Measurements of Thermophysical Characteristics of Thin Film Metal Filters for Extreme Ultraviolet Radiation

Knowledge of the emissivity and thermal conductivity of thin metal films used in conjunction with multilayer mirrors for spectral selection of radiation in the extreme ultraviolet and soft X-ray wavelength ranges is necessary in order to correctly calculate the heating of film elements at high heat loads. Heating is associated with the absorption in the film of a significant fraction of the incident intensity, and the concept of a high heat load is somewhat arbitrary, since even at an absorbed intensity level of the order of 1 W/cm² a freestanding film can be heated in vacuum by several hundred degrees. In the first approximation, to estimate the thermal conductivity coefficient, one could use tabular values for massive samples of the corresponding metals or use the well-known Wiedemann–Franz law which links the thermal conductivity and the electrical resistivity of the sample – the latter is easier to be measured. However, an analysis of the literature data indicates significant errors that are possible when using any of these approaches. Therefore, in this work, we have measured the thermal conductivity directly by processing the temperature distribution obtained by IR pyrometry over a film sample mounted on a heated frame or heated by a flowing electric current. Thermophysical characteristics (thermal conductivity and emissivity) were determined for samples of film absorption filters based on Mo, Al, and Be of submicron thickness (from 100 nm), as well as for films of copper – a metal whose bulk samples have high thermal and electrical conductivity. As expected, significant differences were found between the thermal and electrical properties of the film materials and the properties of the same metals in monolithic samples.