An ohmic contact structure and a method for manufacturing the same, and a HEMT device

Abstract

The application discloses an ohmic contact structure and a preparation method thereof and a HEMT device, and relates to the technical field of semiconductors. The preparation method of the ohmic contact structure provided by the application comprises the following steps: providing an epitaxial wafer to be prepared with an ohmic contact metal electrode; sequentially evaporating an adhesion layer and a cover layer on the position of the electrode of the epitaxial wafer in a vacuum environment; exposing the epitaxial wafer with the evaporated adhesion layer and cover layer to the atmosphere to oxidize the metal material on the surface of the cover layer to form a barrier layer; sequentially evaporating a connecting layer and a protective layer on the barrier layer in the vacuum environment; and annealing the epitaxial wafer with the formed connecting layer and protective layer in an environment with a temperature of 500-600 DEG C. The ohmic contact structure, the preparation method thereof and the HEMT device provided by the application can be prepared in a low-temperature environment, and the prepared ohmic contact structure has a smooth surface and a relatively thick film layer.

Description

Technical Field

[0001] The present application relates to the field of semiconductor technology, and in particular to an ohmic contact structure and a preparation method thereof, and a HEMT device. Background Art

[0002] Wide-bandgap semiconductor gallium nitride (GaN) boasts advantages such as a large bandgap, high breakdown field strength, and high electron saturation drift velocity. GaN high electron mobility transistors (HEMTs) offer significant advantages in microwave high-power devices and high-voltage power electronics, and have become a current research hotspot. Ohmic contacts are a key technology for GaN HEMTs. The fabrication method and resistance of ohmic contact electrodes directly impact the performance of GaN HEMTs. Poor ohmic contacts not only reduce the transconductance and output power of the HEMT, but also introduce reliability issues.

[0003] There are two existing methods for preparing ohmic contacts for gallium nitride-based HEMT devices. One is to use a titanium / aluminum / nickel / gold multilayer metal structure and rapidly anneal it at high temperatures (750-900°C), i.e., a high-temperature ohmic contact. The other is to anneal a gold-free structure such as titanium / aluminum / titanium nitride or titanium / aluminum / tungsten at low temperatures (500-600°C), i.e., a low-temperature ohmic contact. The first high-temperature annealing ohmic process can achieve lower contact resistance, but the melting and diffusion of aluminum metal at high temperatures causes the surface of the titanium / aluminum / nickel / gold electrodes to become rough and uneven, affecting the subsequent photolithography overlay process. Furthermore, bumps are easily formed on the electrode edges, which can easily short-circuit the source and gate electrodes and introduce reliability issues. The second low-temperature ohmic contact can achieve a smooth surface morphology, but the protective layer metal used cannot be gold, which easily diffuses and forms a gold semi-contact under low-temperature annealing. The protective layer is generally a metal dielectric layer such as titanium nitride, silicon nitride, tungsten, or aluminum oxide. Existing low-temperature ohmic processes, on the one hand, can only be produced through sputtering, not electron beam evaporation equipment, requiring a sputtering machine and increasing production line equipment costs. On the other hand, the sputtering process makes it difficult to strip the photoresist from thick metals, while the thin protective layer is easily etched and perforated during subsequent processing, reducing process yield and potentially causing device failure. Existing low-temperature ohmic devices generally use titanium nitride, which has a higher resistivity than conventional gold, resulting in higher contact resistance. Summary of the Invention

[0004] The purpose of the present application is to provide an ohmic contact structure of a HEMT device and a preparation method thereof, which can prepare an ohmic contact structure with a smooth surface and a thick film layer at a low temperature annealing temperature.

[0005] On the one hand, an embodiment of the present application provides a method for preparing an ohmic contact structure, including: providing an epitaxial wafer for preparing an ohmic contact metal electrode; sequentially vapor-depositing an adhesion layer and a covering layer at positions corresponding to the electrodes on the epitaxial wafer in a vacuum environment; exposing the epitaxial wafer on which the adhesion layer and the covering layer are vapor-deposited to the atmosphere to oxidize the metal material on the surface of the covering layer to form a barrier layer; sequentially vapor-depositing a connecting layer and a protective layer on the barrier layer in a vacuum environment; and annealing the epitaxial wafer on which the connecting layer and the protective layer are formed in an environment of 500-600°C.

