Improving the optical and mechanical characteristics of aluminum thin-film filters by adding thin cap layers

15 Dec.,2022

 

Cap Filter Mesh

Aluminum (Al) filters for solar astronomy of the extreme ultraviolet (EUV) wavelength range are the first spectral film filters used in space observations of the Sun [1]. Aluminum spectral filters remain most commonly used in solar astronomy due to the fact that they have a wide transmission region (17.1–70 nm) behind the L2.3 absorption edge at 17.1 nm and a high degree of blocking of UV, visible and IR radiation (the visible light rejection is >106 times for a 150 nm Al filter). The disadvantage of the Al film filter is that the transmission of the filter in the EUV range is reduced due to oxidation when it is stored at standard atmospheric conditions [2]. In addition, Al films are relatively fragile. To solve the problem with oxidation, it is proposed to store Al filters in dry air or dry nitrogen. However, the degradation of the optical characteristics of the Al filter is also observed in space, as was shown in comparison of transmission of Al filters with triple layer carbon/zirconium/carbon (C/Zr/C) composite filters at the same wavelength of 17.1 nm [3]. To protect Al filters from oxidation it was proposed to use protective cap layers, for example, carbon layers [4]. However, the effectiveness of such cap layers remains to be determined. In addition, in the long-wave part of the EUV wavelength range, thick layers of C significantly reduce the transmission. Thin films (typical thickness of spectral film filters is 150–200 nm) are fragile and can be damaged under the influence of vibration and acoustic loads during the spacecraft launch. These membranes are usually reinforced by a supporting metal mesh. The small size of the mesh (typical size is 70 lines per inch) can lead to distortion of the image caused by light diffraction. Filters on a mesh with larger cells, as well as large aperture entrance filters, require a special chamber for launching the filters under vacuum [5], which complicates and increases the cost of the assembly.

To overcome the disadvantages of Al filters, Al-based multilayer film filters have been developed [6]. In space telescopes of the Russian solar observatory TESIS (2009) [6,7] and in the rocket experiment Hi-C (2012) [8], multilayer Al/Si films stretched and glued to grids were used as entrance spectral filters. The Al/Si filter on a supporting mesh with 5 mm mesh size has withstood the acoustic and vibration loads that take place during the rocket launch without being preliminarily placed in vacuum conditions [8]. The disadvantages of multilayer Al/Si filters are additional absorption in the EUV wavelength range produced by Si interlayers, as well as low thermal stability of the Al/Si films. That limits the possibility of using Al/Si filters in telescopes operating in the long-wave part of the EUV range (silicon strongly absorbs radiation at wavelengths λ > 50 nm), as well as in telescopes operated in orbits close to the Sun [6].

In new projects of high resolution solar EUV telescopes, filters on a supporting mesh with a high transmission in the EUV range and minimal diffracting effect caused by the supporting grid are required. In this case, the use of multilayer filters with interlayers giving additional absorption in the EUV range is unacceptable. As an alternative to the multilayer film approach, in this article we will consider the possibility of improving the optical and mechanical properties of Al monolayer films by adding nanometer thicknesses cap layers. By the examples of several types of cap layers (molybdenum disilicide (MoSi2), aluminum nitride (AlN)), we demonstrate that the use of protective cap layers can increase not only the resistance of Al films to oxidation, but also their mechanical strength.

The purpose of this paper is to compare the properties of Al filters with and without cap layers. We studied the influence of cap layers on the stability of Al filter transmission in the EUV range and on the value of the ultimate pressure difference between the sides of the film at which it is damaged. In order to elucidate the mechanisms of the observed difference in the strength of Al films with a cap layer and without, a more detailed comparison of the behaviour of films under mechanical loads was carried out using the bulge method [9,10]. The film structure of Al with MoSi2 cap layers, which had shown the best result in mechanical tests, glued to a special mesh with different cell sizes, was tested on a bench simulating the emerging acoustic loads during rocket launch.