Variation of the strength and fracture mode of a grain and a grain boundary in polycrystalline copper thin films

In this study, variation of the strength of electroplated copper thin films depending on the crystallinity was quantitatively evaluated by EBSD (electron back scatter diffraction) and a micro tensile test using FIB (focused ion beam) technologies. For the crystallinity evaluation, the order of the atomic arrangement was numerically evaluated by using the IQ (image quality) value obtained from the EBSD method. For the strength evaluation, tensile strength, yield stress and CRSS (critical resolved shear stress) were measured by the micro tensile test on bicrystal specimens fabricated from electroplated copper thin films. From the results of these experiments, both the intergranular fracture and the transgranular fracture were observed in the bicrystal specimens. It was also found that there is a critical IQ value at which the fracture mode of the bicrystal specimen changes from brittle intergranular fracture at the grain boundary to ductile transgranular fracture within the grain. The intergranular strength monotonically decreases with decreasing the IQ value because the total number of atomic bonds decreases in the grain boundary with low IQ value where the lattice mismatch occurs and the atomic density is relatively low, and thus, the bonding strength between grains decreases. On the other hand, as the order of the atomic arrangement increases, dislocation movement occurs easily, so that the transgranular strength monotonically decreases as the IQ value increases. It is clarified that the strength and the fracture modes of a grain boundary and a grain drastically change as a function of crystallinity.