Aging of Al thin film extreme ultraviolet filters

Thin film Al filters, very popular for their high transmittance in the wavelength range 17 to 67 nm and simultaneously low transmittance in the visible and near UV region, are prone to oxidation. The amorphous Al2O3 layers on the Al surfaces have much smaller transmittance than the bulk Al material, and, therefore, they strongly influence the total transmittance of the filter. This paper gives not only the transmittance of very old Al filters, but also maps the transmittance development of Al filters in two years span since their delivery in not controlled atmosphere. Surprisingly, it turned out that while the transmission of surface layers (present on filters since the very beginning) slightly increases with time, the average attenuation rate per unit filter thickness of the bulk material (Al) with time dramatically rises.