High-reflective Coatings for Ground and Space Based Applications

High-reflective coatings are indispensable in order to manufacture mirrors with highest possible reflectivity. The maximum reflectivity can be achieved by all-dielectric coatings; however, the spectral bandwidth of these mirrors is limited. For astronomical applications metal based coatings (Al, Au, Ag) are commonly applied, as they allow high reflectivity and at the same time a broad spectral bandwidth. Silver based coatings are the only possible choice for numerous applications, as from all metals silver provides the highest reflectivity from VIS to IR. The optical properties of sputtered and of evaporated silver films are related to their morphology. A model has been developed which relates the reflectivity of as deposited samples and samples after aging to the surface morphology of the Ag films and allows the determination of effective optical constants for the respective silver films. During aging, corrosion products like Ag2S and AgCl are formed and reduce the reflectivity of silver mirrors. Thus, a protective layer is required to prevent the corrosion and preserve high reflectivity of a silver reflector. Even space-based telescopes require a protection because assembling, transport and storage takes place at ground under various environmental conditions. The protective layer on the mirror front-side influences the optical properties of the film, therefore transparent dielectric materials like SiO2, Al2O3, etc. are commonly applied as protective layers. However, even the deposition of a pinhole-free protective layer seems to be only a necessary but not a sufficient condition for the effective protection of silver films: hydrophobic particles containing Cl and/or S in combination with a humid ambient can potentially harm even silver coatings with defect free protective layer. For some applications the optical performance of protected silver coatings is not sufficient. In these cases enhanced Ag-coatings might be a suitable option. For example, a considerably higher reflectivity in the spectral range 350 – 450 nm can be achieved with an UV-enhanced Ag-coating while the coating still maintains a high reflectivity in the IR range. This UV-enhancement can be achieved by a multilayer approach for the protective coating which at the same time considerably increases the stability of protected silver coatings. In a separate part of this work the properties of dielectric coatings deposited by plasma enhanced atomic layer deposition (PEALD) were investigated. ALD is a very promising technology for the optical coating of components with complex shapes, however, there are not many data available on the optical properties of ALD deposited films. It will be shown, that ALD coatings have a high potential for a successful application in optical coatings, however, some process optimization will still have to be made.