Method and device for removing parasitic polycrystals on HVPE-GaN substrate carrier

Abstract

The invention belongs to the technical field of gallium nitride single crystal growth, and particularly relates to a method and device for removing parasitic polycrystals on an HVPE-GaN substrate carrier. The method comprises the following steps: placing the substrate carrier in a reaction furnace, firstly introducing inert gas, then introducing hydrogen, raising the temperature to 1000-1050 DEG C for reaction, and removing parasitic polycrystals on the substrate carrier. The method has remarkable advantages, can utilize thermal chemical reaction, accurately control temperature, gas components and treatment time, can efficiently and completely remove parasitic polycrystals, avoids damage to the substrate carrier, eliminates the problem of environmental pollution from the source, can inhibit secondary deposition of polycrystals, is suitable for substrate carriers made of various materials and having different sizes, and has wide application prospects. The quality and efficiency of gallium nitride single crystal growth are greatly improved, and the production cost is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of gallium nitride single crystal growth, and particularly relates to a method and device for removing parasitic polycrystals on a HVPE-GaN substrate carrier. BACKGROUND

[0002] Gallium nitride (GaN) is a representative of the third generation of semiconductor materials, and is widely used in optoelectronics, power electronics and other fields due to its wide band gap (3.4 eV), high electron mobility and excellent thermal conductivity. At present, hydride vapor phase epitaxy (HVPE) is the mainstream technology for growing gallium nitride single crystals, which is usually carried out in a substrate carrier at normal pressure and about 1000 DEG C. However, during the growth process, gallium nitride polycrystalline particles are generated on the surface of the graphite substrate carrier due to the side reaction of the reaction gas (such as NH3 and GaCl). These parasitic polycrystals not only reduce the quality of the single crystal, but also cause the substrate carrier to break due to thermal stress accumulation, greatly increasing the production cost.

[0003] In the prior art, polycrystal removal mainly depends on mechanical scraping or acid washing, but the former easily damages the surface of the substrate carrier, and the latter has the risk of environmental pollution. Therefore, there is an urgent need for an efficient, environmentally friendly and non-destructive removal method. SUMMARY

[0004] In view of the deficiencies of the prior art, the application provides a method and device for removing parasitic polycrystals on a substrate carrier for HVPE growth of gallium nitride single crystals to solve the problem of parasitic polycrystals on the graphite substrate carrier.

[0005] A method for removing parasitic polycrystals on a HVPE-GaN substrate carrier, the substrate carrier is placed in a reaction furnace, first inert gas is introduced, then hydrogen gas is introduced, and then heated to 1000-1050 DEG C for reaction to remove the parasitic polycrystals on the substrate carrier.

[0006] Preferably, the reaction furnace is a high-temperature tube furnace.

[0007] Preferably, the reaction furnace is preheated to 500-600 DEG C and maintained for 15-20 minutes to remove water vapor and impurities that may exist in the furnace chamber.

[0008] Preferably, the hydrogen gas flow is 10-15 L / min.

[0009] Preferably, the inert gas flow is 5-8 L / min.

[0010] Preferably, the inert gas is introduced for 3-5 minutes, and then hydrogen gas is introduced.

[0011] Preferably, the heating rate is 5-8 DEG C / min.

[0012] Preferably, the reaction time is 3-6 h.

[0013] Preferably, after the reaction is completed, the furnace cavity of the reaction furnace is naturally cooled, and the cooling rate is controlled at 3-5 ℃ / min; when the temperature of the furnace cavity is reduced to 300-350 ℃, the inert gas and hydrogen gas are stopped; when the temperature is reduced to 50-100 ℃, the substrate carrier is taken out of the reaction furnace.

[0014] A device for removing parasitic polycrystal on HVPE-GaN substrate carrier, comprising a control assembly, a furnace body, a gas inlet assembly and a tail gas discharge pipe; the control assembly is electrically connected with the gas inlet assembly, the gas inlet assembly is connected with the furnace body through a pipeline; the tail gas discharge pipe is arranged on the furnace body; the furnace body comprises, from inside to outside, a heat preservation layer, a resistance heating body heating layer and a furnace cavity; the front end of the furnace cavity is provided with a furnace cavity door; a machine table and a large graphite frame are arranged at the bottom of the furnace cavity, a machine table graphite frame is arranged on the machine table, the gas inlet assembly comprises an inert gas bottle connected with a first gas inlet pipe and a hydrogen gas bottle connected with a second gas inlet pipe, and the first gas inlet pipe and the second gas inlet pipe are both connected with the rear end of the furnace cavity through a pipeline. Preferably, the tail gas discharge pipe is connected with the rear end of the furnace cavity.

