Single crystal synthetic diamond material via chemical vapour deposition

Abstract

There is described a single crystal CVD diamond material comprising three orthogonal dimensions of at least 2 mm; one or more regions of low optical birefringence, indicative of low strain, such that in a sample of the single crystal CVD diamond material having a thickness in a range 0.5 mm to 1.0 mm and an area of greater than 1.3 mm1.3 mm and measured using a pixel size of area in a range 11 m.sup.2 to 2020 m.sup.2, a maximum value of n.sub.[average] does not exceed 1.510.sup.4 for the one or more regions of low optical birefringence, where n.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; one or more regions of high optical birefringence, indicative of high strain, such that in said sample of the single crystal CVD diamond material and measured using said pixel size, n.sub.[average] is greater than 1.510.sup.4 and less than 310.sup.3; and is wherein every 1.3 mm1.3 mm area of the sample of the single crystal CVD diamond material comprises at least one of said regions of high optical birefringence. There is also described a method of making the CVD diamond material.

Claims

1. A single crystal CVD diamond material comprising: three orthogonal dimensions of at least 2 mm; one or more regions of low optical birefringence, indicative of low strain, such that in a sample of the single crystal CVD diamond material having a thickness in a range 0.5 mm to 1.0 mm and an area of greater than 1.3 mm1.3 mm and measured using a pixel size of area in a range 11 m.sup.2 to 2020 m.sup.2, a maximum value of n.sub.[average] does not exceed 1.510.sup.4 for the one or more regions of low optical birefringence, where n.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; one or more regions of high optical birefringence, indicative of high strain, such that in said sample of the single crystal CVD diamond material and measured using said pixel size, n.sub.[average] is greater than 1.510.sup.4 and less than 310.sup.3; and wherein every 1.3 mm1.3 mm area of the sample of the single crystal CVD diamond material comprises at least one of said regions of high optical birefringence.

2. The single crystal CVD diamond material according to claim 1, wherein the one or more regions of high optical birefringence each comprise a plurality of dislocations extending laterally in a line across the single crystal CVD diamond material.

3. The single crystal CVD diamond material according to claim 1, wherein the one or more regions of high optical birefringence each comprise a bundle of dislocations at a point localized in lateral dimensions of the single crystal CVD diamond material.

4. The single crystal CVD diamond material according to claim 1, wherein the one or more regions of high optical birefringence each have a width in a range 5 to 500 m.

5. The single crystal CVD diamond material according to claim 1, wherein the regions of high optical birefringence have a spacing in a range 50 m to 1.3 mm.

6. The single crystal CVD diamond material according to of claim 1, wherein the regions of high optical birefringence are configured in a regular pattern.

7. The single crystal CVD diamond material according to of claim 1, wherein the regions of high optical birefringence are equally spaced.

8. The single crystal CVD diamond material according to claim 1, wherein n.sub.[average] of the one or more regions of low optical birefringence does not exceed 810.sup.5.

9. The single crystal CVD diamond material according to of claim 1, wherein n.sub.[average] of the one or more regions of high optical birefringence is greater than 2.510.sup.4.

10. The single crystal CVD diamond material according to of claim 1, wherein the single crystal CVD diamond material has a neutral single substitutional nitrogen)(N.sub.s.sup.0) concentration in a range 310.sup.15 atoms/cm.sup.3 to 510.sup.17 atoms/cm.sup.3 as measured by electron paramagnetic resonance.

11. The single crystal CVD diamond material according to claim 1, wherein the single crystal CVD diamond material has a low optical absorption such that said sample of the single crystal CVD diamond material has an optical absorption coefficient at a wavelength of 1.06 m of less than 0.09 cm.sup.1.

12. The single crystal CVD diamond material according to claim 1, wherein the single crystal CVD diamond material is colourless or near colourless with a colour grade in a range D to J.

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

14. A method of manufacturing the single crystal CVD diamond material according to claim 1, the method comprising: preparing a plurality of single crystal diamond substrates, each substrate having a growth surface comprising: one or more regions for nucleating dislocations to form said one or more regions of high optical birefringence in the single crystal CVD diamond material grown thereon; and one or more low defect regions to form said one or more regions of low optical birefringence in the single crystal CVD diamond material grown thereon, wherein the one or more low defect regions have a density of defects such that surface etch features related to defects formed by a revealing plasma etch is below 510.sup.3/mm.sup.2; and growing at least 2 mm2 mm2 mm of the single crystal CVD diamond material on the growth surface of each of the single crystal diamond substrates while controlling CVD diamond growth conditions in order to fabricate the single crystal CVD diamond material of claim 1.

