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.
In the present study the preparation and characterization of Al2O3/AlF3 material mixture coatings is described. The main focus is on the correlation between the optical properties (refractive index, extinction coefficient), elemental composition and mechanical properties, depending on deposition conditions. The thin mixture films prepared with E-Beam gun evaporation with and without plasma ion assistance show a continuous change in the refractive index between the basic materials extrema (n=1.4 - 1.75), depending on the constituents volume filling factors. At the same time, extinction coefficients vary between less than 1x10-4 and 2x10-3. In addition, it can be shown that in dependency of the applied plasma ion assistance, the residual stress inside the mixture layers is tunable. Finally, several multilayer coatings has been prepared and investigated.
Metal mirrors are an attractive solution for scan mirrors working with ultra-short pulse lasers. Small mechanical inertia and a small mirror mass are required. Therefore, the mirrors have to be very stiff and a high quality optical surface has to be provided. This can be achieved with lightweight AlSi based mirrors with diamond-turned NiP polishable plating.
Different coating options were evaluated in order to provide the necessary high reflectivity and a satisfactory laser damage threshold for ultrashort laser pulses in the few ps to fs regime at λ = 1030 nm. High-reflective metal layers enhanced by dielectric HfO2/SiO2 stacks were found to be the most advantageous coating option due to their comparatively small thickness and measured damage thresholds above 1 J/cm2@8ps.