Multiple plasma source combined surface wave plasma chemical vapor deposition apparatus

Abstract

The present application belongs to the field of microwave plasma chemical vapor deposition, and particularly relates to a multiple plasma source combined surface wave plasma chemical vapor deposition device. The device comprises a 5.8 GHz microwave power source, a rectangular waveguide, a three-pin tuner, a short-circuit piston, a copper antenna, a quartz dielectric cavity, a stainless steel outer wall, an observation window, a horizontally rotatable substrate table structure, and a substrate heating system. The copper antenna is installed on the rectangular waveguide, and the microwave is coupled by the copper antenna to excite plasma and accompany surface waves below the quartz dielectric cavity. Three plasma sources are arranged at an angle of 120° to achieve large-area plasma discharge. The generated plasma diffuses downward in the quartz dielectric cavity, and the substrate for diamond deposition or surface modification is placed at the bottom of the reaction chamber. The substrate table can be horizontally rotated to improve the deposition uniformity. The device is mainly applied to the preparation of diamond coatings on heat-sensitive materials, large-size diamond homoepitaxy, and surface functional group modification of materials.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of microwave plasma chemical vapor deposition, and particularly relates to a surface wave plasma chemical vapor deposition device with multiple plasma sources. TECHNICAL BACKGROUND

[0002] A surface wave plasma (SWP) device is a kind of plasma equipment constructed by using surface wave coupling technology, which can realize the ionization of gas at very low radio frequency power and low deposition pressure (1 Pa~100 Pa). Due to the lower microwave power input, the surface wave plasma will not cause thermal damage to the sample, and can be applied in some fields sensitive to sample damage. The report on the surface wave plasma was first proposed by Moisan et al. in 1975, and then this plasma technology was applied to diamond deposition, surface modification and preparation of other carbon materials.

[0003] Compared with the typical microwave plasma, the SWP can be used for cleaning the surface of materials, removing organic matter, increasing surface activity, changing surface energy and roughness, etc. It can provide higher energy and concentration to enhance the chemical reaction and physical process of the surface of materials. This makes the SWP have potential in applications such as surface coating, functional modification, ceramic bonding, etc. In addition, the SWP is widely used for low-temperature deposition of diamond thin films. Compared with the traditional high-temperature pyrolysis method, the SWP can deposit diamond thin films with high quality, uniformity and high adhesion at a lower temperature. This low-temperature deposition method is very useful for micro-nano processing and biomedical applications on thin films.

[0004] In addition, with the application of single crystal diamond materials in detectors, sensors and optical instruments, the market demand for large-size single crystal diamond is increasing. At present, the method for obtaining large quantities of large-size (2 inch) single crystal diamond is to deposit a single crystal diamond film on a single crystal diamond by homoepitaxy on a single crystal diamond by heteroepitaxy. The effective deposition area of a traditional 2.45 GHz microwave plasma chemical vapor deposition device is 50 mm, and the effective deposition area of a 915 MHz microwave plasma chemical vapor deposition device can reach 120 mm, but the high cost limits the development of the technology.

[0005] In summary, in order to meet the needs of depositing diamond coating on heat-sensitive materials at low deposition temperature, surface functional modification of materials and homoepitaxy of large-size single crystal diamond, it is very meaningful to design a surface wave plasma chemical vapor deposition device with multiple plasma sources, which has simple device structure, relatively low cost, high microwave coupling efficiency and large plasma excitation area. ​​​SUMMARY

[0006] In order to solve the above problems, the purpose of the present application is to provide a multiple plasma source combined surface wave plasma chemical vapor deposition device, which can realize deposition of diamond coating at low cavity pressure and low deposition temperature, surface modification of materials and homoepitaxy of large-size single crystal diamond, has simple structure, relatively low cost, large plasma excitation area, stable operation and easy-to-control parameters.

[0007] Since the surface wave plasma does not cause thermal damage to the sample, the device can be applied in some fields sensitive to sample damage, such as deposition of diamond coating on heat-sensitive materials, or etching, surface modification, etc. In addition, the large-area plasma discharge generated by the arrangement and combination of multiple plasma sources can be applied to homoepitaxy of large-size single crystal diamond.

