Field of Invention

The present invention relates to single crystal diamond substrates for synthesis of single crystal diamond material. Particular arrangements relate to a method of treating single crystal diamond substrates for synthesis of single crystal diamond material.

Background of Invention

HPHT synthesis of single crystal diamond material is well known in the art. Standard processes for manufacturing small crystals of diamond, i.e. diamond grit, involve mixing a graphite powder with a powdered metal catalyst comprising, for example, cobalt and iron (advantageously in a ratio at, or close to, the eutectic composition - 65% Co : 35% Fe). Other catalyst compositions are also known comprising, for example, Co, Fe, Ni, and/or Mn. A micron scale diamond powder may also be included in the reaction mixture to form seeds for diamond growth although spontaneous nucleation is possible.

The reaction mixture is transferred into a capsule and loaded into a press where it is subjected to a pressure of approximately 5 GPa and a temperature of approximately 1500°C. Such pressures and temperatures are in the region of the carbon phase diagram where diamond is the thermodynamically stable form of carbon and diamond growth occurs to form a large number of small diamond grit particles.

While the aforementioned process is successful for manufacturing small diamond grit particles, the process is not suitable for manufacturing larger single crystal diamonds.

The standard method for manufacturing larger single crystal HPHT diamond material is known in the art as the temperature gradient method. This method is similar to the previously described method in that the reaction mixture comprises a graphite powder and a powdered metal catalyst. However, instead of using a micron scale diamond powder to seed the reaction mixture, a seed pad is manufactured comprising a plurality of single crystal diamond seeds or substrates anchored to, or embedded in, a holder which may be formed by a ceramic disk. The seeds themselves are larger in size than the micron size diamond powder used to seed grit processes.

The seed pad is introduced into a capsule and the reaction mixture is disposed over the seed pad within the capsule. The capsule is then loaded into a press and subjected to a HPHT treatment. The HPHT treatment differs from that previously described in that the capsule is heated to a higher temperature at the top of the capsule than at the bottom of the capsule. The resultant temperature gradient within the capsule causes convection currents which transport carbon material from an upper portion of the capsule to the diamond seed pad at a lower portion of the capsule where diamond growth occurs. The fact that the seeds are anchored to a pad in a lower portion of the HPHT capsule ensures that the seeds have a fixed and well defined orientation relative to the applied temperature and pressure. That is, the growing diamond crystals are prevented from floating within the metal solvent during synthesis and this allows the crystals to grow with a well defined single crystal morphology. If the seeds are allowed to float in the melted reactants during synthesis, this leads to misshapen growth. Therefore anchoring is required to form good morphology, large single crystal stones. The temperature gradient allows carbon to be transported to the anchored crystals via convection currents to achieve relatively large single crystal growth. Diamond growth is driven by the temperature differential. Hence this method is known as the temperature gradient method.

Chemical vapour deposition (CVD) processes for synthesis of diamond material are also 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, diamond 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.

CVD synthesis of both poly crystalline and single crystal diamond material is known. Polycrystalline CVD diamond material may be grown on a range of substrates including silicon and refractory metals such as tungsten and molybdenum. Single crystal diamond material is generally grown via homoepitaxial growth on single crystal diamond substrates. As such, single crystal diamond substrates are utilized to grow synthetic single crystal diamond material using both HPHT and CVD techniques.

One problem which may occur during single crystal diamond synthesis is cracking of the single crystal diamond substrates. While diamond material is extremely hard it is relatively brittle. Diamond's limited ability to plastically deform under stress can lead to cracking of the substrate and overlying synthetic diamond material during a growth cycle. This problem can occur in both HPHT and CVD diamond synthesis, both methods exposing the single crystal diamond substrates to extreme physical conditions which can create stress in the single crystal substrates. The stress may be generated during initiation, propagation, or termination of a synthetic diamond growth cycle. It is an aim of certain embodiments of the present invention to reduce the problem of cracking of single crystal diamond substrates during a synthetic diamond growth cycle.

Summary of Invention

According to a first aspect of the present invention there is provided a method of growing synthetic single crystal diamond material, the method comprising:

providing a single crystal diamond substrate; and

growing synthetic single crystal diamond material on said single crystal diamond substrate,

wherein said single crystal diamond substrate is irradiated prior to growing synthetic single crystal diamond material thereon, and

wherein the irradiation comprises irradiating the diamond material to a depth of 5 μιη or greater.

According to a further aspect of the present invention there is provided a composite substrate assembly comprising:

a carrier substrate; and

a plurality of irradiated single crystal diamond substrates mounted on the carrier substrate,

wherein the irradiated single crystal diamond substrates are irradiated to a depth of 5 μιη or greater.

