WO2025202275A1

WO2025202275A1 - Microwave plasma reactor and method of operation

Info

Publication number: WO2025202275A1
Application number: PCT/EP2025/058249
Authority: WO
Inventors: Michael Andrew Cooper; Firooz Nasser-Faili; Nicola Louise PALMER; Michael Ian PEARSON; Christopher John Howard Wort; Spencer Charles WITTER
Original assignee: Element Six Technologies Ltd
Current assignee: Element Six Technologies Ltd
Priority date: 2024-03-27
Filing date: 2025-03-26
Publication date: 2025-10-02
Anticipated expiration: 2026-09-27
Also published as: GB202504384D0

Abstract

A microwave plasma reactor for manufacturing synthetic diamond material. The microwave plasma reactor comprises a microwave generator configured to generate microwaves, a plasma chamber, 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 first sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

Description

MICROWAVE PLASMA REACTOR AND METHOD OF OPERATION

FIELD OF THE INVENTION

This disclosure relates to a microwave plasma chemical vapour deposition (CVD) reactor, in particular a microwave plasma CVD reactor for the production of synthetic diamond, and methods of operating a microwave plasma chemical vapour deposition reactor.

BACKGROUND

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 in a 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).

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.

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 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.

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.

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 early stages of CVD diamond growth are critical in determining whether the resultant grown diamond will be of sufficient quality to provide an acceptable yield. As diamond growth can take many hours or days, it is therefore desirable to assess the quality of the diamond in the early stages of growth so operation parameters can be adjusted or the growth run can be abandoned before committing to a full growth run, which is time consuming and requires a large amount of power. SUMMARY OF THE INVENTION

It is an object to provide a method and apparatus for assessing OVD diamond growth in an early stage of a growth run.

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; 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 first sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

The microwave plasma reactor optionally comprises a second sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

As an option, the first sensor senses interference from a first area of the deposited diamond and the second sensor senses interference from a second area of the deposited diamond.

As an option, the first sensor comprises a pyrometer.

As an option, the first sensor is configured to measure a temperature, wherein the interference is in the form of variations in the measured temperature.

The microwave plasma reactor optionally further comprises a controller, the controller configured to, in use, analyse interference data.

The controller is optionally further configured, in use, to dynamically adjust a microwave power from the microwave generator on the basis of the analysed interference data. The controller is optionally further configured, in use, to dynamically adjust any of a gas pressure, a gas flow and a gas composition from the gas control system on the basis of the analysed interference data.

The controller is optionally further configured to calculate a deposited synthetic diamond material growth rate using the sensed interference data.

The microwave plasma reactor is optionally configured to grow polycrystalline CVD diamond.

As an alternative option, the microwave plasma reactor is configured to grow single crystal CVD diamond.

According to a second aspect, there is provided a method of operating 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; 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; using a first sensor to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface; and using the sensed interference of emitted infrared radiation, determining whether to adjust parameters of diamond deposition.

As an option, the parameters comprise any of gas flow, gas composition, power, and whether to continue with diamond deposition.

The method optionally further comprises using a second sensor to sense emitted infrared radiation from the substrate with reflections from the diamond plasma interface. The method optionally further comprises using the first sensor to sense interference from a first area of the deposited diamond and using the second sensor to sense interference from a second area of the deposited diamond.

The first sensor optionally comprises a pyrometer.

As an option, the method further comprises using the first sensor to measure a temperature, wherein the interference is in the form of variations in the measured temperature.

As an option, the method further comprises using a controller configured to analyse interference data.

The method optionally further comprises using the controller to dynamically adjust a microwave power from the microwave generator on the basis of the analysed interference data.

The method optionally further comprises using the controller to dynamically adjust any of a gas pressure, a gas flow and a gas composition from the gas control system on the basis of the analysed interference data.

As an option, the method further comprises using the controller to calculate a deposited synthetic diamond material growth rate using the sensed interference data.

According to a third aspect, there is provided a controller for a microwave plasma reactor for manufacturing synthetic diamond material as described in the first aspect, the controller comprising: a first input configured to receive first interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface; a microprocessor configured to analyse the received first interference data.

