Microwave plasma reactor

Abstract

A a microwave plasma reactor for manufacturing synthetic diamond material. The microwave plasma reactor comprises a microwave generator configured to generate microwaves, a plasma chamber having an internal wall comprising aluminium, a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber, a gas flow system for feeding process gases into the plasma chamber and removing them therefrom, a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use, and a corrosion-resistant cooling system in thermal contact with the plasma chamber.

Description

[0001] MICROWAVE PLASMA REACTOR

[0002] FIELD OF THE INVENTION

[0003] This disclosure relates to the field of microwave plasma reactors, and in particular to microwave plasma reactors for manufacturing synthetic diamond material.

[0004] BACKGROUND

[0005] Chemical vapour deposition (CVD) processes for synthesis of diamond material are well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found, for example, the review article by R.S Balmer et al. which gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21 , No. 36 (2009) 364221.

[0006] Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), in the form of a carbon containing gas, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, synthetic diamond material can be deposited.

[0007] Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.

[0008] Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.

[0009] In light of the above, it has been found that microwave plasma is an effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.

[0010] A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave field to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.

[0011] A range of possible microwave plasma reactors for synthetic diamond film growth using a CVD process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.

[0012] The plasma chamber is typically manufactured from stainless steel. A cooling system is important for the plasma chamber to avoid overheating. The cooling system typically relies on a flow of water through the cooling system to remove heat from the plasma chamber.

[0013] For reasons of cost, manufacturing ease and thermal conductivity, it may be desirable to use an aluminium plasma chamber, but this leads to problems managing corrosion of the aluminium chamber where the cooling system is provided as part of the chamber. SUMMARY OF THE INVENTION

[0014] It is an object to provide a microwave plasma reactor for manufacture of synthetic diamond with a plasma chamber manufactured primarily from aluminium or an aluminium alloy, while reducing problems associated with corrosion.

[0015] According to a first aspect, there is provided a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition. The microwave plasma reactor comprises: a microwave generator configured to generate microwaves; a plasma chamber having an internal wall comprising aluminium; a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use; a corrosion-resistant cooling system in thermal contact with the plasma chamber.

[0016] As a first option, the corrosion resistant cooling system comprises channels for carrying cooling fluid in the plasma chamber walls, an internal surface of the channels configured to be in contact with a cooling fluid, the internal surface comprising anodized aluminium.

[0017] As a second option, the corrosion resistant cooling system comprises a jacket in thermal contact with an outer wall of the plasma chamber, the jacket comprising channels for carrying cooling fluid.

[0018] The channels of the second option optionally have an internal surface configured to be in contact with a cooling fluid, the internal surface comprising a corrosion resistant steel.

[0019] For the first and second options, a path length of each channel optionally has a greater proportion that follows a linear path than the proportion that follows a non-linear path. This improves ease of cleaning, accessibility and maintenance.

[0020] As an option, the corrosion-resistant cooling system comprises a plurality of sub cooling systems, each sub-cooling system in thermal contact with a different portion of the plasma chamber. This has the advantage of reducing channel paths around difficult or inaccessible areas, and allows separate cooling regimes to be applied to different portions of the plasma chamber.

[0021] As an option, the plasma chamber has an internal wall comprising an alloy of aluminium.

[0022] According to a second aspect, there is provided a cooling system for a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the cooling system comprising a jacket configured to be, in use, in thermal contact with an outer wall of a plasma chamber of the microwave plasma reactor, the jacket comprising channels for carrying cooling fluid. This allows a cooling system to be retro-fitted to an existing microwave plasma reactor.

[0023] As an option, the channels have an internal surface configured to be in contact with a cooling fluid, the internal surface comprising a corrosion resistant steel.

[0024] As an option, a path length of each channel has a greater proportion that follows a linear path than the proportion that follows a non-linear path.

[0025] As an option, the jacket comprises a plurality of sub cooling systems, each sub-cooling system in thermal contact with a different portion of the plasma chamber.

[0026] According to a third aspect, there is provided a method of manufacturing a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the method comprising: providing a microwave generator configured to generate microwaves; providing a plasma chamber having an internal wall comprising aluminium; providing a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; providing a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; providing a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use; providing a corrosion-resistant cooling system in thermal contact with the plasma chamber. The method optionally comprises providing a corrosion resistant cooling system that comprises channels for carrying cooling fluid in the plasma chamber walls, an internal surface of the channels configured to be in contact with a cooling fluid, the internal surface comprising anodized aluminium.

