Measurement of absorptance of optical coatings for F2 lithography

An absorptance measurement system has been developed for evaluation of the absorption loss of optical coatings at the wavelength of F2 laser(157nm). Calorimetry was adopted as measurement method because of its high reliability. In the system, the calorific values generated by irradiation has estimated by comparison with those generated by an electric heater in order to obtain the high accuracy of measurement. The repeatability of measurement has been attained so far to be +/- 0.02%. We have found out with the system that the absorptance is increased by measurement in the vacuum compared with in nitrogen and decreased by irradiation of F2 laser light due to its contamination cleaning effect. We have measured the absorptance of samples with anti-reflection coatings that several suppliers fabricated by their own methods.