In-Situ Observation and Mechanical Criterion on Interface Cracking in Nano-Components

Abstract

ABSTRACTWe have investigated the criterion of interfacial crack initiation in nanometer-scale components (nano-components) by means of a loading facility built in a transmission electron microscope (TEM). Three types of experiments are conducted in this project. (1) In order to clarify the applicability of conventional continuum mechanics to the nano-components, we prepare cantilever specimens with different size, which introduce different stress fields, containing an interface between a 20 nm-thick copper (Cu) thin film and a silicon (Si) substrate. These demonstrate the validity of the “stress” criterion even for the nano-scale fracture. (2) In order to examine the effect of microscopic structure on the mechanical property, we fabricate a bending specimen in the nano-scale with thin Cu bi-crystal (the thickness of about 100 nm) formed on Si substrate, of which understructure can be observed in situ by means of a TEM during the mechanical experiment. The initial plastic deformation takes place near the interface edge in a grain with a high critical resolved shear stress and expands preferentially in the grain. Then, the plasticity appears near the between Cu grain boundary and Cu/Si interface, and this development brings about the interfacial cracking from the junction. These indicate the governing influence of understructure on the mechanical property in the nano-components. (3) In order to investigate the fatigue behavior of metal in a nano-component, a cyclic bending experiment is carried out using nano-cantilever specimens with a 20 nm-thick Cu constrained by highly rigid materials (Si and SiN). The high strain region is in the size of 20-40 nm near the interface edge. The specimen breaks along the Cu/Si interface before the maximum load under the fatigue loading. The load-displacement curve shows nonlinear behavior and a distinct hysteresis loop, indicating plasticity in the Cu film. Reverse yielding appearing after the 2nd cycle suggests the development of a cyclic substructure in the Cu film. These indicate that the crack is caused by characteristic understructure owing to fatigue cycles.