Stress evolution in a Ti/Al(Si,Cu) dual layer during annealing
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Stress evolution in a Ti/Al(Si,Cu) dual layer during annealing Ola Bostrom* **, Patrice Gergaud*, Olivier Thomas* and Philippe Boivin** Laboratoire TECSEN, UMR CNRS 6122, Université d'Aix-Marseille III 13397 Marseille Cedex 20 ** STMicroelectronics 6", Rousset, France *
ABSTRACT Mechanical stress and stress evolution in interconnections may cause reliability problems in IC circuits. It is thus of great importance to understand the origin of this stress. In this paper, the stress evolution during the solid state reaction between blanket titanium and aluminum films has been studied by in-situ substrate curvature measurements. Whereas the formation of TiAl3 is expected to induce large tensile stress because of a global volume decrease of 6-8%, curvature measurements of titanium/aluminum dual layers during annealing at 450°C suggests the formation of a compressive compound. The evolution of the average force per unit width of the layer during the solid state reaction is interpreted on the basis of a phenomenological model used to describe stress evolution during silicide formation.
INTRODUCTION The plasticity of thin films has been extensively studied by numerous authors [1,2,3,4] but the effect of a solid state reaction on stress needs further investigation. In the case of a solid state reaction, such as precipitation inside a matrix, the global volume change when the reactants are replaced by the reaction product may be considered. If the result of a solid state reaction is a global volume decrease, the reaction product is assumed to be in tension. In this paper, we have studied the effect of the solid state reaction between aluminum and titanium blanket films on the average stress in the thin film system. This system has been chosen because of its importance in the microelectronic industry. The two metals are deposited at different interconnection levels and often a solid state reaction occurs during further processing. This modifies the stress-state in the metal lines and may result in reliability problems. According to the phase diagram [5], heating of a titanium/aluminum dual layer may result in numerous compounds. However, observations often reveal that TiAl3 is the only phase formed in thin film systems. This may be explained by reaction kinetics [6]. The TiAl3 phase exists in three different structures (L12, DO23 or DO22) including the stable tetragonal DO22 observed at high reaction temperatures. The formation of any of the TiAl3 phases results in a global volume decrease of 6-8%. The tensile stress evolution observed by some authors as a result of TiAl3 formation is explained by this volume change [7]. Note however that the systems studied in [7] consisted of a multiple depositions of very thin titanium layers (3at% Ti) between successive aluminum depositions and not two relatively thick ones as in the present study. In the special case of a reaction between a blanket metal film and a silicon substrate, the silicide formation is often accompanied by a global volume decrease. This should then result in a ten
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