For optical applications it is necessary to generate surfaces that are as pure as possible. Even the smallest degree contamination can reduce or even completely destroy the effectiveness of the entire layer. Dust is a particular challenge for optical applications because the technical effort necessary for its total avoidance in coating systems increases markedly as the desired degree of purity increases. Via a particle simulation developed at the Fraunhofer IST it is now possible to predict the contamination of dust in plasma coating systems. Thus, an important contribution is made towards minimizing the number of defects in the course of subsequent optimization of coating processes.
The solution approach
Dust cannot be completely avoided, consequently one strategy to minimize the number of defects is to tolerate the dust, but keep it away from the surfaces. To do this a good understanding of the behavior of dust in the plasma of a coating system is necessary. However, this is made more difficult through a large number of phenomena resulting from the interaction between plasma and dust. To understand and predict all influences in sufficiently good approximation it is useful to execute simulations in this regard. For this purpose a special program has been developed at the Fraunhofer IST for the simulation of dust particles.
The dust particles in the simulation are modelled in a simplified form as homogeneous spheres, which can carry either a positive or a negative charge. They are located in a virtual system, whose geometry is comprised of triangular surfaces, and the particles are generated at random points on the geometry.
The characteristic parameters of the plasma, such as density, temperature, and flow velocity are obtained from separate plasma simulations, likewise developed at the Fraunhofer IST. From these parameters ultimately the surface currents and forces on the dust particles can be calculated, from which a dynamic change in the charge, as well as the velocity and thus the location of the particles results. This is executed at regular time intervals, through which the precision of the simulation can be controlled. Relevant effects on dust particles are:
- The direct drag through neutral gas and plasma
- The repulsion and attraction of the electrostatic potentials between charged dust and ions
- The tendency of the particles to follow heat flows
Macroscopic forces, such as gravitation and electromagnetism are likewise taken into account in the simulation, as well as the interaction of dust with surfaces through reflection or absorption of the particles.
Utilization of the results
If the simulation is run with a variety of start points for different particle types, then locations can be identified at which statistically a particularly high number of dust particles gathers, or from which a high number of particles emanate. Individual strategies for effective avoidance of contamination can be derived from this simulation, these strategies in turn can be optimized through additional simulations.
By using the program for dust particle simulation it could be possible to generate optical coatings that are significantly more precise and pure. The precision can even be further increased by supplementing additional forces and influences, so that the processes that are responsible for the dust load can be better investigated. An extension of the concept to other application areas with plasmas is likewise conceivable.