Methods for manufacturing semiconductor devices

Abstract

This invention relates to a method for manufacturing a semiconductor device. The method includes the following steps: forming a first photoresist film on a semiconductor substrate, and forming a second photoresist film having a higher acidity than the first photoresist film on the first photoresist film; forming an opening for exposing a surface of the semiconductor substrate by patterning the first and second photoresist films; coating a shrinkage material onto the upper surface of the second photoresist film and the interior of the opening, and reacting the shrinkage material and the second photoresist film inside the opening by heat-treating the first photoresist film, the second photoresist film, and the shrinkage material; and removing unreacted shrinkage material from the upper surface of the second photoresist film and the interior of the opening that did not react with the second photoresist film.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing a semiconductor device, for example, a method for manufacturing a semiconductor device including the step of forming an opening in a photoresist film. Background Technology

[0002] In semiconductor devices using compound semiconductors such as GaN and GaAs (e.g., high electron mobility transistors (HEMTs) and monolithic microwave integrated circuits (MMICs)), a lift-off method is used to form electrodes, such as gate electrodes, on a semiconductor substrate. To form a gate electrode of a desired shape at a desired location on the semiconductor substrate, a photoresist film is applied to the upper surface of the semiconductor substrate. An opening is formed in the photoresist film to expose the semiconductor substrate, and a lift-off method is performed. The gate electrode is formed according to the shape of the opening.

[0003] Stripping is made easier by making the opening length on the lower surface of the photoresist film smaller than the opening length on the upper surface. As a method for forming such openings, it is known that two photoresist films with different photosensitivity are formed, irradiated with light, and then exposed and developed, thereby making the opening length of the lower photoresist film larger than the opening length of the upper photoresist film (e.g., Patent Document 1: Japanese Patent Application No. 2008-242247). It is known that openings with small opening lengths are formed in the photoresist film by exposing the photoresist with an electron beam (EB) (e.g., Patent Document 2: Japanese Patent Application No. 2009-198587). It is known that a chemical solution that swells the photoresist film is coated onto the photoresist film to form openings with small opening lengths and an inverted conical shape (e.g., Patent Document 3: Japanese Patent Application No. 2005-107116). Summary of the Invention

[0004] By using a chemical solution that swells the photoresist film, as in Patent Document 3, it is possible to form an opening with a small opening length and an inverted conical shape using light such as an i-line. However, for example, when the opening length is reduced to shorten the gate length of the HEMT, it is insufficient to have an inverted conical cross-sectional shape for forming the gate electrode by a stripping method.

[0005] Therefore, the purpose of this disclosure is to form a photoresist film with an opening having a shape suitable for a stripping method.

[0006] The method for manufacturing a semiconductor device according to this disclosure includes the following steps: forming a first photoresist film on a semiconductor substrate, and forming a second photoresist film having a higher acidity than the first photoresist film on the first photoresist film; forming an opening for exposing a surface of the semiconductor substrate by patterning the first and second photoresist films; coating a shrinkage material on the upper surface of the second photoresist film and the interior of the opening, and reacting the shrinkage material and the second photoresist film inside the opening by heat-treating the first photoresist film, the second photoresist film, and the shrinkage material; and removing unreacted shrinkage material that has not reacted with the second photoresist film from the upper surface of the second photoresist film and the interior of the opening. Attached Figure Description

[0007] Figure 1 A, Figure 1 B Figure 1 C Figure 1 D, Figure 1 E is a cross-sectional view illustrating a method for forming a photoresist film with openings according to a first embodiment.

[0008] Figure 2 This is a view of the SEM image of the opening formed in the photoresist film in Experiment 2.

[0009] Figure 3A , Figure 3B , Figure 3C , Figure 3D This is a cross-sectional view illustrating a method for manufacturing a semiconductor device according to a second embodiment (part 1).

[0010] Figure 4A , Figure 4B , Figure 4C This is a cross-sectional view illustrating a method for manufacturing a semiconductor device according to a second embodiment (part 2). Detailed Implementation

[0011] Some embodiments will now be described.

