PhD Thesis Defence Presentations - Ioannis Tsigaras

Low pressure Capacitively Coupled Plasma discharges have been widely used since the 1970s due to their application in semiconductors and thin film solar cells. Due to their complex nature, plasma processes should be monitored for the optimization of power transfer from the generator to the plasma and the achievement of stable, reliable and reproducible discharge conditions. As a result, several in situ non – intrusive plasma diagnostic techniques should be implemented with the use of mathematical models for the estimation of the characteristics of interest. The determination of plasma microscopic characteristics can provide useful knowledge about the mechanisms by which RF power is dissipated and can lead to the optimization of the process.

The aim of the present thesis is the implementation of different electrical and optical diagnostic techniques for the calculation of the microscopic characteristics of the discharge. In that direction, three different experimental techniques were developed and applied for the characterization of the reactors parasitic impedance and the subsequent calculation of the electrical characteristics of an argon discharge. An investigation was also performed about the error propagation through the electrical measurements in classical plasma conditions and the factors affecting the power coupling efficiency of capacitively coupled discharges. Finally, electrical measurements together with Optical Emission Spectroscopy were implemented for the calculation of the electron temperature of argon discharges.

The experimental results indicate that both the external RF circuit and the discharge conditions play a crucial role as they affect the accuracy of the implemented methods and the power coupling to the discharge. For lower chamber pressures in which the discharge impedance is high the power coupling efficiency to the discharge is low and the errors in the calculation of the dissipated power, which depend on the measured VSWR, are high. The accuracy of the electrical calculations and the power coupling efficiency is increased in higher pressures due to a decrease of the discharge impedance. Finally, when only a qualitative calculation of the electron temperature is needed mainly for monitoring of the process, the line ratio method, implemented in this thesis can be used efficiently.