A High-Selection-Ratio Back-Hole Etching Method for GaN HEMT Based on ICP Etching

Abstract

The present invention discloses a high-selectivity back-hole etching method for GaN HEMT based on ICP etching, belonging to the technical field of transistors. Firstly, the SiC layer is etched for the first time using the first etching parameters, and the first etching uses a mixed gas of SF6 and O2. Secondly, the GaN layer is etched for the second time using the second etching parameters, and the second etching uses a mixed gas of Cl2, BCl3, and Ar. Thirdly, the AlGaN layer is etched for the third time using the third etching parameters, and the third etching uses a mixed gas of Cl2 and BCl3. Based on the second etching parameters, the proportion of Cl2 is increased to reduce the combination of residual oxygen molecules and surface metal dangling bonds in the first etching and the second etching, so as to reduce the N vacancies on the surface of the AlGaN layer. The present invention takes into account the material characteristics of different layers and provides a supporting selection scheme for the etching parameters in different etching stages. While achieving high selectivity, it completes the preparation of through holes with good morphology and smaller wall roughness, improves the performance and reliability of GaN HEMT devices, and has broad prospects for industrial production applications.

Description

Technical Field

[0001] The present invention relates to the technical field of transistors, and particularly to a high-selectivity GaN HEMT back-hole etching method based on ICP etching. Background Art

[0002] SiC-based AlGaN / GaN HEMT devices are widely used in the fields of power electronics and microwave power. In order to achieve the performance of high current, high power, high temperature, high frequency, and high efficiency, the preparation of the back via structure of the SiC-based AlGaN / GaN HEMT device has become a particularly critical process step.

[0003] In the field of high-performance GaN HEMT, currently, GaN materials are epitaxially grown on SiC substrates. The prepared GaN HEMT has the characteristics of high operating temperature, high application frequency, large output power, high gain, etc. The via structure is a hole-like structure that penetrates the substrate, buffer layer, barrier layer, and passivation layer of the GaN HEMT device, and has an important impact on the frequency characteristics, parasitic capacitance, and inductance of the device. A reasonably designed and well-prepared via grounding structure can reduce the parasitic inductance at the source end, reduce the transmission impedance, optimize the overall frequency characteristics of the device, improve the heat dissipation performance of the device, and improve the high-temperature reliability of the device.

[0004] SiC has a good lattice match with GaN, and has a relatively large bandgap width, high electron and hole mobility, and high thermal conductivity, so it is commonly used as the substrate material for GaN HEMT devices. SiC is composed of group-IV elements Si and C, and the basic structural unit is a regular tetrahedron composed of Si atoms and C atoms. The bond energy of Si-C is relatively strong, and the stability of the overall structure results in extremely high hardness and good chemical corrosion resistance of SiC, which determines that it is difficult to etch SiC by wet etching. Generally, laser or plasma dry etching is used in the industry for its processing.

[0005] GaN has a relatively wide bandgap width, a relatively high breakdown field strength, and a very high electron saturation drift velocity. These properties enable the device to have characteristics such as high power density, small volume, and high temperature resistance. The crystal structure of GaN in the stable state is mostly wurtzite structure, and the lattice symmetry is relatively low. The GaN / AlGaN heterojunction interface composed of it and AlGaN generates a strong polarization (spontaneous polarization and piezoelectric polarization) effect, and a two-dimensional electron gas (2DEG) with a very large surface density and extremely high mobility is generated in the heterojunction quantum well. This is the main advantage of the GaN / AlGaN heterostructure forming GaN HEMT.

[0006] The purpose of the back-hole process for SiC-based GaN HEMT is to fabricate low-damage vias that penetrate the GaN / AlGaN heterojunction interface. Therefore, it is required to etch the GaN layer and the AlGaN layer sequentially on the basis of etching through the SiC layer. Common GaN and AlGaN etching methods include electron cyclotron resonance plasma (ECR), reactive ion etching (RIE), inductively coupled plasma (ICP) etching, magnetically neutral loop discharge plasma (NLD) etching, etc.

