Mark Kushner1
University of Michigan1
The role of plasmas in achieving sustainability goals can be generally classified as providing one or more of higher energy efficiency, better chemical selectivity, use of renewable (non-fossil fuel) power, providing better logistical solutions (point of use) or providing a unique solution that is not otherwise available (microelectronics fabrication and PFAS removal from water). With the exception of chemical conversion (e.g., methane up-conversion), the majority of these plasma-focused solutions involve plasma-surface interactions, though chemical conversion through plasma catalysis is also dominated by plasma surface interactions. The approaches followed towards controlling the plasma-surface interactions at low- and high-pressure are fundamentally different. At the pressures used in microelectronics fabrication, it is in principle possible to decouple reactant generation from the delivery of those reactants, in the form of energetic ions, to surfaces to activate surface processes. This is the basis of multi-frequency capacitively coupled plasmas, and the use of biased substrates in inductively coupled plasmas. In spite of products of surface reactions returning to the plasma, the surface is generally passive with respect to reactant generation, secondary electron emission aside. At high (atmospheric) pressures, it is less clear whether such distinct separation between producing gas phase reactants and activating surface processes can be achieved. The shorter mean free paths and close coupling of adjacent plasmas to the surface (e.g., surface ionization waves) makes that separation difficult, in addition to the difficulty of generating, on demand, ions with high enough energies to significantly modify surface properties. In this talk, results from computational investigations will be used to discuss methods to separately control the delivery of reactants and activation energy to surfaces in atmospheric pressure plasmas. Examples will be drawn from plasma jets incident onto dielectrics and liquids, and surface ionization waves. As a baseline, the methods used for this purpose in semiconductor processing will be reviewed.<br/>Work was supported by the National Science Foundation, Department of Energy Office of Fusion Energy Sciences and the Army Research Office Multidisciplinary University Research Initiative.