[0027] As an alternative option, the method comprises providing a jacket in thermal contact with an outer wall of the plasma chamber, the jacket comprising channels for carrying cooling fluid.

[0028] As an option, a path length of each channel has a greater proportion that follows a linear path than the proportion that follows a non-linear path.

[0029] The method optionally comprises providing a plurality of sub cooling systems, each subcooling system in thermal contact with a different portion of the plasma chamber.

[0030] As an option, the plasma chamber has an internal wall comprising an alloy of aluminium.

[0031] According to a fourth aspect, there is provided use of the microwave plasma reactor described above in the first aspect, for manufacturing synthetic diamond material.

[0032] BRIEF DESCIPTION OF THE DRAWINGS

[0033] For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0034] Figure 1 illustrates schematically a cross-sectional view of a microwave plasma reactor configured to deposit synthetic diamond material using a chemical vapour deposition technique;

[0035] Figure 2 illustrates schematically a cross-sectional view of the microwave plasma reactor shown in Figure 1 with a plasma chamber cooling system according to a first embodiment of the invention;

[0036] Figure 3 illustrates schematically a cross-sectional view of the microwave plasma reactor shown in Figure 1 with a plasma chamber cooling system according to a second embodiment of the invention; and Figure 4 is a flow diagram illustrating steps of providing a microwave plasma reactor shown in Figures 2 and 3.

[0037] The figures are not drawn to scale. Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity.

[0038] DETAILED DESCTIPION

[0039] Figure 1 shows an embodiment of a microwave plasma reactor 1 according to an embodiment of the present invention. The microwave plasma reactor 1 comprises the following basic components: a plasma chamber 2; a substrate holder 4 disposed in the plasma chamber for holding a substrate 6; a microwave generator 8, for forming a plasma 10 within the plasma chamber2; a microwave coupling configuration 12 for feeding microwaves from the microwave generator 8 into the plasma chamber 2; and a gas flow system comprising a gas inlet 13 and a gas outlet 16 for feeding process gases into the plasma chamber 2 and removing them therefrom.

[0040] It should also be noted that while the microwave plasma reactor 1 illustrated in Figure 1 has a separate substrate holder disposed in the plasma chamber, the substrate holder may be formed by the base of the plasma chamber. The use of the term “substrate holder” is intended to cover such variations. Furthermore, the substrate holder may comprise a flat supporting surface which is the same diameter (as illustrated) or larger than the substrate. For example, the substrate holder may form a large flat surface, formed by the chamber base or a separate component disposed over the chamber base, and the substrate may be carefully positioned on a central region of the flat supporting surface. In one arrangement, the flat supporting surface may have further elements, for example projections or grooves, to align, and optionally hold, the substrate. Alternatively, no such additional elements may be provided such that the substrate holder merely provides a flat supporting surface over which the substrate is disposed.

[0041] The microwave coupling configuration 12 comprises a coaxial waveguide 14 configured to feed microwaves from a rectangular waveguide 16 to an annular dielectric window 18. The coaxial waveguide 14 comprises an inner conductor 20 and an outer conductor 22. The inner conductor 20 is a floating post in the illustrated embodiment which is not attached to an upper wall of the rectangular waveguide 16 but rather terminates within the waveguide at a transition region between the rectangular waveguide 16 and the coaxial waveguide 14. The annular dielectric window 18 is made of a microwave permeable material such as quartz. It forms a vacuum-tight annular window in a top portion of the plasma chamber 2. The microwave generator 8 and the microwave coupling configuration 12 are configured to generate a suitable wavelength of microwaves and inductively couple the microwaves into the plasma chamber 2 to form a standing wave within the plasma chamber 2 having a high energy anti-node located just above the substrate 6 in use.

[0042] During operation, the plasma 10 generates a significant amount of heat, and this must be managed. The plasma chamber 2 is therefore provided with a cooling system to externally cool the plasma chamber 2. The cooling system has an inlet 30 and an outlet 32 to allow a cooling fluid to be passed through the cooling system 28 in order to dissipate heat from the plasma chamber 2 walls. Typically, water is used as the cooling fluid owing to its availability and cost. As described below, the cooling system 28 may be in the form of a jacket surrounding at least a portion of the external walls of the plasma chamber 2, or could be a series of cooling channels provided directly in the walls of the plasma chamber 2.

