POLYCRYSTALLINE SYNTHETIC DIAMOND MATERIAL

Abstract

A method of fabricating a polycrystalline CVD synthetic diamond wafer is disclosed. A first polycrystalline CVD synthetic diamond wafer is grown using a CVD process to a first thickness on a substrate. A second smaller wafer is cut from the polycrystalline CVD synthetic diamond wafer. The second smaller wafer is located on a carrier, and further polycrystalline CVD synthetic diamond material is grown on the second smaller wafer to a second thickness to give a polycrystalline CVD synthetic diamond material having a total thickness of the combined first and second thicknesses.

Claims

1. A method of fabricating a polycrystalline CVD synthetic diamond wafer, the method comprising: locating a substrate in a first CVD reactor; introducing process gases into the first CVD reactor, wherein the process gases within the CVD reactor comprises a carbon containing gas and hydrogen, and using the process gases to form a plasma; growing a first polycrystalline CVD synthetic diamond wafer to a first thickness on the substrate; removing the first polycrystalline CVD synthetic diamond wafer from the first CVD reactor; removing the first polycrystalline CVD synthetic diamond wafer from the substrate; cutting at least one second smaller wafer from the polycrystalline CVD synthetic diamond wafer; locating the at least one second smaller wafer on a carrier; locating the carrier and second smaller wafer in a second CVD reactor; introducing process gases into the second CVD reactor, wherein the process gas within the second CVD reactor comprises nitrogen, a carbon containing gas, and hydrogen, and using the process gases to form a plasma; and growing further polycrystalline CVD synthetic diamond material on the second smaller wafer to a second thickness to give a polycrystalline CVD synthetic diamond material having a total thickness of the combined first and second thicknesses.

2. The method according to claim 1, wherein the second CVD reactor is the first CVD reactor.

3. The method according to claim 1 wherein the second smaller wafer is cut from at least a central area of the first polycrystalline CVD synthetic diamond wafer, wherein the central area is at least 70% of a total area of the first polycrystalline CVD synthetic diamond wafer, and wherein the first polycrystalline CVD synthetic diamond wafer is substantially crack free over at least the central area thereof such that the central area has no cracks which intersect both external major faces of the first polycrystalline CVD synthetic diamond wafer and extend greater than 2 mm in length.

4. (canceled)

5. The method according to claim 1, wherein the carrier comprises a polycrystalline CVD synthetic diamond surface.

6-7. (canceled)

8. The method according to claim 1, further comprising cutting a plurality of second smaller wafers from the polycrystalline CVD synthetic diamond wafer and affixing at least one of the plurality of second smaller wafers to the carrier.

9-10. (canceled)

11. The method according to claim 1, wherein the substrate is a carbide-forming metal.

12. The method according to claim 1, further comprising processing a surface of the at least one second smaller wafer prior to growth of further polycrystalline diamond on the second smaller wafer.

13. The method according to claim 12, wherein the surface that is processed is the surface that was initially adjacent to the substrate.

14. The method according to claim 1, further comprising growing the first polycrystalline CVD synthetic diamond wafer and growing the further polycrystalline CVD synthetic diamond material using different conditions selected from any of time, power density, pressure and gas composition such that the first polycrystalline CVD synthetic diamond and the further polycrystalline CVD synthetic diamond material have different properties.

15. The method according to claim 1, wherein any of the first and CVD reactor and the second CVD reactor is a microwave plasma CVD reactor.

16. A polycrystalline CVD synthetic diamond material having: an average thermal conductivity at room temperature through a thickness of the polycrystalline CVD synthetic diamond material of between 1700 and 2400 Wm.sup.−1K.sup.−1; a total thickness of at least 2.5 mm; and wherein the thickness comprises a first thickness of first polycrystalline diamond material grown during first growth conditions, and a second thickness of second polycrystalline diamond material grown during second growth conditions, the first thickness and the second thickness being separated by a growth event interface.

17. The polycrystalline CVD synthetic diamond material according to claim 16, wherein the material has two opposing external major faces and the material is substantially crack free such that there are no cracks that intersect both external major faces of the polycrystalline CVD synthetic diamond material.

18. (canceled)

19. The polycrystalline CVD synthetic diamond material according to claim 16, wherein the polycrystalline CVD synthetic diamond material has a largest linear dimension selected from any of at least 10 mm, at least 15 mm, at least 20 mm, at least 25 mm, at least 30 mm, at least 40 mm, at least 50 mm, at least 75 mm and at least 100 mm.

