Inductively Coupled Plasma Sources at Low Driving Frequencies

Inductively Coupled Plasmas (ICPs) can be maintained over a wide range of driving frequencies, from 50 Hz to GHz. We have investigated the specifics of ICP operation at different frequencies which address: a) nonlinear plasma dynamics due to the disparity of time scales for ion transport and electron energy relaxation, b) the absence of time-varying magnetic field in plasma for certain ICP configurations, and c) the absence of a skin effect at low frequencies. Numerical simulations have been performed to demonstrate the spatial distributions of the electric and magnetic fields for different coil topologies when using ferromagnetic cores. We investigated three regimes of ICP operation with respect to angular frequency ω, the time scale for ion transport τa and the electron energy relaxation time τε. Since it is usually the case that τa >> τε , one can distinguish quasi-static (ωτa<1), dynamic (τε > ω >τa ), and high-frequency (ωτε >1) regimes. An example of an ICP having ferrite cores forming a closed magnetic path is shown in Fig. 1, The calculated electron temperature and plasma density at different ω in Argon gas at a pressure of 2 Torr and coil current of 0.1 A are also shown, from ω = 10 το 10. In the high-frequency regime, the electron density and the electron temperature, Te, are essentially constant over the period. A highly nonlinear behavior is observed in quasi-static regime. In the dynamic regime, the plasma density varies slightly over the field period, but the Te and the Electron Energy Distribution Function (EEDF) could change significantly over the field period. Detailed studies of the dynamic regime have not been performed so far, and our work [1] appears to be the first step in this direction. As an example of important kinetic effects, we have observed dynamic constriction of the ICP column at low frequencies due to Maxwellization of the EEDF by Coulomb collisions.