[0043] In a first embodiment, and illustrated in Figure 2, the plasma chamber 2 is made from aluminium or an aluminium alloy. It is provided with a cooling system 28 that comprises a channel or a plurality of channels located in the walls of the plasma chamber 2. The cooling system 28 has an inlet 30 and an outlet 32 configured to allow cooling fluid such as water to flow through the first cooling system 28.

[0044] In order to mitigate corrosion of the channels, the internal surface of the channels is anodized. Anodizing of aluminium is a well-known process. Briefly, anodizing is a process that involves creating a controlled oxide layer on the surface of aluminium. This oxide layer serves to protect the aluminium from corrosion. It may also be used to improve its surface hardness.

[0045] Prior to anodizing, the surfaces are cleaned to remove any contaminants such as dirt, oil or grease. This cleaning process is required to ensure uniform anodizing and good adhesion of the oxide layer. In some cases, the aluminium surface may be chemically etched to remove any surface irregularities or to prepare it for better anodizing. The surfaces may also be rinsed, deoxidized, or have another suitable chemical treatment to prepare the surface for anodizing.

[0046] The surface is then anodized using an acidic electrolyte solution, such as sulfuric acid, which acts as the medium for the anodizing process. The concentration and temperature of the electrolyte solution are carefully controlled. The aluminium is electrically connected to an external power source such that the aluminium parts serve as an anode while a cathode is immersed in the electrolyte solution. When an electric current is passed through the electrolyte solution, oxygen ions from the electrolyte react with the aluminium surface, forming a layer of aluminium oxide.

[0047] After a desired thickness of the aluminium oxide layer is achieved, the anodized aluminium parts are rinsed with water to remove any residual electrolyte solution. The rinsing step not only removes the electrolyte but closes open pores and improves corrosion resistance. There are various sealing steps and chemical treatments that may be carried out at this stage. The aluminium oxide layer on the surface of the cooling channels is much more resistant to corrosion than an aluminium or aluminium alloy surface.

[0048] It may be desirable to only anodise a portion of a surface. For example, only the internal surfaces and other areas around cooling channels may be anodised. The skilled person will appreciate that there are several ways to only anodise a portion of a surface. For example, the surface may be masked before anodising so only the parts exposed through the mask will be anodised. Alternatively, the entire surface may be anodised and then the anodised surface over portions that are not required to be anodised are machined away.

[0049] Turning now to Figure 3, a second embodiment is illustrated in which the plasma chamber 2 is also made from aluminium or an aluminium alloy. In this embodiment, a cooling jacket 34 is used as a cooling system. The cooling jacket is fitted to an external surface of the plasma chamber 2, and in thermal contact with the external surface of the plasma chamber 2. The cooling jacket 34 is provided with a channel or a plurality of channels located within the cooling jacket 34. The cooling jacket 34 has an inlet 36 providing access to the cooling channel and an outlet 38 configured to allow cooling fluid such as water to flow through the first cooling system 28.

[0050] The cooling jacket 34 may be manufactured from a stainless steel alloy to provide corrosion resistance or, similarly to the first embodiment, may have channels with internal surfaces of anodized aluminium.

[0051] Advantages of using a cooling jacket 34 include allowing the separation of the cooling system from other parts of the plasma chamber, such as the microwave and vacuum components, and ensures that the cooling fluid doesn’t come into contact with the plasma chamber at all. It may be retrofitted to existing reactors, and can be easily removed for cleaning or servicing. The external walls of the plasma chamber 2 are typically substantially cylindrical. For both embodiments, it is preferred that the majority of the path length of the cooling channel lies in a direction substantially parallel to a main axis of the plasma chamber 2 rather than substantially circumferentially around the walls of the plasma chamber. This ensures that the majority of the path length of the channel is linear, providing more linear portions in the path length of the cooling channels. This facilitates cleaning and servicing.

[0052] Figure 4 is a flow diagram illustrating steps of providing exemplary microwave plasma reactors shown in Figures 2 and 3. The following numbering corresponds to that of Figure 4.

[0053] 51 . A microwave generator is provided that is configured to generate microwaves. There are various types of microwave generators available, for example magnetrons or solid-state microwave generators.

[0054] 52. A plasma chamber is provided that has an internal wall comprising aluminium or an aluminium alloy.

[0055] 53. A microwave coupling configuration is provided for feeding microwaves from the microwave generator into the plasma chamber.