The controller optionally further comprises a second input configured to receive second interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

As an option, the microprocessor is further configured to calculate a deposited synthetic diamond material growth rate using the sensed interference data. As an option, the microprocessor is further configured to calculate a desired microwave power on the basis of the received first interference data, the controller further comprising a first output in functional communication with the microwave generator for controlling the microwave power.

As an option, the microprocessor is further configured to calculate any of any of a desired gas pressure, gas flow a gas composition on the basis of the received first interference data, the controller further comprising a second output in functional communication with the gas control system for controlling any of the gas pressure, a gas flow and a gas composition.

According to a fourth 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, 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 first sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface, and a second sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

BRIEF DESCIPTION OF THE DRAWINGS

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:

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 in accordance with an embodiment of the present invention;

Figure 2 is four graphs showing temperature measured at the centre and edges of growing deposited diamond for various samples;

Figure 3 illustrates schematically a side elevation cross section view of diamond growing on a silicon substrate; Figure 4 is a graph showing linear growth rate plotted against run time;

Figure 5 is a graph showing modelled fringing and measured fringing;

Figure 6 illustrates schematically in a block diagram an exemplary controller for a microwave plasma reactor; and

Figure 7 is a flow diagram showing exemplary steps for operating the microwave plasma reactor illustrated in Figure 1.

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.

DETAILED DESCTIPION

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 chamber 2; 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.

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. 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.

A first sensor 26 is provided for sensing interference of emitted infrared radiation from the substrate with reflections from the interface between the deposited diamond surface and the plasma, as described below. This is provided outside the plasma chamber 2 and so a viewing port 28 is also provided in the wall of the plasma chamber 2 to allow direct line of sight from the first sensor 26 to the substrate 6 and the diamond deposited on the substrate.

As described below, a second sensor (not shown) may also be used for looking at a different area of the substrate. More sensors can be added if required.

An example of a first sensor is a pyrometer. A pyrometer is a device used to measure high temperatures, and operates on the principle of detecting thermal radiation emitted by a hot object. When the substrate 6 and diamond deposited on the substrate are heated to high temperatures, they emit electromagnetic radiation in the form of infrared (I R) radiation. The intensity and wavelength distribution of this radiation depend on the object's temperature. The pyrometer contains an optical system, usually comprising lenses or mirrors, that focuses the infrared radiation onto a detector. The detector in the pyrometer is sensitive to infrared radiation. Examples of detectors include thermopiles, bolometers, a semiconductor device, or some other type of sensor capable of converting IR radiation into an electrical signal. The electrical signal produced by the detector is proportional to the intensity of the incoming infrared radiation, which in turn is related to the temperature of the object being measured. Pyrometers are calibrated to relate this electrical signal to temperature, usually through a known relationship or a calibration curve. Pyrometers must be carefully calibrated and adjusted for factors such as emissivity (the efficiency with which the object emits thermal radiation), distance from the object, and ambient temperature. Additionally, the optical system must be designed to accurately focus the infrared radiation onto the detector.

Different types of pyrometers exist, including single-wavelength pyrometers (which measure temperature based on the intensity of radiation at a specific wavelength), two-colour pyrometers (which utilize the ratio of intensities at two different wavelengths to compensate for variations in emissivity), and ratio pyrometers (which compare the intensity of the radiation emitted by the hot object to a reference source of known temperature). Each type may be used, but for the sake of brevity, the following description assumes a single wavelength pyrometer.

Other types of sensor may be used. For example, an IR imaging camera working at ~2.2um could analyse an entire substrate surface. The following description assumed that one or more pyrometers are used.

When growing polycrystalline diamond in a CVD reactor, during the initial stages of growth (nucleation and coalescence), “fringing” is observed in the pyrometry temperature profile due to Fabry-Perot interference between the growing film and the substrate.

Fringing is caused by destructive and constructive interference between emission from the substrate on which the diamond is grown, along with reflections at the diamond/plasma interface. It is this reflection that contributes to the fringing. The degree of reflection at the diamond/silicon interface is around 4% and so to simplify the analysis below, this reflection is neglected.

