Ohmic contact electrode based on ultrathin barrier layer and preparation method thereof

Abstract

The invention relates to an ohmic contact electrode based on an ultrathin barrier layer and a preparation method thereof. Comprising a GaN heterojunction substrate, a Ta barrier layer, an insertion layer, a Ti metal layer, an Al metal layer, a Ni metal layer and an Au metal layer which are sequentially arranged from bottom to top, and the material of the insertion layer is a IVA group metal or semiconductor material in the periodic table of chemical elements. Therefore, on one hand, the Ta barrier layer can inhibit an excessive diffusion phenomenon between Al and Au metals during high-temperature annealing, and the surface roughness is improved; and on the other hand, the insertion layer can form a low-resistance alloy structure to further deplete the barrier layer, so that the tunneling effect of electrons is promoted to form effective ohmic contact, and the resistance is reduced.

Description

Technical Field

[0001] This application relates to the field of semiconductor electronic device technology, and in particular to an ohmic contact electrode based on an ultrathin barrier layer and its fabrication method. Background Technology

[0002] Since the 20th century, group III nitride semiconductor electronic devices have attracted increasing attention due to their excellent performance. Among them, gallium nitride (GaN) material has characteristics such as large bandgap, high electron concentration and mobility, and high breakdown field strength. High electron mobility transistor (HEMT) devices made from it have advantages such as high frequency, high power, high temperature and high pressure resistance, and radiation resistance, and have become a research hotspot.

[0003] Ohmic contacts are crucial for fabricating high-quality GaN HEMT devices. Currently, electron beam evaporation is commonly used to obtain four layers of Ti / Al / Ni / Au metal, followed by rapid high-temperature annealing above 800℃ to form an alloy. This method is easy to implement and produces ohmic electrodes with relatively low contact resistance, making it the mainstream process for fabricating ohmic contact electrodes. In the four-layer Ti / Al / Ni / Au metal structure, the alloy structure of the bottom Ti / Al layer is one of the key factors in forming ohmic contacts. The Ti / Al metal ratio is typically around 1:5 to 1:7 to achieve optimal ohmic characteristics. It is generally believed that TiN, formed by the solid-state reaction of the bottom Ti metal with N atoms in the AlGaN / GaN material surface at high temperatures, plays a key role in the formation of high-quality ohmic contacts. TiN is an alloy with good conductivity, and its work function is very low at only 3.74 eV, which facilitates the tunneling effect of electrons in the semiconductor, thereby forming ohmic contacts.

[0004] It is worth noting that Al is a highly reactive metal that readily undergoes oxidation in air to form an aluminum oxide film. This increases resistivity and hinders the preparation of high-quality alloys. Therefore, a high vacuum level (typically less than 10⁻⁶) is required within the chamber during the evaporation of electrode metals. -4 (Pa). In addition, Au is a metal with excellent conductivity and low metallic reactivity, making it particularly suitable as the top cap metal to prevent the metal below from oxidizing. However, Al and Au readily diffuse into each other at high temperatures to form an Al-Au alloy, leading to a violent reaction between the metals and forming an alloy with an extremely rough surface. This severely affects the contact resistance of the device, and in more serious cases, the metal may even splash outwards, causing short circuits and parasitic phenomena. Therefore, a barrier layer needs to be added between Al and Au to prevent excessive reaction between the two. Common barrier layer metals include Ni, Ti, and Mo, etc.

[0005] The main reason for the rough surface morphology of Ti / Al / Ni / Au metal structures after annealing is that the Ni barrier layer is unable to fully suppress the intense diffusion reaction between Al and Au metals, resulting in an uneven surface morphology and metal burrs. In severe cases, metal particles are also splashed between the source and drain, which has an adverse effect on the subsequent gate fabrication process and seriously affects the performance of GaN RF devices.

