A New Type of Dislocation Mechanism in Ultrathin Copper Films
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A New Type of Dislocation Mechanism in Ultrathin Copper Films T. John Balk, Gerhard Dehm and Eduard Arzt Max-Planck-Institut für Metallforschung, Seestrasse 92, 70174 Stuttgart, Germany ABSTRACT In this study of thin film plasticity, the relationship between thermomechanical behavior and dislocation motion has been investigated in copper constrained by a silicon substrate. The stresstemperature behavior as determined from wafer curvature experiments has been directly compared to deformation microstructures observed during in situ thermal cycling of plan-view specimens in the transmission electron microscope. The flow stress of copper films with thicknesses ranging from 100 nm to 400 nm was found to be constant, indicating that strengthening mechanisms may be saturated in this thickness regime. Moreover, unexpected dislocation glide on a plane parallel to the film surface, which should experience no resolved shear stress, provides potential evidence for the occurrence of constrained diffusional creep in a 270 nm film. INTRODUCTION The stresses that exist in thin film metallizations can be significantly higher than those at which bulk metals yield. Moreover, the maximum stress that develops during thermal cycling increases significantly with decreasing film thickness [1]. Although the thermomechanical behavior of many thin film systems has been characterized, the exact mechanisms that control deformation are not known. In order to test and confirm existing models, e.g. the dependence of film strength on film thickness [1-3], and in order to create new models, knowledge of the contribution of dislocation-based plasticity to the deformation of thin films is necessary. Several studies have utilized transmission electron microscopy (TEM) to investigate dislocation behavior in thin films of various face-centered cubic metals, including copper [4]. For copper, it was found that dislocations undergo jerky glide and tangling as the film is cooled, producing high densities of dislocations. Moreover, the dislocation densities measured at a given temperature do not appear to increase as a result of repeated thermal cycling, indicating that the deformation microstructure in the thin film is alternately healed and regenerated during successive heating and cooling cycles [4]. In their study of 200 nm thick Al films that had been deposited on single crystalline Si coated with amorphous SiNx, Allen et al. [5] conducted in situ thermal cycling experiments in the TEM. They observed that the expansion and contraction of dislocation loops within a -oriented grain was reversible and highly repeatable, over successive heating and cooling cycles. Such observations of the deformation microstructure correlate well with thermal cycling results of thin films, which exhibit very repeatable stresstemperature curves. Finally, it has been observed that dislocations that glide within unpassivated films are often emitted from grain boundaries [4,6], presumably facilitated by the lattice discontinuities at such defects. Gao et al. [7] have proposed a
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