[0006] As an practicable method, an adhesion layer and a covering layer are sequentially evaporated at positions corresponding to electrodes on the epitaxial wafer in a vacuum, including: placing the epitaxial wafer in a coating chamber of an electron beam evaporation coating machine and evacuating the coating chamber; evaporating titanium metal at positions corresponding to electrodes on the epitaxial wafer to form an adhesion layer; evaporating aluminum metal at positions corresponding to electrodes on the epitaxial wafer to form a covering layer, and the covering layer covers the adhesion layer.

[0007] As an practicable method, an epitaxial wafer with an adhesion layer and a covering layer evaporated thereon is exposed to the atmosphere so that the metal material on the surface of the covering layer is oxidized to form a barrier layer, including: the covering layer is formed using aluminum metal; the aluminum metal on the surface of the covering layer reacts with oxygen in the atmosphere in the atmosphere to form an aluminum oxide layer as a barrier layer.

[0008] As an practicable manner, the epitaxial wafer on which the adhesion layer and the cover layer are evaporated is exposed to the atmosphere for a preset time so that the thickness of the barrier layer is between 1-10 nm.

[0009] As an practicable method, sequentially vapor-depositing a connecting layer and a protective layer on the barrier layer in a vacuum environment includes: placing an epitaxial wafer provided with a barrier layer in a coating chamber of an electron beam evaporation coating machine and evacuating the coating chamber; vapor-depositing nickel metal on the barrier layer to form a connecting layer; and vapor-depositing gold material on the connecting layer to form a protective layer.

[0010] Another aspect of the embodiments of the present application provides an ohmic contact structure, which is disposed on an epitaxial wafer of a HEMT device. The ohmic contact structure is disposed at the source and drain of the HEMT device. The ohmic contact structure includes an adhesion layer, a covering layer, a barrier layer, a connecting layer, and a protective layer sequentially disposed on the epitaxial wafer, wherein the barrier layer is an oxide of the metal material of the covering layer.

[0011] As an practicable manner, the adhesion layer is a titanium layer, the cover layer is an aluminum layer, the barrier layer is an aluminum oxide layer, the connection layer is a nickel layer, and the protective layer contains gold.

[0012] As an practicable method, the thickness of the adhesive layer is The thickness of the covering layer is between The thickness of the barrier layer is between The thickness of the connecting layer is between The thickness of the protective layer is between between.

[0013] As an practicable manner, the thickness ratio of the adhesive layer to the covering layer is between 1:5 and 1:50.

[0014] The beneficial effects of the embodiments of the present application include:

[0015] The present application provides a method for preparing an ohmic contact structure, comprising: providing an epitaxial wafer of a device, sequentially evaporating an adhesion layer and a covering layer at positions corresponding to electrodes on the epitaxial wafer in a vacuum environment; exposing the epitaxial wafer with the adhesion layer and the covering layer to the atmosphere to oxidize the metal material on the surface of the covering layer to form a barrier layer; sequentially evaporating a connection layer and a protective layer on the barrier layer in a vacuum environment; annealing the epitaxial wafer with the connection layer and the protective layer in an environment of 500-600°C. In the embodiment of the present application, the annealing is performed in an environment of 500-600°C, and the annealing temperature is relatively low, thereby avoiding the melting of the metal in each layer of the ohmic contact structure and making the surface of the ohmic contact smooth; due to the use of the evaporation method, a thicker metal film layer can be obtained than sputtering, thereby forming a thicker ohmic contact structure, which is beneficial to improving the yield of subsequent processes and reducing the resistivity of the ohmic electrode. Therefore, the embodiment of the present application can prepare a low-resistivity ohmic contact structure with a smooth surface and a thicker film layer in a low-temperature environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0017] Figure 1 A flow chart of a method for preparing an ohmic contact structure provided in an embodiment of the present application;

[0018] Figure 2 One of the state diagrams of a method for preparing an ohmic contact structure provided in an embodiment of the present application;

[0019] Figure 3 A second state diagram of a method for preparing an ohmic contact structure provided in an embodiment of the present application;

[0020] Figure 4 A third state diagram of a method for preparing an ohmic contact structure provided in an embodiment of the present application;

[0021] Figure 5 An EDS single-point scanning spectrum of a barrier layer provided in an embodiment of the present application;

[0022] Figure 6 This is one of the EDS surface scans of a barrier layer provided in an embodiment of the present application;

[0023] Figure 7 This is the second EDS scan of a barrier layer provided in an embodiment of the present application;

[0024] Figure 8 The voltage-current curve of the gold semi-contact formed after low-temperature annealing of the Ti-Al-Ni-Au metal structure;

[0025] Figure 9 This is a voltage-current curve of the ohmic contact formed after low-temperature annealing of Ti-Al-AlOx-Ni-Au in an embodiment of the present application.

