ELSEVIER Thin SolidFilms288 (1996) 330-333 Elastic light scattering and electrochemical durability of electrochromic tungsten-oxide-based films L. Kullman *, D. R~Snnow, C.G. Granqvist Department of Technology, Uppsala University, P.O, Box 534. S.751 21 Uppsala, Sweden Received6 December 1995; accepted 20 February 1996 Abstract Electrechromic W-oxide-based films were produced by reactive d.c. magnetmn sputtering. Fluorination and subsrtate bias were used to modify the film properties. Spectral measurements of the total and diffuse light scattering showed that the diffuse component remained at much less than 1% in the visible, irrespective of electrochemical degradation, which is low enough for smart windows applications. Keyword~: Electrochemistry; Opticalproperties;Spunering; Tungstenoxide 1. Introduction In this study, we investigate spectrally resolved elastic light scattering from electrochromie W-oxide-based films. The scattering depends strongly on the deposition technology, and can be well below 10 - 3of the incident beam. Electrochemical degradation does not modify the scattering level. Electroehromic thin films are relevant to different kinds of optical technology [ 1,2], with transmittance modulation in energy efficient smart windows being of particular interest. The pertinent window coatings are of muhilayer type, and the optical changes are contingent on reversible insertion/ extraction of small ions and charge balancing electrons in the electrochromie layer. From a practical perspective, the smart windows should allow large alterations in the optical prop- erties during periods of the order of minutes, and this ability should persist for many years. A less obvious, yet very impor- tant, requirement is that light scattering should be insignifi- cant and remain so during the lifetime of the device; as a rule of thumb, the scattering must be less than 1% of the incident light in the visible part of the spectrum, W-oxide-based films are considered to be the most viable option for electrochromie smart windows [ 1,2], and it is appropriate that a study of light scattering in relation to electrochromie performance is first conducted on this material. Section 2 reports the thin film deposition by reactive d.c. magnetron sputtering under a variety of conditions, following recent work [ 3]. Electrochemical characterization by cyclic * Corresponding author. 0040-6090/961515.00© 1996Elsevier ScienceS.A. All rightsreserved PIIS0040-6090(96)08809-8 voltammetry and degradation under repeated voltage sweeps arerepurtedinSection 3.Section 4isdevotedtoelasticfight scattering and describes equipment as well as results. Heavily disordered W-oxide and Wooxyfiuoride films demonstrate a scattering level of much less than I0- 2 across the luminous spectrum irrespective of electrochemical degradation. Crys- talline and electrochemically stable oxide films, deposited under strong electron irradiation, show significantly stronger scattering. Section 5 summarizes the main results. 2, Film deposition Films were made in a versatile deposition system based on a Balzers UTT 400 unit capable of evacuation to approxi- mately 10 -s Pa by turbomolecular pumping. Reactive d.c. magnetron sputtering was performed from a 5 em diameter metallic W (99,95%) target in an atmosphere of Ar (99.998%), 02 (99.998%) and, for some films, CF4 (99.995%). Oxide films were made with and without an applied positive bias Ubl,- The discharge power density during sputtering was 14 W cm- z counted per full target area. The pressure was main- tained at a value p during introduction of the gas via mass. flow-controlled regulators, and the Ar/O2 flow ratio was set at/'. Oxyfluurid¢ films were made by keeping a constant CF 4 admixture. Parameter values for the films analysed below are given in Table I. The target-substrate distance was 13 era. Substraxes were 7-8 cm 2 glass plates precoated with trans-