Single crystal synthetic diamond material by chemical vapor deposition

Abstract

The application discloses a single crystal synthetic diamond material by chemical vapor deposition. The material comprises a total nitrogen concentration of at least 3 ppm as measured by secondary ion mass spectrometry (SIMS); and a low optical birefringence such that, in a sample of the single crystal CVD diamond material having an area of at least 1.3 mm * 1.3 mm and using pixel dimensions of an area in the range of 1 * 1 [mu] m2 to 20 * 20 [mu] m2, the maximum value of [Delta] n [average] does not exceed 1.5 * 10-4, and the maximum value of [Delta] n [average] does not exceed 1.5 * 10-4, in a sample of the single crystal CVD diamond material having an area of at least 1.3 mm * 1.3 mm. Wherein [Delta] n [average] is an average value averaging the difference in refractive index over the sample thickness for light polarized parallel to the slow and fast axes. Methods of making the materials are also disclosed.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201780074414.0, filed on November 30, 2017, entitled "Synthesis of Single Crystal Diamond Material by Chemical Vapor Deposition". TECHNICAL FIELD

[0002] The present invention relates to single crystal chemical vapor deposition (CVD) synthetic diamond material and in particular to the synthesis of single crystal CVD synthetic diamond material layers containing high levels of nitrogen dopant. BACKGROUND

[0003] In the 1980s and 1990s, many groups around the world carried out extensive research into the synthesis of single crystal CVD diamond material. Much of this work disclosed the growth of thin layers of single crystal CVD diamond material by homoepitaxial growth on single crystal diamond substrates. While there was a desire to produce relatively thick high quality single crystal CVD synthetic diamond material layers, this proved difficult to achieve in practice. The synthesis of single crystal CVD diamond material requires extreme conditions, where the extreme conditions need to be created and then maintained in a stable manner for long periods of time to successfully grow thick high quality single crystal CVD synthetic diamond material layers. Furthermore, the properties of the synthetic diamond material are sensitive to many synthesis parameters which form a complex multi-dimensional synthesis parameter space. Only a small area of this multi-dimensional synthesis parameter space enables thick high quality single crystal CVD diamond material layers. Finding these synthesis regimes and developing methods to produce the correct combination of parameters required to produce and maintain stable growth within one of these synthesis regimes is far from trivial.

[0004] In the early 2000s, Element Six Limited (the De Beers group) filed a series of patent applications relating to the growth of high quality single crystal CVD synthetic diamond material with many different types. These patent applications were based on extensive research over many years which developed an understanding of the multi-dimensional synthesis parameter space for single crystal CVD diamond material and developed methods to produce and maintain the correct combination of parameters required to produce and maintain stable growth within selected synthesis regimes.

[0005] Important synthesis parameters for single crystal CVD diamond growth were found to include substrate type, substrate processing and growth surface preparation, substrate geometry, substrate temperature and thermal management, microwave power, gas pressure, gas composition and flow. The need to select, produce and maintain the correct combination of these parameters in a stable manner and the fact that many of these parameters are interlinked such that if one parameter is changed then others must also be changed in the correct manner in order to remain in a stable growth regime. Some examples of the Element Six Limited patent applications filed in the 2000s are briefly discussed below.

[0006] For some applications it is desirable to minimise the number of defects, or at least some types of defects, within the diamond lattice structure. For example, for some electronic applications, such as radiation detectors or semiconductor conversion devices, it is desirable to minimise the number of charge carriers inherent in the diamond material and to increase the mobility of the charge carriers intentionally introduced into the material in use. Such materials can be designed by manufacturing single crystal CVD synthetic diamond material with a low concentration of impurities which would otherwise introduce charge carriers into the diamond lattice structure. Patent literature relating to such electronic / detector grade single crystal CVD synthetic diamond materials includes WO 01 / 096633 and WO 01 / 096634.

[0007] For some optical applications it is desirable to provide materials with low optical absorption and low optical birefringence. Such materials can be designed by manufacturing single crystal CVD synthetic diamond material with a low concentration of impurities which would otherwise increase the optical absorption of the material and a low concentration of extended defects which would otherwise introduce anisotropic strain into the diamond lattice structure causing birefringence. Patent literature relating to such optical grade single crystal CVD synthetic diamond materials includes WO 2004 / 046427 and WO 2007 / 066215.

