ICMCTF2009 Session C1: Recent Advances in Optical Thin Films

Thursday, April 30, 2009 1:30 PM in Room Royal Palm 4-6

Thursday Afternoon

Start	Invited?	Item
1:30 PM	Invited	C1-1 Unusual Optical Thin Film Solutions Based on Advances in Materials and Deposition Processes J.A. Dobrowolski (NRC, Canada) Optical thin film systems must meet a specified optical performance, as well as satisfy a number of auxiliary specifications that are equally important. These may include stability of the optical performance over time and at high and low temperatures, mechanical robustness, chemical inertness, resistance to damage by radiation and by high power lasers. These auxiliary requirements are often much harder to meet than the specified optical performance. The spectral and angular characteristics of thin film systems depend on the number of layers and their thicknesses, on the overall thickness of the layer system and on the optical constants of the available coating and substrate materials. Examples will be given in this paper showing how the optical performance of designs has been improved in the past through the use of unusual properties of coating materials, or through the use of deposition techniques that resulted in films with effective optical constants that lie outside those that nature has provided. However, a layer system with the calculated optimum optical performance may not meet the auxiliary requirements. Also shown will be examples in which both the thin film designs and the deposition process had to be adjusted in order to meet both the auxiliary requirements and the required optical performance. The layer systems that will be used for illustration purposes will include antireflection coatings, reflectors, cut-off filters, narrow band pass filters and polarizing beam splitters and they will include examples taken from the extreme ultraviolet to the far infrared spectral regions.
2:10 PM		C1-3 Thermal Stability of SiO_xN_y Thin Films with Tailored Refraction Index: Microstructural, Chemical and Mechanical Properties V. Godinho (Instituto de Ciencia de Materiales de Sevilla-CSIC/US, Spain); C. Fernandez-Ramos (Institute for Prospective and Technological Studies-JRC European Commission; Instituto de Ciencia de Materiales de Sevilla-CSIC/US); M.C. Jimenez de Haro (Instituto de Ciencia de Materiales de Sevilla-CSIC/US); M.P. Delplancke-Ogletree (Université Libre de Bruxelles, Belgium); A. Fernandez (Instituto de Ciencia de Materiales de Sevilla, Spain) Silicon oxynitride is transparent in the visible range and the refraction index can be varied from that of pure silicon dioxide (1.47) to the one of pure silicon nitride (2.3). A continuous change in its composition allows to obtain films with different optical, electrical and also mechanical properties@super 1,2 studied theoretically the possibility of tuning the optical properties of thin films of similar composition introducing nanopores with different shape, size and spatial distribution. The introduction of pores decreases the refraction index of the coatings. In a previous work³ we showed that the formation of close porosity on SiO@sub x@N@sub y@ thin films, in the form of nano-voids (3-15 nm in size), allowed tuning the dielectric constant of silicon oxynitride films in a wide range of values without significant change in their composition. In this work we show the tuning of refraction index and focus on the thermal stability (chemic al and microstructural) of these coatings under different atmospheres considering their possible applications. Electron microscopy (SEM-FEG), elipsometry, X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), as well as nanoindentation results will be discussed demonstrating the control of the optical properties together with a good thermal stability of the amorphous thin films for optical applications up to 900°C. ¹M. I. Alayo, D. Criado, L. C. D. Gonçalves, I. Pereyra, Journal of Non-Crystalline Solids 338-340 (2004) 76-80. ²A. Navid, L. Pilon, Thin Solid Films 516 (2008) 4159-4167. ³V. Godinho, V.N. Denisov, B.N. Mavrin, N.N. Novikova, E.A. Vinogradov, V.A. Yakovlev, C. Fernández-Ramos, M.C. Jiménez de Haro, A. Fernández (submitted).
2:30 PM		C1-4 Improved Contrast and Reflectivity of Near Normal Incidence Reflective Mjltilayer Optics for FLASH and Next Generation Soft X-ray Lithography T. Tsarfati, E. Zoethout, R.W.E. van de Kruijs, F. Bijkerk (FOM Institute for Plasma Physics Rijnhuizen, Netherlands) For applications in e.g. FLASH, next generation lithography, soft x-ray spectroscopy, fluorescence analysis and imaging, we performed an experimental feasibility study of multilayer reflective optics for use in the 6.7 nm wavelength range. A B₄C/La combination offers a reflection maximum of around 40% and FWHM of ~0.06 nm. It is observed to yield chemical reactivity and LaB₆ interlayer formation at the interfaces, severely limiting optical contrast and reflection. We have now successfully passivated the individual interfaces. Simultaneously, we managed to increase optical contrast ^1,2. Extrapolations of experimental results with 50 period multilayers suggest more than 1/3 higher reflection maximum and FWHM than currently reported record values for 200 period multilayers. Calculations show that even further improvement of FWHM can be achieved by replacing B₄C with B and La with Th or U³. ¹T. Tsarfati, E. Zoethout, R. W. E. van de Kruijs, F. Bijkerk, submitted to PRL ²T. Tsarfati, E. Zoethout, E. Louis, F. Bijkerk, patent ³T. Tsarfati, E. Zoethout, R. W. E. van de Kruijs, F. Bijkerk, submitted to APL