[0012] (1) An embodiment of this disclosure is a method for manufacturing a semiconductor device, the method comprising the steps of: forming a first photoresist film on a semiconductor substrate, and forming a second photoresist film having a higher acidity than the first photoresist film on the first photoresist film; forming an opening for exposing a surface of the semiconductor substrate by patterning the first and second photoresist films; coating a shrinkage material onto the upper surface of the second photoresist film and the interior of the opening, and reacting the shrinkage material and the second photoresist film inside the opening by heat-treating the first photoresist film, the second photoresist film, and the shrinkage material; and removing the unreacted shrinkage material that did not react with the second photoresist film from the upper surface of the second photoresist film and the interior of the opening. Thus, a photoresist film having an opening with a shape suitable for a peel-off method can be formed.

[0013] (2) Preferably, the shrinkage material includes polyvinyl alcohol, the first photoresist film and the second photoresist film include a novolac resin, the first photoresist film and the second photoresist film also include ethyl lactate, and the weight concentration of ethyl lactate in the second photoresist film is higher than the weight concentration of ethyl lactate in the first photoresist film.

[0014] (3) Preferably, the shrinkage material includes polyvinyl alcohol, the first photoresist film and the second photoresist film include phenolic varnish resin, the second photoresist film also includes ethyl lactate, and the first photoresist film does not include ethyl lactate.

[0015] (4) Preferably, the opening length of the second photoresist film that reacts with the shrinkage material through heat treatment is smaller than the opening length of the first photoresist film.

[0016] Specific examples of methods for manufacturing semiconductor devices according to the present disclosure will be described below with reference to the accompanying drawings. It should be noted that the present disclosure is not limited to these examples, but is indicated by the claims, and the present disclosure is intended to include all modifications equivalent to the meaning and scope of the claims.

[0017] (First Embodiment)

[0018] The first embodiment describes an example of a method for forming a photoresist film with openings. Figure 1 A, Figure 1 B Figure 1 C Figure 1 D, Figure 1E is a cross-sectional view illustrating a method for forming a photoresist film with openings according to a first embodiment. For example, these cross-sectional views illustrate the cross-sectional structure of a semiconductor device when it is cut along the film formation direction. Figure 1 As illustrated in Figure A, a semiconductor substrate 10 is fabricated. For example, the semiconductor substrate 10 is a substrate made of a semiconductor material, or a substrate made of an insulator or semiconductor with a semiconductor layer formed thereon. For example, a substrate made of a semiconductor material is a silicon substrate or a GaAs substrate. A substrate made of an insulator or semiconductor with a semiconductor layer formed thereon is, for example, a GaAs substrate with at least one of GaAs, AlGaAs, and / or InGaAs layers formed on it, or a substrate with at least one of GaN, AlN, AlGaN, and / or InGaN layers formed on it, or a silicon substrate, GaN substrate, SiC substrate, or sapphire substrate.

[0019] A photoresist film 20 is coated on a semiconductor substrate 10, and a photoresist film 22 is coated on top of the photoresist film 20. For example, the direction in which the respective films 20 and 22 are formed on the semiconductor substrate 10 in the order of photoresist film 20 is the film formation direction. For example, the thickness of each of photoresist films 20 and 22 is 0.1 μm to 1 μm. Each of photoresist films 20 and 22 primarily comprises a photosensitizer such as phenolic varnish resin and anaphthoquinone azide derivative, as well as an organic solvent. Photoresist film 20 has a higher acidity than photoresist film 22.

[0020] like Figure 1 As illustrated in Figure B, photoresist films 20 and 22 are selectively irradiated with ultraviolet rays using an i-line stepper or the like. Then, by developing the photoresist films with a developing solution, openings 32 and 34 are formed in the photoresist films 20 and 22, respectively. Opening 30 includes openings 32 and 34. Here, opening 30 refers to the entire aperture or recessed space from the opening portion (opening) on ​​the upper surface of the photoresist film 22 to the upper surface of the semiconductor substrate 10. Opening 30 has opening 32 in the photoresist film 20 and opening 34 in the photoresist film 22. Specifically, the term "opening" refers only to the portion of the opening on the upper surface of the photoresist film 22 within opening 30.

