Method and apparatus for processing gallium compounds

Abstract

To provide a processing method and device for a gallium compound that can process a gallium compound by plasma etching with high efficiency without using a fluorine-based gas or a chlorine-based gas that have safety and environmental impact problems.SOLUTION: In a processing method of processing a surface of a gallium compound by plasma etching, an oxidizing gas having an action to oxidize metal gallium is added to an etching gas consisting of an inert gas and a hydrogen gas to form a process gas, and plasma is generated to plasma-etch the surface of the gallium compound. For example, the gallium compound is gallium nitride or gallium oxide, and the oxidizing gas is a water vapor or oxygen gas.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for processing gallium compounds and an apparatus therefor, which have both high efficiency and high practicality.

Background Art

[0002] Gallium nitride (GaN) and gallium oxide (Ga2O3) have attracted attention as materials for low-loss power semiconductor devices. However, since GaN has high hardness and Ga2O3 has high cleavage property, high-efficiency processing has been difficult in the prior art, which is one of the factors that increase the device cost.

[0003] Patent Document 1 describes that in the dry etching of a compound film such as GaN, a mixed gas of at least one halogen gas or halide gas selected from Cl2, BCl3, HCl, HCl4 and HBr as an etching gas for plasma generation and a dilution gas is used.

[0004] Patent Document 2 describes a method for manufacturing a substrate having at least a gallium nitride layer on the surface, in which a plasma etching apparatus equipped with an inductively coupled plasma generation apparatus is used, a fluorine-based gas is introduced, and the surface of the gallium nitride layer is dry-etched.

[0005] On the other hand, silicon carbide (SiC), which has also attracted attention as a material for low-loss power semiconductors, is also difficult to process with high efficiency because of its high hardness and chemical stability. However, high-efficiency processing at 15 μm / min has been achieved by plasma etching using a high-radical density plasma using SF⁶ gas (Non-Patent Document 1). Similarly, in the case of GaN, high-efficiency processing using atmospheric pressure plasma has been studied (Non-Patent Document 2). Although a high processing speed of about 9 μm / min was obtained, chlorine gas (diluted with helium) was used as the reaction gas, so it is necessary to consider safety and the corrosiveness of the apparatus, and it has not reached practical use. In addition, regarding Ga₂O₃, sufficient studies on plasma etching have not been conducted. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 5033361 [Patent Document 2] Patent No. 5832058 [Non-patent literature]

[0007] [Non-Patent Document 1] ECS J. Solid State Sci. Technol. 10, 014005 [Non-Patent Document 2] Surf. Interface Anal.40, 1556 [Overview of the project] [Problems that the invention aims to solve]

[0008] Therefore, in view of the above circumstances, the present invention aims to provide a method and apparatus for processing gallium compounds that can process gallium compounds with high efficiency by plasma etching without using fluorine-based gases or chlorine-based gases, which have safety and environmental impact issues. [Means for solving the problem]

[0009] To solve the aforementioned problems, the present invention provides a method and apparatus for processing gallium compounds as described below.

[0010] (1) A processing method for processing the surface of a gallium compound by plasma etching, An etching gas consisting of an inert gas and hydrogen gas is combined with an oxidizing gas that has the effect of oxidizing metallic gallium to create a process gas, and a plasma is generated to plasma etch the surface of a gallium compound. A method for processing gallium compounds.

[0011] (2) The gallium compound is gallium nitride or gallium oxide. The processing method of the gallium compound according to (1).

[0012] (3) The oxidizing gas is water vapor or oxygen gas. The processing method of the gallium compound according to (1) or (2).

[0013] (4) The hydrogen gas concentration is 0.5 to 10% with respect to the inert gas. The processing method of the gallium compound according to (1).

[0014] (5) A processing apparatus for processing the surface of a gallium compound by plasma etching, comprising: A holding table that holds the gallium compound and also serves as a lower electrode; An electrode provided at a predetermined interval with respect to the gallium compound on the holding table;​​​​​​​​​​​​​​​​​​​​​The gas supply system mixes the inert gas supplied from an inert gas cylinder via a mass flow controller and the hydrogen gas supplied from a hydrogen gas cylinder via a mass flow controller to form an etching gas, and passes the etching gas through the water in an airtight water tank to vaporize and mix water vapor as the oxidizing gas to prepare a process gas. The processing apparatus for a gallium compound according to claim 5 or 6.

