Method of producing CVD single crystal diamond material

Abstract

Method of producing CVD single crystal diamond material using a chemical vapour deposition process, the method comprising: providing a single crystal diamond material having a largest linear dimension

Description

Field The invention relates to the field of methods of producing CVD single crystal diamond material. Background Diamond materials may be categorized into three main types: natural diamond materials; HPHT (high pressure high temperature) synthetic diamond materials, and CVD (chemical vapour deposited) synthetic diamond materials. These categories reflect the way in which the diamond materials are formed. Furthermore, these categories reflect the structural and functional characteristics of the materials. This is because while natural, HPHT synthetic, and CVD synthetic diamond materials are all based on a theoretically perfect diamond lattice, the defects in these materials are not the same. For example, CVD synthetic diamond contains many defects unique to the process of CVD, and whilst some defects are found in other diamond forms, their relative concentration and contribution is very different. As such, CVD synthetic diamond materials are different to both natural and HPHT synthetic diamond materials. Diamond materials may also be categorized according to their physical form. In this regard, diamond materials may be categorized into three main types: single crystal diamond materials; polycrystalline diamond materials; and composite diamond materials. Single crystal diamond materials are in the form of individual single crystals of various sizes ranging from small “grit” particles used in abrasive applications through to large single crystals suitable for use in a variety of technical applications as well for gemstones in jewellery applications. Polycrystalline diamond materials are in the form of a plurality of small diamond crystals bonded together by diamond-to-diamond bonding to form a polycrystalline body of diamond material such as a polycrystalline diamond wafer. Such polycrystalline diamond materials can be useful in various applications including thermal management substrates, optical windows, and mechanical applications. Composite diamond materials are generally in the form of a plurality of small diamond crystals bonded together by diamond-to-diamond or a nondiamond matrix to form a body of composite material. Various diamond composites are known including diamond containing metal matrix composites, particularly cobalt metal matrix composites known as polycrystalline diamond (PCD), and skeleton cemented diamond (ScD) which is a composite comprising silicon, silicon carbide, and diamond particles. It should also be appreciated that within each of the aforementioned categories there is much scope for engineering diamond materials to have particular concentrations and distributions of defects in order to tailor diamond materials to have particular desirable properties for particular applications. The present disclosure is concerned with CVD single crystal synthetic diamond materials. CVD processes for synthesis of diamond material are well known. 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, CVD synthetic diamond material can be deposited. Polycrystalline CVD diamond material may be formed on a non-diamond substrate such as a refractory metal or silicon substrate. Single crystal CVD synthetic diamond material may be formed by homoepitaxial growth on a single crystal diamond substrate. Atomic hydrogen present in the process 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 synthetic diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma. A problem with prior art methodologies is how to achieve large area single crystal CVD synthetic diamond material. It has been found that large area single crystal diamond can be grown by a process known as “heteroepitaxial growth”. This is where diamond nucleates and grows epitaxially on a non -diamond substrate. Iridium has been found to be a suitable substrate to allow diamond nucleation and growth, but other substrates such as silicon, silicon carbide, copper, nickel, rhenium and titanium carbide have been investigated. US 7,396,408 describes such a process. In this case, diamond is grown in a CVD process using a silicon carbide or silicon single crystal wafer that has a layer of iridium deposited on its surface. This is used as a substrate on which to heteroepitaxially deposit and grow diamond. During the growth process, diamond crystallites nucleate on the iridium film. These crystallites grow and merge to form a single crystal layer, which is continued until a single crystal diamond wafer of the desired thickness is formed. Typically the dislocation density reduces via dislocation interactions (fusion and annihilation) as growth proceeds, leading to a single crystal diamond wafer that has a higher dislocation density adjacent to the original nucleation face compared to the growth face. A problem is the cost of materials. Large area single crystal diamond material is relatively time consuming and expensive to produce, and so reducing this cost is important. Summary For certain applications of CVD diamond requiring large area single crystals, it is desirable to provide a process for homoepitaxially growing single crystal diamond on a heteroepitaxially grown single crystal CVD diamond substrate. Such substrates are expensive. It is an object of the present invention to provide a method of growing CVD