ICMCTF1998 Session C3: Structure/Property Relationships in Dielectric Films

Tuesday, April 28, 1998 1:30 PM in Sunset Room

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		C3-1 Optical Properties of Chiral and Porous Thin Films M.J. Brett, K. Robbie (University of Alberta, Canada) Chiral thin solid films of dielectric materials have been deposited using the Glancing Angle Deposition (GLAD) process¹. This new technique utilizes oblique incidence deposition and computer controlled substrate motion to fabricate films with helical or vertical microstructures and with porosities from 10 to 94%. Refractive indices of porous films were found to correlate with film density according to standard linear mixing rules. The film porosity may be changed continuously during growth, leading to controlled variation of refractive index with film thickness. This has been accomplished while maintaining vertical film microstructures and has been applied to the fabrication of a 22-layer, 50 nm bandpass rugate filter consisting of MgF₂ of refractive index ranging from 1.11 to 1.34². Chiral films have been fabricated with GLAD with helical pitches of 50 nm to 11 micrometers in metal oxides and fluorides. Films with a chiral microstructure have been found to rotate the plane of polarization of light (340 degrees/mm) in a manner similar to cholesteric liquid crystals (CLCs). It is expected that these films will exhibit circular Bragg reflection (utilized in CLC displays) where one incident circular polarization state is preferentially reflected while the other is transmitted. The large range of control over film microstructure may prove advantageous in applications for which CLC's are currently used. ¹K. Robbie and M.J. Brett, Journal of Vacuum Science and Technology A15, 1460 (1997). ²K. Robbie, A.Hnatiw, M. Brett, R. MacDonald and J. McMullin, Electronics Letters 33, 1213 (1997).
1:50 PM	Invited	C3-2 Correlation of Predicted and Observed Optical Properties of Multilayer Thermal Control Coatings D.A. Jaworske (NASA Lewis Research Center) Thermal control coatings on spacecraft will be increasingly important, as spacecraft grow smaller and more compact. New thermal control coatings will be needed to meet the demanding requirements of next generation spacecraft. Computer programs are now available to design optical coatings and one such program was used to design several thermal control coatings consisting of alternating layers of WO₃ and SiO₂. The coatings were subsequently manufactured with electron beam evaporation and characterized with both optical and thermal techniques. Optical data were collected in both the visible and infrared regions of the spectrum. Predictions of solar absorptance and infrared emittance were successfully correlated to the actual thermal control properties. Functional performance of the coatings was verified in a bench top thermal vacuum chamber.
2:30 PM		C3-4 Processing Directed Chemical State Alteration in Doped ZnO Films G.J. Exarhos, C.F. Windisch, Jr. (Pacific Northwest National Laboratory) Enhanced electronic conductivity in II-VI semiconductors is achieved by introducing defect levels in close proximity to the conduction band. These levels, which derive from localized impurities or intentionally introduced dopants, become populated upon reduction of the materials and serve as source electronic states from which free carriers are generated. The chemical state of the film, which is dependent upon deposition conditions and post deposition processing, is associated with an increase in film conductivity and infrared reflectivity. In many cases, these impurity levels are prone to reoxidation leading to time dependent properties variations. The introduction of dopants such as Pt or Au, which tend to resist oxidation, is but one means by which the conductivity can be stabilized in both spin cast or rf sputtered films. Following post deposition reduction in hydrogen or cathodic reaction in an electrochemical cell, partial reduction of resident cations in the lattice results which leads to gross changes in the Raman allowed E1 mode frequency, linewidth, and intensity. The extent of reduction also is manifested by alteration of dopant chemical state as determined from XPS traces. Current voltage measurements obtained in an electrochemical cell (pH 7 aqueous buffer solution) also show changes in resident oxidation state upon polarization of the electrode. Results indicate a reversible charge state transformation for certain cations but contrast with experiments on films containing more noble cation dopants which show little tendency to reoxidize once they have been reduced. Work reported here has implications for stabilizing film conductivity and for the development of new sensor materials.
2:50 PM		C3-5 Dependence of the Sheet Resistance of Indium-Tin-Oxide Thin Films on Grain Size and Grain Orientation Determined from X-ray Diffraction Techniques. A.K. Kulkarni, K.H. Schulz, M.A. Khan, T.S. Lim (Michigan Technological University) The microstructure of Indium-Tin-Oxide (ITO) films greatly affects the sheet resistance. A unique relationship is shown to exist between grain orientation and grain size in ITO thin films deposited on glass, PET, and polycarbonate substrates by rf deposition technique. As determined from X-ray diffraction results, the ratio of the (400) orientation of grains to all other orientation is directly related to the grain sizes of the grains. As the (400) orientation ratio changes from 0 to 0.9, the grain size varies from 10 to 60 nm in a non-linear fashion. The sheet resistance decreases from 10⁴ ohms per square for 10 nm grain size films to 10³ ohms per square for a film with 60 nm grain size. The relationship between grain size, grain orientation, and sheet resistance is explained on the basis of thin films microstructure and conductivity of the film.
