Narrowband and tunable anomalous transmission filters for spectral monitoring in the extreme ultraviolet wavelength region

We present the first experimental demonstration of a novel type of narrowband and wavelength-tunable multilayer transmission filter for the extreme ultraviolet (EUV) region. The operating principle of the filter is based on spatially overlapping the nodes of a standing wave field with the absorbing layers within the multilayer structure. For a wavelength with a matching node pattern, this increases the transmission as compared to neighboring wavelengths where anti-nodes overlap with the absorbing layers. Using Ni/Si multilayers where Ni provides strong absorption, we demonstrate the proper working of such anomalous transmission filter. The demonstration is carried out at the example of 13.5 nm wavelength and at normal incidence, providing a 0.27 nm-wide transmission peak. We also demonstrate wavelength tunability by operating the same Ni/Si filter at different wavelengths by varying the angle of incidence. As the multilayer filter is directly deposited on the active area of an EUV-sensitive photodiode, this provides an extremely compact device for easy spectral monitoring in the EUV. The transmission spectrum of the filter is modeled and found to be in good agreement with the experimental data. The agreement proves that such filters and compact monitoring devices can be straightforwardly designed and fabricated, as desired, also for other EUV wavelengths, bandwidths and angles of incidence, thereby showing a high potential for applications.