[0006] To improve the surface morphology of alloys, commonly used solutions include:

[0007] Using a gold-free ohmic contact process: On one hand, the barrier layer can be thinned to about 1 nm by dry etching, followed by deposition of metals with structures such as Ti / Al / Ti / TiN and annealing at a relatively low temperature of about 600℃ to obtain a smooth ohmic contact electrode. However, the main problem with this approach is the difficulty in precisely controlling the stability of the etching depth. On the other hand, metals with structures such as Ti / Al / Ni / Pt can be deposited and annealed at a high temperature above 900℃ to obtain a smooth ohmic contact electrode. The problem with this approach is that excessively high temperatures can easily cause degradation of the GaN material properties. This problem is even more severe for Si-based GaN HEMT materials, and the resulting ohmic contact has a relatively high resistance.

[0008] Therefore, there is an urgent need to develop an ohmic contact electrode with a smooth surface morphology and low contact resistance. Summary of the Invention

[0009] Therefore, it is necessary to provide an ohmic contact electrode based on an ultrathin barrier layer that has a smooth surface morphology and low contact resistance, and a method for its fabrication, in order to address the above-mentioned technical problems.

[0010] An ohmic contact electrode based on an ultrathin barrier layer, the ohmic contact electrode based on the ultrathin barrier layer comprising, in sequence from bottom to top: GaN heterojunction substrate (1), Ta barrier layer (2), insertion layer (3), Ti metal layer (4), Al metal layer (5), Ni metal layer (6) and Au metal layer (7);

[0011] The material of the insertion layer (3) is a metal or semiconductor material belonging to Group IVA of the periodic table of chemical elements.

[0012] In one embodiment, the material of the insertion layer (3) is either Ge or Si.

[0013] In one embodiment, the thickness of the Ta barrier layer (2) is 2nm to 5nm.

[0014] In one embodiment, the thickness of the insertion layer (3) is 2 nm to 5 nm.

[0015] In one embodiment, the thickness of the Ni metal layer (6) is 30nm-50nm.

[0016] In one embodiment, the thickness of the Au metal layer (7) is 40 nm to 50 nm.

[0017] A method for fabricating an ohmic contact electrode based on an ultrathin barrier layer, characterized in that the fabrication method includes:

[0018] The source and drain regions are defined on a GaN heterojunction substrate using photolithography.

[0019] A sample was obtained by sequentially depositing a Ta barrier layer, an insertion layer, a Ti metal layer, an Al metal layer, a Ni metal layer, and an Au metal layer on a GaN heterojunction substrate using electron beam evaporation.

[0020] The sample was peeled off and subjected to rapid thermal annealing to obtain an ohmic contact electrode.

[0021] In one embodiment, the chamber temperature during electron beam evaporation is below 60°C, and the vacuum level is below 2 × 10⁻⁶. - 5 Pa, evaporation rate is lower than

[0022] In one embodiment, the gas used in the rapid thermal annealing process is N2.

[0023] In one embodiment, the rapid thermal annealing process is performed at a temperature of 750°C-850°C for 25-40 seconds.

[0024] The aforementioned ohmic contact electrode based on an ultrathin barrier layer comprises, from bottom to top, a GaN heterojunction substrate (1), a Ta barrier layer (2), an insertion layer (3), a Ti metal layer (4), an Al metal layer (5), a Ni metal layer (6), and an Au metal layer (7); the material of the insertion layer (3) is a metal or semiconductor material belonging to Group IVA of the periodic table. Thus, on the one hand, the Ta barrier layer can suppress excessive diffusion between Al and Au metals during high-temperature annealing, improving surface roughness; on the other hand, the insertion layer can form a low-resistance alloy structure to further deplete the barrier layer, thereby promoting electron tunneling to form an effective ohmic contact and reducing resistance. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of an ohmic contact electrode based on an ultrathin barrier layer in one embodiment;

[0026] Figure 2This is a schematic diagram of the structure after defining the source and drain regions on the GaN heterojunction substrate during the fabrication of an ohmic contact electrode based on an ultrathin barrier layer in one embodiment.

[0027] Figure 3 This is a schematic diagram of a sample obtained during the fabrication of an ohmic contact electrode based on an ultrathin barrier layer in one embodiment.

[0028] Figure 4 The image is a scanning electron microscope image of the surface of the ohmic contact electrode prepared in Example 1.

