Prospects for the enhancement of PIAD processes by plasma diagnostics

The purpose of this paper is to present concepts for an improved control of plasma ion assisted deposition (PIAD) processes which are employed for the production of optical interference coatings. While the well established PIAD technique typically comprises methods for in situ monitoring of thin film properties, there is no detailed knowledge about plasma parameters which are the foundation of magnitude and stability of plasma assistance, however. We adopt optical emission spectroscopy (OES) and active plasma resonance spectroscopy (APRS) and present schemes for controlling radiance and electron density on a batch coater equipped with an Advanced Plasma Source (APS). In a repeatability experiment of a 5-layer quarterwave stack (QWS, SiO₂/TiO₂), characteristics of two plasma based control schemes are compared to those of a conventional approach. For the conventional process we find systematic drifts and shifts in time traces of monitored plasma paramaters which correlate to properties of the layer stack. By using the novel concepts, stability of plasma paramaters can be improved by a factor of up to 6, while repeatability of in situ QWS transmission is strongly enhanced, exhibiting no spectral shift and minimal variation in reflectivity.