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30 June 1999 Evolution of sculptured thin films
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Sculptured thin films (STFs) are columnar thin films in which the growth direction is altered instantaneously by variations in the vapor incidence angle. By fixing the orientation of the substrate-surface to a glancing angle (approximately 1-30° from the substrate plane) where atomic self-shadowing effects are enhanced, and then rotating around the plane normal, it is possible to engineer a wide range of STF nanostructural shapes which have been classified generally as thin film helicoidal bianisotropic mediums (TFHBMs). If the column size remains constant with increasing film thickness (i.e. matchstick morphology), then the TFHBMs will have constant density with evolution, and existing theories can describe their optical behavior. Furthermore, many practical applications will require constancy with evolution. In this paper we show that essentially constant column size (widths are typically 10 - 100 nm) is obtained for simple motions of the substrate for TFHBM growth, whereas more complex motions involving rapid and abrupt changes in the angular velocity result in column size increases with evolutionary development (widths up to 200 - 500 nm for film thicknesses of 2 - 4μm). As the column sizes approach optical wavelengths, the assumption that the STF is a rotationally non-homogeneous continuum is invalid and will lead to complicated optical behavior and modeling. A classification scheme is proposed to understand the underlying mechanisms for column expansion, and it is based upon the deposition parameters of column growth rate in the column direction, angular rotation rate, and the vapor incidence angle, and their combined effects on the anisotropy of the atomic self-shadowing process. Approaches to controlling and possibly eliminating this column expansion are discussed and include ion bombardment during growth and its effects on the shadowing processes.
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Russell F. Messier, Paul D. Sunal, and Vijayakumar C. Venugopal "Evolution of sculptured thin films", Proc. SPIE 3790, Engineered Nanostructural Films and Materials, (30 June 1999);

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