[0029] Figure 5 The image is a scanning electron microscope image of the surface of the ohmic contact electrode prepared in Example 2.

[0030] Figure 6 The image is a scanning electron microscope image of the surface of the ohmic contact electrode prepared in Example 3.

[0031] Figure 7 The image is a scanning electron microscope image of the ohmic contact electrode surface prepared in Comparative Example 1. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0033] In one embodiment, such as Figure 1 As shown, an ohmic contact electrode based on an ultrathin barrier layer is provided. The ohmic contact electrode based on the ultrathin barrier layer includes, in sequence from bottom to top: GaN heterojunction substrate 1, Ta barrier layer 2, insertion layer 3, Ti metal layer 4, Al metal layer 5, Ni metal layer 6 and Au metal layer 7; the material of the insertion layer 3 is a metal or semiconductor material of Group IVA in the periodic table of chemical elements.

[0034] The thickness of both the Ta barrier layer 2 and the insertion layer 3 does not exceed 5 nm.

[0035] The thickness ratio of Ti metal layer 4 to Al metal layer 5 can be 1:5 to 1:7.

[0036] The thickness of the Ti metal layer 4 can be 20 nm.

[0037] The thickness of the Al metal layer 5 can be 135 nm.

[0038] The thickness of the Ni metal layer 6 can be 30nm-50nm.

[0039] The thickness of the Au metal layer 7 can be 40nm-50nm.

[0040] Among them, the traditional Ti / Al / Ni / Au structure is improved by adding a Ta bottom barrier layer, which suppresses the excessively violent reaction between metals during high-temperature annealing. This not only avoids the generation of metal burrs at the electrode edge after annealing, but also greatly reduces the surface roughness.

[0041] The introduced insertion layer can form a low-resistance alloy structure to further deplete the barrier layer, thereby promoting electron tunneling, effectively reducing the ohmic contact of the device, and improving the reliability and stability of the RF device.

[0042] The GaN heterojunction substrate can be an AlGaN / GaN substrate, an AlN / GaN substrate, an InAlN / GaN substrate, etc.

[0043] AlN represents aluminum nitride, AlGaN represents aluminum gallium nitride, and InAlN represents indium aluminum nitride.

[0044] Among them, Ta represents tantalum, Ge represents germanium, Si represents silicon, Ti represents titanium, Al represents aluminum, Ni represents nickel, and Au represents gold.

[0045] It should be understood that during annealing, the insertion layer 3 can diffuse downwards to reduce the barrier height at the metal-semiconductor contact and form compounds that facilitate ohmic contacts, thereby lowering the temperature required for high-temperature alloying and thus reducing the ohmic contact resistance. Simultaneously, the Ta metal, acting as barrier layer 2, can prevent excessive downward diffusion of the Al metal layer 5 and Au metal layer 7 under intense alloying reactions, improving the alloy surface morphology.

[0046] The aforementioned ohmic contact electrode based on an ultrathin barrier layer comprises, from bottom to top, a GaN heterojunction substrate 1, a Ta barrier layer 2, an insertion layer 3, a Ti metal layer 4, an Al metal layer 5, a Ni metal layer 6, and an Au metal layer 7; the material of the insertion layer 3 is a metal or semiconductor material belonging to Group IVA of the periodic table. Thus, on the one hand, the Ta barrier layer can suppress excessive diffusion between Al and Au metals during high-temperature annealing, improving surface roughness; on the other hand, the insertion layer can form a low-resistance alloy structure to further deplete the barrier layer, thereby promoting electron tunneling to form an effective ohmic contact and reducing resistance.

[0047] In one embodiment, the material of the insertion layer 3 is either Ge or Si.

[0048] In one embodiment, the thickness of the Ta barrier layer 2 is 2 nm to 5 nm.

[0049] The thickness of the Ta barrier layer 2 is preferably 3 nm.

[0050] In one embodiment, the thickness of the insertion layer 3 is 2nm to 5nm.

[0051] The thickness of the insertion layer 3 is preferably 3 nm.

[0052] In one embodiment, the thickness of the Ni metal layer 6 is 30nm-50nm.

