Microwave plasma reactors and substrates for synthetic diamond manufacture

A method of manufacturing synthetic diamond material via chemical vapour deposition, the method comprising:

• providing a microwave plasma reactor comprising:
• a microwave generator configured to generate microwaves at a frequency f;
• a plasma chamber comprising a base, a top plate, and a side wall extending from said base to said top plate defining a resonance cavity for supporting a microwave resonance mode between the base and the top plate;
• a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber;
• a gas flow system for feeding process gases into the plasma chamber and removing them therefrom;
• and
• a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate;
• locating a substrate over the supporting surface of the substrate holder, the substrate having a growth surface on which the synthetic diamond material is to be deposited,
• feeding process gases into the plasma chamber;
• and
• feeding microwaves into the plasma chamber to form a plasma over the growth surface of the substrate and grow synthetic diamond material on the growth surface of the substrate,
• wherein the substrate dimensions and location within the resonance cavity are selected to generate a localized axisymmetric Ez electric field profile across the growth surface in use, the localized axisymmetric Ez electric field profile comprising a substantially flat central portion bound by a ring of higher electric field, the substantially flat central portion extending over at least 60% of an area of the growth surface of the substrate and having an Ez electric field variation of no more than ±10% of a central Ez electric field strength, the ring of higher electric field being disposed around the central portion and having a peak Ez electric field strength in a range 10% to 50% higher than the central Ez electric field strength.