Mechanisms of Grain Growth in Free-Standing Nanograined Gold Thin Films
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Mechanisms of Grain Growth in Free-Standing Nanograined Gold Thin Films J. A. Gregg1; K. Hattar1; C. H. Lei2; I. M. Robertson1 1 Materials Science and Engineering, University of Illinois, 1304 W. Green St. Urbana, IL 61801 USA 2 Center for Microanalysis of Materials, University of Illinois, 104 S. Goodwin Ave. Urbana, IL 61801 USA ABSTRACT Retention of the enhanced properties reported for nanograined metallic systems requires that the nanostructure be insensitive to temperature and deformation. In situ transmission electron microscopy annealing experiments were employed to investigate the structural changes associated with the formation of micron-sized grains in nanograined evaporated gold thin films. This abnormal grain growth occurs randomly throughout the film. Twinning but not dislocation slip occurs in the growing grains until the grain size is in the hundreds of nanometer range. The twins appear to hinder growth and for grain growth to continue the twins must either be annihilated or be able to grow with the grain concurrently. INTRODUCTION The thermal and mechanical properties of nanograined metallic thin films have become of increasing importance as these films are implemented in a greater variety of applications within the microelectronics and microelectromechanical systems industries. The microstructural evolution of nanograined films over time or at elevated temperature can potentially degrade their unique properties. Grain growth is generally divided into two categories, normal and abnormal grain growth. Normal grain growth is the process wherein all grains develop at similar rates to achieve the equilibrium six sided grain structure with 120° between grain boundaries at the triple junctions[1]. This is in contrast to abnormal grain growth in which a limited number of grains grow at elevated rates resulting in a bimodal grain size distribution. Abnormal grain growth has been attributed to either preferred growth of selected grains, which is dependent on grain boundary energy[2, 3], substrate effect[4-6], and applied load[7,8], or grain boundary energy variations[8], contamination[10-12], deposition conditions[5, 13] and grain boundary grooving[14]. Twinning has been associated with texture evolution in constrained evaporated gold thin films and has been shown to be a function of both substrate and film thickness [6]. This in situ transmission electron microscope (TEM) study examines the stability and evolution of free-standing nanograined gold thin films at elevated temperatures. EXPERIMENTAL PROCEDURE Gold thin films were prepared by evaporating 99.95% pure gold onto glass slides, precleaned with a piranha (1:H2O2::3:H2SO4) solution, at a base pressure of 10-7 Torr. Free-standing 20 nm thick samples were then produced by the following procedure: coating the gold film with a polyacrylic acid (PAA) and ethanol solution, allowing the ethanol to evaporate, removing the gold-PAA film, dissolving the PAA in water, and placing the gold film on a copper TEM grid. A Dektak 3030 profilometer was u
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