Brief Description 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 the basic steps involved in performing a method of manufacturing synthetic single crystal CVD diamond material according to an embodiment of the present invention; and Figure 2 illustrates the basic steps involved in performing a method of manufacturing synthetic single crystal HPHT diamond material according to an embodiment of the present invention.

Detailed Description of Certain Embodiments

It is known that irradiating diamond material can change its colour. See, for example, EP 0 615 954 Al, EP 0 316 856 and "The Type Classification System of Diamonds and Its Importance in Gemology", Gems and Gemology, Vol. 45, No. 2, pp96-l l l, 2009. Furthermore, it is known that irradiating diamond material can change the wear characteristics of diamond tools. See, for example, US4184079, GB1588445, GB1588418, US4012300, SU1346418, US4273561, SU1813812, US4533812, US4319889 and JP2006021963.

In contrast to the above, the present inventors are proposing to use irradiation to reduce the problem of cracking of single crystal diamond substrates during a synthetic diamond growth cycle. While not been bound by theory, the present inventors believe that the problem of cracking in single crystal diamond substrates during a synthetic diamond growth cycle is a result of thermally induced stress build-up in the substrate and that irradiating the substrates prior to growing synthetic single crystal diamond material thereon introduces vacancy defects into the substrates which may function as crack-stops to prevent, or at least reduce, crack propagation through the substrates due to thermally induced stresses. Introducing vacancy defects may also alleviate lattice mismatches or stress induced by twinned growth around the periphery of the single crystal CVD diamond material. The vacancies may also aid in evening out the strain in a substrate that might contain multiple growth sectors (i.e. areas of different strain) which can be one of the causes of intrinsic substrate strain and cracking. This invention will thus allow an increase in the yield of synthetic diamond material recoverable from a synthetic diamond growth process.

Figure 1 illustrates the basic steps involved in performing a method of manufacturing synthetic single crystal CVD diamond material according to an embodiment of the present invention. The method comprises: forming a plurality of single crystal diamond substrates; irradiating the plurality of single crystal diamond substrates to increase their crack-resistance; mounting the plurality of irradiated single crystal diamond substrates on a carrier substrate; loading the carrier substrate comprising the plurality of irradiated single crystal diamond substrates into a CVD reactor; and growing synthetic single crystal CVD diamond material on the single crystal diamond substrates.

Figure 2 illustrates the basic steps involved in performing a method of manufacturing synthetic single crystal HPHT diamond material according to an embodiment of the present invention. The method comprises: forming a plurality of single crystal diamond substrates; irradiating the plurality of single crystal diamond substrates to increase their crack-resistance; mounting the plurality of irradiated single crystal diamond substrates on a carrier substrate; loading the carrier substrate comprising the plurality of irradiated single crystal diamond substrates into a capsule along with a source of carbon; loading the capsule into a HPHT press; and growing synthetic single crystal HPHT diamond material on the single crystal diamond substrates.

In each of the aforementioned embodiments a plurality of single crystal diamond substrates are formed and then the single crystal diamond substrates are irradiated. However, in an alternative embodiment a single crystal diamond material may be first irradiated and then the plurality of single crystal diamond substrates can be formed from the irradiated single crystal diamond material. Indeed, this alternative may be advantageous in reducing unwanted cracking or chipping of the diamond material during formation of the single crystal diamond substrates.

In each of the aforementioned embodiments a plurality of irradiated single crystal diamond substrates is mounted on a carrier substrate. Of course, it is possible to perform diamond synthesis on only a single irradiated substrate. However, to increase yield per growth run it is preferred, when possible, to grow a plurality of single crystal diamonds in a single growth run. In such synthesis methods, it is required to mount single crystal diamond substrates at different locations within a synthesis apparatus. As such, it is possible that variations in stress will occur across the plurality of single crystal diamond substrates during diamond synthesis thereon. Accordingly, the use of irradiated substrates can be particularly beneficial in such synthesis methods to avoid some of the plurality of single crystal diamond substrates from cracking during the growth run due to increased stress in certain regions of the synthesis apparatus and thus increase yield per growth run.

In light of the above, according to a further aspect of the present invention there is provided a composite substrate assembly comprising:

a carrier substrate; and

a plurality of irradiated single crystal diamond substrates mounted on the carrier substrate.

The carrier substrate may comprise one or more of: a refractory metal disk (e.g. molybdenum or tungsten); a silicon wafer; a ceramic disk or holder; and a polycrystalline diamond wafer or layer.

