Room temperature plasma-etching and surface passivation of far-ultraviolet Al mirrors using electron beam generated plasmas

Abstract

The development of optical systems operating in the far ultraviolet range (FUV, λ=100-200 nm) is limited by the efficiency of passivated aluminum (Al) mirrors. Although it is presently possible to obtain high-reflectivity FUV mirrors through physical vapor deposition, the process involves deposition with substrates at high temperatures, which is technically challenging for large optical elements. A novel passivation procedure for bare Al mirrors is reported. The treatment consisted of using a low-temperature electron-beam generated plasma produced in a gas mixture of Ar and SF6 to etch away the native oxide layer from the Al film, while simultaneously promoting the generation of a thin aluminum tri-fluoride (AlF3) layer on the Al surface. In the first section we analyze the effect of varying both ion energy and SF6 concentration on the FUV reflectance, thickness, composition, and surface morphology of the resulting AlF3 protective layers. In the second section, the reflectivity of samples is optimized at selected important FUV wavelengths for astronomical observations. Notably, samples attained state-of-the-art reflectances of 75% at 108.5 nm (He Lyman γ), 91% at 121.6 nm (H Lyman α), 90% at 130.4 nm (OI), and of 95% at 155.0 nm (C IV). The stability over time of these passivated mirrors is also investigated.

Environmental and polarization characterizations of E-beam plasma-based AlF3-passivated aluminum mirrors for astronomical telescopes

“…4, which shows steps in the reduction of oxygen content as a function of treatment time in these films, and consequent enhancements of FUV reflectivity, reaching samples with no detectable O content and reflectivity ∼54% at 105 nm and reflectivity over ∼83% at wavelengths longer than 115 nm. 1,13 This is quite remarkable since this is obtained from samples that previously had a Al 2 O 3 layer, which were then treated at LAPPS. Once the process parameters were optimized, a series of experiments were designed to evaluate the uniformity of processed samples all of them done at the same time.…”

Electron beam-generated plasmas in NF3 environments for the passivation of UV/O/IR mirrors

“…All bare Al mirrors were deposited on soda-lime glass slides of 50 mm x 50 mm area with the conditions stated in Ref. [18] for Al, but for the present investigation the chamber baked out prior to the aluminum coating deposition was omitted. Most of the bare Al coatings shown in this research were fabricated in the same coating run with samples distributed over an area of 200 mm x 160 mm.…”

“…The LAPPS and associated process considerations are described in detail in previous works [18,22]. Unlike these, the present work does not employ the use of an additional inductively couple plasma (ICP) source on the gas inlet.…”

“…FUV optical characterization was performed with the same instrument described in Ref. [18], using similar measurement configuration.…”

“…To address these challenges, we have developed a novel passivation procedure for the plasma-based fluorination of bare aluminum [17,18,19,20]. The passivation layer that protects the Al from oxidation is composed of AlF3.…”