Size And Constituent Effects On Mechanical Behaviors Of Metallic Multilayers

The single-/multi-layered films are widely used as essential components of high performancemicroelectronics, microelectromechanical systems and interconnect structures owing to theiroutstanding mechanical/electric properties. The deformation and fracture during themanufacture and service processes has been identified as an important factor influencingtheir reliability. The urgent demand for understanding the complex properties of thenanostructured multilayers and their constituent are both for scientific aspects and in theinterest of the engineering application. In this work, the mesoscaled mechanical/electronicproperties and the related physical mechanisms of nanocrystalline Cu films andnanostructured Cu/X (X=Cr, Nb, Zr) multilayers that typically used in microelectronicshave been investigated.Uniaxial tensile test and mechanical fatigue test was respectively performed on themesoscaled Cu thin films to systemically investigate their mechanical properties. It isrevealed from the results that the yield strength of Cu thin films as well as the fatiguebehavior (fatigue lifetime and fatigue damage) is length scale-dependent. Maxima areobserved for both the yield strength and fatigue lifetime in mesoscaled Cu thin films at somecritical dimension. At room temperature and low strain rates, the nanocrystalline Cu thinfilms exhibit the double-inverse deformation twinning behavior with respect to the normalHall-Petch grain size dependence, which is semiquantitatively explained by the classicdislocation theory and dislocation stimulated slip model.With reducing the characteristicdimensions of Cu thin films, the fatigue damage of mesoscaled Cu thin films show thetransition from dislocation glide-induced extrusions/intrusions to boundary related damagedue to the inhibition of dislocation mobility and the limited availability and activation ofdislocation sources.Uniaxial tensile test with combination of in-situ measurement the change of electricalresistance method was carried out to investigate the deformation and fracture behavior ofnanostructured Cu/X (X=Cr, Nb, Zr) multilayers. It is found that the yield strength of Cu/Xmultilayers strongly depends on the characteristic parameters (modulation period, λ andmodulation ratio, η), i.e., it increases with decreasing the modulation period or increasing themodulation ratio (or increasing the volume fraction of hard phase), accompanied with thetransition of deformation mechanism. This consists with the prediction by the confined layer slip model and the rule of mixture. Due to the constraint effects of soft/ductile phase (Cu) onhard/brittle phase (X), the nanostructured Cu/X multilayers present singularity ductility aswell as fracture toughness, and also show the transition of fracture mode from shearing toopening related to the length scale. The fracture behavior in Cu/X multilayers isquantitatively assessed using a fracture mechanism-based micromechanical model. Moreover,at a constant modulation period, Cu/X multilayers exhibit ductility scaling linearly with yieldstrength that is varied with modulation ratio. The scaling slope for Cu/X multilayers withmore interfaces is much sharper than that of ones with fewer interfaces, owing to a strongerinterface constraint caused by more interfaces.The nanoindentation test was adopted to explore the effect of interface structure ofnanostructured Cu/X (X=Cr, Nb) multilayers on hardness and modulus. It is revealed thatthe hardness is also length scale-dependent, similar to that of yield strength. Different fromthe monotonic modulation period dependence known for Cu/X multilayers with clearinterface, a maximum indentation modulus is found in the Cu/X multilayers with theinterfacial intermixing (amorphous) layer. This unusual behavior is explained by consideringthe intermixing layer-induced competing effects of the compressed out-of-plane interplanarspacing of the constituent layers and free volume.By using mechanical fatigue test associated with the in-situ measurement the change ofelectrical resistance method, the fatigue behavior of nanostructured Cu/X (X=Cr, Nb)multilayers was studied. The results show that a similar Coffin-Manson fatigue relationshipobserved commonly in bulk materials is found to be still operative in the multilayers,consisting with the results of Cu thin films. The suitable synergy between strength andductility can be achieved the maximum fatigue lifetime of thin film materials. Because thenucleation and motion of dislocations within the multilayers are strongly suppressed byincreased layer-to-layer interfaces, the fatigue damage of nanostructured Cu/X multilayers isboundary/interface dominated.The length scale-dependent electric transport properties of nanostructured Cu/X (X=Cr, Nb)multilayers were measured using four-point probe technique. Both the theoretical analysisand experimental results show that the resistivity increases with decreasing the modulationperiod. The interface structure can remarkably influence the resistivity of nanostructuredCu/X multilayers. The best combination of mechanical-electrical properties can be achievedby artificially tailoring the microstructure of multilayers.