SINGLE CRYSTAL SYNTHETIC DIAMOND MATERIAL VIA CHEMICAL VAPOUR DEPOSITION

Abstract

A single crystal CVD diamond material is disclosed, the material comprising 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 measured using a pixel size of area in a range 1×1 μm.sup.2 to 20×20 μm.sup.2, a maximum value of Δn.sub.[average] does not exceed 1.5×10.sup.−4, where Δn.sub.[average] is an average value of a difference between refractive index for light polarised parallel to slow and fast axes averaged over the sample thickness. A method of making the material is also disclosed.

Claims

1. A single crystal CVD diamond material comprising: 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 measured using a pixel size of area in a range 1×1 μm.sup.2 to 20×20 μm.sup.2, a maximum value of Δn.sub.[average] does not exceed 1.5×10.sup.−4, where Δn.sub.[average] is an average value of a difference between refractive index for light polarised parallel to slow and fast axes averaged over the sample thickness.

2. A single crystal CVD diamond material according to 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. A single crystal CVD diamond material according to claim 1, wherein the sample of single crystal CVD diamond material used to measure birefringence has a thickness in a range 0.5 mm to 1.0 mm.

4. A single crystal CVD diamond material according to claim 1, 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. A single crystal CVD diamond material according to claim 1, wherein the maximum value of Δn.sub.[average] does not exceed 8×10.sup.−5.

6. A single crystal CVD diamond material according to claim 1, wherein the maximum value of Δn.sub.[average] does not exceed 5×10.sup.−5.

7. A single crystal CVD diamond material according to claim 1, wherein the optical birefringence is measured in a direction of highest birefringence to within ±10°.

8. A single crystal CVD diamond material according to claim 1, wherein the single crystal CVD diamond material has a neutral single substitutional nitrogen (N.sub.s.sup.0) concentration greater than 5×10.sup.17 atoms/cm.sup.3, 8×10.sup.17 atoms/cm.sup.3, or 1×10.sup.18 atoms/cm.sup.3 as measured by electron paramagnetic resonance.

9. A single crystal CVD diamond material according to claim 1, wherein the single crystal CVD diamond material is coloured brown, yellow, blue, or pink.

10. A single crystal CVD diamond material according to claim 1, wherein the single crystal CVD diamond material is in the form of a cut gemstone.

11. A method of fabricating a single crystal CVD diamond material according to claim 1, the method comprising: preparing a plurality of single crystal diamond substrates by mechanically processing the substrates and then etching the substrates to remove mechanical processing damage, wherein a growth surface of each substrate has a density of defects such that surface etch features related to defects formed by a revealing plasma etch is below 5×10.sup.3/mm.sup.2; growing a first layer of single crystal CVD diamond material on the growth surface of each single crystal diamond substrate, and growing a second layer of single crystal CVD diamond material on the first layer of single crystal CVD diamond material, 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.

12. A method according to claim 11, wherein the first layer of single crystal CVD diamond material is grown with a synthesis atmosphere containing less than 5 ppm, 3 ppm, 1 ppm, or 0.8 ppm of nitrogen.

13. A method according to claim 11, wherein the second layer of single crystal CVD diamond material is grown with a synthesis atmosphere containing more than 5 ppm, 7 ppm 10 mm, 15 ppm, 20 ppm, or 30 ppm of nitrogen.

14. A method according to claim 11, wherein the first layer is grown to a thickness of at least 5 micrometres.

15. A method according to claim 11, wherein the first layer is grown to a thickness of no more than 200 micrometres.

16. A method according to claim 11, wherein the second layer of single crystal CVD diamond material is yellow or brown.

17. A method according to claim 11, wherein the second layer of single crystal CVD diamond material is irradiated to produce a blue coloured material.

18. A method according to claim 11, wherein the second layer of single crystal CVD diamond material is irradiated and annealed to produce a pink coloured material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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:

[0027] FIG. 1 illustrates the basic steps involved in fabricating a single crystal CVD diamond material according to the present invention.

DETAILED DESCRIPTION

[0028] As described in the summary of invention section of this specification, the key to achieving the present invention is to provide a methodology which achieves a single crystal CVD diamond product material with both high nitrogen content and also low strain and birefringence.

[0029] The basic methodology is illustrated in FIG. 1. In step 1, a substrate 10 is mechanically processed to a desired geometry and surface finish. Mechanical processing comprises lapping to a desired thickness and then polishing to a desired surface roughness and flatness. Such mechanical processing produces surface and sub-surface damage 12 to the growth surface of the substrate 10. This surface and sub-surface damage 12 can nucleate dislocations and generate strain in single crystal CVD diamond material grown on such a surface. Accordingly, in step 2 an etch process is applied to the growth surface 10 of the substrate to remove this damage. While this etch process removes surface and sub-surface 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 substrate 10. Such pits 14 are not usually problematic for single crystal CVD diamond material grown on such a surface using a synthesis atmosphere with a low and controlled nitrogen concentration as the single crystal CVD diamond material tends to fill in the pits without nucleating dislocations and generating strain. However, when a high nitrogen, fast growth rate synthesis process is utilized the single crystal CVD diamond material tends to over-grow pits in the substrate without filling leading to a discontinuity in the diamond lattice generating dislocations and strain. To solve this problem, in step 3 a thin layer 16 of single crystal CVD diamond material is grown using a low and controlled nitrogen concentration to fill in pits 14 on the substrate 10 before moving into a higher nitrogen growth process in step 4 yielding high nitrogen, low strain diamond material 18.