15. The method according to claim 14, wherein the single crystal diamond substrates are prepared by ablating the growth surface of each of the single crystal diamond substrates with ablated regions forming the one or more regions for nucleating dislocations.

16. The method according to claim 15, wherein the ablated regions have a width in a range 5 to 500 m.

17. The method according to claim 15, wherein the ablated regions have a depth in a range 5 to 100 m.

18. The method according to claim 15, wherein the ablated regions have a spacing in a range 50 m to 1.3 mm.

19. The method according to claim 14, wherein the single crystal diamond substrates are prepared by implanting the growth surface of each of the single crystal diamond substrates with implanted regions forming the one or more regions for nucleating dislocations.

20. The method according to claim 14, wherein the single crystal diamond substrates are prepared by etching the growth surface after forming the one or more regions for nucleating dislocations, the etching being controlled to remove processing damage from the one or more low defect regions but not to fully remove the one or more regions for nucleating dislocations such that said regions for nucleating dislocations are in a form capable of generating sufficient dislocations in the single crystal CVD diamond material grown thereon that n.sub.[average] is greater than 1.510.sup.4 and less than 310.sup.3 thus forming the regions of high optical birefringence.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 illustrates the basic steps involved in fabricating a single crystal CVD diamond material according to the present invention.

DETAILED DESCRIPTION

(3) 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 controlled regions of high, but not too high, birefringence separated by low defect material.

(4) The basic methodology is illustrated in FIG. 1. Steps (a) to (c) are concerned with creating the correct density and form of substrate structures for nucleating the correct amount of dislocations to produce high strain regions while nucleating few dislocations outside these regions.

(5) In step (a), a single crystal diamond substrate 2 is selected which has a low concentration of defects at a growth surface 4 thereof. For example, a single crystal diamond substrate is selected which has a low density of dislocations intersecting the growth surface. The substrate selection is similar to that described in WO2004/046427 for fabricating low birefringence material. Such a substrate will still have some surface and sub-surface damage 6 at the growth surface as a result of polishing for example.

(6) In step (b), one or more regions 8 for nucleating dislocations are processed into the growth surface 4 of the single crystal diamond substrate 2. The single crystal diamond substrates may be prepared by ablating the growth surface of each of the single crystal diamond substrates with ablated regions forming the one or more regions 8 for nucleating dislocations. Pits or trenches can be ablated into the growth surface by laser ablation, electron beam, or another suitable processing technique. A masked plasma etching technique may also be used to form pits or trenches in the single crystal diamond substrates. Furthermore, it has been found to be advantageous to provide non-circular features and most preferably features which are oriented along one of the principle crystallographic directions of the diamond substrate and overgrown single crystal CVD diamond material, e.g. {100}, {110}, etc.

(7) The depth, width, shape and spacing of the features is critical as this will determine the amount of strain generated in single crystal CVD diamond material grown thereon. For example, a substrate surface feature 8 may have a depth in a range 5 to 100 m, a width in a range 5 to 500 m, and a spacing in a range 50 m to 1.3 mm. For example, while laser scoring of the substrate surface can introduce sources of dislocations and fine laser grooves can give rise to high dislocation densities and samples not prone to cracking, coarse laser grooves can lead to sample cracking (excess strain induced) when a thick layer of single crystal CVD diamond material is grown.

(8) The features may have tapered walls which can be formed using the natural shape of a cutting beam or via a benching routine. Alternatively, if a highly collimated cutting beam is utilized then the walls of the surface features may be substantially vertical.

(9) As an alternative to ablated pits or trenches, the one or more regions for nucleating dislocations may be formed by implanting, e.g. ions or via other irradiation, the growth surface of each of the single crystal diamond substrates to from highly damaged regions which form the one or more regions for nucleating dislocations.

(10) Key to the processing is to form isolated regions of the growth surface which generate the desired amount of stress or strain, via dislocation nucleation, in the single crystal CVD diamond grown thereon while the surrounding regions of single crystal CVD diamond retain a low stress, strain, and birefringence.

