Heuristic modeling of the doping efficiency in sputtered TCO layers

• The DSMC method is extended by a reactive wall model for modeling reactive processes.
• The model is applied on reactive sputtering of ZnO:Al.
• A qualitative comparison with experiments is carried out.
• The reactive wall formalism enables to join atomistic film growth models with reactor scale simulation.

The increased demand on throughput and precision of PVD sputter processes promote the development of modeling tools describing relevant process mechanisms in detail and allowing for finding efficient optimization strategies. For the rarefied gas flow conditions typical in PVD processes, particle based simulation methods such as the Direct Simulation Monte Carlo (DSMC) method for transport of neutral particles and Particle-in-Cell Monte Carlo (PIC-MC) method for plasma discharges can be used.While these methods allow for determining the deposition profile, film stoichiometry as well as to get a first idea on the ion bombardment distribution, they do not draw further conclusions about the resulting film quality. In order to bridge the gap between transport and film growth simulation we present a heuristic model of the dopant incorporation during growth of doped TCO layers such as ZnO:Al. For such materials, a good doping efficiency can usually be obtained only within a certain parameter range of the reactive sputtering process.In the DSMC/PIC-MC simulation tool, we address this issue by introducing an extended wall reaction model. This model involves several coating material phases with dynamic surface fractions assigned to the mesh elements of the reactor geometry. We present a set of concurrent surface reaction mechanisms which reproduce the overall behavior of the reactive ZnO:Al deposition process and its doping efficiency in good qualitative agreement with experimental data.