20. (canceled)

21. The polycrystalline CVD synthetic diamond material according to claim 16, wherein the growth event interface has a thickness selected from any of no more than 5% of the first thickness, no more than 2% of the first thickness and no more than 1% of the first thickness.

22. The polycrystalline CVD synthetic diamond material according to claim 16, wherein the first thickness of first polycrystalline diamond material has a different composition to the second thickness of second polycrystalline diamond material.

23. The polycrystalline CVD synthetic diamond material according to claim 16, wherein the first thickness of first polycrystalline diamond material has a different thickness to the second thickness of second polycrystalline diamond material.

24. A polycrystalline CVD synthetic diamond material made according to the method of claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0047] Non-limiting embodiments will now be described by way of example and with reference to the accompanying drawings in which:

[0048] FIG. 1 is a flow diagram showing exemplary steps;

[0049] FIG. 2 illustrates schematically in plan view the process of FIG. 1;

[0050] FIG. 3 is a cross-section micrograph showing polycrystalline CVD synthetic diamond material grown according to the method of FIG. 1;

[0051] FIG. 4 is a cropped and higher magnification image of the polycrystalline CVD synthetic diamond of FIG. 2;

[0052] FIG. 5 is a Diamondview™ image of the cross-section of FIG. 3; and

[0053] FIG. 6 is a differential interference contrast (DIC) image of the polished growth surface.

[0054] FIG. 7 illustrates schematically in a side elevation view (not to scale) the variation in thickness of an as grown polycrystalline CVD diamond material and the effect of processing the surface of the polycrystalline CVD diamond material; and

[0055] FIG. 8 illustrates schematically in a side elevation view (not to scale) the effect of using the surface of the wafer adjacent to the substrate as a new growth surface.

DETAILED DESCRIPTION

[0056] The inventor has realised that higher yields and early identification of failed growth runs can be achieved by growing a polycrystalline CVD diamond wafer to a first thickness, cutting usable parts of this wafer into one or more smaller polycrystalline CVD diamond wafer and then continuing diamond growth on the smaller polycrystalline CVD diamond wafers. In this way, if a problem has occurred at the start of a growth run, it is identified early when the first polycrystalline CVD diamond wafer is processed to form the smaller wafers. Furthermore, if the continued diamond growth on the smaller wafers has a yield of, say, 80%, this is an improvement over the prior art where the entire run would give no usable yield. A further advantage of the method is that diamond can be grown with different properties at different points through the thickness of the final material. A further advantage of the method is that the polycrystalline CVD diamond can be grown at near net-shape, thereby requiring less processing to form a final product.

[0057] The process is illustrated in the flow diagram of FIG. 1, wherein the following numbering corresponds to that of the flow diagram:

[0058] S1. A substrate is located in a first CVD reactor. An example of a suitable CVD reactor is described in WO 2104/026930. Any suitable substrate may be used, such as a refractory carbide-forming metal, a substrate coated with diamond, as well known by the person of skill in the art. The substrate is processed to obtain a low surface roughness. Prior to CVD diamond growth, the refractory metal substrates may be cleaned to ensure all contamination from the processing has been removed and/or seeded to aid nucleation for diamond growth thereon. Typical sizes for the substrate range from 60 mm to 200 mm, 80 mm to 150 mm, 90 mm to 110 mm or 95 mm to 105 mm.

[0059] S2. Process gases are introduced, which are ionized to form a plasma close to the substrate. The process gases include hydrogen and a carbon containing gas such as methane. Other gases may also be added if it is required to dope the polycrystalline diamond. Examples of other types of gas include boron-containing gas and nitrogen containing gas.

[0060] The plasma may be formed by any suitable method known to the skilled person, such as using any of a microwave plasma CVD reactor, a hot filament reactor, a plasma jet reactor, and a DC cathode arc jet reactors.

[0061] While high power density and high pressure conditions have been found to be advantageous for synthesis of certain grades of polycrystalline diamond material at high growth rates, if the power density and pressure are made too high then the growth conditions become less stable and more difficult to control in a uniform manner.

[0062] The temperature of the substrate may be controlled, partly by the microwave power and partly by the use of cooling gases flowing adjacent to the substrate. Typically, the temperature difference between the edge and a centre point of the substrate is no more than 60° C., 49° C., 20° C. or 10° C. A low temperature difference is desirable to reduce strain within the substrate and the diamond material grown on the substrate, but it can be difficult to achieve this. As the largest linear dimension of the substrate increases, it becomes harder to control the temperature difference.