(11) In step (c), the growth surface 4 of the single crystal diamond substrate 2 is etched, e.g. via plasma etching, after forming the one or more regions 8 for nucleating dislocations and prior to growth of single crystal CVD diamond thereon. The etching is controlled to remove processing damage 6 from the one or more low defect regions to ensure that the single crystal CVD diamond material overlying these regions will have a low birefringence in a similar manner to WO2004/046427. For example, the one or more low defect regions on the growth surface of the substrate may have a density of defects such that surface etch features related to defects formed by a revealing plasma etch is below 510.sup.3/mm.sup.2.

(12) However, it is also critical not to fully remove the one or more regions 8 for nucleating dislocations during etching such that these regions are in a form capable of generating sufficient dislocations in the single crystal CVD diamond material grown thereon to form the high birefringence regions. The ability to control birefringence in the high strain regions is a function of both the size and shape of the features 8 processed into the growth surface 4 in step (b) and the way these features are modified by the etching in step (c). For example, etching can reduce the depth of the features 8 and round edges of the features, both of which will tend to lower the amount of stress which is generated in product material grown thereon. In step (d), at least 2 mm2 mm2 mm of single crystal CVD diamond material 10 is grown on the growth surface 4 the single crystal diamond substrates 2. For example, CVD diamond growth conditions may be controlled in accordance with the following parameters: a gas phase composition comprising a carbon source gas and hydrogen, wherein the carbon source gas forms between 3 and 7% of the gas phase composition and nitrogen is present in a gas phase concentration of 300 ppb to 2 ppm; a substrate temperature in a range 700 C. to 1050 C.; a pressure in a range 90 to 400 torr; and a microwave power in a range 3 to 60 kW for substrate diameters in a range 25 to 300 mm.

(13) Early stage CVD diamond growth should be controlled to ensure that the correct amount of dislocations are nucleated in high strain regions 12. For example, certain growth processes, such as very slow growth processes using a high purity CVD process, where diamond growth and etching are finally balanced, can result in the substrate surface features being etched or filled in during the early growth phase without nucleating sufficient dislocations to generate the desired amount of strain. Similarly, certain growth processes such as fast growth rate processes using significant levels of nitrogen, can overgrow the nucleating structures in such a way as to generate too much stress in the overgrown single crystal CVD diamond material. As such, it has been found that the ability to control birefringence in the high strain regions 12 is not only a function of both the size and shape of the features 8 processed into the growth surface 4 in step (b) and the way these features are modified by the etching in step (c) as previously indicated, but also a function of the parameters used in the early stages of single crystal CVD diamond growth.

(14) Furthermore, after the earlier stage of single crystal CVD diamond growth, even if the correct levels of dislocations and strain is generated, the CVD diamond growth process must be controlled to suppress dislocation multiplication and thus control the level of strain through a thick layer of single crystal CVD diamond material 10. A different growth process may be utilized for the early CVD diamond growth phase as compared to the process conditions used for the majority of the single crystal CVD diamond material 10 to ensure that the correct number of dislocations is generated prior to switching to a growth regime which suppresses dislocation multiplication during the main growth phase. For example, according to certain embodiments it has been found to be advantageous to grow a thin layer of high purity single crystal CVD diamond material initially (e.g. according to WO2001/096633) and then move to a synthesis processes which uses low and controlled nitrogen addition (e.g. according to WO2004/046427).

(15) After growth the original substrate 2 can be removed (e.g. via laser cutting, electron beam, or some other method) to yield a free-standing single crystal CVD diamond product 10 as illustrated in step (e) of FIG. 1. Alternatively, the single crystal CVD diamond material 10 can be retained on the substrate 2 and processed into a product incorporating the substrate and the single crystal CVD diamond material.

(16) The single crystal CVD diamond material 10 fabricated using the methodology describe herein comprises: three orthogonal dimensions of at least 2 mm; one or more regions of low optical birefringence 14, indicative of low strain, such that in a sample of the single crystal CVD diamond material having a thickness in a range 0.5 mm to 1.0 mm and an area of greater than 1.3 mm1.3 mm and measured using a pixel size of area in a range 11 m.sup.2 to 2020 m.sup.2, a maximum value of n.sub.[average] does not exceed 1.510.sup.4 for the one or more regions of low optical birefringence, where n.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; one or more regions of high optical birefringence 12, indicative of high strain, such that in said sample of the single crystal CVD diamond material and measured using said pixel size, a maximum value of n.sub.[average] is greater than 1.510.sup.4 and less than 310.sup.3; and wherein every 1.3 mm1.3 mm area of the sample of the single crystal CVD diamond material comprises at least one of said regions of high optical birefringence.

