A Fundamental Approach Towards Improved Optical Coatings

Abstract

The potential afforded by molecular beam epitaxy as a means of producing thin film materials in a highly controlled manner has been exploited using an ultra-high vacuum facility dedicated to the examination of all facets of thin-film technology as applied to optical coatings. The equipment allows deposition on ion-beam cleaned substrates either from molecular beams or by RF sputtering, and allows in situ characterisation by mass spectrometry, Auger Electron Spectroscopy and laser calorimetry. Application of the equipment is described with reference to the fabrication of ZnSe coatings. The composition of the residual atmosphere within the coating chamber is discussed with reference to the rate of adsorption of impurities onto the substrate whilst growth of the coating is underway. By ensuring the reduction of the partial pressures of water vapour and carbon monoxide to the low 10−10 mbar range, ZnSe films have been produced of up to 3 μm in thickness which have negligible values of absorptance at 10.6 μm. Such layers, although micro-polycrystalline, are mirror smooth and featureless, and of (111) orientation. Growth rates in excess of 1 μm/h are comparable with conventional coating techniques.