A method for fabricating an ohmic contact for a gallium nitride device

Abstract

This invention discloses a method for manufacturing an ohmic contact for a gallium nitride (GaN) device. The method includes a substrate layer on which a buffer layer, a GaN channel layer, an aluminum nitride (ANH) isolation layer, an ANH barrier layer, and a cap layer are sequentially epitaxially grown. Through cleaning, photolithography, and etching processes, a window is created from the cap layer to the ANH barrier layer. Source and drain metals are deposited using an electron beam evaporation stage or an ion sputtering machine. The source and drain metals are sequentially composed of an alloyed contact metal, a barrier layer metal, and an anti-oxidation protective layer metal. The alloyed contact metal is a multilayer metal alloy structure of Sc / Ti / Al. The source and drain metals are annealed at high temperature in a nitrogen atmosphere using a rapid annealing furnace. The alloying of AlGaN with Sc / Ti / Al yields the ohmic contact. The ohmic contact proposed in this invention can reduce the ohmic contact resistance by 20%–50% and lower the ohmic contact annealing temperature by 50–100 degrees Celsius compared to existing processes, while also improving surface flatness. This results in a corresponding improvement in the stability and reliability of the ohmic contact, as well as the device's operating current and output power.

Description

Technical Field

[0001] This invention belongs to the field of gallium nitride device ohmic contact technology, and particularly relates to a method for manufacturing an ohmic contact for gallium nitride devices. Background Technology

[0002] Gallium nitride (GaN) devices possess advantages such as high temperature resistance, high voltage resistance, high power, high frequency, and radiation resistance, making them widely applicable in power electronics and communication systems. Ohmic contact technology is one of the key technologies for manufacturing high-performance GaN devices. The magnitude of the source-drain ohmic contact resistance is crucial to the performance of AlGaN / GaN HEMT devices. Good ohmic contacts result in low on-state resistance, high saturation current, high output power, and good stability and reliability.

[0003] The choice of metal for ohmic contacts, the structure of the metal layers, the thickness of each metal layer, the annealing temperature, the annealing time, and surface treatment processes all directly affect the magnitude of the ohmic contact resistance and the surface flatness of the device, thus affecting device performance and yield. The inventors believe that because gallium nitride (GaN) has a very high bandgap, the Schottky barrier of the ohmic contact alloy is also correspondingly high. Multilayer GaN alloying requires very high annealing temperatures, but high-temperature annealing can cause unevenness on the metal surface, increasing the difficulty of subsequent surface passivation and electroplating processes, thereby affecting the yield. Therefore, a method for manufacturing ohmic contacts for gallium nitride devices has been designed.

[0004] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art. Summary of the Invention

[0005] The inventors discovered through research that high-temperature annealing causes unevenness on the surface of the source and drain metals, which increases the difficulty of subsequent surface passivation and electroplating processes, thereby affecting the yield.

[0006] In view of at least one of the above-mentioned technical problems, this disclosure provides a method for manufacturing an ohmic contact for a gallium nitride device, the specific technical solution of which is as follows:

[0007] A method for fabricating an ohmic contact for a gallium nitride (GaN) device includes a substrate layer on which a buffer layer, a GaN channel layer, an aluminum nitride (ANH) isolation layer, an ANH barrier layer, and a cap layer are epitaxially grown sequentially. Through cleaning, photolithography, and etching processes, a window is created in the cap layer up to the ANH barrier layer. Source and drain metals are deposited using an electron beam evaporation stage or an ion sputtering machine. The source and drain metals are sequentially composed of an alloyed contact metal, a barrier layer metal, and an anti-oxidation protective layer metal. The alloyed contact metal is a multilayer metal alloy structure of Sc / Ti / Al. The source and drain metals are annealed at high temperature in a nitrogen atmosphere using a rapid annealing furnace. The alloying of AlGaN with Sc / Ti / Al yields the ohmic contact.

[0008] In some embodiments of this disclosure, the substrate is a sapphire, silicon, or silicon carbide substrate.

[0009] In some embodiments of this disclosure, the buffer layer is an aluminum nitride or gallium nitride buffer layer.

[0010] In some embodiments of this disclosure, the cap layer is a silicon nitride or gallium nitride layer.

[0011] In some embodiments of this disclosure, the barrier layer metal is Ti, Ni, or Mo, and the thickness ranges from 20 nm to 200 nm.

[0012] In some embodiments of this disclosure, the anti-oxidation protective layer metal is Au, and the thickness ranges from 20 nm to 200 nm.

[0013] In some embodiments of this disclosure, the high-temperature annealing conditions are 400–900°C and the annealing time is 30–120 seconds.

