ICMCTF2004 Session C2/E6: Mechanical Characteristics of Optical Films

Wednesday, April 21, 2004 8:30 AM in Room California

Wednesday Morning

Start	Invited?	Item
8:30 AM	Invited	C2/E6-1 Stress and Structure Formation Upon Reactive Sputtering of Different Transition Metal Oxides M. Wuttig, J. Ngaruiya, O. Kappertz, S. Venkataraj (RWTH Aachen, Germany) Thin films of transition metal oxides find many important applications in the area of optical functional coatings. Direct current reactive magnetron sputtering continues to be the commercial method of choice at least for large area applications due to its ease in automation and scaling as well as the resulting superior film properties. Nevertheless the properties of these films very often depend on the growth conditions. Hence we have performed a comparative study on room temperature reactive sputtering deposition of eight of the transition metal oxide films. This work reveals a systematic trend not only for the stress in the different oxides but also for the structural properties. Films of ZrO₂ and HfO₂ showed by far the highest levels of compressive stresses, while other oxides had much lower stress levels. Those oxides which showed the highest stress levels typically were crystalline (oxides of group IV, i.e. TiO₂, ZrO₂ and HfO₂) while those in groups V and VI (V₂O₅, Nb₂O₅, Ta₂O₅, MoO₃ and WO₃) showed lower stress levels and were amorphous. A review of the prevailing literature provides no model to explain this observation. We hereby propose an atomistic model to explain the foregoing observations. We further use the approach to account for the re-sputtering phenomena observed during growth. The systematic trends in the film properties are attributed to the impact of energetic particles originating from the oxidized target during sputtering. This scenario is supported by the measured target characteristics and the variation in the activation energy for oxygen ion formation.
9:10 AM		C2/E6-3 Tailoring the Mechanical Properties of Optical Films on Plastic Substrates J.E. Klemberg-Sapieha, P. Jedrzejowski, S. Larouche, L. Martinu (Ecole Polytechnique de Montreal, Canada) Optical coating systems fabricated on plastic substrates are increasingly applied in advanced technological applications including displays, glazings for energy control, security devices, instrumentation optics, consumer products and others. However, optimization of the performance of inorganic optical films on polymers is rather challenging due to their incompatibility associated with a large difference in the coefficients of thermal expansion, and the nature of plastics characterized by low service temperature, low surface energy, and high content of water and contaminants. In this respect, plasma-based processes allow one to tailor the plasma-surface interactions in terms of ion bombardment, vacuum ultraviolet radiation and radical chemistry in order to control interfacial adhesion, film stress and mechanical performance. In this work, we particularly focus on the stabilization of interfaces by plasma-induced crosslinking, and by the application of optical films with a graded index. We report a substantial enhancement of adhesion, elasto-plastic and tribological properties for technologically important systems such as PECVD Si₃N₄, TiO₂, Nb₂O₅ and SiO₂ on polymethyl-methacrylate, polycarbonate and cyclopolyolefines. The mechanical properties, based on depth-sensing indentation, micro- and nanowear and microscratch measurements are related to the microstructural characteristics obtained by a multitechnique approach using XPS, AFM and spectrollipometry.
