Innovative plasma diagnostics and control of process in reactive low-temperature plasmas

Abstract

By means of two independent diagnostic methods, the plasma parameters in a RIE system (ALCATEL GIR 300) were measured. Density and collision rate of electrons and the power dissipated in the plasma body in He, Ar, O 2, CF 4, and SF 6 were determined. The first completely new diagnostic method is the self-excited electron plasma resonance spectroscopy (SEERS), which is based on the occurrence of harmonics in the discharge current due to the non-linearity of the sheath at the powered electrode. Owing to the inductive behaviour of the plasma body (bulk, ∈<0) and the capacitive nature of the rf sheath, a non-linear and damped series resonance can be observed and used for plasma diagnostics. Its eigenfrequency depends directly on the Langmuir frequency and the electron density, respectively. In order to perform the measurement, the plasma monitoring system HERCULES, containing a robust numerical algorithm, was used. The second method is a completely automatic Langmuir probe (LP). The compensation of the rf signal was achieved by means of a triaxial setup. The measurement time of one I–V pair takes only 19.5 μs. Retracting the probe tip pneumatically prevents undesirable coating. The I–V curve is smoothed by several FIR-filters to support the automatic calculation of the plasma parameters. Both methods have shown an agreement concerning the electron density and the possibility of advanced process control by innovative plasma diagnostics.