Crystallinity-induced variation of the yield strength of electroplated copper thin films

The quality of grains and grain boundaries of polycrystalline copper thin films was analyzed by using image quality (IQ) value obtained from the observed Kikuchi pattern by applying electron back-scatter diffraction (EBSD) analysis. It is considered that the IQ value strongly correlates with the order of atomic configuration in the observed area, in other words, density of various defects, and thus, the area with high IQ value was defined as the area with high crystallinity. The yield strength of a grain was measured by using micro tensile test system in a scanning electron microscope. A bicrystal structure which had two grains with different IQ values was cut from a copper thin film by using focus ion beam (FIB) and the sample was fixed to a single-crystalline silicon beam and a micro probe, respectively, by tungsten deposition. Finally it was thinned to 1µm and stretched to fracture at room temperature. In this micro tensile test, however, the tungsten deposition on the side surface of the test samples caused serious error on the measured strength. Therefore, in this study, the experimental method was improved by the development of an effective method for elimination the excess tungsten deposition. During the tensile test, a mass of plastic deformation and necking phenomenon were obviously observed. Ductile fracture always occurred in the grain with higher Schmidt factor. It was found that the yield strength of a copper grain decreased monotonically with the increase in the IQ value when the IQ value at the grain boundary was larger than 3500.