The standard VUV broadband mirrors are based in an Al film protected with a thin film of MgF2.
An increase in VUV reflectance is obtained by depositing the protective MgF2 film over a heated Al film, following the procedure developed in Quijada et al., Proc. SPIE 8450, 84502H (2012).
GOLD has investigated the influence of deposition temperature and annealing temperature on the FUV reflectance of Al/MgF2 mirrors. The right figure displays the reflectance of an Al/MgF2 mirror prepared at GOLD. The figure shows the reflectance increase as a function of MgF2 deposition temperature. An optimum temperature of ~250°C resulted in a 90% reflectance at 121.6 nm.
MgF2 deposited on hot Al
MgF2 layer deposited on the Al film heated at various temperatures
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A secondary procedure was developed in which the MgF2 protective coating was deposited at room temperature and the mirror was then annealed in vacuum. The next figure displays the reflectance of post-annealed mirrors at various temperatures. We obtained a reflectance enhancement almost as high as for the hot-deposited MgF2 procedure.
RT deposited and post-annealed mirrors
Al/MgF2 mirror deposited at room temperature and post-annealed in vacuum
AlF3 protective layer deposited on the Al film heated at various temperatures
AlF3 has been also demonstrated as an effective protective film for broadband Al mirrors. The next figure displays the reflectance of Al mirrors protected with AlF3 deposited and/or post-annealed at various temperatures. Measurements were performed after two years of ageing.
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In situ reflectance of an IBS-SiC film
At wavelengths shorter than 100 nm, MgF2, LiF, and AlF3 strongly absorb radiation, so that there is no protective layer for Al. Then single layers of SiC or B4C have the largest reflectance.
GOLD has prepared single layer mirrors of SiC and B4C prepared by ion-beam-sputtering.
In situ reflectance and optical constants of ion-beam-sputtered SiC films in the 58.4 to 149.2 nm region
Mónica Fernández-Perea, José A. Méndez, José A. Aznárez, and Juan I. Larruquert
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