ICMCTF2014 Session F5: Coatings for Compliant Substrates

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.

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 80-120^oC. 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

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.

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.