[0008] In contrast to the low defect materials described above, for some applications it is desirable to intentionally introduce a significant but controlled number, type and distribution of defects into the diamond lattice structure. For example, by introducing boron into the diamond lattice by providing a boron containing gas within the CVD process gas provides acceptor energy levels within the energy band structure of the diamond material forming a p-type semiconductor. If very high levels of boron are introduced into the diamond lattice structure the material exhibits similar conductivity to a metal. Such materials can be used as electrodes, electrochemical sensing electrodes and can be used in electronic applications. Patent literature relating to such boron doped single crystal CVD synthetic diamond materials includes WO 03 / 052174.

[0009] Another example is nitrogen doped single crystal CVD synthetic diamond material. Nitrogen is one of the most important dopants in the synthesis of CVD diamond material as it has been found that providing nitrogen in the CVD process gas increases the growth rate of the material and can also influence the formation of crystalline defects such as dislocations. As such, nitrogen doping of single crystal CVD synthetic diamond material has been extensively researched and reported in the literature. Nitrogen doped CVD synthetic diamond material tends to be brown in colour. As such, for the applications discussed previously, such as optical applications, it has been found to be advantageous to develop techniques to intentionally exclude nitrogen from the CVD process gas. However, for applications which do not involve optical, electronic and quantum coupling parameters, such as mechanical applications, nitrogen doping to a significant level can be used to enable the growth of thick layers of CVD synthetic diamond material. Patent literature relating to such nitrogen doped single crystal CVD synthetic diamond materials includes WO 2003 / 052177.

[0010] For some applications, it has also been found advantageous to utilize a synthesis method that includes the introduction of two or more dopants into the CVD synthesis process. For example, as previously described, nitrogen-doped CVD synthetic diamond material tends to be brown in color. However, it has been found that if a co-dopant such as boron or silicon is introduced into the synthesis process along with nitrogen, it is possible to produce colorless or near colorless single crystal CVD diamond material at nitrogen levels that would otherwise result in a brown color. Patent literature relating to such co-doped single crystal CVD synthetic diamond material includes WO 2006 / 136929.

[0011] Co-doping can also be used by intentionally introducing one or more layers of a different dopant material into the single crystal CVD diamond as a way of qualifying the material as synthetic without adversely affecting the visual quality of the material. For example, it is possible to produce colorless or near colorless single crystal CVD diamond having one or more layers of a co-doped material that is not visible under normal viewing conditions but is visible under fluorescent conditions. Such an approach is described in WO 2005 / 061400.

[0012] Finally, EP 2985368 (Sumitomo) proposes the incorporation of a range of different types of defects into single crystal CVD diamond material for mechanical tool applications in order to inhibit chipping. In order to achieve such a mechanical tool component, a grooved substrate, ion implantation and relatively high levels of methane and nitrogen are utilized to produce a range of defects within the product material. A single crystal CVD diamond product having varying lateral dimensions but at a relatively low thickness of 0.7 mm is achieved.

[0013] With the above in mind, it will be apparent that single crystal CVD diamond material is becoming available in a range of different forms and can be designed to have a range of different properties for particular applications. SUMMARY

[0014] One of the most important synthesis regimes for commercial applications is described in WO 2004 / 046427. As described in the background section of this specification, WO 2004 / 046427 relates to the production of single crystal CVD diamond material having low optical absorption and low optical birefringence. While it has been found that such material is required for some optical applications, it has been found that the synthesis method as described therein can also be used for applications that do not necessarily require all of the advantageous optical qualities of the product material. For example, even for applications that do not require low optical birefringence, it has been found that the synthesis method as described in WO 2004 / 046427 can be advantageous for commercial production as it allows the consistent production of high quality, thick single crystal CVD diamond at a relatively good growth rate compared to other methods and with a relatively high yield.

[0015] The implementation scheme of WO2004 / 046427 describes the preparation of a single-crystal CVD diamond layer having essentially no high birefringence regions and containing, as measured by electron paramagnetic resonance spectroscopy (EPR), a 3 × 10⁻⁶ ohm region. 15 atoms / cm 3 Up to 5 × 10 17 atoms / cm 3 Monosubstituted nitrogen in a concentration range. Such materials, exhibiting low and controlled nitrogen levels and low strain, are described as being manufactured using chemical vapor deposition (CVD) techniques, wherein a low and controlled gaseous nitrogen level in the concentration range of 300 ppb to 5 ppm is introduced into the synthesis atmosphere. The inventors recognize that for some applications, it will be desirable to manufacture low-strain single-crystal CVD diamond materials with higher nitrogen concentrations than those described in the embodiments of WO2004 / 046427. However, it has been found that increasing the nitrogen level in the synthesis atmosphere to increase the nitrogen concentration in the single-crystal CVD diamond product material increases the strain and birefringence in the product material. Furthermore, the increased strain can also lead to increased cracking during synthesis or post-synthesis processing, thereby reducing yield.

