Plasma dynamics of microwave excited microplasmas in a sub-millimeter cavity

Abstract

Summary form only given. Capacitively coupled microplasmas in dielectric cavities have a range of applications from VUV lighting sources for surface treatment to radical production. Due to the large surface-to-volume ratio of these devices, the wall mediated dynamics of plasma transport are important to the uniformity and confinement of the plasma. For example, there may be applications where a plume of ionized gas is desired from the microcavity - whereas other applications may require a confined plasma emitting only VUV photons.In this paper, we will discuss results from a computational investigation of the plasma dynamics in microwave excited micro plasma VUV lighting sources. A 2dimensional hydrodynamics model, the Hybrid Plasma Equipment Model, has been used in which radiation and electron energy transport are addressed using Monte Carlo techniques. The microdischarges have widths of:: 1 mm and lengths of :: 1 cm, operate at pressures of 1-20 Torr, with microwave power of 2-10s Watt at 2.5 GHz and a flow rate of several sccm. Gases are either pure rare gases or mixtures of rare gases. We found that the plasma operates in a mode that has both normal-glow and abnormal glow characteristics. Under usual operation in argon, plasmas are produced with a peak electron density of 1013 cm-3. The plasma may not fill the microdischarge cavity at low power. As the power is increased, the plasma expands to fill the cavity. In this regard, the plasma operates as a normal glow. The current density, however, increases with increasing power, and so in this regard, the plasma resembles an abnormal glow. The expansion of the plasma will eventually overfill the cavity, at which time a plasma plume is formed. These plasma dynamics are sensitive to gas mixture. The scaling of plasma confinement and VUV production as a function of aspect ratio, power and gas mixture will be discussed.