[0014] In some embodiments of this disclosure, in the Sc / Ti / Al multilayer metal alloy structure, the thickness of Sc ranges from 2 to 20 nm, the thickness of Ti ranges from 2 to 20 nm, and the thickness of Al ranges from 10 to 100 nm.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The Sc / Ti / Al multilayer metal alloy structure proposed in this invention reduces the Schottky barrier of the ohmic contact through the low work function of Sc metal. Sc / Ti / Al is annealed at high temperature in a nitrogen atmosphere. On the one hand, the TiAl alloy and ScAl alloy diffuse to enhance the conductivity of the material and reduce the Schottky barrier. On the other hand, Sc and Ti diffuse into the gallium nitride material. After N in GaN combines with Sc and Ti to form ScN and TiN respectively, the holes of N provide conductive charges, thereby further reducing the Schottky barrier and forming an ohmic contact between the metal and gallium nitride, thereby reducing the contact resistance. The ohmic contact performance of gallium nitride devices directly affects the saturation current and output power of the device. The lower the ohmic contact resistance, the higher the transconductance of the device, the larger the saturation current, and the larger the output power of the device.

[0017] This invention is mainly applied to the ohmic contacts of gallium nitride HEMT devices and deep ultraviolet LED devices. The ohmic contacts proposed in this invention can reduce the ohmic contact resistance by 20% to 50% and the ohmic contact annealing temperature by 50 to 100 degrees Celsius on the basis of existing processes, improve the surface flatness, and correspondingly improve the stability and reliability of the ohmic contacts, as well as the device operating current and output power.

[0018] The process menu of this invention is simple and has good repeatability, which can fully utilize the characteristics of gallium nitride power devices such as high frequency, high breakdown voltage, and high current density.

[0019] This invention can be applied to the production and manufacturing of gallium nitride power devices, radio frequency devices, and deep ultraviolet LED devices. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of Embodiment 1 of the structure of the present invention.

[0021] The labels in the figure are as follows: 1. Substrate layer; 2. Buffer layer; 3. Gallium nitride channel layer; 4. Aluminum nitride isolation layer; 5. Aluminum gallium nitride barrier layer; 6. Cap layer; 7. Alloyed contact metal; 8. Barrier layer metal; 9. Anti-oxidation protective layer metal. Detailed Implementation

[0022] To better understand the purpose, structure, and function of this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments.

[0023] The component numbers used in this document are solely for distinguishing the objects described and have no sequential or technical meaning. The term "connection" in this disclosure, unless otherwise specified, includes both direct and indirect connections. It should be understood that directional terms such as "upper" and "lower" indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. These terms are used solely for the convenience of describing this application and for a brief description, and do not indicate or imply that the device or unit referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0024] As shown in the attached diagram. Figure 1 As shown, a method for fabricating an ohmic contact for a gallium nitride (GaN) device is designed, including a substrate layer 1. On the substrate layer 1, a buffer layer 2, a gallium nitride channel layer 3, an aluminum nitride isolation layer 4, an aluminum gallium nitride barrier layer 5, and a cap layer 6 are epitaxially grown sequentially. Through cleaning, photolithography, and etching processes, a window is opened from the cap layer 6 to the aluminum gallium nitride barrier layer 5. Source and drain metals are deposited using an electron beam evaporation stage or an ion sputtering machine. The source and drain metals are sequentially composed of an alloyed contact metal 7, a barrier layer metal 8, and an anti-oxidation protective layer metal 9. The alloyed contact metal 7 is a multilayer metal alloy structure of Sc / Ti / Al. The source and drain metals are annealed at high temperature in a nitrogen atmosphere using a rapid annealing furnace. The alloying of AlGaN with Sc / Ti / Al obtains the ohmic contact.

[0025] The specific principle is as follows: Gallium nitride (GaN) has a bandwidth of 3.4 eV, while aluminum nitride (ANU) has a bandwidth of 6.2 eV. The bandwidth of ANU is very large and varies with the aluminum composition. Between 3.4 eV and 6.2 eV, the work function of the metal and GaN is relatively large. Since the Schottky barrier height is directly related to the metal's work function, the barrier height of metallic GaN is relatively large, making the ohmic contact process for GaN more difficult. To obtain a better ohmic contact, a lower Schottky barrier is better. Therefore, we need to choose metals with low work functions, such as titanium, aluminum, and chromium. The corresponding work functions are: titanium 4.33 eV, aluminum 4.28 eV, and chromium 4.50 eV, etc. Table 1 compares the work functions of metals and GaN.

