Berlin 2001 – scientific programme

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P: Plasmaphysik

P 12: Plasma Wall Interaction, Diagnostic and Low Temperature Discharges (Poster Session)

P 12.22: Poster

Friday, April 6, 2001, 12:30–15:00, AT2

Measurements of molecular radical densities in a CF₄ — •U. Czarnetzki, P. Fendel, A. Francis, and H.F. Döbele — Institut f. Laser- und Plasmaphysik, Universität Essen, 45117 Essen, Germany

An asymmetric capacitively coupled RF-discharge is operated in CF₄ at pressures between 20 Pa and 100 Pa and RF-power of the order of several 10 Watts. The stainless steel electrodes have a diameter of 8 cm with a 2.5 cm separation. The discharge can be operated either cw or pulsed with a switch-off time of about 10 µs. CF and CF₂ radical densities are measured by laser induced fluorescence spectroscopy. Excitation and observation wavelengths are at λ=233 nm/ λ=255 nm (CF) and λ=262 nm/ λ=271 nm (CF₂), respectively. The spatial and temporal (after switching off the RF-power) profiles of the radical densities are investigated. From these data the sticking coefficients at the electrode surfaces and the effective source strength distributions in the plasma volume are inferred. Under our experimental conditions the effective sources for both CF and CF₂ are within the plasma volume while the electrodes are sinks even when the discharge is powered. The effective CF sources are located mainly within the sheath close to the powered electrode while the effective CF₂ sources are closer to the grounded electrode. Decay time measurements as a function of pressure give an indication for an additional volume loss process for CF, while the CF₂ densities decay by diffusion to the walls only.