single crystal diamond material with reduced cost and reduced energy and material use. According to a first aspect, there is provided a method of producing CVD single crystal diamond material using a chemical vapour deposition process. The method comprises providing a single crystal diamond material having a largest linear dimension of at least 50 mm. The single crystal diamond material is processed to produce a plurality of substrates from the single crystal diamond material, each substrate of the plurality of substrates having a largest linear dimension of at least 50 mm. Each of the plurality of substrates is used to grow a plurality of further single crystal diamond products, each further single crystal diamond product having a largest linear dimension of at least 50 mm. As an option, the method comprises using at least one of the plurality of single crystal diamond products as a substrate on which to grow a further single crystal diamond product. The processing optionally comprises forming in the single crystal diamond material a damage layer comprising sp2 bonded carbon, wherein the presence of the damage layer defines a first diamond layer on a first side with respect to the damage layer and a second diamond layer on a second, opposing side with respect to the damage layer, and electrochemically etching the damage layer to separate the first layer from the second layer. This allows very thin layers to be separated and used as substrates. The method optionally further comprises, prior to electrochemically etching the damage layer, removing material from a lateral side of the single crystal diamond material. This step is to ensure that the etchant can access the damage layer through the lateral side. As an option, each further single crystal diamond product has a desired largest linear dimension, and the largest linear dimension of the provided single crystal diamond material is greater than the desired largest linear dimension. By creating an oversized product initially, any lateral dimension losses caused by processing mean that the resultant products still have the required lateral dimensions. As an option, the provided single crystal diamond material has a largest linear dimension of at least 50 mm, the largest linear dimension oriented within 5° of a {100} plane. As an option, the method further comprises providing the single crystal diamond material having a largest linear dimension of at least 50 mm by growing the single crystal diamond material on a first substrate such that lateral overgrowth is achieved, and removing material from lateral edges of the single crystal diamond material prior to processing the single crystal diamond material to produce the plurality of substrates from the single crystal diamond material, wherein the amount of material removed is equal to or less than the amount of lateral overgrowth. This technique ensures that substrates can be cloned indefinitely, as there is no overall loss in lateral dimensions. As a further option, the method comprises, prior to growing the single crystal diamond material on the first substrate, masking at least one lateral edge of the first substrate to prevent lateral overgrowth from the lateral edge of the first substrate. This ensures that lateral overgrowth does not overgrow a damage layer introduced into the diamond prior to the overgrowth, and so allows the etchant to access the damage layer. As an option, the single crystal diamond material has a largest linear dimension selected from any of at least 75 mm and at least 100 mm. As an option, each substrate of the plurality of substrates has a thickness selected from any of at least 10 pm, at least 50 pm, at least 100 pm, and at least 200 pm. Each substrate of the plurality of substrates optionally has a thickness selected from any of no more than 5 mm, no more than 2 mm, no more than 1 mm, no more than 500 pm, no more than 400 pm, and no more than 300 pm. The method optionally comprises processing a surface of each substrate of the plurality of substrates prior to growing a plurality of further single crystal diamond products. Examples of such processing include polishing, chemical mechanical polishing, etching, and laser processing. The method optionally comprises, prior to growing a plurality of further single crystal diamond products, disposing a first buffer layer on a surface of each substrate of the plurality of substrates, the buffer layer configured to limit the propagation of dislocations. This step is important where a low dislocation density is required, as dislocations tend to propagate from the substrate into the overgrown diamond. As an example each of the further single crystal diamond products has a surface dislocation density of less than 105 cm-2. Brief Description of Drawings Non-limiting embodiments will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a flow diagram showing prior art exemplary steps to produce CVD single crystal diamond material from a substrate; Figure 2 is a flow diagram showing steps of a first exemplary embodiment to produce CVD single crystal diamond material from a substrate; Figure 3 illustrates schematically a side elevation cross section view of a single crystal diamond substrate with diamond grown on a surface and showing lateral overgrowth; Figure 4 illustrates schematically a side elevation cross section view of a masked single crystal diamond substrate with diamond grown on a surface and showing lateral growth; and Figure 5 is a flow diagram showing a second