Characterization of a CH 4-RF-plasma by ion flux, Langmuir probe, and optical emission spectroscopy measurements

Abstract

Methane plasmas excited with 13.56 MHz have been used for a-C:H deposition. Plasma parameters are studied by Langmuir probes and optical emission spectroscopy. In order to correlate macroscopic film properties and the plasma parameters, the ion energy distributions and the different ion masses impinging on the substrate have been determined by a 90° energy analyser and a quadrupole mass spectrometer. A very important parameter for the plasma and consequently for the a-C:H properties is the flow rate of CH 4 into the reactor. Various experiments have been performed at constant pressure and varying flow rate in the range from 21 seem to 1 seem by reducing the pumping speed. For an a-C:H film with 1300 kgf/mm 2 Knoop hardness, 1.55 g/cm 3 mass density and 1.3 GPa internal stress details of the ionic flux rate have been determined. The ionic flux rate is rather high with 0.3 mA/cm 2 and contains 65% CH − x and 30% C 2H − x ions. The ions have a mean energy of 170 eV (4.4 × 10 −2 mbar pressure, 21 seem flow rate, residence time of the gas in the plasma∼1 × 10 −2s). The ionic flux rate is about 32% of the total particle flux to the surface, which forms the film. In the plasma a rather high portion of the gas is transformed into C 2H 2 and C 2H 4, both in the order of 5–10% of the CH 4 pressure.