AVS2002 Session TF-MoM: Optical Thin Films

Monday, November 4, 2002 8:20 AM in Room C-101

Monday Morning

Start	Invited?	Item
8:20 AM	Invited	TF-MoM-1 Progress in Optical Thin-Films: A Telecommunications Perspective G. Ockenfuss (JDSU/OCLI) Thin-Film filters are widely used in fiber-optic telecommunication networks. Important applications include channel and band selection, gain-flattening, and pump isolation/combining. As telecommunications networks evolve toward more spectral efficiency and higher data rates, filter manufacturers will be challenged to produce thin-film filters with better performance at a lower cost. This talk will detail the evolution of filter performance and complexity, and report on recent progress toward the development of extremely thick filters with very low stress. Such filters enable very good performance at a low cost. Some of the most significant results of this research will be presented, including the characteristics and resulting optical performance of a 94+ micrometer thick "8-skip-0 100GHz" filter for DWDM bandsplitting applications.
9:00 AM		TF-MoM-3 Hybrid Deposition of Sputtered and Evaporated Multilayer Thin Films P.M. Martin, L. Olsen, J.W. Johnston (Pacific Northwest National Laboratory); D.M. DePoy (Knolls Atomic Power Laboratory) Si:H/CaF₂ high reflectors with as many as 27 layers were deposited by a hybrid process that combined reactive magnetron sputtering and electron beam evaporation. The ultimate goal of this work was to deposit interference filters with low NIR and LWIR absorption for use in thermophotovoltaic systems. The high index Si:H layers were deposited by reactive magnetron sputtering in mixtures of Ar + H₂. Electron beam evaporation was the preferred deposition method for the CaF₂ layers.¹ The multilayer hybrid coatings were deposited in the same chamber by sequentially rotating the substrate over the Si sputtering target and the CaF₂ evaporation source. The deposition rate of the evaporated CaF₂ was 600 Å/min, compared to 25 Å/min for sputtered CaF₂. The deposition rate of the Si:H layers was 56 Å /min. The optical performance of the hybrid deposited and fully sputter-deposited coatings will be compared. The refractive index of the SiH at NIR wavelengths was 3.25, with extinction coefficient < 0.001. The refractive index of the CaF₂ in the same wavelength range was 1.34, with an extinction coefficient < 0.0001. The resulting multilayer coatings had lower physisorbed water, lower optical absorption and lower mechanical stress than those with sputtered CaF₂ layers. Advantages of this hybrid deposition method were reduced deposition times, optimum deposition process for each layer material, and improved optical and mechanical properties. This deposition method could be used for any type of multilayer optical or non-optical coating design, including but not limited to high reflectors, notch filters, beam splitters, antireflection, edge filters, polarizers, Fabrey-Perot filters, tuned absorbers, heterojunction semiconductor devices, thin film photovoltaic devices, superlattice devices, and thermoelectric devices. ¹J. D. Traylor Kruschwitz and W. T. Pawlewicz, Appl. Opt. (1997), 36(10), 2157-2159.
9:20 AM		TF-MoM-4 Electron Cyclotron Resonance Sputtering Apparatus for Optical Band-pass Filters for Wavelength Division Multiplexing Y. Jin, M. Shimada (NTT Telecommunications Energy Laboratories, Japan) We have developed a new apparatus for fabricating narrow band-pass multilayer filters for dense wavelength division multiplexing (DWDM) that uses electron cyclotron resonance (ECR) sputtering with two plasma sources. ECR sputtering with Ar/O₂ gas and a metallic target has a stable metal-mode deposition. The target is sputtered in a metallic state, and this is followed by oxide film formation, which is enhanced by ECR plasma irradiation on the substrate.¹ The ECR sputtering of metal-mode SiO₂ and Ta₂O₅ films is suitable for the fabrication of optical multilayer filters because it provides surface smoothness, low optical loss, and stability of the refractive index. These depositions were carried out at room temperature. The sputtering system consists of two ECR plasma sources, which are coupled with divided microwaves (2.45 GHz), and cylindrical sputtering targets (Si: 99.999%, Ta: 99.99%) biased by using rf-power (13.56 MHz). The deposition-thickness uniformities are ±0.65% for SiO₂ and ±0.44% for Ta₂O₅ over a 200-mm diameter. The refractive indices of the SiO₂ and Ta₂O₅ films are 1.47 and 2.10 at a wavelength of 1550 nm. This technique produces a typical surface roughness of 0.12-nm rms. To achieve precise deposition of these films, a new optical thickness monitoring system was developed. This system directly measures spectral transmission curves during film deposition using incandescent light and an infrared spectrometer, and detects the endpoint of the demanded thickness. Using this method, we fabricated a narrow band-pass filter with 43 layers on a 100-mm quartz substrate, and obtained a half bandwidth with 0.27-nm ±0.0253-nm band-pass filtering characteristics with low optical loss over the whole 100-mm diameter. ECR sputtering deposition in the metal mode is thus effective for DWDM optical band-pass filter fabrication. ¹ M. Shimada et al., Vacuum 59, 727 (2000).
