M.I. Ridge

Deposition of transparent heat-reflecting coatings of metal oxides using reactive planar magnetron sputtering of a metal and/or alloy☆

Abstract Highly conducting visually transmitting filters were made on glass and these also showed a high IR reflectance. The filters were made by planar magnetron sputtering of InSn and Cd 2 Sn in an ArO 2 atmosphere which was activated by an r.f. discharge to give very high rates of deposition (with excellent properties) onto substrates which were at room temperature. The key to the achievement of these properties lay in the control of the activating field and the rate of deposition compared with the rate of admission of oxygen gas. The properties of InSn oxide and Cd 2 Sn oxide films in their visual transparency, electrical conductivity, electrical mobility and IR reflectance match those made by conventional vacuum techniques and chemical vapour deposition and they do not require any heating of the substrate. The high rates obtained with this method, with no requirement for substrate heating, offer an economical technique of coating glass for the suppression of radiation loss in glazing elements.

Production of transparent electrically conducting films by ion plating

Abstract In general there are two ways of producing a highly transparent film which is also electrically conducting: (1) the use of a highly degenerate semiconductor film which has a sufficiently high energy gap to be transparent in the visible region; (2) the use of a very thin metal film with the onset of the high reflection suppresed by interference supported by matching dielectric layers on either side. The only semiconductors which have large energy gaps and sufficiently high electrical mobilities and carrier concentrations to give good electrical conductivity have proved to be the oxides of indium, tin and cadmium and alloys of these metals. Films of indium tin oxide and cadmium tin oxide were made by reactively ion plating at controlled rates of the metal and oxygen to give films with sheet resistance less than 100 Ω/□ and transmittances of 80%. The metal films of interest are generally restricted to copper, gold and silver films. These films must be very thin (of the order of 15 nm) and are sandwiched between layers of a suitable dielectric, such as TiO2 or ZnO. This study of the performance that is possible with conducting oxides and thin metals showed that visible windows can be made with sheet resistances of less than 10 Ω/□. The important choices of material and technique for the oxide are those that give a high electrical mobility, i.e. Cd2SnO4 and In2O3 are preferable to indium tin oxide. Copper is shown to be a material which, although it does not have as high an average visible transmittance for a given sheet resistance as silver, is more neutral in its wavelength-selective characteristics.

Properties of conducting transparent oxide films produced by ion plating onto room‐temperature substrates

Indium and indium 10% tin oxide films have been produced on room‐temperature insulating substrates and have exhibited high conductivities which compare with the best reported for films made at substrate temperatures of 450 °C and higher. They were made by the evaporation and sputtering of the metals in an atmosphere of argon and oxygen onto a substrate which was one electrode of a rf discharge providing surface bombardment of the substrate. The properties of the films were a function of the oxygen partial pressure, the rf bias voltage, the rate of formation, and gas throughput. It was possible to produce films with mobilities up to 40 cm2 V−1 s−1 with carrier densities up to 3×1020 cm−3 and with conductivities up to 5×104 (Ω m)−1, all of which were transparent in the visible region of the spectrum.

Temperature dependence of Hall mobility in indium–tin oxide thin films

Carrier concentrations and mobilities of vacuum‐prepared indium–tin oxide films have been measured to determine the effects of annealing. The apparent variation of electron mobility with temperature in these films is interpreted in terms of a grain‐boundary barrier model. No grain growth was observed in the films studied; all changes in electron density and mobility may be explained by oxygen diffusion in the grain boundaries and, from them, into the grains themselves.

Formation of transparent heat mirrors by ion plating onto ambient temperature substrates

Abstract Transparent conducting oxide films were produced by the reactive ion plating of indium onto glass and plastic substrates at room temperature. The electrical properties of these films were sufficient, with high values of electrical mobility and carrier density, to give a sharp plasma reflectance edge in the near infrared. These properties are ideally suited for the production of the visibly transmitting, IR-reflecting surfaces needed for heat mirror applications. Optimum properties were achieved with careful control of the gas mixture and the amount of substrate bias acquired by the substrate from the radio frequency used for the discharge. Satisfactory films were made using evaporation or planar magnetron-sputtering sources.