[0026] Icons: 10-Ohm contact structure; 11-Adhesion layer; 12-Cover layer; 13-Barrier layer; 14-Connection layer; 15-Protective layer; 20-Substrate; 30-Epitaxial wafer. DETAILED DESCRIPTION

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Generally, the components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.

[0028] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for protection, but merely represents selected embodiments of the present application. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments in the present application without making any creative efforts shall fall within the scope of protection of the present application.

[0029] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0030] It should also be noted that, in the description of this application, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to direct connections, indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0031] The present application provides a method for preparing an ohmic contact structure 10, such as Figure 1 As shown, including:

[0032] S10: Figure 2 As shown, an epitaxial wafer 30 on which an ohmic contact metal electrode is to be prepared is provided;

[0033] S20: Vapor-depositing an adhesion layer 11 and a cover layer 12 in sequence at positions corresponding to electrodes on the epitaxial wafer 30 in a vacuum environment;

[0034] Adhesion layer 11 is made of a metal with a low work function and good adhesion, which can effectively bond with the semiconductor material, thereby improving the yield of the ohmic contact structure in the electrode preparation process (such as the lift-off process). The present embodiment does not impose any specific restrictions on the material of adhesion layer 11. Those skilled in the art can make appropriate selections based on the semiconductor material of epitaxial wafer 30, including but not limited to metals such as titanium, nickel, and tungsten.

[0035] Covering layer 12 overlies adhesion layer 11 to complement adhesion layer 11. Its upper surface should be capable of being oxidized by atmospheric air in subsequent steps to form barrier layer 13. The metals of adhesion layer 11 and covering layer 12 should be mutually soluble in the subsequent low-temperature annealing process, and the work function of the mutually soluble metal compound should be further reduced to achieve ohmic contact with epitaxial wafer 30. Covering layer 12 dynamically changes depending on the choice of adhesion layer 11, and may include, but is not limited to, metals such as aluminum and germanium.

[0036] In addition, if Figure 2 As shown, the epitaxial wafer 30 is usually arranged on the substrate 20 , and the substrate 20 serves as a base and supports the epitaxial wafer 30 .

[0037] It should be understood that the ohmic contact structure 10 provided in the embodiment of the present application, as a medium for current conduction, should have a relatively high electrical conductivity after being prepared.

[0038] S30: Figure 3 As shown, the epitaxial wafer 30 with the adhesion layer 11 and the cover layer 12 evaporated thereon is exposed to the atmosphere to oxidize the metal material on the surface of the cover layer 12 to form a barrier layer 13; the thickness and composition of the barrier layer 13 will vary depending on the exposure time to the atmosphere.

[0039] The barrier layer 13 can prevent the connection layer 14 and the protection layer 15 from diffusing to the positions of the adhesion layer 11 and the cover layer 12 during annealing, thereby preventing the formed ohmic contact from degenerating into a gold semi-Schottky contact.

[0040] S40: Figure 4 As shown, a connecting layer 14 and a protective layer 15 are sequentially evaporated on the barrier layer 13 in a vacuum environment;

[0041] The adhesion layer 11, cover layer 12, connection layer 14, and protective layer 15 are formed by vapor deposition. When forming the ohmic contact electrode, metal evaporation combined with a photoresist stripping process can be used, solving the problem of difficult vapor deposition of the barrier layer 13. Those skilled in the art can adjust the thickness of each layer according to actual needs, and the connection layer 14 and protective layer 15 can be thickened. This avoids the problem of the protective layer 15 being too thin in existing low-temperature ohmic processes, which can be easily etched and perforated in subsequent processes, reducing process yield, and easily leading to device failure.

[0042] S50: annealing the epitaxial wafer 30 formed with the connection layer 14 and the protection layer 15 in an environment of 500-600°C.