[0008] The technical solution of the present application is:

[0009] A multiple plasma source combined surface wave plasma chemical vapor deposition device, characterized in that it comprises a 5.8 GHz microwave power source, a rectangular waveguide, a three-pin tuner, a short-circuit piston A, a short-circuit piston B, a copper antenna, a quartz dielectric cavity, a sealing rubber ring, an air inlet, a stainless steel outer wall, plasma, an observation window, a horizontally rotatable substrate table structure, and an air outlet; the microwave energy generated by three 5.8 GHz microwave power sources propagates along the rectangular waveguide, enters the quartz dielectric cavity after being coupled by the copper antenna; the three-pin tuner and the short-circuit pistons A and B are adjusted to minimize the microwave reflection coefficient; the sealing rubber ring is arranged between the quartz dielectric cavity and the stainless steel outer wall, the air inlet is arranged above the quartz dielectric cavity, and the air outlet is located outside the substrate table; a strong electric field region is formed inside the quartz dielectric cavity, which excites the thin precursor gas to form plasma and accompanies the transmission of surface waves, the generated plasma diffuses downward in the quartz dielectric cavity to the horizontally rotatable substrate table structure, and the observation window is used for real-time temperature measurement during diamond growth.

[0010] Further, the 5.8 GHz microwave power source, rectangular waveguide, three-pin adapter, short-circuit piston A, short-circuit piston B, and copper antenna constitute a plasma source, and three such plasma sources are arranged at an angle of 120° to each other to achieve large-area plasma discharge; the copper antenna is directly installed on the WR159 rectangular waveguide, and the microwave coupled by the copper antenna excites plasma and generates surface waves below the quartz dielectric cavity; the generated plasma diffuses downward in the quartz dielectric cavity, and the substrate for diamond deposition or surface modification is placed at the bottom of the reaction chamber; the substrate table structure can rotate horizontally around the central axis to achieve uniform plasma surface modification or diamond coating deposition on a large-size substrate; the gas inlet is located above the quartz dielectric cavity, and the gas outlet is located outside the substrate table to ensure that the gas flow drives the plasma to diffuse to the substrate.

[0011] Further, the copper antenna is directly installed on the WR159 rectangular waveguide, and a cylindrical copper antenna is used to directly couple the microwave, thereby improving the microwave coupling efficiency, and the length of the copper antenna is Satisfies: wherein = 20.193 mm is the height of the WR159 rectangular waveguide.

[0012] Further, the relative dielectric constant of the quartz dielectric cavity is 4.2; the quartz dielectric cavity structure is directly integrated below the rectangular waveguide, and plasma is excited in the quartz dielectric cavity and generates surface waves; there is a hollow chamber above and below the quartz dielectric cavity, the upper chamber is used to accommodate the copper antenna, and the lower chamber is used to excite plasma inside the chamber, and the gas flow will drive the plasma to diffuse downward along the lower chamber.

[0013] Further, the horizontally rotatable substrate table structure is provided with a servo rotary motor, which can rotate in the horizontal direction to achieve uniform plasma surface modification, diamond coating deposition, and large-size single crystal diamond homoepitaxy on a large-size substrate; in addition, the horizontally rotatable substrate table structure can also move up and down in the vertical direction to change the distance between the substrate and the plasma.

[0014] Further, the observation window is located on the side of the stainless steel outer wall to observe and measure the substrate temperature in real time.

[0015] Further, the stainless steel outer wall is cooled by water to avoid local overheating and ensure long-term operation of the equipment.

[0016] Further, the horizontally rotatable substrate table structure is provided with a substrate heating system to ensure the temperature uniformity of a large-area substrate.

[0017] Further, the quartz medium cavity is connected with the stainless steel outer wall at a sealing groove, a rubber ring is placed in the sealing groove, and the quartz medium cavity and the stainless steel outer wall are fixed to form a vacuum chamber.

