Microstructural influence on the cyclic electro mechanical behaviour of ductile films on polymer substrates

When ductile metal films on compliant polymer substrates are strained in tension catastrophic failure can be suppressed by the substrate, thus allowing 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. Studies of strained films on polymer substrates tend to have more emphasis on the electrical properties and thickness effects than on the film microstructure or deformation behaviour. To address both the electrical degradation and deformation behaviour of metal films supported by polymer substrates two types of combined electro-mechanical in-situ tests were performed. First, is a combination of in-situ resistance measurements with in-situ confocal scanning laser microscopy imaging of the film surface during cycling. The 4 point probe resistance measurements allow for the examination of the changes in resistance with strain, while the surface imaging permits the visualization of extrusion and crack formation. Second, is the combination of in-situ resistance with in-situ X-ray diffraction measurements of the film stresses during cycling. The combination of electrical measurements, surface imaging, and stress measurements allow for a complete picture of electromechanical behaviour needed for the improvement and future success of flexible electronic devices

Annealing effects on the film stress and adhesion of tungsten titanium barrier layers

Tungsten titanium WTi alloys are important barrier materials in microelectronic devices. Thus the adhesion of WTi to silicate glass substrates influences the reliability of these devices. One factor that affects the adhesion of barrier layers are thermal treatments during and after fabrication. To address the impact of annealing, WTi films deposited on silicate glass substrates were subjected to different annealing treatments. The stress development in the WTi film has been monitored with wafer curvature and X ray diffraction. Quantitative measurements of the adhesion energies were performed using scratch testing to induce interface delamination. Imaging with atomic force microscopy provided the dimensions of the buckles to quantify adhesion energies. Focused ion beam crosssections were used to verify the failing interfaces and to inspect any deformation in the film and the substrate caused by scratch testing. It was found that as the annealing duration increased, the residual compressive stresses in the film and the adhesion energy increase

Mechanical and optical degradation of flexible optical solar reflectors during simulated low earth orbit thermal cycling

Multilayer thin film systems on flexible polymer substrates are used as flexible optical solar reflectors or thermal insulation of satellites and spacecraft. During one year of operation, a satellite in low earth orbit typically encounters 6000 thermal cycles of ±100 °C. Due to the different coefficients of thermal expansion between the individual layers and the substrate it is important to investigate the thermo-mechanical stability of the multilayers as a function of the cyclic heat load. Scanning electron microscopy and focused ion beam cross-sectioning revealed that Inconel-Ag bilayers on fluorinated ethylene propylene (FEP) substrate severely degrade during thermal cycling of ±150 °C in a gaseous N2 atmosphere. After only 100 cycles through thickness cracks and subsurface voids in the Ag layer form as a result of equi-biaxial thermal stresses caused by the large difference in thermal expansion between film and substrate. Transmission Kikuchi Diffraction (TKD) before and after thermal cycling also revealed grain growth and twin widening in the Ag layer. Cracking and void formation are detrimental to application relevant material properties including corrosion protection (Inconel) and reflectivity (Ag). Reflectance measurements revealed that the amount of reflected energy as well as the reflection mode (specular vs. diffuse) significantly change during the first 100 cycles. Saturation of reflection characteristics was observed after 25 cycles, which correlates to a turning point in the evolution of Ag voids. Results of this study indicate that special focus should be directed towards thermal stress control (Δα) and tailoring of the metal-polymer interface to improve resistance of versatile metal-polymer systems against thermal cycling. © 2020 IA

Ductile film delamination from compliant substrates using hard overlayers

AbstractFlexible electronic devices call for copper and gold metal films to adhere well to polymer substrates. Measuring the interfacial adhesion of these material systems is often challenging, requiring the formulation of different techniques and models. Presented here is a strategy to induce well defined areas of delamination to measure the adhesion of copper films on polyimide substrates. The technique utilizes a stressed overlayer and tensile straining to cause buckle formation. The described method allows one to examine the effects of thin adhesion layers used to improve the adhesion of flexible systems

Improved fracture resistance of Cu Mo bilayers with thickness tailoring

The fracture toughness of Mo in Cu Mo bilayers on polyimide was assessed with in situ X ray diffraction during uniaxial tensile straining. The fracture resistance of Mo acting as an adhesion layer greatly depends on the thickness of the Cu layer exhibiting a toughening effect with increasing Cu layer thickness. In contrast, the presence of the Mo interlayer greatly decreases the apparent KIc of the Cu layers. The quantification of KIc for Mo with a Cu top layer provides further evidence that when brittle layers are used, a thicker ductile layer is advantageous to create fracture resistant stretchable system