[0063] S3. A first polycrystalline CVD synthetic diamond wafer is grown to a first thickness on the substrate.

[0064] S4. The first polycrystalline CVD synthetic diamond wafer is removed from the first CVD reactor. It may also be removed from the substrate.

[0065] S5. As shown in FIG. 2a, the polycrystalline CVD synthetic diamond wafer 1 is processed to cut a second, smaller wafer 2 from the polycrystalline CVD synthetic diamond wafer. FIG. 2a shows a plan view and a side elevation view of the polycrystalline CVD synthetic diamond wafer 1. Any suitable cutting technique may be used, such as laser cutting, Electrical Discharge Machining (EDM) if the diamond is of a suitably electrically conducting composition), mechanical cutting and so on. The second smaller wafer 2 may then also be cleaned to remove graphite and other contaminants, and may be further processed to give a surface with a required roughness, flatness and uniform (or controlled) thickness. As shown in FIG. 2a, a plurality of second smaller wafers 2 may be cut from the polycrystalline CVD synthetic diamond wafer 1. Note that not all of the polycrystalline CVD synthetic diamond wafer 1 may be of sufficient quality for further diamond growth. Cracks, impurities and inclusions may make sections of the polycrystalline CVD synthetic diamond wafer 1 unsuitable. Typically, such regions will be towards the circumference of the polycrystalline CVD synthetic diamond wafer and only a central portion of the polycrystalline CVD synthetic diamond wafer 1 may be usable.

[0066] S6. As shown in FIG. 2b, the second smaller wafer 2 is located on a carrier 3 by any suitable means. Examples of such means include brazing, soldering and mechanically affixing. In most cases, it is important to have a uniform thermal contact between the second smaller wafer 2 and the carrier 3 to ensure that the temperature and temperature variations across the surface of the second smaller wafer 2 can be controlled, and that hot-spots do not appear between the second smaller wafer 2 and the carrier 3. Carriers may include other smaller wafers, carbide-forming metals, carbide-forming metals with a coating of polycrystalline diamond or other suitable materials as known to the skilled person. Note that a plurality of second smaller wafers 2 may be located on the carrier.

[0067] S7. The carrier 3 and the second smaller wafer 2 (or wafers) are located in a second CVD reactor. The second CVD reactor may be the same CVD reactor as used to grow the first polycrystalline CVD synthetic diamond wafer, or it may be a different CVD reactor.

[0068] S8. Process gases are introduced into the reactor and ionized to form a plasma close to the substrate. The process gases include a carbon containing gas such as methane, hydrogen, and the process gas forms a plasma. Other gases may also be added if it is required to dope the polycrystalline diamond. Examples of other types of gas include boron-containing gas and nitrogen containing gas. Growth conditions are typically similar to those described above in S2 of the flow diagram. However, note that the growth conditions may differ significantly to give diamond with different properties. For example, it may be desirable to have a diamond material that has a first layer of diamond that has a high abrasion resistance, and a second layer that has a high thermal conductivity.

[0069] S9. Further polycrystalline CVD synthetic diamond material 4 is grown on the second smaller wafer 2 to a give a second total thickness of polycrystalline CVD synthetic diamond material.

[0070] S10. The second smaller wafer 2 is removed from the second CVD reactor and if necessary removed from the carrier for further processing to form a finished product such as a heat spreader or optical window. Alternatively, the second smaller wafer may be further processed and steps S6 to S9 repeated to give a diamond material with three or more layers.

[0071] When examining the resultant diamond material, a growth event interface is apparent between the layer of diamond grown in S2 and S3, and the layer of diamond grown in steps S8 and S9. This interface is apparent even where the growth conditions are identical. This may be observed as a change in luminescence or texture. Even if the growth conditions are identical in steps S2 and S8, the ramp up to the growth conditions will leave a visible growth interface.

[0072] In an exemplary process, a substrate in the form of a carbide-forming metal disc with a diameter of 100 mm was polished to give a surface roughness R.sub.a of less than 1 μm. The diamond synthesis process used a microwave plasma CVD reactor with a power density of 3.5 W mm.sup.−2, a pressure of around 200 torr and process gases including hydrogen and a carbon containing gas. A first polycrystalline CVD diamond wafer was produced with a thickness of around 2.5 mm.

[0073] The first polycrystalline CVD diamond wafer was laser cut into a plurality of second smaller wafers, each having a diameter of 25 mm. Each second smaller wafer was cleaned by boiling in acid to remove graphite and other contaminants.