[0043] The epitaxial wafer 30 formed with the connection layer 14 and the protection layer 15 is annealed in an environment of 500-600°C. The annealing temperature is lower than the melting point of each metal layer, which can avoid the melting of metals in each layer of the ohmic contact structure 10 and make the surface of the ohmic contact metal smooth.

[0044] Specifically, the annealing temperature condition of the epitaxial wafer 30 for forming the connection layer 14 and the protection layer 15 is not limited in the embodiment of the present application. For example, the epitaxial wafer 30 can be annealed at 550° C. for 60 seconds in an inert gas atmosphere.

[0045] The method for preparing the ohmic contact structure 10 provided in this application utilizes annealing in an environment of 500-600°C. The relatively low annealing temperature prevents melting of the metal in each layer of the ohmic contact structure 10, resulting in a smooth ohmic contact surface. Due to the use of evaporation, there is no photoresist stripping process, allowing each layer to be slightly thicker, resulting in a thicker ohmic contact structure 10. Therefore, the embodiments of this application can produce an ohmic contact structure 10 with a smooth surface and a relatively thick film layer in a low-temperature environment.

[0046] Optionally, the epitaxial wafer 30 on which the adhesion layer 11 and the cover layer 12 are evaporated is exposed to the atmosphere for a preset time, so that the thickness of the barrier layer 13 is between 1 nm and 10 nm.

[0047] The surface of the covering layer 12 is oxidized to form an oxide, which serves as a barrier layer 13. The oxide has a dense structure. On the one hand, it can prevent the connecting layer 14 and the protective layer 15 from diffusing downward during annealing. Because the protective layer 15 uses a metal that is not easily oxidized in the atmosphere and has a large work function, when the metal of the protective layer 15 diffuses downward and contacts the semiconductor material of the epitaxial wafer 30, the contact is not an ohmic contact but a Schottky contact, which affects the current transmission of the electrode structure. Therefore, the thickness of the barrier layer 13 cannot be too small; on the other hand, the oxide usually has a large resistivity. When the thickness of the barrier layer 13 is small, the carriers can pass through the barrier layer 13 by means of drift diffusion, tunneling, thermal electron emission, etc. to realize current transmission. When the thickness of the barrier layer 13 is too large, the carriers will not be able to pass through the barrier layer 13, making the resistance of the ohmic contact structure 10 too large. Therefore, the thickness of the barrier layer 13 cannot be too large. Based on the above two reasons, the embodiment of the present application sets the thickness of the barrier layer 13 between 1-10nm.

[0048] Among them, the preset time for the epitaxial wafer 30 with the adhesion layer 11 and the covering layer 12 evaporated thereon to be exposed to the atmosphere is not specifically limited in the embodiment of the present application. Those skilled in the art can make specific settings based on the oxidation rate of the covering layer 12 in the atmosphere and the oxygen content in the atmosphere. For example, the covering layer 12 in the embodiment of the present application is made of aluminum metal, and the barrier layer 13 is aluminum metal exposed to the atmosphere for 10 minutes to 30 minutes to form a 7nm thick aluminum oxide layer.

[0049] In one achievable manner of the embodiment of the present application, sequentially evaporating the connection layer 14 and the protection layer 15 on the barrier layer 13 in a vacuum environment includes:

[0050] S41: Place the epitaxial wafer 30 provided with the barrier layer 13 in the coating chamber of the electron beam evaporation coating machine and evacuate the coating chamber to a vacuum degree of 1.0×10 -3 Below Pa;

[0051] S42: evaporating nickel metal on the barrier layer 13 to form a connecting layer 14;

[0052] The connection layer 14 is made of nickel metal, which has good adhesion and can be well adhered to the barrier layer 13 .

[0053] S43 : forming a protective layer 15 by evaporating a gold material on the connection layer 14 .

[0054] Because the adhesion layer 11, the covering layer 12 and the connecting layer 14 are not resistant to the dry etching process, in order to prevent the adhesion layer 11, the covering layer 12 and the connecting layer 14 from being damaged after the subsequent device process, which increases the resistance of the ohmic contact, the embodiment of the present application evaporates gold material on the connecting layer 14 as a protective layer 15. The gold material has good corrosion resistance and is not easily oxidized in the atmosphere, thereby achieving the purpose of protecting the adhesion layer 11, the covering layer 12 and the connecting layer 14.

