Probing plasmas used for chemical vapour deposition of diamond thin films

The group has longstanding interests in the development and application of new or improved detection methods for gas phase species.

Multiphoton ionisation spectroscopy is one such technique. It relies on the fact that the probability of a molecule undergoing a multiphoton excitation, and thereby being ionised, is greatly enhanced if there is a real excited state resonant at (say) the energy of two absorbed photons. In such a case, the resonance enhanced multiphoton ionisation (REMPI) spectrum obtained by measuring the ion yield as a function of laser excitation wavelength will display structure associated with the two photon transition of the neutral species of interest. The technique offers high sensitivity (ions can be detected with high efficiency), and high species selectivity since the spectra so obtained constitute a unique 'fingerprint' for the species under investigation.

2+1 REMPI methods have been used to make spatially resolved measurements of both H atom and CH₃ radical relative number densities, as a function of process conditions, in a purpose designed and built hot filament (HF) reactor used for chemical vapour deposition (CVD) of thin film diamond. These measurements, when considered in conjunction with the results of complementary modelling studies of the gas phase chemistry prevailing in HF activated hydrocarbon/H₂ gas mixtures diamond CVD reactors (by colleague Mankelevich at Moscow State University) afford a much improved understanding of key gas phase and gas-surface reactions involved in diamond CVD using such weakly activated gas mixtures. In particular, this program helped clarify a longstanding puzzle regarding diamond growth from C₂H₂/H₂ gas mixtures and demonstrated the (hitherto neglected) importance of H atom addition reactions in driving the C₂H₂ → CH₄ conversion.

Cavity ring down spectroscopy (CRDS) offers a route to obtaining spatially resolved absolute column densities of a range of gas phase species present in HF, microwave and DC arc jet reactors used for diamond CVD. Recent examples of work in Bristol include:

• the use of pulsed CRDS methods to quantify CH and NH radical densities in HF activated CH₄/H₂ and CH₄/NH₃/H₂ process gas mixtures, and of continuous wave (cw) CRDS (in the infrared) to monitor C₂H₂ in HF activated CH₄/H₂ mixtures;
• pulsed CRDS studies of C₂ radicals (in both the ground (X) and first excited (a) states), of ground state CH radicals, and of electronically excited H(n=2) atoms in the highly activated plume in a <10 kW DC arc jet reactor operating with CH₄/H₂/Ar gas mixtures;
• pulsed CRDS studies of C₂(a) and CH(X) radicals, of electronically excited H(n=2) atoms in microwave activated hydrocarbon/H₂/Ar gas mixtures, and of these species and B atoms and BH(X) radicals in microwave activated B₂H₆/CH₄/H₂/Ar gas mixtures, as functions of process conditions and height above the substrate.

We also use continuous wave, single pass infrared laser absorption spectroscopy (IRLAS, with a quantum cascade laser operating at wavelengths ~8 μm, in conjunction with the Photonics Group at the University of Strathclyde) to make spatially resolved measurements of CHsub>4 and C₂H₂ column densities in a range of microwave activated hydrocarbon/H₂/Ar gas mixtures, as a function of process conditions.

All such measurements have been, or are in the process of being, tensioned against appropriate 2-D or 3-D modelling of the chemistry occurring in these activated gas mixtures. Examples of recent work can be found in our publications list.

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