5 June 2018 Stable, durable, low-absorbing, low-scattering MgF2 films without heat or added fluorine
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Abstract
The goal achieved by this work has been to produce stable, durable, low-absorbing, low-scattering magnesium fluoride (MgF2) films without additional heat or fluorine. This was done with IAD in a chamber that could do production on a commercial basis. Stability with respect to humidity and temperature shifts and durability with respect to abrasion resistance depend on the density of the films. Absorption depends on the stoichiometry of the films. Scattering partially depends on stress cracking due to mismatch of the coefficients of thermal expansions of the substrates and the coatings. In recent decades, MgF2 has become a preferred material in the DUV to wavelengths as short as 180 nm. It has been shown that MgF2 often suffers from a fluorine deficiency when deposited by energetic processes. It has been demonstrated that much of the resulting absorption can be eliminated by ultraviolet annealing which provides enough energy to give mobility to fluorine atoms to reunite with nearby magnesium atoms. This implies that the fluorine deficiency is due to dislocations of the fluorine atoms, but that they are not all lost from the matrix of the film and pumped away by the vacuum process. It is believed that energetic ions from sputtering or IAD sources cause the dissociation of the F atoms from the Mg if their energy level exceeds some threshold. Present results provided "fully" dense films for hardness, low temperature/humidity shifts, and low absorption and scattering by using IAD at 160 eV or less and no added fluorine.
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Ronald R. Willey, Ronald R. Willey, R. Shakoury, R. Shakoury, } "Stable, durable, low-absorbing, low-scattering MgF2 films without heat or added fluorine", Proc. SPIE 10691, Advances in Optical Thin Films VI, 106910C (5 June 2018); doi: 10.1117/12.2309812; https://doi.org/10.1117/12.2309812
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