[0053] In one embodiment, the thickness of the Au metal layer 7 is 40 nm to 50 nm.

[0054] The aforementioned ohmic contact electrode based on an ultrathin barrier layer presents a novel structure and fabrication method on the existing Ti / Al / Ni / Au alloy structure. On one hand, the Ta barrier layer suppresses excessive diffusion between Al and Au metals during high-temperature annealing, improving surface roughness. On the other hand, the intercalation layer forms a low-resistance alloy structure that further depletes the barrier layer, thereby promoting electron tunneling to form an effective ohmic contact, reducing resistance, and improving the reliability and stability of GaN HEMT devices. It can also improve surface morphology while reducing resistance, and boasts advantages such as simple processing and low fabrication cost. Furthermore, it reduces the ohmic contact resistance of GaN HEMT devices and significantly improves the alloy surface morphology, contributing to enhanced DC characteristics, frequency characteristics, and stability of GaNHEMT devices.

[0055] A method for fabricating an ohmic contact electrode based on an ultrathin barrier layer, characterized in that the fabrication method includes:

[0056] The source and drain regions were defined on the GaN heterojunction substrate using photolithography. A Ta barrier layer, an insertion layer, a Ti metal layer, an Al metal layer, a Ni metal layer, and an Au metal layer were sequentially deposited on the GaN heterojunction substrate using electron beam evaporation to obtain a sample. The sample was then stripped and subjected to rapid thermal annealing to obtain an ohmic contact electrode.

[0057] The GaN heterojunction substrate can be an AlGaN / GaN substrate, an AlN / GaN substrate, an InAlN / GaN substrate, etc.

[0058] The material of the insertion layer is either Ge or Si.

[0059] In one embodiment, the chamber temperature during electron beam evaporation is below 60°C, and the vacuum level is below 2 × 10⁻⁶. -5 Pa, evaporation rate is lower than

[0060] In one embodiment, the gas used for rapid thermal annealing is N2.

[0061] In one embodiment, the rapid thermal annealing temperature is 750℃-850℃, and the rapid thermal annealing time is 25s-40s.

[0062] The preferred temperature for rapid thermal annealing is 780℃.

[0063] The preferred time for rapid thermal annealing is 30 seconds.

[0064] The above-mentioned method for fabricating ohmic contact electrodes based on ultrathin barrier layers, compared with traditional high-temperature alloying processes, can effectively reduce ohmic contact resistance and significantly improve surface morphology; compared with gold-free ohmic processes such as thinning of barrier layers, the above-mentioned method for fabricating ohmic contact electrodes based on ultrathin barrier layers is more stable and has better repeatability; compared with secondary epitaxy n + GaN or Si + Compared to ion implantation activation, this process is easier to implement and has lower manufacturing costs. It can be applied to the source and drain electrode fabrication processes of GaN HEMT devices on substrates such as Si, SiC, and sapphire.

[0065] In one embodiment, a method for fabricating an ohmic contact electrode based on an ultrathin barrier layer is provided, wherein an ohmic contact electrode on an AlGaN / GaN substrate is fabricated, such as... Figures 1-3 As shown, the specific steps include:

[0066] 1) Using photolithography, a layer of PMGI SF3 photoresist 8 is first spin-coated onto the upper surface of the AlGaN / GaN substrate 1, followed by a layer of AZ7908 photoresist 9. Exposure is performed using a photolithography machine, followed by development with 3038 positive photoresist developer. This allows for the definition of the source and drain regions on the AlGaN / GaN substrate 1, resulting in the following: Figure 2 The structure shown.

[0067] 2) The chamber vacuum level of the electron beam evaporation stage is better than 2×10⁻⁶. -5 When the crystal oscillator lifetime is greater than 95%, electron beam evaporation is used to sequentially deposit the crystal onto the AlGaN / GaN substrate 1. The rate of deposition of Ta barrier layer 2, at 3. Deposit Si or Ge insertion layers at a rate of 3. 4. Deposit Ti metal layer at a rate of 5. Deposit Al metal layer at a rate of 5. 6. Deposit Ni metal layer at a rate of The Au metal layer 7 was deposited at a rate that yielded the following result: Figure 3 The sample shown.