The plurality of irradiated single crystal diamond substrates may be mounted on the carrier substrate using a bonding technique which is robust to the physical conditions imparted on the composite substrate during diamond growth thereon. A braze join has been found to be suitable for certain applications as it provides a join which is stable up to high temperatures and provides a certain degree of flexibility when compared with more rigid solder joins to alleviate thermally induced stress which may lead to failure of the bond or cracking of the single crystal diamond substrates.

The single crystal diamond substrate may be selected from one of: a HPHT diamond material having a total equivalent isolated nitrogen concentration in the range 1 to 800 ppm; a CVD diamond material having a total equivalent isolated nitrogen concentration in the range 0.005 to 100 ppm; and a natural diamond material having a total nitrogen concentration in the range 1 to 2000 ppm.

It is proposed that there may be an interaction between the mechanism by which irradiation increases the crack resistance of diamond material and the nitrogen content within the diamond crystal matrix. Although this mechanism is not fully characterized, one possibility is that irradiation introduces a relatively even distribution of vacancy defects within the crystal matrix which can act as crack stops and/or introduce regions of stress/strain within the diamond crystal matrix which may act to inhibit crack propagation. Nitrogen impurities within the crystal matrix may function to trap vacancies introduced by the irradiation to form N-V-N or N-V centres. During manufacture, and in use, a single crystal diamond substrate becomes hot. As such, vacancies introduced by irradiation may become mobile within the crystal matrix. However, it is desirable to provide a relatively even distribution of vacancies within the crystal matrix to act as crack stops. As such, a relatively even distribution of vacancies introduced by irradiation may be maintained by ensuring that a suitable concentration of nitrogen is present within the crystal matrix to prevent the vacancies migrating through the diamond crystal structure.

In addition to the above, it has been recognized that CVD, HPHT, and natural diamond are structurally different materials with, for example, different distributions of nitrogen. Natural diamond for example, tends to have aggregated nitrogen defects (Type la) whereas synthetic CVD and HPHT diamond material tends to have isolated nitrogen defects (Type lb). Materials having different types and distributions of nitrogen defects behave differently after being subjected to irradiation. Furthermore, the nitrogen content can affect other characteristics such as CVD diamond growth. As such, the optimum amount of nitrogen required to be present in the diamond material to interact with vacancy defects introduced by irradiation will vary according to the type of diamond material which is irradiated.

In light of the above, and in accordance with the present invention, it is proposed that for irradiated single crystal diamond substrates, the optimum isolated nitrogen concentration for HPHT diamond material lies in the range 1 to 800 ppm, the optimum isolated nitrogen concentration for CVD diamond material lies in the range 0.005 to 100 ppm, and the optimum isolated nitrogen concentration for natural diamond material lies in the range 1 to 2000 ppm. Using such materials, the irradiation and the nitrogen act in a compatible manner to provide a more crack resistant substrate.

The HPHT diamond material may have a total equivalent isolated nitrogen concentration in the range 1 to 800 ppm, 1 to 600 ppm, 10 to 300 ppm, 10 to 200 ppm, 50 to 250 ppm, 100 to 200 ppm, 10 to 100 ppm, or 10 to 50 ppm. The CVD diamond material may have a total equivalent isolated nitrogen concentration in the range 0.01 to 50 ppm, 0.05 to 20 ppm, 0.08 to 5 ppm, or 0.1 to 2 ppm.

The natural diamond material may have a total nitrogen concentration in the range 200 to 2000 ppm, 500 to 1500 ppm, 800 to 1300 ppm, or 1000 to 1200 ppm.

It is to be noted that the nitrogen concentrations discussed above are measured as an average concentration over a majority volume of the diamond substrate material. The majority volume may be greater than or equal to 50%, 60%>, 70%>, 80%>, or 90%> of the total volume of the diamond substrate material. This is to account for the fact that different diamond growth sectors have different rates of nitrogen uptake leading to concentration variations.

The total equivalent isolated nitrogen concentration for a diamond substrate material can be measured by techniques known by persons skilled in the art, for example the concentration can be calculated by deconvoluting the absorption spectrum of the one phonon part of the FTIR spectrum. The total concentration of nitrogen including aggregated nitrogen may be determined using secondary ion mass spectroscopy (SIMS).

The irradiation may comprise electrons, neutrons, X-rays, gamma radiation, protons, alpha particles, or ions.

