Abstract

Aluminum thin-film spectral filters are widely used in telescopes for space observations of the Sun in the extreme ultraviolet range of wavelengths. The main purpose of film filters is to block radiation in the UV, visible, and near-IR spectral ranges. In connection with the development of new projects for the observation of the Sun from close distances, the thermal stability of the entrance film filter is an important characteristic. In this paper, the thermal stability of Al films with 2.5 nm thick ${\mathrm{MoSi}}_2$ protective cap layers has been studied. ${\mathrm{MoSi}}_2$ was chosen as a coating material because ${\mathrm{MoSi}}_2$ caps effectively protect the Al film from oxidation and simultaneously increase the mechanical strength of the Al film. Vacuum annealing of ${\mathrm{MoSi}}_2/\mathrm{Al}/{\mathrm{MoSi}}_2$ films was carried out at temperatures up to 300°C. It has been demonstrated that at an annealing temperature of more than 200°C for 24 h, a noticeable decrease in the blocking properties of the ${\mathrm{MoSi}}_2/\mathrm{Al}/{\mathrm{MoSi}}_2$ film is observed in the visible wavelength range, which is caused by the appearance of semi-transparent crystalline silicon dendritic structures that are tens of micrometers in size in the film. In the annealed area of the film specimen, the intermetallic ${\mathrm{Al}}_{12}\mathrm{Mo}$ phase was detected by electron diffraction structure analysis, indicating a possible reason for the appearance of silicon atoms needed for dendrite growth as a result of the chemical interaction of Al and ${\mathrm{MoSi}}_2$. Due to the requirements for a high degree of visible radiation blocking (${10}^6$ to ${10}^7$ times), the appearance of even one dendritic structure significantly reduces the blocking properties of the film filter and is, therefore, not permissible. Based on the measurement of the transmission of ${\mathrm{MoSi}}_2\text{-}2.5\mathrm{nm}/\mathrm{Al}\text{-}72\mathrm{nm}/{\mathrm{MoSi}}_2\text{-}2.5\mathrm{nm}$ films at 633 nm for isothermal annealing at 200°C–250°C, the activation energy for the formation of dendritic structures ($E=1.55\pm 0.1\mathrm{eV}$) was measured and the maximum permissible film temperature (130°C) at which dendritic structures did not appear during a 5-year mission was predicted.

© 2018 Optical Society of America

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