Papers

ICMCTF2005 Session C2/E6: Mechanical Characteristics of Optical Films

Tuesday, May 3, 2005 8:30 AM in Room Royal Palm 4-6

Tuesday Morning

Start	Invited?	Item
8:30 AM		C2/E6-1 A Concept for Measurment and Design of Optical Films in Respect to their Mechanical Strength T. Chudoba (ASMEC GmbH, Germany) The design of optical coatings and film stacks is normally determined by their original optical purpose. However, they also have to meet some mechanical requirements. Therefore the design of optical films should additionally be done with respect to the mechanical strength of the whole film stack by exhausting the remaining scope for choosing film thickness, materials and deposition process. This requires firstly accurate measurements of the respective mechanical film and substrate properties and secondly an appropriate model for the optimization of the mechanical behavior. Relevant mechanical parameters are for instance Young's modulus, yield strength, tensile strength, adhesion and internal stress. A short overview about measurement capabilities and limitations will be given. Especially some new possibilities of using depth sensing indentation measurements will be explained. Then a model is introduced which allows the fast calculation of elastic stress fields and deformations in coated systems for the conditions of a surface contact or for the impact of particles. With the help of the model a mechanical optimization of film stacks is possible, provided that the required materials parameter are available. Some examples for such an approach will be given.
9:10 AM		C2/E6-3 Structural and Mechaincal Properties of ITO Films - Effect of Energetic Conditions J.E. Klemberg-Sapieha, M. Dudek, O. Zabeida, L. Martinu (Ecole Polytechnique de Montreal, Canada) Indium tin oxide (ITO) has found extensive applications in various optoelectronic devices, low emissitivity windows, transparent electrodes in flat panel displays, solar cells, touch-sensitive panels, heat mirrors, LCD, photo luminescent displays, and many others. In many of these applications there is a need to simultaneously optimize the electrical, optical, and mechanical properties of ITO films, especially in the case of flexible plastic substrates. In this work we systematically studied ITO films deposited by reactive sputtering from an indium-tin alloy target. We found that additional substrate biasing allows one to effectively control the level of stress as well as its crystalline structure and composition. Stress-free films possessing high transparency and conductivity were deposited under moderate ion bombardment. Ion-induced changes in the refractive index and extinction coefficient in the visible and near-infrared regions were revealed by spectroscopic ellipsometry and spectrophotometry. We discuss possible mechanisms related to the modification of the film properties due to the ion bombardment, plasma pulsing, and other phenomena.
9:30 AM		C2/E6-4 Improvement of Mechanical Properties of Photo-Spacer for TFT LCD J.H. Hahn (Korea Research Institute of Standards and Science, South Korea); G.S. Kim (HanKuk Aviation Unversity, South Korea); Y. Pae, H.-J. Cha (ADMS-Tech Co., Ltd., South Korea) In thin-film-transistor liquid-crystal-displays (TFT LCDs), spacers play a role of maintaining cell gap between the two glass panels facing each other. Generally, uniform sized ball or cylinder type spacers made from polymetacrylate or silica are applied by dispersing between two panels. However, with this method, it is not easy to maintain uniform cell gap, not only during panel processing but also while being used because of many problems such as inhomogeneous distribution of spacers due to partial agglomeration and gravitation and defects, especially in large sized panel. Recently, patterning method have been developed which provides fixed column spacers (photo-spacer) between two panels using photo-mask and UV irradiation. Photo-spacers have the advantage of maintaining a uniform cell gap over the entire range of LCDs. However, the drawback of photo-spacers is its mechanical properties such as rigidity (low softness and low elastic recovery rate). And reliable test method to measure the mechanical properties of single photo-spacer which is integral to develop reliable spacers is not well established. Thus, reliable test method using nanoindenter was developed after series of pre-test to measure the mechanical properties of photo-spacer such as elastic recovery rate and softness. And series of photo-spacers were prepared with different types of binders and cross-linkers to improve elastic recovery and softness and their mechanical properties were measured by developed test method. The elastic recovery of photo-spacer was most largely improved by functional cross-linker with six reactive groups showing relatively small decrease of softness than that of the photo-spacer with other types of cross-linker. Detailed analysis results of the reliability of test method and the mechanical properties of photo-spacers will be presented.