Advantageously, the energy and dosage of irradiation is controlled to form a single crystal diamond substrate having a concentration of vacancy point defects in the range: 1 x 10 ¹⁴ to 1 x 10 ²¹ vacancies/cm ³ ; 1 x 10 ¹⁵ to 1 x 10 ²¹ vacancies/cm ³ ; 5 x 10 ¹⁵ to 1 x 10 ²⁰ vacancies/cm ³ ; 1 x 10 ¹⁶ to 5 x 10 ¹⁹ vacancies/cm ³ ; or 5 x 10 ¹⁶ to 1 x 10 ¹⁹ vacancies/cm ³ .

The irradiation should be of sufficient energy to generate isolated vacancies or relatively small cluster defects in the diamond material which can act as crack stops. If the energy if the radiation is relatively high or the radiation comprises relatively heavy particles, carbon atoms are knocked off their lattice sites with enough energy to knock further carbon atoms of their lattice sites resulting in what is known as cascade damage. This results in clusters of defects and a region of stress/strain within the diamond crystal matrix which may act to inhibit crack propagation. Small cluster defects are acceptable. However, if the energy of the radiation is too high, the cascade damage becomes too extensive and crack resistance may be reduced. Furthermore, extensive damage to the single crystal diamond substrates can potentially reduce the quality of the overgrown diamond material. Conversely, if the energy of the radiation is too low, the radiation may not penetrate sufficiently into the diamond material to provide a bulk treatment of the diamond substrates.

In light of the above, it is advantageous to irradiate the single crystal diamond substrate material in order to form a large number of relatively evenly spaced isolated vacancies or small cluster defects without the individual clusters becoming too large in size. If cluster defects are formed, they should preferably have a maximum length no greater than 50 atoms, 20 atoms; 10 atoms; or 5 atoms in length. The size of the cluster defects can be measured using transmission electron microscopy (TEM) or positron annihilation techniques. If cluster defects are formed, the single crystal diamond substrate material may have a concentration of cluster defects in the range: 1 x 10 ¹⁴ to 1 x 10 ²¹ clusters/cm ³ ; 1 x 10 ¹⁵ to 1 x 10 ²¹ clusters/cm ³ ; 5 x 10 ¹⁵ to 1 x 10 ²⁰ clusters/cm ³ ; I x l0 ¹⁶ to 5 x l0 ¹⁹ clusters/cm ³ ; or 5 x 10 ¹⁶ to 1 x 10 ¹⁹ clusters/cm ³ .

The energy of the radiation will depend on the type of radiation and the mechanism of energy transfer between the radiation and a carbon atom it hits within the diamond crystal matrix. The dose of radiation will also depend on the type of radiation and the number of vacancies produced per particle of radiation.

An iterative process can be used to find optimum vacancy defect levels. A diamond material can be irradiated, tested, re-irradiated, and so on to find the optimum defect levels for a particular diamond material for use as a substrate for single crystal diamond growth thereon.

According to certain embodiments, the irradiation may be above an energy and dose rate which leads to a change in colour of the diamond material. It is also advantageous that the irradiation is kept below an energy and dose rate which would lead to amorphization of the diamond material. Amorphization has a detrimental effect on the mechanical properties of the diamond material. In general, the longer the irradiation dose the more vacancy defects will be introduced. However, the rate of vacancy incorporation may vary according to the nature of the starting material and the nature of the irradiation.

For electrons, the irradiation may have an energy: 30 keV or greater; in the range 0.1 MeV to 12 MeV; in the range 0.5 MeV to 10 MeV; in the range 1 MeV to 8 MeV; 2 MeV to 8 MeV or in the range 4 MeV to 6 MeV. The dosage of electron irradiation

15 2 16 2 19 2 may be: 1 x 10 e ^" / cm or greater; in the range 1 x 10 e ^" / cm to 1 x 10 e ^" / cm ;

17 2 19 2 18 2 in the range 1 x 10 e ^" / cm to 1 x 10 e ^" / cm ; or in the range 2 x 10 e ^" / cm to 1 x 10 ¹⁹ e / cm ² .

For neutrons, the irradiation may have an energy: in the range 1.0 keV to 12 MeV; in the range 1.0 keV to 10 MeV; in the range 100 keV to 8 MeV; in the range 100 keV to 6 MeV; or in the range 500 keV to 4 MeV. The neutrons will tend to be distributed over a range of energies. Accordingly, at least 50%, at least 60%, at least 70%>, or at least 80% of the neutrons fall within one of the aforementioned ranges. The dosage of neutron irradiation may be: 1 x 10 ¹⁴ neutrons / cm ² or greater; in the range 1 x 10 ¹⁴

2 18 2 15 2 neutrons / cm to 1 x 10 neutrons / cm ; in the range 1 x 10 neutrons / cm to 5 x

17 2 15 2 17 2

10 neutrons / cm ; or in the range 1 x 10 neutrons / cm to 1 x 10 neutrons / cm .