exemplary embodiment to produce CVD single crystal diamond material from a substrate. Detailed description Turning to Figure 1, there is shown a flow diagram illustrating known exemplary steps to produce CVD single crystal diamond material from a substrate. The following numbering corresponds to that of Figure 1: S1. A single crystal diamond substrate is provided. This substrate typically has a growth surface that has been processed to reduce the number of surface defects, as surface defects on the growth surface or within the substrate typically propagate into the single crystal diamond material that is homoepitaxially grown on the substrate growth surface. Large area single crystal diamond substrates (which typically have a largest linear dimension of at least 50 mm) may be obtained, for example, by heteroepitaxial growth of diamond on a non-diamond substrate, as described in US 7,396,408. S2. The single crystal diamond substrate is located in a CVD reactor, and single crystal CVD diamond is homoepitaxially grown on the single crystal diamond substrate. In order to grow the single crystal CVD diamond, process gases are fed into the CVD reactor. Such process gases typically include a carbon-containing gas such as methane, and hydrogen. A plasma is formed from the gases and the single crystal CVD diamond grows on the substrate. CVD synthesis conditions are typically controlled such that the single crystal diamond material is held at a desired temperature (typically between 800°C and 1200°C). If the temperature is too low, then growth rates are low. An upper limit to the growth temperature of 1200°C is generally required to avoid detrimental defect formation in the single crystal CVD diamond such as twinning. Furthermore, the temperature, in combination with other parameters such as carbon containing gas concentration, affects the morphology of the homoepitaxially grown diamond material and thus can be selected and controlled to achieve a desired morphology. CVD synthesis conditions are typically controlled such that a CVD synthesis atmosphere comprises a carbon containing gas (e.g. methane) at a concentration by volume in a range 3 to 8%. If the carbon containing gas concentration is too low, then growth rates are too low. If the carbon containing gas concentration is too high, then cracking may occur and / or the material may have a poor optical quality. Furthermore, as previously stated, carbon containing gas concentration, in combination with other parameters such as the single crystal CVD diamond temperature, affects the morphology of homoepitaxially grown diamond material and thus is selected and controlled to achieve the desired morphology close to net shape of the final processed product. CVD synthesis conditions are further controlled to provide a high power density across the substrate of at least 150 W / cm2. The power density will generally be less than 600 W / cm2. In the context of this specification, power density is defined as the total microwave input power divided by the area of the substrate, or the substrate holder, whichever has the greater area. After growth of the single crystal CVD diamond, the substrate may be left in place or may be removed from the grown single crystal CVD diamond. Either way, the substrate is used only once, and a substrate needs to be manufactured for each single crystal CVD diamond that is to be grown. A single sample must undergo all processing and quality improvement steps, giving a risk to yield losses. Figure 2 is a flow diagram showing steps of the invention. The following numbering corresponds to that of Figure 2: S3. A single crystal diamond material having a largest linear dimension of at least 50 mm is provided. S4. The single crystal diamond material is processed to produce a plurality of substrates from the single crystal diamond material, each substrate of the plurality of substrates having a largest linear dimension of at least 50 mm. As all of the substrates come from the same sample, they will be substantially uniform. One way to produce several substrates from the single crystal diamond material is to use an etch and lift off technique. Work by Parikh et. al. (Appl. Phys. Lett. 1992, 61 (26), 3124-3126) has shown that a combination of ion implantation and etching (either wet or dry) can be used to produce thin, uniform diamond membranes that can be used as substrates. This approach involves ion implantation of the diamond, to create a damaged layer within the diamond, where the ions come to rest, which is at a depth dependent on the energy used. This implantation was followed by annealing at 950°C for 30 minutes, during which this damage layer becomes graphitic (sp2 bonded) in nature. This graphitic layer can then be removed using either a wet etch using acid, or a dry etch by annealing at 550°C under oxygen. Both removal techniques free the thin layer of diamond above the graphitic layer. An advantage of this technique is that the ion implantation step creates an etchable layer at a controlled and fixed distance beneath the surface, ensuring a membrane of uniform thickness. S5. Each of the plurality of substrates is then used to grow a plurality of further single crystal diamond products, each further single crystal diamond product having a largest linear dimension of at least 50 mm. S6. At least one of the further single crystal diamond products may be used as a substrate for further growth, provided that its lateral dimensions are still at a required specification. By using a