An experimental and theoretical study of the positive column of the CF4 DC glow discharge is presented. It includes the experimental determination of the reduced electric field Ez /N and the electron impact excitation rate constant KAr of the line ( lambda =750.3 nm, epsilon th=13.5 eV) of the actinometric addition (Ar) in CF4 plasma for a wide range of parameters (pressure p=0.2-2.3 Torr, current density j=4-35 mA cm-2). All experimental values of the reduced electric field can be described within the limits of experimental error by the universal dependence Ez/N on r0N (r0=0.9 cm, the discharge tube radius). Also the experimental dependence of kAr on Ez/N has been obtained. These results have been used to test different theoretical models of the CF4 discharge positive column. Comparison of the experimental and calculated values of Ez /N and KAr has shown the validity of the theoretical model developed in this work. This model takes into account the large negative ion concentration in the positive column and predicts its division into regions of ion-ion plasma (in the centre) and electro-ion plasma (near the tube wall). The model also accounts for the effect of non-locality of the electron energy distribution function. It has been shown that this effect can be accounted for by solving the Boltzmann equation for the EEDF in the spatially non-uniform case. The incorrectness of the notion of ambipolar diffusion in the plasma of a strongly electronegative gas with high negative ion concentrations has been reaffirmed.

The influence of the phase shift between rf voltages applied to the powered electrodes on plasma parameters and etch characteristics was studied in a very high-frequency ͑VHF͒ capacitively coupled plasma ͑CCP͒ triode reactor. rf voltages at 100 MHz were simultaneously applied to the top and bottom electrodes having a controlled phase shift between them, which could be varied between 0°and 360°. Several plasma and process characteristics were measured as a function of the phase shift: ͑i͒ radial profiles of plasma-emission intensity, ͑ii͒ line-of-sight averaged plasma-emission intensity, and ͑iii͒ radial profiles of blanket SiO 2 etching rate over a 300 mm wafer. Radial profiles of plasma emission were obtained using the scanning optical probe. It has been shown that all the measured characteristics strongly depend on the phase shift: ͑i͒ plasma-emission intensity is minimal at phase shift equal to 0°and maximal at 180°for all radial positions, while the emission radial profile changes from bell-shaped distribution with considerable nonuniformity at 0°to a much more flattened distribution at 180°; ͑ii͒ line-of-sight averaged plasma-emission intensity shows a similar dependence on the phase shift with minimum and maximum at 0°and 180°, respectively; and ͑iii͒ the etch-rate radial profile at 180°shows a much better uniformity as compared to that at 0°. Some of these results can be qualitatively explained by the redistribution of plasma currents that flow between the electrodes and also from the electrodes to the grounded wall with the phase shift. We suggest that the phase-shift effect can be used to improve the plasma and etch-rate spatial uniformity in VHF-CCP triode reactors.

Plasma spatial non-uniformities were studied experimentally in a very high frequency (100 MHz) capacitive triode-type reactor used for etching of 300 mm wafers. It has been shown that in the traditional mode of operation there is considerable plasma non-uniformity due to the electromagnetic effects, namely at a low power, the plasma density profile is determined by the standing-wave effect, while at a high power the skin effect dominates. The influence of phase-shift control on plasma uniformity was examined. Phase-shift control means applying to the top and the bottom electrodes very high frequency voltages with a controlled phase shift between them. The experiments were carried out in process (C4F8/O2/Ar) plasma in a wide range of pressures and powers. It has been shown that the phase-shift control can considerably improve the plasma uniformity under a wide range of experimental conditions.

Plasma spatial nonuniformities in the 100MHz rf driven capacitively coupled reactor used for reactive ion etching of 300mm substrates were experimentally studied using a linear scanning optical emission spectroscopy probe. Radial profiles of plasma emission intensity were measured both in argon and fluorocarbon-containing gas mixtures in the pressure interval of 10–80mTorr and the rf power range of 500–1250W. It was demonstrated that the plasma emission profiles strongly depend on the working gas composition and pressure. The profiles have a bell-like shape at pressures about 10mTorr for all gases. As the pressure increases, the profile shape becomes more complex with the central and peripheral peaks, and the amplitudes of the peaks strongly depend on the working gas composition. It is suggested that the emission profiles show plasma spatial nonuniformities that can influence the etching rate profiles obtained with such systems. According to the existing theoretical models, the most probable reasons for these plasma nonuniformities are charged particle radial diffusion at low pressures (about 10mTorr), as well as the standing wave and skin and edge effects at higher pressures. Using the experimental emission profiles, the working conditions have been found that allow one to achieve the most uniform plasma for discharges in argon and fluorocarbon-containing gas mixtures.

Remote plasma sources (RPSs) are being developed for low damage materials processing during semiconductor fabrication. Plasmas sustained in NF3 are often used as a source of F atoms. NF3 containing gas mixtures such as NF3/O2 and NF3/H2 provide additional opportunities to produce and control desirable reactive species such as F and NO. In this paper, results from computational investigations of RPS sustained in capacitively coupled plasmas are discussed using zero-dimensional global and two-dimensional reactor scale models. A comprehensive reaction mechanism for plasmas sustained in Ar/NF3/O2 was developed using electron impact cross sections for NF2 and NF calculated by ab initio molecular R-matrix methods. For validation of the reaction mechanism, results from the simulations were compared with optical emission spectroscopy measurements of radical densities. Dissociative attachment and dissociative excitation of NFx are the major sources of F radicals. The exothermicity from these Franck–Condon dissociative processes is the dominant gas heating mechanism, producing gas temperatures in excess of 1500 K. The large fractional dissociation of the feedstock gases enables a larger variety of end-products. Reactions between NFx and O atom containing species lead to the formation of NO and N2O through endothermic reactions facilitated by the gas heating, followed by the formation of NO2 and FNO from exothermic reactions. The downstream composition in the flowing afterglow is an ion–ion plasma maintained by, in oxygen containing mixtures, [F−] ≈ [NO+] since NO has the lowest ionization potential and F has the highest electron affinity among the major neutral species.