[0017] (8) The gas supply system mixes the inert gas supplied from an inert gas cylinder via a mass flow controller and the hydrogen gas supplied from a hydrogen gas cylinder via a mass flow controller to form an etching gas, and mixes oxygen gas as the oxidizing gas supplied from an oxygen gas cylinder via a mass flow controller into the etching gas to prepare a process gas. The processing apparatus for a gallium compound according to claim 5 or 6.

[0018] (9) The hydrogen gas concentration is 0.5 to 10% with respect to the inert gas. The processing apparatus for a gallium compound according to (5). [Advantages of the Invention]

[0019] According to the present invention thus constituted, plasma etching of gallium nitride (GaN) and gallium oxide (Ga2O3) can be realized with high efficiency without using chlorine-based gases or fluorine-based gases, and it can be used not only for planarization and smoothing of gallium nitride substrates and gallium oxide substrates, but also by application to processes such as back thinning after power device formation, etc., it is expected to contribute to the realization of low-cost and low-loss power devices. [Brief Description of the Drawings]

[0020] [Figure 1] It is a schematic diagram of the processing apparatus for a gallium compound of the present invention. [Figure 2]The following are specific examples of gas supply systems: (a) shows a supply system for etching gas mixed with He gas and H2 gas; (b) shows a gas supply system in which water vapor is added as an oxidizing gas to etching gas mixed with He gas and H2 gas; and (c) is a piping diagram of a gas supply system in which O2 gas is added as an oxidizing gas to etching gas mixed with He gas and H2 gas. [Figure 3] This is a diagram of the exhaust system piping. [Figure 4] This graph shows the processing rate of GaN under the following processing conditions: He:H2 = 90:10, RF power 130W, processing time 5 minutes, and gas flow rate 500 sccm. [Figure 5] This graph shows the processing rate of Ga2O3 under the following processing conditions: He:H2=90:10, RF power 130W, processing time 3 minutes, and gas flow rate 500 sccm. [Figure 6] This is a scanning electron microscope image of the GaN surface under the following processing conditions: He:H2=90:10, RF power 130W, processing time 5 minutes, and gas flow rate 100 sccm. [Figure 7] This is a scanning electron microscope image of the Ga2O3 surface under the following processing conditions: He:H2=96:4, RF power 130W, processing time 5 minutes, and gas flow rate 500 sccm. [Figure 8] This graph shows the GaN processing rate when the processing conditions are fixed at He:H2=95:5, RF power 180W, and processing time 5 minutes, and the gas flow rate is varied to 100, 500, and 800 sccm. [Figure 9] This graph shows the GaN processing rate when the processing conditions are fixed at RF power of 180W, processing time of 5 minutes, and gas flow rate of 500 sccm, and the hydrogen gas concentration is varied to 1%, 5%, and 10%. [Figure 10] This graph shows the GaN processing rate when the processing conditions are fixed at He:H2=90:10, processing time 5 minutes, and gas flow rate 100 sccm, while the RF power is varied to 130, 150, 180, and 200 W. [Figure 11]This graph shows the GaN processing rate when the processing conditions are fixed at He:H2=90:10, RF power 130W, processing time 5 minutes, and gas flow rate 500 sccm, while the temperature of the holding stage 2 (sample stage) is changed to 30, 50, and 200°C. [Figure 12] This graph shows the processing rate of Ga2O3 when the processing conditions are fixed at He:H2=90:10, RF power 130W, processing time 5 minutes, and gas flow rate 75 sccm, and the temperature of the holding stage 2 (sample stage) is changed to 30, 50, 70, and 100°C. [Figure 13] This is a scanning electron microscope image of the GaN surface under the following processing conditions: He:H2=95:5, RF power 180W, processing time 5 minutes, and gas flow rate 800 sccm. [Figure 14] This is a scanning electron microscope image of the GaN surface under the following processing conditions: process gas is etching gas He:H2=95:5 with added water vapor, RF power 200W, processing time 3 minutes, gas flow rate 800 sccm, and temperature 150°C. [Figure 15] This is a scanning electron microscope image of the Ga2O3 surface under the following processing conditions: process gas is etching gas He:H2=90:10 with added water vapor, RF power 80W, processing time 5 minutes, gas flow rate 500 sccm, and temperature 50°C. [Figure 16] This is a scanning electron microscope image of the GaN surface under the following processing conditions: He:H2:O2=92:4:4, RF power 100W, processing time 3 minutes, gas flow rate 500 sccm, and temperature 80°C. [Figure 17] This is a scanning electron microscope image of the Ga2O3 surface under the following processing conditions: He:H2:O2=92:4:4, RF power 80W, processing time 3 minutes, gas flow rate 500 sccm, and temperature 50°C. [Modes for carrying out the invention]

