Method for manufacturing semiconductor device

Abstract

With the present disclosure, surface alteration of a gallium nitride layer is suppressed even when activation annealing is performed. The present disclosure involves a step for forming a cap film on a gallium nitride layer, wherein the cap film is formed by a film formation device that uses plasma generated utilizing an electron cyclotron resonance phenomenon.

Description

Semiconductor device manufacturing method

[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.

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

[0003] Japanese Patent Application Laid-Open No. 2020-122178

[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 and therefore have a higher breakdown voltage than silicon. As a result, the breakdown voltage can 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, and therefore the on-resistance can be reduced.

[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 semiconductor region or a p-type semiconductor region within the gallium nitride layer. For this purpose, for example, conductive impurities (donors or acceptors) are introduced into the gallium nitride layer by 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.

[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.

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

[0010] 1 is a diagram explaining the surface alteration of a GaN layer by activation annealing. 2 is a diagram explaining a technique for performing activation annealing with a cap film formed on the surface of a GaN layer. 3 is a diagram illustrating the configuration of an ECR plasma film formation apparatus. 4 is a flowchart explaining the flow of film formation operations. 5 is a diagram illustrating the configuration of a target used in an ECR plasma film formation apparatus. 6 is a diagram explaining that the use of a disk-shaped target makes it easier to damage a film formation target. 7 is a diagram explaining that the use of a cylindrical target can reduce damage to a film formation target. (a) to (f) are verification results showing that surface alteration of a GaN layer can be suppressed by forming a 100 nm thick aluminum nitride film on a GaN layer. (a) to (f) are verification results showing that surface alteration of a GaN layer can be suppressed by forming a 300 nm thick aluminum nitride film on a GaN layer. (a) to (f) are verification results showing that surface roughness is hardly affected by annealing after forming a 100 nm thick aluminum nitride film on a GaN layer. (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. (a) to (f) are verification results showing that the crystallinity of GaN and aluminum nitride is hardly affected by annealing after forming a 100 nm thick aluminum nitride film on a GaN layer. (b) to (f) are verification results showing that the crystallinity of GaN and aluminum nitride is hardly affected by annealing after forming a 300 nm thick aluminum nitride film on a GaN layer.

[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 Improvement> FIGS. 1( a) and 1(b) are diagrams illustrating the alteration of the surface of a GaN layer when activation annealing is performed. 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, for example, by 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), when activation annealing is performed, nitrogen constituting GaN is 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. 2B, a cap film 300 is formed on the GaN layer 200. Therefore, even when activation annealing is performed, desorption of nitrogen constituting 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 when activation annealing is performed, a Ga-rich region 200A is not formed on the surface of the GaN layer 200. That is, surface alteration of the GaN layer 200 is prevented.

[0016] 2C, when the activation annealing is completed, 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, the technique of performing activation annealing in a state where 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 alteration of the GaN layer 200. That is, when forming an aluminum nitride film using a magnetron sputtering apparatus or a CVD film formation apparatus, there is a risk of alteration 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 concept of this embodiment in which an improvement is made will be described below.

[0020] <Basic Concept of the Embodiment> The basic concept of the present embodiment is a semiconductor device manufacturing method including a step of forming a cap film on a gallium nitride layer, in which the cap film is formed using a film formation apparatus (sometimes referred to as an ECR plasma film formation apparatus) that uses plasma generated by electron cyclotron resonance (ECR). That is, the plasma density of plasma generated by electron cyclotron resonance (ECR) is high. Therefore, the ECR plasma film formation apparatus can form a high-quality cap film with excellent high-temperature resistance without using a high heating temperature. Specifically, the ECR plasma film formation apparatus can form a cap film at a temperature lower than 1000°C. As a result, according to the 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. That is, by forming the cap film using an ECR plasma film formation apparatus, alteration of the GaN layer can be prevented.

[0021] The following describes an embodiment that embodies the basic concept.

[0022] <Configuration of ECR ​​Plasma Film Forming Apparatus> FIG. 3 is a diagram showing a schematic configuration of an ECR plasma film forming apparatus.

