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Methane plasmas excited at 25 kHz and 13.56 MHz used for a-C:H deposition have been studied by optical emission spectroscopy. At 25 kHz the emitted bands and lines, namely CH (431.4 nm), H (486.1 nm) and H2 (463.4 nm), have been found to result from dissociative excitation of the CH4 molecule under electron impact. It is supposed that the fast electron beam formed by the accelerated secondary electrons is predominant. In this discharge a correlation between the CH emission and the deposition rate is found. In contrast to the 13.56 MHz discharge the Hbeta emission appears to result from both dissociative excitation from CH4 and electron impact excitation of H atoms present in the bulk of the discharge. A correlation between the IH/ICH intensity ratio and the bulk electron density measured by double probes has been found. However, no correlation has been found between the emission intensities of various species and the deposition rates. It is thought that processes in the vicinity of the RF electrode on which deposition occurs are predominant.

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