Semiconductor device manufacturing method

Abstract

To restrain surface transformation of a gallium nitride layer even when performing activation annealing.SOLUTION: According to the present disclosure, in a step of forming a cap film on a gallium nitride layer, a cap film is formed by a film deposition device using a plasma created by an electron cyclotron resonance phenomenon.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a semiconductor device, and relates to a technique that is effective when applied to, for example, a technique for forming a film by utilizing plasma. [Background technology]

[0002] Japanese Patent Publication No. 2020-122178 (Patent Document 1) describes a technology in which ions contained in plasma generated using the electron cyclotron resonance phenomenon are collided with a target member, causing target particles ejected from the target member to adhere to the object to be filmed, thereby forming a film on the object to be filmed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-122178 Summary of the Invention [Problem to be solved by the invention]

[0004] In power devices, for example, an epitaxial layer is formed on a substrate. It is desirable for the epitaxial layer to achieve both a reduced on-resistance and an improved breakdown voltage, which are in a trade-off relationship. To address this issue, the use of wide-bandgap semiconductor materials with a larger bandgap than silicon for the epitaxial layer is being considered.

[0005] This is because wide bandgap semiconductor materials have a larger bandgap than silicon, resulting in a higher breakdown voltage than silicon. This allows the breakdown voltage to be ensured even if the thickness of the semiconductor chip (thickness of the drift layer) is made thinner than when silicon is used. In other words, when a semiconductor chip is made from a wide bandgap semiconductor material, the thickness of the semiconductor chip can be made thinner than when the semiconductor chip is made from silicon, thereby reducing the on-resistance.

[0006] For example, gallium nitride (GaN) is an example of a wide bandgap semiconductor material. When forming an epitaxial layer from a gallium nitride layer, it is necessary to form an n-type or p-type semiconductor region within the gallium nitride layer. For this purpose, conductive impurities (donors or acceptors) are introduced into the gallium nitride layer by, for example, ion implantation. Then, activation annealing is performed to activate the introduced conductive impurities.

[0007] In this regard, activation annealing is performed at a temperature higher than the crystal growth temperature of the gallium nitride layer. As a result, the surface of the gallium nitride layer may be altered by activation annealing. Therefore, there is a need for a method that can suppress the surface alteration of the gallium nitride layer even when activation annealing is performed. [Means for solving the problem]

[0008] In one embodiment, a method for manufacturing a semiconductor device includes a step of forming a cap film on a gallium nitride layer, and the cap film is formed in a film formation apparatus that uses plasma generated by utilizing the electron cyclotron resonance phenomenon. [Effects of the Invention]

[0009] According to one embodiment, surface deterioration of the gallium nitride layer can be suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating the surface of a GaN layer being altered by activation annealing. [Figure 2] 1A and 1B are diagrams illustrating a technique for performing activation annealing in a state where a cap film is formed on the surface of a GaN layer. [Figure 3] FIG. 1 is a diagram showing the configuration of an ECR plasma film forming apparatus. [Figure 4] 10 is a flowchart illustrating the flow of a film forming operation. [Figure 5] FIG. 1 is a diagram showing the configuration of a target used in an ECR plasma film forming apparatus. [Figure 6] FIG. 10 is a diagram illustrating that when a disk-shaped target is used, the object to be film-formed is more likely to be damaged. [Figure 7] 10A and 10B are diagrams illustrating that damage to a film-forming target can be reduced when a cylindrical target is used. [Figure 8] (a) to (f) are verification results showing that the surface alteration of the GaN layer can be suppressed by forming an aluminum nitride film with a thickness of 100 nm on the GaN layer. [Figure 9] (a) to (f) are verification results showing that the surface alteration of the GaN layer can be suppressed by forming an aluminum nitride film with a thickness of 300 nm on the GaN layer. [Figure 10] (a) to (f) are verification results showing that the surface roughness is hardly affected by annealing after forming a 100 nm thick aluminum nitride film on a GaN layer. [Figure 11] (a) to (f) are verification results showing that the surface roughness is hardly affected by annealing after forming a 300 nm thick aluminum nitride film on a GaN layer. [Figure 12] This is a verification result showing that the crystallinity of GaN and aluminum nitride is hardly affected by annealing after forming a 100 nm thick aluminum nitride film on the GaN layer. [Figure 13]This is a verification result showing that the crystallinity of GaN and aluminum nitride is hardly affected by annealing after forming a 300 nm thick aluminum nitride film on the GaN layer. DETAILED DESCRIPTION OF THE INVENTION

[0011] In all the drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated explanations thereof will be omitted. In addition, hatching may be used even in plan views to make the drawings easier to understand.

