ELECTROMAGNETIC-FIELDS IN A RADIOFREQUENCY INDUCTION PLASMA

The electromagnetic fields which drive a radio-frequency induction plasma are both modeled and measured. The plasma source consists of a planar, square coil separated from a low pressure plasma chamber by a 2.54-cm-thick quartz window. A small loop antenna, which is sealed in a pyrex tube, is immersed in the discharge to determine the magnitude and direction of the rf magnetic field. The measured B field is primarily radial and axial. Typical rf field strengths vary from 2 to 7 G for rf powers of 0.1-1 kW. The radial B field decays exponentially in the axial direction. The skin depth of the electromagnetic field is 1.6-3.6 cm which is consistent with Langmuir probe measured ion densities (typically 3 X 10(11) cm-3) in argon. Invoking Maxwell's equations to deduce the rf electric field from the measured B field, we find the E field to be primarily azimuthal. Peak field strengths increase from 100 V/m at 100 W to 200 V/m at 600 W where they saturate for higher powers. Finally, we present a 3D finite element solution for the fields produced by this plasma source which employs a cold, collisionless plasma model to relate the relative plasma permittivity epsilon(r) to the electron plasma frequency, omega(pe), using epsilon(r) = 1 - (omega(pe)/omega)2. The measured fields support this numerical solution.