For gamma rays, the irradiation may have an energy in the range 0.1 to 12 MeV, 0.2 to 10 MeV, or 0.3 to 8 MeV. The dosage of gamma ray irradiation may be 5 x 10 ¹⁶ γ- ray/cm 2 or greater, in the range 1 x 1017 to 5 x 1021 γ-ray/cm2 , or in the range 5 x 1017 to 1 x 10 ²¹ γ-ray/cm ² .

During irradiation according to certain embodiments of the present invention, the temperature of the diamond material is kept relatively low. For example, the temperature may be: 500°C or less; 400 °C or less; 300 °C or less; 200 °C or less; 100 °C or less; or 50 °C or less. In order to keep the temperature down, the diamond material may be actively cooled during irradiation. It is advantageous to keep the temperature relatively low as an increase in temperature can result in a decrease in the number density of vacancy defects. The method may also comprise the optional step of annealing the diamond material in addition to treatment by irradiation. The annealing step may be performed before, during or after the irradiation step, or any combination thereof. In certain applications it may be preferred to perform an annealing step before irradiating as an annealing step after irradiating can result in a decrease in vacancy defects. The annealing may be performed at a temperature equal to or greater than 800°C, 1200°C, 1400°C, 1600°C, 1800°C, 2200°C, or 2400°C. Embodiments of the present invention may include a combination of irradiating and a relatively low temperature anneal, or a combination of irradiating and a high pressure high temperature anneal. Embodiments also envisage the possibility of iterative doses of radiation and/or iterative annealing. That is, more than one annealing and/or irradiation step may be performed. For example, the diamond material may be annealed, then irradiated, and then annealed. Further alternating irradiation and annealing steps may also be performed. Alternatively, the diamond material may not be exposed to any substantial annealing step, at least after irradiation. By substantial annealing step, we mean an annealing step which substantially and measurably changes the properties of the material. Annealing below 1800°C can be conducted in an inert atmosphere whereas annealing above 1800°C may require stabilising pressures, especially if a long anneal is performed. The annealing is typically conducted for 30 seconds up to 50 hours. By inert atmosphere, we mean an atmosphere under which the diamond will not significantly degrade during annealing. Examples include Argon and Neon.

A relatively low temperature anneal may be advantageous for certain applications. In use, the diamond substrates get hot, and methods of mounting the diamond substrates may also include brazing at, for example, 900°C. As such, a low temperature anneal can be useful to ensure a consistent performance of the diamond substrate in use. For example, a low temperature anneal at a temperature of 1500°C or less, 1300°C or less, 1200°C or less, 1100°C, or less, or approximately 1000°C can be useful.

The irradiation may be performed before, during, or after processing to form one or more single crystal diamond substrates. The processing may involve treating, grinding, cutting and/or shaping the diamond material to form one or more diamond substrates, each substrate having a growth surface for diamond growth thereon. In addition to improving the crack resistance of the single crystal diamond substrates, an increase in crack resistance can allow the single crystal diamond substrates to be processed in different ways. For example, an increase in crack resistance can allow the single crystal diamond substrate to be processed to have a low surface roughness without cracking or chipping during processing. As such, irradiated substrates can provide a route to better quality growth surfaces which enables high quality, low defect CVD diamond material to be grown thereon. In this regard, a plasma etch may be applied to the growth surface after grinding/polishing the growth surface and prior to CVD growth thereon. Defects such as microcracks formed during grinding/ polishing can result in a non-flat surface after etching. Irradiating prior to processing can reduce surface damaged caused by grinding/ polishing and thus result in a flatter surface after etching in addition to reducing surface defects.

As an alternative to the above, in some instances it may be desirable to intentional introduce defects into the growth surface of the substrate. In this instance, a high level of irradiation may be used to form a growth surface comprising radiation damage which can be used to nucleate dislocation defects in CVD diamond material grown thereon.

The diamond material can be irradiated to a depth of 5 μιη or greater, 10 μιη or greater, 100 μιη or greater, 500 μιη or greater or 1 mm or greater. The diamond material can advantageously be irradiated throughout a total thickness of the diamond material using, for example, electrons, neutrons, or electromagnetic radiation. It should be noted that this type of irradiation is distinguished over techniques which involves implanting ions just under the growth surface to form a release layer at a depth of around Ιμιη such that after growth on the substrate the substrate can be severed at the release layer to release the material grown thereon. Such a surface treatment does not result in a bulk change in the material properties of the substrate.

