In high quality optical coating systems for the DUV-spectral range, MgF2 is one of the preferred deposition materials [1]. MgF2-coatings exhibit relatively low optical losses as well as high stability and laser induced damage thresholds. In the present joint research effort of several European laboratories, the potentiality of MgF2 is evaluated in respect to the production of improved optical coatings for applications in laser technology and semiconductor lithography. For this purpose, single, layers of MgF2 were deposited on superpolished fused silica and CaF2-substrates by ion beam sputtering (IBS, [2]), boat (bPVD) and e-beam (ePVD) evaporation in different laboratories. Besides photometric inspections, the samples were characterised by an optical scatter measurement facility at 193 nm [3] and 633 nm. The structural properties were assessed using AFM, XRD, and adapted TEM-techniques involving conventional thinning methods for the layers. For the measurement of mechanical stress in the coatings, special silicon substrates were coated and analysed. The optical constants of the MgF2-layers, which were determined by two independent data reduction algorithms from the spectral behaviour, are in good agreement with values published in the open literature [4]: The refractive indices range between 1.40 and 1.41 at 248 nm for the PVD and IBS processes. The IBS-process achieves best homogeneity and lowest surface roughness around 1 nm(rms) among the processes compared. In contrast to PVD coatings, which exhibit tensile mechanical stress ranging approximately from 300 to 400 MPa, high compressive stress of up to 910 MPa occurs in the IBS-coatings. The experimental findings are discussed in respect to the microstructural properties and the surface topography of the coatings [5].
