Direct-current low-pressure supersonic plasma jet used for diamond deposition

Abstract

A study of diamond deposition in a supersonic plasma jet at mbar pressures is described. Methane as carbon containing species was mixed into the decaying plasma by injection either close to the exit of the Laval nozzle or into the background of the chamber. From coatings deposited on pre-treated molybdenum, substrates at distances ranging from 3.5 to 20 cm growth rates as well as diamond phase purity were derived. At the conditions investigated, a growth rate of about 6 µm/h at a distance of 5–10 cm from the nozzle was determined for methane injection at the exit of the nozzle. The growth rate decreases for shorter and longer distances. With increasing distance from the nozzle the crystallite size decreases and the diamond phase purity as reflected in the Raman spectra deteriorates. When methane is injected into the background gas, approximately eight times as much methane gas flow is needed to achieve a comparable growth rate at a comparable diamond film quality. These results can be explained by mass spectrometric investigations accompanied with calculations performed with a numerical model of the gas-phase reactions in the jet under simplifying assumptions for the transport of the species to the substrate.