9:40 AM		TF-MoM-5 A New Dual Ion Beam Sputter Deposition System for the Production of Complex Optical Telecommunication Filters D. Siegfried, C. Montcalm, R. Blacker, D. Burtner, J.D. Deakins, A. Dummer, T.A. Erguder, J. George, C. Heizer, I. Kameyama, S.M. Lee, D. Walters (Veeco Instruments, Inc.) Dual-ion-beam sputter deposition (DIBSD) is a technique used to produce various coatings for telecommunication applications in the near-infrared wavelength region (1300-1600 nm) such as anti-reflection coatings, low loss laser mirrors, dense wavelength division multiplexing filters, and gain flattening filters. This deposition technology benefits from inherently stable deposition rates and produces dense, bulk-like films that are stable in a wide range of environmental conditions. However, compared to competing technologies such as ion-assisted electron-beam evaporation or magnetron sputtering, this technology typically has lower deposition rates and limited deposition uniformity over large coating areas. These two main limitations have been addressed by developing an entirely new DIBSD system. The deposition rates in this new system are >0.5 nm/s for Ta₂O₅ and >1.0 nm/s for SiO₂ over a 300 mm diameter substrate, which are now comparable to those obtained in electron beam evaporation systems used for similar applications. The increased rates have been achieved with several process and hardware changes, while preserving the favorable optical and physical properties of the films generally produced by DIBSD. Specific examples include the use of reactive deposition processes and the development of a new high current ion source. Simultaneously, the chamber geometry was optimized to achieve uniform films, both in terms of thickness and index, while the overall stability and robustness of the process was increased by judicious placement of all critical components. Finally, improvements in the vacuum pumping system have reduced the pumpdown time by a factor of 2 and have eliminated the need for lengthy cryopump regenerations. We describe these major improvements that have led to large yields and reduced production cost for optical filters.
10:00 AM		TF-MoM-6 Optical Thin Film Formation by Oxygen Gas Cluster Ion Beam Assisted Depositions N. Toyoda, I. Yamada (Himeji Institute of Technology, Japan) High-quality Ta₂O₅/SiO₂ and Nb₂O₅/SiO₂ were deposited with oxygen gas cluster ion assisted deposition at low-temperature for optical filters. As one cluster ion has thousands of O₂ molecules, equivalently low-energy ion irradiations are realized at several keV of total acceleration energy. Due to the dense energy deposition of cluster ions, high-temperature and high-pressure conditions are realized at the impacted area, which enables to deposit high quality thin films without heating the substrate. Also, GCIB shows significant surface smoothing effects, which realizes very flat surface and interfaces for multi-layered structures. In this study, O₂-GCIB was applied to form high quality optical films. With gas cluster ion assisted deposition, high refractive index and very smooth surface of Ta₂O₅ films were deposited. The optimum cluster ion energy and cluster ion current density for Ta₂O₅ films were found to be 7keV and 0.5uA/cm², respectively. The structure of film was very uniform and no porous or columnar structures were observed. The surface or interfaces of Ta₂O₅/SiO₂ films were also very flat by surface smoothing effect of cluster ion beams. Very smooth surface can be realized even though the bottom surface was rough. There was no wavelength shift of filters after environmental tests, which indicates that dense oxide films were formed at low-temperature with O₂ cluster ion assisted deposition.