[0023] 3, the ECR plasma film formation apparatus 1 has a chamber 10, which is a film formation chamber. A holder 11 is disposed in the chamber 10, and this holder 11 holds a film formation target SUB, such as a substrate. The holder 11 is connected to a mechanism 12 disposed close to the chamber 10 and is operable by the mechanism 12. The chamber 10 is provided with a gas inlet 10a and a gas outlet 10b.

[0024] Next, the chamber 10 is provided with a plasma generating unit 13 at a position facing the film-forming target SUB held by the holding unit 11. This plasma generating unit 13 is configured to generate plasma, and a magnetic field generating unit 14, for example, composed of a coil, is arranged around the plasma generating unit 13. A waveguide 15 is connected to the plasma generating unit 13, and microwaves propagating through the waveguide 15 are introduced into the plasma generating unit 13. Furthermore, a target TA, for example, having a cylindrical shape, is arranged between the holding unit 11 and the plasma generating unit 13 and in close proximity to the plasma generating unit 13, and this target TA is electrically connected to a high-frequency power supply 16. This allows a high-frequency voltage from the high-frequency power supply 16 to be applied to the target TA. This target TA is fixed by a fixing unit 17.

[0025] The ECR plasma film forming apparatus 1 is configured as described above.

[0026] <Film Forming Operation in Film Forming Apparatus> Next, a film forming operation in the film forming 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 by the magnetic field generating unit 14 disposed around the plasma generating unit 13, electrons contained in the gas are subjected to the Lorentz force and undergo circular motion. 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 resonate with the microwaves, and the energy of the microwaves 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 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 light-mass electrons can follow the radio-frequency voltage applied to the target TA, while heavy-mass positive ions cannot. 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 an average negative potential, and collide with the target TA (S103 in FIG. 4 ).

[0030] Subsequently, when the positive ions collide with the target TA, the target particles constituting the target TA receive some of the kinetic energy of the positive ions and are ejected from the target TA into the internal space of the chamber 10 (S104 in FIG. 4). After that, some of the target particles ejected into the internal space of the chamber 10 adhere to the surface of the film-forming target SUB held by the holder 11 (S105 in FIG. 4). Then, by repeating this phenomenon, a large number of target particles adhere to the surface of the film-forming target SUB, and as a result, a film is formed on the surface of the film-forming target SUB (S106 in FIG. 4).

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

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

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

[0034] <Advantages of the ECR Plasma Film Forming Apparatus> The ECR plasma film forming apparatus 1 is a method in which a plasma flow created by utilizing the electron cyclotron resonance phenomenon (ECR) and a diverging magnetic field is irradiated onto a film-forming 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-forming target SUB. If this film formation method is called the ECR sputtering method, this ECR sputtering method has the following advantages.

[0035] For example, in the magnetron sputtering method, －３ Torr (10 －３ On the other hand, 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 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 causing particles (positive ions) in the plasma to hit the target TA using a high-frequency voltage.

[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 an energy 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 made large. Therefore, the ions in the ECR plasma flow impart energy to the raw material particles that have been sputtered and landed on the film-forming target SUB, and also promote a bonding reaction between the raw material particles and oxygen, thereby improving the quality of the film deposited on the film-forming target SUB by the ECR sputtering method. A particular advantage of such an ECR sputtering method is that it is possible to form a high-quality film on the film-forming target SUB 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, it can be said that the ECR plasma film-forming apparatus 1 is excellent in that it can form a high-quality film. In particular, it can be said that the ECR plasma film-forming apparatus 1 is extremely excellent in that it can form a high-quality film on the surface of the film-forming object SUB without exposing the film-forming object SUB to high temperatures. In other words, the ECR plasma film-forming apparatus 1 is extremely excellent in that it can form a high-quality film on the surface of the film-forming object SUB while reducing damage to the film-forming object SUB.

[0039] Advantages of a Cylindrical Target 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 the backing tube 20 by a bonding material (adhesive) not shown.

[0040] The cylindrical target TA configured as described above can reduce damage to the film-forming target SUB compared to when a commonly used disk-shaped target is used. The advantages of this will be described below.

