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Free-standing multilayer films consisting of Si, Zr, Mo and silicides of both metals have been fabricated and studied as spectral purity filters (SPF) for extreme ultraviolet (EUV) (13.5 nm) lithography tools. Comparative tests of multilayer SPF structures of various compositions have been performed at high power loads. It was found that a Mo∕ZrSi 2 structure with MoSi 2 capping layers is featured with capability to withstand prolonged heating in vacuum (10 −7 mbar) at 900-950°C. A technique of fabrication of large aperture free-standing multilayers was developed, and a pilot sample of the above film structure of 160 mm in diameter, 50 nm thick, with transparency at 13.5 nm above 70% was fabricated as a conceptual prototype of SPFs with large dimensions.

Al, with a passband in the wavelength range of 17-60 nm, and Zr, with a passband in the wavelength range of 6.5-17 nm, thin films on a support grid or support membrane are frequently used as UV, visible, and near-IR blocking filters in solar observatories. Although they possess acceptable optical performance, these filters also have some shortcomings such as low mechanical strength and low resistance to oxidation. These shortcomings hinder meeting the requirements for filters of future telescopes. We propose multilayer thin film filters on the basis of Al, Zr, and other materials with improved characteristics. It was demonstrated that stretched multilayer films on a support grid with a mesh size up to 5 mm can withstand vibration loads occurring during spacecraft launch. A large mesh size is preferable for filters of high-resolution solar telescopes, since it allows image distortion caused by light diffraction on the support grid to be avoided. We have investigated the thermal stability of Al/Si and Zr/Si multilayers assuming their possible application as filters in the Intergelioprobe project, in which the observation of coronal plasma will take place close to the Sun. Zr/Si films show high thermal stability and may be used as blocking filters in the wavelength range of 12.5-17 nm. Al/Si films show lower thermal stability: a significant decrease in the film's transmission in the EUV spectral range and an increase in the visible spectrum have been observed. We suppose that the low thermal stability of Al/Si films restricts their application in the Intergelioprobe project. Thus, there is a lack of filters for the wavelength range of λ>17  nm. Be/Si and Cr/Si filters have been proposed for the wavelength range near 30.4 nm. Although these filters have lower transparency than Al/Si, they are superior in thermal stability. Multilayer Sc/Al filters with relatively high transmission at a wavelength of 58.4 nm (HeI line) and simultaneously sufficient rejection in the wavelength range near 30.4 nm (HeII line) have been fabricated. They are planned to be used in the project KORTES, whose telescopes will have an EUV channel at 58.4 nm.

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The technique for fabrication of thin film filters with high mechanical strength, capable of withstanding the prolonged heat load of 1 W/cm 2 , has been developed. Freestanding multilayer Zr/Si filters of size 20 150 mm 2 with high transparency of 76% at wavelength = 13 nm were manufactured for EUV lithography tool. We have also developed and fabricated various designs of filters (freestanding or mesh supported) with lower transparency of 40-50% for experiments with intensive EUV sources. The tests of differential pressure withstandability and heat-resistance of filter samples were fulfilled. In order to model the influence on the filter of intensive radiation of the lithography source we have tested Zr/Si film samples by the Joule heating in vacuum at residual pressure of 10 -8 Torr. The testing consisted in continuous heating of Zr/Si films at the electrical power per area unit from 0.5 W/cm 2 to 6 W/cm 2 during long period of time (up to 2 months). The influence of the long-term heat load on the transparency of samples at = 13 nm and within wavelength region 0.3 -2 m was investigated.