Experimental characteristics of rf parallel-plate discharges: Influence of attaching gases

Abstract

Electrical characteristics of parallel-plate rf discharges were measured with the goal of obtaining quantitative information of the properties of the discharges, in particular the electrode sheaths. Measurements of the impedance characteristics at variable electrode spacing suggest a transition of the power deposition process from volume to electrode dominated at about 1 Torr in argon. By changing the secondary electrode emission coefficient of the electrodes, it could be shown that the power into electrons emitted from the electrodes is small compared to the total power input. Adding the attachers CF4, C2F6, and SF6 caused a large increase of the discharge impedance in the high-pressure (volume power deposition) regime and shifted the transition to the electrode-dominated power deposition regime to lower pressures. The impedance behavior versus pressure was modeled with a simple equivalent electrical circuit from which the characteristics of the electrode sheaths and their influence on the electrical characteristics could be derived. The time-averaged sheath thickness was measured optically and compared with the results using the equivalent circuit. The sheath thickness, decreasing on addition of the attaching gases CF4 and SF6, is quite different for these two added gases, which have about the same ionization, but very different attachment cross sections. In contrast, the impedance with added CF4 is only slightly less than with SF6, indicating an average electron energy of at least 5 eV where the attachment rates of the two gases become similar. Using variable electrode spacing, the peak electric field was measured both in the electrode sheaths and in the discharge volume. For argon with 23% added SF6, at 1 Torr pressure, the field in the sheath reached 569 and 27 V/cm in the volume; at 0.15 Torr the fields were 300 V/cm in the sheath and below the measurement accuracy in the volume.