[0021] The opening lengths of openings 32 and 34 are substantially the same and are L1. For example, the opening length L1 is 0.3 μm to 1 μm. For example, the opening length L1 is set according to the gate length of the HEMT. The opening length L1 is, for example, the length of opening 30 on the upper surface of the photoresist film 22 formed on the HEMT along the direction from the source electrode to the drain electrode (i.e., the opening length of opening 30) (for example, the source electrode and the drain electrode correspond to...). Figures 3B to 3D Ohmic electrode 14).

[0022] like Figure 1 As illustrated in Figure C, shrinkage material 24 is applied to the upper surface of the photoresist film 22 and the interior of the opening 30. Shrinkage material 24 fills the opening 30. For example, shrinkage material 24 primarily comprises polyvinyl alcohol and solvents (organic solvents and / or water).

[0023] like Figure 1 As illustrated in Figure D, the semiconductor substrate 10 is heated to perform heat treatment (baking) on ​​the photoresist film 20, photoresist film 22, and shrinkage material 24. For example, the heat treatment temperature is from 80°C to 150°C. The photoresist film 22 and the shrinkage material 24 react with each other to form a shrinkage film 26. The heat treatment causes almost no reaction between the photoresist film 20 and the shrinkage material 24.

[0024] like Figure 1 As illustrated in Figure E, for example, the remaining shrinkage material 24 (i.e., unreacted shrinkage material 24) after it has not reacted with the photoresist films 20 and 22 is removed by washing it with water. The unreacted shrinkage material 24 is water-soluble. The shrinkage film 26 produced by the reaction is exposed on the upper surface of the photoresist film 22 and the inner surface of the opening 34. The opening 34 of the photoresist film 22 is contracted by the shrinkage film 26 (the opening length decreases). Since the photoresist film 20 and the shrinkage material 24 do not react with each other, the opening 32 of the photoresist film 20 hardly contracts (the opening length does not decrease).

[0025] As a result, the opening length L2 of the opening 34 of the photoresist film 25 (i.e., the photoresist film 22 with the shrinkage film 26) is smaller than the opening length L3 of the opening 32 of the photoresist film 20. For example, the opening length L2 is smaller than... Figure 1 The opening length L1 of B is less than 0.1 μm to 0.5 μm. For example, the opening length L2 is 0.05 μm to 0.3 μm.

[0026] This section will describe why shrinkage material 24 reacts with photoresist film 22 but not with photoresist film 20. Shrinkage material 24 comprises polyvinyl alcohol. Figure 1The heat treatment in the D component induces polymerization reactions between polyvinyl alcohols in the photoresist and / or crosslinking reactions between polyvinyl alcohols and polymer resins (e.g., phenolic varnish resins). These reactions are influenced by the H component in the photoresist. + Therefore, when the acidity of the photoresist film 22 becomes greater than that of the photoresist film 20, the photoresist film 22 and the shrinkage material 24 react with each other to form a shrinkage film 26. When the acidity of the photoresist film 20 is reduced, the reaction between the photoresist film 20 and the shrinkage material 24 is suppressed, and no shrinkage film is formed in the opening 32 of the photoresist film 20.

[0027] The phenolic varnish resin in photoresist films 20 and 22 is a very weak acid. However, due to the extremely low acidity of the phenolic varnish resin, the reaction between the shrinkage material 24 and the phenolic varnish resin is hardly accelerated. The naphthoquinone azide derivative in photoresist films 20 and 22 is transferred after photosensitization, combines with H2, and is converted to indenecarboxylic acid. However, due to the extremely low acidity of indenecarboxylic acid, the reaction between the shrinkage material 24 and the phenolic varnish resin is hardly accelerated.

[0028] Based on the above, it is believed that the organic solvents in photoresists contribute to accelerating the reaction between the shrinkage material 24 and the photoresist. For example, ethyl lactate contains hydroxyl groups. Ethyl lactate has high acidity because H... + Dissociation from the hydroxyl group. For example, 2-heptanone or 1,4-dioxane do not contain a hydroxyl group and have low acidity.