[0021] Next, the present invention will be described in more detail based on the embodiments shown in the attached drawings. Figure 1 shows a processing apparatus used to demonstrate the processing characteristics of the present invention. The present invention is not limited to the illustrated embodiments, and it is possible to configure a processing apparatus that uniformly plasma etches the entire surface of a workpiece, or a PCVM apparatus that locally plasma etches a part of the surface of a workpiece to create an arbitrary shape. In the present invention, the workpiece is a gallium compound such as gallium nitride (GaN) or gallium oxide (Ga2O3). In this embodiment, atmospheric pressure plasma is used, but reduced pressure plasma may also be used. Here, "atmospheric pressure" means a range of pressure slightly higher than atmospheric pressure because it is necessary to generate the flow of process gas. In other words, the pressure of the process gas in the present invention is in the range of a few kPa to atmospheric pressure.

[0022] In this invention, hydrogen gas is used as the reaction gas in the plasma etching gas for gallium compounds. Incidentally, when a fluorine-based gas is used as the reaction product of the gallium compound, GaF3 (boiling point 1000°C) is produced, when a chlorine-based gas is used, GaCl3 (boiling point 201°C) is produced, and when hydrogen gas is used, Ga2H6 (boiling point -50°C) is produced. Thus, using hydrogen gas is advantageous because the reaction product volatilizes without heating the gallium compound, but a new problem arises as metallic gallium adheres to the surface, as will be described later.

[0023] The processing apparatus shown in Figure 1 comprises a holding base 2 that holds a gallium compound 1 and also serves as a lower electrode, an electrode 3 provided at a predetermined distance from the gallium compound 1 on the holding base 2, a high-frequency power supply 4 that generates a process gas plasma by applying a high-frequency voltage between the holding base 2 and the electrode 3, a gas supply system 5 that supplies a process gas to at least the space between the gallium compound 1 and the electrode 3, which is an etching gas consisting of an inert gas and hydrogen gas to which an oxidizing gas that has the effect of oxidizing metallic gallium is added, and an exhaust system 6 that exhausts the process gas.

[0024] The processing apparatus of this embodiment is equipped with a temperature control system combining a heater and an appropriate cooling device, and a thermometer 7 consisting of a thermocouple, on the holding base 2. The upper surface of the holding base 2 is equipped with an XY stage that can hold or scan the substrate-shaped gallium compound 1. A conductive upper plate 9 is held by the holding base 2 via an insulating cylindrical body 8. A part of the upper plate 9 protrudes downward to form a pipe-shaped electrode 3. Part or all of the cylindrical body 8 is made of a translucent glass material so that the inside can be observed.

[0025] The chamber is formed by the holder 2, the cylindrical body 8, and the upper plate 9. Process gas from the gas supply system 5 is ejected from the central hole 11 of the electrode 3 through the supply passage 10 inside the upper plate 9, while process gas in the chamber is exhausted from the exhaust system 6 through the exhaust passage 12 inside the holder 2. The holder 2 is grounded, and a high-frequency voltage is applied to the upper plate 9 from the high-frequency power supply 4, generating high-frequency plasma in the space between the electrode 3 and the gallium compound 1. In this invention, the structure of the electrode 3 and the method of plasma generation are not particularly limited.

