Experimental Characterization of Instabilities in Inductively Coupled Argon Plasma

Abstract

Summary form only given. The phenomenon of instabilities in a low pressure inductively coupled Ar plasma has been characterized by using various diagnostic tools, including a home-made RF impedance meter, a Langmuir probe, and optical emission spectroscopy. The impedance meter was designed to extract fast variations in amplitudes of RF voltage, current and phase. It consisted of voltage and current sensors, along with signal processing circuits. The voltage and current sensors were a capacitive voltage divider and a magnetic flux pick-up coil, respectively. Bridge rectifiers were used to determine the voltage and current amplitudes while the relative phase signal was extracted by using a double balance mixer. The meter was calibrated against a commercial impedance meter under wide operation conditions to ensure the measurement accuracy. It also has been tested under pulsed discharge condition with modulation frequency up to 20 kHz to check the time response of signal processing circuits. A multilayer feed-forward neural network was developed for the phase angle estimation against the values obtained by a commercial impedance meter and accuracy around plusmn0.1deg was obtained. Measurement results show that, at 5 mtorr Ar, instabilities were observed only at certain special matchbox settings (L-type, consists of a shunt capacitor CL and a series capacitor CT) and resulting in two kinds of plasma density oscillations. The first kind has a lower oscillation frequency at about 150 Hz which is independent of CT positions and forward power setting of the RF generator. The other is a higher frequency oscillation and the oscillation frequency decreases from 15 kHz to 10 kHz with increasing of CT position from the best matching position (no reflect power). There is a time delay between plasma density variation and the absorbed power obtained by the impedance meter. In both cases, the oscillation amplitude increases with CT position and forward RF power. In contrast to the source and downstream instabilities in electronegative ICP discharges, the origin of these oscillations in the electropositive Ar plasmas are believed to be a result of mode switching between low density capacitive and high density inductive couplings, which is induced by mismatch between the RF generator and the plasma load whose impedance is determined by plasma density. Comparison of probe and impedance meter measurements to OES measurements and the effects of different Ar flow rate and pressure will also be presented