further single crystal diamond product as a substrate for further growth, the substrate is effectively cloned and there is no need to produce a new substrate from scratch, thereby removing a synthesis step for the dedicated production of a new substrate. Steps S3 to S6 can be repeated as many times as required ensuring a supply of substrates from the grown material without having to manufacture any new substrates. A problem with the technique shown in Figure 2 is that over time, owing to processing, the lateral dimensions of the substrates can reduce. The reasons for this are illustrated in Figure 3. Figure 3 illustrates schematically a side elevation cross section view of a single crystal diamond substrate with diamond grown on a surface and showing lateral overgrowth. A substrate 1 is provided, on which further single crystal diamond 2 is overgrown. Depending on the crystallographic orientation of the growth surface, and the growth conditions (for example temperature), lateral overgrowth can occur such that the further single crystal diamond material extends laterally beyond the edges of the substrate 1. For example, if the growth surface lies substantially in a {100} crystallographic orientation, a significant amount of lateral overgrowth can be expected. A damage layer 3 is introduced just below the surface of the substrate 1, typically at a depth of a few microns, for a subsequent etch and lift-off process as described above. This is done before the overgrowth of the further single crystal diamond 2. However, the lateral overgrowth partly initiates from the lateral sides of the substrate and can grow over the edges of the damage layer 3 that would otherwise be exposed at the lateral sides of the substrate 1. This makes the subsequent etching process slow or impossible, because the etchant does not have access to the damage layer 3. This makes lift-off difficult or impossible. In order to address this, the lateral sides of the substrate 1 and further single crystal diamond 2 are removed, as shown by the dotted lines 4, 5. This has the effect of reducing the lateral dimensions of the substrate 1 and further single crystal diamond 2. With each subsequent use of the substrate 1 or further single crystal diamond 2 if it us used as a substrate), the lateral dimensions reduce until they are below the specification, at which point a new substrate is needed. One way to address this issue is to start with a single crystal diamond material having a largest linear dimension significantly larger than a desired largest linear dimension. However, eventually the lateral dimensions of substrates will be reduced below the desired size. Figure 4 shows an alternative way to address this issue. It involves using a substrate cloning step from the overgrowth material that doesn’t lose lateral dimensions and so can theoretically continue to produce new substrates. In the example of Figure 4, a mask 6, 7 is applied to the lateral sides of the substrate 1 prior to overgrowth of the single crystal diamond material 2. This ensures that lateral overgrowth does not initiate from the sides of the substrate 1, but only from the growth surface. After growth, the mask is removed. The damage layer 3 is therefore not obscured by lateral overgrowth and so and the sides of the substrate 1 and further single crystal diamond 2 can be processed without the loss of lateral dimensions. The mask 6, 7 may be a layer applied to the sides of the substrate 1. Alternatively the sides of the substrate 1 can be masked by locating the substrate 1 in a recessed pocket which covers the sides of the substrate 1 and so prevents any overgrowth from the sides of the substrate, and retains the access to the damage layer 3. Figure 5 is a flow diagram showing a second exemplary embodiment to produce CVD single crystal diamond material from a substrate. S7. A single crystal diamond material having a largest linear dimension of at least 50 mm is provided. S8. The single crystal diamond material is processed to produce a plurality of substrates from the single crystal diamond material, each substrate of the plurality of substrates having a largest linear dimension of at least 50 mm. As all of the substrates come from the same sample, they will be substantially uniform. Where an etch and lift off process is to be subsequently used, a damage layer is introduced into each of the plurality of substrates. S9. Each of the plurality of substrates is then used to grow a plurality of further single crystal diamond products, each further single crystal diamond product having a largest linear dimension of at least 50 mm. In some embodiments, the largest linear dimension is at least 75 mm or at least 100 mm. Each substrate of the plurality of substrates may have a thickness selected from any of at least 10 pm, at least 50 pm, at least 100 pm, and at least 200 pm, and no more than 500 pm, no more than 400 pm, and no more than 300 pm. S10. Where the further single crystal diamond product is to be used as a substrate for further diamond growth, a damage layer is introduced and the sides of the at least one further single crystal diamond product are masked, for example using gold or a recessed pocket, to ensure that there is no subsequent lateral overgrowth initiating from the sides of the at least one further single crystal diamond product. S11. Further diamond is grown in the further single crystal diamond product. S12. If a mask was used, for example gold, this is removed. Where gold is used, it can be stripped