High-reflectance, sputter-deposited aluminum alloy thin thin films for micro-electro-mechanical systems

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Title: High-reflectance, sputter-deposited aluminum alloy thin thin films for micro-electro-mechanical systems
Author: Barron, Lance
Abstract: This study comprises an investigation of highly reflective thin film for microelectro- mechanical systems (MEMS) consisting of reflective components. For these applications, the desired film system must have (1) low stress, (2) high specular reflectivity, (3) improved nano-hardness relative to pure aluminum, (4) compatibility with traditional semiconductor fabrication techniques. This study is an in depth investigation of both the specular reflectance and mechanical properties of thin film reflectors formed by low-thermal (<200°C) processing. Six different aluminum films (containing Cu, Ti, Cr) were chosen based on extensive literature research. Each film was characterized by a variety of optical, electrical, and mechanical analytical techniques to obtain data relating microstructure to the film’s reflectivity and mechanical properties. A complete dielectric function for each deposited aluminum alloy has been developed utilizing Drude-Lorentz parameters and effective media approximations (EMA) to account for film surface topography. Results show that copper solute addition generate films that maintain much of the bulk reflectance of pure aluminum while refining surface morphology to create a more specular surface consisting of smaller, more uniform grains. Electron and atomic force microscopy and x-ray diffraction studies reveal that copper inclusion into the aluminum lattice cause both a reduction in the preferred orientation of the film and change in the lattice parameter. Copper concentrations of 1.0% and above lead to spatial variation in copper content within the films, with copper precipitating out of solution. Film hardness, resistivity, and stress increase as a function of copper content, with the absolute magnitude of stress being acceptable for MEMS applications. The addition of titanium and chromium to Al-1.0%Cu films cause further microstructure refinement as well as increased stress, resistivity, and hardness. Al-2.0%Ti-1.0%Cu and Al-2.0%Cr-1.0%Cu exhibit reflectivity spectra that differ significantly in both inter- and intraband absorption from that of pure aluminum. The Drude-Lorentz/EMA dielectric function model has been successfully applied to all alloy films explaining the observed reflectivity and showing an excellent agreement between measured and Drude resistivity. This study will enable to model and predict the optical response of thin films in terms of their intrinsic and extrinsic properties
Record URI: http://hdl.handle.net/1850/5195
Date: 2005-09

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