Correlation between the characteristics of a pulsed microwave plasma used for diamond growth and the properties of the produced films
Résumé
Previous studies have pointed out the advantages of using a pulsed discharge for diamond growth by microwave plasma assisted chemical vapour deposition (MPACVD): increase of the growth rate and/or enhancement of the diamond quality. It has been shown that the duration of the pulse and that of the pause between two pulses have a particular influence on the deposition process. Optimal values can be determined that depends mainly on the geometry and size of the reactor and on the working conditions. In the present work, new results are obtained in a MPACVD reactor working in a pulsed plasma mode, H2/CH4 gas mixture and 6 kW peak power. These conditions are similar to those of industrial reactors. The most important species in such process are CHx, C2Hy and H, which are considered, respectively, as the key species for diamond deposition, graphite deposition and graphite etching. A particular interest was devoted to the atomic hydrogen evolution during the discharge phase and the afterglow. Quantitative measurements of H-atom ground state concentration are carried out by two-photon Laser Induced Fluorescence (LIF). The study of the kinetic processes which are responsible of atomic hydrogen creation and loss permits us to determine the production rate of H-atoms in pulsed mode as compared with the continuous one (CW), and leads to a better understanding of its influence on the diamond growth rate and its chemical quality as estimated by micro-Raman spectroscopy. The correlation established between the results of the diamond film analysis and the plasma characteristics points out the benefit of this study for the optimisation and the control of the diamond growth process by pulsed MPACVD.