The diamond material may also be exposed to radiation on more than one side of the material. For example, a diamond plate may be exposed on both main faces to achieve an even exposure of radiation. Rotation of the sample during irradiation, or repeated rotation followed by irradiation, can assist in achieving irradiation throughout the volume of diamond material and/or assist in achieving a relatively even distribution of vacancy defects. Alternatively, irradiation from one side may be used to generate a gradient of damage through the material, particularly when applied to material already prepared as a substrate. For example, in some instances it may be desirable to impart a greater degree of damage at the growth surface to nucleate defects as previously described. In other instances, it may be desirable to impart a lower degree of damage at the growth surface so as to avoid nucleation of defects. In this case, the substrate material may be irradiated from a rear side only or from a lateral side spaced back from the growth surface. In such cases, annealing may be used in conjunction with irradiation to remove damage. If the damage is non-uniform it is possible to substantially remove damage from one or more portions of the material which have a lower initial level of damage while retaining damaged regions which initially had a higher level of damage due to a non-uniform irradiation process.

An advantage of the types of irradiation discussed herein is that they can provide a bulk treatment of the diamond material rather than just a surface treatment. Accordingly, the irradiation can be done before processing the diamond material into a substrate. Furthermore, bulk treatment can be applied to a large volume of material pieces with relatively simple handling requirements. For example, diamond pieces do not need to be carefully mounted in a certain orientation as is required for many surface treatments. In contrast, prior art ion implantation methods need to be performed after processing of the diamond material. This is because ion implantation methods generally result in a surface treatment only. Processing of the material into a substrate by, for example, cutting, shaping and/or grinding the diamond material will remove the treated surface of such materials.

The single crystal diamond substrates may be natural diamond or synthetic diamond. The synthetic diamond may be formed by a high pressure high temperature (HPHT) method or by a chemical vapour deposition (CVD) method. The single crystal diamond substrate may have at least one dimension of: 0.5 mm or greater; 1 mm or greater; 2 mm or greater 3 mm or greater; 5 mm or greater or 10 mm or greater. The invention is particularly suited for application to HPHT and CVD diamond. However, certain embodiments may also be applied to natural diamond. It will be understood that natural diamond, HPHT diamond and CVD diamond have their own distinctive structural and functional characteristics and thus the terms "natural", "HPHT" and "CVD" not only refer to the method of formation of the diamond material but also refer to specific structural and functional characteristics of the materials themselves.

The diamond material used in embodiments of the present invention may be Type la, Type lb, Type Ila, or Type lib. Type la and Type lb diamonds contain nitrogen. Although Type Ila diamonds are usually defined as containing no nitrogen, in practice they can contain low concentrations of nitrogen. In Type la, the nitrogen atoms form various types of aggregate defect whereas in Type lb diamonds, the nitrogen atoms tend to be isolated as single impurities. Type la diamonds can be colourless, brown, pink, and violet. Natural Type lb diamonds can be deep yellow ("canary"), orange, brown or greenish. The colour of a diamond is determined by the number, type and distribution of defects within the crystal structure. Crystal defects include dislocations, microcracks, twin boundaries, point defects, and low angle boundaries. As such, for example, the colour of diamond will depend on the type and distribution of impurities such as nitrogen as well as the type and distribution of other defects such as dislocations. There is a large number of different types and subclasses of defects within diamond. For example, there are numerous different types of nitrogen defect alone, each having its own spectral characteristics.

The diamond material in the single crystal diamond substrate may be configured into a number of possible crystallographic orientations including a growth surface which corresponds to a {110}, {111 }, {113} or {100} crystallographic plane (within 20°, 10°, 5°, 3°, 2°, or 1°).

Examples

Electron Irradiation

Electron irradiation (for example, using electrons with energies less than or equal to 12 MeV) typically introduces vacancies in an isolated form. These may be in the neutral (V ) and negative charge states (V ). The total vacancy concentration ([V _T ] = [V°] + [V ]) post irradiation should preferably be in the range: 1 x 10 ¹⁴ to 1 x 10 ²¹ vacancies/cm 3 ; l x l015 to 1 x 1021 vacancies/cm 3 ; 5x1015 to 1 x 1020 vacancies/cm 3 ; 1 x 10 ¹⁶ to 5 x 10 ¹⁹ vacancies/cm ³ ; or 5 x 10 ¹⁶ to 1 x 10 ¹⁹ vacancies/cm ³ . Such a concentration of defects can be formed, for example, using electron irradiation having a dose rate: 1 x 10 ¹⁵ e ^" / cm ² or greater; in the range 1 x 10 ¹⁶ e ^" / cm ² to 1 x 10 ¹⁹ e ^" /

2 17 2 19 2 17 2 cm ; in the range 1 x 10 e ^" / cm to 1 x 10 e ^" / cm ; or in the range 2 x 10 e ^" / cm to 1 x 10 ¹⁹ e / cm ² .

