Material influence on GHz split-ring resonator plasma ignition performance

Abstract

Split-ring resonators have been popularized by their application in metamaterials, but their ability to concentrate electric fields has also made them useful as microwave plasma generators. Despite the existence of much work on plasma generation using ring resonators, a comparative study of the effect of different materials on plasma generation performance has been absent. This work focuses on the study of material effects on ring resonators' microwave properties and plasma generation performance at pressures ranging from 4 to 100 Torr. To achieve this end, screen-printed silver and gold ring resonators are studied due to their high conductivity, relatively low reactivity, and differences in conductivity and work function. The surface morphology and chemistry of the ring resonators are studied using optical profilometry, scanning electron microscopy, and X-ray photoelectron spectroscopy. It is found that the main factor influencing performance between these two materials is Q-factor, which is determined using both conventional bandwidth measurements and measurements of conductivity. Q-factor is further isolated by modifying a silver ring resonator such that its Q-factor matches gold ring resonators. In addition, a film formed on the silver resonators after plasma exposure provides an opportunity to study a material, which, unlike gold, is quite different from silver. With the film present, plasma generation performance is decreased with increasing severity as pressure is decreased—20% more power is required for breakdown at 4 Torr. This change is qualitatively consistent with a model of microwave plasma breakdown where boundary effects are expected to increase as pressure is decreased.