Megan Jo Cordill

Electrical Resistance of Metal Films on Polymer Substrates Under Tension

The coupling of electrical and mechanical property measurements for thin films is a relatively new area of research. It is particularly useful to know how electrical resistance could be altered with mechanical straining for flexible electronic applications. While coupled resistance measurements have been made in conjunction with tensile straining, a clear description of the measurement technique is rarely provided. Explained here is a four-point probe resistance measurement that has been incorporated into tensile straining grips for accurate and repeatable in situ resistance measurements. Also described is an analytical explanation of how to correct for constant resistances that do not change during the experiment. The new technique has been employed on Cu films on polyethylene terephthalate (PET) to illustrate its use and to discuss how resistance will recover after 24 h.

Microstructure and adhesion of as-deposited and annealed Cu/Ti films on polyimide

Abstract The ability to measure the adhesion energy of metal thin films on polymer substrates is important for the design of reliable flexible electronic devices. One technique is to create well-defined areas of delamination (buckles) as a consequence of lateral compressive stresses induced by tensile straining of the film–substrate system. The adhesion energy is calculated from the buckle dimensions. In order to improve the adhesion between the metal film and polymer substrate, thin adhesion layers can be incorporated. However, interdiffusion and reactions can occur between the adhesion layer and the metal film when subjected to elevated temperatures. This is detrimental for the interfacial adhesion, as will be discussed for Cu films on polyimide with a Ti interlayer subjected to annealing at 350°C.

Interface failure and adhesion measured by focused ion beam cutting of metal–polymer interfaces

New developments in flexible electronics require metal films to adhere to polymer substrates. Measuring the interfacial adhesion of these systems is challenging, requiring the formulation of new techniques and models. A strategy to measure the adhesion of Cr–polyethylene terephthalate (PET) interfaces using tensile straining and buckle formation is presented in this article. Focused ion beam cross-sectioning of the buckles reveals that the polymer substrate can locally fail, which may lead to an overestimate of adhesion. Cr–PET adhesion energy of 9.4u2009±u20091.6u2009J/m2 is determined with the present approach.

Adhesion measurement of a buried Cr interlayer on polyimide

A fundamental knowledge and understanding of the adhesion behaviour of metal–polymer systems is important as interface failure leads to a complete breakdown of flexible devices. A combination of in situ atomic force microscopy for studying topological changes and in situ synchrotron based stress measurements both during film tensile testing were used to estimate the adhesion energy of a thin bilayer film. The film systems consisted of 50–200u2009nm Cu with a 10u2009nm Cr adhesion layer on 50u2009μm thick polyimide. If the Cu film thickness is decreased to 50u2009nm the Cr interlayer starts dominating the system behaviour. An apparent transition from plastic to predominantly brittle deformation behaviour of the Cu can be observed. Then, compressive stresses in the transverse direction are high enough to cause delamination and buckling of the Cr interlayer from the substrate. This opens a new route to induce buckling of a brittle interlayer between a ductile film and a compliant substrate which is used to determine the interfacial adhesion energy.

Robust mechanical performance of chromium-coated polyethylene terephthalate over a broad range of conditions

Mechanical properties of metal films on polymer substrates are normally studied in terms of the fracture and adhesion of the film, while the properties of the polymer substrate and testing conditions are overlooked. Substrate orientation and thickness, as well as strain rate and temperature effects, are examined using Cr films deposited onto polyethylene terephthalate substrates. A faster strain rate affects only the initial fracture strain of the Cr film and not the crack and buckle spacings in the high strain condition. The substrate orientation slightly changes the average crack spacing while the substrate thickness has little effect on the cracking and buckling behaviour. Straining experiments at high temperature increased the average crack spacing and led to a change in buckling mode. The lack of sizeable changes in the mechanical behaviour over the large range of testing procedures leads to a resilient material system for flexible applications.

