Modelling of microwave plasma sources: potential and applications

Low-temperature, non-equilibrium plasmas form the basis of a growing variety of plasma-related processes. The demand for high-density plasmas over a wide pressure range has stimulated the development and use of microwave plasma sources in the last few years. Depending on the specific application, quite different and specialized sources have emerged. Other than empirical trial and error methods, computer simulations drastically reduce the time and effort needed to optimize the power coupling and distribution into a given gas. The computation of electromagnetic fields in plasma sources, including the plasma as a lossy dielectric, is a practical (though not self-consistent) approach yielding valuable insight on a short time-scale. Finite integral methods (FIMs) have proven to be powerful tools because they may be interpreted as a discrete analogue representation of Maxwell's equations in the computational grid. We have already developed and optimized a whole family of slot antenna microwave plasma sources (SLANs) based on such an approach. Our work included three-dimensional numerical simulations of the coupling structures and impedance matching in the time and frequency domains. For the smallest source mu SLAN geometry-dependent resonances were also identified, suppressed or enhanced to improve plasma ignition and power coupling. In that case the driving force was to use these sources more efficiently for time-modulated power flow, which is becoming very attractive for advanced plasma-based materials processing. The insight gained from our smallest source type mu SLAN has also been successfully applied to model larger plasma sources with diameters of up to more than 0.5 m and predict their performance at realistic working parameters before construction. Examples of these approaches as well as performance data will be given.