Multiple cracking events in metal bi-layers on polymer substrates

Metal films on polymer substrates are used in a variety of applications such as flexible electronics, sensors, medical devices and aerospace including multilayer insulators and surface mirrors on satellites. A common way to assess the mechanical behavior of metal-polymer systems is with fragmentation testing, which strains the system under uniaxial tension. During straining cracks or localized deformation (necks) develop perpendicular to the loading direction and buckle delaminations occur parallel to the loading. From the crack spacing the fracture behavior can be determined and the interface adhesion energy can be measured from the buckles. Fragmentation testing has been used on single and multilayer films and has shown that brittle adhesion layers next to the substrate, can cause brittle cracking of normally ductile overlying films. A similar fracture behavior was observed here for the Inconel-Ag-Teflon system, but in this system, the top 30 nm Inconel film is the brittle layer inducing brittle cracking of the underlying 150 nm Ag film. Inconel acts as a corrosion protection for the Ag layer in surface mirrors on satellites in low earth orbit, where the material should not develop cracks upon mechanical loading. Observation of the Inconel surface during in-situ tensile straining revealed crack formation in the Inconel layer at less than 1% strain, which continues with increasing strain (primary cracks). At approximately 3% strain, the primary cracks in the Inconel overcoat act as stress concentrators and generate through thickness cracks in the Ag film (secondary cracks). The primary Inconel cracks had a saturation spacing of 1.5 µm, while the secondary Inconel-Ag saturation crack spacing was much larger at 12 µm. In-situ fragmentation experiments performed through the transparent Teflon substrate revealed only the secondary through thickness cracks and cross-sectional focused ion beam characterization provides further evidence for the two-stage cracking behavior. Using the shear lag model the interfacial shear stresses of the Inconel and Inconel-Ag layers were determined from the saturation crack spacings and observed fracture strains. These results further illustrate that brittle layers at any position are detrimental to the functionality of multi-layered metal-polymer systems and should be carefully considered for any application. Please click Additional Files below to see the full abstract

Evolution of thickness dependent buckle geometries

Interfaces determine the overall reliability of multi-material components since they have to bear the distinct physical and chemical properties of the different adhering materials. In microelectronic applications, where several materials are implemented at small length scales, the main interest is on identifying the weakest interface, since it dictates the overall reliability of the implemented packages. The focus of the present study is set on a multi-layer stack composed of a rigid Si substrate with dielectric borophosphosilicate glass (BPSG) and a thin TiW film acting as adhesion promoter and diffusion barrier to the copper film, which are finally covered with 6 µm of polyimide (PI). Of main interest is a thorough characterization of the delamination of the various interfaces, which allow for a better understanding of the adhesion and the stress states present in the complex material stack. As a first step to study the interfacial behaviour, a peeling test was carried out to reveal the weakest interfaces resulting in three different delamination zones. Zone 1 delaminated at the BPSG-TiW interface and Zone 2 delaminated at the copper-PI interface (Fig 1a). An intermediate Zone 3 (Fig 1a) was identified, where straight buckles formed in the Cu-TiW layer parallel to the peeling direction at the TiW-BPSG interface (Fig 1b). Using these Zone 3 delaminations, the evolution of the buckle shape as a function of film thickness and layer stress was investigated using atomic force microscopy and X-ray diffraction. Of great interest is that with the Cu layer the buckles have a straight geometry (Fig 1b) indicating an isotropic stress. However, when the Cu layer is removed with chemical etching, the buckle morphology changes to a telephone cord geometry (Fig. 1c), maintaining the outer boundaries from the previous straight buckles shape. The change in geometry could be due to the change in film stress from isotropic to biaxial as well as the fact that the out of plane plasticity is constrained while the copper film is present. Both topics will be further discussed along with how the interfacial adhesion measurements may also be influenced by the change in buckle geometry. Please click Additional Files below to see the full abstract

Role of film microstructure on interface stability: in-situ and ex-situ investigations

Thin film adhesion is an important measure to quantify the stability of thin film – substrate interfaces. Film thickness, film microstructure, residual stresses as well as chemical reactions at the interface determine this value and it is often difficult to decouple each individual factor to study their influence on interface adhesion. In the following study the role of grain size on the interface stability was investigated for a model system at constant film thickness with comparable residual stresses. Therefore, 100 nm thin Cu films were sputtered on glass substrates. The film microstructure was tuned by a change of Argon pressure during deposition and by isothermal heat treatments post-deposition. To quantify the adhesion of the obtained Cu films, 500 nm thick, highly stressed Mo overlayers were deposited on the films leading to a spontaneous delamination at the Cu-glass interface in the shape of straight and telephone cord buckles. The model of Hutchinson & Suo could then be extended to a bilayer problem, allowing to determine adhesion for each Cu-glass system. The small grained films revealed improved adhesion compared to the large grained films. Detailed characterizations of the Cu film microstructures as well as the particular interfaces were conducted by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Finally, to study the plasticity mechanisms upon delamination, cyclic bending experiments were conducted in-situ in the TEM to observe the crack propagation towards the Cu-glass interface as a function of the film microstructure. Please click Additional Files below to see the full abstract

Buckling-induced delamination: Connection between mode-mixity and Dundurs parameters