[0029] Therefore, the organic solvent of photoresist film 22 includes a significant amount of ethyl lactate, while the organic solvent of photoresist film 20 contains almost no ethyl lactate and almost no materials like ethyl lactate used to increase acidity. This allows shrinkage material 24 to react with photoresist film 22 and almost no reaction with photoresist film 20.

[0030] (Experiment 1)

[0031] To demonstrate the above effects, the following experiment was conducted. AZR200, manufactured by Merck, was used as the shrinkage material 24. PFI-89B8, manufactured by Sumitomo Chemical, and THMRiP-3500, manufactured by Tokyo Ohka Kogyo, were used as photoresists.

[0032] The main components and their concentrations in AZR200 are as follows.

[0033] Isopropanol: 5% to 10% by weight

[0034] Water: 85% or less by weight

[0035] Polyvinyl alcohol

[0036] The main components and their concentrations in PFI-89B8 are as follows.

[0037] Ethyl lactate: 34% to 36% by weight

[0038] 2-Heptanone: 34% to 36% by weight

[0039] 1,4-Dioxane: less than 1% by weight

[0040] Phenolic varnish resin: 22% to 26% by weight

[0041] Naphthoquinone azide derivative: 6% by weight

[0042] The main components and their concentrations in THMRiP-3500 are as follows.

[0043] 2-Heptanone: 45% to 95% by weight

[0044] 1,4-Dioxane: less than 1% by weight

[0045] Cresol: less than 1% by weight

[0046] Phenolic varnish resin

[0047] Naphthoquinone azide derivatives: 1% to 15% by weight

[0048] Here, the undescribed concentration value of a component is considered to be less than or equal to the remaining amount (wt%) obtained from the sum of the wt% of the other components.

[0049] Approximately half of the organic solvent in PFI-89B8 is ethyl lactate. The organic solvent in THMRiP-3500 does not include ethyl lactate and is primarily 2-heptanone and 1,4-dioxane. Therefore, it is believed that PFI-89B8 reacts with AZR200 to form a shrinkage film 26, but THMRiP-3500 hardly reacts with AZR200.

[0050] The reaction of each of PFI-89B8 and THMRiP-3500 with AZR200 was examined. The following two samples were prepared.

[0051] Sample A: THMRiP-3500 was coated onto a semiconductor substrate to a thickness of approximately 0.3 μm, forming a photoresist film. Openings were formed in the photoresist film by exposure and development using an i-line stepper. The opening length was 367 nm. After coating the photoresist film with shrinkage material, the entire semiconductor device was heat-treated at 110°C for 70 seconds. Unreacted shrinkage material was removed by washing with water. The opening length in the photoresist film was 481 nm. The upper part of the photoresist film shrank, and the opening length increased by approximately 115 nm.

[0052] Sample B: PFI-89B8 was coated onto a semiconductor substrate to a thickness of approximately 0.3 μm, forming a photoresist film. Openings were formed in the photoresist film by exposure and development using an i-line stepper. The opening length was 489 nm. After coating the photoresist film with shrinkage material, the entire semiconductor device was heat-treated at 110°C for 70 seconds. Unreacted shrinkage material was removed by washing with water. The opening length in the photoresist film was 351 nm. The opening length was reduced by approximately 140 nm.

[0053] Experiment 1 confirmed that THMRiP-3500 hardly reacts with AZR200, while PFI-89B89 reacts with AZR200 and the opening length is reduced.

[0054] (Experiment 2)

[0055] Considering the results of Experiment 1, the opening is based on the following reference. Figure 1 A, Figure 1 B Figure 1 C Figure 1 D, Figure 1 The method described in E is formed by using AZR200 manufactured by Merck as shrinkage material 24, PFI-89B8 manufactured by Sumitomo Chemical Co., Ltd. as photoresist film 22, and THMRiP-3500 manufactured by Tokyo Ohka Kogyo Co., Ltd. as photoresist film 20.

