Electroplated copper thin films have been used for thin-film interconnections and through-silicon vias (TSVs) in advanced semiconductor devices because of its low electrical resistivity and high thermal conductivity. However, as the diameter of interconnection structure is extraordinarily tiny, it has been getting harder to guarantee the long-term reliability of products because of the various degradation phenomena such as very high local Joule heating and electromigration (EM). Electroplated copper thin films often contain porous grain boundaries and the volume ratio of porous grain boundaries in the copper thin films is much larger than that in bulk copper. Since the degradation process of interconnections caused by EM is mainly dominated by the diffusion of copper atoms along porous grain boundaries, the lifetime of the interconnection components is strongly dominated by the strength of grain boundaries. Therefore, it is necessary to quantitatively evaluate the grain boundary strength of electroplated copper films for estimating the lifetime of the interconnection in order to assure the product reliability.  In this study, relationship between the strength and crystallinity of electroplated copper thin films was investigated experimentally and theoretically. The crystallinity variation of the strength of the bicrystal specimens was evaluated by applying both the micro tensile test using FIB (Focused Ion Beam) technologies and electron back-scatter diffraction (EBSD) analysis. In addition, molecular dynamics (MD) simulations were applied to analyze the deformation behavior and fracture mode of a bicrystal sample and its strength.

Micro scale bicrystal specimens of electroplated copper consisting of two grains with different crystallographic orientation were used for this evaluation. The grain boundary in the center part of the bicrystal specimen was roughly normal to the applied tensile stress. The micro tensile test indicated that the yield strength for all bicrytal specimens was almost equal to each other even when there was a difference in the IQ value between the specimens. On the other hand, the ductility significantly decreased with the decrease of IQ value around a grain boundary, in other words, the decrease of the crystallinity around the grain boundary. 

MD simulations were performed to investigate the deformation behavior for seven bicrystal structure models with different crystallographic orientation between the upper and the lower grains which reproduce the actual orientation relationship of the  the experiment specimens used for the micro tensile test. As was observed in the micro tensile tests of various bicrystal specimens, both the brittle-like intergranular cracking and ductile transgranular cracking appeared in the MD simulations. The brittle intergranular cracking tended to occur at a grain boundary which consists of grains with large difference in the crystallographic orientation, in other words, large mismatch in the lattice constant. In addition, accumulation of vacancies around a grain boundary enhanced the embrittlement of the grain boundary and decreased its strength drastically. The initially generated dislocations around the grain boundary could not diffuse into the nearby grains because vacancies around the grain boundary played a role of pinning centers of the movement of dislocations. Therefore, the deformation behavior of bicrystal specimens in which the ductility decreased with the decrease of crystallinity around a grain boundary was attributed to the higher defect density that impedes further propagation of slip initiated near the grain boundary. As a result, the intergranular cracking should start at grain boundaries with large mismatch and large density of defects such as dislocations and vacancies. Therefore, it is concluded that the accumulation of vacancies around a grain boundary is one of the dominant factors of the embrittlement of a grain boundary and thus, the drastic decrease in its strength, and therefore, the acceleration of intergranular cracking.


Crystallinity Dependence of Grain and Grain Boundary Strength of a Bicrystal Structure of Copper

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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