RF models for plasma-surface interactions
The behavior of plasmas near metallic and dielectric surfaces in the presence of oscillating fields is critical for both the plasma processing industry and fusion reactors. In magnetic fusion research, sheath formation near the radiofrequency (RF) antenna structures used to heat the core plasma causes metallic ions emitted from the surfaces to contaminate the core plasma, reducing fusion power output and possibly causing ignition failure of the reactor. In semiconductor manufacturing processes that utilize plasma etching, precise knowledge and control of the plasma steady-state parameters is necessary to obtain a proper etch; these equilibrium parameters are sensitive to variation in the width, electric potential gap, and other parameters associated with the faster physics of the sheath. The complex, nonlinear physics of sheath formation includes the rapid electron timescale (10-11 sec), but the practical consequences of this fast motion on the bulk plasma properties (density, impurity content, etc.) are on much longer (10-6 sec) timescales. Modeling which resolves the electron timescale is impractical, yet the need to calculate the nonlinear effects imparted by the sheath remains. To enable the numerical optimization of fusion reactor antennas and plasma processing reactors, the development of rapid, accurate simulation methods which remove the need to model this timescale is needed. The timescale issue is addressed through the use of sub-grid models to average over the faster timescales and smaller length scales. We will implement these models into the Vorpal Particle-In- Cell code to improve the physics modeling of plasma-wall interactions in an oscillating field. Commercial applications and other benefits: The subgrid sheath models that we will add to Vorpal enable its use as a design tool for plasma- etch-based semiconductor manufacturing equipment. In such scenarios, plasmas are formed in the region between an anode and cathode (both of which may be coated with dielectric material) and evolve to steady-state conditions. Adjustment of anode/cathode properties and materials enables optimization of the steady-state plasma parameters to accomplish the desired etching process. The proposed reduced-order sheath models in Vorpal will allow industrial plasma processing customers to more rapidly model the longer time scale equilibrium physics of the etching plasma while retaining the essential properties of the sheath physics, and will enable efficient and rapid design and optimization of equipment for the desired manufacturing processes. By studying the optimization of the ITER antennas, we will also be able to greatly improve the efficiency, and perhaps even enable the success, of a $10B experiment.
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