[0055] This application utilizes atmospheric oxidation to form the barrier layer 13, and then employs an electron beam evaporation coating machine to deposit the connecting layer 14 and protective layer 15 on the barrier layer 13. This avoids the existing low-temperature ohmic technology, which requires sputtering to deposit the cap layer dielectric, thereby circumventing the technical issue of photoresist stripping difficulties associated with sputtering. Furthermore, the production line eliminates the need for sputtering equipment, which is relatively expensive, thus reducing production line equipment costs.

[0056] Optionally, sequentially evaporating the adhesion layer 11 and the cover layer 12 at positions corresponding to the electrodes of the epitaxial wafer 30 in a vacuum comprises:

[0057] S21: Place the epitaxial wafer 30 in the coating chamber of the electron beam evaporation coating machine and evacuate the coating chamber to a vacuum degree of 1.0×10 -3 Below Pa;

[0058] S22: evaporating titanium metal at positions corresponding to electrodes on the epitaxial wafer 30 to form an adhesion layer 11;

[0059] S23 : Aluminum metal is evaporated on the adhesive layer 11 to form a cover layer 12 .

[0060] When the ohmic contact structure 10 is applied to a gallium nitride-based HEMT device, the adhesion layer 11 is set to titanium metal. This is because after the epitaxial wafer formed with the connection layer 14 and the protective layer 15 is annealed, the Ti metal of the adhesion layer 11 and the Al metal of the cover layer form a TiAlx compound. The TiAlx compound has a low work function and is easy to form an ohmic contact with the nitride of the epitaxial wafer. During the subsequent annealing process, the titanium metal reacts with the gallium nitride system, capturing nitrogen from the gallium nitride system. The reaction generates titanium nitride, which causes the gallium nitride system to re-form nitrogen vacancies. The nitrogen vacancies enable N-type doping in the gallium nitride system, which is also conducive to forming a good ohmic metal and reducing the resistance of the ohmic contact structure 10.

[0061] In one achievable method of the embodiment of the present application, the epitaxial wafer 30 on which the adhesion layer 11 and the cover layer 12 are evaporated is exposed to the atmosphere to oxidize the metal material on the surface of the cover layer 12 to form the barrier layer 13, including:

[0062] The covering layer 12 is formed of aluminum metal;

[0063] The aluminum metal on the surface of the cover layer 12 reacts with oxygen in the atmosphere to generate an aluminum oxide layer, which serves as the barrier layer 13 .

[0064] Aluminum material is easily oxidized in the atmosphere, and forms a dense aluminum oxide layer after oxidation. The surface of the aluminum oxide layer is dense. On the one hand, the aluminum oxide layer can prevent the aluminum metal from being further oxidized, and on the other hand, it can also prevent the connecting layer 14 and the protective layer 15 from diffusing into the aluminum metal layer during annealing.

[0065] The ratio of the number of atoms of aluminum and oxygen in the oxide layer is not limited in the embodiment of the present application. For example, Figure 5 、 Figure 6 and Figure 7 As shown, Figure 5 According to the EDS single-point scanning spectrum of the barrier layer 13 provided in an embodiment of the present application, it is calculated that aluminum atoms account for 49.7%, oxygen atoms account for 50.3%, and the atomic ratio of aluminum and oxygen elements in the aluminum oxide layer is approximately 1:1. Figure 6 and Figure 7 An EDS surface scan provided in the embodiment of the present application reveals the distribution of oxygen and aluminum elements in the ohmic contact structure. Figure 6 A is oxygen element, Figure 7 Point B is aluminum element.

[0066] In addition, in order to further verify the performance of the ohmic contact structure 10 provided in the embodiment of the present application in the application of gallium nitride-based HEMT devices, the present application compares the voltage-current relationship of the Ti / Al / Ni / Au structure of the existing low-temperature ohmic technology and the Ti / Al / AlOx / Ni / Au ohmic contact structure 10 provided in the embodiment of the present application. Both structures adopt the annealing conditions of 550°C and 60 seconds, and the epitaxial growth is the same nitride HEMT epitaxial wafer. Figure 8 It can be seen that the Ti / Al / Ni / Au structure in the prior art forms a gold-semi-Schottky contact after low-temperature annealing, from which it can be seen that the gold material of the protective layer 15 diffuses to the position of the gold-semi-contact; Figure 9 It can be seen that the Ti / Al / AlOx / Ni / Au ohmic contact structure 10 provided in the embodiment of the present application remains an ohmic contact after annealing. Therefore, the AlOx layer in the embodiment of the present application, as a barrier layer 13, can block the diffusion of gold and nickel metal, thus achieving the purpose of the present application. The embodiment of the present application uses a two-stage evaporation method to form the ohmic contact structure 10. The aluminum oxide layer is formed by natural oxidation of the Al layer in the atmosphere, resulting in a dense structure that can block the annealing diffusion of the Ni and Au layers.

