Simulation assisted deposition of optical filters onto 3D substrates by magnetron-sputtering
The direct deposition of optical filters onto curved 3D substrates such as convex lenses is beneficial for fabricating optical devices with a minimized number of components and internal reflections. However, in order to account for spectral shifts resulting from a variation in the angle of incidence across the surface, the film thickness profile of the filter has to be carefully adapted to the surface curvature. For e. g. convex lenses, this would imply an increasing film thickness from the center towards the edge, while the natural deposition profile on such substrates in sputter deposition processes has the opposite shape. The deposition task is realized on a dual-cylindrical magnetron-sputtering compartment equipped with a sub-rotating substrate holder on a rotating turntable and specialized uniformity masks. A multi-scale simulation and optimization approach determines the shape of the uniformity masks: First, 3D Particle-in-Cell Monte Carlo (PIC-MC) simulations result in the relative erosion profile on the cylindrical sputter targets. Subsequently, the transport of sputtered material through the coater geometry is modelled via the Direct Simulation Monte Carlo (DSMC) method. Finally, a fast algorithm projects the deposition flux onto the moving and rotating substrate for arbitrary angles of the turntable rotation. The optimization scheme has been successfully validated for a band pass filter onto a spherical lens and is currently being applied on aspherical lenses. Further extension of the coupled simulation framework towards different coater and 3D substrate geometries is ongoing.