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

The 1.5 kW MW reactorPlasma-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.

The Plasma Balls

We have done extensive work studying CH4/H2 plasmas, which produce a light blue/purple plasma ball as shown on the right. CH4/H2 plasma - click for enlargement
Methane/Hydrogen plasma
We have also studied CO2/CH4 gas mixtures, as these allow diamond growth down to temperatures as low as 400°C. A 100% CO2 plasma is deep blue with a whitish centre. 100% CO2 plasma - click for enlargement
100% CO2 plasma
For CO2-rich plasmas (upto 50% CO2) the plasma becomes more light blue in colour, but there is still no diamond growth. Diamond growth is confined to a very narrow window around the 50:50 mixture. 50%CO2/50%CH4 plasma - click for enlargement
CO2-rich plasmas
With increasing CH4 concentration, the plasma develops a yellow/orange halo around its extremeties due to blackbody emission from macroscopic soot particles as well as excited C2. Close to the wafer surface there is a violet/purple haze. The plasma also starts to deposit soot around the cooler parts of the chamber walls. 30%CO2/70%CH4 plasma - click for enlargement
70% CH4/30% CO2 plasma
For 100% CH4 plasmas, the ball is very bright white in the centre with the orange halo now very pronounced and extending a long way into the chamber. The soot production rate is very fast, making it difficult to sustain these plasmas for more than a few minutes. 100% CH4 plasma - click for enlargement
CH4 plasma
A 1% CH4 in 99% Ar plasma is much colder than other plasmas, but is also quite sooty. These plasmas are used to grow UNCD films. 1% CH4 in Ar plasma - click for enlargement
Ar CH4 plasma
A 5% CH4/10%Ar/85%H2 Ar plasma running at pressures over 150 Torr looks much greener, due to emission from the C2 radical. These plasmas are used to grow polycrystalline diamond films at higher rates. High pressure CH4/H2(/Ar) plasma ball - greenish