Papers

ICMCTF2001 Session C1-1: Recent Advances in Optical Coating Technology

Tuesday, May 1, 2001 8:30 AM in Room Sunset

Tuesday Morning

Start	Invited?	Item
8:30 AM		C1-1-1 Achieving Narrow Bandpass Filters Which Meet the Performance Required for DWDM R.R. Willey (Willey Optical, Consultants) Some of the most difficult optical coating requirements at this time are for narrow bandpass filters to be used in dense wavelength division multiplexing (DWDM). The design of these filters is relatively straightforward, but aids are given for estimating the design details needed to achieve specific bandwidth requirements. In the control or monitoring during the production of such designs, the classical techniques benefit tremendously by self compensation effects of errors in spite of the hypersensitivity of the results to small individual uncompensated errors. The effects of errors and drifts in the control system are shown. A monitoring strategy and algorithm are given to maximize performance utilizing more of the available information and computing power. The compromises in the choice of materials for the filters is discussed. The principle limitation to reasonable production yields for such filters is the material deposition uniformity . Test and adjustment procedures to correct uniformity errors are discussed. DWDM filter production contains extreme contrasts between the error forgiveness in some areas and requirements for rigid adherence to stable control in others.
9:10 AM		C1-1-3 Thermochromic VO₂ Deposited by Active Control of Direct Current Magnetron Sputtering K Le, R.O. Dillon, N Ianno (University of Nebraska at Lincoln) The objective of this work is to deposit and characterize coatings containing primarily the VO₂ phase. This phase has a thermochromic semiconductor to metal transition at 68 °C. Above the transition temperature, VO₂ is more reflective and less resistive compared to the insulator state. One device application uses the change in infrared optical properties for thermal switches near room temperature. Precise stoichiometry is required to deposit VO₂ coatings because of competition from other oxide phases. To achieve this stoichiometric control we deposited VO₂ by actively controlling the plasma emission ratio of vanadium and oxygen. The emission ratio was processed in real time for feedback control. The feedback involved increasing or decreasing the oxygen flow to maintain a desired ratio. Direct current reactive magnetron sputtering was used with a constant current power supply. We varied the deposition temperature between 450 and 625 °C and the amount of oxygen injected into the system. We will present X-ray, resistance, and reflectance measurements to verify that the majority phase of the coating is VO₂. The resistance results showed a change of about three orders of magnitude due to the semiconductor to metal transition. The reflectance results showed emittance changes in the infrared between 60 to 90%, so this is an excellent material for a thermal switch. This work has been supported by NASA under Grant No. NCC5-169.
9:30 AM		C1-1-4 Tribological Properties of PECVD Optical Coatings M.-A. Raymond, S. Larouche, R. Vernhes, L. Martinu, J.E. Klemberg-Sapieha (Ecole Polytechnique de Montreal, Canada) Beside their optical performance the optical coatings must frequently satisfy numerous requirements regarding their mechanical characteristics. This includes, on the one hand, the basic mechanical properties such as stress, hardness, and Young modulus; on the other hand, this involves active functional tribological properties such as scratch-, wear-, and abrasion resistance in different environments. In the present work, plasma enhanced chemical vapor deposition (PECVD) was applied to fabricate high index (TiO2, SiN1.3), intermediate index (SiOxNy) and low index (SiO2) single layer materials, as well as complex multilayer and inhomogeneous optical filters. The mechanical and tribological properties of the film systems are compared with the behavior of different optical substrates such as glass, silica and optical polymers, using single-pass and multi-pass scratch testing, depth-sensing indentation, and pin-on-disc techniques. The failure mechanisms are evaluated by using different characterization methods such as SEM, AFM, microraman spectroscopy and electron microprobe analysis. Using a large load range (1 ?N - 10 mN) we found a microhardness for individual SiN1.3, TiO2 and SiO2 coatings to be 20, 11 and 10 GPa, respectively. The tribological behavior will be correlated with the mechanical characteristics such as microhardness, stress and stress distribution, and with the microstructural features including film density, quality of interfaces and stress distribution. Consequences for the optical film design and multifunctional character of the coatings are discussed.
