Comparison of optical emission spectrometric measurements of the concentration and energy of species in low-pressure microwave and radiofrequency plasma sources

Abstract

Understanding of specific plasmachemical reactions occurring in plasma sources with the addition of complicated molecules requires knowledge of the particular plasma conditions. The subject of the present paper is a comparison of the spectrometrically measured relative concentration distributions and energies of species in low-pressure argon discharges containing an organosilicon compound, hexamethyldisiloxane (HMDSO), which has a large molecular size. The optical diagnostics were performed with three different plasma sources: a special planar microwave plasma source (v= 2.45 GHz), an r.f. planar reactor (v= 13.56 MHz) and a capacitively coupled r.f. model discharge tube (v= 460 kHz). The energy distribution functions of each plasma are not the same, but their general forms are similar to a Maxwell–Boltzmann distribution. The results of comprehensive, spatially resolved measurements of the relative concentrations of atoms and radicals (H, Si, CH and C₂), the neutral species gas temperature, rotational temperature and optical excitation temperature are reported. The use of the same gas mixture in three plasma sources, each of distinct construction, excited by different frequencies, once again clearly indicates that the comparison and interpretation of optical diagnostic results has to be done very carefully, taking into consideration the discharge conditions. Results obtained by actinometry show considerably different particle density gradients in the plasmas. Gradients of the excitation temperatures (T_exc= 0.45–0.7 eV) are less, but the neutral gas temperatures also exhibit large spatial gradients (T_g= ambient temperature, about 2000 K). This clearly indicates the absolute necessity of spatially resolved optical emission measurements for the purpose of comparisons between different plasma sources.