The fringing observed varies significantly depending on seeding of the substrate, substrate type and process conditions. In addition, under some conditions the initial fringing amplitude is very small (as low as ~6°C has been observed) and thus fitting a model accurately is not possible, particularly since the noise from the pyrometers can be of the order ±1.5°C. Furthermore, the initial growth rate (which can be measured using fringing as described below) is considerably different to the growth rate measured at 160h by mass increase.

As fringing is largely determined by nucleation and early stages of growth, fringing can be used for optimisation of these stages. For example, it might be expected that high amplitude fringing shows dense nucleation and a fully coalesced uniform film. Similarly, in-phase centre and edge fringing is indicative that nucleation and initial growth is occurring very similarly in the two areas of the substrate, which is desirable.

To illustrate the invention, four substrates were prepared. Sample 1 was a substrate formed of silicon. Sample 2 was a thermal SiNx coated Si substrate. Sample 3 was a spin on glass (SoG) and SiNx coated Si substrate. Sample 3 was a GaN substrate. All the substrates had a growth face with a diameter of 4” (102 mm). The samples were lightly dusted with diamond grits before use to assist in seeding subsequent polycrystalline CVD diamond growth.

Each substrate was then placed in a microwave plasma reactor for subsequent polycrystalline CVD diamond growth using the conditions described in WO 2014/026930.

At the start of growth, the temperature at the centre of the substrate was measured using a first pyrometer and the temperature at the edge of the substrate was measured using a second pyrometer. The emissivity for the pyrometers was set to 0.9.

Figure 2 shows fringing for three different 4 inch substrate types. There are a number of features to note:

The centre and edge fringing is generally not in phase. This is indicative of nucleation and early stage growth occurring differently, or at least at a different rate, at the centre and edge. Comparing the two SoG substrates, it can be seen one shows more in phase fringing than the other. It is proposed that this is due to seeding differences between the two substrates, in particular, non-uniformity of seeding. The change of refractive index of diamond with temperature is insufficient to explain the difference in fringing frequency between the centre and edge of the substrate.

The fringing starts with a peak once reaching process conditions for the samples 2 and 4 but on the sample 3, by the time process conditions are achieved, the fringing cause the observed temperature to drop. This indicates that nucleation occurs more quickly on the SoG substrates. This is likely due to the rougher nature of the surface of the SoG substrates.

For sample 4, the initial amplitude of the fringing is higher in the centre than the edge indicating a more uniform diamond surface at the centre thus perhaps better nucleation. This could be as a result of uneven seeding, differences in plasma species concentration or temperature variation. The fringing decay time varies. It is expected that the longer the decay time, the more uniform the surface of the growing diamond as it is thought that the fringing decay is caused by scatter on the non-uniform diamond growth surface as the grain facets get larger and thus there is less interference.

There is more noise on the SoG substrates, attributed to the rougher surface of the SoG substrates compared to the GaN and SiNx.

A simple fringing model has been developed in an attempt to better understand the early stages of nucleation and growth. It is based on a diamond film growing in a planar regime on the surface of silicon, and is illustrated in Figure 3.

Emission from the silicon is accounted for, as are reflections at the diamond/air interface. It is this reflection towards the silicon that causes the fringing. The degree of reflection at the diamond/silicon interface is small (4%) and so for simplicity this reflection is neglected.

The observed emissivity of the sample is modelled according to:

The variables and values used in equation 1 are summarised in Table 1.

Table 1 The bulk of the plasma is H2 plasma, and so this is used as an approximation for the plasma as a whole.

The emissivity can then be converted to a temperature using the Stefan- Boltzmann law: Eq. 2

Where P is the radiated power, A the radiating area and a the Stefan-Boltzmann constant.

