Papers

ICMCTF2008 Session C1: Recent Advances in Optical Thin Films

Friday, May 2, 2008 8:00 AM in Room Royal Palm 1-3

Friday Morning

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8:00 AM		C1-1 Optical Coatings in Aerospace: Requirements, Performance and Limitations A.F. Stewart (Boeing Directed Energy Systems) Optical coatings perform a variety of functions in aerospace. Coatings serve to reduce or enhance reflection from surfaces with performance enhanced for specific spectral bands. Applications may include enhancement of power generation in solar cells, enhanced throughput in optical systems, and control of surface reflectance or emittance. In these systems, optical coatings are enabling technology that provides value, efficiency and capability. The performance of coatings in these applications depends on many factors. The operational environment determines external factors which may interact with the coating. At the most basic level, particulate contamination leads to obscuration and a loss of functionality in many applications. Coating materials may react with one another or with materials which migrate onto the surface. These are the most common factors which must be managed. In space, the radiation environment and atomic oxygen are present and present greater challenges. In most applications, the rate of degradation of optical coatings may be predicted based on known factors in the environment. Recent results from optical materials on the Materials on the International Space Station Experiment (MISSE) will be discussed. The surprising result from these experiments is the good condition of the coatings and the residual functionality this implies. As one example, aerospace applications for optical coatings in space must be carefully designed to address the influence of these environmental factors and their possible impact on functionality during a mission. Many of these same issues apply directly to performance and function of optical coatings in terrestrial applications.
8:40 AM		C1-3 Effects of Interfacial Properties and Residual Stress in SiC/Si₃N₄ Multilayer Stacks on Mirror Replication M.Y. Chen, L.E. Matson (Air Force Research Laboratory); H.D. Lee, T.I. Mah, R. Bhattacharya (UES, Inc) The fabrication of light weight mirror assemblage via replication technique offers a great potential to eliminate the high cost associated with the grinding and polishing steps of conventional glass or SiC mirrors. A replicating mandrel is polished to an inverse figure shape, which then coated with an appropriate reflective layer and the optical membrane and finally reinforced with a substructure before replication. Successful replication of the entire coating stack depends on how one controls the film separation at the desired location. In the current study, the Al reflective layer was deposited on the semi-conductor grade silicon wafer using e-beam evaporation. The multilayer stacks of SiC/Si₃N₄ deposition using RF magnetron sputtering was followed on the Al reflective layer. The Sebastian pull tests were performed to measure the bond strength between the reflective layer and the silicon wafer. The deposition induced residual stress and the introduction of various release layers on the silicon wafer was examined and the effects of them will be discussed.
9:00 AM		C1-4 Maintenance and Use of Robust Reflectance Standards in the EUV L. Strein, D.D. Allred, N. Brimhall, J.B. Muhlestein, S.B. Mitchell, R.S. Turley (Brigham Young University) We describe maintaining standards useful in VUV/EUV reflectance characterization namely SiO₂ /Si and Si/Ru ultrathin bilayer films. Such optical standards are useful for astronomy and lithography applications. We discuss the necessary criteria for practical, robust standards for wavelengths from about 8 to 60 nm (or 20 to 150 eV). Results from XPS and ellipsometry are presented for cleaning procedures to remove carbon and oxidation contamination while minimizing overall surface roughness via various solvents along with exposure to Resonance Ltd. excimer VUV lamp (172nm, 7.2 eV) irradiation. Thickness and surface composition data is presented for the effect of various solvents along in conjunction with the effect of varying the exposure time to the lamp. This data is determined using a Surface Physics SSX-100 ESCA Spectrometer and a JA Woollam (M-2000D) spectroscopic ellipsometer. Storage options (wet versus dry), the effect of repeated cleaning and other maintenance procedures for the standards are set forth. Such procedures are important for those who will use the standards in EUV measurements of unknown materials.
