Modelling and Simulation of the Advanced Plasma Source

Plasma ion assisted-deposition (PIAD) is a combination of conventional thermal evaporation deposition and plasma-beam surface modification; it serves as a well-established technology for the creation of high quality coatings on mirrors, lenses, and other optical devices. It is closely related to ion-assisted deposition to the extent that electrons preserve quasineutrality of the ion beam. This paper investigates the Advanced Plasma Source (APS), a plasma beam source employed for PIAD. A field enhanced glow discharge generates a radially expanding plasma flow with an ion energy of about 80-120 eV. Charge exchange collisions with the neutral background gas (pressure 0.1 Pa and below) produce a cold secondary plasma, which expands as well. A model is developed which describes the primary ions by a simplified Boltzmann equation, the secondary ions by the equations of continuity and momentum balance, and the electrons by the condition of Boltzmann equilibrium. Additionally, quasineutrality is assumed. The model can be reduced to a single nonlinear differential equation for the velocity of the secondary ions, which has several removable singularities and one essential singularity, identified as the Bohm singularity. Solving the model yields macroscopic plasma features, such as fluxes, densities, and the electrical field. An add-on Monte-Carlo simulation is employed to calculate the ion energy distribution function at the substrate. All results compare well to experiments conducted at a commercial APS system.