[0056] exist Figure 1 In step A, THMRiP-3500 is coated onto a semiconductor substrate 10 to form a photoresist film 20 with a thickness of approximately 0.3 μm on the semiconductor substrate 10. Furthermore, PFI-89B8 is coated onto the photoresist film 20 to form a photoresist film 22 with a thickness of approximately 0.3 μm. Figure 1 In B, an i-line stepper is used to achieve an exposure of 1650 J / cm. 2Ultraviolet rays are exposed to photoresist films 20 and 22. An opening 30 with an opening length L1 of about 0.4 μm is formed in the photoresist films 20 and 22 by developing with an alkaline developer.

[0057] exist Figure 1 In C, AZR200 is applied to the photoresist film 22 and inside the opening 30 using a spin coating method at a rotation speed of 1500 rpm to form a shrinkage material 24. Figure 1 A, Figure 1 B Figure 1 C Figure 1 D, Figure 1 The figure only shows a portion of the cross-section, but the semiconductor device in wafer form is actually being processed. Figure 1 As illustrated in Figure D, the photoresist films 20 and 22 and the shrinkage material 24 are heat-treated (baked) at 100°C. This causes the photoresist film 22 and the shrinkage material 24 to react. Figure 1 As illustrated in Figure E, shrinkage material 24 that did not react (change) with the photoresist film 22 is removed by washing with water.

[0058] The cross-section of opening 30 was observed using SEM (scanning electron microscopy). Figure 2 The figure shows a SEM image of a cross-section taken along the film formation direction of the opening formed in the photoresist film in Experiment 2. Figure 2 As illustrated, the opening 32 formed in the photoresist film 20 has a positive conical shape, and the opening length L3 on the lower surface of the opening 32 is approximately 0.4 μm. The openings 32 and 34 at the interface between the photoresist films 20 and 22 are substantially identical. The opening 34 formed in the photoresist film 22 has an inverted conical shape, the opening length L2 on the upper surface of the opening 34 is approximately 0.16 μm, and the contraction L4, which is the difference between the lower and upper surfaces of the opening 34, is approximately 0.12 μm on one side.

[0059] The reason why opening 34 is thought to have an inverted conical shape is that organic solvent mixing occurs near the interface between photoresist films 20 and 22. For example, due to mixing, the acidity of a portion of photoresist film 22 near its interface with photoresist film 20 (i.e., the lower part of the inverted conical shape) can be lower than the acidity of another portion of photoresist film 22 near its surface (i.e., the upper part of the inverted conical shape). The reason why opening 32 has a slightly conical shape is that the interface between photoresist film 20 and semiconductor substrate 10 is fixed to be in close contact with semiconductor substrate 10 and approximately maintained. Figure 1 The shape of opening 32 in B (i.e., opening length L1).

[0060] By making the opening length of the lower photoresist film wider than that of the upper photoresist film, as in Patent Document 1, electrodes can be easily formed by peeling. However, since the photoresist film is exposed using light such as ultraviolet rays, the opening length of the upper photoresist film cannot be reduced. As in Patent Document 2, the opening length can be reduced when the photoresist film is exposed using an electron beam. However, the throughput of electron beam drawing is extremely poor. Therefore, this is undesirable in terms of productivity and manufacturing cost.

[0061] As in Patent Document 3, the use of a chemical solution that swells the photoresist film and light, such as i-lines, allows for high throughput and the formation of openings with small opening lengths and inverted conical shapes. In a third embodiment of Patent Document 3, films with high affinity for the resist pattern swelling material and films with low affinity for the resist pattern swelling material are formed by ashing or pre-baking the photoresist film. However, in such a method using the same photoresist film, the underlying photoresist film also shrinks to some extent. Therefore, the opening is not ideal for use in stripping. Additionally, when the ashing process is performed, the layer below the opening (e.g., a semiconductor layer) is oxidized.

[0062] Patent document 3 describes the amount of shrinkage as depending on, for example Figures 3A-3D The type of resin in the photoresist film illustrated varies. However, if the resin is different, the developer is also different. Therefore, the manufacturing method becomes complicated in order to laminate photoresist films with different resins. As a result, optimal manufacturing conditions may not be obtained. As mentioned above, since the mechanism is not sufficiently explained in Patent Document 3, it is difficult to form an opening suitable for the peeling method, in which the inverted conical shape is located on the upright conical shape.