[0068] 3) After step 2, the sample is peeled off and subjected to rapid thermal annealing to obtain the following result: Figure 1The GaN HEMT ohmic contact electrode is shown. The gas used for rapid thermal annealing is N2, the temperature for rapid thermal annealing is 750℃-850℃, and the time for rapid thermal annealing is 25s-40s.

[0069] Based on the above method for preparing ohmic contact electrodes based on ultrathin barrier layers, the following three embodiments are given, but are not limited to these embodiments, as well as Comparative Example 1 prepared using existing technology to verify the effect.

[0070] Example 1: An ohmic contact electrode structure was fabricated with a Ta barrier layer and a Si insertion layer thickness of 3 nm, and Ti / Al / Ni / Au thicknesses of 20 nm, 135 nm, 40 nm, and 45 nm, respectively. The process is as follows:

[0071] 1) Define the source and drain regions on the AlGaN / GaN substrate using photolithography.

[0072] 2) Ta / Si / Ti / Al / Ni / Au metals with thicknesses of 3nm, 3nm, 20nm, 135nm, 40nm, and 45nm were deposited by electron beam evaporation.

[0073] 3) The stripped sample was subjected to rapid thermal annealing at 780℃ for 30s to obtain an ohmic contact electrode.

[0074] Example 2: An ohmic contact electrode structure was fabricated with a Ta barrier layer and a Si insertion layer thickness of 5 nm, and Ti / Al / Ni / Au thicknesses of 20 nm, 135 nm, 40 nm, and 45 nm, respectively. The process is as follows:

[0075] 1) Define the source and drain regions on the AlGaN / GaN substrate using photolithography.

[0076] 2) Ta / Si / Ti / Al / Ni / Au metals with thicknesses of 5nm, 5nm, 20nm, 135nm, 40nm, and 45nm were deposited by electron beam evaporation.

[0077] 3) The stripped sample was subjected to rapid thermal annealing at 780℃ for 30s to obtain an ohmic contact electrode.

[0078] Example 3: An ohmic contact electrode structure was fabricated with a Ta barrier layer and a Ge insertion layer thickness of 3 nm, and Ti / Al / Ni / Au thicknesses of 20 nm, 135 nm, 40 nm, and 45 nm, respectively. The process is as follows:

[0079] 1) Define the source and drain regions on the AlGaN / GaN substrate using photolithography.

[0080] 2) Ta / Ge / Ti / Al / Ni / Au metals with thicknesses of 3nm, 3nm, 20nm, 135nm, 40nm, and 45nm were deposited by electron beam evaporation.

[0081] 3) The stripped sample was subjected to rapid thermal annealing at 780℃ for 30s to obtain an ohmic contact electrode.

[0082] Comparative Example 1: Defining source and drain regions on an AlGaN / GaN substrate using photolithography.

[0083] 2) Ti / Al / Ni / Au metals with thicknesses of 20 nm, 135 nm, 40 nm, and 45 nm were deposited by electron beam evaporation.

[0084] 3) The stripped sample was subjected to rapid thermal annealing at 780℃ for 30s to obtain an ohmic contact electrode.

[0085] The performance evaluation of the ohmic contact electrodes obtained in Examples 1-3 and Comparative Example 1 is as follows:

[0086] like Figure 4-7 As shown, SEM observation of the morphology of Example 1 and Comparative Example 1 after annealing revealed that the existing Ti / Al / Ni / Au structure has a rough surface morphology and is accompanied by metal particle spatter. In contrast, the Ta / Si / Ti / Al / Ni / Au structure of this application can reduce the temperature required for high-temperature alloying, effectively reduce the surface roughness after alloying, and greatly improve the phenomenon of metal spattering. Furthermore, the ohmic contact electrodes prepared in Examples 1-3 and Comparative Example 1 were subjected to four-probe resistance testing using a TLM model, and the specific contact resistance R was obtained. c The sizes are shown in Table 1:

[0087] Table 1. Comparative contact resistance R of the embodiments and comparative examples c size

[0088] <![CDATA[Specific contact resistance R c (Ω·mm)]]> Example 1 0.48 Example 2 0.61 Example 3 0.53 Comparative Example 1 0.69

[0089] The following points can be observed from Table 1:

[0090] (1) Examples 1-3 and Comparative Example 1 can all form effective ohmic contacts.

