Characterization of an ECR etching plasma with a microwave interferometer, Langmuir probes and a FTIR

Abstract

Cavity perturbation techniques have been around since the birth of microwave engineering. Cavity perturbation refers to the shift in the resonant conditions of a microwave cavity caused by the presence of a load within the cavity. By precisely measuring these shifts, one may determine certain characteristics of the load. Cavity perturbation has become a diagnostic widely applied to plasma discharges, because the technique quickly provides measurements of the electron density and collision frequency of the discharge. However, in its most fundamental form, cavity perturbation is applicable only to a limited range of electron densities (<lo9 cm-3).Work completed by S.C. Brown in 19601 attempted to extend the range of applicable electron densities by modeling the electromagnetic fields present within the resonant cavity. However, the work failed to address the problems which arise when the electromagnetic fields become complex. The fields are real when the collision frequency is neglected but inclusion of a collision frequency (important for higher pressure discharges) results in complex fields. The paper presented here introduces a new perturbation analysis which accounts for the complex electromagnetic fields. This is done by modeling the discharge as a lossy dielectric, and allowing the microwave frequency to become complex. It is shown that the imaginary portion of the frequency represents the change in the cavity quality factor due to the discharge while the real portion is simply the change in the resonant frequency of the cavity. By measuring these two quantities, the electron density and collision frequency of the discharge may be obtained.The results of the diagnostic are applied to an Asmussen style microwave cavity operating in the TM012 mode. The electron densities determined from this technique are compared to those measured from double Langmuir probes. The densities ranged from 10" to 10l2 cm-3.