[0015] Preferably, the first gas inlet pipe, the second gas inlet pipe and the tail gas discharge pipe are all made of aluminum alloy.

[0016] The machine table graphite frame and the large graphite frame are both used for processing polycrystal on graphite substrate carriers, quartz materials or silicon carbide materials of different sizes.

[0017] The furnace cavity is heated by a resistance heating body, and the target temperature range is 1000-1050 ℃, and a heat preservation layer is arranged around the furnace cavity to maintain the temperature of the furnace cavity.

[0018] In the present application, the large graphite frame is mainly used for removing polycrystal on large devices such as quartz cavities, and the machine table graphite frame is used for removing polycrystal on small devices such as cover and carrier disc substrate carriers. Since the large graphite frame is close to the gas inlet pipe at the tail of the device, the gas concentration is higher, and when a device with polycrystal and large volume is placed, the contact between the carrier and the gas is more uniform, which can significantly improve the removal rate. The machine table graphite frame is used for placing substrate carriers (small devices) covered with polycrystal, and does not need too high gas concentration, and too high concentration will damage the furnace cavity wall and the surface of the carrier.

[0019] The present application uses a graphite frame to place devices used for HVPE-GaN growth, which can avoid the cracking of the device caused by the large thermal capacity difference between the substrate carrier and other materials.

[0020] Advantages The traditional mechanical scraping and pickling method has obvious disadvantages in removing the parasitic polycrystal on the substrate carrier for growing gallium nitride single crystal by HVPE. The mechanical scraping can damage the surface of the substrate carrier and affect its service life. The pickling method can remove the polycrystal, but has the risk of environmental pollution. The removal device and working method of the present application have obvious advantages. The device uses a thermal chemical reaction to accurately control the temperature, gas composition and processing time, can efficiently and completely remove the parasitic polycrystal, avoid damage to the substrate carrier, eliminate the environmental pollution problem from the source, and can also inhibit the secondary deposition of the polycrystal. The device is suitable for various materials and different size substrate carriers, greatly improves the quality and efficiency of gallium nitride single crystal growth, and reduces the production cost. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 A high-temperature heating tube furnace for removing the parasitic polycrystal on the substrate carrier of the present application is shown in the figure; Figure 2 An optical microscope picture of a graphite substrate carrier without removing the parasitic polycrystal in Example 2 of the present application is shown in the figure; Figure 3 An optical microscope picture of a graphite substrate carrier with removing the parasitic polycrystal in Example 2 of the present application is shown in the figure; Figure 4 An optical microscope picture of a silicon carbide substrate carrier without removing the parasitic polycrystal in Example 3 of the present application is shown in the figure; Figure 5 An optical microscope picture of a silicon carbide substrate carrier with removing the parasitic polycrystal in Example 3 of the present application is shown in the figure; Figure 6 An optical microscope picture of a graphite substrate carrier without removing the parasitic polycrystal in Comparative Example 1 of the present application is shown in the figure; Figure 7 An optical microscope picture of a graphite substrate carrier with removing the parasitic polycrystal in Comparative Example 1 of the present application is shown in the figure; Figure 8 An optical microscope picture of a graphite substrate carrier without removing the parasitic polycrystal in Comparative Example 2 of the present application is shown in the figure; Figure 9 An optical microscope picture of a graphite substrate carrier with removing the parasitic polycrystal in Comparative Example 2 of the present application is shown in the figure; 1, control assembly, 2, furnace body, 3, gas inlet assembly, 4, tail gas discharge pipe, 21, insulation layer, 22, resistance heating body heating layer, 23, furnace cavity, 231, furnace cavity door, 24, machine table, 25, large graphite frame, 26, machine table graphite frame, 31, first gas inlet pipe, 32, inert gas bottle, 33, second gas inlet pipe, 34, hydrogen gas bottle. DETAILED DESCRIPTION

[0022] The present application aims at the problem that the parasitic polycrystal on the HVPE substrate carrier is difficult to remove efficiently, and provides a removal method based on thermal chemical reaction, which realizes complete removal of the polycrystal and inhibits its secondary deposition by precisely controlling temperature, gas composition and processing time, thereby prolonging the service life of the substrate carrier.

