Enhanced PVD process control by online substrate temperature measurement

In physical vapor deposition (PVD) processes pressure and temperature are of outstanding importance for process control and coating performance. The pressure measurement and its integration into the process control are state of the art in industrial coating units. In contrast, the precise determination of the substrate temperature Tsub presents an unequally greater challenge. In PVD processes the temperature control has a great importance in two respects. On the one hand, the morphology of the coating as well as the adhesion between the coating and substrate directly depend on Tsub. On the other hand, the process temperature is limited by the maximum permissible temperature Tmax as for example of heat-treated steel. Exceeding can easily lead to a critical loss of hardness and size accuracy of the substrate and production reject. The online measurement of Tsub using currently applicable systems such as drag pointer or pyrometer is either extremely complicated or simply not possible. This is particularly the case for large-volume industrial coating units with a rotating substrate table. Therefore, in coating processes, the influence of the different process parameters on Tsub is usually not quantified. Hence, Tsub is often several tens centigrades below Tmax. Within the scope of this paper, a temperature measuring system developed at the Surface Engineering Institute (IOT) of the RWTH Aachen University is used in an industrial coating unit with rotating substrate table. With this, the online temperature measurement of rotating substrates is possible throughout the entire coating process, from the heating to the etching over the coating to the cooling process phase. In the heating, etching and coating process phases, the influence of the heating power on Tsub was analyzed and described by characteristic trend lines. The results were used to improve the substrate temperature profile of an industrial-like coating process with regard to Tmax. Here the temperature difference between Tmax of the heat-treated steel and the realized Tsub was reduced to a feasible minimum over the entire process time. Investigations on the compound properties show an improved adhesion between coating and substrate.