Microwave Plasma Enhanced Chemical Vapour Deposition of Diamond
Microwave plasma enhanced chemical vapour deposition (MWPECVD) is used at Bristol to deposit CVD diamond under varying growth conditions. Films can then be analysed ex situ by Scanning Electron Microscopy (SEM), Laser Raman Spectroscopy (LRS), Field Emission studies, AES, SIMS etc. The chemistry of CVD diamond growth can be studied in situ via a Molecular Beam Mass Spectrometer attached to the microwave chamber.
Plasma-assisted deposition methods
Plasma-assisted deposition techniques are very popular methods for growing CVD diamond. Plasma systems offer uniform films over larger substrate areas than hot filament CVD growth, with the possibilty of industrial scale up by using more powerful reactors. Many different forms of plasma deposition system have been developed and some are used to deposit CVD diamond e.g. microwave plasma, electron cyclotron resonance microwave plasma, and d.c. arc-jet plasmas. The plasma in the microwave system is detached from any reactor surface hence no impurities from reactor construction materials enter the film bulk during deposition. In hot filament assisted CVD incorporation of filament materials into the film occurs during deposition and this (combined with limited deposition area) make hot filament methods useless for high purity commercial applications of diamond. However hot filament reactors are cheaper and simpler than microwave reactors and a lot of useful research is done in them.
Our Microwave system
Our reactor (shown right) consists of a 1.5 kW ASTeX-style microwave generator coupled to the top of a cylindrical, water cooled, stainless steel chamber. A resonant electromagnetic field pattern (mode), created by the microwaves is supported in the chamber and the reactant gases are heated and excited to form a plasma ball. The substrate sits ~1 mm below the visible edge of the plasma ball, on top of molybdenum substrate holder. The reactor is also equipped with diagnostic ports, such as the one mentioned earlier to which we have attached a Molecular Beam Mass Spectrometer.
This reactor is capable of growing high quality polycrystalline diamond films at rates of around 6 μm h-1 uniformly over an area of 2x2 cm Si substrate. Films of many 10's of μm can easily be grown in a few hours, or even 100's of μm by combining together several day's worth of growth runs.