[0007] In the current ICP etching process of SiC, common working gases are fluorine-based gases such as CF4, SF6, NF3, C2F6, etc. O2 or Ar is mixed in a specific ratio as an auxiliary gas to increase the etching rate and reduce etching damage. Using a CF4 / O2 mixed gas as the etching gas, the ICP source power is 200W - 1000W, the RF power is 100W - 150W, the CF4 flow rate is 15 sccm, the CF4 / O2 flow rate ratio is controlled from 1 to 0.68, and the working pressure is 0.25 Pa. The common working gas for the ICP etching process of GaN is Cl2 / BCl3, the total gas flow rate is 100 sccm. To protect the front structure, an ICP power of 500 - 1000W is mostly selected, the RF power is 250W - 500W, and the working pressure is 1.07 Pa. The above working gas schemes only consider the best etching rate and selectivity of single SiC or GaN. However, during the process preparation, the diameter and depth of the via are restricted by the epitaxial layer material and thickness. When the depth of the general hole exceeds 6 times the hole diameter, it is impossible to ensure that the hole wall can be uniformly coated with a metal layer. The etching process involves the etching rates of different layer materials and the by-products brought by the etching of different layers. The etching of the upper layer will affect the etching effect of the lower layer. Therefore, it is necessary to develop a back-hole etching process that takes into account the etching efficiency and quality between different layers. Summary of the Invention

[0008] The purpose of the present invention is to overcome the problems existing in the back-hole etching in the prior art, consider the overall etching rate, effect and selectivity of the AlGaN / GaN / SiC material, and provide a high-selectivity GaN HEMT back-hole etching method based on ICP etching.

[0009] The purpose of the present invention is achieved through the following technical solutions:

[0010] Mainly provide a high-selectivity GaN HEMT back-hole etching method based on ICP etching. On the basis of etching through the SiC layer, the GaN layer and the AlGaN layer are etched sequentially. The method includes:

[0011] Using the first etching parameters to perform the first etching on the SiC layer, and the first etching uses a mixed gas of SF6 and O2 for plasma etching;

[0012] After the first etching is completed, the GaN layer is subjected to a second etching using second etching parameters. The second etching is a plasma etching using a mixed gas of Cl2, BCl3, and Ar.

[0013] After the second etching is completed, the AlGaN layer is subjected to a third etching using third etching parameters. The third etching is a plasma etching using a mixed gas of Cl2 and BCl3, and based on the second etching parameters, the proportion of Cl2 is increased to reduce the combination of residual oxygen molecules and surface metal dangling bonds in the first and second etchings, so as to reduce the N vacancies on the surface of the AlGaN layer.

[0014] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the first etching parameters are: the gas flow ratio of SF6 and O2 includes but is not limited to 4:1, 5:1, 6:1, 8:1, 16:1; the second etching parameters are: the gas flow ratio of Cl2, BCl3, and Ar includes but is not limited to 20:2:1, 16:2:1, 8:2:1; the third etching parameters are: the gas flow ratio of Cl2 and BCl3 includes but is not limited to 1:1, 2:1, 4:1, 8:1.

[0015] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the total gas flow in the first etching parameters is 20 sccm - 130 sccm, and the working pressure is 0.34 Pa - 1.2 Pa; the total gas flow in the second etching parameters is 90 sccm - 130 sccm, and the working pressure is 0.6 Pa - 1.3 Pa; the total gas flow in the third etching parameters is 25 sccm - 100 sccm, and the working pressure is 0.3 Pa - 1.1 Pa.

[0016] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the etching thickness of the first etching is 80 μm - 200 μm.

[0017] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the etching thickness of the second etching is 1 μm - 3 μm.

[0018] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the etching thickness of the third etching is 10 nm - 200 nm.