Modern electronics, micromechanical devices and applications demanding high reliability to weight or cost ratio consist of various combinations of multilayered thin films on rigid and compliant substrates, whereas the used materials can differ in their mechanical properties. In recent years, differences in the elastic moduli and Poisson’s ratios of such structures are becoming more pronounced. Therefore, a strong push to investigate interface stability with a more in-depth view on the elastic material properties mismatch influence is needed. Measurements of the adhesion of thin films on different substrate materials can be easily performed by the spontaneous buckling method described by Hutchinson and Suo. However, the original approach assumes several simplifications. One is to omit the changes of the influence of the elastic mismatch between the thin film and substrate on the basis of small variations in then-used materials, which is not true for modern materials combinations with vastly different elastic properties. The elastic mismatch on the interface between two different materials can be described by the Dundurs parameters. In this work, finite element modelling is combined with analytical solutions according to general description of the original model to extend the usability of the Hutchinson and Suo method for use with more different materials with higher accuracy. Obtained results point out the fact that disregarding the Dundurs parameters introduces significant errors in evaluating adhesion energy in relation to loading mode, proving the necessity to properly include elastic mismatch

Estimation of the in-situ elastic constants of wood pulp fibers in freely dried paper via AFM experiments

Atomic force microscopy-based nanoindentation (AFM-NI) enables characterization of the basic mechanical properties of wood pulp fibers in conditions representative of the state inside a paper sheet. Determination of the mechanical properties under different loads is critical for the success of increasingly advanced computational models to understand, predict and improve the behavior of paper and paperboard. Here, AFM-NI was used to indent fibers transverse to and along the longitudinal axis of the fiber. Indentation moduli and hardness were obtained for relative humidity from 25 % to 75 %. The hardness and the indentation modulus exhibit moisture dependency, decreasing by 75 % and 50 %, respectively, over the range tested. The determined indentation moduli were combined with previous work to estimate the longitudinal and transverse elastic modulus of the fiber wall. Due to the relatively low indentation moduli, the elastic constants are also low compared to values obtained via single fiber testing

Modeling of friction-induced deformation and microstructures.

Frictional contact results in surface and subsurface damage that could influence the performance, aging, and reliability of moving mechanical assemblies. Changes in surface roughness, hardness, grain size and texture often occur during the initial run-in period, resulting in the evolution of subsurface layers with characteristic microstructural features that are different from those of the bulk. The objective of this LDRD funded research was to model friction-induced microstructures. In order to accomplish this objective, novel experimental techniques were developed to make friction measurements on single crystal surfaces along specific crystallographic surfaces. Focused ion beam techniques were used to prepare cross-sections of wear scars, and electron backscattered diffraction (EBSD) and TEM to understand the deformation, orientation changes, and recrystallization that are associated with sliding wear. The extent of subsurface deformation and the coefficient of friction were strongly dependent on the crystal orientation. These experimental observations and insights were used to develop and validate phenomenological models. A phenomenological model was developed to elucidate the relationships between deformation, microstructure formation, and friction during wear. The contact mechanics problem was described by well-known mathematical solutions for the stresses during sliding friction. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provided an estimate of the characteristic microstructural feature size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material provided a mechanism for lubrication, and model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to lubrication were in good qualitative agreement with experimental evidence. A nanomechanics-based approach has been developed for characterizing the mechanical response of wear surfaces. Coatings are often required to mitigate friction and wear. Amongst other factors, plastic deformation of the substrate determines the coating-substrate interface reliability. Finite element modeling has been applied to predict the plastic deformation for the specific case of diamond-like carbon (DLC) coated Ni alloy substrates

Electro-Mechanical Testing of Conductive Materials Used in Flexible Electronics

The use of flexible electronics has increased in recent years. In order to have robust and long lasting flexible displays and sensors, the combined electro-mechanical behavior needs to be assessed. The most common method to determine electrical and mechanical behavior of conductive thin films used in flexible electronics is the fragmentation test, or uniaxial tensile straining of the film and substrate. When performed in situ fracture and deformation behavior can be determined. The use of in situ electrical resistance measurements can be informative about the crack onset strain of brittle layers, such as transparent conductors, or the stretchability of metal interconnects. The combination of in situ electrical measurements with in situ X-ray or confocal laser scanning microscopy can provide even more information about the failure mechanisms of the material systems. Lattice strains and stresses can be measured with X-rays, while cracking and buckle delaminations can be studied with confocal laser scanning microscopy. These new combinations of in situ methods will be discussed as well as methods to quantify interfacial properties of conductive thin films on polymer substrates. The combined techniques provide valuable correlated electrical and mechanical data needed to understand failure mechanisms in flexible devices

Materials Engineering for Flexible Metallic Thin Film Applications

More and more flexible, bendable, and stretchable sensors and displays are becoming a reality. While complex engineering and fabrication methods exist to manufacture flexible thin film systems, materials engineering through advanced metallic thin film deposition methods can also be utilized to create robust and long-lasting flexible devices. In this review, materials engineering concepts as well as electro-mechanical testing aspects will be discussed for metallic films. Through the use of residual stress, film thickness, or microstructure tailoring, all controlled by the film deposition parameters, long-lasting flexible film systems in terms of increased fracture or deformation strains, electrical or mechanical reliability, can be generated. These topics, as well as concrete examples, will be discussed. One objective of this work is to provide a toolbox with sustainable and scalable methods to create robust metal thin films for flexible, bendable, and stretchable applications