[0023] Embodiment 1 A device for removing parasitic polycrystal on a substrate carrier for HVPE growth of gallium nitride single crystal is a high-temperature heating tube furnace, comprising a control assembly 1, a furnace body 2, a gas inlet assembly 3 and a tail gas discharge pipe 4; the control assembly 1 is electrically connected with the gas inlet assembly 3, the gas inlet assembly 3 is connected with the furnace body 2 through a pipeline, the tail gas discharge pipe 4 is arranged on the furnace body 2; the furnace body 2 comprises, from inside to outside, a heat preservation layer 21, a resistance heating body heating layer 22 and a furnace cavity 23; the furnace cavity 23 is provided with a furnace cavity door 231 at the front end, the furnace cavity 23 is provided with a machine table 24 and a large graphite rack 25 at the bottom, the machine table 24 is provided with a machine table graphite rack 26, the gas inlet assembly 3 comprises an inert gas bottle 32 connected with a first gas inlet pipe 31 and a hydrogen gas bottle 34 connected with a second gas inlet pipe 33, the first gas inlet pipe 31 and the second gas inlet pipe 33 are both connected with the rear end of the furnace cavity 23 through a pipeline. The tail gas discharge pipe 4 is connected with the rear end of the furnace cavity 23.

[0024] The first gas inlet pipe 31, the second gas inlet pipe 33 and the tail gas discharge pipe 4 are all made of aluminum alloy.

[0025] A method for removing polycrystal using the above device for removing parasitic polycrystal on a substrate carrier for HVPE growth of gallium nitride single crystal comprises the following steps: (1) Ensure that the resistance heating body heating layer 22, the gas inlet assembly 3, the tail gas discharge pipe 4 and the like are undamaged, and that the furnace cavity door 231 is well sealed. Connect the inert gas bottle 32 and the hydrogen gas bottle 34 to the corresponding first gas inlet pipe 31 and second gas inlet pipe 33 respectively, and check whether the gas delivery pipeline is tightly connected and free of leakage. Turn on the power supply and the related control assembly 1 of the device, preheat the furnace cavity 23, slowly raise the temperature of the furnace cavity 23 to 500-600℃, and maintain for 15-20 minutes to remove the water vapor and impurities possibly existing in the furnace cavity 23.

[0026] (2) After the furnace cavity 23 is preheated and the temperature is lowered by 50-100℃, open the furnace cavity door 231, and carefully place the substrate carrier with polycrystal distributed after HVPE growth on the large graphite rack 25 or the machine table graphite rack 26 (select the appropriate graphite rack according to the size of the substrate carrier), and ensure that the substrate carrier is stably placed to avoid shaking or displacement during the processing. After the substrate carrier is placed, close the furnace cavity door 231 to ensure that the furnace cavity 23 is well sealed to prevent gas leakage.

[0027] (3) Set the flow rate of the inert gas (N2or Ar) introduced through the first gas inlet pipe 31 to be 5-8 L / min and the flow rate of the hydrogen gas (H2) introduced through the second gas inlet pipe 33 to be 10-15 L / min by controlling the components 1. First, open the valve of the first gas inlet pipe 31 to fill the furnace cavity 23 with the inert gas and to expel the air in the furnace cavity 23 to avoid the reaction between the impurities in the air and the reaction gas and to affect the treatment effect. After continuously introducing the inert gas for 3-5 minutes, slowly open the valve of the second gas inlet pipe 33 to mix the hydrogen gas and the inert gas uniformly.

[0028] (4) Turn on the resistance heating body heating layer 22 and increase the temperature of the furnace cavity 23 from the current temperature to the target temperature interval of 1000-1050℃ at a temperature increasing rate of 5-8℃ / min. During the temperature increasing process, closely observe the temperature change of the furnace cavity 23 to ensure the stability of the temperature increasing rate and to avoid the temperature fluctuation affecting the treatment effect. When the temperature of the furnace cavity 23 reaches the target temperature, keep the temperature constant for 4-6 hours. During this period, the hydrogen gas and the gallium nitride polycrystal have a thermal chemical reaction to convert the gallium nitride polycrystal into gaseous substances which are discharged from the furnace cavity 23 through the tail gas discharge pipe 4, thereby realizing the removal of the parasitic polycrystal. At the same time, the inert gas plays a role of protective atmosphere to prevent the substrate carrier from being oxidized at high temperature and to help maintain the stability of the air pressure in the furnace cavity 23 and to promote the smooth discharge of the gaseous reaction products.

