supplying nitrogen radicals 47 to a substrate W placed in a chamber 11 (film deposition chamber 11) maintained in a reduced-pressure atmosphere, and sputtering the gallium target (target material T1) with a magnetron plasma generated by applying a discharge voltage of 200V or less and a magnetic field to the liquid-phase gallium target (target material T1), thereby forming a gallium nitride film on the substrate W.

[0065] In other words, in this embodiment, a reactive magnetron sputtering method using highly reactive nitrogen radicals 47 is employed to reduce the amount of nitrogen introduced into the chamber 11, thereby suppressing the growth of the film, which is a reaction product, on the liquid surface of the gallium target. Furthermore, by sputtering the target material T1 with a discharge voltage of 200V or less, the energy of the recoil ions incident from the target material T1 toward the substrate W is reduced, thereby suppressing changes in film quality due to the influence of these particles. This makes it possible to suppress film growth on the target liquid surface while simultaneously suppressing changes in film quality due to the influence of high-energy particles incident from the target material T1.

[0066] The term "film properties" refers to the film properties of the deposited gallium nitride, such as its crystallinity and conductivity, and in this embodiment, it refers to the in-plane distribution of resistivity. The second sputtering source 70 sputters the target material T2 to add impurity elements to the gallium nitride film, thereby adjusting the resistivity of the gallium nitride film to a target value.

[0067] Furthermore, the discharge voltage refers to the potential difference (so-called Vdc) between the plasma P1 and the target material T1, and is typically the input voltage from the sputtering power supply 35. By keeping this potential difference below a predetermined voltage (200V), the energy of particles such as recoil ions incident from the target material T1 toward the substrate W is limited, thereby suppressing changes in the film quality of the gallium nitride film formed on the substrate W. The above predetermined voltage is preferably 150V or less, more preferably 120V or less.

[0068] In this case, it is preferable that the strength of the magnetic field B1 leaking onto the surface (liquid surface) of the target material T1 is 0.06 Tesla (600 Gauss) or higher. This increases the density of the plasma P1, thereby enabling stable sputter deposition. The discharge voltage can be adjusted by the strength of the magnetic field B1, and typically, to lower the discharge voltage, the strength of the magnetic field B1 is increased. For example, when the discharge voltage is around 200V, the strength of the magnetic field B1 is 0.06 Tesla, when it is around 150V, the strength of the magnetic field B1 is 0.08 Tesla, and when it is around 120V, the strength of the magnetic field B1 is 0.1 Tesla. The magnetic field strength is adjusted, for example, by the type of magnetic material constituting the magnet section 51, or by the distance between the container 31 and the magnetic circuit 50. Hereinafter, the strength of the magnetic field B1 will also be referred to as the magnetic field strength of the magnetic circuit 50.

[0069] On the other hand, the amount of nitrogen gas introduced to generate nitrogen radicals 47 is, for example, 5% to 25% of the partial pressure ratio of the nitrogen gas supplied to the radical gun relative to the pressure in the deposition chamber 11, when the pressure in the deposition chamber 11 is an argon atmosphere of 0.1 Pa to 1 Pa. If the partial pressure ratio of nitrogen gas is less than 5%, it is difficult to form a gallium nitride film of the desired composition. Also, if the partial pressure ratio of nitrogen gas exceeds 25%, a film, which is a reaction product, is likely to grow on the liquid surface of the gallium target (target material T1), making it difficult to suppress the occurrence of abnormal discharges.

[0070] Furthermore, it is preferable to adjust the partial pressure ratio of nitrogen gas according to the magnetic field strength of the magnetic circuit 50. For example, if the magnetic field strength of the magnetic circuit 50 is high, the reactivity with nitrogen increases due to the increased density of plasma P1, resulting in the growth of a film consisting of the reaction products on the liquid surface of the gallium target (making abnormal discharges more likely to occur). For this reason, when the magnetic field strength of the magnetic circuit 50 is relatively high, the upper limit of the nitrogen gas supply amount may be set lower than when the magnetic field strength of the magnetic circuit 50 is relatively low. Also, when the magnetic field strength of the magnetic circuit 50 is low, although the growth of the above film is suppressed, the reactivity with gallium decreases, making it difficult to obtain a gallium nitride film of the desired composition. For this reason, when the magnetic field strength of the magnetic circuit 50 is relatively low, the lower limit of the nitrogen gas supply amount may be set higher than when the magnetic field strength of the magnetic circuit 50 is relatively high.

[0071] [Example of experiment] The following describes experimental examples conducted using the sputtering apparatus 100 of this embodiment.

[0072] (Experimental Example 1) A gallium nitride film was formed on a substrate W by sputtering a gallium target (target material T1) with the magnetic field strength of the first sputtering source 30 set to 0.10 Tesla (1000 Gauss), the discharge voltage set to 112 V, the rotation speed of the magnetic circuit 50 set to 10 rpm (eccentricity ΔC of 10 mm), and the nitrogen supply amount (partial pressure ratio of nitrogen gas) in the reaction gas source 40 set to 11%. The sputtering conditions for the second sputtering source 70, the temperature of the substrate W, and the rotation speed were the same for all experimental examples.