(17) The one or more regions of low optical birefringence may preferably have a maximum value of n.sub.[average] which does not exceed 810.sup.5 or more preferably 510.sup.5.

(18) The one or more regions of high optical birefringence may have a n.sub.[average] value which is greater than 2.510.sup.4 or 510, a n.sub.[average] value which is less than 2.510.sup.3, 210.sup.3, or 110.sup.3, or a n.sub.[average] value within a range defined by any combination of these upper and lower values.

(19) The one or more regions of high optical birefringence each comprise a plurality of dislocations extending laterally in a line across the single crystal CVD diamond material or may each comprise a bundle of dislocations at a point localized in lateral dimensions of the single crystal CVD diamond material. The one or more regions of high optical birefringence may each have a width in a range 5 to 500 m. Where a plurality of the regions of high optical birefringence are provided, the regions of high optical birefringence can be spaced apart by a distance in a range 50 m to 1.3 mm. The geometry of the regions of high optical birefringence will depend on the geometry of the features provided in the substrate on which the single crystal CVD diamond material is grown. As such, the regions can be controlled so as to be configured in a regular pattern rather than a random distribution. For example, the regions of high birefringence are equally spaced.

(20) The optical birefringence in the regions of low and high optical birefringence may be measured in a direction of highest birefringence to within 10. This will generally correspond to the growth direction of the single crystal CVD diamond material and corresponds to the primary direction of dislocations propagating through the material.

(21) The one or more regions of high optical birefringence may each have an area of at least 100 m100 m. When measuring the birefringence of the regions of high optical birefringence with an array of small pixels it is possible that not every one of the pixels will exhibit high birefringence. However, at least 80% of the pixels in the one or more regions of high optical birefringence have n.sub.[average] greater than 1.510.sup.4 and less than 310.sup.3. Furthermore, in terms of an entire sample comprising regions of high and low birefringence, at least 2% of a total number of pixels in every 1.3 mm1.3 mm area of a sample of the single crystal CVD diamond material form the one or more regions of high optical birefringence.

(22) Optionally, the single crystal CVD diamond material has a neutral single substitutional nitrogen) (N.sub.s.sup.0) concentration in a range 310.sup.15 atoms/cm.sup.3 to 510.sup.17 atoms/cm.sup.3 as measured by electron paramagnetic resonance and for certain applications a tighter range of less than 210.sup.17 atoms/cm.sup.3 and/or more than 110.sup.16 atoms/cm.sup.3 is desirable. Furthermore, the single crystal CVD diamond material preferably has a low optical absorption such that a sample of a specified thickness (0.5 mm to 1.0 mm) has an optical absorption coefficient at a wavelength of 1.06 m of less than 0.09 cm.sup.1, preferably less than 0.07 cm.sup.1. The present invention also enables the fabrication of thick, colourless or near colourless single crystal CVD diamond material which is of high crystal quality with good optical characteristics except for those regions of high stress and birefringence which have been controllably engineered into the material. For example, the single crystal CVD diamond material can be colourless or near colourless with a colour grade in a range D to J. In many respects, embodiments of the present invention comprise portions of material meeting the requirements of WO2004/046427 separated by portions of high birefringence material.

(23) Stress patterns can be observed in birefringence measurements of single crystal CVD diamond material according to embodiments of the invention. Induced dislocations propagate close to the <100> direction and can be perturbed by step flow. For example, fine <110> oriented grooves can be provided in a {100} growth surface of a substrate and the dislocations propagate upwards through the single crystal CVD diamond material grown thereon in a <100> direction with minor perturbations due to step flow. The high strain regions can be separated by an amount such that the dislocations in adjacent high strain regions do not affect each other.

(24) The single crystal CVD diamond material according to the present invention may be used in a range of applications including optical applications, thermal applications, jewellery applications, and as substrates for further CVD diamond growth (e.g. via vertical slicing to form substrates with low defect growth surfaces). For example, the single crystal CVD diamond material may be in the form of a cut gemstone. The high strain regions can also be intentionally patterned to form a distinctive mark viewable, for example, under luminescent conditions, which can function as an indicator of the source of the material or as a personalized design for an end customer. Furthermore, the engineered strain patterns can be used for controlling optical and spin properties of the diamond material.

(25) 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.