[0029] (5) After the reaction time ends, turn off the resistance heating body heating layer 22 and stop heating. Let the furnace cavity 23 naturally cool down at a cooling rate of 3-5℃ / min. When the temperature of the furnace cavity 23 drops to 300-350℃, close the valves of the first gas inlet pipe 31 and the second gas inlet pipe 33 and stop introducing the reaction gas. Continue to wait until the temperature of the furnace cavity 23 drops to 50-100℃, open the furnace cavity door 231 and carefully take out the treated substrate carrier using a high-temperature-resistant tool.

[0030] (6) After the substrate carrier is taken out, clean the furnace cavity 23 and remove the impurities and reaction products remaining in the furnace cavity 23 using a graphite brush or other suitable tools.

[0031] Example 2 The polycrystal removal treatment method of the graphite carrier includes the following steps: Select a graphite carrier which has been used for many times and has a serious surface parasitic polycrystal. Observe the surface of the graphite carrier using an optical microscope and the result is shown in FIG. 2. The surface polycrystal coverage rate is about 35% according to the preliminary detection. Figure 2 Place the graphite carrier on the machine graphite frame 26, adjust the position to make the carrier be at the center position of the furnace cavity 23 to ensure uniform heating.

[0032] 1. Set the parameters and start heating The flow rate of hydrogen introduced through the first inlet pipe 31 is set to 12 L / min, and the flow rate of nitrogen introduced through the second inlet pipe 33 is set to 8 L / min, with a hydrogen to nitrogen volume ratio of 3:2.

[0033] The target temperature of the high-temperature furnace chamber 23 is set to 1020℃, and the resistance heating element heating layer 22 is started to heat up at a rate of 8℃ / min. At the same time, the air inlet assembly 3 is turned on to allow hydrogen and nitrogen to enter the furnace chamber 23 at the set flow rate.

[0034] 2. Processing procedure Once the temperature inside furnace chamber 23 reaches 1020℃, maintain this temperature for 3 hours. During this period, record the temperature, pressure, and gas flow rate data inside furnace chamber 23 every 15 minutes to ensure that all parameters remain stable. Observation of the inside of furnace chamber 23 through the observation window revealed no abnormalities.

[0035] 3. Cooling and removal After processing, turn off the heating power and the air inlet assembly 3, and allow the high-temperature furnace to cool naturally at a rate of 3°C / min. When the temperature of the furnace chamber 23 drops to 300°C, stop the introduction of inert gas and hydrogen. When the temperature inside the furnace chamber 23 drops below 80°C, open the furnace door 231 and remove the graphite carrier.

[0036] Detection and Analysis The surface of the graphite substrate was observed using an optical microscope, and the results are as follows: Figure 3 As shown, parasitic polycrystalline material was significantly reduced. After treatment, the polycrystalline coverage on the substrate carrier surface decreased to approximately 1.5%.

[0037] Example 3 A method for removing polycrystalline silicon from a silicon carbide substrate includes the following steps: A silicon carbide substrate was selected as the carrier, and the results were observed using an optical microscope. Figure 4 As shown, its surface polycrystalline coverage was measured to be approximately 32%. The silicon carbide substrate carrier was placed on a suitable graphite holder, ensuring good contact between the substrate carrier and the graphite holder to facilitate heat transfer.

[0038] 1. Set parameters and start heating The flow rate of hydrogen introduced through the first intake pipe 31 is set to 12 L / min, and the flow rate of nitrogen introduced through the second intake pipe 33 is set to 8 L / min. The volume ratio of hydrogen to nitrogen is also 3:2.

[0039] Set the target temperature of the high-temperature furnace to 1020℃, start the heating device, and control the heating rate at 8℃ / min. At the same time, turn on the gas supply system.

