Growth behavior of evaporated porous thin films
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T. Smy Department of Electronics, Carleton University, Ottawa, Ontario, Canada K1S 5B6
M.J. Brett Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 (Received 18 March 2002; accepted 21 August 2002)
Recent experimental work by a number of researchers has demonstrated that unusual high porosity thin films may be obtained in physical deposition systems by combining glancing angle deposition with in situ substrate motion control. The microstructure of these films consists of isolated columns engineered into shapes such as helices, posts, or chevrons. Due to the isolated nature of the columns, the films present a unique opportunity to study fundamental thin film growth behavior and, in particular, the influence of the self-shadowing mechanism in three dimensions. Apart from this academic motivation, there is the need to characterize the physical constraints imposed on the engineering of these films. In particular, this study will have implications for the realization of isolated, periodically arranged nanostructures envisioned for certain applications such as photonic band gap crystals. Results from an ongoing study of growth dynamics, morphology, porosity, and scaling behavior, and the dependence of these features on deposition parameters are presented.
I. INTRODUCTION
It has long been recognized that thin films grown under conditions of oblique vapor incidence exhibit structural anisotropy.1 This structural anisotropy can in turn induce corresponding anisotropy in the optical, magnetic, and electrical properties of films.2 For example, Motohiro and Taga demonstrated that thin film quarter wave plates could be produced by the deposition of multilayers of chevron-shaped nanostructures.3 If, in addition to oblique deposition, dynamic modification of the substrate to source orientation is introduced, structural chirality may be created. In a pioneering work, Young and Kowal used substrate rotation to introduce optical chirality into thin films.4 The recent development of the glancing angle deposition (GLAD)5–15 technique has rekindled an interest in this area. GLAD combines extremely oblique vapor incidence with dynamic control of substrate orientation to produce unique highly porous films. Considerable control over the film density and shape of the isolated columnar structure has been demonstrated,16 and a number of possible applications for the films, including embedding matrices for optically active media,17 a)
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J. Mater. Res., Vol. 17, No. 11, Nov 2002 Downloaded: 13 Mar 2015
enhanced surface areas for sensor devices,18 perforated thin films,19 and catalytic surfaces,13 have been proposed. A necessary condition for the production of GLAD films is that the incident deposition angle (defined from the substrate normal) be greater than about 80°. The term sculptured thin films (STFs) was originally introduced to describe porous films grown by GLAD.20
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