Abstract

We investigated the diamond synthesis in a microwave plasma (MPCVD) using Ar/CH₄/O₂, Ar/C₂H₂/O₂ and Ar/CH₄/CO₂ gas mixtures. These mixtures do not contain molecular hydrogen and are more typical for flame deposition methods in which diamond is deposited, for example, in an ethine/oxygen combustion flame. We focus in the present study to regions of a C/H/O gas composition in the Bachmann „diamond phase diagram“ which to our knowledge had not been investigated with MPCVD until now. The ternary C/H/O diagram proposed by Bachmann demonstrates that diamond synthesis takes place only in a limited range of gas compositions. With the commonly used hydrogen/hydrocarbon/oxygen gas mixtures with a hydrogen concentration near 95-99% the experiments are limited to the H-comer of the C/H/O ternary diagram (C/H atomic ratio<0.025). In our experiments, the C/H atomic ratio is kept constant, 1:4 for Ar/CH₄/O₂ and 1:1 for Ar/C₂H₂/O₂. For each system we determined the range of gas phase composition in which diamond deposition was detected. This was achieved by Raman spectroscopic analysis of the deposited films. The second goal concerned the analysis of the plasma processes by mass spectroscopy, optical emission spectroscopy and model calculations using a program to calculate the thermodynamic equilibrium concentrations. The calculations revealed that a drastic reduction of ethine occurs for carbon mole fractions X_c=C/(C+O) near and above 0.5, this is due to the formation of CO and CO₂ with a high negative ΔG.