Considering the experimental setup where the emissivity is set to a constant value of 0.9, the observed temperature (Jobs can be expressed as: Eq. 3

And so it follows that, by combination of Equation 2 and Equation 3:

Substituting for the emissivity:

Consideration must be given to how the thickness of the diamond (z) varies with respect to time. It can be seen in Figure 4 that the growth rate is not constant. Experimental data has been fitted with a third order polynomial function to approximate the growth rate change over the course of the run. This fits well at early growth time which is the area in question for fringing.

The growth rate (g) can be used to define the thickness of the diamond in terms of run time (0: The model can now be fitted to experimental data by selection of appropriate values for the real temperature (which defines the final temperature after fringing decay), the decay factor (which defines the rate at which the fringing decays) and by altering the growth rate function (which determines the frequency of the fringing). It is worth noting that the decay factor mostly takes into account reduction in fringing as a result of reduced coordinated interference as the diamond surface becomes rougher during growth. It will also take into account any absorption of emission from the silicon in the diamond layer.

Figure 5 shows the logs from the central pyrometer of sample 2 with the fringing model fitted using a real temperature of 800°C, a decay factor of 10000 and a growth rate according to that displayed for the fitted curve in Figure 4. The fit seems quite good however the fringing frequency from ~12h can be seen to be getting out of phase, indicating that the experimental growth rate is not constant over this period.

It should be noted that the model considers only a bilayer structure consisting of diamond and silicon. There will be some impact to the emissivity of the substrates as a result of the addition of the various layers in the GaN, SiNx and SoG substrates. However, the observed pyrometer temperatures under the same growth conditions for the different types of substrates is very similar. It is assumed that under the same conditions, the real substrate temperature is very similar between substrate types and thus this is indicative that the different layers do not have a significant impact on emissivity over the diamond/silicon model.

Figure 6 illustrates schematically in a block diagram an exemplary controller 30 for a microwave plasma reactor 1. The controller 30 has a microprocessor 32. A first input 34 is provided for receiving interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface. As described above, a second input 36 may also be provided for receiving second interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface. The data is typically taken from the centre of a substrate and the edge of a substrate. This provides a good indication of the uniformity of early stage growth on the substrate. The microprocessor 32 is configured to analyse the received data.

If dynamic control is required, then a first output 38 is provided in functional communication with the microwave generator 8 for controlling the microwave power. The microprocessor 32 calculates a desired power from the received interference data and sends signals to the microwave generator 8 using the first output 38 accordingly. A second output 40 in functional communication with the gas control system for controlling the gas pressure, gas flow or gas composition may also be provided. The microprocessor 32 calculates a desired pressure or mix of gases from the received interference data and sends signals to the gas flow system accordingly.

The microprocessor 32 may also be configured to calculate a deposited synthetic diamond material growth rate using the sensed interference data.

A display (not shown) may also be provided for providing information to an operator. For example, the microprocessor 32 may determine that a run should be aborted, for example if the interference data indicates a sufficiently large degree of non-uniformity between the growth rates at the edge and centre of the substrate, and the display can be used to alert an operator to abort the run or take remedial action.

Figure 7 is a flow diagram showing exemplary steps for operating the microwave plasma reactor illustrated in Figure 1. The following numbering corresponds to that of Figure 7:

51 . A microwave plasma reactor 1 as described above is provided.

52. A substrate 6 is disposed over the substrate holder 4. The substrate may have single crystal substrates attached to it for homoepitaxial growth of single crystal diamond, or may be seeded with diamond grit for growth of polycrystalline diamond, or may be a non-diamond substrate for heteroepitaxial growth of diamond.

53. A microwave generator 8 is used to generate microwaves which are fed into the plasma chamber 2 via the microwave coupling configuration 12.

54. A gas flow system is used to feed process gases into the plasma chamber 2. Typical process gases include hydrogen and a carbon-containing gas, along with any dopant gases.

55. A first sensor 26 is used to measure interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface. As described above, a second sensor may also be used to measure interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface. Typically, centre and edge interference will be measured. 56. The sensed interference of emitted infrared radiation is used to determine whether to adjust parameters of diamond deposition. These parameters may include whether continue with the deposition, gas flow, gas composition, and power. This may be done by a controller for dynamic process control of the gas flow system and the microwave generator.