[0016] The aforementioned problem was solved by growing a thin layer of low-nitrogen single-crystal CVD diamond material over a substrate and then moving to a high-nitrogen growth process for high-nitrogen single-crystal CVD diamond products. While not bound by theory, it is believed that high-nitrogen single-crystal CVD diamond materials cause overgrowth of pits in the substrate (e.g., formed by plasma etching to remove substrate damage) without properly filling the pits, leading to strain / dislocation formation. An initial layer of low-nitrogen, low-growth-rate material fills these pits before moving to a higher-nitrogen, higher-growth-rate synthesis. In this way, it is possible to fabricate high-nitrogen-concentration single-crystal CVD diamond products that also have low strain. Of course, an alternative approach to avoid the problem of pits in the substrate formed during substrate etching before growth is to reduce or avoid the use of substrate etching that forms pits. However, substrate etching methods are designed to remove surface and subsurface substrate damage caused by machining. If this substrate damage is not removed by etching, it also causes dislocation formation and strain. The solution presented in this paper therefore retains the substrate etching step to remove machining damage from the substrate growth surface, but then uses a low-nitrogen, low-growth-rate synthesis process to fill pits and irregularities in the substrate surface after etching and before moving to a higher-nitrogen, higher-growth-rate synthesis process. In this way, it is possible to achieve single-crystal CVD diamond products with high nitrogen content and low birefringence.

[0017] According to a first aspect of the present invention, a single-crystal CVD diamond material is provided, comprising:

[0018] Total nitrogen concentration of at least 3 ppm as measured by secondary ion mass spectrometry (SIMS); and

[0019] Low optical birefringence, such that: in a sample of the single crystal CVD diamond material having an area of at least 1.3 mm x 1.3 mm, and using pixel size measurements over an area in the range 1 x 1 μm 2 to 20 x 20 μm 2 the maximum value of Δn [平均] does not exceed 1.5 x 10 -4 -7 [平均] where Δn [平均] is the average value averaged over the thickness of the sample of the difference in refractive index for light polarised parallel to the slow and fast axes.

[0020] According to a second aspect of the application there is provided a method of manufacturing a single crystal CVD diamond material according to the first aspect of the application, the method comprising:

[0021] preparing a plurality of single crystal diamond substrates by machining a plurality of single crystal diamond substrates and then etching the substrates to remove machining damage,

[0022] wherein the growth surface of each substrate has a defect density such that the surface etch feature associated with defects formed by a revealing plasma etch is less than 5 x 10 3 / mm 2 ;

[0023] growing a first layer of single crystal CVD diamond material on the growth surface of each single crystal diamond substrate; and

[0024] growing a second layer of single crystal CVD diamond material on the first layer of single crystal CVD diamond material,

[0025] wherein the second layer of single crystal CVD diamond material is grown under higher nitrogen conditions than the first layer of single crystal CVD diamond material, the synthesis conditions being controlled so as to achieve a single crystal CVD diamond material according to the first aspect of the application.

[0026] The single crystal CVD diamond product material has high nitrogen content and low strain and can be fabricated into thick layers. The synthesis conditions can be controlled so as to form material in the growing state that is yellow in color or to remove the brown color after an annealing treatment. The growing state product material can be irradiated to produce material that is blue in color. Alternatively, the growing state material can be irradiated and annealed to produce material that is pink in color. Such material can be fabricated into cut gemstones for jewelry applications. Alternatively, such material can be used in quantum sensing and information processing applications, where strain reduction can result in more stable nitrogen vacancy defects and increased sensitivity. Still alternatively, such material can be used in mechanical applications. In all cases, lower strain can result in higher synthesis yield and improved surface finishing quality and yield. BRIEF DESCRIPTION OF DRAWINGS

[0027] For a better understanding of the present application and to show how it can be carried into effect, embodiments of the present application will now be described purely by way of example, with reference to the accompanying drawings in which:

[0028] Figure 1 The basic steps involved in manufacturing single crystal CVD diamond material in accordance with the present application are illustrated. DETAILED DESCRIPTION

[0029] As described in the SUMMARY section of the present specification, the key to implementing the present application is to provide a method that achieves single crystal CVD diamond product material that has high nitrogen content and also low strain and birefringence.