[0012] <Consideration of improvements> 1(a) and 1(b) are diagrams illustrating how activation annealing alters the surface of a GaN layer. First, as shown in FIG. 1(a), a GaN layer 200 is formed on a sapphire substrate 100. Conductive impurities are introduced into the GaN layer 200 by, for example, ion implantation. In this state, activation annealing is performed. The temperature of the activation annealing is 1000°C or higher. As shown in FIG. 1(b), activation annealing causes nitrogen, which constitutes GaN, to be desorbed from the surface of the GaN layer 200. As a result, a Ga-rich region 200A is formed on the surface of the GaN layer 200. This means that the formation of the Ga-rich region 200A alters the surface of the GaN layer 200. Therefore, it is desirable to be able to suppress the formation of the Ga-rich region 200A due to nitrogen desorption, even when activation annealing is performed at 1000°C or higher.

[0013] Therefore, it has been considered to perform activation annealing in a state where a cap film is formed on the surface of the GaN layer 200. This technique will be described below.

[0014] 2(a) to 2(c) are diagrams illustrating a technique for performing activation annealing with a cap film formed on the surface of a GaN layer 200. As shown in FIG. 2(a), the GaN layer 200 is formed on a sapphire substrate 100. A cap film 300 is formed on the GaN layer 200. The cap film 300 is, for example, an aluminum nitride film. Conductive impurities are introduced into the GaN layer 200 by, for example, ion implantation. In this state, activation annealing is performed. The temperature for activation annealing is 1000°C or higher.

[0015] As shown in FIG. 2(b), a cap film 300 is formed on the GaN layer 200. Therefore, even if activation annealing is performed, desorption of nitrogen, which constitutes GaN, from the surface of the GaN layer 200 is suppressed. That is, by forming the cap film 300 on the GaN layer 200, the cap film 300 exhibits the function of suppressing desorption of nitrogen. As a result, even if activation annealing is performed, a Ga-rich region 200A is not formed on the surface of the GaN layer 200. That is, surface deterioration of the GaN layer 200 is prevented.

[0016] 2(c), when the activation annealing is completed, the cap film 300 is removed. The cap film 300 is removed by, for example, etching.

[0017] From the above, in order to suppress the surface alteration of the GaN layer 200, a technique of performing activation annealing in a state in which the cap film 300 is formed on the surface of the GaN layer 200 is useful.

[0018] However, according to the inventors' investigations, there is room for improvement in the technology for forming the cap film 300. Specifically, the cap film 300 is formed, for example, from an aluminum nitride film. The aluminum nitride film is formed using a magnetron sputtering apparatus or a CVD (Chemical Vapor Deposition) film formation apparatus. At this time, heating to 1000°C or higher is required when forming the aluminum nitride film using the magnetron sputtering apparatus or the CVD film formation apparatus. That is, when forming the cap film 300, heat of 1000°C or higher is applied to the GaN layer 200. Therefore, this heat may cause deterioration of the GaN layer 200. That is, when forming the aluminum nitride film using the magnetron sputtering apparatus or the CVD film formation apparatus, there is a risk of deterioration of the GaN layer 200.

[0019] Therefore, in this embodiment, an improvement is made to the method of forming the cap film 300. The technical idea behind this improved embodiment will be described below.

