Mohammad M. Hamasha

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.

Electrical and Optical Degradation Studies on AZO Thin Films Under Cyclic Bending Conditions

Aluminum-doped zinc oxide (AZO) thin film is a promising candidate for low-cost transparent conductive oxide applications. AZO thin films have good electrical and optical properties; however, their reliability must be carefully considered in manufacturing and usage of flexible devices such as flexible solar cells. Cyclic bending leads to both electrical and optical degradation of AZO thin films. Therefore, this paper was designed to investigate the effects of the bending frequency, bending diameter, number of bending cycles, and film thickness on the electrical and optical degradation of AZO films, which were deposited on polyimide (PI) substrates using a radio frequency-magnetron sputtering technique. The cycling was conducted on identical specimens under controlled study parameters. The design of experiments technique was used to determine the significant factors on the percentage change in electrical resistance (PCER) and the degradation of average transmittance (DAT), considering a wavelength range from 660 to 950 nm. The result shows that the PCER is very sensitive to number of bending cycles, particularly in the first 300 cycles. After 1000 cycles, the mean PCER of 425- and 600-nm AZO films are about 300% and 600%, respectively. The DAT is sensitive to number of bending cycles, bending diameter, and bending frequency. The mean DAT is about 2.8% after 1000 bending cycles.

Moisture-Induced Surface Corrosion in AZO Thin Films Formed by Atomic Layer Deposition

Aluminum-doped zinc oxide (AZO) thin film is a viable alternative to tin-doped indium oxide, the dominant transparent conducting oxide used in solar cells. The durability of the AZO thin films grown by atomic layer deposition technique, which is known to form layers with atomic layer precision, is studied. The AZO films were subjected to the harsh environmental conditions of varying temperatures and humidity, and their changes in surface morphology and conductivity are investigated. Four different combinations of temperature (100°C and 20°C) and relative humidity (100% and 20%) were used. It was found that the films exposed to the high-moisture and temperature conditions resulted in surface corrosion and lowered conductivity. However, SEM cross-sectional images showed that the bulk of the film was unaffected. The corroded surface had contaminants deposited from the measurement chamber as observed from XPS elemental analysis. Detailed phase analysis showed the presence of zinc hydroxide and zinc carbonate inside the corroded regions.

Reliability of Sputtered Aluminum Thin Film on Flexible Substrate Under High Cyclic Bending Fatigue Conditions

Aluminum thin films on flexible substrates are very popular as a back electrode in solar photovoltaic technology. However, during their manufacturing and use, the package is subject to cyclic bending, which leads to cracks in the conductive thin film and ultimately failure of the package. This paper investigates the effect of film thickness, bending diameter (BD), and number of cycles (NOC) on crack development and the percentage change in electrical resistance (PCER) of aluminum thin films under cyclic bending conditions. PCER-NOC diagrams are constructed at all considered factor-level combinations. These curves are used in comparisons between high and low levels of BD and film thickness. The Design of Experiment tool is used to investigate the effect and significance of film thickness, BD, NOC, and the interactions between them on the PCER. In this regard, all factors are found to be significant. Furthermore, thickness-NOC and BD-NOC interactions are significant, while thickness-BD interaction is not significant. Moreover, a finite element model is built to investigate the area of the highest stress on the aluminum thin film, in other words, the area with the most fatigue potential.

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.

Practitioner advice: Approximation of the cumulative density of left-sided truncated normal distribution using logistic function and its implementation in Microsoft Excel

ABSTRACT The left-sided truncated normal distribution is especially important in quality engineering where we remove the left side of the distribution, for example, due to estimation of the life of used products. Although the theoretical background of truncated normal distribution is already established, there is very little work on mathematical approximation of the probability density and the cumulative probability density functions. In this article, a high accuracy mathematical approximation of the left-sided truncated normal distribution is proposed. A full analysis of errors and recommendations for implementation using Microsoft Excel are provided at the end of this article.

Optimisation of single-queue service delivery systems using a Markovian approach

Service delivery systems, such as banks and airport checking terminals, are faced with the daunting task of processing an increasing demand of customers. At the same time, customers have to make accept, reject or reprocess decisions for the service being offered (when given a choice). Single-server systems are simple and can be utilised as preliminary models, but most organisations generally use multi-server service delivery systems, which require more complex modelling. Modelling of the systems state using Markovian and queuing models provides a more robust approach to better understand the dynamics of the service delivery system, thereby enabling improved services based on anticipated demand variations. In addition, it provides a means to ensure an effective cost minimisation and an increased profit for the service delivery organisation. Therefore, this research proposes a Markovian-based conceptual model to optimise general service delivery systems. To illustrate the model, a numerical example for both...

Multimachine Flexible Manufacturing Cell Analysis Using a Markov Chain-Based Approach

In this paper, a stochastic model is developed to analyze the performance of a flexible manufacturing cell (FMC). The FMC considered in this paper consists of a single conveyor, a single robot, and one or more machine(s). The conveyor belt delivers the working part to the robot, which loads it onto the machine. A Markov chain model is constructed for one-machine and two-machine FMCs, after which the model is generalized to an FMC with n machines. Most importantly, the model provides an estimate of the overall machine utilization and production rate for the FMC under consideration and also illustrates the effect of different operational factors on machine utilization and production rate. The results indicated that the overall machine utilization increases with conveyor belt and robot delivery rates and decreases with machine rate, as expected. However, this decrease or the increase in the overall machine utilization is sharp at low levels of each parameter (e.g., conveyor belt delivery and robot loading), but it gradually stabilizes at higher levels of the parameters. Finally, the production rate increases sharply at low levels of each parameter and gradually stabilizes at higher levels.