Investigation of Micro-hollow Cathode Plasma Discharges

Abstract

Plasma technology is an essential instrument of many fields, from the semi-conductor industry to exciting new applications in areas such as bio-medicine, agriculture and more. As these new avenues are explored, plasma processing must be able to act on large sample areas to be financially competitive to traditional methods. This brings us to a core challenge of plasma technology: scalability. The maximum area that a plasma can treat is pre-determined by the physical length of the system. As opposed to making reactors larger, one solution that has been considered is arrays of numerous micro-plasmas. The scaling of these systems is much simpler and comes with the advantage of being able to operate over a wide range of pressures. Of particular interest is the micro-hollow cathode discharge due to simplicity of design. However, as of right now, understanding of these discharges is limited due to its complex temporal and spatial behaviour.

This thesis aims to shed some light on argon and nitrogen plasmas ignited in these micro-hollow cathode geometries. To observe spatial variations in plasma properties, a two-dimensional fluid model was developed in the finite element solver software COMSOL operating with argon and nitrogen chemistry. Results from argon simulations highlighted the importance of the cathode sheath region and demonstrated that excited plasma species could exist at quite extended distances away from the cathode hole itself. To examine argon plasma behaviour under different conditions, investigation of species properties with over a range of current and pressures was conducted. Preliminary results from the nitrogen fluid model were also presented, with particular focus on the nitrogen ion and excited neutral dynamics due to their relevance to plasma processing. To validate the findings of the model, plasma diagnostics on an experimental micro-hollow cathode system were also performed. Firstly, small amounts of hydrogen gas was leaked into the chamber to perform Stark broadening measurements to obtain electron density in argon. These values compared well with the model and the literature in different pressure and current regimes. Next, gas temperatures were estimated using the second positive band in nitrogen and found to be relatively (Tg < 400 K) cool and constant despite different conditions. This justified the use of a constant gas temperature in simulation. Finally, a collisional radiative model for argon was developed and the line-ratio method was used to determine values for electron density and temperature. The electron density obtained from this diagnostic technique showed significant differences to values found in both the Stark method and simulation but compared well to electron temperatures. Differences here were linked to difficulty determining the plasma length of the micro-hollow cathode. Overall, the model was found to be in reasonable agreement with experimental methods. Show more