Chemical vapour deposition processes for synthesis of diamond materials are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S Balmer et al. 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).
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%), typically in the form of methane although other carbon containing gases may be utilized, 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.
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.
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.
In light of the above, it has been found that microwave plasma is the most 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.
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 field. 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.
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.
The present applicant has previously filed a number of patent applications directed to microwave plasma activated CVD reactor hardware and CVD diamond synthesis methodology for achieving high quality, thick CVD diamond growth of both single crystal and polycrystalline CVD diamond materials over relatively large areas and relatively high growth rates. These patent applications include patent applications describing:                (i) certain aspects of the structure and geometry of the microwave plasma chamber (e.g. WO2012/084661 which describes the use of a compact TM011 resonance mode plasma chamber configuration and WO2012/084657 which describes the provision of a plasma stabilizing annulus projecting from a side wall of the plasma chamber);        (ii) certain aspects of the microwave power coupling configuration (e.g. WO2012/084658 which describes a microwave power delivery system for supplying microwave power to a plurality of microwave plasma reactors and WO2012/084659 which describes a microwave coupling configuration comprising an annular dielectric window, a coaxial waveguide, and a waveguide plate comprising a plurality of apertures disposed in an annular configuration for coupling microwaves towards the plasma chamber);        (iii) certain aspects of the substrate preparation, geometry, and temperature control configurations within the microwave plasma chamber (e.g. WO2012/084655 which describes how to prepare, locate, and control substrate parameters within a microwave plasma reactor to achieve desirable electric field and temperature profiles); and        (iv) certain aspects of the gas flow configuration and gas flow parameters within a microwave plasma chamber (e.g. WO 2012/084661 which describes a microwave plasma reactor with a multi-nozzle gas inlet array having a desirable geometric configuration for achieving uniform diamond growth over large areas and WO2012/084656 which describes the use of high gas flow rates and injection of process gases with a desirable Reynolds number to achieving uniform doping of synthetic diamond material over large areas).        
By providing a microwave plasma reactor incorporating the features as described in the above identified patent applications the present applicant has achieved high quality, thick CVD diamond growth of both single crystal and polycrystalline CVD diamond materials over relatively large areas and relatively high growth rates.
That said, there is still an on-going need to further improve upon prior art arrangements in order to provide a robust, efficient, and high yield synthesis platform for synthetic diamond products. In this regard, it is an aim of embodiments of the present invention to provide a system which has improved robustness, efficiency, and yield.