10:20 AM		TF-MoM-7 Plasma Deposited Inhomogeneous Optical Filters S. Larouche, A. Amassian, H. Szymanowski, J.E. Klemberg-Sapieha, L. Martinu (Ecole Polytechnique, Canada) Plasma enhanced chemical vapor deposition (PECVD) of inhomogeneous optical filters with a continuously varying refractive index profile (for example, rugate filters) offers a unique opportunity to combine the advantageous optical (elimination of harmonics, suppression of sidelobes), mechanical (compensation of stress, scratch- and wear resistance), and technological (high deposition rate) characteristics. In the present work, we study the optical and structural properties of mixed TiO2/SiO2 materials obtained by PECVD from TiCl4 and SiCl4 precursors. We evaluate the effect of the fabrication conditions on the refractive index, deposition rate, chemical structure and morphology at every moment of the film growth using numerous complementary techniques; this includes in-situ and ex-situ wide-range spectroscopic ellipsometry (UV-VIS-NIR-IR), spectrophotometry, FTIR, XPS, XRD, AFM, and other methods. Detailed knowledge of the microstructural evolution, combined with real-time in-situ spectroellipsometric process monitoring, is then used to fabricate model single-band and multiband optical filters in the visible and NIR regions. Optical and mechanical performance, environmental stability, and advanced applications of such filters are evaluated and discussed.
10:40 AM		TF-MoM-8 Scandium and Vanadium Multilayer Mirrors: Working towards High Reflectivity in the Extreme Ultraviolet G.A. Acosta, D.D. Allred, N.D. Webb (Brigham Young University) Despite bulk reflectivities of materials in the EUV being typically less than 7%, it is possible to design a multilayer mirror using thin films to achieve reflectivities in the vicinity of 30-40%. Inspired by the 1998 Uspenski paper (Optics Letters, Vol. 23, No.10) which theorized 72% reflectance of 42 nm light, we have been working on developing a design scheme that uses the rare earth metal scandium to achieve such high reflectivities. To characterize our thin film samples, they were studied using X-ray diffraction, atomic force microscopy, ellipsometry, X-ray photoelectron spectroscopy, and an EUV scanning monochromator. Optical constants were found experimentally over the 800-200nm range, as well as in the EUV.
11:00 AM		TF-MoM-9 Measuring Optical Losses in Thin Films with Ringdown Cavity G. Vaschenko, Y. Godwal, C.S. Menoni (Colorado State University); C. Montcalm, R. Blacker, D. Siegfried (Veeco Instruments Inc.) Optimization of multilayer optical components (such as wavelength division multiplexing (WDM) filters for fiber optics communications) often requires measuring the losses in the individual films composing these optical elements. Ion beam sputtering yields films with optical losses on the scale of parts per million (ppm), which is significantly smaller than the sensitivity threshold of conventional analytical techniques such as ellipsometry or spectrophotometry. In this contribution we demonstrate a new, highly sensitive experimental configuration for the characterization of losses in thin films on optically transparent substrates. Our method is based on synchronous pumping of the ringdown cavity by the pulses of a mode-locked Ti:Sapphire laser. The length of the ringdown cavity is selected to match that of the laser, therefore the pulse inside the ringdown cavity is significantly boosted on every roundtrip. The thin film sample grown on a transparent substrate is introduced in the cavity and aligned exactly perpendicular to the axis of the cavity, so that the light reflected from the sample interfaces is recycled and the reflection loss does not contribute to the total measured loss of the film. The losses are determined by measuring the time dependent decay of the ringdown signal. This method allows us to obtain high signal-to-noise ratio decay signals with small pulse energy and without signal integration over large number of shots. Using this technique we have detected record low single pass loss of only 64 ppm in a 2 mm fused silica substrate and 79 ppm loss in a 4 µm tantalum pentoxide film fabricated with a SPECTOR ion beam sputter system.
