Plasma processes in methane discharges during r.f. plasma-assisted chemical vapour deposition of a-C:H thin films

Abstract

Methane-hydrogen capacitively coupled r.f. (13.56MHz) discharges were studied using a mass spectrometer. The relative concentrations of neutral species (CH 4, C 2H x ) were investigated in a broad range of discharge parameters (pressure 1.5–50 Pa, residence time 1–45 s, and r.f. power 0–120 W) and correlated with deposition rate measurements. It is shown that the chemical plasma processes (C 2H x ) formation and deposition characteristics (deposition rate) are strongly correlated with the degree of methane dissociation. These observations are interpreted in terms of the occurrence of dominant CH y –CH 4 reaction pathways for the production of C 2H x and a simple growth model based on a combined mechanism of CH 3 radical adsorption and ion bombardment. The pressure dependence of the degree of dissociation of methane is correlated with the existence of various discharge regimes (α and γ).