9:50 AM		C2/E6-5 Assessment of the Toughness of Thin Coatings using Nanoindentation Under Displacement Control J. Chen, S.J. Bull (University of Newcastle, United Kingdom) Modern optical coatings, such as solar control coatings, are usually designed based on their optical properties but the major failure mechanisms can be mechanical (e.g. scratch damage). Many of these coatings are multilayer structures and different coating architectures are possible (i.e. different layer materials, thickness and stacking order). Using nanoindentation techniques the hardness behaviour of an individual coating layer on glass can be determined provided that the coating thickness is not too small (typically greater than 200nm is necessary with peak test loads of a few hundred µN). At higher loads the effect of coating fracture during indentation becomes significant. In this paper the use of displacement control to assess the onset of cracks during the nanoindentation loading cycle will be discussed and compared to other techniques for crack detection (e.g. acoustic emission, AFM, etc.). In displacement control the load drops if fracture occurs and this even can be identified more precisely than the changes in displacement on the onset of fracture which occur in conventional open loop indentation cycles. The need to incorporate the fracture of the coating into any performance model for the multilayer stack will be highlighted.
10:10 AM		C2/E6-6 The Influence of Deposition Parameters on the Structure and Properties of Magnetron Sputtered Titania Coatings A.A. Onifade (Salford University, United Kingdom); P.J. Kelly (University of Salford, United Kingdom) TiO₂ films are widely used as high refractive index dielectric layers in multi-layer optical devices and functional films. The optical properties of titania films, and their stability, are critically dependent on the film structure, which in turn, is dependent on the deposition conditions used. Titania can exist in three forms, amorphous, anatase and rutile. Titania films were grown under systematically controlled conditions using variable field strength planar magnetrons. The effects of varying deposition conditions, including the magnetic field strength of the magnetrons, degree of unbalance of the magnetrons, pulse frequency, use of alternative reactive gas compositions and the use of ion sources on titania films were investigated. The influence of these process variables was determined in terms of their impact on both deposition rate and process stability. Film properties were analyzed in terms of structure, crystallinity, grain size, refractive index and extinction co-efficient. Results to date are discussed here.
10:30 AM		C2/E6-7 Optical and Mechanical Properties of Tantalum Oxynitride Thin Films Deposited by Reactive Magnetron Sputtering O. Banakh, P.-A. Steinmann, L. Dumitrescu-Buforn (University of Applied Sciences, Le Locle, Switzerland) Ta-O-N thin films (650 - 1100 nm thick) were deposited by D.C. reactive magnetron sputtering from a metallic Ta target in a mixed O₂/N₂/Ar atmosphere. The ratio between Ar and the reactive gas mixture in the plasma was 1:1, and the O₂/N₂ ratio varied between 0.08 and 1.33. The depositions were performed without substrate heating. The chemical composition of the Ta-O-N films was determined by Wavelength Dispersive X-ray spectroscopy. There is no linear dependence between oxygen-to-nitrogen ratio in plasma and that in the films. Increasing O₂ concentration in plasma results in a much faster increase of its concentration in the films, as a result of higher (as compared to nitrogen) oxygen affinity towards tantalum. Physical and chemical properties of the Ta-O-N films strongly depend on their O/N ratio. Three groups of the films can be distinguished: films with 1) low (0.07-0.44) 2) intermediate (0.78-1.76) and 3) high (13.67-25.56) O/N ratio in the films. Ta-O-N films with low O/N have a metallic character and are opaque. Their optical properties show similar behavior as that of Ta-N films, while the optical properties of the films with high O/N are identical with those of insulating Ta₂O₅. The index of refraction of the films with intermediate O/N decreases from 2.5 to 2.1 with increasing O/N. The crystallographic structure of the films was investigated by X-ray Diffraction. All the films are poorly crystallized thus exact phase identification is impossible. Film amorphization becomes more pronounced with the increasing O/N. Hardness and Young's modulus of the films were determined by nanoindentation technique. The value of hardness decreases with increasing O/N in the films and varies from 27GPa for O/N=0.07 to 6GPa for O/N=1.33, fluctuating around 12GPa for the films with intermediate O/N.
10:50 AM		C2/E6-8 Improved Properties of Optical Coatings Through Substrate Pre-Treatment G.T. West, P.J. Kelly (University of Salford, United Kingdom) Optical films were deposited onto float glass substrates following initial pre-treatment via a Hall-type ion source within a conventional magnetron sputtering system. Films of titania, and zinc oxide were deposited via reactive magnetron sputtering. Film properties were analysed using a range of techniques including AFM, XRD, spectrophotometry, scratch testing and wear resistance. Coatings deposited onto glass that was pre-treated with the ion source exhibited enhanced mechanical properties in comparison with those deposited onto untreated glass. In particular, film durability was increased substantially and surface roughness was reduced, without any detrimental effect on the optical properties of the films. The effects of ion source gas species, ion energy and exposure time were studied systematically, and optimum conditions were derived to provide maximum film property enhancement. This study has demonstrated the potential to enhance coatings via the incorporation of an ion source for pre-treatment of glass within a standard production system.