[0067] The present invention is applied to a gallium nitride-based HEMT device, providing an epitaxial wafer 30 on which an ohmic contact metal electrode is to be prepared, wherein the epitaxial wafer comprises a heterojunction consisting of a channel layer and a barrier layer; the barrier layer can be aluminum nitride, aluminum indium nitride, aluminum gallium nitride, indium gallium nitride, or aluminum indium gallium nitride, etc.

[0068] Another aspect of the present invention provides an ohmic contact structure 10, such as Figure 4 As shown, the ohmic contact structure 10 is arranged at the source and drain of the HEMT device. The ohmic contact structure includes an adhesion layer 11, a covering layer 12, a barrier layer 13, a connecting layer 14 and a protective layer 15, which are sequentially arranged on the epitaxial wafer 30. The barrier layer 13 is an oxide of the metal material of the covering layer 12.

[0069] The ohmic contact structure 10 provided in the embodiment of the present application is prepared using the above-mentioned method for preparing the ohmic contact structure 10, wherein the barrier layer 13 is an oxide of the metal material of the covering layer 12, and has a relatively dense structure, which can prevent the connecting layer 14 and the protective layer 15 from diffusing to the position where the metal and the semiconductor contact during annealing, thereby forming a good ohmic contact.

[0070] On the other hand, the thickness of the adhesion layer 11, the covering layer 12, the connecting layer 14 and the protective layer 15 can be set according to actual needs, thereby preventing the thinner ohmic contact structure 10 from being easily etched and perforated in subsequent processes, reducing the process yield and causing device failure.

[0071] Optionally, the adhesion layer 11 includes a titanium layer having a thickness of The covering layer 12 includes an aluminum layer with a thickness of The barrier layer 13 is an aluminum oxide layer, and the barrier layer 13 has a thickness of Between, the connecting layer 14 includes a nickel layer with a thickness of Between, the protective layer 15 comprises gold, with a thickness of between.

[0072] When the ohmic contact structure 10 is applied to a gallium nitride-based HEMT device, the adhesion layer 11 is set to a titanium layer formed by titanium metal, and aluminum metal is evaporated on the titanium layer to form a capping layer 12. Because the work function of the titanium-aluminum compound is low, it is easy to form an ohmic contact with the nitride. At the same time, during the subsequent annealing process, the titanium metal reacts with the gallium nitride system, capturing nitrogen elements from the gallium nitride system. The reaction generates titanium nitride, which causes the gallium nitride system to reform nitrogen vacancies. The nitrogen vacancies enable N-type doping in the gallium nitride system, which is also conducive to forming a good ohmic metal and reducing the resistance of the ohmic contact structure 10.

[0073] The barrier layer 13 is formed by oxidation of the covering layer 12 in the atmosphere. When the covering layer 12 is an aluminum layer, the barrier layer 13 is an aluminum oxide layer. The surface of the aluminum oxide layer is dense. The aluminum oxide layer can prevent the aluminum metal from being further oxidized, and can also prevent the connecting layer 14 and the protective layer 15 from diffusing into the aluminum metal layer during annealing.

[0074] The connection layer 14 is formed of nickel metal to form a nickel layer. Nickel metal has a relatively dense structure and can prevent the downward diffusion of gold material.

[0075] The protective layer 15 is formed of gold material, which has good corrosion resistance and is not easily oxidized in the atmosphere, thereby protecting the adhesion layer 11, the covering layer 12 and the connecting layer 14.

[0076] Optionally, the thickness ratio of the adhesion layer 11 to the covering layer 12 is between 1:5 and 1:50.