[0074] A second tungsten disc was provided that had been coated with polycrystalline CVD synthetic diamond. The second smaller wafer was attached to the coated surface of the tungsten disc.

[0075] The second tungsten disc with the second smaller wafer was returned to the CVD reactor and a growth run was carried out to grow a second layer to a thickness of 0.8 mm, making a total thickness of 3.3 mm.

[0076] FIG. 3 is a cross-section micrograph showing the final polycrystalline CVD synthetic diamond material 5. FIG. 4 is a cropped and higher magnification image of the polycrystalline CVD synthetic diamond 1 of FIG. 3.

[0077] The first layer 6 of diamond material included some microfeatures and other defects. Microfeatures are described in WO 2013/087702. Optical microscopy of polycrystalline diamond samples in general reveals the presence of microscopic crack-like features, termed “microfeatures” formed during synthesis within individual grains, which are most probably a result of inter-grain stress. These microfeatures have diverse shapes, however typically have a radius of approximately 50-100 μm, and have been shown to have a negative impact upon certain physical properties of the diamond material. Microfeatures can be inspected under a microscope at a magnification of around ×50.

[0078] The second layer 7 of further diamond material had fewer microfeatures than the first layer 2. The interface 8 between the first layer 6 and the second layer 7 was found to be substantially free of voids and no delamination occurred between the first layer 6 and second layer 7, even when the surface of the second layer 3 was mechanically processed.

[0079] FIG. 5 is a Diamondview™ image of the cross-section of FIG. 4, where the original material has a characteristic blue and red colouration. Diamondview reveals surface fluorescence by illuminating diamond with short wave ultra-violet light. Red colouration arises from nitrogen-vacancy centres within the grains of the polycrystalline diamond. Blue colouration arises from dislocations and grain boundaries. Two layers are observed; the first layer 6 contains some red colouration and the second layer 7 contains very little red colouration.

[0080] FIG. 6 is a differential interference contrast (DIC) image of the polished growth surface of the second layer 7. This shows a surface as expected for a high quality material with few flaws, in that there are no discernible features other than some grain exposed and no voids.

[0081] When polycrystalline CVD diamond is growth, the variation in the thickness across the surface of the diamond is typically around 10% of the thickness. FIG. 7a illustrates schematically a small wafer 2 in side elevation view. The surface 9 of the small wafer is rough and the thickness varies by around 10% of the average thickness.

[0082] As shown in FIG. 7b, once further diamond 10 is grown on the smaller wafer 2, a growth even interface 11 can be seen between the smaller wafer 2 and the further diamond 10. The growth event interface typically has a thickness of around 10% of the average thickness of the smaller wafer 2. A growth event interface will also be observed if instead of following steps S1 to S10, the growth conditions are simply changed between growing the two layers.

[0083] If the surface of the second wafer is processed to reduce the surface roughness on the growth surface and to reduce variations in the thickness of the second wafer 2, a much thinner growth event interface can be observed, as shown in FIG. 7c. This thickness may be no more than 5% of the first thickness, no more than 2% of the first thickness or no more than 1% of the first thickness. Such processing is well known to a person of skill in the art, and may include lapping, grinding, polishing and other well-known techniques.

[0084] Note that this processing may be applied to the surface of each smaller wafer 2, or may be applied to the surface of the polycrystalline CVD synthetic diamond wafer 1 before it is cut into smaller wafers 2.

[0085] In a further embodiment, the surface of the smaller wafer 2 that is processed and forms the growth surface for the further diamond material 10 is the surface that was initially adjacent to the substrate in the initial growth stage. As CVD polycrystalline diamond is grown, the grains coarsen and so grains adjacent to the substrate at the start of growth are typically smaller than grains at the opposite surface to the surface adjacent to the substrate. This is illustrated in FIG. 8.

[0086] FIG. 8a shows a smaller wafer 2 with a rough surface 9 and an opposite surface 11 that was originally adjacent to substrate. Note that the opposite surface will also have surface roughness (not shown) that will need to be processed. The smaller wafer 2 is turned over and both of the major surfaces 9, 11 are processed to give a low surface roughness and a uniform thickness (shown in FIGS. 8b and 8c).

[0087] The average grain size at the rough surface 9 is larger than the average grain size at the opposite surface. As a smaller grain size gives rise to a more uniform polycrystalline diamond material, the opposite surface 11 is used as a surface on which to grow the further polycrystalline CVD diamond 10.

[0088] While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendant claims.