Electric Field and Impedance Measurements of Glow Discharges Used for Plasma Etching
Abstract
Non-equilibrium plasmas such as those generated in DC and RF glow discharges are extremely complicated but widely used in industrial applications such as sputtering, plasma etching and deposition. Experimental measurements have been a leading factor in promoting understanding of the physical phenomena in such discharges. A principal aim of this work is to develop relatively simple diagnostic techniques to measure key glow discharge parameters such as electric field, impedance, power dissipation, and electron concentration, that can be applied in an industrial setting. Electric fields in the cathode fall of DC glow discharges in He and mixtures of He/CF_4 have been measured using optical emission of Stark -enhanced forbidden transitions. An experimental technique was developed to derive the local electric field from the forbidden to allowed transition intensity ratio. A Stark mixing model was formulated and calculations made to relate the forbidden-to-allowed intensity ratio to the local electric field. Comparison of experimental measurements with theoretical calculations show good agreement. Discharge impedance and true RF power dissipation have been measured on a commercial plasma etcher as functions of gas pressure and input RF power for both electronegative (SF_6) and electropositive (Ar) gases as well as mixtures of the two. Electronegative and electropositive discharges are found to have quite different impedance magnitudes and phases, which is explained by the effect of ion inertia in electronegative discharges. Using a modified circuit model, the charged particle concentrations are derived from the measured impedance. By mapping the measured impedance into the plane of the settings of the two tuning capacitors in the matching network, real-time monitoring of the discharge impedance is possible. The stray impedance between the powered electrode and the matching network was characterized and found to be mainly capacitive. It was found that the RF power efficiency (i.e. the fraction of the generator power actually dissipated in the discharge) can be as low as 30% depending on the nature of the discharge. Therefore, the normal practice of monitoring only the RF power from the power supply can be misleading. As a remedy, a simple method for calculating the power loss to the matching network between the RF generator and the powered electrode has been developed. Good agreement was achieved with the embedded network calculations of the total RF power loss.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT.......233S
- Keywords:
-
- DIAGNOSTICS;
- Engineering: Electronics and Electrical; Engineering: Mechanical; Physics: Fluid and Plasma