[0020] <Basic Concept of the Embodiment> The basic concept of this embodiment is to form a cap film on a gallium nitride layer using a film-forming apparatus (sometimes referred to as an ECR plasma film-forming apparatus) that uses plasma generated by electron cyclotron resonance (ECR). Specifically, the plasma density of plasma generated by electron cyclotron resonance (ECR) is high. Therefore, the ECR plasma film-forming apparatus can form a high-quality cap film with excellent high-temperature resistance without using a high heating temperature. Specifically, the ECR plasma film-forming apparatus can form a cap film at a temperature lower than 1000°C. As a result, according to this basic concept, application of heat of 1000°C or higher to the GaN layer during cap film formation can be suppressed. Therefore, alteration of the GaN layer due to heat applied during cap film formation can be suppressed. Specifically, forming a cap film using an ECR plasma film-forming apparatus can prevent alteration of the GaN layer.

[0021] Next, an embodiment that embodies the basic idea will be described.

[0022] <Configuration of ECR Plasma Deposition Apparatus> FIG. 3 is a diagram showing a schematic configuration of an ECR plasma deposition apparatus.

[0023] In FIG. 3, an ECR plasma deposition apparatus 1 has a chamber 10 that is a deposition chamber. In this chamber 10, a holding part 11 is disposed, and by this holding part 11, a deposition object SUB typified by a substrate, for example, is held. The holding part 11 is connected to a mechanism part 12 disposed close to the chamber 10 and is configured to be operable by the mechanism part 12. In this chamber 10, a gas inlet 10a and a gas outlet 10b are provided.

[0024] Next, in the chamber 10, a plasma generation part 13 is provided at a position facing the deposition object SUB held by the holding part 11. This plasma generation part 13 is configured to generate plasma, and around the plasma generation part 13, a magnetic field generation part 14 composed of, for example, a coil is disposed. Further, a waveguide 15 is connected to the plasma generation part 13, and microwaves propagating through the waveguide 15 are introduced into the plasma generation part 13. Furthermore, between the holding part 11 and the plasma generation part 13 and at a position close to the plasma generation part 13, a target TA having, for example, a cylindrical shape is disposed, and this target TA is electrically connected to a high-frequency power source 16. Thereby, the target TA is configured such that a high-frequency voltage from the high-frequency power source 16 is applied. This target TA is fixed by a fixing part 17.

[0025] As described above, the ECR plasma deposition apparatus 1 is configured.

[0026] <Film Deposition Operation in Film Deposition Apparatus> Subsequently, the film deposition operation in the film deposition apparatus 1 will be described.

[0027] FIG. 4 is a flowchart illustrating the flow of the film forming operation.

[0028] First, in FIG. 3, a gas, such as argon gas, is introduced into the plasma generating unit 13. When a magnetic field is generated from the magnetic field generating unit 14 disposed around the plasma generating unit 13, electrons contained in the gas introduced into the plasma generating unit 13 undergo a circular motion due to the Lorentz force. When microwaves (electromagnetic waves) having the same period (or frequency) as the period (or frequency) of the circular motion of the electrons are introduced into the plasma generating unit 13 from the waveguide 15, the circularly moving electrons and the microwave resonate with each other, and the energy of the microwave is efficiently supplied to the circularly moving electrons (electron cyclotron resonance phenomenon) (S101 in FIG. 4). As a result, the kinetic energy of the electrons contained in the gas increases, and the gas separates into positive ions and electrons. This generates plasma consisting of the positive ions and electrons (S102 in FIG. 4).

[0029] Next, referring to FIG. 3, a radio-frequency voltage is supplied from the radio-frequency power supply 16 to the target TA. In this case, a positive potential and a negative potential are alternately applied to the target TA to which the radio-frequency voltage is supplied. Among the positive ions and electrons that constitute the plasma, only the electrons with a low mass can follow the radio-frequency voltage applied to the target TA, while the positive ions with a high mass cannot follow the radio-frequency voltage. As a result, the positive potential that attracts the following electrons is offset by the negative charge of the electrons, while the negative potential remains, so the average value of the radio-frequency power shifts from 0 V to a negative potential. This means that even though a radio-frequency voltage is applied to the target TA, it can be considered as if a negative potential is applied to the target TA. As a result, the positive ions are attracted to the target TA, which is considered to have a negative potential applied to it on average, and collide with the target TA (S103 in FIG. 4).