[0019] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the method further includes a pretreatment step before the first etching, and the pretreatment includes back-hole area lithography and back-side mask preparation.

[0020] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the back-hole area lithography includes:

[0021] Using photoresist as a back-hole position mask, lithographically forming a back-hole pattern.

[0022] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the back-side mask preparation includes:

[0023] Electroplating a Ni metal mask layer on the back-hole pattern, depositing a mask layer with a thickness of 2μm - 5μm (including but not limited to).

[0024] As a preferred option, a high-selectivity GaN HEMT back-hole etching method based on ICP etching, the method further includes:

[0025] After the third etching, stripping the back-hole metal mask and depositing a metal layer on the via hole surface.

[0026] It should be further noted that the technical features corresponding to the above options can be combined or replaced with each other without conflict to form a new technical solution.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] (1) When etching the back hole, the present invention takes into account the material characteristics of different layers, considers the etching rate, effect and selectivity of the overall AlGaN / GaN / SiC material, provides a selection scheme for etching parameters supporting different etching stages, while achieving high selectivity, obtains through holes of SiC-based GaN HEMT with high uniformity and high surface quality, completes the preparation of through holes with good morphology and smaller wall roughness, and improves the performance and reliability of GaN HEMT devices.

[0029] (2) The first etching is carried out by plasma etching with a mixed gas of SF6 and O2, so that the selectivity ratio of SiC to the Ni mask is greater than 40:1, and the selectivity ratio of SiC to GaN is greater than 60:1; the second etching is carried out by plasma etching with a mixed gas of Cl2, BCl3 and Ar, so that the selectivity ratio of GaN to SiC is greater than 3:1; the third etching is carried out by plasma etching with a mixed gas of Cl2 and BCl3, and based on the second etching parameters, the proportion of Cl2 is increased to reduce the combination of residual oxygen molecules and surface metal dangling bonds in the first etching and the second etching, so that the N vacancies on the surface of the AlGaN layer are reduced, the surface roughness is decreased, and the etching quality is improved.

[0030] (3) The second etching introduces Ar to improve the etching ignition process, and at the same time adjusts the composition and quality of the working gas plasma, which can reduce the micro-mask effect in the etching process and improve the etching quality. Description of the Drawings

[0031] Figure 1 is a flowchart of a high-selectivity GaN HEMT back hole etching method based on ICP etching shown in the present invention;

[0032] Figure 2 shows the lithography of the back hole area on the back of the SiC-based GaN HEMT wafer in the present invention, and the back via pattern is formed by lithography and development;

[0033] Figure 3 shows the preparation of the mask on the back of the wafer in the present invention, a Ni metal mask is formed on the back by electroplating, and the glue is removed to expose the via position;

[0034] Figure 4 shows the schematic of etching the SiC layer, GaN layer and AlGaN layer in sequence in the present invention;

[0035] Figure 5 shows the schematic of forming an interconnection by metal deposition on the surface of the via and the front structure in the present invention;

[0036] Figure 6 shows the external morphology of the via without the deposited metal layer in the present invention;

[0037] Figure 7 shows the internal morphology of the via without the deposited metal layer in the present invention. Detailed Embodiments

[0038] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the scope of protection of the present invention.

[0039] In the description of the present invention, it should be noted that the directions or positional relationships indicated by terms such as "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are based on the directions or positional relationships shown in the drawings. This is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0040] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0041] Embodiment 1

[0042] In an exemplary embodiment, a high-selectivity GaN HEMT back-hole etching method based on ICP etching is provided. On the basis of etching through the SiC layer, the GaN layer and the AlGaN layer are etched in sequence, as Figure 1 shown, the method includes:

[0043] Performing a first etching on the SiC layer using first etching parameters, and the first etching is performed by plasma etching using a mixed gas of SF6 and O2;

[0044] After the first etching is completed, performing a second etching on the GaN layer using second etching parameters, and the second etching is performed by plasma etching using a mixed gas of Cl2, BCl3, and Ar;

[0045] After the second etching is completed, performing a third etching on the AlGaN layer using third etching parameters, and the third etching is performed by plasma etching using a mixed gas of Cl2 and BCl3, and based on the second etching parameters, the proportion of Cl2 is increased to reduce the combination of residual oxygen molecules and surface metal dangling bonds in the first etching and the second etching, so as to reduce the N vacancies on the surface of the AlGaN layer.

