Plasticity-Related Phenomena in Metallic Films on Substrates
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Plasticity-Related Phenomena in Metallic Films on Substrates M. Legros1, G. Dehm2, T.J. Balk2, E. Arzt2, J. R., O. Bostrom3, P. Gergaud3, O. Thomas3, B. Kaouache4 1 CEMES-CNRS, 29 rue J. Marvig, 31055 Toulouse -France 2 MPI für Metallforschung, Heisenbergstr. 3, 70569 Stuttgart - Germany 3 TECSEN Laboratory, Faculté des Sciences de St Jerôme, 13397 Marseille -France 4 LPM, Ecole des Mines, Parc de Saurupt, 54042 Nancy, France ABSTRACT. Plastic deformation due to thermal stresses has been investigated for different metallic films deposited on Si or α-alumina substrates. We conducted post-mortem TEM and SEM investigations of samples that underwent thermal cycles in order to capture the microstructural changes imposed by thermal stresses. The ultimate goal is to determine the dominant plasticity mechanisms responsible for such changes. In-situ thermal cycles performed inside the TEM allowed direct and real-time observations of dislocation behaviour under stress. It is shown that dislocation density drops in Al/Si, Au/Si and in Cu/α-alumina thin film systems. Except in the case of pseudo-epitaxial Cu on sapphire, the interaction of dislocations with the interfaces (passivation, oxide, adhesion layer) is attractive and leads to the disappearance of interfacial dislocations. In this light, the generalized observation of high tensile stresses that arise in metallic films at the end of cooling is explained in terms of insufficient dislocation sources instead of classic strain hardening. Diffusional processes can substitute for a lack of dislocation, but the low relaxation strain rate that would be excpected should lead to high stresses during the cooling stages of thermal cycles.
INTRODUCTION There are sometimes more than 100 different materials in modern integrated circuits. Many of them are chosen for their electric or magnetic capabilities, but a growing number serve as interfacial material to prevent undesired interaction between active compounds. Diffusion of Cu into Si is one of the many examples which the microelectronic industry had to overcome to maintain the integrity of its processors. Among all the materials science puzzles to be solved by engineers and researchers, the control of stress in these structures becomes a major issue. Mismatched coefficients of thermal expansion (CTE) are one of the main sources of stress when assembled dielectrics, semiconductors and metallizations have to go through annealing or high temperature processes [1]. Having a lower yield stress, the metallic connections are often the first element to endure plastic deformation, even if the size reduction extends their elastic regime towards higher stresses [2]. A now common way to investigate thermal stresses in metallic films deposited onto rigid substrates is to measure, using laser profilometry, the curvature imposed by the film to the substrate. In the case of films that are much thinner than the substrate, Stoney's equation [3] simplifies to:
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