Eexperimental And Nnumericalanalysis Of Bifurcation Of Nano-scale Metal Films Under Tension

The flexible electronic products have a lot of thin film components and film interconnects. The flexible electronic products in use process will be stretched, curled and folded repeatedly. Taking film/substrate as an essential structure, it is necessary to research the deformation bifurcation and failure of the films under tension. It also helps to optimize film device and raise the service life and reliability.A tensile loading appliance suitable for films based on in-situ observation under the microscope is designed. The appliance is made of three parts:central align loading part by opposite screw, compliant hinge loading frame with force sensor and displacement sensor. The compliant hinge loading frame with force sensor use elliptical hinge and is made into a whole with force sensor. The frame can convert compressive load into tensile load. The compliant hinge loading frame with force sensor can eliminate clearance caused by hinge or screw during the transmission and reduce experimental space. The appliance has advantages of simple structure, compact conformation, low cost and easy operation. It is able to tackle difficult clamp, situ observation, small load increment and small strain. The tensile device can meet tensile experiments for thin film/substrate structure.The different thickness Cu films and Al films were sputtered on PET substrates by magnetron sputtering technique. The bifurcation evolutionary process of metal films under tension is observed in situ under the optical microscope. The experiments indicate that the metal films under tension are experienced three stages:uniform deformation, the microcracks initiation and propagation, microcracks saturation. The critical strain began to increase and then decrease with increasing the thickness of the films. Both Cu films and Al films under tension exist obvious size dependent phenomena and optimum thickness. The critical strain of Cu film is smaller than Al film at the same condition. The microcracks spacing will vary linearly with film thickness after microcracks saturation. The interfacial shear strength is agreement with critical strain. The elastic modulus is independent of the thickness of Cu film when the thin film structures are continuous film. The elastic modulus of nano-scale Cu films is114.6GPa, the result is equal to a calculation of the modulus based on the measured texture and single-crystal elastic constants. And the elastic modulus of Al films is smaller than aluminum block, about28.5GPa. Moreover, mechanical properties of PET substrate affected critical strain and crack speed of film:the crack speed of film on larger strain hardening rate substrate towards was fast.Based on the power hardening materials, two-dimensional simulation on the tensile bifurcation phenomenon of the film bonded substrate is done by the finite element. According to match the hardening relation between film and substrate, the simulation indicated that there were three types of tensile behavior:a single neck deformation, multiple necks deformation and deformation uniformly. The hardening match relation decide bifurcation mode under the well-bonded interface condition. This situation of multiple necks can be formed only in such a manner that weakly hardening thin film is bonded the steeply hardening substrate, and the hardening stress in the film is larger than that in the substrate. Three tensile deformation modes can mutually transform with decrease of interfacial stiffness:deformation uniformly mode can transform into multiple necks deformation, and further transform into the single neck deformation. The position of other defects around the nonuniform deformation can alter the wavelength of bifurcation, neck numbers and critical strain. The necks are periodic and the secondary imperfection makes the film form secondary necks, which is usually regarded as an important reason to cause new cracks between cracks.