Constitutive Response of Passivated Copper Films: Experiments and Analyses
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Constitutive Response of Passivated Copper Films: Experiments and Analyses Y.-L. Shen and U. Ramamurty1 Department of Mechanical Engineering, University of New Mexico Albuquerque, NM 87131, U.S.A. 1 Department of Metallurgy, Indian Institute of Science Bangalore -560 012, INDIA ABSTRACT The constitutive behavior of passivated copper films is studied. Stresses in copper films of thickness ranging from 1000 nm to 40 nm, passivated with silicon oxide on a quartz or silicon substrate, were measured using the curvature method. The thermal cycling spans a temperature range from −196 to 600°C. It is seen that the strong relaxation at high temperatures normally found in unpassivated films is nonexistent for passivated films. The copper film did not show any rate-dependent effect over a range of heating/cooling rate from 5 to 25°C/min. Further analyses showed that significant strain hardening exists during the course of thermal loading. In particular, the measured stress−temperature response can only be fitted with a kinematic hardening model, if a simple constitutive law within the continuum plasticity framework is to be used. The analytic procedures for extracting the film properties are presented. Implications to stress modeling of copper interconnects in actual devices are discussed. INTRODUCTION This study concerns the temperature dependent elastic−plastic response of passivated copper (Cu) films within the continuum framework. It has been illustrated that passivated Cu films behave very differently from their unpassivated counterpart [1-4]. This is in direct contrast with aluminum (Al) films. In the present work we seek to explore the mechanical properties of passivated Cu films in more detail. The Cu films, sandwiched between a substrate and a thin passivation layer, are subject to temperature excursions. Special attention is given to the plastic hardening characteristics during cyclic loading. Results obtained from this study can be used for stress modeling in Cu interconnects. EXPERIMENTAL PROCEDURES The experiments were performed with electron beam deposited films on quartz substrates. The Cu films with thicknesses of 400 nm, 250 nm and 40 nm were passivated with a silicon oxide layer having a thickness 1/5 that of Cu. A 15 nm-thick chromium interlayer is applied between the Cu film and adjacent materials to serve as diffusion barrier. After thermally cycled to a stabilized condition, the Cu films showed columnar grains with an average grain size approximately equal to the film thickness. Many annealing twins and dislocations were also existent [5]. In addition to the above specimens, 1000 nmthick Cu films were also studied. The films were sputter deposited on oxidized silicon wafers and passivated with 50 nm silicon oxide. In this case the interlayer is 50 nm-thick tantalum. The standard curvature measurement technique was employed for extracting the evolution of stress in the Cu film during thermal cycles. Unless otherwise stated, a nominal heating/cooling rate of 10 °C/min was used. The temperature range conside
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