Numerical optimization of baffles for sputtering optical precision filters

The optimization of the coating uniformity of precision optical filters generally is a critical and time consuming procedure. The present paper demonstrates this optimization procedure on a new optical precision sputter coater “Enhanced Optical Sputtering System (EOSS)” at Fraunhofer IST. The coater concept is based on dual cylindrical sputtering sources and a rotating turn-table as sample-holder. For compensating non-uniformity introduced by the particle flux profile and the radially dependent track speed on the turntable, baffle elements have to be designed and inserted beneath the substrates. For that purpose the distribution of the particle flow from the cylindrical magnetron as well as the resulting thickness profile for different shaper designs is simulated using Direct Simulation Monte Carlo (DSMC) transport simulation. For comparison, experimentally obtained film thickness profiles are evaluated by spectrophotometry and ellipsometry. The simulation model is used for optimization of the baffle geometry as well as investigation on the role of long term drifts caused by target erosion and mechanical tolerances. •An optical precision coater is simulated via 3D Direct Simulation Monte Carlo method•The simulated deposition profiles are in good agreement with experiments•The simulation enables numerical optimization of the uniformity mask•Long term drifts and the role of mechanical tolerances can be analyzed in detail•Numerical optimization yields ±0.35 % of rate homogeneity on 200mm substrates