9:30 AM		C2/E6-4 Property Change in ZrN_xO_y Thin Films: Effect of the N/O Ratio F. Vaz, P. Carvalho, L. Cunha, L. Rebouta, C. Moura (Minho University, Portugal); E. Alves (ITN, Portgual); A. Cavaleiro (Coimbra University, Portgual); Ph. Goudeau, J.P. Riviere (University of Poitiers, France) The main purpose of this work consists on the preparation of single layered zirconium oxynitride, ZrN_xO_y, thin films. The presence of the oxygen allows tailoring the film properties between those of pure covalent zirconium nitride and those of the correspondent ionic oxide. Varying the oxide/nitride ratio allows tuning the band-gap, bandwidth, and crystallographic order between oxide and nitride and consequently electronic, mechanical and optical properties of the material. Films were deposited on steel, silicon and glass substrates at a constant temperature of 300°C by rf reactive magnetron sputtering. The depositions were carried out from a pure Zr target varying the process parameters such as substrate bias voltage and flow rate of reactive gases. X-ray diffraction (XRD) results revealed the occurrence of a face-centred cubic phase (ZrN type) with <111> orientation and traces of some oxide phases. The chemical composition and the microstructure strongly influence the hardness in the range from 20 to 40 GPa as determined by nanoindentation experiments. Good adhesion to the substrates was observed, with similar trends to those of the hardness. The chemical composition throughout the entire thickness was determined by Rutherford Backscattering Spectrometry (RBS). Residual stresses are compressive in the range from -0.5 to -5 GPa. Scanning electron microscopy (SEM) revealed a mixture of very dense and columnar type structures. All these results have been analysed and will be presented as a function of the deposition parameters, the chemical composition and in the structure of the films.
9:50 AM		C2/E6-5 Mechanical Modelling of Multilayer Optical Coatings S.J. Bull, E. G-Berasategui (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 many different coating architectures are possible (i.e. different layer materials, thickness and stacking orders). Testing all the possible combinations is expensive and time consuming and thus there is a need for a modelling approach which can be used to predict the mechanical behaviour of any proposed coating design. Using nanoindentation techniques the hardness behaviour of coated glass can be determined provided that the coating thickness is not too thin. Data generated by this approach can be used in modelling the hardness of a multilayer coating; an energy-based predictive model has been developed and tested for a range of coatings in glass. This paper will introduce the successful application of the model to a multilayer optical coating stack and highlights the need to incorporate the effects of fracture into the model in order that its output better matches experimental data. The potential for expanding the model to predict scratch performance will be discussed.
10:10 AM		C2/E6-6 The Properties of TiN-doped Indium Oxide Films Prepared by Pulsed Magnetron Sputtering from Powder Targets Y. Zhou, P.J. Kelly (University of Salford, United Kingdom) Abstract Mixed indium oxide and tin oxide powder targets have been used to prepare transparent semi-conductive films on glass by pulsed magnetron sputtering. The influences of dopant concentration in the target and oxygen concentration in the plasma were investigated. The structure, crystallinity, optical properties and electrical properties of the films were investigated using a range of techniques, including SEM, XRD, spectrophotometry and four-point probe. Also, the properties of the coatings were compared before and after annealing under various conditions of temperature and atmosphere. The main preferred orientations of the crystal are (100) and (400). The transparent lines of the coatings, which were deposited with additional oxygen in the plasma, showed blue shifts after annealing, with the average transmittance within the visible range being in the range 80-85 %. Also, coating resistivities of lower than 10-4 cm were obtained. The results to date demonstrate that the pulsed magnetron sputtering of Sn-doped In₂O₃ films from powder targets is a versatile, novel technique for the deposition of high quality transparent conductive oxide (TCO) materials. Due to the flexibility of this technique, preliminary experiments of multi-component indium and zinc oxide coatings doped with tin or aluminium oxide are also included here.
10:30 AM		C2/E6-7 Multi-parameter Modeling of Visco-plastic Mechanical Behavior of Polymer Thin-films by Nanoindentation and PC-based Finite Element Simulations T.C. Ovaert (University of Notre Dame); J. Wang (ABB Robotics) The nanoindentation method is utilized to characterize the mechanical properties of a wide variety of materials. In this investigation, a four-parameter visco-plastic constitutive model has been used to develop a PC-based finite element simulation of spherical (axisymmetric) indentation. The model is used in conjunction with experimental nanoindentation tests, also utilizing a spherical indenter. The parameters simultaneously account for visco-elastic and plastic response in a thin layer, and are determined by a process that matches the experimental load vs. indentation depth plot from the nanoindentation test with the load vs. indentation depth plot from the finite element simulation. Once characterized, the individual layers are then assembled into two- or three-layered structures and subject to micro-scratch tests, where a critical normal indentation load (evidenced by the appearance of visible surface cracks) is determined for the layered structures. Experimental scratch tests were carried out with three different spherical scratch tip radii. Top-layer tensile stresses of the layered structures were then estimated by a second finite element (scratching simulation) model, utilizing the individual layer properties, top layer friction coefficient, and micro-scratch test critical indentation load. The tensile stresses form a basis of correlation with abrasive wear resistance of different multi-layered coating structures. The method may be used in the design of a wide variety of multi-layered structures, such as flooring, coated optics, and automotive finishes.