away using aqua regia. S13. An electrochemical lift operation is performed. In all of the embodiments described above, an initial substrate is required having a largest lateral dimension of at least 50 mm. As discussed above, such substrates can be grown using techniques such as heteroepitaxial growth. Heteroepitaxially grown diamond typically has a high density of dislocations. For example, type lb single crystals typically have between 104 and 106 dislocations per cm2. In contrast, figures of greater than 107 dislocations per cm2 have been reported for heteroepitaxially grown diamond in Shreck et. al., Appl. Phys. Lett. 78, 192 (2001). A further problem is that over time, and multiple ‘cloning’ of a particular single crystal diamond substrate, the number of surface defects will increase, which leads to an increase in the number of dislocations in subsequently grown material that initiate from surface defects. This is because new growth can introduce dislocations but not remove them. For certain applications, for example electronic applications, it is desirable to significantly reduce the concentration of dislocations in the diamond. It is known to use heteroepitaxially grown diamond as a substrate on which to grow single crystal diamond homogeneously (e.g. WO2020 / 008044). However, surface defects in the heterogeneously grown diamond substrate propagate into the homogeneously grown diamond and so the resultant homogeneously grown diamond will typically have a similar surface dislocation density to the heterogeneously grown diamond substrate. It may therefore be desirable to use techniques known for limiting the propagation of dislocations from a diamond substrate into subsequently grown diamond. Ohmagari et. al., Phys. Status Solidi A 2019, 216, 1900498 describes a technique in which the propagation of dislocations from a single crystal diamond substrate into a subsequently grown diamond material is suppressed using a technique referred to as metal-assisted termination. In this technique, a layer of metal impurities that possess larger covalent radii than that of the carbon atoms in the diamond lattice is randomly incorporated during epitaxial growth by CVD. The presence of these larger metal impurities suppresses the propagation of dislocations into the subsequently grown diamond material. Tang et. al., Appl. Phys. Lett. 108, 052101 (2016) describes a technique in which a gold masking layer having openings is deposited on the surface of the substrate prior to subsequent growth of diamond material. Dislocations in the substrate below the gold layer cannot propagate into the subsequently grown diamond material, and lateral overgrowth above the gold layer ensures that the subsequently grown diamond material is continuous without gaps. Techniques such as these can be used to reduce the concentration of dislocations in subsequently grown diamond material, and may be used either for the initial substrate or for the further single crystal diamond material that is processed to become further substrates. GB202316754.7 describes a process in which a starting diamond surface has a surface dislocation density of at least 107 cm-2. A first buffer layer is disposed on the surface to limit the propagation of dislocations during subsequent growth on that surface. Further diamond material is grown which, thanks to the buffer layer, has a reduced surface defect density compared to the starting diamond surface. The grown further diamond then has a buffer layer applied and still further growth is performed. This process is repeated until the surface defect density is reduced to less than 105 cm-2. Iterated use of the buffer layer to filter or restrict propagation of dislocations from one layer to a subsequently grown layer of diamond allows single crystal diamond with much reduced concentrations of dislocations to be achieved. Various types of buffer layer are described. One example is a mask with openings, the mask being arranged to only allow dislocation growth through the openings. Another example is a diamond buffer layer which contains metal impurities having a larger atomic radius than the atomic radius of carbon. A further example is a layer of surface depressions, such as trenches, voids or apertures, or surface protrusions, such as nano-pillars. A process such as that described in GB202316754.7 allows multiple single crystal diamond growth from an initial substrate with the same or a reduced surface defect density. In the example of Figure 3, this process could be performed between steps S9 and S10. Of course, this type of process would also apply to the embodiment shown in Figure 2, and could be applied between steps S5 and S6. If required, a surface processing operation is performed on the substrate or further substrates to reduce surface damage. Examples of processing techniques include one or more of cutting, cleaving, lapping, polishing, scaife polishing and / or etching. This may reduce any surface roughness that arises during the growth process. The surface dislocation density may be measured by applying an oxygen-containing plasma to the surface of the diamond. This forms etch pits that reveal the presence of dislocations. The number of etch pits in a predetermined area can then be counted to determine the dislocation density. 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, while the examples describe a heteroepitaxially grown diamond substrate in the initial stages, other single crystal diamond material substrates could be used.

