Modeling of the film thickness distribution along transport direction in in-line coaters for reactive sputtering

For large area architectural coatings such as Low-E or solar control systems strong demands on color uniformity are to be fulfilled. Typically, a precision of delta a*, delta b* < +- 1 is required. For single-Ag based Low-E coatings this implies a film thickness tolerance of about +- 2 %, while for double or multi-AG low-E coatings the thickness variation should be less than +- 1 %. A common problem of in-line sputter coaters is a certain film thickness deviation of about 2 % along the glass transport direction. It is caused by varying flow conditions due to the glass substrate movement within the compartments. In order to solve this problem, we performed a three dimensional Monate Carlo gas flow simulation of a jumbo sized sputter compartment together with adjacent pumping compartments and glass transport slots. The sputter compartment comprises two 3.83 m sized twin rotatable magnetrons equipped with Zn-targets. Based on the Monte Carlo simulation, a heuristic model of the reactive ZnO sputtering process has been implemented. Experimental verification shows that this model properly describes (i) the pressure with respect to substrate position, (ii) the ZnO target voltage vs. oxygen flow charcteristics at different conditions as well as (iii) the film thickness profile on a substrate after dynamic deposition. This model is a starting point towards novel controlling algorithms which allow for film thickness homogeneity better than 1 % within jumbo sized in-line sputtering coaters.