[0030] In Figure 1The basic method is illustrated in Figure 1. In Step 1, the substrate 10 is machined to the desired geometry and surface finish. The machining includes grinding to the desired thickness and then polishing to the desired surface roughness and flatness. Such machining creates surface and subsurface damage 12 to the growth surface of the substrate 10. This surface and subsurface damage 12 can become dislocation cores and create strain in the single crystal CVD diamond material grown on such a surface. Therefore, in Step 2, an etching process is applied to the growth surface of the substrate 10 to remove this damage. While this etching process removes the surface and subsurface damage, it also causes pits 14 to form in the growth surface of the substrate 10, particularly where defects such as dislocations are located in the growth surface of the substrate 10. Such pits 14 are generally not a problem for single crystal CVD diamond material grown on such a surface using a synthesis atmosphere with low and controlled nitrogen concentration because the single crystal CVD diamond material tends to fill in the pits rather than become dislocation cores and create strain. However, when utilizing a high nitrogen, fast growth rate synthesis process, the single crystal CVD diamond material tends to overgrow the pits in the substrate rather than fill them in, which results in discontinuities in the diamond lattice and thus dislocations and strain. To address this problem, in Step 3, a thin layer of single crystal CVD diamond material 16 is grown using a low and controlled nitrogen concentration to fill in the pits 14 on the substrate 10 prior to moving to Step 4 to create a high nitrogen, low strain diamond material 18 in a higher nitrogen growth process.

[0031] Generally, the method according to the present invention comprises the following steps:

[0032] preparing a plurality of single crystal diamond substrates by machining a plurality of single crystal diamond substrates and then etching the substrates to remove machining damage, wherein the growth surface of each substrate has a defect density such that the surface etching feature associated with defects formed by exposure to the plasma etch is less than 5 x 10 3 / mm 2 ;

[0033] growing a first layer of single crystal CVD diamond material on the growth surface of each single crystal diamond substrate; and

[0034] growing a second layer of single crystal CVD diamond material on the first layer of single crystal CVD diamond material,

[0035] wherein the second layer of single crystal CVD diamond material is grown under higher nitrogen conditions than the first layer of single crystal CVD diamond material.

[0036] The first single-crystal CVD diamond material layer can be grown in a synthetic atmosphere containing less than 5 ppm, 3 ppm, 1 ppm, or 0.8 ppm nitrogen. According to some embodiments, a thin single-crystal CVD diamond material layer 16 can be fabricated using a high-purity synthesis process (e.g., according to WO2001 / 096633) or a synthesis process using low and controlled nitrogen addition (e.g., according to WO2004 / 046427).

[0037] The second single-crystal CVD diamond material layer is grown using a synthetic atmosphere containing nitrogen at concentrations greater than 5 ppm, 7 ppm, 10 ppm, 15 ppm, 20 ppm, or 30 ppm, optionally not exceeding 300 ppm. The first layer can be grown to a thickness of at least 5 micrometers and / or not exceeding 200 micrometers. The first layer should be grown under conditions that ensure filling of defects in the substrate while maintaining good continuity of the crystal lattice.

[0038] After growth, the original substrate 10 and the thin low-nitrogen single-crystal CVD diamond material layer 16 can be removed (e.g., by laser cutting, electron beam, or some other method) to produce a separate single-crystal CVD diamond product material 18 with high nitrogen and low strain. The single-crystal CVD diamond material comprises: a total nitrogen concentration of at least 3 ppm as measured by secondary ion mass spectrometry (SIMS); and low optical birefringence, such that: in a sample of this single-crystal CVD diamond material having an area of ​​at least 1.3 mm × 1.3 mm, and used in a 1 × 1 μm... 2 Up to 20 × 20 μm 2 Pixel size measurement of the area within the range, Δn [平均] The maximum value does not exceed 1.5 × 10 -4 , where Δn [平均] This is the average value of the difference in refractive index for light polarized parallel to the slow and fast axes across the sample thickness. Some embodiments may have a refractive index not exceeding 8 × 10⁻⁶. -5 Or even 5 × 10 -5 or smaller Δn [平均] The maximum value for Δn. [平均] In terms of the maximum value, the nominal lower limit can be 1 × 10. -7 The single-crystal CVD diamond material fabricated using the method described herein can have a thickness of at least 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.7 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, or 5 mm and optionally not greater than 20 mm. For thicker embodiments exceeding 1 mm, samples of such material have a thickness in the range of 0.5 mm to 1.0 mm and can be removed and used to measure birefringence properties.