[0091] (2) Adding a Ta metal barrier layer to the bottom of the traditional Ti / Al / Ni / Au structure helps suppress excessive diffusion between Al and Au metals, improves the surface morphology of the alloy, and can also form a TaN structure with excellent electrical properties to further deplete the N atoms in the barrier layer and reduce the specific contact resistance Rc. Si and Ge intercalation layers can diffuse downwards during annealing to reduce the barrier height in the metal-semiconductor contact and form compounds that facilitate ohmic contacts, thereby reducing the specific contact resistance Rc. c It can also reduce the temperature required for high-temperature alloys by nearly 100°C.

[0092] (3) The thickness of the Ta barrier layer should be appropriate. If it is too thin, it will be difficult to form a film. If it is too thick, it will affect the formation of effective ohmic contacts.

[0093] In summary, a high-flatness, low-resistance ohmic contact electrode based on an ultrathin barrier layer and its fabrication method are presented. Adding a Ta barrier layer and a Si insertion layer to the bottom of the traditional Ti / Al / Ni / Au structure improves the alloy surface morphology, reduces the temperature required for high-temperature alloys, and also lowers the specific contact resistance Ro. c The process is simple and easy to implement.

[0094] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0095] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. An ohmic contact electrode based on an ultrathin barrier layer, characterized in that, The ohmic contact electrode based on the ultrathin barrier layer includes, in sequence from bottom to top: GaN heterojunction substrate (1), Ta barrier layer (2), insertion layer (3), Ti metal layer (4), Al metal layer (5), Ni metal layer (6) and Au metal layer (7); The material of the insertion layer (3) is a metal or semiconductor material belonging to Group IVA of the periodic table of chemical elements.

2. The ohmic contact electrode based on an ultrathin barrier layer according to claim 1, characterized in that, The material of the insertion layer (3) is either Ge or Si.

3. The ohmic contact electrode based on an ultrathin barrier layer according to claim 1, characterized in that, The thickness of the Ta barrier layer (2) is 2nm to 5nm.

4. The ohmic contact electrode based on an ultrathin barrier layer according to claim 1, characterized in that, The thickness of the insertion layer (3) is 2nm to 5nm.

5. The ohmic contact electrode based on an ultrathin barrier layer according to claim 1, characterized in that, The thickness of the Ni metal layer (6) is 30nm-50nm.

6. The ohmic contact electrode based on an ultrathin barrier layer according to claim 1, characterized in that, The thickness of the Au metal layer (7) is 40 nm to 50 nm.

7. A method for fabricating an ohmic contact electrode based on an ultrathin barrier layer as described in any one of claims 1-6, characterized in that, The preparation method includes: The source and drain regions are defined on a GaN heterojunction substrate using photolithography. A sample was obtained by sequentially depositing a Ta barrier layer, an insertion layer, a Ti metal layer, an Al metal layer, a Ni metal layer, and an Au metal layer on a GaN heterojunction substrate using electron beam evaporation. The sample was peeled off and subjected to rapid thermal annealing to obtain an ohmic contact electrode.

8. The method for fabricating an ohmic contact electrode based on an ultrathin barrier layer according to claim 7, characterized in that, During electron beam evaporation, the chamber temperature is below 60°C and the vacuum level is below 2×10⁻⁶. -5 Pa, evaporation rate is lower than 9. The method for fabricating an ohmic contact electrode based on an ultrathin barrier layer according to claim 7, characterized in that, The gas used in the rapid thermal annealing process is N2.

10. The method for fabricating an ohmic contact electrode based on an ultrathin barrier layer according to claim 7, characterized in that, The rapid thermal annealing process is performed at a temperature of 750℃-850℃ for 25s-40s.

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