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cal behavior. For example, when a gold film has a tantalum adhesion layer between the gold and the polymer, the sample is more prone to failure, compared to just a single gold film. In the rigid electronic industry it was historically thought that gold and copper have low adhesion to polymers, and so additional layers were introduced. Dr Cordill explains: “Although this was not the case looking back to the literature of the 1980’s, it was assumed that the interlayer would still be needed in the flexible electronic industry. Our work is instead showing that these interlayers are actually damaging.” For example, gold films can buckle and delaminate from the polymer substrate. On unloading the sample, these buckles form as the polymer springs back and the film becomes detached from the substrate. As higher loads are applied and removed, the buckles increase in size, or new buckles can form. Together with the film thickness, measuring the height and width with CLSM calculates the adhesion energy of the metal and polymer interface. Figure 3 shows how the gold film with a tantalum adhesion layer delaminates from the polymer substrate. “Because we can see so many more buckles with the increased field of view compared with the AFM, I can make a lot of measurements just from one image. Actually, these buckles are too high for the AFM to measure.” Because of its higher range, the LEXT can be used to analyze a wider range of samples, and from these measurements, Dr Cordill’s research concludes that the adhesion of the gold layer is actually worsened in the presence of the tantalum adhesion layer. Summary Flexible electronic devices must withstand bending, stretching and twisting. The innovative work of Dr Cordill’s group has reinvented the design of flexible electronic devices, demonstrating how the inclusion of a tantalum adhesion layer is detrimental, leading to buckling and delamination of gold films. Shedding light on the behaviour of these devices in more detail than ever before, this discovery has been possible through coupling in situ testing with the fast and Figure 3: Measuring adhesion energy between layers. Gold film with a tantalum adhesion layer on a polyimide substrate was strained to 6% and during unloading, buckles (delaminated areas) formed. Using the Olympus LEXT OLS4100 to generate a laser image for visualization (A), and height map of buckles (B) provided height and width information, plotted in graph C. easy-to-use high-resolution imaging platform of the Olympus LEXT OLS4100. CLSM provides a fast means to generating new information, especially when compared to AFM, where a wider dynamic range means samples with varying feature heights can also be investigated. “Manufacturers can now determine when cracks form, and respond to this information,” says Dr Cordill, who goes on to conclude: “Our approach is instrumental in guiding the optimization of materials and fabrication processes, leading us towards robust, market-ready devices.” - Figure 2: In situ testing links mechanical and electrical properties. During a cyclic experiment applying 2% maximum strain with 114 cycles, the electrical resistance (R/Ro) increased by 20% over time (A), but confusingly exhibited no crack damage (B). This is explained by the behaviour over a cycle of loading/ unloading, where the polymer relaxes and hides the cracks that are otherwise visible when observed in situ (C). www.electronics-eetimes.com Electronic Engineering Times Europe October 2015 37


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