[0046] Specifically, the present invention uses a mixed gas of SF6 and O2 to etch the SiC layer, and the fluorine-containing plasma and O2 form charged SiF x O ylayer, attracting reactive ions and increasing the etching rate. In addition, SiF x The base product has a relatively low bond energy and is easy to remove. The etching effects of SF6 and CF4 on the roughness of the etched hole walls are similar, but the etching rate of SF6 is higher than that of CF4. Therefore, it is more suitable for practical production applications. By controlling other relevant parameters, the self-stopping of the etching process can be achieved at the SiC / GaN interface, and then the high selectivity etching of SiC and GaN can be realized.

[0047] Etch the GaN layer using a mixed gas of Cl2, BCl3, and Ar. The ionized Cl - reacts with GaN to form GaCl x , GaCl x + , and products such as N2. Through high-kinetic energy ion bombardment, the chemical reaction is promoted and the reaction products are desorbed to remove the residues on the sample surface. Among them, introducing Ar improves the etching start-up process. At the same time, adjusting the composition and quality of the working gas plasma can reduce the micro-masking effect during the etching process and improve the etching quality.

[0048] Furthermore, when performing the third etching on the AlGaN layer, adjust the ratio of Cl2 / BCl3. Compared with the gas parameters in the second etching, increase the ratio of Cl2 and other corresponding parameters, reduce the possibility of the residual O in the first and second etchings combining with the surface metal dangling bonds, reduce the N vacancies on the AlGaN surface, decrease the surface roughness, improve the etching quality, and obtain a better through-hole etching effect of the AlGaN layer.

[0049] Based on the ICP etching mechanism, a staged ICP back-hole etching process gas scheme is designed for different layer material characteristics of the SiC-based epitaxial wafer. By changing relevant parameters such as the working gas, gas flow ratio, and working pressure, the etching rate and sidewall tilt angle are controlled, so that each layer of material can achieve a high selectivity between different materials, a small roughness of the tube wall, and a good surface quality under the condition of ensuring a relatively stable etching rate. A high-quality through-hole preparation process with precise control can be realized, improving the high-temperature reliability of the device and then increasing the yield of the overall device.

[0050] Example 2

[0051] Based on Example 1, a high-selectivity GaN HEMT back-hole etching method based on ICP etching is provided, which further includes a pretreatment step before the first etching. The pretreatment includes back-hole area lithography and back-side mask preparation, and after the third etching, the back-hole metal mask is peeled off, and a metal layer is deposited on the through-hole surface.

[0052] Specifically, please refer to Figures 2 - 7 , and the complete steps of this method are as follows:

[0053] Step 1: Lithography of the back hole area

[0054] As Figure 2 shown, use photoresist as the mask for the back hole position. The lithography method includes but is not limited to contact exposure, step exposure, etc., to form the back hole pattern.

[0055] Step 2: Preparation of the back mask

[0056] As Figure 3 shown, electroplate the Ni metal mask layer on the back hole pattern formed in Step 1, and deposit a mask layer with a thickness of 2μm - 5μm, including but not limited to this range.

[0057] Step 3: Etching of the back hole

[0058] As Figure 4 shown, under the protection of the Ni metal mask formed in Step 2, perform etching. First, etch the SiC layer. The first etching parameters are: the gas flow ratio of SF6 and O2 includes but is not limited to 4:1, 5:1, 6:1, 8:1, 16:1; the total gas flow in the first etching parameters includes but is not limited to 20sccm - 130sccm, the working pressure of the chamber includes but is not limited to 0.34Pa - 1.2Pa; the RF power includes but is not limited to 125W - 450W, and the ICP power includes but is not limited to 800W - 1800W. The etching rate is controlled at 0.2μm.min -1 -1.3μm.min -1 , and the etching thickness is related to the wafer thinning situation, including but not limited to 80μm - 200μm. Adjust the etching time according to the thickness.