[0026] Chlorine-based and fluorine-based gases, which have been conventionally used as etching gases, offer high processing rates but have the disadvantage of high environmental impact, resulting in costly handling and recovery processes. Therefore, in this invention, we considered using an etching gas prepared by diluting hydrogen gas with an inert gas. In this embodiment, He was used as the inert gas, but Ar may also be used. A specific example of the gas supply system 5 is shown in Figure 2, and a specific example of the exhaust system 6 is shown in Figure 3. As shown in Figure 2(a), the etching gas is prepared by mixing helium gas supplied from a helium gas cylinder 13 via a mass flow controller 14 with hydrogen gas supplied from a hydrogen gas cylinder 15 via a mass flow controller 16. As shown in Figure 3, the exhaust system 6 consists of a vacuum pump 17 and an abatement cylinder 18 for initial exhaust of the chamber. When the process gas is at atmospheric pressure, two valves 19 and 20 are used to switch the exhaust flow path so that the gas can be exhausted directly into the atmosphere through the abatement cylinder 18 without going through the vacuum pump 17. Reference numeral 21 in the figure indicates a pressure gauge.

[0027] We investigated the possibility of using hydrogen gas (diluted with helium) as a reaction gas and processing small pieces of GaN and Ga2O3 using the processing apparatus shown in Figure 1. As a result, we found that high processing speeds could be obtained for GaN at 4 μm / min (processing conditions: He:H2=90:10, RF power 130W, processing time 5 minutes, gas flow rate 500 sccm) as shown in Figure 4, and for Ga2O3 at 60 μm / min (processing conditions: He:H2=90:10, RF power 130W, processing time 3 minutes, gas flow rate 500 sccm) as shown in Figure 5. However, the sample surface, which had been polished to a mirror finish before processing, became cloudy after processing, and as shown in Figures 6 and 7, both GaN and Ga2O3 were covered with spherical deposits. The processing conditions were as follows: for GaN, He:H2=90:10, RF power 130W, processing time 5 minutes, gas flow rate 100 sccm; and for Ga2O3, He:H2=96:4, RF power 130W, processing time 5 minutes, gas flow rate 500 sccm. Analysis of the deposits revealed that they were metallic Ga. It is thought that the metallic Ga formed by surface reduction and re-deposition of reaction products remained on the surface after processing because its etching rate by hydrogen radicals was lower than that of GaN and Ga2O3.

[0028] Next, for GaN, the processing rate was evaluated by varying the gas flow rate and hydrogen gas concentration using the etching gas described above. Figure 8 is a graph showing the processing rate when the processing conditions were fixed at He:H2=95:5, RF power 180W, and processing time 5 minutes, and the gas flow rate was varied to 100, 500, and 800 sccm. It can be seen that the processing rate increases with increasing gas flow rate. This can be understood as an increase in the amount of H2 receiving energy from the plasma, leading to an increase in hydrogen radicals. Figure 9 is a graph showing the processing rate when the processing conditions were fixed at RF power 180W, processing time 5 minutes, and gas flow rate 500 sccm, and the hydrogen gas concentration was varied to 1%, 5%, and 10%. It can be seen that the processing rate is maximized when the hydrogen concentration relative to helium is 5%. It is thought that when the hydrogen gas concentration is low, there are insufficient hydrogen radicals to contribute to the reaction, resulting in a low processing rate, and conversely, when the hydrogen gas concentration is high, the energy in the plasma decreases, leading to a decrease in the processing rate. Furthermore, even if the hydrogen gas concentration is low, the processing rate can be increased by increasing the gas flow rate, so a hydrogen gas concentration in the range of 0.5 to 10% is preferable. In practical terms, a hydrogen gas concentration in the range of 2 to 10% is more preferable, and even more preferable is in the range of 3 to 7%.

[0029] Furthermore, for GaN, the processing rate was evaluated by varying the RF power and the temperature of the holder 2 (sample stage) using the etching gas described above. Figure 10 is a graph showing the processing rate when the processing conditions were fixed at He:H2=90:10, processing time of 5 minutes, and gas flow rate of 100 sccm, and the RF power was varied to 130, 150, 180, and 200 W. It can be seen that the processing rate increases with increasing power. This is presumed to be because the number of hydrogen radicals increases with increasing power, as well as the temperature of the GaN substrate rises. Figure 11 is a graph showing the processing rate when the processing conditions were fixed at He:H2=90:10, RF power of 130 W, processing time of 5 minutes, and gas flow rate of 500 sccm, and the temperature of the holder 2 (sample stage) was varied to 30, 50, and 200°C. The processing rate increases with increasing temperature, but shows a tendency to saturate.

