Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF2 radicals in capacitively coupled 13.56 MHz plasmas in pure CF4 at 50 and 200 mTorr. Under these conditions (pure CF4, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces. Fluorine-poor conditions will be considered in a following paper. Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink. The summed fluxes of CF and CF2 produced at this surface were found to be comparable to the incident ion flux. We propose therefore that CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ ions under these conditions. This mechanism often predominates over the gas-phase production of these species by direct dissociation of CF4, and accounts for the unexpectedly high concentrations observed. The difference in behavior between the powered and grounded electrode surfaces is explained by the difference in the incident ion energy and mass distributions.

A novel electrostatic probe method is described in which the ion flux is determined from the discharging of an RF-biased capacitance in series with the probe. By using a large-area planar probe, with a guard ring and located in or on other surfaces, edge effects and perturbations to the plasma volume can be kept small. The ion flux to the probe can be determined even when its surface is coated with insulating material from the plasma itself. Results are reported for ion fluxes in RF-excited plasmas in Ar and in CF 4 in a RIE reactor. In Ar, ion fluxes to the earthed surfaces increase with pressure and power over the ranges 50-200 mTorr and 30-200 W. In CF 4 , over the same ranges the ion fluxes to the surfaces decrease with increasing pressure.

The measurement of radical densities in atmospheric-pressure plasmas has gained increasing importance in recent years in view of their crucial role in many applications. In this paper we present absolute OH density measurements by broadband UV absorption in diffuse atmospheric-pressure RF glow discharges in mixtures of He and H 2 O. The use of a 310 nm light-emitting diode as a light source and a very high resolution spectrometer (2.6 pm resolution) made the estimation of the total OH density possible by simultaneously measuring the absorption rates of different spectrally resolved rotational lines of the OH(A-X) transition. For different RF powers and water concentrations, OH densities and gas temperatures ranging between 6 × 10 19 and 4 × 10 20 m −3 and 345 and 410 K, respectively, were obtained. The gas temperature T g was also measured by three different methods. T g deduced from the rotational temperature of N 2 (C-B) emission, nitrogen being present as a trace impurity, provided the most reliable value. The rotational temperature T r of the ground state OH(X) presented values with a maximum deviation of 25 K compared with T g . To obtain the gas temperature from the emission intensities of OH(A-X) rotational lines, the recorded intensities of different lines must be corrected for the effect of self-absorption inside the plasma.

Articles you may be interested inStudy of Ti etching and selectivity mechanism in fluorocarbon plasmas for dielectric etchThe study of CF and CF 2 radical production and loss mechanisms in capacitively-coupled 13.56 MHz CF 4 plasmas has been extended to CF 4 plasmas with an Si substrate, and to C 2 F 6 plasmas, conditions where the atomic fluorine concentration is lower and where more polymer deposition occurs on the reactor surfaces. Processes in the gas phase and at the reactor surfaces were investigated by time resolved axial concentration profiles obtained by laser induced fluorescence, combined with absolute calibration techniques. The results for CF were similar to those observed in the fluorine rich case, whereas the results for CF 2 were strikingly different and more complex. This paper focuses on the CF 2 radical, which, under these conditions is produced at all of the surfaces of the reactor, apparently via a long-lived surface precursor. The results can only be explained if large polymeric ions and/or neutrals are produced by polymerization in the gas phase. The gas-phase CF 2 concentration is high, causing the otherwise slow gas-phase concatenation reactions C X F Y (CF 2 ) n ϩCF 2 →C X F Y (CF 2 ) nϩ1 to occur. These processes produce high-mass neutrals ͑and ions͒ which are the real polymer precursors. The CF 2 radical therefore circulates in a closed cycle between the surface and the gas phase. The degree of polymerization is controlled by the fluorine atom concentration, which simultaneously controls the concentrations of CF 2 , of chain initiating species such as CF 3 and of dangling bonds on the growing oligomers. This model appears to apply to fluorocarbon discharges in general, and agrees well with other results presented in the literature.