[0018] The key of the implementation process of the present application is:

[0019] The present application provides a multiple plasma source combined surface wave plasma chemical vapor deposition device. It mainly comprises a 5.8 GHz microwave power source, a rectangular waveguide, a three-pin tuner, a short-circuit piston, a copper antenna, a quartz medium cavity, a stainless steel outer wall, a horizontally rotatable substrate table structure, and a substrate heating system. The 5.8 GHz microwave power source, the rectangular waveguide, the three-pin tuner, the short-circuit piston A, the short-circuit piston B, and the copper antenna constitute a plasma source, and there are three such plasma sources arranged at an angle of 120° each, which are combined to achieve large-area plasma discharge. Three microwave power sources generate microwaves with a frequency of 5.8 GHz, which are transmitted by a WR159 rectangular waveguide, coupled by a copper antenna, transmitted along the quartz medium cavity, and form a strong electric field region inside the quartz medium cavity, which excites the thin precursor gas to form plasma and accompanies the transmission of surface waves. The generated plasma diffuses downward in the quartz medium cavity, and the substrate for diamond deposition or surface modification is placed at the bottom of the reaction chamber. The substrate table structure can be horizontally rotated to improve the deposition uniformity. The gas inlet is located above the quartz medium cavity, and the gas outlet is located outside the substrate table to ensure that the gas flow drives the plasma to diffuse towards the substrate.

[0020] The device is mainly used for preparing diamond coatings on heat-sensitive materials, plasma surface modification of materials, and homoepitaxy of large-size single crystal diamonds.

[0021] The key of generating plasma and performing diamond deposition or surface modification is to place the substrate on a horizontally rotatable substrate table structure, vacuumize the chamber to 1.0 10 -3 Pa, introduce hydrogen and methane into the chamber after vacuumizing to the predetermined vacuum, and maintain the chamber pressure at 10-100 Pa. Turn on the 5.8 GHz microwave power source, adjust the output power to 0.1-1 kW. Adjust the three-pin tuner and the short-circuit piston to minimize the microwave reflection coefficient. Achieve the ideal plasma discharge state of the film deposition process. After the film deposition is completed, turn off the power and the gas, and turn off the machine after vacuumizing to the limit.

[0022] The present application has the following beneficial effects compared with the prior art:

[0023] The application proposes a multiple plasma source combined surface wave plasma chemical vapor deposition device. Compared with the traditional microwave plasma chemical deposition device, three plasma sources in the application are arranged at an angle of 120°, and are combined to achieve large-area plasma discharge. A copper antenna is installed on a rectangular waveguide, and the microwave coupled by the copper antenna excites plasma under the quartz dielectric cavity and accompanies surface waves. The generated plasma diffuses downward in the quartz dielectric cavity, and the substrate for diamond deposition or surface modification is placed at the bottom of the reaction chamber. The substrate table structure can rotate along the central axis in the horizontal direction to realize uniform plasma surface modification, diamond coating deposition and large-size single crystal diamond homoepitaxy on a large-size substrate. The device is mainly applied to the preparation of diamond coating on heat-sensitive materials, plasma surface modification of materials and large-size single crystal diamond homoepitaxy. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a three-dimensional structure simplified schematic diagram of a multiple plasma source combined surface wave plasma chemical vapor deposition device.

[0025] Figure 2 It is a waveguide transmission part structure schematic diagram of a multiple plasma source combined surface wave plasma chemical vapor deposition device.

[0026] In the figure, 1 is a 5.8 GHz microwave power supply, 2 is a rectangular waveguide, 3 is a three-pin tuner, 4 is a short-circuit piston, 5 is a short-circuit piston, and 6 is a copper antenna.

[0027] Figure 3 It is a vacuum chamber part structure schematic diagram of a multiple plasma source combined surface wave plasma chemical vapor deposition device.

[0028] In the figure, 7 is a quartz dielectric cavity, 8 is a sealing rubber ring, 9 is an air inlet, 10 is a stainless steel outer wall, 11 is plasma, 12 is an observation window, 13 is a horizontally rotatable substrate table structure, and 14 is an air outlet.

[0029] Figure 4 It is the microwave electric field intensity distribution result inside the quartz dielectric cavity of a multiple plasma source combined surface wave plasma chemical vapor deposition device proposed based on the application.