Reactive ion plating of metal oxides onto insulating substrates

Abstract SnO2 and In2O3 films were produced by evaporation and by magnetron sputtering of the elements in an atmosphere containing oxygen. The insulating substrate of glass or plastic was water cooled and made one electrode of an r.f. discharge in the gas. The optical and electrical properties of the films indicated that the energy of the bombarding ions, measured by the d.c. standing voltage that developed on the substrate due to r.f. discharge, gave properties that had only previously been obtained using substrate temperatures of 450°C and above. The SnO2 films were insulating. The In2O3 films were conducting, a minimum value of the sheet resistivity ( 187 Ω □ being achieved with evaporated films at a bias of −400 V and a value of 630 Ω □ being obtained with sputtered films at a bias of −160 V with a 30 vol.% O2−70 vol.% Ar gas mixture. All films were about 80 nm thick. The ultraviolet absorption edge position proved to be related to the refractive index and the conductivity. Hall effect measurements gave a carrier concentration of 4 × 1025 m−3 with a mobility of 1.9 × 10−3 m2 V−1 s−1 for the best sputtered film.

Transparent conducting oxides of metals and alloys made by reactive magnetron sputtering from elemental targets

Abstract Oxide films were made in a continuous process onto plastic sheet would from reel to reel over a drum exposed to the sputtering material and residual active gas. Single and double coaxial magnetron sources were used to create conducting elemental oxides of indium, cadmium, tin and zinc and alloy oxides of a combination of these materials. Optimum stoichiometric composition of oxygen was determined for each material using continuous monitoring of electrical conductivity and visual transparency. The composition of the films was determined with XPS, AES and electron microprobe analysis. The films were assessed for electrical conductivity, visual transparency and the Hall effect parameters of carrier density and mobility. Indium and cadmium oxides proved the most conducting and stable with resistivities of 4.10−6Ωm being obtainable with good visual transparency. The addition of a small quantity of tin to cadmium oxide had the effect of considerably changing the blue transmittance, removing the yellow appearance from the film. For the less conducting oxides of tin and zinc film resistivities of approximately 2.10−4Ωm were obtained.

Reactive ion plating of TiO2

Abstract TiO2 films have been produced from a variety of sources utilising various ion assisted techniques to provide surface energy. It was found that reactive rf bias ion plating using electron beam evaporated titanium metal could give films of refractive index of close to 2.5 and equivalent to those made by conventional reactive evaporation onto substrates at elevated temperatures. Neutralised ion beam and oxygen plasma guns were not so successful, as was the use of sources of TiO2 and TiO. Reactive planar magnetron sputtering of titanium metal could be equally successful if the source was operated in the metal sputtering mode, this could be achieved with sufficient stability with the use of a pulsed flow of the oxy gen supply.

Composition control in conducting oxide thin films

Abstract A reactive d.c. magnetron technique for providing electrically conducting oxides of indium, InSn and CdSn on room temperature plastic substrates is described. Using continuous monitoring of a coating made onto a plastic sheet under high rate deposition conditions, with precisely controlled sputtering power and gas admission rates, it is shown to be possible to create highly conducting oxide films of all systems. The ratio of cadmium to tin in the films is shown to change as a function of the oxygen partial pressure in the system. It is concluded that the control of stoichiometry under high rate high energy conditions leads to high quality and stable films. Rates of deposition of 1.5 μm min−1 were attained for all films with Hall mobilities greater than 20 × 10−4m2V−1 s−1 giving sheet resistances of the order of 20 Ω/□ for the 300 nm thicknesses that were prepared.

The application of ion plating to the continuous coating of flexible plastic sheet

Abstract Ion plating onto plastic substrates was demonstrated to give considerable advantages in that adhesion is increased and properties can be obtained which would normally require higher temperatures than the substrate is capable of withstanding. In order to coat large areas of sheet plastic it is necessary to achieve high deposition rates without creating high substrate temperatures. This is done in commercial apparatus by pulling the sheet substrate tightly over a very smooth cooled drum and transferring the plastic from an initial roll to a final roll over this rotating drum. Conventional resistance-heated sources are generally used, though induction and electron beam heating have also been applied. Simple diode sputtering results in damage to the plastic from both heat and electron bombardment. This can be avoided if a planar magnetron source is used. We created an apparatus which allows plastic sheet passing over a rotating drum to be subjected to an r.f. discharge either before, or simultaneously with, the deposition of material from a planar magnetron source. The material may be deposited directly from the source, or reactive processes may be used with carefully chosen reactive gas inlet points. The r.f. power is applied to the roll, which is isolated by carefully designed insulation techniques whereby the storage rolls and the drum are mounted within an r.f. cage. All motor drive shafts are insulated and only the surface of the drum over which the plastic is rolled is exposed to the general chamber; the rest of the apparatus is protected by a closely spaced earth shield. The results of experiments using planar magnetron sputtering sources, including the successful use of ion plating to give transparent conducting oxide films of In-10%Sn onto flexible plastic sheet, are given.