DUV light scattering and morphology of ion beam sputtered fluoride coatings

Optical thin films are nowadays major obstacles in deep UV lithography. Scattering and absorption losses are the dominant factors which limit their optical performances. The reduction of scattering requires the development of specific deposition techniques. As a new way of manufacturing optical thin films for 193 nm applications, we applied Ion Beam Sputtering (IBS) to fluoride coatings previously developed for 355 nm. In order to check the ability of IBS to reduce the scattering losses of coatings, various quarterwave and halfwave coatings as well as multilayer stacks deposited onto fused silica and CaF2 substrates were produced and characterized by total scattering (TS) measurements at 193 nm. An AFM was used for direct imaging of the surface features and calculation of rms surface roughnesses. Subsequent data processing provided power spectral density curves containing more detailed roughness information. The evolution of the surface roughness and scattering with increasing film thickness or number of layers was analyzed and the results were discussed in order to determine the origin of the scattering and to highlight the benefits which can be expected from IBS processes.