57. Synthetic diamond is deposited on the substrate.

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. For example, the above description refers to polycrystalline diamond, but the same principles apply to early stage growth for single crystal diamond.

Claims

1. 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; 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 first sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

2. The microwave plasma reactor according to claim 1 , further comprising a second sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

3. The microwave plasma reactor according to claim 2, wherein the first sensor senses interference from a first area of the deposited diamond and the second sensor senses interference from a second area of the deposited diamond.

4. The microwave plasma reactor according to any one of claims 1 to 3, wherein the first sensor comprises a pyrometer.

5. The microwave plasma reactor according to any one of claims 1 to 4, wherein the first sensor is configured to measure a temperature, wherein the interference is in the form of variations in the measured temperature.

6. The microwave plasma reactor according to any one of claims 1 to 5, further comprising a controller, the controller configured to, in use, analyse interference data.

7. The microwave plasma reactor according to claim 6, wherein the controller is further configured, in use, to dynamically adjust a microwave power from the microwave generator on the basis of the analysed interference data.

8. The microwave plasma reactor according to any of claims 6 or 7, wherein the controller is further configured, in use, to dynamically adjust any of a gas pressure, a gas flow and a gas composition from the gas control system on the basis of the analysed interference data.

9. The microwave plasma reactor according to any one of claims 6 to 8, wherein the controller is further configured to calculate a deposited synthetic diamond material growth rate using the sensed interference data.

10. The microwave plasma reactor according to any one of claims 1 to 9, wherein the microwave plasma reactor is configured to grow polycrystalline CVD diamond.

11. The microwave plasma reactor according to any one of claims 1 to 9, wherein the microwave plasma reactor is configured to grow single crystal CVD diamond.

12. A method of operating 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; 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; using a first sensor to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface; using the sensed interference of emitted infrared radiation, determining whether to adjust parameters of diamond deposition.

13. The method according to claim 12, wherein the parameters comprise any of gas flow, gas composition, power, and whether to continue with diamond deposition.

14. The method according to claim 12 or claim 13, further comprising using a second sensor to sense emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

15. The method according to claim 14, further comprising using the first sensor to sense interference from a first area of the deposited diamond and using the second sensor to sense interference from a second area of the deposited diamond.

16. The method according to any one of claims 12 to 15, wherein the first sensor comprises a pyrometer.

17. The method according to any one of claims 12 to 16, further comprising using the first sensor to measure a temperature, wherein the interference is in the form of variations in the measured temperature.

18. The method according to any one of claims 12 to 17, further comprising using a controller configured to analyse interference data.

19. The method according to claim 18, further comprising using the controller to dynamically adjust a microwave power from the microwave generator on the basis of the analysed interference data.

20. The method according to any one of claims 18 or 19, further comprising using the controller to dynamically adjust any of a gas pressure, a gas flow and a gas composition from the gas control system on the basis of the analysed interference data.

21. The method according to any one of claim 18 to 20, further comprising using the controller to calculate a deposited synthetic diamond material growth rate using the sensed interference data.

22. A controller for a microwave plasma reactor for manufacturing synthetic diamond material according to any one of claims 1 to 11, the controller comprising: a first input configured to receive first interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface; a microprocessor configured to analyse the received first interference data.

23. The controller according to claim 22, the controller further comprising: a second input configured to receive second interference data of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.

24. The controller according to claim 22 or 23, wherein the microprocessor is further configured to calculate a deposited synthetic diamond material growth rate using the sensed interference data.

25. The controller according to any one of claims 22 to 24, the microprocessor is further configured to calculate a desired microwave power on the basis of the received first interference data, the controller further comprising a first output in functional communication with the microwave generator for controlling the microwave power.

26. The controller according to any one of claims 22 to 25, wherein the microprocessor is further configured to calculate any of a desired gas pressure, gas flow and gas composition on the basis of the received first interference data, the controller further comprising a second output in functional communication with the gas control system.

27. 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; 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 first sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface; and a second sensor configured to sense interference of emitted infrared radiation from the substrate with reflections from the diamond plasma interface.