[0041] FIG. 6 is a schematic diagram illustrating how the use of a disk-shaped target increases the likelihood of damaging the film-forming target. In FIG. 6, a disk-shaped target TA1 is disposed facing the film-forming target SUB. This disk-shaped target TA1 includes a support member 30 and a target member 31 disposed 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, as shown in FIG. 6, when the disk-shaped target TA1 is used, the film-forming target SUB is disposed at a position facing the disk-shaped target TA1. Therefore, as shown in FIG. 6, the recoiled argon ions 40 are also likely to collide with the film-forming target SUB. That is, when a disc-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 the disc-shaped target TA1 to form a film on the surface of a film-forming target SUB disposed opposite the target TA1, the probability that the recoiled argon ions will collide with the film-forming target SUB increases, and as a result, the recoiled argon ions are likely to cause damage to the film-forming target SUB.

[0042] In contrast to this, FIG. 7 is a diagram for explaining 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. At this time, 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 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 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 the cylindrical target TA, the probability that recoiled argon ions will collide with the film-forming target SUB is reduced, and as a result, it is possible to reduce damage to the film-forming target SUB caused by the recoiled argon ions colliding with the film-forming target SUB.

[0044] From the above, the cylindrical target TA shown in FIG. 7 has the advantage of being able to reduce damage to the film-forming target SUB compared to the use of the commonly used disk-shaped target TA1 (see FIG. 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. In other words, 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. In other words, surface alteration 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] The following describes 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 results showing 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] Fig. 8(a) is a micrograph showing the surface of a GaN layer. Fig. 8(b) is a micrograph showing the state in which an aluminum nitride film is formed on a GaN layer. Fig. 8(c) is a micrograph showing the state in which an aluminum nitride film is formed on a GaN layer and then annealed at 1000°C for 10 minutes. Fig. 8(d) is a micrograph showing the state in which an aluminum nitride film is formed on a GaN layer and then annealed at 1000°C for 30 minutes. Fig. 8(e) is a micrograph showing the state in which an aluminum nitride film is formed on a GaN layer and then annealed at 1300°C for 10 minutes. Fig. 8(f) is a micrograph showing the state in which an aluminum nitride film is formed on a GaN layer and then annealed at 1300°C for 30 minutes.

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

[0051] In contrast, in Fig. 8 , looking at the area 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 film as a cap film. Therefore, the verification results shown in Fig. 8 support the idea that forming a 100 nm thick aluminum nitride film on the GaN layer can suppress surface alteration of the GaN layer. That is, 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 an aluminum nitride film with a thickness of 300 nm on the GaN layer.

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

[0054] 9, the region surrounded by the dotted line indicates a non-film-formed region where the aluminum nitride cap film is not formed. Looking at the non-film-formed region surrounded by the dotted line, surface deterioration of the GaN layer occurs. In other words, if the aluminum nitride cap film is not formed on the GaN layer, the surface of the GaN layer will be deteriorated if annealing is performed at a temperature of 1000° C. or higher.

[0055] In contrast, in Fig. 9 , looking at the area 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 film, which serves as a cap film. Therefore, the verification results shown in Fig. 9 support the idea 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] <<Test Results Regarding Surface Roughness>> Figure 10 shows micrographs taken with an atomic force microscope, illustrating the test results that demonstrate that annealing after forming a 100 nm thick aluminum nitride film on a GaN layer has almost no effect on the surface roughness.

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

[0059] The surface roughness (sq) of the GaN layer without the 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 when annealing at 1000°C for 10 minutes or 30 minutes, the surface roughness is 0.39 nm or 0.34 nm. This indicates that even when annealing at 1000°C for 10 minutes or 30 minutes, the surface roughness remains almost unchanged (FIG. 10(c) or FIG. 10(d)). Furthermore, even when annealing at 1300°C for 10 minutes or 30 minutes, the surface roughness remains 0.31 nm or 0.35 nm. This indicates that even when annealing at 1300°C for 10 minutes or 30 minutes, the surface roughness remains almost unchanged (FIG. 10(e) or FIG. 10(f)).

[0061] This proves that forming an aluminum nitride film with a thickness of 100 nm on a GaN layer has almost no effect on the surface roughness.