9:20 AM		C1-7 Infrared-Reflecting Thin Film Coatings A.N. Ranade, M.E. Graham, Y.W. Chung (Northwestern University) The heating of vehicles such as cars and planes is directly dependent on the absorption of solar radiation. Approximately half of the energy that is incident upon vehicles can be attributed to near infrared (IR) radiation of wavelengths ranging from 700-2500 nm. Reflecting instead of absorbing IR radiation would lower cabin temperatures considerably, thereby improving passenger comfort and fuel economy. An inorganic coating that is transparent to visible, absorbing to UV, and reflecting to IR radiation is desired to meet these goals. Substitutionally doping a TiO₂ matrix with a cation of a higher valence than titanium produces a film that acts as a plasma with specific electromagnetic properties. By controlling the dopant concentration, one can tune the wavelength at which electromagnetic radiation is reflected. This paper discusses the properties of TiO₂ films with varying substitutional dopant concentrations. The films are made by reactive magnetron sputtering and evaluated by AFM (surface roughness), XRD (structure), and UV-VIS-IR spectroscopy (optical properties).
9:40 AM		C1-8 Irreversible Effects of Annealing on Magnetron-Sputtered Tantalum Pentoxide Films A. Bergeron, R.B. Sargent (JDSU) Thin films of tantalum pentoxide produced by magnetron sputtering were annealed at temperatures between 200°C and 500°C. The annealing process was found to induce a small irreversible increase in the optical thickness of the films. The physical thickness and refractive index of annealed layers were characterized, and the optical thickness increase was found to be largely attributable to an increase in the physical thickness of the films. A relaxation model based on a spectrum of activation energies was developed to describe the optical thickness changes as a function of time and temperature exposure. The model accurately describes the behavior of tantalum oxide films subjected to a wide range of time and temperature annealing sequences.
10:00 AM		C1-10 Recent Progress in Modification of Film Structure and Optical and Electrical Properties in rf-Plasma-Assisted Pulsed Sputtering E. Kusano (Kanazawa Institute of Technology, Japan) Modification of film structure and properties in rf-plasma-assisted pulsed magnetron sputtering has been one of the interesting issues in the fields of optical coatings or electronic coatings. In this presentation, structure modification observed for Ti, TiN, and Ta₂O₅ films deposited by dc, pulsed, rf-plasma-assisted-dc, and rf-plasma-assisted pulsed sputtering is discussed. By cross-sectional observation by scanning electron microscopy, it was clearly shown that structure of Ti and Ta₂O₅ films changed from voided to dense columnar and that structure of TiN films changed from columnar to dense granular by pulsing or assisting the cathode plasma. In addition smoothening of film surface was observed by atomic force microscopy for the films deposited by rf-plasma-assisted pulsed sputtering. Normal reflectance of Ti films deposited by rf-plasma-assisted pulsed sputtering was higher than those of films deposited by dc or pulsed sputtering. The increase in the energy transferred to the surface of a growing film in rf-plasma-assisted dc or pulsed sputtering was observed by energy analysis of ions using an energy-resolved mass spectrometer. Increases in plasma potential and ion fraction in the flux arriving to substrate resulted in the increase in the energy transferred to the substrate. It should be emphasized that by the rf-plasma assisting to the pulsed cathode plasma the energy transferred to the substrate increase by two orders or more compared to that for conventional dc sputtering. The combination of rf-assisting and pulsing in sputtering plasma is a promising technology to modify film structure.
10:40 AM		C1-9 Enhanced Formation of Er³⁺-Yb³⁺ Codoped Al₂O₃ Thin Films by a Non-Aqueous Sol-Gel Method Z.H. Zhu, M.K. Lei (Dalian University of Technology, China) 1 mol%Er³⁺-10 mol%Yb³⁺ codoped Al₂O₃ thin films on thermally oxidized SiO₂/Si(110) substrates have been prepared by dip-coating process from a polyvinglpyrrolidone (PVP) assisted sol in non-aqueous sol-gel method under an environment composed of isopropanol and ethylalcohol, as drying control physical additives (DCPA). The viscosity of the PVP assisted sol increases from 2.3 mPa s to 31.5 mPa s with a molar ratio of PVP: Al(OC₃H₇) up to 1:1, corresponding to increase of the thickness of the sol film by about one order of magnitude for a dip-coating cycle relative to the original sol, which is calculated according to the Landau-Levich-Derjaguin formula. Cross-section scanning electron microscopy (SEM) observation of the Er³⁺-Yb³⁺ codoped Al₂O₃ films shows a compact, uniform and crack-free films about 1.6 µm thick for 10 dip-coating cycles at the sintering temperature of 1000°C. It is suggested that the DCPAs together with PVP into the sol can provide a long-lasting structural relaxation and therefore, decreases the stress of non-uniform shrinkage and suppresses the cracks formation for gel film during gelation. The strong photoluminescence (PL) spectra with a broadband extending from 1.400 to 1.700 µm centered at 1.533 µm is obtained for the Er³⁺-Yb³⁺ codoped Al₂O₃ films, and have no significant change in the PL intensity with PVP assistant. The enhanced formation of the r³⁺-Yb³⁺ codoped Al₂O₃ thin films on SiO₂/Si(110) substrates has possibility for use in planar optical waveguide.