Microwave plasma reactor for manufacturing synthetic diamond material

A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising:

• a plasma chamber;
• a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use;
• a microwave coupling configuration for feeding microwaves from a microwave generator into the plasma chamber;
• and
• a gas flow system for feeding process gases into the plasma chamber and removing them therefrom;
• wherein the microwave plasma reactor further comprises an electrically conductive plasma stabilizing annulus disposed around the substrate holder within the plasma chamber when viewed down a central axis of the plasma chamber,
• wherein the electrically conductive plasma stabilizing annulus is in the form of a projecting ring which protrudes into the plasma chamber from a side wall of the plasma chamber,
• wherein the electrically conductive plasma stabilising annulus has an upper surface, a lower surface, and an end portion which forms the projecting ring which protrudes into the plasma chamber from the side wall of the plasma chamber,
• wherein the upper surface of the electrically conductive plasma stabilising annulus is spaced apart from the a microwave coupling configuration, the electrically conductive plasma stabilizing annulus being positioned at a height within 50 mm, of a height of the supporting surface of the substrate holder, and
• wherein the electrically conductive plasma stabilizing annulus has an axial depth relative to a height of the plasma chamber in the range 1% to 30%.