[0030] Subsequently, when positive ions collide with the target TA, target particles constituting the target TA receive part of the kinetic energy of the positive ions and fly out from the target TA into the internal space of the chamber 10 (S104 in FIG. 4). Then, a part of the target particles that have flown out into the internal space of the chamber 10 adheres to the surface of the film formation target object SUB held by the holding part 11 (S105 in FIG. 4). And as such a phenomenon is repeated, as a result of a large number of target particles adhering to the surface of the film formation target object SUB, a film is formed on the surface of the film formation target object SUB (S106 in FIG. 4).

[0031] In the above manner, the film formation operation in the ECR plasma film formation apparatus 1 is realized.

[0032] For example, when the target TA is made of aluminum, the target particles become aluminum atoms, and the film formed on the film formation target object SUB becomes an aluminum film. However, when the above-described film formation operation is performed while introducing oxygen gas or nitrogen gas from the gas inlet 10a provided in the chamber 10 of the film formation apparatus 1 shown in FIG. 1, an aluminum oxide film or a aluminum nitride film can be formed on the surface of the film formation target object SUB.

[0033] Similarly, for example, when the target TA is made of silicon, the target particles become silicon atoms, and the film formed on the film formation target object SUB becomes a silicon film. However, when the above-described film formation operation is performed while introducing oxygen gas or nitrogen gas from the gas inlet 10a provided in the chamber 10 of the film formation apparatus 1 shown in FIG. 1, a silicon oxide film or a silicon nitride film can be formed on the surface of the film formation target object SUB.

[0034] <Advantages of the ECR Plasma Film Formation Apparatus> The ECR plasma film formation apparatus 1 is a method in which a plasma flow created by using electron cyclotron resonance (ECR) and a diverging magnetic field is irradiated onto a film formation target SUB, and at the same time, a high-frequency voltage is applied between the target TA and the ground, causing ions in the plasma to collide with the target TA, thereby forming a film on the film formation target SUB. This film formation method, which we will call the ECR sputtering method, has the following advantages.

[0035] For example, in the magnetron sputtering method, -3 Torr(10 -3 In contrast, in the ECR sputtering method, stable ECR plasma can be obtained only when the pressure is on the order of 10 -4 Torr(10 -4 × 133.32 Pa). In addition, in the ECR sputtering method, a film can be formed on the film-forming object SUB at a low pressure because sputtering is performed by applying a high-frequency voltage to particles (positive ions) in the plasma and hitting the target TA.

[0036] In the ECR sputtering method, an ECR plasma flow and sputtered particles are irradiated onto a film-forming target SUB. Ions in the ECR plasma flow have energies of 10 eV to several tens of eV, and because of the low pressure, the ion current density of the ions reaching the film-forming target SUB can be large. Therefore, the ions in the ECR plasma flow impart energy to the sputtered raw material particles that have landed on the film-forming target SUB, and also promote a bonding reaction between the raw material particles and oxygen, improving the quality of the film deposited on the film-forming target SUB by the ECR sputtering method. A particular advantage of this ECR sputtering method is that it can form a high-quality film on the film-forming target at a low substrate temperature (temperature of the film-forming target SUB).

[0037] Specifically, the cap film can be formed at a temperature lower than 1000° C. For example, the cap film can be formed at a temperature of 250° C.

[0038] From the above, the ECR plasma film formation apparatus 1 is excellent in that it can form a high-quality film. In particular, it can be said that the ECR plasma film formation apparatus 1 is extremely excellent in that it can form a high-quality film on the surface of the film formation target SUB without exposing the film formation target SUB to high temperatures. In other words, the ECR plasma film formation apparatus 1 is extremely excellent in that it can form a high-quality film on the surface of the film formation target SUB while reducing damage to the film formation target SUB.

