Buckling-delamination And Fracture Of Micro-nano Thin Metal Films On A Polymer Substrate

Thin films are so widely used in MEMS, nuclear, space, magnetic devices, lowradiation windows, aerospace and medicine, that receive attention in many fields, suchas mechanics, material science, physics, microelectronics, optics and biomedicine. Filmdamage, including buckling-delamination and fracture, will exert serious influence onreliability of device. Work was carried out to study the deformation and damagemechanisms in thin metal films on a polymer substrate, under applied load and solventenvironment. Thin metal films of different thickness were deposited on PMMAsubstrates by DC-magnetron sputtering, moreover, in-situ compressive(compressive-thermal coupled) testing was carried out under an optical microscope.Straight-sided buckling propagation (tunneling and sided) of thin films undercompression has been studied using the decohesion mechanics of compressed films.Furthermore, loading-unloading in-situ compressive experiments were conducted toCu/PMMA samples. During the unloading process, the straight-sided buckles becomeunstable and evolve to telephone-code ones. After unloaded completely, the buckles alltranslate into bubbles. The study shows that secondary buckling is connected with filmresidual stress, Poisson’s ratio of the substrate and stress ratio of components in thelongitudinal and transversal directions in the film.Estimation of interface toughness of thin films deposited on a polymer substrate isan important topic. We propose an approach combining a uniaxial compression testwith a non-linear numerical analysis for adhesion determination of thin titanium filmson polymethylmethacrylate substrates. During the test, tunneling film buckling-delamination were formed and after that, transverse film cracking across the bucklecoupled with further interface delamination occurred under increasing strain due todeformation of the substrate in longitudinal direction. A three-dimensional modelincorporating a cohesive zone along the film/substrate interface is then developed tosimulate the process of film buckling and transverse cracking concomitant withinterfacial delamination. Parameter studies of interface properties are conducted andsimulation results are compared with the experimental data. Consistency between theexperiment and the developed numerical analysis has been established, enabling theinterface adhesion parameters to be ascertained.Damage and failure process of titanium films under thermal-mechanical loadingswere investigated. The relationships between saturated crack spacing, interfacialstrength and film thickness under increasing temperature were studied. Finite element(FE) modelling was carried out to develop a deeper understanding of the stress transferand evolution mechanisms in the thin films under applied tensile load. Furthermore,with FE-modelling the effects of different geometric parameters on the stress transfer to the film from the substrate have been studied. The stress distribution and evolutionin different cracked coating segments under substrate’s deformation was elucidated.Based on the study of thin film buckling under the above physics conditions, apreliminary study under the chemical condition was also conducted. Solvent-inducedbuckling delamination of thin titanium films was directly observed. They were shownto nucleate and grow in blisters, but propagate in the form of telephone cord structures.Moreover, the observed onset, propagation and the resulting patterns of film bucklesare strongly depend on the film thickness. Observation of buckling instability suggestsa mechanism that the interface fails due to solvent diffusion, while an energyminimization conspire to generate final branched buckles.The above investigations of the deformability and failure mechanisms of thin metalfilms on polymeric substrates can complement technologies and methodologies forcurrent understanding and research in thin film mechanics.