Morphological stability of copper-silver multilayer thin films at elevated temperatures
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I.
INTRODUCTION
MULTILAYER thin-film systems are of interest in a wide variety of technologies and have bilayer periods covering many orders of magnitude.[1–13] Examples range from those on a nanometer scale,[1–5] such as Fe-Cu and Cu-Ni microlaminates that exhibit enhanced magnetic properties,[6] to those several microns in size,[7,8] such as Nb-Nb3Al or Cr2Nb-Nb(Cr) multilayer composites, which are of interest for structural skins on gas turbine airfoils.[9] In the majority of applications, interface definition and stability are crucial to the performance of the multilayer structure. However, morphological instabilities can arise as the system attempts to reach a lower-energy state. For example, interfacial roughening has been observed in Cr2Nb-Nb(Cr) microlaminates following anneals at 1673 K[14] and in Ni-Ag microlaminates heated to approximately 973 K in tension.[15] The system energy may be lowered by reducing the surface area of layer interfaces, grain boundaries, and free surfaces;[16,17] by reducing strain energy from applied or residual stresses;[3,13,18–20] or by reducing the chemical energy in seeking a chemical equilibrium.[21] The evolution of surfaces and interfaces has been studied in relatively simple systems,[3,13,16–19] such as single-crystal arrays (e.g., eutectics[22]) or layers formed by interdiffusion (e.g., oxidation[21,23,24]). However, little is known regarding the specific mechanism or combination of mechanisms that affect the morphological evolution of interfaces in systems with a more complex microstructure. The limited available literature indicates that grain boundaries can play a large role H. LUDTKE KNOEDLER, formerly Graduate Student, Department of Chemical Engineering, University of California, Santa Barbara, is Senior Process Engineer, Skyworks Solutions Inc., Newbury Park, CA 91320. Contact e-mail: [email protected] G.E. LUCAS, Acting Executive Vice Chancellor, and C.G. LEVI, Vice Chair and Professor, Department of Materials Engineering, and Professor, Department of Mechanical Engineering, are with the University of California, Santa Barbara, CA 91306. Manuscript submitted August 20, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
in the morphological evolution of plate like structures with internal boundaries[16,25,26] and polycrystalline mutilayer structures.[15,27,28] However, the interplay between layer interfaces and grain boundaries in such a system and their effect on the morphological evolution of the interfaces have received little attention. This article elucidates the roles of microstructure and length scales on the breakdown kinetics of a model Cu-Ag polycrystalline multilayer structure due to capillary instability and examines the conditions leading to such layer breakdown. II.
EXPERIMENTAL APPARATUS AND PROCEDURES
The Cu-Ag microlaminates were fabricated using a dualgun electron-beam (e-beam) deposition system. The system was equipped with a substrate heater, piezoelectric quartz crystals to monitor the deposition rate and thickness of each mat
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