[0039] <Advantages of cylindrical targets> Fig. 5 is a diagram showing the external configuration of the target TA used in the ECR plasma film formation apparatus 1. As shown in Fig. 5, the target TA has a cylindrical shape. Specifically, the target TA has a cylindrical backing tube (support member) 20 made of, for example, a copper material, and a cylindrical target member 21 made of, for example, aluminum is bonded to the inner wall of this backing tube 20 by a bonding material (adhesive) not shown.

[0040] The cylindrical target TA configured in this manner can reduce damage to the film-forming target SUB compared to when a commonly used disk-shaped target is used. This advantage will be described below.

[0041] FIG. 6 is a schematic diagram illustrating how a disc-shaped target is more likely to damage a film-forming target. In FIG. 6, a disc-shaped target TA1 is positioned facing the film-forming target SUB. This disc-shaped target TA1 includes a support member 30 and a target member 31 positioned on the support member 30. For example, as shown in FIG. 6, argon ions having kinetic energy collide with the target member 31, causing target particles 50 to fly out of the target member 31 and adhere to the surface of the film-forming target SUB. This results in the formation of a film composed of target particles on the surface of the film-forming target SUB. However, at this time, argon ions 40 that collide with the target member 31 also recoil. However, when the disc-shaped target TA1 is used, the film-forming target SUB is positioned opposite the disc-shaped target TA1, as shown in FIG. 6. Therefore, as shown in FIG. 6, the recoiled argon ions 40 are also more likely to collide with the film-forming target SUB. That is, when a disk-shaped target TA1 is used to form a film on the surface of a film-forming target SUB disposed opposite the target TA1, not only the target particles 50 that are components of the film but also the recoiled argon ions 40 are likely to collide with the surface of the film-forming target SUB. For this reason, in a film-forming apparatus configured to use a disk-shaped target TA1 to form a film on the surface of a film-forming target SUB disposed opposite the target TA1, the probability that recoiled argon ions will collide with the film-forming target SUB increases, and as a result, the recoiled argon ions are likely to damage the film-forming target SUB.

[0042] In contrast to this, FIG. 7 is a diagram illustrating that when a cylindrical target is used, damage to the film-forming target can be reduced.

[0043] In FIG. 7, a cylindrical target TA is disposed at a position facing the film-forming target SUB. In the cylindrical target TA, a cylindrical target member 21 is disposed on the inner wall of a cylindrical backing tube 20. Therefore, in the target TA shown in FIG. 5, the target member 21 is not disposed facing the film-forming target SUB. In this case, as shown in FIG. 7, even in the cylindrical target TA, argon ions 40 having kinetic energy collide with the target member 21, causing target particles 50 to fly out of the target member 21 and adhere to the surface of the film-forming target SUB. As a result, even when using the cylindrical target TA, a film made of the target particles 50 can be formed on the surface of the film-forming target SUB. Meanwhile, in the cylindrical target TA shown in FIG. 7, unlike the disk-shaped target TA1 shown in FIG. 6, the target member 21 itself is not disposed facing the film-forming target SUB. 7, in the case of a cylindrical target TA, the probability that argon ions 40 that recoil after colliding with the target member 21 will collide with the film-forming target SUB is reduced. Therefore, in a film-forming apparatus configured to form a film on the surface of the film-forming target SUB using a cylindrical target TA, the probability that recoiled argon ions will collide with the film-forming target SUB is reduced, and as a result, damage to the film-forming target SUB caused by the recoiled argon ions colliding with the film-forming target SUB can be reduced.

[0044] From the above, the cylindrical target TA shown in Figure 7 has the advantage of being able to reduce damage to the film-forming target SUB compared to using the commonly used disk-shaped target TA1 (see Figure 6).

[0045] According to an embodiment, a cap film (aluminum nitride film) can be formed on a film-forming target SUB (GaN layer) by using an ECR plasma film-forming apparatus 1. The ECR plasma film-forming apparatus 1 allows a high-quality film to be formed at a low temperature without damaging the surface of the GaN layer due to the synergistic effects of high-density plasma and the use of a cylindrical target. For example, the thickness of the cap film is 100 nm or more and 300 nm or less.