[0073] Next, the resistivity and in-plane distribution of the formed gallium nitride film were measured. The resistivity was measured at the edges (substrate edge) and center (substrate center) of the substrate W, with the substrate edge being 15 mm from the peripheral edge of the substrate W. The resistivity was calculated from the product of the film thickness and the sheet resistance. The film thickness was measured using a scanning electron microscope (SEM), and the four-terminal method was used to measure the sheet resistance.

[0074] The measurement results are shown in Table 1. The resistivity at the edge of the substrate was 345 μΩcm, the resistivity at the center of the substrate was 341 μΩcm, and the average of these values ​​was 343 μΩcm. The in-plane distribution of resistivity was 0.6%.

[0075] (Experimental Example 2) A gallium nitride film was formed on the substrate W under the same conditions as in Experimental Example 1, except that the magnetic field strength of the first sputtering source 30 was set to 0.08 Tesla (800 Gauss) and the discharge voltage was set to 143 V. The resistivity and its in-plane distribution were measured for the formed gallium nitride film using the same method as in Experimental Example 1. The measurement results are shown in Table 1. The resistivity at the substrate edge was 344 μΩcm, the resistivity at the center of the substrate was 330 μΩcm, and their average value was 337 μΩcm, with an in-plane resistivity distribution of 2.1%.

[0076] (Experimental Example 3) A gallium nitride film was formed on the substrate W under the same conditions as in Experimental Example 1, except that the magnetic field strength of the first sputtering source 30 was set to 0.06 Tesla (600 Gauss), the discharge voltage to 177 V, and the nitrogen gas partial pressure ratio to 15%. The resistivity and its in-plane distribution were measured for the formed gallium nitride film using the same method as in Experimental Example 1. The measurement results are shown in Table 1. The resistivity at the substrate edge was 380 μΩcm, the resistivity at the center of the substrate was 345 μΩcm, their average value was 363 μΩcm, and the in-plane distribution of resistivity was 4.8%.

[0077] (Experimental Example 4) A gallium nitride film was formed on the substrate W under the same conditions as in Experimental Example 1, except that the magnetic field strength of the first sputtering source 30 was set to 0.04 Tesla (400 Gauss), the discharge voltage to 214 V, and the nitrogen gas partial pressure ratio to 17%. The resistivity and its in-plane distribution were measured for the formed gallium nitride film using the same method as in Experimental Example 1. The measurement results are shown in Table 1. The resistivity at the substrate edge was 443 μΩcm, the resistivity at the center of the substrate was 339 μΩcm, their average value was 391 μΩcm, and the in-plane distribution of resistivity was 13.3%.

[0078] [Table 1]

[0079] Figure 4 is a graph showing the correlation between discharge voltage and resistivity obtained from the measurement results of Experimental Examples 1 to 4. Figure 5 is a graph showing the correlation between discharge voltage and resistivity distribution obtained from the measurement results of Experimental Examples 1 to 4. As shown in Table 1, Figure 3, and Figure 4, in Experimental Examples 1-3, where the discharge voltage was 200V or less, the in-plane distribution of resistivity was 5% or less in all cases (the difference in resistivity between the substrate edge and the substrate center was small). From this, it was confirmed that the incident energy of ions in the plasma P1 on the substrate W can be reduced by setting the discharge voltage to 200V or less.

[0080] On the other hand, in Experimental Example 4, where the discharge voltage exceeded 200V, the difference in resistivity between the substrate edge and the center of the substrate was much larger than in Experimental Examples 1-3, and the in-plane distribution also exceeded 10%. This indicates that at discharge voltages exceeding 200V, variations in the film quality of the gallium nitride film are unavoidable due to the influence of high-energy particles incident on the substrate.

[0081] Furthermore, as shown in the results of Experimental Examples 1-3, it was confirmed that the lower the discharge voltage, the smaller the difference in resistivity between the substrate edge and the substrate center, and the improved resistivity distribution of the gallium nitride film. From these results, the discharge voltage of the gallium target is preferably 200V or less, more preferably 150V or less, and most preferably 120V or less. The lower limit of the discharge voltage is not particularly limited as long as it is a voltage that can stably form plasma P1, for example, 100V.

[0082] Furthermore, by increasing the magnetic field strength of the magnetic circuit 50 to increase plasma density and lowering the discharge voltage, stable sputter deposition can be achieved. In addition, by decreasing the nitrogen gas partial pressure ratio as the magnetic field strength increases, the growth of reaction products (films) on the target liquid surface due to the increase in plasma density can be suppressed, thereby reducing the occurrence of abnormal discharges (arc discharges or splashes).

