Structural Control of Lithium Fluoride Thin Films
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Structural Control of Lithium Fluoride Thin Films O.G. Yazicigil, D. Rafik†, V. Vorontsov† and A.H. King School of Materials Engineering Purdue University, West Lafayette, IN 47907-2044 † also at Department of Materials Science & Engineering Imperial College, London, England
ABSTRACT Polycrystalline lithium fluoride thin films have a number of existing and potential uses, but the optimization of their microstructure has not yet been addressed systematically. We have developed a means of measuring the porosity in LiF films, and a method for performing detailed electron-microscopical studies on this normally beamsensitive material. These techniques have been applied to assess the structure of LiF films immediately after deposition from the vapor phase, and also after subsequent annealing.
INTRODUCTION Lithium Fluoride thin films are often used as an "electron photoresist," and they have many other potential uses because of their unique optical and electrical properties [1]. A barrier to the application of LiF thin films, however, is the poor understanding of how the film structure is affected by the processing conditions. Structural “zone models” are widely used to describe how the structure of metallic thin films is affected by their processing [2,3,4] and Kaiser et al. [5] have attempted to apply a similar description to LiF films, but were only able to use C/Pt replicas of the film surfaces to study in the transmission electron microscope (TEM), presumably because of the beam-sensitivity of LiF. Nunzio et al. [6] have used scanning electron microscopy to study the microstructures of LiF films, and they report structures that differ significantly from those reported by Kaiser et al. In this paper, we demonstrate a technique for the direct study of LiF in the TEM, and use it in conjunction with atomic force microscopy (AFM) to elucidate the structures of LiF films deposited on unheated substrates, and also to study their microstrucural development on annealing. MATERIALS and METHODS LiF films were prepared by thermal evaporation onto amorphous 50nm thick Si3N4 membrane substrates, and also onto glass microscope slides. A Quartz Crystal Thickness Monitor was used to measure the thickness of the films. Films of nominal thickness 30 nm, 50 nm and 90 nm, were deposited on unheated substrates for TEM analysis, and additional LiF films of thicknesses of 100 nm and 600 nm were grown only on glass substrates. The films were studied in the as deposited condition, and also after 250°C anneals of 10 minutes and 6 hours duration. The films were annealed in a controlled atmosphere of Ar-5%H2 flowing approximately at
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100sccm, with a heating ramp rate of 1000°C/h followed by cooling to room temperature at a rate of 600°C/hr. A Digital Instruments Multimode Atomic Force Microscope (AFM) was used in tapping mode to study the surface topography of LiF thin films deposited onto glass substrates. Film thickness measurements were made with the AFM. A JEOL JEM 2000FX Electron Microscope, equipped with a LaB6 cathode,
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