10:50 AM	Invited	C2/E6-8 Use of Intrinsic Stress in Optical Films to Shape Membrane Optics C. Jenkins (South Dakota School of Mines and Technology) The emerging gossamer spacecraft technology will enable very large, ultra-lightweight systems for future space missions such as: 1) Very large telescopes for imaging extra-solar planets, studying formation of large-scale structure in the early universe, and continuously monitoring the Earth from distant vantage points. 2) Large deployable and inflatable antennas for space-based radio astronomy, high-bandwidth communications from deep space, and Earth remote sensing with radar and radiometers. 3) Solar sails for low-cost propulsion, station keeping in unstable orbits, and precursor interstellar exploration missions. 4) Large solar power collection and transmission systems for human and robotic exploration missions, and for the commercial development of space. Gossamer spacecraft are typically realized as membrane structures. Membrane structures are those structures (load-carrying artifacts or devices) comprised of highly flexible (compliant) plate or shell-like elements. This usually implies thin, low-modulus materials, such as polymer films. Membrane structures have very little inherent stiffness, and do not lend themselves well to carrying compressive loads. Thus, they are often found either in tensioned-planar or inflated-curved configurations. In this paper we present a unique approach to shaping a particular class of gossamer spacecraft, namely membrane mirrors. The intrinsic stress inherent in the optical coatings applied to the membrane surface are investigated for use in "stress stiffening" the membrane. We first introduce membrane mirror basics, and then discuss computational modeling efforts to predict the necessary coating prescription to define the mirror shape. Next, we discuss experimental methods and efforts to measure the coating stress in thin, highly-flexible membranes. We conclude with some comparison between the numerical predictions and the applied coating stress.
11:30 AM		C2/E6-10 Mechanical Properties of Photocatalytic TiO₂ Films Deposited by Cathodic Arc Plasma Ion Plating J.T. Chang, C.W. Su, J.L. He, K.C. Chen (Feng Chia University, Taiwan, R.O.C.) Cathodic arc plasma ion plating is known to be capable of producing high quality film at low deposition temperature used for protective coating. It is therefore worth of evaluation on the application to photocatalytic TiO₂ film. The success can embody the usage of soda-lime-silica glass, which would be distored after suffering high deposition temperature or post heat treatment of some other coating processes. In the study, photocatalytic titanium dioxide films were deposited on glass substrate by cathodic arc plasma ion plating technique using a pulsed-DC bias power supply. Deposition time, substrate bias voltage, and deposition atmosphere were varied to reveal the effect of these three most influential deposition parameters on structural properties of TiO₂ films. The amount of anatase phase in TiO₂ films is strongly dependent on deposition time and oxygen partial pressure. Film thickness is monotonously increased with deposition time, and decreased with increasing substrate bias voltage and oxygen partial pressure. The as-deposited TiO₂ films exhibit a higher intrinsic hardness than glass substrate leading to an overall increase in apparent hardness of the coated specimens. The scratch test results showed that the strong film adhesion can be obtained in all cases, as also revealed as conformal and tensile cracking mode of the scratch scar after test. This study demonstrates the possibility of using cathodic arc plasma ion plating technique to obtain high quality photocatalytic TiO2 films at low deposition temperature.