[0046] The GaN layer contains conductive impurities. After forming a cap film using the ECR plasma deposition apparatus 1, annealing is performed to activate the conductive impurities. This annealing is performed at a temperature of 1000°C or higher. However, in this embodiment, a cap film is formed on the GaN layer. Therefore, even when activation annealing is performed, desorption of nitrogen, which constitutes GaN, from the surface of the GaN layer is suppressed. That is, by forming a cap film on the GaN layer, the cap film exhibits the function of suppressing nitrogen desorption. As a result, even when activation annealing is performed, a Ga-rich region is not formed on the surface of the GaN layer. That is, surface deterioration of the GaN layer is prevented. Thereafter, when activation annealing is completed, the cap film is removed. The cap film is removed by, for example, etching.

[0047] Below, a description will be given of the verification results that show that the GaN layer is well protected by using the aluminum nitride film formed by the ECR plasma film-forming apparatus 1 as the cap film.

[0048] <Verification results> <<Verification results regarding the effectiveness of the cap film>> FIG. 8 is a micrograph illustrating the verification result that the surface alteration of the GaN layer can be suppressed by forming a 100 nm thick aluminum nitride film on the GaN layer.

[0049] Figure 8(a) is a micrograph showing the surface of a GaN layer. Figure 8(b) is a micrograph showing an aluminum nitride film formed on the GaN layer. Figure 8(c) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 10 minutes. Figure 8(d) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 30 minutes. Figure 8(e) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 10 minutes. Figure 8(f) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 30 minutes.

[0050] In Figure 8, the area surrounded by the dotted line indicates a non-film-formed area where no aluminum nitride cap film is formed. Looking at the non-film-formed area surrounded by the dotted line, surface deterioration of the GaN layer occurs. In other words, if no aluminum nitride cap film is formed on the GaN layer, annealing at a temperature of 1000°C or higher will cause the surface of the GaN layer to deteriorate.

[0051] In contrast, in Figure 8, looking at the areas other than the non-film-formed area surrounded by the dotted line, the surface of the GaN layer is not altered due to the presence of the aluminum nitride cap film. Therefore, the verification results shown in Figure 8 confirm that forming a 100 nm thick aluminum nitride film on the GaN layer can suppress surface alteration of the GaN layer. In other words, the 100 nm thick aluminum nitride film can suppress surface alteration of the GaN layer caused by annealing at 1000°C or higher.

[0052] FIG. 9 is a micrograph illustrating the verification result that the surface alteration of the GaN layer can be suppressed by forming a 300 nm thick aluminum nitride film on the GaN layer.

[0053] Figure 9(a) is a micrograph showing the surface of a GaN layer. Figure 9(b) is a micrograph showing an aluminum nitride film formed on the GaN layer. Figure 9(c) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 10 minutes. Figure 9(d) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 30 minutes. Figure 9(e) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 10 minutes. Figure 9(f) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 30 minutes.

[0054] In Figure 9, the area surrounded by the dotted line indicates a non-film-formed area where no aluminum nitride cap film is formed. Looking at the non-film-formed area surrounded by the dotted line, surface deterioration of the GaN layer occurs. In other words, if no aluminum nitride cap film is formed on the GaN layer, annealing at a temperature of 1000°C or higher will cause the surface of the GaN layer to deteriorate.

[0055] In contrast, in Figure 9, looking at the areas other than the non-film-formed area surrounded by the dotted line, the surface of the GaN layer is not altered due to the presence of the aluminum nitride cap film. Therefore, the verification results shown in Figure 9 confirm that forming a 300 nm thick aluminum nitride film on the GaN layer can suppress surface alteration of the GaN layer. In other words, the 300 nm thick aluminum nitride film can suppress surface alteration of the GaN layer caused by annealing at 1000°C or higher.

[0056] 8 and 9, an aluminum nitride film having a thickness of 100 nm or more and 300 nm or less can suppress the surface deterioration of the GaN layer caused by annealing at 1000° C. or more.