[0030] In general, methods according to the present invention comprise the following steps: [0031] preparing a plurality of single crystal diamond substrates by mechanically processing the substrates and then etching the substrates to remove mechanical processing damage, wherein a growth surface of each substrate has a density of defects such that surface etch features related to defects formed by a revealing plasma etch is below 5×10.sup.3/mm.sup.2; [0032] growing a first layer of single crystal CVD diamond material on the growth surface of each single crystal diamond substrate, and [0033] growing a second layer of single crystal CVD diamond material on the first layer of single crystal CVD diamond material, [0034] 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.

[0035] The first layer of single crystal CVD diamond material can be grown under a synthesis atmosphere containing less than 5 ppm, 3 ppm, 1 ppm, or 0.8 ppm of nitrogen. According to certain embodiments the thin layer of single crystal CVD diamond material 16 may be fabricated using a high purity synthesis process (e.g. according to WO2001/096633) or a synthesis process which uses low and controlled nitrogen addition (e.g. according to WO2004/046427).

[0036] The second layer of single crystal CVD diamond material is grown with a synthesis atmosphere containing more than 5 ppm, 7 ppm 10 mm, 15 ppm, 20 ppm, or 30 ppm of nitrogen, optionally no more than 300 ppm. The first layer can be grown to a thickness of at least 5 micrometres and/or no more than 200 micrometres. The first layer should be grown under conditions to ensure that defects in the substrate are filled in while retaining good continuity of the crystal lattice.

[0037] After growth the original substrate 10 and the thin layer of low nitrogen single crystal CVD diamond material 16 can be removed (e.g. via laser cutting, electron beam, or some other method) to yield a free-standing single crystal CVD diamond product of high nitrogen, low strain material 18. 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 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 measured using a pixel size of area in a range 1×1 μm.sup.2 to 20×20 μm.sup.2, a maximum value of Δn.sub.[average] does not exceed 1.5×10.sup.−4, where Δn.sub.[average] is an average value of a difference between refractive index for light polarised parallel to slow and fast axes averaged over the sample thickness. Certain embodiments can have a maximum value of Δn.sub.[average] that does not exceed 8×10.sup.−5 or even 5×10.sup.−5 or lower. Nominally a lower limit for the maximum value of Δn.sub.[average] may be 1×10.sup.−7. The single crystal CVD diamond material fabricated using the methodology describe herein may 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 no more than 20 mm. For the thicker embodiments in excess of 1 mm thickness, a sample of such a material have a thickness in a range 0.5 mm to 1.0 mm may be extracted and used to measure the birefringence characteristics.

[0038] The optical birefringence can be measured in a direction of highest birefringence to within ±10° which will generally correspond to the growth direction of the single crystal CVD diamond material as dislocations tend to propagate through the material in the growth direction.

[0039] The single crystal CVD diamond material may have a total nitrogen concentration of at least 5 ppm, 7 ppm 10 mm, 15 ppm, 20 ppm, or 30 ppm as measured by secondary ion mass spectrometry (SIMS) and optionally no more than 50 ppm. The single crystal CVD diamond material may have a neutral single substitutional nitrogen (N.sub.s.sup.0) concentration greater than 5×10.sup.17 atoms/cm.sup.3, 8×10.sup.17 atoms/cm.sup.3, or 1×10.sup.18 atoms/cm.sup.3 as measured by electron paramagnetic resonance and optionally no more than 1×10.sup.20 atoms/cm.sup.3.

[0040] The as-grown product material can be coloured brown similar to that described in WO2003/052177. Alternatively, the as-grown product material can be coloured yellow, e.g. similar to that described in WO2011/076643. The as-grown material can be treated after synthesis by applying annealing treatments as described in WO2004/022821. Blue coloured material can be fabricated via irradiation in a similar manner to that described in WO2010/149779. Pink coloured material can be fabricated via 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 with the colourless or near colourless product material of WO2004/046427.

[0041] The single crystal CVD diamond material according to the present invention may be used in a range of applications including optical applications, thermal applications, jewelry applications in the form of a cut gemstone, quantum sensing and information processing applications, and as substrates for further CVD diamond growth (e.g. via vertical slicing to form substrates with low defect growth surfaces).

[0042] While this invention has been particularly shown and described with reference to 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 appending claims.