[0059] Then etch the GaN layer. The second etching parameters are: the gas flow ratio of Cl2, BCl3 and Ar includes but is not limited to 20:2:1, 16:2:1, 8:2:1; the total gas flow in the second etching parameters includes but is not limited to 90sccm - 130sccm, the working pressure of the chamber includes but is not limited to 0.6Pa - 1.3Pa; the RF power includes but is not limited to 50W - 350W, and the ICP power includes but is not limited to 600W - 1200W. The etching rate is controlled at 100nm.min -1 -800nm.min -1 , and the etching thickness is related to the epitaxial layer design, including but not limited to 1μm - 3μm. Adjust the etching time according to the thickness.

[0060] Finally, the AlGaN layer is etched. The third etching parameters are as follows: the gas flow ratio of Cl2 and BCl3 includes but is not limited to 1:1, 2:1, 4:1, 8:1. The total gas flow in the third etching parameters is 25 sccm - 100 sccm, and the working pressure is 0.3 Pa - 1.1 Pa. The working pressure includes but is not limited to 0.3 Pa - 1.1 Pa, the RF power includes but is not limited to 5 W - 30 W, and the ICP power includes but is not limited to 100 W - 400 W. The etching rate is controlled at 20 nm.min -1 -200 nm.min -1 , and the etching thickness is related to the epitaxial layer design, including but not limited to 10 nm - 200 nm. The etching time is adjusted according to the thickness.

[0061] Step 4: As Figure 5 shown, the Ni metal mask in Step 2 is wet-etched and a metal layer including but not limited to Ti, Au, Ni, Ag is deposited on the surface of the through-hole by sputtering and electroplating to achieve the interconnection between the through-hole and the front side.

[0062] By adjusting different etching conditions, this method can prepare through-holes with high controllability and good quality, improve the quality of the subsequent backside metallization process, and further reduce parameters such as the overall parasitic inductance and transmission impedance of the device, while improving the yield of device preparation and optimizing the overall frequency characteristics of the device. Specifically, in the first etching, a mixed gas of SF6 and O2 is used for plasma etching to make the selectivity ratio of SiC to the Ni mask greater than 40:1 and the selectivity ratio of SiC to GaN greater than 60:1; in the second etching, a mixed gas of Cl2, BCl3, and Ar is used for plasma etching to make the selectivity ratio of GaN to SiC greater than 3:1; in the third etching, a mixed gas of Cl2 and BCl3 is used for plasma etching, and based on the second etching parameters, the proportion of Cl2 is increased to reduce the combination of residual oxygen molecules and surface metal dangling bonds in the first and second etchings, reduce the N vacancies on the surface of the AlGaN layer, decrease the surface roughness, and improve the etching quality.

[0063] Example 3

[0064] Based on Example 2, a high-selectivity GaN HEMT back-hole etching method based on ICP etching is provided. Specifically, electroplating of a Ni metal mask layer is performed on the back-hole pattern formed in Step 1, and a 5-μm mask is deposited. Under the protection of the Ni metal mask formed in Step 2, etching is carried out. The etching process is divided into three stages: In the first stage, the SiC layer is etched. The working gas for etching is SF6 and O2, the working chamber temperature is 20 degrees Celsius, and the etching thickness is 100 μm. In the second stage, the GaN layer is etched. The working gas for etching is changed to Cl2, BCl3, and Ar, and the etching thickness is 2 μm. In the third stage, the AlGaN layer is etched. The working gas for etching is adjusted to Cl2 and BCl3, and the etching thickness is 20 nm. Then, hydrochloric acid and deionized water are used to wet-etch the Ni metal mask in Step 2. By means of sputtering and electroplating, a Ti / Au metal layer with a thickness of 7 μm is deposited on the through-hole surface.