[0040] 2. Processing procedure After the temperature in the furnace chamber 23 reached 1020°C, the temperature was maintained for 3 hours. The temperature, pressure and gas flow data in the furnace chamber 23 were recorded every 15 minutes to ensure that the parameters were stable.

[0041] The gas composition in the furnace chamber 23 was monitored in real time by the control assembly 1 to ensure that the reaction environment was stable.

[0042] 3. Cooling and removal After the treatment was completed, the heating and gas supply were stopped, and the high-temperature furnace was allowed to cool at a rate of 3°C / min. When the temperature in the furnace chamber 23 dropped to 300°C, the inert gas and hydrogen supply were stopped. When the temperature dropped to 80°C, the furnace chamber door 231 was opened, and the silicon carbide substrate carrier was removed.

[0043] Detection and analysis The surface of the removed silicon carbide substrate carrier was observed using an optical microscope, and it was found that the parasitic polycrystals were significantly reduced. The polycrystal coverage on the surface of the substrate carrier after treatment was reduced to about 1.5%. Figure 5

[0044] Comparative Example 1 The polycrystal removal treatment method for the graphite carrier included the following steps: A graphite carrier that had been used multiple times and had a serious surface parasitic polycrystal problem was selected. The surface of the graphite carrier was observed using an optical microscope, and the results are shown in Figure 2 The surface polycrystal coverage was about 35% after preliminary detection. The graphite carrier was placed on the machine graphite holder 26, and the position was adjusted so that the carrier was in the center of the furnace chamber 23 to ensure uniform heating.

[0045] 1. Set parameters and start heating The flow rate of hydrogen gas through the first gas inlet pipe 31 was set to 12 L / min, and the flow rate of nitrogen gas through the second gas inlet pipe 33 was set to 8 L / min. The volume ratio of hydrogen gas to nitrogen gas was 3:2.

[0046] The target temperature of the high-temperature furnace chamber 23 was set to 950°C, and the electric resistance heating body heating layer 22 was started to heat up at a rate of 8°C / min. At the same time, the gas inlet assembly 3 was turned on, and hydrogen and nitrogen were supplied to the furnace chamber 23 at the set flow rates.

[0047] 2. Treatment process When the temperature in the furnace chamber 23 reached 950°C, the temperature was maintained for 3 hours. During this period, the temperature, pressure and gas flow data in the furnace chamber 23 were recorded every 15 minutes to ensure that the parameters were stable. The furnace chamber 23 was observed through the observation window, and no abnormal phenomena were found.

[0048] 3. Cooling and removal ​After the treatment, the heating power and the gas inlet assembly 3 are turned off, and the high-temperature furnace is naturally cooled. When the temperature in the furnace chamber 23 is below 80°C, the furnace chamber door 231 is opened, and the graphite carrier is taken out.

[0049] Detection and analysis The surface of the graphite substrate carrier is observed using an optical microscope, and the results are shown in Figure 7 The polycrystal coverage of the surface of the substrate carrier after the treatment is about 15%, and although the polycrystals are removed, the surface is still covered with polycrystal particles. Therefore, the polycrystal coverage is significantly higher than 1.5% in Example 2.

[0050] Comparative Example 2 The polycrystal removal treatment method of the graphite carrier includes the following steps: A graphite carrier that has been used for multiple times and has a serious surface parasitic polycrystal is selected. The surface of the graphite carrier is observed using an optical microscope, and the results are shown in Figure 8 The polycrystal coverage of the surface of the graphite carrier is about 35% according to the preliminary detection. The graphite carrier is placed on the graphite rack 26 of the machine, and the position is adjusted so that the carrier is in the center of the furnace chamber 23 to ensure uniform heating.

[0051] 1. Setting parameters and starting heating The flow rate of hydrogen gas introduced through the first gas inlet pipe 31 is set to 12 L / min, and the flow rate of nitrogen gas introduced through the second gas inlet pipe 33 is set to 8 L / min. The volume ratio of hydrogen gas to nitrogen gas is 3:2.

[0052] The target temperature of the high-temperature furnace chamber 23 is set to 1020°C, and the heating layer 22 of the resistance heating body is started to heat up at a rate of 15°C / min. At the same time, the gas inlet assembly 3 is turned on, and hydrogen gas and nitrogen gas are introduced into the furnace chamber 23 according to the set flow rates.