The electron irradiation may have an energy of: 30 keV or greater; in the range 0.1 MeV to 12 MeV; in the range 0.5 MeV to 10 MeV; or in the range 1 MeV to 8 MeV. The preferred energy is that which introduces a near uniform concentration of vacancies in a nitrogen doped diamond, while minimizing the formation of cascade damage e.g. vacancy chains. For the results reported here it was found that 4.5 MeV provided a good compromise between these two factors.

Factors such as diamond temperature, beam energy, beam flux, and even the starting diamond's properties can affect the [V _T ] produced for a fixed experimental irradiation set-up and time. Irradiation is typically carried out with the sample mounted under ambient conditions -300 K with only minimal temperature rise during the irradiation dose (e.g. less than 100 K). However, factors such as beam energy and beam flux can lead to sample heating. Preferably the sample is held as cold as possible (with even cryogenic cooling at 77 K being advantageous under some circumstances) to enable high dose rates without compromising temperature control and thus minimize the irradiation time. This is advantageous for commercial reasons.

The vacancy concentration can be measured spectroscopically. For example, to measure concentrations of isolated vacancies, spectra are obtained at 77 K, using liquid nitrogen to cool the samples, since at that temperature sharp peaks at 741 nm and 394 nm are seen that are attributable to neutral and negatively charged isolated vacancies respectively. The coefficients that are used for the calculations of concentrations of isolated vacancies in the present specification are those set out by G. Davies in Physica B 273-274 (1999) 15-23, as detailed in Table 1 below. In Table 1, "A" is the integrated absorption (meV cm ¹ ) in the zero phonon line of the transition, measured at 77 K, with the absorption coefficient in cm ^"1 and the photon energy in meV. The concentration is in cm ^"3 .

Table 1

Neutron Irradiation

Neutron irradiation tends to knock carbon atoms off their lattice sites with enough energy to knock further carbon atoms of their lattice sites resulting in what is known as cascade damage. This results in clusters of defects and a region of stress/strain within the diamond crystal matrix which may act to inhibit crack propagation and increase toughness. If the energy of the neutrons is too high, the cascade damage becomes too extensive and toughness and/or wear resistance is reduced.

In light of the above, it is advantageous to irradiate the diamond material in order to form a large number of isolated and/or relatively small cluster defects without the individual clusters becoming too large in size. It has been found that a suitable size of cluster defects can be formed using neutron radiation having an energy: in the range 1.0 keV to 12 MeV; in the range 1.0 keV to 10 MeV; in the range 100 keV to 8 MeV; in the range 100 keV to 6 MeV; or in the range 500 keV to 4 MeV. The neutrons will tend to be distributed over a range of energies. Accordingly, at least 50%, at least 60%, at least 70%, or at least 80% of the neutrons fall within one of the aforementioned ranges.

Neutron irradiation according to the present invention can introduce a near uniform concentration of isolated vacancies and/or small cluster defects, while minimizing the formation of extensive cascade damage e.g. long vacancy chains. It is difficult to measure the concentration of cluster defects. However, the concentration of isolated defects can be readily characterized spectroscopically. Vacancy point defects may be in the neutral (V°) and negative charge states (V ^" ). The total isolated vacancy concentration ([V _T ] = [V°] + [V ]) may be in the range: 1 x 10 ¹⁴ to 1 x 10 ²⁰ vacancies/cm 3 ; 1 χ 1015 to 1 x 1019 vacancies/cm 3 ; l x l015 to 1 x 1018 vacancies/cm 3 ;

15 17 3 16 17 3

1 x 10 to 1 x 10 vacancies/cm ; or 1 x 10 to 1 x 10 vacancies/cm . The presence of cluster defects can be detected by a broadening of the absorption peak for isolated vacancies. Such a concentration of vacancy defects can be formed, for example, using neutron irradiation having a dose rate: 1 x 10 ¹⁴ neutrons / cm ² or

1 2 18 2

greater; in the range 1 x 10 neutrons / cm to 1 x 10 neutrons / cm ; in the range 1 x

15 2 17 2 15 2

10 neutrons / cm to 5 x 10 neutrons / cm ; or in the range 1 x 10 neutrons / cm to 1 x 10 ¹⁷ neutrons / cm ² .

