Stress Relaxation during Isothermal Annealing at Elevated Temperatures in Electroplated Cu Films
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Stress Relaxation during Isothermal Annealing at Elevated Temperatures in Electroplated Cu Films Soo-Jung Hwang1, 2, Young-Chang Joo1 and Junichi Koike2 1 School of Materials Science & Engineering, Seoul National University, Seoul 151-742, Korea 2 Dept. of Materials Science & Engineering, Tohoku University, Sendai 980-8579, Japan
ABSTRACT Deformation mechanisms of electroplated Cu thin films on TaN/SiO2/Si were investigated by performing isothermal annealing above 200 oC. Stress relaxation behavior during isothermal annealing was analyzed by curve fitting using exponential decay equations. During heating, fast relaxation and subsequent slow relaxation processes were observed. In contrast, during cooling, only slow relaxation process was observed. Among possible mechanisms for stress relaxation, diffusion creep was found to be the most plausible mechanism based on the obtained values of the activation energy. It was suggested that the slow relaxation process observed both in the heating and in the cooling processes was attributed to a grain-boundary diffusion creep. On the other hand, the fast relaxation process observed during heating was attributed to a surfacediffusion controlled mechanism. The surface diffusion mechanism was considered to be characteristic to Cu thin films that did not form stable surface oxide. INTRODUCTION Recently, Cu has received increasing attention because of its possible advantages as a metallization material to replace aluminum in microelectronic circuits. During fabrication processes of integrated circuits, interconnect materials are subject to many thermal cycles. Difference in thermal expansion among the various materials in the integrated circuits gives rise to a large thermal stress and may lead to stress-induced voiding and hillocks. Therefore, understanding the evolution of such stresses and controlling them is of importance for the reliability of such thin-film systems. It has been reported that the thermo-mechanical properties of Cu thin films are different from that of conventional Al films [1]. Thermal stress in Al films was found to be larger than in Cu thin films because of the presence of a surface oxide layer in Al [1]. The oxide layer can hamper deformation mechanisms in various manners. For example, dislocation motion within the film
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may be restricted by elastic interaction with the hard oxide layer [2]. Dislocation segments may be trapped at the interface between the film and the oxide layer [3]. Diffusion along grain boundaries and surface would also be restricted by the oxide layer. Since Cu does not form a stable surface oxide, not only restriction effects may be less on dislocation motion but also additional deformation mechanisms are expected in relation to surface diffusion. In this paper, the deformation mechanisms on Cu thin films were investigated in light of the effects of surface diffusion. Stress relaxation behavior during isothermal annealing was measured and analyzed using exponential decay functions. The derived relaxation parameters wer
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