DUV scattering measurements as a tool for characterization of UV-optical surfaces

A sensitive total-scattering measurement setup for the DUV spectral range is presented, which allows precise determination of both forward and backward scatter losses from optical components for excimer lasers with a sensitivity below 10 ppm for 248 nm and 50 ppm for 193 nm. Scattering from several different coated and uncoated DUV optical surfaces was monitored. For uncoated samples, the backward scatter losses are in good agreement with the predictions of scalar scattering theory, indicating that in this case scattering is mainly determined by surface effects. Although forward and backward scatter losses are of the same order of magnitude for uncoated samples, they differ by up to two orders of magnitude for high-reflection- and by one order of magnitude for anti-reflection-coated samples. The experimental data demonstrate that the anti-reflection coatings suffer from substantial losses due to forward scattering, whereas backward scattering is the predominant loss channel for high-reflection coatings. In addition, the strong influence of defects and impurities on the total scattering is demonstrated.