[0030] Figure 5 It is the microwave electric field intensity distribution result on the horizontal cross section inside the deposition chamber of a multiple plasma source combined surface wave plasma chemical vapor deposition device proposed based on the application. It can be seen that a high microwave electric field intensity is excited inside each of the three quartz dielectric cavities arranged at an angle of 120°. The substrate diameter shown in the figure is 80 mm. DETAILED DESCRIPTION

[0031] The technical solutions of the present application are further described below in combination with the drawings and specific embodiments:

[0032] As shown in Figure 1 , Figure 2 and Figure 3 , the present application proposes a multiple plasma source combined surface wave plasma chemical vapor deposition device. The microwave energy generated by three 5.8 GHz microwave power sources (1) propagates along the rectangular waveguide (2), is coupled into the quartz dielectric cavity (7) through the copper antenna (6). Adjust the three-pin tuner (3) and the short-circuit piston (4, 5) to minimize the microwave reflection coefficient. A sealing rubber ring (8) is provided between the quartz dielectric cavity (7) and the stainless steel outer wall (10), and an air inlet (9) is provided above the quartz dielectric cavity (7), and an air outlet (14) is located outside the substrate table (13). The quartz dielectric cavity (7) forms a strong electric field region inside, excites a thin precursor gas to form a plasma (11) and accompanies the transmission of surface waves, and the generated plasma diffuses downward in the quartz dielectric cavity (7) to the horizontally rotatable substrate table structure (13), and the observation window (12) is used for real-time temperature measurement during diamond growth.

[0033] Example 1

[0034] Place 60 mm polymethyl methacrylate substrates on the horizontally rotatable substrate table structure (13), and vacuum the chamber to 1.0 10 -3 Pa, after pumping to the predetermined vacuum, introduce H2 into the resonant cavity as the working gas, and adjust the cavity pressure to 10 Pa. Turn on three 5.8 GHz microwave power sources (1), gradually adjust the output power to 200 W, simultaneously increase the cavity pressure to 100 Pa, adjust the three-pin tuner (3), short-circuit piston (4, 5) to minimize the microwave reflection coefficient. Introduce CH4 to deposit diamond thin film. During the deposition of the diamond thin film, the horizontally rotatable substrate table structure (13) is kept rotating at an angular velocity of 10 / min to ensure the uniformity of thin film deposition. After the thin film deposition is completed, turn off the power and close the gas, and after pumping to the limit vacuum, turn off the machine.

[0035] Example 2

[0036] Place 16 pieces of 10 10 mm 2 SiO2 substrates on the horizontally rotatable substrate table structure (13) in a 4 4 arrangement, vacuum the chamber to 1.0 10 -3Pa, after pumping to a predetermined vacuum, H2 is introduced into the resonant cavity as working gas, and the cavity pressure is adjusted to 10 Pa. Three 5.8 GHz microwave power sources (1) are turned on, and the output power is gradually adjusted to 150 W. The cavity pressure is simultaneously increased to 100 Pa, and the three-pin tuner (3), shorting piston (4, 5) are adjusted to minimize the microwave reflection coefficient. CH4 is introduced to deposit a diamond film. During the deposition of the diamond film, the horizontally rotatable substrate table structure (13) is rotated at an angular velocity of 10 radians per second to ensure uniformity of film deposition. After the film deposition is complete, the power is turned off, the gas is turned off, and the system is shut down after being pumped to a limit vacuum.

[0037] Example 3

[0038] Sixteen 10 mm 2 Si substrates are arranged in a 4 row pattern on the horizontally rotatable substrate table structure (13), and the chamber is pumped to a vacuum of 1.0 10 -3 Pa, after pumping to a predetermined vacuum, H2 is introduced into the resonant cavity as working gas, and the cavity pressure is adjusted to 10 Pa. Three 5.8 GHz microwave power sources (1) are turned on, and the output power is gradually adjusted to 150 W. The cavity pressure is simultaneously increased to 100 Pa, and the three-pin tuner (3), shorting piston (4, 5) are adjusted to minimize the microwave reflection coefficient. CH4 is introduced to deposit a diamond film. During the deposition of the diamond film, the horizontally rotatable substrate table structure (13) is rotated at an angular velocity of 10 radians per second to ensure uniformity of film deposition. After the film deposition is complete, the power is turned off, the gas is turned off, and the system is shut down after being pumped to a limit vacuum.