A Mechanical Method for Preparing TEM Samples from Brittle Films on Compliant Substrates

Abstract Preparing transmission electron microscopy (TEM) samples from thin films is technically challenging and traditional preparation routes can sometimes introduce unacceptable artefacts or even prove impossible. A novel method of preparing plan view TEM samples from thin films by a purely mechanical method is assessed. Two examples of films prepared by this route are briefly presented, a Cr film on PET and an amorphous AlxOy film on Cu. The application of this method allows for TEM analysis of the Cr film without the problems associated with a polymer such as PET disintegrating under the electron beam. For the AlxOy films it is demonstrated that this purely mechanical preparation prevents crystallisation of the film resulting from conventional ion milling preparation routes. The technique also allows for an upper bound of thickness approximation for these films.

Adhesion Thin Ductile Films Using Stressed Overlayers and Nanoindentation

Differently stressed films of tungsten on silicon dioxide have been studied to determine the interfacial fracture toughness and the Mode I fracture energy release rate of tungsten on glass. Tungsten films with a low compressive stress (less than 1GPa) had nanoindentation tests performed on them to induce buckling. Using mechanics based models and the dimensions of the buckles the fracture energy release rate and the phase angle of loading (Ψ) were calculated to be between 3.8 and 13 J/m 2 . By varying the residual stress in the film it was possible to examine regions of pure shear (Mode II) interfacial fracture as well as mixed mode interfacial fracture toughness of this system. A similar tungsten film was then used as stressed overlayer on sputtered Pt films on silicon dioxide to determine the fracture energy release rate. Nanoindentation was required to induce buckling, as the overlayer alone did not cause spontaneous buckling. The stressed overlayer method and nanoindentation were used to determine the interfacial toughness of the Pt/silica system to be 1.4 J/m 2 .

In-situ AFM and SEM Investigation of Slip Steps Evolving during Nanoindentation

Since plasticity in metallic materials is usually attributed to the motion of dislocations, the knowledge of the evolution of the dislocation structure during deformation is the key to understanding the mechanical properties on all length scales. Especially for samples in the micrometer regime, the evolution of the slip-step pattern can be directly linked to the active dislocation sources. Depending on the number of operating dislocation sources on the different slip systems (slip planes) a characteristic slip-step pattern forms at the surfaces. The collective motion of dislocations and the formation of complex dislocation structures have been studied with nanoindentation using slip-step analysis. Scanning electron micrographs can give only information on slip-step orientation and slip-step distribution with no quantitative information on their heights. The heights are needed to estimate the number of dislocations that have reached the surface. Therefore, new correlated microscopy techniques are needed to further understand the origins of plasticity.

Improved Understanding of Material Behavior using Correlative In-situ Techniques

In order to advance flexible electronic technologies it is important to study the combined electromechanical properties of thin metal films on polymers substrates under mechanical load. Ductile films and lines are an integral part of flexible electronics because they allow current flow between semiconducting islands and other operating features. When ductile films on polymer substrates are strained in tension the substrate can suppress the catastrophic failure that allows for their use in flexible electronics and sensors. However, the charge carrying ductile films must be of an optimum thickness and microstructure for the suppression of cracking to occur [1,2]. In order to improve mechanical and electrical properties of these complex material systems, more work at characterizing the processingstructure-property relationships should be performed. Studies of strained films on polymer substrates tend to emphasize only the electrical properties and thickness effects more than the role of film microstructure or deformation behavior. The microstructure of the film not only determines the mechanical behavior but also influences the electrical behavior and could be optimized if studied in connection with the mechanical behavior.

Grain Size Effects on the Adhesion of Thin Ductile Films

Grain size can be controlled by varying the process conditions used to deposit the film and post growth processing. This study examines how the grain size will impact the mechanical properties, hardness and adhesion, of thin ductile films on brittle substrates. The grain structures of copper and tin films were examined using OIM and AFM. The interfacial fracture toughness of each film was calculated using a tungsten overlayer and mechanics based models. The hardnesses of the films were correlated to the measured interfacial fracture toughness, and demonstrate that increased hardness correlates to an increased sensitivity to the Mode II component of loading.