ICMCTF2011 Session F6: Coatings for Compliant Substrates

Functional films underpin many electronic and opto-electronic devices, including flat panel displays, electrochromic coatings, OLED’s, image sensors, thin film photovoltaic solar cells, etc. Increasingly, manufacturers of such devices are looking to move away from rigid glass substrates to flexible polymers to reduce the weight and cost of the devices and to increase their durability. Further substantial gains in productivity would be made, and new markets opened up, if the devices could be deposited directly onto polymeric web in very large throughput roll-to-roll coaters. In all cases, though, device performance depends on the quality of the individual layers and the coating/substrate system as a whole. Of the deposition techniques available, the newly introduced HIPIMS (high power impulse magnetron sputtering) technique is attracting attention for these types of applications due to the low thermal load at the substrate and highly ionised metal flux observed in this mode.

In recent years, significant advances have been made in the analysis of mechanical and tribological properties at the nano-scale. Widely used functional films, such as TiN, ZrN, TiO₂, Al₂O₃ and AZO (aluminium doped zinc oxide) deposited on rigid substrates (tool steel, glass, etc.) have been characterised using nano-indentation, nano-scratch and nano-impact testing. However, much less is known about the suitability of these techniques for analysing coatings deposited onto compliant substrates, and it is recognised that for ‘hard’ coatings deposited onto polymeric substrates the coating/substrate system is very different in nature and the mechanisms of failure may also be very different. In the work reported here we are, therefore, seeking to extend the analysis of the mechanical and tribological properties of functional ceramic coatings to include those deposited onto compliant substrates. We have investigated a range of metal nitride and oxide coatings deposited by HIPIMS (and other sputtering processes) onto commonly available polymer substrates. The effects of film thickness, elasto-plastic properties of the film and the substrate, etc. on failure mechanisms are considered and comparisons are made with the same coating materials deposited onto rigid substrates.

The objective of the present work is to study the co-deformation of composite exhibiting strong mechanical contrast components: nanostructured metallic thin film deposited onto polymeric substrate. The mechanical characterization of such structures and the relationship with the microstructure still required for further understanding both from fundamental and technological applications. In order to mimic the stress field of thin films in actual applications, we used a biaxial tensile device dedicated to the DiffAbs beamline at the French synchrotron SOLEIL. The machine allows for controlling equi- or non-equi-biaxial loading onto thin films supported onto compliant substrates, e.g. polyimide substrates. The applied strains are measured in situ both by X-ray diffraction (XRD) and by digital image correlation (DIC). Those two methods are non-destructive: XRD strain is related to the shift of Bragg peaks and DIC strain is obtained from photography of the surface of the sample. Metallic thin films are produced using Physical Vapor Deposition technique onto the center of a cruciform-shaped polyimide substrate (Kapton ®). The film-substrate composites are then deformed in the Diffabs X-ray goniometer. The mechanical modeling using homogenization methods is specific for thin films; it has to take into account both the crystallographic texture and morphology of the metallic films. But grain interaction model is very simple in the case a perfect local elastic isotropy such as W. This thin film allows for directly comparing microstrains (measured at the microscopic scale by X-ray diffraction) to macrostrains (measured at the macroscopic scale by DIC). The first results concerning W thin films in situ deformed with this biaxial tensile modulus will be presented.

The objective of the present work is to study the deformation of metallic thin films deposited on polyimide substrates. The mechanical characterization of such structures and the relationship with the microstructure are still required for further understanding both from fundamental and technological applications. Noticeably, it is important to know how deforms the film during continuous testing of the composite film/substrate. This behaviour depends on the adhesion between the film and the substrate.

Metallic thin films (Au, Ni) are produced using Physical Vapor Deposition technique on polyimide substrates (Kapton®). For a few years, we develop in situ tensile tests combined with synchrotron x-ray diffraction [1]. In this study, the film-substrate composites were deformed in the D2AM X-ray goniometer (European Synchrotron Radiation Facility (ESRF), Grenoble (FRANCE)). Strain analysis was achieved during continuous mechanical testing using a 2D detector that allows recording Debye rings every few seconds [2]. Moreover, the semi-crystalline structure of polyimide was probed during the mechanical testing.

First results show the co-deformation (in the elastic and elasto-plastic regime) of thin film and Kapton® substrate.

[1] D. Faurie, O. Castelnau, R. Brenner, P.-O. Renault, E.Le Bourhis, PH. Goudeau, Journal of Applied Crystallography 42, (2009)

[2] G. Geandier, P.-O. Renault,E. Le Bourhis, PH. Goudeau, D. Faurie, C. Le Bourlot, PH. Djemia, O. Castelnau, S. M. Cherif, Applied Physics Letters 96, 041905 (2010)

In this research, paraffin wax is employed in passivation layer of the bottom gate amorphous indium–gallium–zinc oxide thin-film transistors (a-IGZO TFTs), and it is formed by coating in the atmosphere. Comparing with passivation-free a-IGZO TFTs, the threshold voltage (Vth) and subthreshold swing (SS) of paraffin wax passivation a-IGZO TFTs are improved. In the generalized case, positive gate bias stress can induce hump characteristic phenomenon. The hump phenomenon can be relieved after device passivation with a paraffin wax layer. In addition, the effects of bias stress on the passivation-free and paraffin wax passivation layer a-IGZO TFTs are investigated by capacitance-voltage and current-voltage measure. The hump characteristic in transfer curve depends on the drain voltage after positive bias stress, inferring that the a-IGZO backchannel appears with the drain-induced depletion electrical field.