Khalid Alzoubi

Bending Fatigue Study of Sputtered ITO on Flexible Substrate

Recently, there has been a tremendous rise in production of portable electronic devices. To produce flexible devices, flexible substrates should replace conventional glass substrates. Indium-tin-oxide (ITO) is the preferred transparent conducting layer used in the display technology. Although ITO has excellent sheet resistance and very good optical properties, ITO can crack at very low tensile strains which might cause failure in the conductive layer because of the unusual structure of a very thin film of brittle ceramic material applied to a polymer substrate. Therefore, the mechanics of ITO on flexible substrates has been thoroughly considered in the design and manufacturing of flexible devices. In a typical roll-to-roll manufacturing process, many challenges exist during the travel of the coated web over the rollers which produce bending stresses that might cause failure even if the stresses are below the yield strength of the material. Therefore, the high cycle bending fatigue of ITO thin films on flexible substrates is of a significant importance. In this work, high cycle bending fatigue experiments were conducted on ITO coated PET substrate. The effect of bending diameter, bending frequency, and sample width on the change in electrical resistance was investigated. High magnification images were obtained to observe crack initiation and propagating in the ITO layer. The goal of this work is to establish a baseline for a comprehensive reliability study of ITO thin films on flexible substrate. It was found that bending diameters as well as the number of bending cycles have a great influence on the electrical conductivity of the ITO layer.

Stability of ITO Thin Film on Flexible Substrate Under Thermal Aging and Thermal Cycling Conditions

Indium-tin-oxide (ITO) is a transparent conductive thin film that is widely used as a top conducive layer in photovoltaic solar cells. However, ITO is sensitive to environmental conditions and the electrical conductivity degrades as a consequence of harsh environmental conditions. Furthermore, the thermal expansion coefficient mismatch between the ITO film and the substrate creates stress/strain on the film when the package is subjected to fluctuating temperatures. This could create micro-cracks and consequently damage the film. Therefore, this study was designed to study the effect of the thermal cycling and thermal aging on the ITO thin films to simulate the effect of continuous high temperatures and fluctuating temperatures that may be applied on the thin films during the usage. In this study, two sets of experiments were conducted on a 60Ω/□square sputter-deposited ITO on 127 μm heat stabilized Poly Ethylene Terephthalate (PET) substrate. The first set of experiments contained four samples which were thermally aged at 100°C for 30 days and the other set of experiments contained another four samples which were thermally cycled for 900 cycles. The thermal profile consisted of a high temperature of 100°C, a low temperature of 0°C, dwell time of 10 minutes, and ramp rate of 10°C/min , as depicted in Fig. 1. The initial results showed that the ITO thin film is not stable in the thermal aging experiment and the electrical resistivity gradually increased for all samples until the end of the 30 days. The degradation happened during the thermal cycling as well. However, SEM images show that the morphology of the ITO surface is stable under both conditions. Energy-dispersive X-ray (EDX) spectroscopy analysis showed stability in the ITO thin film in terms of composition. XRD spectra confirmed the improved crystallinity for the thermally aged films, which corresponded to the increased transmission in the visible region.

Behavior of Sputtered Indium–Tin–Oxide Thin Film on Poly-Ethylene Terephthalate Substrate Under Stretching

In this work, two different sheet resistances, 100 and 60 Ω per square of DC-Magnetron sputtered indium tin oxide (ITO) thin film on poly ethylene terephthalate (PET) substrate were stretched up to 15% of the original length under three different strain rates, 0.01, 0.1, and 1.0 min-1. The cracks development during stretching was monitored using optical microscope. Two types of cracks were observed: in the first type, cracks initiated perpendicularly to the tensile load and propagated towards the edge of the sample, which was observed at 4% strain for both sheet resistances and different strain rates. In the second type, cracks initiated from the first type cracks and propagated perpendicularly to it towards the next original crack. The cracks intensity for both types of cracks was investigated. The intensity of both cracks type increases with the strain and sheet resistance. The electrical resistance was measured every 1% strain during stretching. The relative electrical resistance change (ΔR/R0) was plotted against strain at different strain rates. It could be concluded that, the relative electrical resistance change under high strain rate is higher at the beginning of the stretching process, but after a certain strain, the relative electrical resistance change under the lower strain rate accelerated till the film becomes non-conductive while the relative electrical resistance change under high strain rate is in acceleration process until reaching non-conductive condition. Analysis of Variance (ANOVA) results showed that the strain and the sheet resistance are significant factors on the relative electrical resistance change at 95% confidence level. The strain rate was not significant factor in the range considered. No two factor interactions are significant as well.

Experimental and Analytical Studies on the High Cycle Fatigue of Thin Film Metal on PET Substrate for Flexible Electronics Applications

This paper addresses the behavior of thin-film metal coated flexible substrates under high cyclic bending fatigue loading. Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are widely used substrates in the fabrication of microelectronic devices. Factors affecting the fatigue life of thin-film coated on a flexible PET substrates were studied, including thin-film thickness, film material, bending radius, temperature, and humidity. A series of experiments for sputter-deposited copper and aluminum coated on a PET substrate were conducted. Electrical resistance and crack growth rate were monitored during the experiments at specified time intervals. In addition, a finite element model was built to simulate the bending of thin-films on flexible substrate structure. Layered shell elements were used in the model. Stress intensity and stress distribution across the film were obtained and compared with the experiments. Initial results of copper-coated PET showed a great agreement between the model and the experimental results.

Stability of Interdigitated Microelectrodes of Flexible Chemiresistor Sensors

One of the main challenges in flexible sensors is their performance degradation under different environmental conditions. In this work, high cycle bending fatigue experiments were conducted on a flexible sensor array deposited on a Polyethylene Terephthalate (PET) substrate. These sensors were designed and fabricated for detecting different types of chemical vapors. Molecularly-mediated thin film assemblies of gold nanoparticles were deposited on interdigitated microelectrodes with different line widths and spaces. The behavior of the sensor array was studied under repeated mechanical and thermal loadings. This work focuses on studying the failure modes when such devices are subjected to bending fatigue stresses, high temperature, and high humidity environments. The initial results showed that these devices were very stable under mechanical, thermal, and environmental loadings.

Effect of lamination on the bending fatigue life of copper coated PET substrate

While todays display technology is mainly based on rigid-based substrate, flexible display technology has been significantly growing since the last decade. However, Flexible displays are susceptible to many types of stresses during processing and usage. Thin films on flexible substrate are sensitive to ambient conditions. Therefore devices are usually laminated with a protective layer. In this study high cyclic bending fatigue experiments were conducted on 2000 oA thick copper thin films sputter deposited on 127 μm polyethylene terephthalate (PET) substrate laminated with another 127 μm PET layer. High magnification images were used to observe crack initiation and propagation in the thin film layer. Initial results showed a great influence of laminate layer on stress reduction in the thin film. Furthermore, a lamination layer causes cracks to spread out on a larger area with fine cracks and therefore reduce the chance of the cracks to meet and grow.