[0083] The upper and lower limits of the nitrogen gas partial pressure ratio can be determined in relation to the magnitude of the magnetic field strength. For example, as shown in Table 2, when the magnetic field strength is relatively high at 0.08 Tesla, it is preferable that the nitrogen gas partial pressure ratio be between 5% and 21%. If the nitrogen gas partial pressure ratio is less than 5%, the amount of nitrogen radical 47 generated is too small, and the gallium nitride film cannot be stably formed. On the other hand, if the nitrogen gas partial pressure ratio exceeds 21%, the amount of nitrogen radical 47 generated is too large, causing reaction products (films) to grow on the liquid surface of the gallium target, making abnormal discharge more likely. In Table 2, the symbols "〇", "△", and "×" indicate the type of composition of the film formed: "〇" is a gallium nitride film, "△" is a mixed film of gallium and gallium nitride, and "×" is a gallium film ("-" indicates not performed).

[0084] [Table 2]

[0085] Furthermore, in the experimental results shown in Table 2, the nitrogen gas partial pressure ratio is preferably 8% to 21% when the magnetic field strength is 0.06 Tesla, and similarly, it is preferably 15% to 24% when the magnetic field strength is 0.05 Tesla, and 21% to 25% when the magnetic field strength is 0.04 Tesla.

[0086] The experimental results in Table 2 show the case where the power of the high-frequency power supply 40 in the reaction gas source 40 (radical gun) is 400W. However, Table 3 shows the relationship between the magnetic field strength and the characteristics of the formed film when the power is 0W, i.e., when nitrogen gas is supplied to the deposition chamber 11 instead of nitrogen radicals.

[0087] [Table 3]

[0088] As shown in Table 3, when nitrogen gas is used instead of nitrogen radicals as the reactive gas, the nitrogen gas partial pressure ratio at which a gallium nitride film can be stably formed is very limited, requiring strict control of the nitrogen gas partial pressure ratio. In contrast, according to this embodiment, which uses nitrogen radicals as the reactive gas, the degree of freedom (robustness) in adjusting the nitrogen gas partial pressure ratio is increased, and a gallium nitride film can be stably formed even with a nitrogen gas partial pressure ratio of 21% or less when the magnetic field strength is 0.05 Tesla or higher (see Table 2).

[0089] Although embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the embodiments described above and can be modified in various ways.

[0090] For example, in the above embodiment, gallium (target material T1) is sputtered onto the magnetic circuit 50 in the first sputtering source 30 while it is rotated eccentrically with respect to the container 31. However, the invention is not limited to this, and gallium may be sputtered onto the magnetic circuit 50 without rotating it.

[0091] Furthermore, in the above embodiments, a multi-source sputtering method using a first sputtering source 30 and a second sputtering source 70 was described as an example of a film deposition method, but the second sputtering source 70 may be omitted. In such a film deposition method, the quality of the gallium nitride film can be evaluated, for example, by evaluating the crystallinity of the gallium nitride film, and by setting the discharge voltage to 200V or less, changes in the crystallinity of the gallium nitride film can be suppressed. [Explanation of symbols]

[0092] 10... Chamber 11... Film formation chamber 20... Circuit board support section 21... Circuit board holder 23a... Axis of rotation (second axis of rotation) 25...Heating part 30...First sputtering source 31…Container 32…recess 40… Reaction gas source 50…Magnetic circuit 60... Rotation mechanism 62a... Axis of rotation (second axis of rotation) 70...Second sputtering source 100... Sputtering device P1, P2... Plasma T1, T2… Target material W... Circuit board

Claims

1. Nitrogen radicals are supplied to a substrate placed in a chamber maintained in a reduced-pressure atmosphere. A method for forming a gallium nitride film on a substrate by sputtering a gallium target with a magnetron plasma generated by applying a discharge voltage of 200 V or less and a magnetic field to a gallium target in a liquid phase state, A radical gun is used as the source of the nitrogen radicals, and the partial pressure ratio of the nitrogen gas supplied to the radical gun to the pressure in the reduced-pressure atmosphere is 5% or more and 25% or less. Film formation method.

2. A film formation method according to claim 1, The magnetic field strength leaking onto the surface of the gallium target is 0.06 Tesla or greater. Film formation method.

3. A film formation method according to claim 2, The aforementioned reduced pressure atmosphere is an argon gas atmosphere with a pressure of 0.1 Pa or more and 1 Pa or less. Film formation method.

4. A film formation method according to claim 3, The discharge voltage is 150V or less. Film formation method.

5. A film formation method according to any one of claims 1 to 4, The gallium target in liquid phase is housed in a metal container. The gallium target in liquid phase is sputtered while a magnetic circuit that applies a magnetic field to the gallium target is rotated around a rotation axis eccentric with respect to the center of the container. Film formation method.

6. A film formation method according to any one of claims 1 to 4, The gallium target in liquid phase is housed in a metal container. The gallium target is sputtered while the substrate is rotated around a rotation axis eccentric with respect to the center of the container. Film formation method.

7. A film formation method according to any one of claims 1 to 4, The gallium target is sputtered while sputtering a target material consisting of impurity elements doped into the gallium nitride film. Film formation method.

8. A film formation method according to any one of claims 1 to 4, The gallium target is sputtered while the substrate is heated at a temperature of 300°C to 900°C. Film formation method.

9. A film formation method according to any one of claims 1 to 4, The power supply for the discharge voltage includes at least one of a DC power supply and a high-frequency power supply. Film formation method.

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