Microwave plasma reactors and substrates for synthetic diamond manufacture

1. A substrate for use in a microwave plasma reactor, the substrate comprising:

a cylindrical disc of a carbide forming refractory metal having a flat growth surface on which CVD diamond is to be grown and a flat supporting surface opposed to said growth surface,

wherein the cylindrical disc has a diameter of 80 mm or more,

wherein the growth surface has a flatness variation no more than 100 μm, and

wherein the supporting surface has a flatness variation no more than 100 μm.

2. A substrate according to claim 1 , wherein the flatness variation of the growth surface is no more than 75 μm, 50 μm, 40 μm, 30 μm, 20 μm, 10 μm, 5 μm, or 1 μm.

3. A substrate according to claim 1 , wherein the flatness variation of the supporting surface is no more than 75 μm, 50 μm, 40 μm, 30 μm, 20 μm, 10 μm, 5 μm, or 1 μm.

4. A substrate according to claim 1 , wherein the carbide forming refractory metal is selected from one of molybdenum, tungsten, niobium, or alloys thereof.

5. A substrate according to claim 1 , wherein the cylindrical disc comprises no more than 0.5%, 0.1%, 0.075%, 0.05%, 0.025%, 0.01%, 0.005%, or 0.001% by weight of graphite forming impurities at the growth surface.

6. A substrate according to claim 1 , wherein the cylindrical disc is formed of at least 99%, 99.5%, 99.9%, 99.95%, or 99.99% by weight of a carbide forming refractory metal.

7. A substrate according to claim 1 , wherein the growth surface has a surface roughness R a in the range 1 nm to 1 μm, 1 nm to 500 nm, 10 nm to 500 nm, 10 nm to 200 nm, 10 nm to 100 nm, 10 nm to 50 nm, 20 nm to 100 nm, or 50 nm to 100 nm.

8. A substrate according to claim 1 , wherein an edge of the substrate around the growth surface is one of: sharp; chamfered; or rounded.

9. A substrate according to claim 1 , wherein a circular trench is provided in the growth surface separating an edge of the substrate growth surface from a central region.

10. A method of manufacturing synthetic diamond material using a chemical vapour deposition process, the method comprising:

providing a reactor configured for manufacturing synthetic diamond material;

locating a substrate according to claim 1 over a substrate holder within the reactor;

feeding process gases into the reactor; and

growing synthetic diamond material on the growth surface of the substrate,

wherein the synthetic diamond material is grown to form a polycrystalline diamond wafer having a diameter of at least 80 mm,

and wherein the polycrystalline diamond wafer is spontaneously delaminated from the substrate on cooling after the chemical vapour deposition process is completed to yield a free-standing polycrystalline diamond wafer which is substantially crack free over at least a central region thereof, wherein the central region is at least 70% of a total area of the free-standing polycrystalline diamond wafer, and wherein the central region has no cracks which intersect both external major faces of the free-standing polycrystalline diamond wafer and extend greater than 2 mm in length.

11. A method according to claim 10 , wherein the central region is at least 80%, 80%, 90%, or 95% of a total area of the free-standing polycrystalline diamond wafer.

12. A method according to claim 10 , wherein the polycrystalline diamond wafer has a diameter of at least 120 mm, 140 mm, 160 mm, 200 mm, or 250 mm.

13. A method according to claim 12 , wherein the polycrystalline diamond wafer has a diameter no more than 400 mm or 300 mm.

14. A method according to claim 10 , wherein the polycrystalline diamond wafer is grown to a thickness of at least 1.0 mm, 1.2 mm, 1.5 mm, 2.0 mm, or 2.5 mm.

15. A method according to claim 10 , wherein the reactor is a microwave plasma reactor.