[0026] Table 1 Comparison of work functions of metal-GaN

[0027] Metal Titanium (Ti) Aluminum (Al) Chromium (Cr) Scandium Sc Gold Au NickelNi Work function (eV) 4.33 4.28 4.50 3.50 5.10 5.15

[0028] As shown in Table 1, the work functions of titanium and aluminum are much lower than those of nickel and gold. Therefore, titanium-aluminum can be used as source-drain ohmic contact metals, while nickel-gold has a relatively high work function, so it can be used as a gate Schottky contact metal. The bandgap of aluminum-gallium-nitrogen (Al / GaN) is very wide, and the multilayer metal structure and alloying methods of titanium-aluminum still have many shortcomings. The annealing temperature of Ti / Al-based alloys is relatively high, resulting in significant surface damage, large surface roughness after annealing, and the contact resistance performance needs improvement. Considering that the work function of scandium (Sc) and gallium nitride is only 3.50 eV, much lower than that of titanium (4.33 eV) and aluminum (4.28 eV), this invention proposes novel multilayer metal alloying methods based on Sc / Ti / Al, such as Sc / Ti / Al / Ni / Au, Sc / Ti / Al / Ti / Au, and Sc / Ti / Al / Mo / Au. The multilayer metal film is annealed in a nitrogen atmosphere at a high temperature of 400℃-900℃ for 30 to 120 seconds in a rapid annealing furnace to obtain an ohmic contact with low resistance and good stability.

[0029] Table 2 Comparison of ohmic contact performance of AlGaN / GaN materials with different metal layer structures

[0030] Metal layer TiAlTiAu CrAlNiAu ScAlNiAu TiAlCrMoAu VTiAlNiAu Contact resistance (Ω.mm) 0.54 NA 0.39 NA NA <![CDATA[Specific Resistance (Ω.cm 2 )]]> <![CDATA[7.4x10 -6 ]]> <![CDATA[2.3x10 -5 ]]> NA <![CDATA[1.1x10 -6 ]]> <![CDATA[2.3x10 -6 ]]>

[0031] As shown in Table 2, the contact resistance of the Ti / Al series is much lower than that of the Cr / Al series. This is because the work function of Cr is 4.50 eV, while that of Ti is 4.33 eV. Cr's work function is higher than that of titanium, making the Ti / Al series metal structure the preferred ohmic contact metal for gallium nitride devices. Table 2 also shows that the contact resistance of the Ti / Al series is much higher than that of the Sc / Al series. This is because the work function of Sc is 3.50 eV, which is much lower than that of Ti (4.33 eV). Therefore, Sc is a very promising ohmic contact metal for gallium nitride. However, because Ti / Al is a commonly used and inexpensive metal, it has been widely studied and applied, while Sc, being a precious rare-earth metal, has not received the research and attention it deserves. The ohmic contact resistance of the Ti / Al series is lower than that of the Cr / Al series. This is partly due to the higher work function of Cr compared to titanium, and another important reason is that during high-temperature annealing, titanium and aluminum metals diffuse into gallium nitride (GaN) materials, forming TiAl and TiN alloys. On the one hand, the diffusion of TiAl alloys enhances the conductivity of the material and lowers the Schottky barrier. On the other hand, when Ti diffuses into GaN materials, the N in GaN combines with Ti to form TiN, and the holes in N provide conductive charges, thereby lowering the Schottky barrier and forming an ohmic contact between the metal and GaN.

[0032] Based on the analysis of the ohmic contact resistance and ohmic contact principle of various metal structures such as TiAl, CrAl, ScAl, TiAlCr, VTiAl, and ScTiAl, the main conduction mechanisms are as follows:

[0033] The work function of a metal is crucial for ohmic contacts. The lower the work function, the smaller the ohmic contact resistance. Since the work function of Sc is smaller than that of Ti, Al, Cr, V and other metals, this invention proposes to use Sc as the first contact metal.

[0034] Ti and Al in the TiAl-based metallic structure diffuse into the AlGaN material to form TiAl alloy and TiN alloy, thereby reducing the Schottky barrier and obtaining ohmic contact channels.

[0035] Sc in the ScAl-based metal structure diffuses into AlGaN material to form ScAl alloy and ScN alloy, thereby reducing the Schottky barrier and obtaining ohmic contact channels.

[0036] The ScTiAl-based metal structure combines TiAl and ScAl ohmic contact channels, which not only lowers the ohmic contact annealing temperature and improves surface smoothness, but also further reduces the Schottky barrier, resulting in excellent ohmic contact performance.

[0037] The above embodiments illustrate three examples of implementing the above technical solutions: Example 1

[0038] This embodiment discloses a method for manufacturing an ohmic contact for a gallium nitride (GaN) device, including a substrate layer 1. A buffer layer 2, a GaN channel layer 3, an aluminum nitride (ANH) isolation layer 4, an ANH barrier layer 5, and a cap layer 6 are sequentially epitaxially grown on the substrate layer 1. Through cleaning, photolithography, and etching processes, a window is opened from the cap layer 6 to the ANH barrier layer 5. Source and drain metals are deposited using an electron beam evaporation stage or an ion sputtering machine. The source and drain metals are sequentially composed of an alloyed contact metal 7, a barrier layer metal 8, and an anti-oxidation protective layer metal 9. The alloyed contact metal 7 is a multilayer metal alloy structure of Sc / Ti / Al. The source and drain metals are annealed at high temperature in a nitrogen atmosphere using a rapid annealing furnace. The alloying of AlGaN with Sc / Ti / Al yields the ohmic contact.