[0056] 54. A gas flow system is provided for feeding process gases into the plasma chamber and removing them therefrom. Process gases may include, for example, a carbon containing gas, hydrogen, argon and gases that may allow the introduction of dopants into the grown diamond, such as nitrogen or a boron containing gas.

[0057] 55. A substrate holder is provided that is disposed in the plasma chamber and comprises a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use.

[0058] 56. A corrosion-resistant cooling system in thermal contact with the plasma chamber is provided, as described above. For example, this may take the form of channels for carrying cooling fluid in the plasma chamber walls, an internal surface of the channels configured to be in contact with a cooling fluid, the internal surface comprising anodized aluminium. Alternatively, it may take the form of a jacket in thermal contact with an outer wall of the plasma chamber, the jacket comprising channels for carrying cooling fluid. As described above, it is preferable for the path length of each channel to have a greater proportion that follows a linear path than the proportion that follows a non-linear path. Furthermore, the cooling system may include multiple sub cooling systems in thermal contact with a different portion of the plasma chamber.

[0059] While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS1. A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a microwave generator configured to generate microwaves; a plasma chamber having an internal wall comprising aluminium; a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use; a corrosion-resistant cooling system in thermal contact with the plasma chamber.

2. The microwave plasma reactor according to claim 1 , wherein the corrosion resistant cooling system comprises channels for carrying cooling fluid in the plasma chamber walls, an internal surface of the channels configured to be in contact with a cooling fluid, the internal surface comprising anodized aluminium.

3. The microwave plasma reactor according to claim 1 , wherein the corrosion resistant cooling system comprises a jacket in thermal contact with an outer wall of the plasma chamber, the jacket comprising channels for carrying cooling fluid.

4. The microwave plasma reactor according to claim 3, wherein the channels have an internal surface configured to be in contact with a cooling fluid, the internal surface comprising a corrosion resistant steel.

5. The microwave plasma reactor according to any one of claims 2 to 4, wherein a path length of each channel has a greater proportion that follows a linear path than the proportion that follows a non-linear path.

6. The microwave plasma reactor according to any one of claims 1 to 5, wherein the corrosion-resistant cooling system comprises a plurality of sub cooling systems, each subcooling system in thermal contact with a different portion of the plasma chamber.

7. The microwave plasma reactor according to any one of claims 1 to 6, wherein the plasma chamber has an internal wall comprising an alloy of aluminium.

8. A cooling system for a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the cooling system comprising: a jacket configured to be, in use, in thermal contact with an outer wall of a plasma chamber of the microwave plasma reactor, the jacket comprising channels for carrying cooling fluid.

9. A cooling system according to claim 8, wherein the channels have an internal surface configured to be in contact with a cooling fluid, the internal surface comprising a corrosion resistant steel.

10. The cooling system according to claim 8 or claim 9, wherein a path length of each channel has a greater proportion that follows a linear path than the proportion that follows a non-linear path.

11. The cooling system according to any one of claims 8 to 10, wherein the jacket comprises a plurality of sub cooling systems, each sub-cooling system in thermal contact with a different portion of the plasma chamber.

12. A method of manufacturing a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the method comprising: providing a microwave generator configured to generate microwaves; providing a plasma chamber having an internal wall comprising aluminium; providing a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; providing a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; providing a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use; providing a corrosion-resistant cooling system in thermal contact with the plasma chamber.

13. The method according to claim 12, comprising providing a corrosion resistant cooling system that comprises channels for carrying cooling fluid in the plasma chamber walls, aninternal surface of the channels configured to be in contact with a cooling fluid, the internal surface comprising anodized aluminium.

14. The method according to claim 12, further comprising providing a jacket in thermal contact with an outer wall of the plasma chamber, the jacket comprising channels for carrying cooling fluid.

15. The method according to any one of claims 13 or 14, further comprising providing channels, wherein a path length of each channel has a greater proportion that follows a linear path than the proportion that follows a non-linear path.

16. The method according to any one of claims 12 to 15, further comprising providing a plurality of sub cooling systems, each sub-cooling system in thermal contact with a different portion of the plasma chamber.

17. The method according to any one of claims 12 to 16, wherein the plasma chamber has an internal wall comprising an alloy of aluminium.

18. Use of the microwave plasma reactor according to any one of claims 1 to 7 for manufacturing synthetic diamond material.

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