[0065] The above specific embodiments are detailed descriptions of the present invention. It cannot be determined that the specific embodiments of the present invention are only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the concept of the present invention, several simple deductions and substitutions can still be made, which should all be regarded as belonging to the protection scope of the present invention.

Claims

1. A high selectivity GaN HEMT back hole etching method based on ICP etching, which etches the GaN layer and the AlGaN layer in sequence on the basis of etching through the SiC layer, is characterized in that The method includes: Performing a first etching on the SiC layer using first etching parameters, where the first etching is a plasma etching using a mixed gas of SF6 and O2; After the first etching is completed, performing a second etching on the GaN layer using second etching parameters, where the second etching is a plasma etching using a mixed gas of Cl2, BCl3, and Ar; After the second etching is completed, performing a third etching on the AlGaN layer using third etching parameters, where the third etching is a plasma etching using a mixed gas of Cl2 and BCl3, and based on the second etching parameters, increasing the proportion of Cl2 to reduce the combination of residual oxygen molecules and surface metal dangling bonds in the first etching and the second etching, so as to reduce the N vacancies on the surface of the AlGaN layer; The method further includes a pretreatment step before the first etching, and the pretreatment includes back hole area lithography and back mask preparation; The first etching makes the selectivity ratio of SiC to the back mask greater than 40:1, and the selectivity ratio of SiC to GaN greater than 60:1; the second etching makes the selectivity ratio of GaN to SiC greater than 3:

1.

2. The high-selectivity GaN HEMT back-hole etching method based on ICP etching according to claim 1, characterized in that, The first etching parameters are: the gas flow ratio of SF6 and O2 includes 4:1, 5:1, 6:1, 8:1, 16:1; the second etching parameters are: the gas flow ratio of Cl2, BCl3, and Ar includes 20:2:1, 16:2:1, 8:2:1; the third etching parameters are: the gas flow ratio of Cl2 and BCl3 includes 1:1, 2:1, 4:1, 8:

1.

3. A high-selectivity GaN HEMT back-hole etching method based on ICP etching according to claim 2, characterized in that In the first etching parameters, the total gas flow is 20 sccm - 130 sccm, and the working pressure is 0.34 Pa - 1.2 Pa; in the second etching parameters, the total gas flow is 90 sccm - 130 sccm, and the working pressure is 0.6 Pa - 1.3 Pa; in the third etching parameters, the total gas flow is 25 sccm - 100 sccm, and the working pressure is 0.3 Pa - 1.1 Pa.

4. A high-selectivity GaN HEMT back-hole etching method based on ICP etching according to claim 1, characterized in that The etching thickness of the first etching is 80 μm - 200 μm.

5. A high-selectivity GaN HEMT back-hole etching method based on ICP etching according to claim 1, characterized in that, The etching thickness of the second etching is 1 μm - 3 μm.

6. The high-selectivity GaN HEMT back hole etching method based on ICP etching according to claim 1, wherein The etching thickness of the third etching is 10 nm - 200 nm.

7. A high-selectivity GaN HEMT back-hole etching method based on ICP etching according to claim 1, characterized in that, The back hole area lithography includes: Using a photoresist as a back hole position mask and lithographically forming a back hole pattern.

8. A high-selectivity GaN HEMT back-hole etching method based on ICP etching according to claim 7, characterized in that, The back mask preparation includes: Electroplating a Ni metal mask layer on the back hole pattern and depositing a mask layer with a thickness of 2 μm - 5 μm.

9. A method for etching back holes of a high selectivity GaN HEMT based on ICP etching according to claim 1, characterized in that, The method further includes: After the third etching, stripping the back hole metal mask and depositing a metal layer on the through hole surface.

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