Embodiments of the present invention envisage the possibility of forming a large number of evenly spread isolated vacancies and/or relatively small cluster defects using neutron irradiation, while avoiding large extensive cluster defects formed by extensive cascade damage as a result of neutrons which are too high in energy. This requires the careful selection of a neutron flux of an appropriate energy. It is advantageous to select an energy which results in cluster defects having a maximum size limitation for individual clusters. This is consistent with the understanding that it is desirable to form relatively small, relatively evenly spread defect clusters rather than large sprawling regions of cascade damage. Accordingly, it is preferable that each of a plurality of cluster defects has a maximum size no greater than 50 atoms in length, more preferably no greater than 20 atoms in length, more preferably still no greater than 10 atoms in length, and most preferably no greater than 5 atoms in length. The size of the cluster defects can be measured using transmission electron microscopy (TEM) or positron annihilation techniques.

As previously described, it is advantageous to keep the temperature of the diamond material relatively low during irradiation as an increase in temperature can result in a decrease in the number density of defects. One advantage of neutron irradiation is that it tends not to raise the temperature of the diamond material as much as, for example, electron irradiation. As such, according to certain embodiments of the present invention no active cooling is required.

Another advantage of neutron irradiation is that the diamond material does not usually need to be rotated during neutron irradiation to achieve a relatively even distribution of defects. In fact, one advantage of neutron irradiation over, for example, electron irradiation is that neutrons tend to penetrate more easily through an entire sample to obtain a relatively even distribution of defects without rotation of the sample. It can thus be easier to achieve a high dose of radiation through a sample of diamond in a commercially viable way.

Care needs to be taken when selecting the diamond material to be neutron irradiated so that samples do not remain radioactive for an unreasonably long period of time post irradiation. It is therefore necessary to ensure the diamond material selected for neutron irradiation contains substantially no metallic or other inclusions which will remain radioactive for an unreasonable length of time after exposure to neutron irradiation. In this regard, the diamond material may only be released post neutron irradiation if the radioactivity is less than 4 Bq/g. The diamond material selected for neutron irradiation should therefore preferably contain no metallic inclusions having a size equal to or less 10 μιη, 5 μιη, or 1 μιη. The metallic inclusions should preferably be equal to or less than 0.1%, 0.01%, 0.001%, or 0.0001% of the total mass of the diamond. The diamond material should also preferably be acid cleaned immediately before irradiation to remove any potentially radioactive species from the surface, thereby ensuring that the level of radioactivity falls below 4 Bq/g after being held to 'cool' for equal to or less than 6 months, 4 months, 2 months, 1 month, 2 weeks, or 1 week.

Several CVD diamond samples have been irradiated with neutrons (typically containing approximately 0.1 - 0.5 ppm N). Imperial College's Ur ²³⁵ Consort reactor at Silwood Park, Ascot, UK was used for these treatments (this reactor has now been decommissioned - an alternative can be that found at Delft University, Holland.) The diamond material was typically irradiated for between 14 and 28 hours, with an energy distribution within the reactor which peaked at 1 MeV, 59% of the neutrons falling into the energy range of 0.2 to 2.2 MeV and 86% of neutrons falling into the energy range 0.2 to 12 MeV.

The diamond samples therefore received a dose of approximately 5 x 10 ¹⁵ to 1 x 10 ¹⁶ neutrons / cm ² . A colour change was observed from colourless to yellow-green as a result of the neutron irradiation. Using cold UV- Visible spectroscopic measurements (using the same method of calculation as described above) the concentration of isolated neutral vacancies was measured to be in the range of 0.2 - 0.51 ppm (2 x 10 ¹⁶ to 5.1 x 10 ¹⁶ vacancies / cm ³ ). There was a clear broadening of the GR1 peak compared to corresponding electron irradiated samples, which shows evidence for the formation of vacancy clusters in addition to isolated vacancies.

Gamma Irradiation

Gamma rays can also be used to form vacancy defects within a diamond material. For Gamma radiation, the irradiation may have an energy in the range 0.1 to 12 MeV, 0.2 to 10 MeV, or 0.3 to 8 MeV. The dosage of gamma ray irradiation may be 5 x 10 ¹⁶ γ- ray/cm 2 or greater, in the range 1 x 1017 to 5 x 1021 γ-ray/cm2 , or in the range 5 x 1017 to 1 x 10 ²¹ γ-ray/cm ² .

While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to 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 appendant claims.