[0062] 11 is a photomicrograph 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] FIG. 11(a) is a micrograph showing the surface of a GaN layer. FIG. 11(b) is a micrograph showing the state in which an aluminum nitride film has been formed on a GaN layer. FIG. 11(c) is a micrograph showing the state in which an aluminum nitride film has been formed on a GaN layer and then annealed at 1000°C for 10 minutes. FIG. 11(d) is a micrograph showing the state in which an aluminum nitride film has been formed on a GaN layer and then annealed at 1000°C for 30 minutes. FIG. 11(e) is a micrograph showing the state in which an aluminum nitride film has been formed on a GaN layer and then annealed at 1300°C for 10 minutes. FIG. 11(f) is a micrograph showing the state in which an aluminum nitride film has been formed on a GaN layer and then annealed at 1300°C for 30 minutes.

[0064] The surface roughness (sq) of the GaN layer without the 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 when annealing at 1000°C for 10 minutes or 30 minutes, the surface roughness is 0.61 nm or 0.68 nm. This indicates that even when annealing at 1000°C for 10 minutes or 30 minutes, the surface roughness remains almost unchanged (FIG. 11(c) or FIG. 11(d)). Furthermore, even when annealing at 1300°C for 10 minutes or 30 minutes, the surface roughness remains 0.60 nm or 0.69 nm. This indicates that even when annealing at 1300°C for 10 minutes or 30 minutes, the surface roughness remains almost unchanged (FIG. 11(e) or FIG. 11(f)).

[0066] This proves that the surface roughness is hardly affected by forming an aluminum nitride film with a thickness of 300 nm on the GaN layer.

[0067] <<Verification Results Regarding Crystallinity>> Figure 12 shows verification results indicating that the crystallinity of GaN and AlN (aluminum nitride) is hardly affected by annealing after forming a 100 nm thick aluminum nitride film on a 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 remains almost unchanged even after annealing at 1000° C. for 10 or 30 minutes, which indicates that the aluminum nitride film formed using the ECR plasma deposition apparatus has very high heat resistance.

[0069] Annealing at 1300°C for 10 or 30 minutes does not deteriorate the crystallinity of GaN, but 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 ensured. In other words, the aluminum nitride film formed by the ECR plasma deposition apparatus is able to adequately protect the crystallinity of GaN.

[0070] Figure 13 shows the results of a verification experiment 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 a 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 remains almost unchanged even after annealing at 1000° C. for 10 or 30 minutes, which indicates that the aluminum nitride film formed using the ECR plasma deposition apparatus has very high heat resistance.

[0072] Annealing at 1300°C for 10 or 30 minutes does not deteriorate the crystallinity of GaN, but 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 ensured. In other words, the aluminum nitride film formed by the ECR plasma deposition apparatus is able to adequately protect the crystallinity of GaN.

[0073] From the above verification results, it can be seen that by using an aluminum nitride film formed by an ECR plasma deposition apparatus 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 by an ECR plasma deposition apparatus can provide good protection for the GaN layer.

[0074] <Modification> In the embodiment, a substrate structure in which a GaN layer is formed as an epitaxial layer on a sapphire substrate is described as an example. However, the technical concept of the embodiment is not limited to this. 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.

[0076] REFERENCE SIGNS LIST 1 ECR plasma film formation apparatus 10 Chamber 10a Gas inlet 10b Gas outlet 11 Holding unit 12 Mechanical unit 13 Plasma generation unit 14 Magnetic field generation unit 15 Waveguide 16 High frequency power supply 17 Fixing unit 20 Backing tube 21 Target member 30 Support member 31 Target member 40 Argon ions 50 Target particles 100 Sapphire substrate 200 GaN layer 200A Ga-rich region 300 Cap film SUB Film formation target TA Target TA1 Target

Claims

1. A method for manufacturing a semiconductor device, comprising: (a) forming a cap film on a gallium nitride layer, the cap film being formed using a deposition apparatus that uses plasma generated by utilizing the electron cyclotron resonance phenomenon.

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

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

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

5. In 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. A method for manufacturing a semiconductor device according to claim 1, wherein the gallium nitride layer contains a first conductivity type impurity, and after step (a), annealing is carried out to activate the first conductivity type impurity.

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

8. A method for manufacturing a semiconductor device according to claim 6, wherein after the annealing is performed, the cap film is removed.

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