A one-dimensional fluid model of a pulsed (square-wave power modulated) chlorine discharge was developed in order to study the spatiotemporal evolution of species densities and electron temperatures for various pressures, powers, pulsing frequencies and duty ratios. Simulation results show spontaneous separation of the plasma into an ion-ion core and an electron-ion edge during the power 'on' (active glow) fraction of the cycle. A transition from an electron-dominated plasma to an ion-ion plasma occurs during the power 'off' (afterglow) fraction of the cycle, under the conditions examined. The formation of an ion-ion plasma is favoured at lower power levels, higher pressures, and lower duty ratios. A minimum afterglow time is required for an ion-ion plasma to form and the negative ions to reach the walls. Increasing the afterglow period increases the fraction of time an ion-ion plasma is sustained in the reactor. The evolution of negative ion density profiles is especially complex due to the formation of self-sharpening fronts during power 'on' and subsequent back-propagation of the fronts during the power 'off' stage of the pulse. When possible, simulation results are compared to reported experimental data. In general, good agreement is obtained, except that the measured dependence of electron density on pulse period and duty ratio is more complex than predicted.
A three-dimensional finite element fluid model and a corresponding simulation tool have been developed for studying azimuthal asymmetries and their effect on etch uniformity in inductively coupled plasma ͑ICP͒ reactors. For silicon etching with chlorine in an ICP reactor with a planar coil, four different cases were examined: ͑a͒ uniform power deposition without a focus ring, ͑b͒ uniform power deposition with a focus ring, ͑c͒ nonuniform power deposition without a focus ring, and ͑d͒ nonuniform power deposition with a focus ring. When etching is ion driven, the power deposition profile is most important for etch uniformity, because azimuthal nonuniformities in the ion production rate can persist even down to the wafer level. For uniform power deposition, the effect of asymmetric pumping becomes more important. A focus ring can play an important role in alleviating azimuthal nonuniformities, especially in the nonuniform power deposition cases. Gas inlets pointing parallel to the wafer plane introduce only local disturbances in the species profiles.
Relatively electron-free positive- and negative-ion plasmas (ion–ion plasmas) have been achieved in the afterglow of pulsed-power Cl2 discharges. The application of a low-frequency (20 kHz) bias voltage phase locked to the source power modulation and synchronous with the ion–ion plasma, resulted in alternating fluxes of positive (Cl2+) and negative (Cl−) ions on a substrate. These results qualitatively agree with a one-dimensional fluid model. This technique to produce alternate irradiations could be used to reduce differential charging-induced damage in high-aspect-ratio etching processes.
Negative ion density fronts have been shown to occur in electronegative steady-state plasmas with hot electrons. In this Letter, we report theoretical and numerical results on the spatiotemporal evolution of negative ion density fronts during plasma ignition and extinction (afterglow). During plasma ignition, the negative ion fronts are analogous to hydrodynamic shocks. This is not the case during plasma extinction where, although negative ions diffuse freely in the plasma core, the negative ion front propagates towards the chamber walls with a nearly constant velocity.
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