Claims

1. A method of producing CVD single crystal diamond material using a chemical vapour deposition process, the method comprising:providing a single crystal diamond material having a largest linear dimension of at least 50 mm;processing the single crystal diamond material to produce a plurality of substrates from the single crystal diamond material, each substrate of the plurality of substrates having a largest linear dimension of at least 50 mm;using each of the plurality of substrates to grow a plurality of further single crystal diamond products, each further single crystal diamond product having a largest linear dimension of at least 50 mm.

2. The method according to claim 1, further comprising:using at least one of the plurality of single crystal diamond products as a substrate on which to grow a further single crystal diamond product.

3. The method according to claim 1 or claim 2, wherein the processing comprises: forming in the single crystal diamond material a damage layer comprising sp2 bonded carbon, wherein the presence of the damage layer defines a first diamond layer on a first side with respect to the damage layer and a second diamond layer on a second, opposing side with respect to the damage layer; andelectrochemically etching the damage layer to separate the first layer from the second layer.

4. The method according to claim 3, further comprising, prior to electrochemically etching the damage layer, removing material from a lateral side of the single crystal diamond material.

5. The method according to any one of claims 1 to 4, wherein each further single crystal diamond product has a desired largest linear dimension, and the largest linear dimension of the provided single crystal diamond material is greater than the desired largest linear dimension.

6. The method according to any one of claims 1 to 5, wherein the provided single crystal diamond material having a largest linear dimension of at least 50 mm, the largest linear dimension oriented within 5° of a {100} plane.

7. The method according to any one of claims 1 to 6, further comprising:providing the single crystal diamond material having a largest linear dimension of at least 50 mm by growing the single crystal diamond material on a first substrate such that lateral overgrowth is achieved; andremoving material from lateral edges of the single crystal diamond material prior to processing the single crystal diamond material to produce the plurality of substrates from the single crystal diamond material, wherein the amount of material removed is equal to or less than the amount of lateral overgrowth.

8. The method according to claim 7, further comprising, prior to growing the single crystal diamond material on the first substrate, masking at least one lateral edge of the first substrate to prevent lateral overgrowth from the lateral edge of the first substrate.

9. The method according to any one of claims 1 to 8, wherein the single crystal diamond material has a largest linear dimension selected from any of at least 75 mm and at least 100 mm.

10. The method according to any one of claims 1 to 9, wherein each substrate of the plurality of substrates has a thickness selected from any of at least 10 pm, at least 50 pm, at least 100 pm, and at least 200 pm.

11. The method according to any one of claims 1 to 10, wherein each substrate of the plurality of substrates has a thickness selected from any of no more than 5 mm, no more than 2 mm, no more than 1 mm, no more than 500 pm, no more than 400 pm, and no more than 300 pm.

12. The method according to any one of claims 1 to 11, further comprising processing a surface of each substrate of the plurality of substrates prior to growing a plurality of further single crystal diamond products.

13. The method according to claim 12, wherein the processing comprises any of polishing, chemical mechanical polishing, etching, and laser processing.

14. The method according to any one of claims 1 to 13, further comprising, prior to 5 growing a plurality of further single crystal diamond products:disposing a first buffer layer on a surface of each substrate of the plurality of substrates, the buffer layer configured to limit the propagation of dislocations.

15. The method according to claim 14, wherein each of the further single crystal 10 diamond products has a surface dislocation density of less than 105 cm’2.

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