The Size Effect On The Mechanical Properties Of Metal Films Adherent To Flexible Substrate

Thin metal films are widely used in the field of Micro-Electro-Mechanical Systems (MEMS) and Integrated Circuits (ICs). As the size of the device continuously scales down, the mechanical properties and the working reliability of the metal films are intensively influenced by the feature size (such as thickness, grain size). However, the size effect on the mechanical properties of metal films is still disputable. In addition, the flexible substrates are often widely used in liquid crystal displays and thin film solar cells due to their extraordinary properties. They are light, foldable, easy to transport and are able to suppress the strain localization and necking of thin films. As a result, it becomes an important technical issure in MEMS and ICs to investigate the mechanical properties of thin metal films adherent to flexible substrate.Basing on the above discussion, we chose Cu films, Cu/Ru and Cu/Ta multilayer films deposited onto polyimide (PI) substrates as the aiming materials. The microstructure of the films was characterized by XRD, TEM, SEM and FIB. The mechanical properties such as strength and ductility were measured by uniaxial tension test. The effect of size on the mechanical properties of those films was investigated and the mechanism was also discussed. The main results are summarized as follows:(1) The ductility of nanocrystalline Cu films decreased with increasing the film thickness. As the thickness of Cu films increased, the interfacial shear stress and the residual stress increased, leading to the decrement of the effective interfacial bonding strength, then the Cu films were more likely to debond from the substrate and rupture due to strain localization and necking.(2) The ductiliy of nanocrystalline Cu films decreased with increasing the strain rate. The strain rate sensitivity increased with decreasing the thickness of Cu films. At high strain rate, the micro-crack parallel to the tensile direction appeared on thicker films while the same phenomenon was not observed on thinner films. This was due to larger residual stress exsited in thicker films. The steady-state energy release rate decreased with increasing the strain rate, leading to the rupture of thin films.(3) The ductility of nanocrystalline Cu films decreased with increasing the working temperature and the working temperature dependence of ductility became stronger for thinner film. With increasing working temperature, the elastic modulus of the PI substrates decreased and the substrate was not able to supperess the strain localization, and then the ductility decreased. With the increment of working temperature, the decrement of interfacial bonding strength was larger in thinner films and thicker films were more likely to dobond, leading to a stronger temperature effect on thinner films.(4) The ductility and fracture toughness of the Cu films improved as the annealing temperature increasing. As-deposited Cu films ruptured with debonding and strain localization while as-annealed film ruptured with adhering well to the substrate. With increasing annealing temperature, the residual stress of the Cu films decreased, the microstructure was more stable, a diffusion or compand interface generated. The bonding between film and substrate became better and the strain localization was suppressed more effectively. As a result, the ductility of as-annealed Cu film was larger than that of as-deposited one.(5) The strength, ductility and fracture toughness of Cu/Ru multilayer films were all dependent on the individual thickness. With decreasing the individual thickness from200to20nm, the yield strength and the ductility both increased, the strengthening mechanism was the model basing on single dislocations slip in the confined Cu layers; with further decreasing the individual thickness from20to5nm, the strength was independent of the individual thickness, and the strengthening mechanism was the load-bearing effect. The ductility which depended on the individual thickness was due to the competition between stress intensity fields in Ru layers and the plasticity deformation in Cu layers.(6) A novel Cu/Ta multilayer films whose individual thickness was in gradient distribution were designed. The yield strength and the critical strain of the multilayer films were about2.0GPa and9.5%, respectively. High ductility was obtained in the novel Cu/Ta multilayer films without losing its strength. The high ductility was mainly because the plastic deformation of thicker Cu layers suppressed the propagation of the micro-cracks generated in thinner Cu layers and Ta layers. The design of the novel materials provided a new way to improve the strength and ductility of the multilayer films.