Microstructure of Sputter Deposited TiO 2 /SiO 2 Multilayer Optical Coatings
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Microstructure of Sputter Deposited TiO2/SiO2 Multilayer Optical Coatings E. Sutter (Mateeva), P. Sutter and *J.J. Moore Department of Physics, Colorado School of Mines, Golden, CO 80401, USA. *Department of Metallurgical and Materials Engineering , Colorado School of Mines, Golden, CO 80401, USA. ABSTRACT The microstructure of TiO2/SiO2 multilayer optical filters has been investigated in detail by cross-sectional transmission electron microscopy and related to their optical properties and stability. The amorphous TiO2 layers in the as-deposited multilayers are found to consist of nanocolumns and intercolumnar regions with non-stoichiometric or lower density material. In humid ambients this microstructure absorbs moisture which causes a shift in the absorption edge of the filters. Upon annealing the TiO2 layers are found to recrystallize into the low-temperature anatase modification which leads to significantly improved stability of the optical properties of the filters. INTRODUCTION Thin TiO2 and SiO2 films have been used in a variety of applications, e.g. dielectric layers in microelectronic devices, antireflection coatings in solar cells, catalysts, and optical coatings. Alternating thin films of TiO2 and SiO2 are traditionally being used as high and low refractiveindex layers in multilayer optical filters [1-3]. The quality and stability of the filters strongly depend on the optical and mechanical properties of the TiO2 and SiO2 layers, which in turn are determined by their morphology, density, the surface roughness, degree of inhomogeneity, moisture absorption, and state of stress. TiO2 films deposited by electron beam evaporation have low density and are often found to develop columnar conical morphology with domed tops [4] which makes them prone to moisture absorption resulting in inferior optical and mechanical properties. Relatively dense TiO2 films have been deposited using techniques that involve bombardment with energetic species during deposition such as dc [5] and rf [6] sputtering, or plasma ion-assisted deposition [7]. In most cases the films are grown at room temperature (RT) and are amorphous or consist of columnar-shaped crystallites [7]. The film structure and density are commonly characterized using techniques like X-ray diffraction (XRD) or atomic force microscopy (AFM) that measure the overall crystal structure or the surface morphology but give insufficient insight in the actual microstructure of the layers and distribution of voids. In the TiO2/SiO2 system knowledge of the detailed microstructure and of the factors determining it is very important as the optical properties of the TiO2 films strongly depend on the microstructure, even in amorphous films [8]. Unfortunately in a large number of studies the optical properties of the films have been explained assuming homogeneous, uniform amorphous films [9] or a random distribution of voids, or layer models in which the films consist of several sublayers representing different air-TiO2 or water-TiO2 combinations [10, 11]. In this paper we use cr
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