[0039] Optical birefringence can be measured in a direction up to ±10° of the highest birefringence, which will typically correspond to the growth direction of the single crystal CVD diamond material, as dislocations tend to propagate through the material in the growth direction.

[0040] The single crystal CVD diamond material can have a total nitrogen concentration of at least 5 ppm, 7 ppm, 10 mm, 15 ppm, 20 ppm or 30 ppm and optionally no more than 50 ppm as measured by secondary ion mass spectroscopy (SIMS). The single crystal CVD diamond material can have a concentration of neutral single substitutional nitrogen (N 17 3 ) of greater than 5 x 1016atoms / cm 17 3 , 8 x 1016atoms / cm 18 3 or 1 x 1016atoms / cm 20 3 and optionally no more than 1 x 1016atoms / cm S 0 .

[0041] The as-grown product material can be brown in colour, similar to that described in WO2003 / 052177. Alternatively, the as-grown product material can be yellow in colour, for example similar to that described in WO201 1 / 076643. The as-grown material can be treated after synthesis by applying an annealing treatment as described in WO2004 / 022821. Material which is blue in colour can be made by irradiation in a similar manner to that described in WO2010 / 149779. Material which is pink in colour can be made by irradiation and annealing in a similar manner to that described in WO2010 / 149775. Such coloured products can be similar in colour to those described in the prior art, but with lower strain, more comparable to the colourless or near colourless product material described in WO2004 / 046427.

[0042] The single crystal CVD diamond material according to the present application can be used in a range of applications, including optical applications, thermal applications, jewellery applications in the form of cut gemstones, quantum sensing and information processing applications, and as substrates for further CVD diamond growth (for example to form substrates with low defect growth surfaces by vertical slicing).

[0043] While this application has been particularly shown and described with references to particular embodiments, it will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the scope of the application encompassed by the appended claims.​​​​

Claims

1. Single crystal CVD diamond material comprising: a total nitrogen concentration of at least 3 ppm as measured by secondary ion mass spectroscopy (SIMS); and a low optical birefringence, such that: in a sample of the single crystal CVD diamond material having an area of at least 1.3 mm x 1.3 mm, and using pixel size measurements over an area in the range of 1 x 1 μm 2 to 20 x 20 μm 2 the maximum value of Δn [平均] is no more than 1.5 x 10 -4 where Δn [平均] is the average value averaged over the thickness of the sample of the difference in refractive index for light polarised parallel to the slow and fast axes.

2. The single crystal CVD diamond material of claim 1, wherein the single crystal CVD diamond material has a thickness of at least 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.7 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, or 5 mm.

3. The single crystal CVD diamond material of claim 1 or 2, wherein the sample of single crystal CVD diamond material for measuring birefringence has a thickness in the range of 0.5 mm to 1.0 mm.

4. The single crystal CVD diamond material of any preceding claim, wherein the total nitrogen concentration of the single crystal CVD diamond material is at least 5 ppm, 7 ppm, 10 mm, 15 ppm, 20 ppm, or 30 ppm.

5. The single crystal CVD diamond material of any preceding claim, wherein the Δn [平均] is not more than 8 x 10 -5 .

6. The single crystal CVD diamond material of any preceding claim, wherein the Δn [平均] is not more than 5 x 10 -5 .

7. The single crystal CVD diamond material of any preceding claim, wherein the optical birefringence is measured in a direction within ±10° of the highest birefringence.

8. The single crystal CVD diamond material of any preceding claim, wherein the single crystal CVD diamond material has a concentration of neutral single substitutional nitrogen (N 17 3 8 x 10 17 3 1 x 10 18 3 S 0 atoms / cm by electron paramagnetic resonance measurement.​​​​ 9. The single crystal CVD diamond material of any preceding claim, wherein the single crystal CVD diamond material is a color of brown, yellow, blue, or pink.

10. The single crystal CVD diamond material of any preceding claim, wherein the single crystal CVD diamond material is in the form of a cut gemstone.

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