Improvement of the Harmonic Technique of Probe for Measurements of Electron Temperature and Ion Density

Conventional Langmuir probe techniques usually face the difficulty of being used in processing plasmas where dielectric compounds form, due to rapid failure by surface insulation. A solution to the problem, the so-called harmonic probe technique, had been proposed and shown effectiveness. In this study, the technique was investigated in detail by changing bias signal amplitudes V₀, and evaluated its accuracy by comparing with the conventional Langmuir probe. It was found that the measured electron temperature T_e increased with V₀, but showing a relatively stable region when V₀ >T_e/e in which it was close to the true T_e value. This is contrary to the general consideration that V₀ should be smaller than T_e/e for accurate measurement of T_e. The phenomenon is interpreted by the non-negligible change of the ion current with V₀ at low V₀ values. On the other hand, the measured ni also increased with V₀ due to the sheath expansion, and to improve the accuracy of n_i it needs to linearly extrapolate the n_i-V₀ trend to V₀=0. The results were applied to a diagnosis of the plasmas for chemical vapor deposition of diamond-like carbon thin films and the relationship between plasma parameters and films deposition rates was obtained.