[0039] Wherein, the substrate layer 1 is a sapphire substrate, the buffer layer 2 is an aluminum nitride buffer layer, the cap layer 6 is a silicon nitride layer, the barrier layer metal 8 is Ti with a thickness of 20 nm, the anti-oxidation protective layer metal 9 is Au with a thickness of 20 nm, the high-temperature annealing conditions are 400°C and the annealing time is 120 seconds, and in the Sc / Ti / Al multilayer metal alloy structure, the thickness of Sc is 2 nm, the thickness of Ti is 2 nm, and the thickness of Al is 10 nm. Example 2

[0040] This embodiment discloses a method for manufacturing an ohmic contact for a gallium nitride (GaN) device. The difference between this embodiment and Embodiment 1 is that the substrate layer 1 is a silicon substrate, the buffer layer 2 is a GaN buffer layer, the cap layer 6 is a GaN layer, the barrier layer metal 8 is Ni with a thickness of 200 nm, the anti-oxidation protective layer metal 9 is Au with a thickness of 200 nm, the high-temperature annealing conditions are 900°C and the annealing time is 30 seconds, and in the Sc / Ti / Al multilayer metal alloy structure, the thickness of Sc is 20 nm, the thickness of Ti is 20 nm, and the thickness of Al is 100 nm. Example 3

[0041] This embodiment discloses a method for manufacturing an ohmic contact for a gallium nitride (GaN) device. The difference between this embodiment and Embodiment 1 is that the substrate layer 1 is a silicon carbide substrate, the buffer layer 2 is a GaN buffer layer, the cap layer 6 is a GaN cap layer, the barrier layer metal 8 is Mo with a thickness of 120 nm, the anti-oxidation protective layer metal 9 is Au with a thickness of 100 nm, the high-temperature annealing conditions are 780°C and the annealing time is 75 seconds, and in the Sc / Ti / Al multilayer metal alloy structure, the thickness of Sc is 7 nm, the thickness of Ti is 10 nm, and the thickness of Al is 30 nm.

[0042] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A method for manufacturing an ohmic contact for a gallium nitride device, characterized in that: The substrate includes a substrate (1), on which a buffer layer (2), a gallium nitride channel layer (3), an aluminum nitride isolation layer (4), an aluminum gallium nitride barrier layer (5), and a cap layer (6) are epitaxially grown in sequence. Through cleaning, photolithography and etching processes, a window is opened in the cap layer (6) to the aluminum gallium nitride barrier layer (5), and source and drain metals are deposited using an electron beam evaporation stage or an ion sputtering machine. The source and drain metals are composed of an alloyed contact metal (7), a barrier layer metal (8), and an anti-oxidation protective layer metal (9) in sequence. The alloyed contact metal (7) is a multilayer metal alloy structure of Sc / Ti / Al. The source and drain metals are annealed at high temperature in a nitrogen atmosphere using a rapid annealing furnace. AlGaN is alloyed with Sc / Ti / Al to obtain an ohmic contact.

2. The method for manufacturing an ohmic contact for a gallium nitride device according to claim 1, characterized in that, The substrate (1) is a sapphire, silicon, or silicon carbide substrate.

3. The method for manufacturing an ohmic contact for a gallium nitride device according to claim 1, characterized in that, The buffer layer (2) is an aluminum nitride or gallium nitride buffer layer.

4. The method for manufacturing an ohmic contact for a gallium nitride device according to claim 1, characterized in that, The cap layer (6) is a silicon nitride or gallium nitride layer.

5. The method for manufacturing an ohmic contact for a gallium nitride device according to claim 1, characterized in that, The barrier layer metal (8) is Ti, Ni or Mo, and its thickness ranges from 20 nm to 200 nm.

6. The method for manufacturing an ohmic contact for a gallium nitride device according to claim 1, characterized in that, The anti-oxidation protective layer metal (9) is Au, and the thickness ranges from 20nm to 200nm.

7. The method for manufacturing an ohmic contact for a gallium nitride device according to claim 1, characterized in that, The high-temperature annealing conditions are 400–900°C and annealing time is 30–120 seconds.

8. The method for manufacturing an ohmic contact for a gallium nitride device according to claim 1, characterized in that, In the Sc / Ti / Al multilayer metal alloy structure, the thickness of Sc ranges from 2 to 20 nm, the thickness of Ti ranges from 2 to 20 nm, and the thickness of Al ranges from 10 to 100 nm.

Images