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Test & Measurement In situ testing of flexible electronics: an alternative to AFM TBy Markus Fabich he electronics industry is rapidly moving towards flexible, lightweight devices, and there are many factors to consider in designs that are both robust with a long life expectancy. Manufacturing electronic devices on deformable polymer substrates is the key characteristic of these technologies, and one approach to design is overlaying a thin layer of a conducting material on the polymer. Studying the electro-mechanical behaviour of thin films on polymer substrates for advanced flexible thin film electronic applications is a focus of Dr Megan Cordill’s work at the Erich Schmid Institute for Materials Science of the Austrian Academy of Sciences and the Department of Material Physics at the University of Leoben (Austria). Focus on thin film behavior One research area of Dr Cordill’s group is the mechanical behaviour of ductile materials such as copper or gold under strain. Understanding the relationship of electrical and mechanical properties of thin film devices under strain provides unprecedented insights into their behaviour, and Dr Cordill’s approach of coupling a mechanical straining stage with high-resolution observation allows in situ analysis of both. “We want to study how the microstructure of ductile films affects their electrical and mechanical behaviour during mechanical loading, reflecting the strength and electrical resistance of the film, as well as its adhesion to the substrate,” she comments. “We are particularly interested in the different layers – how they interact when they are stretched or bent. Too many different layers degrades both the electrical and mechanical behaviour.” Interestingly, observing the sample after unloading masks any cracks, as the polymer substrate relaxes to compress the overlaying film, and the sample may appear unaffected. Holding the sample for longer reveals any cracks, and therefore in situ testing is vital for visualizing deformation. With the ability to measure delamination alongside electrical and mechanical properties in situ, CLSM with the Olympus LEXT OLS4100 provides a comprehensive picture of thin film behaviour under loading conditions. Electromechanical testing of ductile films in situ The mechanical straining stage device is controlled automatically for loading or unloading at a certain rate. Through coupling the device to the CLSM system, this allows the measurement of sample displacement and the electrical resistance in parallel, and is therefore termed the ‘in situ squared’ method by Dr Cordill’s group. Cycles of straining replicate the fatigue the device would suffer outside of the laboratory. With the viscoelastic recovery of the polymer, after the sample is unloaded, it has a much lower resistance than it did at the maximum, and it is important Figure 1: ‘In situ squared’ straining system with CLSM. Load is applied automatically to a pre-determined value with known rate, and sample displacement and electrical resistance is measured. With the space of the Olympus LEXT OLS4100 stage, the standard 100x magnification working objectives can be used. to consider this alongside the maximum resistance - see figure 2. “Linking electrical and mechanical behaviour is something we are able to do a lot better now using CLSM and our testing device. We can stop at certain levels of strain, including a hold period of three minutes before acquiring the images.” This has proved beneficial in a range of studies. Dr Cordill’s group has found CLSM provides information previously generated with the AFM, linking crack evolution as a function of strain. “With the AFM, we could only look at a small area – about 20-25 microns. Since the difference between a crack and a neck might only be five microns, you’re looking at 4-5 cracks per image, which doesn’t provide meaningful statistics.” There is also the consideration of speed, and the AFM is slower, with one experiment taking one or two days. “Compared to AFM I would say it takes half the time to learn how to run our CLSM system. Anyone can use an AFM, but to get high quality images, you need more experience, and it can be hard to inspect the same area each time. With CLSM it is much easier to get it right the first time, as image acquisition is straightforward and intuitive.” Layers – less is more From the research of Dr Cordill’s group, it is becoming increasingly clear that in certain film systems with certain microstructures, adhesion layers can prove detrimental to electromechani- Markus Fabich is Product and Application Specialist for Materials Science Microscopy at Olympus SE & CO. KG – www.olympus-europa.com – He can be reached at Markus.Fabich@Olympus-Europa.com 36 Electronic Engineering Times Europe October 2015 www.electronics-eetimes.com


EETE OCT 2015
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