9:50 AM		C1-1-5 Front Surface Filters for Thermo-Photovoltaic Cells P.M. Martin, L.C. Olsen, J.W. Johnston (Pacific Northwest National Laboratory); D.M. DePoy, T. Anderson (Lockheed Martin) Front surface filters are being developed for applications to thin film thermophotovoltaic (TPV) cells. TPV cells are used to convert radiant energy from a heat source to electrical energy in much the same manner as photovoltaic solar cells. The optical performance of front surface filters, which are used to control the spectrum of photons entering the semiconductor TPV cell, is critical to the cell performance. Filters were designed to optimize spectral utilization and energy weighted above bandgap transmission to the TPV cell. The filter design requires high transmittance (>90%) between 1 - 2.4 micron wavelengths, high reflectance (> 90%) between 2.4 - 6 micron wavelengths, and low absorptance (<5%) between 1 - 14 micron wavelengths. The thin films were deposited by rf and dc reactive magnetron sputtering. To this end, materials were investigated to make n_h/n_l for the filters as high as possible, with n_h > 3 and n_l < 1.5 between 1 - 14 micron wavelengths. High index materials evaluated were Si, Si:H alloys, and ZnP. Filters were designed for Si:H and fluorides such as HfF₄, NaF₃, and YF₃ as low index layers. Sputtering gas pressure, substrate temperature, and deposition rate were varied to optimize these optical properties. H content in the coatings was optimized to reduce optical absorption near 1 µm, reduce stress in the Si layers, obtain a refractive index > 3, and minimize Si-H infrared absorptions near 5 and 12 microns. The refractive index of the Si:H films ranged from 3.65 (no H) to 3.4.
10:10 AM		C1-1-6 Deposition of Fluoride Coatings Using Pulsed Ion Assisted Processes W.G. Sainty (SainTech Pty Ltd., Australia); D. Ristau, S. Gunster (Laser Zentrum Hannover, Germany) The benefits that result from ion assistance, during growth, of almost any optical material is well understood and is today widely practised. In general, ion bombardment provides films with close to bulk density resulting in considerable improvements in durability and performance. However, for many commonly used thin film materials, this benefit is accompanied by an undesirable modification to optical properties observed as an increasing absorption coefficient (k) and variability in refractive index (n). For many materials this problem results from a fundamental incompatibility between the bombarding ion species and the depositing material. Argon and oxygen are the most commonly used ion species used in IAD processes. The high momentum of argon provides high packing density although usually results in a chemical reduction of metal oxides and fluorine depletion for many metal fluorides. This results in metal-rich films with a subsequent increase in optical absorption. The use of oxygen ions is well suited to the IAD of metal oxides such as titania and silica etc. With the correct choice of ion current and ion current density, oxygen ion bombardment can provide fully densified films with reduced stress. Problems arise where the very chemically active oxygen ions displace fluorine atoms from depositing molecules immediately prior to their incorporation in the film structure. This leads to the growth of metal oxy-fluorides with a subsequent deterioration of optical performance, particularly in the ultraviolet region of the spectrum. The extent to which this occurs depends on factors such as the ion energy and current density This paper presents the development of a technique that goes well towards reducing this problem and results in highly stoichiometric films with close to bulk properties. Film material is deposited under high vacuum conditions without the influence of ion bombardment until a critical thickness is achieved. The deposit is then bombarded with a high energy pulse of ions. The deposited film is fully densified and the deposition of more material is then resumed. This sequence is continued until the desired film thickness is achieved. The presentation will discuss the results of case studies on several metal fluorides in relation to film stability and optical performance. In particular, very dense, stoichiometric films of magnesium fluoride can be deposited with this technique with optical properties identical with that of the bulk material. These properties can be achieved with no additional heating of the substrate. The relevance of this process to coatings for the deep ultraviolet and far infrared will be discussed as will be the deposition requirements and parameters necessary to achieve these results