[0063] The inventors focused on H in photoresist films 20 and 22 + As in the first embodiment, a photoresist having a higher acidity than the photoresist film 20 is selected as the photoresist film 22. Figure 1 As illustrated in Figure A, a photoresist film 20 (i.e., a first photoresist film) is formed on a semiconductor substrate 10, and a photoresist film 22 (i.e., a second photoresist film) having a higher acidity than the photoresist film 20 is formed on the photoresist film 20. Figure 1 As illustrated in Figure B, the opening 30 for exposing the surface of the semiconductor substrate 10 is formed by patterning photoresist films 20 and 22. Figure 1 As illustrated in Figure C, shrinkage material 24 is coated onto the upper surface of the photoresist film 22 and the interior of the opening 30. Figure 1As illustrated in Figure D, heat treatment of the photoresist film 20, photoresist film 22, and shrinkage material 24 causes the shrinkage material 24 and photoresist film 22 to react inside the opening 34. At this point, the photoresist film 20 hardly reacts with the shrinkage material 24. Figure 1 As illustrated in Figure E, in order to expose the surface of the semiconductor substrate 10, unreacted shrinkage material 24 is removed from the upper surface of the photoresist film 22 and the interior of the opening 30.

[0064] Therefore, as in Experiment 2, the opening length L2 in the photoresist film 22 that reacts with the shrinkage material 24 is less than the opening length L3 in the photoresist film 20. That is, the opening length L2 on the upper surface of the opening 34 of the photoresist film 22 is less than the opening length L3 of the opening 30 in the photoresist film 20. Therefore, peeling becomes easier. In this way, photoresist films 20 and 22 with the shape of the opening 30 can be formed, wherein the cross-section of the opening 34 has an inverted conical shape.

[0065] Furthermore, the opening length L2 of the opening 34 on the surface of the photoresist film 22 is 0.16 μm, and an opening length similar to that drawn by an electron beam can be achieved by using an i-line. This improves throughput. Since the opening 30 is formed without using an ashing process, oxidation of the semiconductor substrate 10 below the opening 30 can be suppressed.

[0066] exist Figure 1 During heat treatment in D, it is important that the acidity of the photoresist film 22 is higher than that of the photoresist film 20. Therefore, the acidity of the organic solvent in the photoresist film 22 is made higher than that of the organic solvent in the photoresist film 20. That is, in Figure 1 In the photoresist before coating A, when the acidity of the organic solvent in the photoresist film 22 is higher than that of the organic solvent in the photoresist film 20, the acidity of the photoresist film 22 becomes higher than that of the photoresist film 20 during heat treatment.

[0067] Shrinkage material 24 comprises polyvinyl alcohol, while photoresist films 20 and 22 comprise phenolic varnish resin. Photoresist films 20 and 22 also comprise ethyl lactate, and the weight concentration of ethyl lactate in photoresist film 22 is higher than that in photoresist film 20. Therefore, the acidity of the organic solvent in photoresist film 22 becomes higher than that of the organic solvent in photoresist film 20.

[0068] To form an opening 30 with an inverted conical shape at the top and a regular conical shape at the bottom of the cross-section, the weight concentration of ethyl lactate in the photoresist film 22 is preferably 5 times or more, more preferably 10 times or more, and even more preferably 20 times or more, of the weight concentration of ethyl lactate in the photoresist film 20. The weight concentration of ethyl lactate in the photoresist film 22 is preferably 10% or more by weight, more preferably 20% or more by weight, and even more preferably 30% or more by weight. Since the photoresist film 22 includes phenolic varnish resin, the weight concentration of ethyl lactate in the photoresist film 22 is preferably 80% or less by weight. The weight concentration of ethyl lactate in the photoresist film 22 is preferably 5% or less by weight, more preferably 1% or more by weight.

