ICMCTF2014 Session F5: Coatings for Compliant Substrates
Monday, April 28, 2014 10:00 AM in Room Royal Palm 4-6
F5-1 Electromechanical Properties of ZNO:Al Thin Films on Polymer Substrates for Optoelectronics Applications
Dilveen Mohammed (University of Birmingham, UK); Rob Waddingham, Andrew Flewitt (University of Cambridge, UK); Stephen Kukureka (University of Birmingham, UK)
Highly-transparent, conducting Al-doped ZnO films have been widely used as transparent electrodes for optoelectronic applications including flat-panel displays, solar cells and thin film transistors (TFT). The advantages of such films in various display technologies include high visible transparency and electrical conduction and they are low cost compared with indium tin oxide (ITO). Investigations of electromechanical properties for Al-doped ZnO films are still relatively limited. However flexible optoelectronic device, in particular rollable electronics, will involve various mechanical deformations such as stretching, bending and twisting depending on their application as well as during fabrication and handling processes. Therefore, we deposited Al-doped ZnO films using RF magnetron sputtering on polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) substrates and investigated their mechanical properties using uniaxial tensile fragmentation, monotonic bending and twisting tests, coupled with in-situ optical microscopy. Changes in electrical resistance were monitored in situ. Also SEM and AFM were used to provide surface characterisation of the mechanically-tested samples. In addition, the effects of acrylic acid on the electromechanical properties of ZnO:AL /PENand ZnO:AL/PET systems were investigated under uniaxial tension, monotonic bending and twisting, in order to evaluate the stability of ZnO:Al in contact with pressure sensitive acrylic acid adhesives, which are employed in the fabrication of flexible electronics devices. Cracking and buckling delimitation failure modes were observed for all samples investigated at critical strains. Fracture behaviour of the Al-doped ZnO films was examined to further understand the failure mechanisms, which depend on the applied stress conditions. Then, a comparison of the properties of ZnO:AL/polymer systems and ITO/polymer system was performed. The results such as values of crack onset strain, critical radius of curvature and stress corrosion of films can help the understanding of such systems and aid the design of more reliable optoelectronic devices.
F5-2 Low Temperature Titanium Dioxide Diffusion Barrier Layers on PEN Using Spatial Atomic Layer Deposition
Morteza Aghaee, Philipp Maydannik (Lappeenranta University of Technology, Finland); Petri Johansson (Tampere University of Technology, Finland); Kimmo Lahtinen, David Cameron (Lappeenranta University of Technology, Finland); Jurkka Kuusipalo (Tampere University of Technology, Finland)
High performance diffusion barriers against water vapour are essential for many applications in flexible optoelectronics such as OLEDs. Spatial atomic layer deposition (SALD) is one technique which has been shown to produce high quality barriers in a roll-to-roll process using aluminium oxide layers [i] [#_edn1] . Titanium dioxide is a material which should have higher stability against long-term degradation than aluminium oxide because of its greater chemical inertness. For high throughput low temperature deposition, high vapour pressure, highly reactive precursors are necessary. Titanium chloride is typically used but has the disadvantage of some chlorine incorporation in the film and also the formation of corrosive byproducts. Titanium tetraisopropoxide (TTIP) is an alternative titanium precursor as it has a high vapour pressure at room temperature compared to other titanium metal organics.
As an initial step before SALD of TiO2 deposition, a study of conventional ALD has been carried out using TTIP and water, ozone or an oxygen plasma precursors to determine the basic process parameters and barrier properties with polyethylene naphthalate (PEN) as substrate material. Deposition took place at temperatures in the range 80-120oC. The highest growth rate (0.055 nm/cycle) and highest refractive index (2.35) films were obtained using plasma oxidation. Water and ozone oxidation gave growth rates of 0.048 and 0.012 nm/cycle and refractive indices of 2.04 and 1.8 respectively. The water vapour transmission rates were approximately 1 × 10-3 g.m-2.day-1 in tropical measuring conditions (38°C, 90% RH) for a film thickness of 45-50 nm.
Based on these results, a low pressure SALD process was carried out using a Beneq TFS200R system. Results will be presented of the performance of the SALD process in terms of speed, growth per cycle, etc. and the properties of the deposited layers will be described.
[i] [#_ednref1] Roll-to-roll ALD process for flexible electronics encapsulation applications
P. S. Maydannik, T. O. Kaariainen, K. Lahtinen, D. C. Cameron, M. Soderlund, P.Soininen, P. Johansson and J. Kuusipalo, submitted for publication
F5-3 Stretch to the Limit: Ductility of Thin Metal Films on Polymer and Elastomer Substrates
Teng Li (University of Maryland, US)
Flexible electronics is an emerging technology with an array of potential applications such as paper-like displays, printable thin-film solar cells, and electronic sensitive skins. Thin metal films deposited on polymer substrates are often used as conductors and interconnects in flexible electronics. Unlike conventional electronic devices, flexible devices are often subject to large deformation (stretches, bending and twists). The mechanical failure of the polymer-supported metal conductors under large deformation poses significant challenge to the functional reliability of flexible electronics. Existing theoretical studies often assume plane strain condition of the deformation of these polymer-supported metal conductors. In reality, however, flexible devices are often subject to large and complicated deformation. For example, the electronic sensitive skins covering the elbow of a robot experience large bi-axial stretches. To decipher the failure mechanisms of polymer-supported metal conductors under various biaxial loading conditions (i.e., different ratios of tensile strains in two in-plane directions), we perform bifurcation analysis to determine the critical tensile strain above which necking sets in the metal conductors. We consider two representative material combinations, namely, thin metal conductors on stiff plastic substrates, and thin metal conductors on compliant elastomer substrates. Also emerging from the analysis is the orientation of the necking respect to the tensile loading directions. The results quantitatively correlate the critical necking limit strain as well as the necking orientation with the mechanical properties and the thickness of the metal conductor and the polymer/elastomer substrate. These results offer understandings on the deformability of polymer-supported metal conductors in flexible electronics; therefore shed light on optimizing the material selection and structural design of deformable metal conductors to achieve better mechanical reliability of flexible electronics.