[0057] <<Surface roughness verification results>> Figure 10 shows a micrograph taken by an atomic force microscope, demonstrating the verification result that the surface roughness is hardly affected by annealing after forming a 100-nm-thick aluminum nitride film on a GaN layer.

[0058] Figure 10(a) is a micrograph showing the surface of a GaN layer. Figure 10(b) is a micrograph showing an aluminum nitride film formed on the GaN layer. Figure 10(c) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 10 minutes. Figure 10(d) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 30 minutes. Figure 10(e) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 10 minutes. Figure 10(f) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 30 minutes.

[0059] The surface roughness (sq) of the GaN layer without an aluminum nitride film is 0.15 nm (Fig. 10(a)). In contrast, when a 100 nm aluminum nitride film is formed on the GaN layer, the surface roughness of the aluminum nitride film becomes 0.33 nm (Fig. 10(b)).

[0060] Even after annealing at 1000°C for 10 minutes or 30 minutes, the surface roughness was 0.39 nm or 0.34 nm. This indicates that the surface roughness remains almost unchanged even after annealing at 1000°C for 10 minutes or 30 minutes (FIG. 10(c) or FIG. 10(d)). Furthermore, even after annealing at 1300°C for 10 minutes or 30 minutes, the surface roughness remains 0.31 nm or 0.35 nm. This indicates that the surface roughness remains almost unchanged even after annealing at 1300°C for 10 minutes or 30 minutes (FIG. 10(e) or FIG. 10(f)).

[0061] This demonstrates that forming a 100-nm-thick aluminum nitride film on a GaN layer has almost no effect on the surface roughness.

[0062] Figure 11 shows a micrograph taken by an atomic force microscope, demonstrating the verification result that the surface roughness is hardly affected by annealing after forming a 300 nm thick aluminum nitride film on a GaN layer.

[0063] Figure 11(a) is a micrograph showing the surface of a GaN layer. Figure 11(b) is a micrograph showing an aluminum nitride film formed on the GaN layer. Figure 11(c) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 10 minutes. Figure 11(d) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1000°C for 30 minutes. Figure 11(e) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 10 minutes. Figure 11(f) is a micrograph showing the state after an aluminum nitride film was formed on the GaN layer and then annealed at 1300°C for 30 minutes.

[0064] The surface roughness (sq) of the GaN layer without an aluminum nitride film is 0.15 nm (Fig. 11(a)). In contrast, when a 300 nm aluminum nitride film is formed on the GaN layer, the surface roughness of the aluminum nitride film becomes 0.59 nm (Fig. 11(b)).

[0065] Even after annealing at 1000°C for 10 minutes or 30 minutes, the surface roughness is 0.61 nm or 0.68 nm. This indicates that the surface roughness remains almost unchanged even after annealing at 1000°C for 10 minutes or 30 minutes (FIG. 11(c) or FIG. 11(d)). Furthermore, even after annealing at 1300°C for 10 minutes or 30 minutes, the surface roughness remains 0.60 nm or 0.69 nm. This indicates that the surface roughness remains almost unchanged even after annealing at 1300°C for 10 minutes or 30 minutes (FIG. 11(e) or FIG. 11(f)).

[0066] This demonstrates that forming a 300 nm thick aluminum nitride film on the GaN layer has almost no effect on the surface roughness.

[0067] <<Verification results regarding crystallinity>> Figure 12 shows the results of a study demonstrating that the crystallinity of GaN and AlN (aluminum nitride) is hardly affected by annealing after forming a 100-nm-thick aluminum nitride film on the GaN layer. Specifically, Figure 12(a) shows the rocking curve of GaN (0002) measured by X-ray diffraction. Figure 12(b) shows the rocking curve of AlN (0002) measured by X-ray diffraction.

[0068] The crystallinity of GaN and AlN hardly changes even after annealing at 1000°C for 10 or 30 minutes. This indicates that the aluminum nitride film formed using the ECR plasma deposition system has extremely high heat resistance.