2:50 PM	Invited	C1-5 Multi-Layer Dielectric Coatings and Processing for Production of Optical Gratings on Large Substrates D.J. Smith (Plymouth Grating Laboratory, Inc.) Laser Pulse Compression methods first use a diffraction grating to stretch a short pulse, then amplify the lengthened pulse in a conventional amplifier, and finally re-compress the pulse using a staged set of large diffraction gratings. Because the gratings are often used at high incidence angles, the required gratings are large rectangles that may be tiled together. Typical grating sizes range to 91cm x 42cm. The gratings may use a gold overcoat to obtain very high fluences or a multi-layer dielectric (MLD) mirror will be coated underneath the grating structure. Since the electric field of the final compressed laser pulse will interact strongly with air at atmospheric pressure, the compression gratings must operate in a high vacuum environment. The gratings are often made on fused silica substrates to provide better thermal stability. The MLD gratings are subjected to an aggressive acid cleaning procedure that removes traces of organic contamination to increase the laser damage threshold. Special ion-assisted deposition coating methods were developed to meet the requirements of coating uniformity, laser damage, stress deformation, and coating failure due to tensile fracture or adhesive failure during fabrication and use. The coatings use hafnia and silica in a multilayer optimized for the final grating wavelength of 1054 nm. Additionally, the coating design must not allow excessive standing waves to develop in the photoresist at the exposure wavelength of 351 nm. Other aspects of exposure, such as scattered light or reflected light from rear surfaces, and efficiency of coupling to the photoresist layer must be considered as well. An organic absorbing layer is used with the MLD and photoresist to meet these requirements to provide the best resolution for the grating structure. An RF ion source is used to make the coatings and can be adjusted to meet the stringent mechanical requirements of the coatings. An RF source is also used to etch the grating structure into the top fused silica layer. Results from some of these large gratings, and some of the pitfalls in manufacturing will be presented.
3:30 PM		C1-7 Visible Light Photocatalysis of N-Doped TiO₂ Films Prepared by Reactive Sputtering Using Air/Ar Mixtures M.-H. Chan (National Chung Hsing University, Taiwan); F.-H. Lu (National Cheng Kung University, Taiwan) Nitrogen-doped titanium dioxide thin films were prepared by reactive sputtering using air/Ar mixtures. The modification of TiO₂ for photocatalysis is desirable to effectively utilize the visible light region of the solar spectrum. Using air as a reactive gas allows the process to perform at a high base pressure (low vacuum) and could then drastically reduce the processing time. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy were used to characterize obtained thin films. XRD patterns show that the films possessed mixed anatase and rutile phases. The constituent of the films was determined by XPS. N-doped TiO₂ films exhibited absorption in the visible light region. The optical band gap of the films calculated from Tauc plots was 3.0 eV, which is lower than that of pure TiO₂. A red shift was clearly observed by introducing the N-dopant.
3:50 PM		C1-8 Low Temperature HIPIMS Deposition of AZO Coatings on Polymeric Web P.J. Kelly, P. Barker, G. West (Manchester Metropolitan University, United Kingdom); J.W. Bradley (University of Liverpool, Untied Kingdom) Functional films, particularly transparent conductive oxides (TCOs), are widely used in devices such as flat panel displays and solar cells. The coatings must have specific optical and electronic properties whilst maintaining good crystalinity, structure and low defect density. These properties are often achieved through post deposition annealing. It would be significant to industry if TCO coatings, such as aluminium-doped zinc oxide (AZO) could be readily deposited onto flexible polymeric web, rather than onto rigid glass substrates. This would provide reductions in weight and cost of the finished products, whilst also increasing throughput and efficiency by utilising roll to roll web coating technology. The thermally sensitive nature of the substrates, though, currently limits the choice of deposition process. However, HiPIMS (high power impulse magnetron sputtering) may provide a solution to this problem. Despite the very high peak powers (up to MWs) achievable in th is mode, the thermal energy flux to the substrate has been shown to be significantly reduced, compared to other magnetron sputtering processes. Furthermore, the process also produces high levels of ionisation of the target material, which offers the potential to produce high quality TCO coatings on polymeric web without the need for post deposition annealing processing. This paper, therefore, discusses the deposition of AZO coatings onto PET web through the use of HiPIMS. The coatings have been characterised in terms of their structural, optical and electrical properties, and these characteristics have been related to specific features in the HiPIMS process.
4:10 PM		C1-9 Fabrication of ZnO Thin Films by Atomic Layer Deposition Using Interrupted Flow-Rate Method C.-S. Ku (National Synchrotron Radiation Research Center, Taiwan); J.-M. Huang (National Hsinchu University of Education, Taiwan); C.-M. Lin (National Hsinchu University of Education,, Taiwan); H.-Y. Lee (National Synchrotron Radiation Research Center, Taiwan) High quality of crystalline ZnO films has been successfully grown on c-plane sapphire substrate by atomic layer deposition (ALD) technique at extremely low temperature. In this work, we obtained the ZnO thin films by ALD technique using interrupted flow-rate method at extremely low temperature with diethylzinc and D. I. water as precursors. The ALD growth window was found to be 50-90°C by using interrupted flow-rate method (IFM). It’s effectual to decrease the growth temperature about 100°C for ALD with general continuous flow rate method. X-ray reflectivity and high-resolution x-ray diffraction measurements were employed to characterize the microstructure of these films. The results show the low growth temperature result higher crystalline quality and to agree with thermodynamically blocked self-compensation processes.