F5-5 Small Diameter Circular Ion Sources for Surface Engineering of Polymers
Frank Papa (Gencoa Ltd., US); Dermot Monaghan, Victor Bellido-Gonzalez, Robert Brown, Alex Azzopardi, Loritz Sorzabal-Bellido (Gencoa Ltd., UK)
Linear Ion sources have been used for more than 15 years as a means to plasma treat surfaces either before, during or after coating. The goals of such treatments are to improve adhesion/activate surfaces, densify coatings and to texture surfaces. Coatings can also be deposited via PACVD methods. Such linear ion sources are generally used in large area web or glass coaters, making process development costly and difficult. In order to accelerate process development, a new small diameter circular ion source has been developed. This source has similar characteristics to linear sources making processes easy to upscale to larger linear systems. Results of plasma etching, DLC deposition and surface nanotexturing on several polymer substrates will be presented.
F5-6 Mechanical Design of Organic Light Emitting Diodes on Polymer Substrates
Steve Bull (Newcastle University, UK)
In many current applications coatings are being developed which do not exist in bulk form and cannot be examined by conventional mechanical tests. This raises problems in the mechanical design of devices based on such materials as no property data is available to include in design calculations. One particular example is the semiconducting layers in organic light emitting diodes (OLEDs) which may consist of evaporated films of organic molecules. The elastic properties of these coatings are often essential for design of devices, particularly if they are deposited on compliant substrates which allow bending during manufacture or service (e.g. in flexible electronics). Whereas it is possible to make good measurements of elastic properties on stiff substrates such as silicon there are serious issues with the reliability of data from coatings on compliant substrates such as the PET used for plastic electronics. This presentation will outline the development of a simple analytic model of the extraction of the contact modulus of a coating from nanoindentation data obtained from a coating/substrate system and analyse the reliability of the data produced. The data will then be used to optimise the design of a multilayer OLED on a compliant substrate subject to bending through the development os an analytic bending model.
F5-7 On the Response of Ti-6Al-4V and Ti-6Al-7Nb Alloys to a Nitron-100 Treatment
Junia Cristina Avelar-Batista Wilson, Sarah Banfield, Jonathan Housden (Tecvac Ltd, UK); Celia Olivero, Patrick Chapon (Horiba Jobin Yvon S.A.S., France)
Titanium alloys have been widely used in automotive, biomedical and aerospace industries due to their high strength-to-weight ratio, outstanding corrosion resistance and biocompatibility. Although alpha-beta alloys such as Ti-6Al-4V (workhorse of aerospace industry) and Ti-6Al-7Nb (much used for surgical implants) exhibit a good combination of mechanical properties, their tribological properties are still limited as they tend to seize and gall when in contact with other surfaces. In this paper, the response of Ti-6Al-4V and Ti-6Al-7Nb alpha-beta alloys to a Nitron-100 treatment is investigated. This treatment, which enhances the load-bearing capacity of titanium alloys, consists of two sequential processes: plasma nitriding using a glow discharge under triode configuration and deposition of a TiN coating. Scanning electron microscopy (SEM), optical surface profilometry, Knoop microhardness measurements and glow discharge optical emission spectroscopy (GDOES) were used to characterise both titanium alloy materials prior to and after the Nitron-100 treatment. Although the Nitron-100 treatment significantly improved the load-bearing capacity of both alloys, the Ti-6Al-7Nb alloy exhibited a superior hardening response under identical processing conditions. This could be attributed to its high niobium content, a strong nitride-forming element.
F5-8 A Comparison of Nanoindentation Pile-up in Bulk Materials and Thin Films
Noushin Moharrami, Steve Bull (Newcastle University, UK)
During nanoindentation testing there are many issues that need to be considered if high quality data is to be obtained when testing both bulk and thin film materials. For soft materials, one of the main issues in determining mechanical properties based on the Oliver and Pharr method is the accuracy of the determined contact area due to the pile-up around the indenter leading to a significant increase in the contact area. During nanoindentation tests for both thin films and bulk materials, the deformation mechanisms, and therefore the governing dislocation nucleation and propagation events, are complex and hence the volume of the pile-up is not always proportional to the indentation load and its shape can vary. Therefore accurate measurement of the Young’s modulus and hardness requires the determination of the contact area using another technique such as atomic force microscopy (AFM) or scanning electron microscopy (SEM) images.
In this study, AFM images made by the indenter tip after the main indentation cycle was complete were analysed to measure the pile-up heights and widths obtained in bulk materials (copper, gold and aluminium) and the results were compared to those from their respective thin films under similar indentation conditions. It was observed that the amount of pile-up that appeared in the thin films was considerably higher than in the bulk materials. Thin films with low hardness values deposited on harder substrates show a different plastic response under the indenter. During the indentation tests, the harder substrate does not deform to the same extent as the softer deposited coating and consequently it has an extreme effect on the degree of pile-up formation for the thin film.