[0069] Annealing at 1300°C for 10 or 30 minutes does not deteriorate the crystallinity of GaN, but it does slightly deteriorate the crystallinity of AlN. Since the crystallinity of GaN does not deteriorate even when annealing at 1300°C for 10 or 30 minutes, the crystallinity of GaN is maintained. In other words, the aluminum nitride film formed in the ECR plasma deposition system is able to adequately protect the crystallinity of GaN.

[0070] Figure 13 shows the results of a study demonstrating that the crystallinity of GaN and AlN (aluminum nitride) is hardly affected by annealing after forming a 300 nm thick aluminum nitride film on the GaN layer. Specifically, Figure 13(a) shows the rocking curve of GaN(0002) measured by X-ray diffraction. Figure 13(b) shows the rocking curve of AlN(0002) measured by X-ray diffraction.

[0071] The crystallinity of GaN and AlN hardly changes even after annealing at 1000°C for 10 or 30 minutes. This indicates that the aluminum nitride film formed using the ECR plasma deposition system has extremely high heat resistance.

[0072] Annealing at 1300°C for 10 or 30 minutes does not deteriorate the crystallinity of GaN, but it does slightly deteriorate the crystallinity of AlN. Since the crystallinity of GaN does not deteriorate even when annealing at 1300°C for 10 or 30 minutes, the crystallinity of GaN is maintained. In other words, the aluminum nitride film formed in the ECR plasma deposition system is able to adequately protect the crystallinity of GaN.

[0073] The above verification results show that by using an aluminum nitride film formed in an ECR plasma deposition system as the cap film, it is possible to suppress surface deterioration caused by annealing at temperatures above 1000°C without affecting surface roughness or crystallinity. In other words, an aluminum nitride film formed in an ECR plasma deposition system can effectively protect the GaN layer.

[0074] <Modification> In the embodiments, a substrate structure in which a GaN layer is formed as an epitaxial layer on a sapphire substrate is used as an example for explanation. However, the technical concept of the embodiments is not limited thereto. For example, the present invention can also be applied to a substrate structure in which a GaN layer is formed as an epitaxial layer on a GaN substrate. In this case, aluminum nitride films (cap films) formed by an ECR plasma film formation apparatus are formed on both sides of the substrate structure in which a GaN layer is formed as an epitaxial layer on a GaN substrate. This makes it possible to suppress deterioration of both the lower surface of the GaN substrate and the upper surface of the GaN layer.

[0075] The invention made by the inventor has been specifically described above based on the embodiments thereof, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention. [Explanation of symbols]

[0076] 1. ECR plasma deposition equipment 10 Chambers 10a Gas inlet 10b Gas exhaust port 11 Holding part 12 Mechanism 13 Plasma generation unit 14 Magnetic field generating unit 15 Waveguide 16 High frequency power supply 17 Fixed part 20 Backing Tube 21 Target material 30 Support member 31 Target material 40 Argon ions 50 target particles 100 Sapphire substrate 200 GaN layers 200A Ga-rich region 300 Cap Membrane SUB Film formation target TA Target TA1 Target

Claims

1. (a) forming a cap film on a gallium nitride layer; The method for manufacturing a semiconductor device, wherein the cap film is formed by a film forming apparatus that uses plasma generated by utilizing an electron cyclotron resonance phenomenon.

2. 2. The method for manufacturing a semiconductor device according to claim 1, The cap film is an aluminum nitride film.

3. 2. The method for manufacturing a semiconductor device according to claim 1, In the step (a), the cap film is formed at a temperature lower than 1000°C.

4. 4. The method for manufacturing a semiconductor device according to claim 3, In the step (a), the cap film is formed at a temperature of 250°C.

5. 2. The method for manufacturing a semiconductor device according to claim 1, The thickness of the cap film is 100 nm or more and 300 nm or less.

6. 2. The method for manufacturing a semiconductor device according to claim 1, the gallium nitride layer contains a first conductivity type impurity; After the step (a), annealing is performed to activate the first conductivity type impurity.

7. 7. The method for manufacturing a semiconductor device according to claim 6, The annealing is carried out at a temperature of 1000° C. or higher.

8. 7. The method for manufacturing a semiconductor device according to claim 6, After the annealing is performed, the cap film is removed.

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