Stress evolution in passivated thin films of Cu on silica substrates
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Stress evolution in passivated thin films of Cu on silica substrates Y-L. Shena) and S. Suresh Departments of Materials Science and Mechanical Engineering, M.I.T., Cambridge, Massachusetts 02139
M. Y. He and A. Bagchi M.I.T. – Harvard Program on Modeling of Materials, Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138
O. Kienzle and M. R¨uhle Max Planck Institut f¨ur Metallforschung, Seestrasse 92, Stuttgart, Germany
A. G. Evans M.I.T. – Harvard Program of Modeling of Materials, Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (Received 5 March 1997; accepted 28 August 1997)
Stresses supported by thin films of Cu passivated by SiOx have been measured upon thermal cycling. Very high stresses have been found, approaching 1 GPa in the thinnest (40 nm) films. Strengthening beyond yield occurs upon both cooling and heating, indicative of strong strain hardening in the Cu. The hardening continues down to at least 77 K. The yielding behavior of the Cu films has been characterized by a kinematic constitutive law, with exceptional strain hardening and a conventional temperature-dependent yield strength. The physical basis for this behavior is ascribed to confined shear bands in the Cu that induce large back stress. Transmission electron microscopy reveals aligned dislocations, which seemingly dictate the inelastic deformations in the shear bands.
I. INTRODUCTION
The residual stresses that arise in thin metal layers can be extremely large.1–8 They are sensitive to such variables as layer thickness, thermal treatment, and the presence of a passivation layer.8–10 Such large stresses motivate failure modes, particularly interface decohesion and cracking.11 It is of practical importance to measure, understand, and control these stresses. Considerable progress has been made for thin Al layers.1,5 Additional phenomena arising in Cu layers are explored in the present study. Thin Cu films on stiff, low expansion substrates (such as Si and SiO2 ) are expected to change their residual stress upon thermal cycling. As-deposited Cu usually has relatively small grains, typically much smaller than the layer thickness.9,10 Upon initial heating, grain growth occurs, until the grain size approaches the layer thickness.12,13 There is an associated reduction in the compressive residual stress caused by a diminution in grain boundary volume, as well as by a texturing that eliminates grain orientations subject to the greatest strain misfit with the substrate.12,13 This phenomenon occurs with or without passivation. At higher temperatures, there are further changes in stress. These are subject to the presence of a passivation layer. The levels reached a)
Present address: Department of Mechanical Engineering, The University of New Mexico, Albuquerque, New Mexico 87131.
1928
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 7, Jul 1998
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are determined by creep and yield
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