Axially and temporally resolved optical emission structures were investigated in the rf sheath region of a parallel plate capacitively coupled rf discharge (13.56 MHz) in pure oxygen and tetrafluoromethane. The rf discharge was driven at total pressures of between 10 and 100 Pa, gas flow rate of 3 sccm and rf power in the range 5–100 W. In particular, the emission of the atomic oxygen at 844.6 nm (3p3P → 3s3S0) and the atomic carbon at 193 nm (3s1P0 → 2p1D) were imaged with a lens onto the entrance slit of a spectrometer and detected by a fast ICCD-camera. The spatio-temporally resolved analysis of the emission intensity during the rf cycle (73.75 ns) provides two significant excitation processes inside the rf sheath: the electron impact excitation at the sheath edge, and heavy particle impact excitation in front of the powered electrode. In oxygen plasma the emission of atomic oxygen was found in both regions whereas in tetrafluoromethane the emission of atomic carbon was observed only in front of the powered electrode. The experimental results reveal characteristic dependence of the emission pattern in front of the powered electrode on plasma process parameters (self-bias voltage, pressure) and allow an estimation of the excitation threshold energy and effective cross section of energetic heavy particle loss.
A laser-induced fluorescence technique, in combination with optical emission spectroscopy, was applied to investigate the spatial distribution of the number densities of neutral species in an asymmetric low-pressure 27.12 MHz discharge in pure nitrogen. The primary targets of this investigation were the three nitrogen triplet states C 3 u , B 3 g and the metastable A 3 + u each in their lowest vibrational levels v = 0, 1, 2 and additionally v = 8 of A 3 + u which can be populated very efficiently in the plasma sheath. The absolute number densities of all these states were determined in a Rayleigh scattering experiment, the results of which have been published elsewhere (Krames B, Glenewinkel-Meyer T and Meichsner J 2001 J. Appl. Phys. 89 3115). The spatial profiles as a function of the discharge parameters were studied by varying the gas pressure between 10 and 100 Pa and the effective rf voltage up to 260 V. These profiles of the different molecular states vary markedly in the plasma bulk as well as in the sheath near the powered electrode. A model, which includes the production, diffusion and quenching rates of contributing species and also fluorescence cascade processes, explains these varieties. A comparison with the experimental data at 50 Pa confirms the results of the model calculations.
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