[0053] 2. Treatment process When the temperature in the furnace chamber 23 reaches 1020°C, the temperature is maintained for 4 hours. During this period, the temperature, pressure, and gas flow rate data in the furnace chamber 23 are recorded every 15 minutes to ensure that the parameters are stable. Through the observation window, no abnormal phenomena are found in the furnace chamber 23.

[0054] 3. Cooling and taking out After the treatment, the heating power and the gas inlet assembly 3 are turned off, and the high-temperature furnace is naturally cooled at a rate of 3°C / min. When the temperature in the furnace chamber 23 is below 300°C, the introduction of inert gas and hydrogen gas is stopped. When the temperature in the furnace chamber 23 is below 80°C, the furnace chamber door 231 is opened, and the graphite carrier is taken out.

[0055] Detection and analysis The surface of the graphite substrate carrier is observed using an optical microscope, and the results are shown inFigure 9 As shown, the polycrystal coverage on the surface of the treated substrate carrier is about 5%, and the polycrystal has been basically removed, but there are obvious cracks on the surface of the carrier due to local thermal stress.

[0056] The application will be further described below in conjunction with specific embodiments. The advantages and features of the application will be more clearly apparent in the description. However, the embodiments are only exemplary and do not constitute any limitation on the scope of the application. Those skilled in the art should understand that the details and forms of the technical solutions can be modified or replaced without departing from the spirit and scope of the application, and these modifications and replacements all belong to the protection scope of the application.

Claims

1. A method for removing on-carrier parasitic poly of HVPE-GaN substrates, characterized in that, The substrate carrier is placed in the reaction furnace, inert gas is introduced first, then hydrogen is introduced, and then the temperature is raised to 1000-1050℃ for reaction to remove the parasitic polycrystal on the substrate carrier.

2. The method of claim 1, wherein the method further comprises: The reaction furnace is a high-temperature tube furnace.

3. The method of claim 1, wherein the method further comprises: The reaction furnace is preheated to 500-600℃ and maintained for 15-20 minutes.

4. The method of claim 1, wherein the method further comprises: The hydrogen flow rate is 10-15 L / min; preferably, the inert gas flow rate is 5-8 L / min.

5. The method of claim 1, wherein the method further comprises: After 3-5 minutes of inert gas introduction, hydrogen is introduced.

6. The method of claim 1, wherein the method further comprises: The rate of temperature rise is 5-8℃ / min.

7. The method of claim 1, wherein the method further comprises: The reaction time is 3-6h.

8. The method of claim 1, wherein the method further comprises: After the reaction is completed, the furnace cavity (23) of the reaction furnace is naturally cooled at a rate of 3-5℃ / min; when the temperature of the furnace cavity (23) drops to 300-350℃, the inert gas and hydrogen are stopped; when the temperature drops to 50-100℃, the substrate carrier is removed from the reaction furnace.

9. An apparatus for removing on-carbon parasitic poly from an HVPE-GaN substrate, the apparatus comprising: a substrate holder; a plasma source; and a gas source. It comprises: A control assembly (1), a furnace body (2), a gas inlet assembly (3), and an exhaust gas discharge pipe (4); the control assembly (1) is electrically connected with the gas inlet assembly (3), the gas inlet assembly (3) is connected with the furnace body (2) through a pipeline; the exhaust gas discharge pipe (4) is arranged on the furnace body (2); the furnace body (2) comprises, from the inside to the outside, a heat preservation layer (21), an electric resistance heating body heating layer (22), and a furnace cavity (23); the front end of the furnace cavity (23) is provided with a furnace cavity door (231); the bottom of the furnace cavity (23) is provided with a machine table (24) and a large graphite rack (25), the machine table (24) is provided with a machine table graphite rack (26), the gas inlet assembly (3) comprises an inert gas bottle (32) communicated with a first gas inlet pipe (31) and a hydrogen bottle (34) communicated with a second gas inlet pipe (33), and the first gas inlet pipe (31) and the second gas inlet pipe (33) are both connected with the rear end of the furnace cavity (23) through a pipeline.

10. The apparatus of claim 9, wherein the apparatus further comprises a laser source. The exhaust gas discharge pipe (4) is connected with the rear end of the furnace cavity (23); preferably, the first gas inlet pipe (31), the second gas inlet pipe (33), and the exhaust gas discharge pipe (4) are all made of aluminum alloy material.

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