3:30 PM		C3-7 In-situ and Ex-situ Ellipsometric Characterization of the Interface Between Plasma Deposited Si-Compound Films and Polymer Substrates. A. Bergeron, L. Martinu, J.E. Klemberg-Sapieha (École Polytechnique, Canada) Plasma-deposited amorphous hydrogenated silicon nitride and dioxide films on polymer substrates can effectively be used as functional, optical, protective and barrier coatings. In our previous work, we investigated the interfacial region between the polycarbonate substrate and plasma-deposited a-SiN:H and SiO₂ films by ex-situ spectroscopic ellipsometry and XPS. In the present work, we study the interfacial region by using in-situ spectroscopic ellipsometry measurements, applied also to polyester substrates. We test various plasma surface pretreatment procedures, such as different gases and exposure duration, and we evaluate the film growth in its initial stage. We determine the structural profile across the polymer/film interfacial region. We found that the optical constants gradually vary over a thickness ranging from few nanometers to several tens of nanometers. The results also point to the effect of surface energy and surface "contamination" during the film nucleation. The ellipsometric measurements are further correlated with the results from cross-sectional TEM observations, XPS analysis and AFM characterization. We point out the effect of the refractive index profile at the interface on the design of optical filters on plastic substrates.
3:50 PM	Invited	C3-8 Fabrication of Durable Protected and/or Enhanced Gold Reflectors R. Lalezari, D.E. Long (Research Electro-Optics, Inc.) Overcoated gold surfaces are used in a variety of applications where broad band high reflectance is desired. Such reflectors are often required to exhibit durability in challenging environments which expose the reflectors to severe temperature cycles and abrasion. Commonly used oxides such as HfO₂ and Y₂O₃ exhibit moderate adhesion to gold. Gold reflectors overcoated with PVD films of these oxides show some durability after deposition but fail abrasion and adhesion tests after exposure to temperature and time. An alternative structure for overcoating the gold reflectors is achieved by using a very thin layer of a metallic binder (5 to 12 angstroms of chrome or tantalum) between the gold and the subsequent oxide layers (patent pending). A structure incorporating the thin binder layer and the oxide films is extremely stable and shows superb adhesion and abrasion resistance even after exposure to multiple rapid thermal cycles and long term exposure to temperature and humidity. Metal films with thickness of the order of 5 to 12 angstroms are substantially transparent and cause minimal degradation of the reflectivity of the underlying gold. Thes structures have been reliably reproduced in volume produciton.
4:30 PM		C3-10 Optical Properties of Sputtered Oxide Materials P. Yashar, A. Lefkow, S.A. Barnett (Northwestern University) Oxides of Al, Mg, Y, and Zr, as well as Y₂O₃/ZrO₂ superlattices, have been deposited on Si and glass slides in an opposed cathode, unbalanced magnetron sputtering system. Pulsed dc power at a frequency of 60 kHz was applied to the targets, which allows insulating materials to be deposited with minimal or no arcing. Using partial pressure control of the reactive gas, the films were deposited at rates of ~ 70% of the metal deposition rate. An rf substrate bias was used to achieve ion bombardment during film growth. Spectroscopic ellipsometry showed that the index of refraction of the films deposited with a substrate bias of ≥ 80 V (dc component) were equal to that of the bulk materials, while films deposited with no substrate bias showed an index of refraction below that of bulk materials indicating porosity in these films. The optical constants of Y₂O₃/ZrO₂ superlattices with Λ ≤ 25 nm were as expected by taking a weighted average of Y₂O₃ and cubic ZrO₂ suggesting that the ZrO₂ layers were epitaxially stabilized by the Y₂O₃ layers. For Λ ≥ 25 nm, the optical constants were equal to the weighted average of Y₂O₃ and monoclinic ZrO₂ indicating a structure change as the ZrO₂ layer reaches a critical thickness. This has also been confirmed by high resolution TEM.
4:50 PM		C3-11 Investigation of Light-emitting DLC Coatings by Time Resolved Photoluminescence N.I. Klyui, V.G. Litovchenko (Institute of Semiconductor Physics, Ukrainian NAS, Ukraine); Yu.P. Piryatinski (Institute of Physics, Ukrainian NAS, Ukraine); A.B. Romanyuk (Institute of Semiconductor Physics, Ukrainian NAS, Ukraine); V.A. Semenovich (Institute for Superhard Materials, Ukrainian NAS, Ukraine) Nitrogen-containing diamond like carbon films (a-C:H:N) deposited by rf (13,56 MHz) plasma decompositions of CH₄-H₂-N₂ gas mixture were studied. Nitrogen content in the gas mixture was changed from 0 to 45% that led to change of relative nitrogen content in the film measured by Auger electron spectroscopy from 0 to 8 at.%. The time-resolved photoluminescence (PL) spectroscopy has been used to investigate the luminescent properties of the nitrogen containing DLC films. It was found that increasing of nitrogen content in the gas mixture in the range of 20-45% results in downshift of main luminecsent band from 450 to 600 nm and in drastic increasing of the main PL band intensity. On the other hand, PL spectra of the nitrogen containing DLC films measured from the lateral face of the the film reveal dramatic narrowing and increasing of intensity of the PL bands. The mechanisms of the effects observed are also discussed. The results obtained show that nitrogen containing DLC films can be used for development of new light-emitting devices.