Unbounded and bounded microwave plasmas generated by a monopole antenna structure

Abstract

Summary form only given. Methods of creating and controlling miniature microwave discharges that operate with low input power levels are being investigated. Various experimental structures to create these small sized and low input power microwave discharges are currently being constructed and measured. One variation creates the plasma discharge by the near fields of a short monopole antenna or electrode structure such that the discharge is attached or nearly attached to the antenna or electrode structure. The objective of this investigation is to explore the behavior of these discharges when they are operated at low microwave powers and with small physical dimensions. The experimental configuration utilized in this investigation is a single, microwave powered, monopole antenna or electrode extending into a discharge chamber. The microwave power supplied to the antenna structure is 2.45 GHz with a double stub tuner used for impedance matching to reduce the reflected power. The microwave discharge for a range of pressures and powers forms at the tip of the antenna with the size and properties of the discharge being dependent on the pressure and power. The discharge formed can be either unbounded where the plasma discharge surrounds the electrode tip, but the discharge is not large enough to touch the walls of the discharge chamber, or bounded where a discharge chamber enclosure confines the discharge. For the bounded discharges, three variations are considered including a spherical confinement, a cylindrical tube confinement and a flat disk shape confinement. The spherical confinement produces a plasma bounded in three directions, the cylindrical confinement produces a plasma bounded in two directions that is free to expand along the third direction, and the disk shaped confinement produces a plasma bounded in one direction and free to expand along the other two directions. The discharges are characterized using diagnostics to determine the discharge shape, size and power density via visual imaging and microwave power measurements, and the discharge charged particle densities and electron temperature via Langmuir probe measurements. Parameters varied in this study include the size and shape of the microwave antenna electrode, the discharge pressure (50 mTorr to 50 Torr) in the chamber, the input microwave power (less than 1 Watt to 100 Watt) and the type of feed gas (Argon and Nitrogen). Discharge sizes studied range from several cubic centimeters down to just a few cubic millimeters.