[0077] The adhesion layer 11 and the covering layer 12 undergo a solid phase reaction to form a metal compound with a reduced work function. Based on the interaction between the adhesion layer 11 and the covering layer 12 , the thickness ratio of the adhesion layer 11 to the covering layer 12 is set between 1:5 and 1:50.

[0078] The present application also discloses a HEMT device comprising a substrate 20, an epitaxial wafer 30 disposed on the substrate 20, and a source and a drain spaced apart on the epitaxial wafer 30, wherein the source and the drain form an ohmic contact structure 10. The ohmic contact structure employs the aforementioned ohmic contact structure. This HEMT device exhibits the same structure and benefits as the ohmic contact structure 10 in the aforementioned embodiment. The structure and benefits of the ohmic contact structure 10 have been described in detail in the aforementioned embodiment and will not be repeated here.

[0079] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A method for preparing an ohmic contact structure, characterized in that: include; Providing an epitaxial wafer on which an ohmic contact metal electrode is to be prepared; sequentially evaporating an adhesion layer and a covering layer at positions corresponding to electrodes on the epitaxial wafer in a vacuum environment; exposing the epitaxial wafer on which the adhesion layer and the cover layer are evaporated to the atmosphere to oxidize the metal material on the surface of the cover layer to form a barrier layer; sequentially evaporating a connecting layer and a protective layer on the barrier layer in a vacuum environment; The epitaxial wafer forming the connecting layer and the protective layer is annealed in an environment of 500-600°C.

2. The method for preparing an ohmic contact structure according to claim 1, wherein: The epitaxial wafer on which the adhesion layer and the cover layer are evaporated is exposed to the atmosphere for a preset time so that the thickness of the barrier layer is between 1-10 nm.

3. The method for preparing an ohmic contact structure according to claim 1, wherein: Sequentially evaporating a connecting layer and a protective layer on the barrier layer in the vacuum environment includes: Placing the epitaxial wafer provided with the barrier layer in a coating chamber of an electron beam evaporation coating machine and evacuating the coating chamber; Vapor depositing nickel metal on the barrier layer to form a connecting layer; A gold material is evaporated on the connection layer to form a protective layer.

4. The method for preparing an ohmic contact structure according to claim 1, wherein: Sequentially evaporating an adhesion layer and a covering layer at positions corresponding to electrodes of the epitaxial wafer in the vacuum environment includes: Placing the epitaxial wafer in a coating chamber of an electron beam evaporation coating machine and evacuating the coating chamber; Vapor-depositing titanium metal at positions corresponding to electrodes on the epitaxial wafer to form an adhesion layer; Aluminum metal is evaporated at positions of the epitaxial wafer corresponding to the electrodes to form a covering layer, and the covering layer covers the adhesion layer.

5. The method for preparing an ohmic contact structure according to claim 1, wherein: The step of exposing the epitaxial wafer on which the adhesion layer and the cover layer are evaporated to the atmosphere so that the metal material on the surface of the cover layer is oxidized to form a barrier layer comprises: The covering layer is formed of aluminum metal; The aluminum metal on the surface of the covering layer reacts with oxygen in the atmosphere to generate an aluminum oxide layer as a barrier layer.

6. An ohmic contact structure provided on an epitaxial wafer of a HEMT device, characterized in that: An ohmic contact structure is provided at the source and drain of the HEMT device, and the ohmic contact structure includes an adhesion layer, a covering layer, a barrier layer, a connection layer and a protective layer stacked in sequence on the epitaxial wafer, wherein the barrier layer is an oxide of the metal material of the covering layer.

7. The ohmic contact structure according to claim 6, wherein: The barrier layer is an aluminum oxide layer, and the protective layer comprises gold.

8. The ohmic contact structure according to claim 6, wherein: The thickness of the adhesive layer is The thickness of the covering layer is between The thickness of the barrier layer is between The thickness of the connecting layer is between The thickness of the protective layer is between between.

9. The ohmic contact structure according to claim 6, wherein: The thickness ratio of the adhesive layer to the covering layer is between 1:5 and 1:

50.

10. A HEMT device, characterized in that: It includes a substrate, an epitaxial wafer arranged on the substrate, and a source and a drain arranged at intervals on the epitaxial wafer, the source and the drain are ohmic contact structures, and the ohmic contact structure of the source or drain adopts the ohmic contact structure as described in any one of claims 6 to 9.

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