[0069] Photoresist film 22 includes ethyl lactate, while photoresist film 20 substantially does not contain ethyl lactate. Therefore, the acidity of the organic solvent in photoresist film 22 becomes higher than that of the organic solvent in photoresist film 20. The weight concentration of ethyl lactate in photoresist film 22 is preferably 10% or more by weight, more preferably 20% or more by weight, and even more preferably 30% or more by weight. The weight concentration of ethyl lactate in photoresist film 22 is preferably 80% or less by weight. The fact that photoresist film 20 substantially does not contain ethyl lactate means that photoresist film 20 can include a small amount of ethyl lactate to the extent that it will not react with the shrinkage material 24. For example, the weight concentration of ethyl lactate in photoresist film 20 is 1% or less by weight.

[0070] (Second Embodiment)

[0071] The second embodiment describes an example of a method for manufacturing a semiconductor device using the method of forming a photoresist film with openings in the first embodiment. Figure 3A , Figure 3B , Figure 3C , Figure 3D and Figure 4A , Figure 4B , Figure 4C This is a cross-sectional view illustrating a method for manufacturing a semiconductor device according to a second embodiment. Figure 3AAs illustrated, the semiconductor substrate 10 has a substrate 11 and a semiconductor layer 12 disposed on the substrate 11. For example, when the semiconductor device is a HEMT (High Mobility Electron Transistor), the substrate 11 is a SiC substrate, a silicon substrate, a GaN substrate, or a sapphire substrate. The semiconductor layer 12 includes, for example, an AlN nucleation layer, a GaN layer, an AlGaN electron supply layer, and a GaN capping layer from the substrate 11 side. The substrate 11 can be a GaAs substrate, and the semiconductor layer 12 can include a GaAs layer, an AlGaAs electron supply layer, and a GaAs capping layer from the substrate 11 side.

[0072] like Figure 3B As illustrated, an ohmic electrode 14 is formed on a semiconductor substrate 10. For example, the ohmic electrode 14 is a source electrode and a drain electrode, and includes titanium and aluminum films from the semiconductor substrate 10 side. For example, the ohmic electrode 14 is formed using a vacuum deposition method and a lift-off method.

[0073] like Figure 3C As illustrated, a protective film 16 is formed on the surface of a semiconductor substrate 10. The protective film 16 is an insulating film, such as a silicon nitride film. For example, a CVD (chemical vapor deposition) method is used to form the protective film 16. Figure 3D As illustrated in the figure, the method of the first embodiment is used to form photoresist films 20 and 25 having openings 30.

[0074] like Figure 4A As illustrated, a portion of the protective film 16 is removed using photoresist films 20 and 25 as a mask, and an opening 36 is formed in the protective film 16. For example, a dry etching method is used to remove a portion of the protective film 16. This exposes the upper surface of the semiconductor substrate 10. Figure 4B As illustrated, a metal film 17 is formed in the opening 36 and on the photoresist film 25 using a vacuum deposition method. The metal film 17 has a laminated structure including, for example, a nickel film, a palladium film, and a gold film from below.

[0075] like Figure 4C As illustrated, the metal film 17 on the photoresist film 25 is stripped away by removing the photoresist films 20 and 25. Therefore, a gate electrode 18 is formed on the semiconductor substrate 10 in the opening 36. In this way, the semiconductor device according to the second embodiment is completed.

[0076] In the second embodiment, the method for forming a photoresist film with an opening according to the first embodiment is used. Therefore, it is possible to form an opening 30 with a small opening length on the upper surface of the opening 30 without using electron beam drawing. Therefore, throughput can be improved. The opening length on the lower surface of the opening 30 is greater than the opening length on its upper surface. This makes it difficult to form... Figure 4BThe metal film 17 is formed on the inner peripheral surface of the opening 30. In particular, the aforementioned effect is further enhanced when the cross-section of the photoresist film 20 has a positive conical shape. Therefore, in Figure 4C During the stripping step, the organic solvent can penetrate the photoresist films 20 and 25 through the opening 30 and remove them. Specifically, because the photoresist film 20 has a conical shape, the organic solvent can easily penetrate inside the opening 30. Furthermore, it is difficult to form burrs or the like on the gate electrode 18. In this way, the opening 30 can be formed with a shape suitable for stripping.