[0030] For comparison, Figure 12 shows a graph of the processing rate for Ga2O3, where the processing conditions were fixed at He:H2=90:10, RF power 130W, processing time 5 minutes, and gas flow rate 75 sccm, and the temperature of the holding stage 2 (sample stage) was varied to 30, 50, 70, and 100°C. The processing rate is maximum at 50°C, but it can be seen that the dependence on temperature is small.

[0031] As mentioned earlier, plasma etching using an etching gas composed of hydrogen gas has been shown to efficiently process gallium compounds. However, the problem of metallic Ga particles adhering to the surface of the gallium compound after etching remains unresolved. Therefore, we investigated what happens to metallic Ga when the gas flow rate is increased for GaN. Figure 13 shows a scanning electron microscope image of the GaN surface under processing conditions of He:H2=95:5, RF power 180W, processing time 5 minutes, and gas flow rate 800 sccm. Compared to Figure 6, although the processing conditions are slightly different, it can be seen that increasing the gas flow rate reduces the particle size of metallic Ga to about 1 / 10, but it does not disappear. Furthermore, even after raising the substrate in the state shown in Figure 6 to a temperature higher than the melting point of Ga (29.76°C) and wiping off the deposits, complete removal was not possible.

[0032] Therefore, this invention proposes adding an oxidizing gas to an etching gas consisting of hydrogen gas to oxidize the metal Ga so that it is easily etched by hydrogen radicals. Specific examples of the oxidizing gas from a practical standpoint include water vapor (H2O molecules) or oxygen gas (O2 molecules). The addition of the oxidizing gas is expected to oxidize the metal Ga, and vaporization is expected to be promoted through reaction with hydrogen radicals.

[0033] The present invention relates to a processing method for processing the surface of a gallium compound by plasma etching, wherein an oxidizing gas that has the effect of oxidizing metallic gallium is added to an etching gas consisting of an inert gas and hydrogen gas to form a process gas, and plasma is generated to process the surface of the gallium compound by plasma etching. In this invention, the oxidizing gas is water vapor or oxygen gas.

[0034] Figure 2(b) shows a gas supply system 5 that adds water vapor as an oxidizing gas to the etching gas mentioned above. This gas supply system 5 is structured to prepare a process gas by mixing helium gas supplied from a helium gas cylinder 13 via a mass flow controller 14 with hydrogen gas supplied from a hydrogen gas cylinder 15 via a mass flow controller 16 to form an etching gas, and then passing the etching gas through the water in an airtight water tank 22 to vaporize and mix it with water vapor.

[0035] Figures 14 and 15 show the results of plasma etching of a gallium nitride substrate and a gallium oxide substrate using a process gas prepared with the gas supply system 5 shown in Figure 2(b) and supplemented with water vapor. Figure 14 is a scanning electron microscope image of the GaN surface under the following processing conditions: process gas He:H2=95:5 with added water vapor, RF power 200W, processing time 3 minutes, gas flow rate 800 sccm, and temperature 150°C. Figure 15 is a scanning electron microscope image of the Ga2O3 surface under the following processing conditions: process gas He:H2=90:10 with added water vapor, RF power 80W, processing time 5 minutes, gas flow rate 500 sccm, and temperature 50°C. These results show that by plasma etching the surface of a gallium compound using a process gas containing water vapor, a clean processed surface without metallic Ga deposition could be achieved.

[0036] Figure 2(c) shows a gas supply system 5 that adds oxygen gas as an oxidizing gas to the etching gas mentioned above. This gas supply system 5 is structured to prepare a process gas by mixing helium gas supplied from a helium gas cylinder 13 via a mass flow controller 14 with hydrogen gas supplied from a hydrogen gas cylinder 15 via a mass flow controller 16 to form an etching gas, and then mixing this etching gas with oxygen gas supplied from an oxygen gas cylinder 23 via a mass flow controller 24.