[0039] Example 4

[0040] Sixty 60 mm mm polyethylene substrates are placed on the horizontally rotatable substrate table structure (13), and the chamber is pumped to a vacuum of 1.0 10 -3 Pa, after pumping to a predetermined vacuum, chlorine oxide is introduced into the resonant cavity as working gas, and the cavity pressure is adjusted to 10 Pa. Three 5.8 GHz microwave power sources (1) are turned on, and the output power is gradually adjusted to 150 W. The cavity pressure is simultaneously increased to 100 Pa, and the three-pin tuner (3), shorting piston (4, 5) are adjusted to minimize the microwave reflection coefficient. The hydrogen atoms on the surface of the polyethylene are replaced by chlorine atoms to form a chlorinated polyethylene (Cl-PE) surface functional group. During the surface functional group modification process, the horizontally rotatable substrate table structure (13) is rotated at an angular velocity of 10 Rotate at angular velocity to ensure uniformity of surface functional group modification. After surface functional group modification is complete, turn off the power and gas supply, evacuate to the ultimate vacuum, and then shut down the machine.

[0041] Example 5

[0042] Will A 60 mm glass substrate was placed on a horizontally rotatable substrate stage structure (13), and the chamber was evacuated to 1.0 mm. 10 -3 After evacuating to a predetermined vacuum, NH3 is introduced into the resonant cavity as the working gas, and the cavity pressure is adjusted to 10 Pa. Three 5.8 GHz microwave power supplies (1) are turned on, and the output power is gradually adjusted to 150 W. The cavity pressure is simultaneously increased to 100 Pa. The three-pin tuner (3) and short-circuit pistons (4, 5) are adjusted to minimize the microwave reflection coefficient. Ammonia (NH3) is introduced into the surface wave plasma to treat the glass surface with plasma, so that it reacts with ammonia and forms amino functional groups on the surface. During the surface functional group modification process, the horizontally rotatable substrate stage structure (13) is maintained at 10 Pa. Rotate at angular velocity to ensure uniformity of surface functional group modification. After surface functional group modification is complete, turn off the power and gas supply, evacuate to the ultimate vacuum, and then shut down the machine.

[0043] Example 6

[0044] 16 pieces of 10 10 mm 2 Polylactic acid (PLA) substrate with 4 The 4 components are arranged on a horizontally rotatable substrate stage structure (13), and the chamber is evacuated to 1.0 oz. 10 -3 After pumping to a predetermined vacuum, O2 and acetone vapor are introduced into the resonant cavity as working gases, and the cavity pressure is adjusted to 10 Pa. The 5.8 GHz microwave power supply (1) is turned on, and the output power is gradually adjusted to 150 W. The cavity pressure is simultaneously increased to 100 Pa. The three-pin tuner (3) and short-circuit piston (4, 5) are adjusted to minimize the microwave reflection coefficient. Surface wave plasma treatment is used to apply high-energy plasma in an atmosphere of oxygen and acetone to introduce oxidized functional groups and enhance the water permeability and cell adhesion of the material surface. During the surface functional group modification process, the horizontally rotatable substrate stage structure (13) is maintained at 10 Pa. Rotate at angular velocity to ensure uniformity of surface functional group modification. After surface functional group modification is complete, turn off the power and gas supply, evacuate to the ultimate vacuum, and then shut down the machine.

[0045] Example 7

[0046] A 3 inch single crystal diamond substrate was placed on the horizontally rotatable substrate table structure (13), the chamber was evacuated to 1.0 10 -3 Pa, after being evacuated to a predetermined vacuum, H2was introduced into the cavity as the working gas, and the cavity pressure was adjusted to 10 Pa. Three 5.8 GHz microwave power sources (1) were turned on, the output power was gradually adjusted to 200 W, the cavity pressure was simultaneously increased to 100 Pa, and the three-pin stub tuner (3), shorting piston (4, 5) were adjusted to minimize the microwave reflection coefficient. CH4was introduced to deposit homoepitaxial single crystal diamond. During the deposition of homoepitaxial single crystal diamond, the horizontally rotatable substrate table structure (13) was rotated at an angular velocity of 10 r / min to ensure uniform deposition. In addition, during long-term homoepitaxial single crystal diamond deposition, the increasing thickness of the diamond causes the distance between its surface and the plasma to change, which can lead to growth instability. Therefore, during deposition, the horizontally rotatable substrate table structure (13) was also moved downward at a rate of 2-10 μm / h to ensure that the distance between the surface of the single crystal diamond and the plasma remained unchanged. After the deposition of homoepitaxial single crystal diamond was completed, the power was turned off, the gas was turned off, and the chamber was evacuated to a limit vacuum before being shut down.