11:20 AM		TF-MoM-10 Efficient Infrared Emission for Zinc Sulphide: Rare Earth Doped Thin Films A.S. Kale, W. Glass, M. Davidson, P.H. Holloway (University of Florida) Infrared emitters (IR) are widely being used for a variety of applications ranging from commercial based fiber optic communication devices and industrial gas sensors to remote controls for televisions. ZnS doped rare earth fluoride thin films typically 1 micron thick have been fabricated by RF sputter deposition in the conventional metal-insulator-semiconductor-insulator-metal configuration as novel structures for emission of IR radiation. The current study investigates three different kinds of phosphors, namely, ZnS:TmF₃, ZnS:NdF₃ and ZnS:ErF₃ for their IR versus visible emission. Electroluminescence has been studied before and after annealing at temperatures up to 510°C to determine the effects on the emission properties. As annealing improves the crystallinity of the film, the emission improves. Emission spectra of the devices have been measured at wavelengths ranging from 0.35 to 1.5 microns, and the films studied for their emission efficiency and luminescent decay times. Methods of enhancing the IR with respect to the visible emission will be reported, including codoping with two or more rare earth elements, interfacial modifications and conditions of annealing.
11:40 AM		TF-MoM-11 Polycrystalline Thin Film Photovoltaics H.S. Ullal (National Renewable Energy Laboratory) Significant technological advances have occurred in the area of polycrystalline thin film photovoltaic technologies based on cadmium telluride (CdTe) and copper indium diselenide (CuInSe₂, CIS). In the past few years, the many advances made in materials research, device development, manufacturing technology, reliability testing of modules and systems, technical R&D issues relating to CdTe & CIS, and early commercialization efforts underway in the United States and worldwide are reported in this paper. A typical thin-film CdTe solar cell structure is glass/SnO₂/CdS/CdTe/contacts. A world-record, total-area efficiency of 16.5% has been achieved by scientists at NREL. One of the most active areas of research is the study of the interdiffusion at the CdS/CdTe interface, where diffusion of S in the range of 4%-12% into the CdTe absorber has been observed by SIMS technique. Other areas of research include developing new transparent conductors such as Cd₂SnO₄ to replace SnO₂2, and the study of back contact stability to develop stable thin film CdTe solar cells. A number of vacuum based thin film growth techniques will also be reported. For thin-film CIS-based solar cells, the typical cell structure is ZnO/CdS/CIS/Mo/glass. Low-cost sodalime glass is normally used for device fabrication. In addition, polymer, metal foils, and stainless substrates are being investigated as alternate substrates. The highest world-record, total-area efficiency achieved thus far by NREL scientists for thin-film CIGS solar cell is 21.5% using a concentration of ~14X by the physical vapor deposition method. The CdS is deposited by the chemical bath deposition (CBD) technique. The process is reproducible and quite robust. The Ga:In ratio is typically 30:70. Areas of research worldwide include the role of Na that diffuses from the glass into the CIGS absorber, the optimum amount of Ga which help increase the bandgap from 0.95 eV to 1.2 eV, such that, there is better match with the solar spectrum, and developing alternate buffer layers to replace CBD CdS. The major players developing thin-film CdTe module fabrication are ANTEC Solar, Germany; BP Solar (BPS), USA, First Solar, USA, and Matsushita Battery, Japan. For thin-film CIS technology, the major players are Energy Photovoltaics, USA, Global Solar Energy (GSE), USA, Honda Engineering, Japan, International Solar Electric Technology, USA, Matsushita Industry, Japan, Showa Shell, Japan, Shell Solar Industries (SSI), USA, and Wurth Solar, Germany. Multi-megawatt manufacturing facilities for module fabrication are currently underway in Germany, Japan, and USA. Early commercial products (5-40 W) of CIS are available from SSI, and GSE for military applications. Prototype thin-film CdTe Apollo module with aperture-area efficiency of 11.0% and power output of 92.5 W has been developed by BPS for commercialization in 2003. This work was supported by NREL under Contract no. DE-AC36-99G010337