[0077] If the opening length of the upper surface of opening 34 is less than the opening length of opening 32, then opening 34 does not need to have an inverted conical shape. For example, the inner peripheral surface of opening 34 can be perpendicular to the upper surface of photoresist film 25. However, to avoid attaching metal film 17 to the inner peripheral surface of opening 34, opening 34 preferably has an inverted conical shape. Opening 32 does not need to have a regular conical shape. For example, the inner peripheral surface of opening 32 can be perpendicular to the lower surface of photoresist film 20. Opening 32 preferably has a regular conical shape such that the upper surface of semiconductor substrate 10 is not exposed or gate electrode 18 is not deposited on protective film 16.

[0078] According to the second embodiment, after removing the unreacted shrinkage material 24, as Figure 4B As illustrated, a metal film 17 is formed on the photoresist film 25 and in the opening 30. Figure 4C As illustrated, a gate electrode 18 made of a metal film 17 is formed on a semiconductor substrate 10 by removing photoresist films 20 and 25. Therefore, a gate electrode 18 with a small gate length can be formed without using electron beam drawing. The method of the first embodiment can also be used to form electrodes such as ohmic electrodes 14. Although HEMT is exemplified as a semiconductor device, the semiconductor device can be a FET (field-effect transistor) other than a HEMT.

[0079] Note that the embodiments disclosed herein are merely illustrative in all respects and should not be considered limiting. The scope of the invention is defined by the scope of the claims, without having the meanings mentioned above, and is intended to include the meanings equivalent to the scope of the claims and all modifications within that scope.

Claims

1. A method for manufacturing a semiconductor device, comprising the following steps: A first photoresist film is formed on a semiconductor substrate, and a second photoresist film having a higher acidity than the first photoresist film is formed on the first photoresist film. An opening for exposing the surface of the semiconductor substrate is formed by patterning the first photoresist film and the second photoresist film; A shrinkage material is coated on the upper surface of the second photoresist film and the inside of the opening. The shrinkage material and the second photoresist film react inside the opening to form a shrinkage film generated by the reaction, while the shrinkage material does not react with the first photoresist film. as well as Remove the unreacted shrinkage material that did not react with the second photoresist film from the upper surface of the second photoresist film and the interior of the opening. in The shrinkage material includes polyvinyl alcohol. The first photoresist film and the second photoresist film comprise phenolic varnish resin. The first photoresist film and the second photoresist film further include ethyl lactate, and The weight concentration of ethyl lactate in the second photoresist film is five times or more than the weight concentration of ethyl lactate in the first photoresist film.

2. The method for manufacturing a semiconductor device according to claim 1, wherein... The opening length in the second photoresist film, which reacts with the shrinkage material through the heat treatment, is smaller than the opening length in the first photoresist film.

3. A method for manufacturing a semiconductor device, comprising the following steps: A first photoresist film is formed on a semiconductor substrate, and a second photoresist film having a higher acidity than the first photoresist film is formed on the first photoresist film. An opening for exposing the surface of the semiconductor substrate is formed by patterning the first photoresist film and the second photoresist film; A shrinkage material is coated on the upper surface of the second photoresist film and the inside of the opening. The shrinkage material and the second photoresist film react inside the opening to form a shrinkage film generated by the reaction, while the shrinkage material does not react with the first photoresist film. as well as Remove the unreacted shrinkage material that did not react with the second photoresist film from the upper surface of the second photoresist film and the interior of the opening. in The shrinkage material includes polyvinyl alcohol. The first photoresist film and the second photoresist film comprise phenolic varnish resin. The second photoresist film also includes ethyl lactate. The first photoresist film does not include ethyl lactate, and The weight concentration of ethyl lactate in the second photoresist film is 10% or more by weight.

4. The method for manufacturing a semiconductor device according to claim 3, wherein... The opening length in the second photoresist film, which reacts with the shrinkage material through the heat treatment, is smaller than the opening length in the first photoresist film.

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