Claims

1. A surface wave plasma chemical vapor deposition apparatus of a combination of a plurality of plasma sources, characterized by, The device comprises a 5.8GHz microwave power source, a rectangular waveguide, a three-pin peg adjuster, a short-circuit piston A, a short-circuit piston B, a copper antenna, a quartz dielectric cavity, a sealing rubber ring, an air inlet, a stainless steel outer wall, plasma, an observation window, a horizontally rotatable substrate table structure and an air outlet. The three 5.8GHz microwave power sources generate microwave energy which propagates along the rectangular waveguide, enters the quartz dielectric cavity through the copper antenna, and is coupled.

2. The multiple plasma source combined surface wave plasma chemical vapor deposition apparatus of claim 1, wherein The three-pin peg adjuster, the short-circuit piston A and the short-circuit piston B are adjusted to minimize the microwave reflection coefficient.

3. The multiple plasma source combined surface wave plasma chemical vapor deposition apparatus of claim 1, wherein The quartz dielectric cavity is provided with the sealing rubber ring between the quartz dielectric cavity and the stainless steel outer wall, and the air inlet is arranged above the quartz dielectric cavity.

4. The multiple plasma source combined surface wave plasma chemical vapor deposition apparatus of claim 1, wherein The quartz dielectric cavity forms a strong electric field region inside, excites a thin precursor gas to form plasma and accompanies the transmission of surface waves, and the generated plasma diffuses downward in the quartz dielectric cavity to the horizontally rotatable substrate table structure.

5. The multiple plasma source combined surface wave plasma chemical vapor deposition apparatus of claim 1, wherein The observation window is used for real-time temperature measurement during diamond growth.

6. The multiple plasma source combined surface wave plasma chemical vapor deposition apparatus of claim 1, wherein, The 5.8GHz microwave power source, the rectangular waveguide, the three-pin peg adjuster, the short-circuit piston A, the short-circuit piston B and the copper antenna constitute a plasma source, and there are three such plasma sources arranged at an angle of 120°. The copper antenna is directly installed on the WR159 rectangular waveguide, and the microwave coupled through the copper antenna excites plasma and accompanies surface waves below the quartz dielectric cavity. The generated plasma diffuses downward in the quartz dielectric cavity, and the substrate for diamond deposition or surface modification is placed at the bottom of the reaction chamber. The substrate table structure can rotate around the central axis in the horizontal direction to realize uniform plasma surface modification or diamond coating deposition on a large-size substrate. The air inlet is located above the quartz dielectric cavity, and the air outlet is located outside the substrate table to ensure that the gas flow drives the plasma to diffuse to the substrate. The copper antenna is directly installed on the WR159 rectangular waveguide, and the cylindrical copper antenna is used to directly couple the microwave, which improves the microwave coupling efficiency. The length l of the copper antenna satisfies 0.5h < l < h, where h = 20.193mm, and h is the height of the WR159 rectangular waveguide. The relative dielectric constant of the quartz dielectric cavity is 4.

2. The quartz dielectric cavity structure is directly integrated below the rectangular waveguide, and plasma is excited in the quartz dielectric cavity and accompanies surface waves. There are one hollow chamber above and below the quartz dielectric cavity, the upper chamber is used to accommodate the copper antenna, and the lower chamber is used to excite plasma inside the chamber. The horizontally rotatable substrate table structure is provided with a servo rotating motor, which can rotate in the horizontal direction to realize uniform plasma surface modification, diamond coating deposition and large-size single crystal diamond homoepitaxy on a large-size substrate. The observation window is located on the side of the stainless steel outer wall to observe and measure the substrate temperature in real time. The stainless steel outer wall is water-cooled to avoid local overheating and ensure long-time operation of the device.

7. The multiple plasma source combined surface wave plasma chemical vapor deposition apparatus of claim 1, wherein The horizontal rotation type substrate table structure is equipped with a substrate heating system, which ensures the temperature uniformity of a large area substrate.

8. The multiple plasma source combined surface wave plasma chemical vapor deposition apparatus of claim 1, wherein, The sealing groove is arranged at the joint of the quartz medium cavity and the stainless steel outer wall, the rubber ring is arranged in the sealing groove, and the quartz medium cavity and the stainless steel outer wall are fixed to form a vacuum chamber.

Images