A noninvasive rf probe for the study of ionization and dissociation processes in technological plasmas

Abstract

A swept frequency absorbance plasma diagnostic technique for measurement of self-resonance frequency, intrinsic plasma-tool distributed capacitance, radiative energy loss, and effective plasma capacitance is described. The ex situ probe measures the plasma properties independently of all contributions from the plasma-tool and transmission line connection to the rf supply. The technique employs a swept frequency source and a balanced equal ratio arm bridge to measure the frequency response of the plasma tool after the plasma has been extinguished under plasma conjugate matching conditions. The resonant frequency of the combination of capacitances due to plasma-tool geometry (intrinsic capacitance, Ci) and the matching network (Cm) exhibits a shift from the excitation frequency (13.56 MHz) that is dependent on the effective plasma capacitance. Resonance frequency shift data are given for He, Ne, Ar, O2, N2, and N2O as a function of both pressure (0.02–0.8 mbar) and incident power (50 and 100 W). This technique allows the differentiation between dissociation and ionization processes within the plasma through a simple noninvasive rf measurement.