[0037] Figures 16 and 17 show the results of plasma etching of gallium nitride substrates and gallium oxide substrates using a process gas prepared with the gas supply system 5 shown in Figure 2(c) and supplemented with oxygen gas. Figure 16 is a scanning electron microscope image of the GaN surface under the following processing conditions: He:H2:O2=92:4:4, RF power 100W, processing time 3 minutes, gas flow rate 500 sccm, and temperature 80°C. Figure 17 is a scanning electron microscope image of the Ga2O3 surface under the following processing conditions: He:H2:O2=92:4:4, RF power 80W, processing time 3 minutes, gas flow rate 500 sccm, and temperature 50°C. These results show that even when using a process gas containing oxygen gas, plasma etching of the gallium compound surface resulted in a clean processed surface without metallic Ga deposition.

[0038] As shown in Figures 14 and 15, when plasma etching of a gallium compound was performed using a process gas consisting of He / H2 / H2O, no spherical deposits were observed on the processed surface, and a glossy processed surface was obtained. Similar effects were observed when oxygen gas was added instead of H2O, suggesting that the metallic Ga formed on the surface during processing was rapidly oxidized by oxygen radicals in the plasma and then etched by hydrogen radicals. [Explanation of Symbols]

[0039] 1. Gallium compounds 2 Holding stand 3 electrodes 4 High frequency power supply 5. Gas supply system 6. Exhaust System 7 Thermometer 8 cylinder 9 Top plate 10 Supply route 11 Center hole 12 Exhaust passage 13 Helium gas cylinders 14 Mass Flow Controller 15 Hydrogen gas cylinder 16 Mass Flow Controller 17 Vacuum pump 18 Harm removal tube 19 valves 20 valves 21 Pressure gauge 22 Airtight water tank 23 Oxygen gas cylinder 24 Mass Flow Controller

Claims

1. A processing method for processing the surface of a gallium compound by plasma etching, An etching gas consisting of an inert gas and hydrogen gas is combined with an oxidizing gas that has the effect of oxidizing metallic gallium to create a process gas, and a plasma is generated to plasma etch the surface of a gallium compound. A method for processing gallium compounds.

2. The gallium compound is gallium nitride or gallium oxide. A method for processing a gallium compound according to claim 1.

3. The oxidizing gas is water vapor or oxygen gas. A method for processing a gallium compound according to claim 1 or 2.

4. The hydrogen gas concentration relative to the aforementioned inert gas is 0.5 to 10%. A method for processing a gallium compound according to claim 1.

5. A processing apparatus for processing the surface of a gallium compound by plasma etching, A holding base that holds the gallium compound and also serves as the lower electrode, Electrodes are provided at predetermined intervals relative to the gallium compound on the aforementioned holding base, A high-frequency power supply that generates a process gas plasma by applying a high-frequency voltage between the holding base and the electrode, A gas supply system provides at least the space between the gallium compound and the electrode with a process gas, which is an etching gas consisting of an inert gas and hydrogen gas to which an oxidizing gas that oxidizes metallic gallium is added. An exhaust system for exhausting the aforementioned process gas, A processing apparatus for gallium compounds composed of the following.

6. The gallium compound is gallium nitride or gallium oxide. The apparatus for processing gallium compounds according to claim 5.

7. The gas supply system has a structure in which an inert gas supplied from an inert gas cylinder via a mass flow controller is mixed with hydrogen gas supplied from a hydrogen gas cylinder via a mass flow controller to form an etching gas, and this etching gas is passed through water in an airtight water tank to vaporize and mix with water vapor as the oxidizing gas to prepare a process gas. The apparatus for processing gallium compounds according to claim 5 or 6.

8. The gas supply system has a structure that mixes inert gas supplied from an inert gas cylinder via a mass flow controller with hydrogen gas supplied from a hydrogen gas cylinder via a mass flow controller to form an etching gas, and then mixes this etching gas with oxygen gas supplied from an oxygen gas cylinder via a mass flow controller as the oxidizing gas to prepare a process gas. The apparatus for processing gallium compounds according to claim 5 or 6.

9. The hydrogen gas concentration relative to the aforementioned inert gas is 0.5 to 10%. The apparatus for processing gallium compounds according to claim 5.

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