R.P. Howson

Characterisation of aluminium oxynitride gas barrier films

In the last decade, metal oxide layers deposited on polymer substrates have been utilised as gas barrier films in food packaging as an alternative to the traditional aluminium foil. The resistance of these composite films to gas transmission is controlled predominantly by nano-scale defects created during the fabrication of the oxide layer. The size and density of these defects are believed to be strongly dependent on the intrinsic properties of the metal oxide layer. Changing the chemical composition of these coatings is one possible method to enhance the gas barrier properties of the films. In this work, aluminium oxynitride films, fabricated by reactive magnetron sputtering on Poly (ethylene terephthalate) substrates, have been investigated using a range of analytical techniques including: scanning proton microprobe; atomic force microscopy; scanning electron microscopy; transmission electron microscopy; uni-axial tensile testing; and gas permeation measurements to characterise the gas barrier properties of the film. The structural observations have been correlated with the measurements of the oxygen and water vapour permeation of the composite. Oxygen transmission rates as low as 1 cm3/m2 day·atm and water vapour transmission rates below 0.2 g/m2 day have been measured, and these competitive values can be explained by the relatively low density of defects in the barrier layers.

A microstructural study of transparent metal oxide gas barrier films

The relationship between the microstructure and the water vapour transmission rates of aluminium oxide and aluminium coatings deposited by magnetron sputtering on polyethylene terephthalate have been investigated. The gas barrier properties of the films have been measured as a function of temperature and a range of techniques used to characterize the coatings including atomic force microscopy, which also provided information on the early growth mechanism. It was found that the Al/PET film showed a better water vapour barrier than the AlOx/PET although the activation energy for water vapour permeation was the same for both. We propose that the interaction of water with the barrier coating plays a significant part in determining the observed gas barrier performance.

The formation and control of direct current magnetron discharges for the high‐rate reactive processing of thin films

The process of reactive dc magnetron sputtering is shown to provide a high‐rate, large‐area deposition system when stability is achieved using optical emission spectroscopy to control the reactive gas flow. The film properties are improved when the isolated substrate is allowed to acquire a bias by immersion in a plasma directed onto it from the source. These techniques give good results at high rates of production without the complications of rf or ion beam systems. Results for oxide systems of indium and titanium are presented. Indium oxide films gave a resistivity of 4×10−6 Ω m and titanium oxide a refractive value of 2.55 at 633 nm when deposited onto room‐temperature substrates.

Pressure stability in reactive magnetron sputtering

Abstract In high rate reactive magnetron sputtering the film deposition results in a substantial pumping rate of the reactive gas. When the depositing film is substoichiometric, the films consumption of the reactive gas is limited by the arrival rate of that gas and so consumption increases with reactive gas partial pressure. When the film is saturated with gas, the reactive gas consumption is limited by the metal arrival rate. As the reactive gas pressure is increased reaction products form on the target (it is “poisoned”) and the metal flux falls, leading to a decreasing consumption of the reactive gas. At pressures where a stoichiometric film is formed the consumption of the reactive gas by the film will be falling. This can lead to an uncontrollable transition between a metallic and a poisoned target that makes the control of optimum deposition conditions difficult. We have investigated this instability and its causes and our results indicate various means of getting a stable deposition system.

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.

Techniques for the sputtering of optimum indium-tin oxide films on to room-temperature substrates

Abstract New ion-assisted, successive-layer plasma anodisation processes are described for the preparation of indium-tin oxide films. They use unbalanced planar magnetron sputtering to provide controlled argon-ion bombardment of the growing film, and also use room-temperature substrates. In both processes the magnetron was used both to deposit the metal and to provide the activated reactive atmosphere, either by moving the substrate between different units or by controlled pulsing of the admission of the oxygen to change the function of the magnetron. These processes have been used to determine the optimum stoichiometry through control of the amount of oxygen incorporated into the metal oxide, and the optimum doping of the indium oxide with tin to produce films with the highest electrical conductivity and visible transparency. The experiments confirm results which were obtained with continuous deposition processes, which demonstrated that the addition of any tin will result in a loss of performance as compared to that achieved with control of the stoichiometry. The continuous processes used both oxide and metal targets with the supply of oxygen being carefully controlled, when necessary, from process parameters. We generally used plasma-emission monitoring (PEM) of the radiation emitted by the sputtered indium, when excited by the plasma, to indicate the partial pressure of oxygen and the state of the target. The continuous sputtering of a compacted oxide target was the least critical of the processes: only a small amount of flow-controlled oxygen had to be added to the sputtering atmosphere to give adequate electrically conducting and visibly transmitting films. Better films could be made more quickly reactively from metal targets, but were much more critical in the control of the conditions which they required. The use of successive ion-assisted plasma anodisation techniques made this a much less critical process, while in all cases the limiting resistivity was found to be around 4×106Ω·m. This was obtained via control of the stoichiometry through the oxygen content of an indium film. The role of tin was to make the achievement of optimum films more easily attainable.

Composition and conductivity of fluorine-doped conducting indium oxide films prepared by reactive ion plating

Abstract Fluorine-doped transparent conducting oxide films were produced by the reactive ion plating of indium onto glass substrates at room temperature. The electrical properties of these films depended on the content of fluorine, which was introduced into the films as a gaseous dopant from an O 2 -fluorocarbon r.f. discharge. The effect of annealing the films in air was also studied in order to elucidate the mechanisms responsible for the electrical conduction. Satisfactory films were fabricated by evaporating indium from boron nitride-titanium diboride evaporation sources.

Substrate effects from an unbalanced magnetron

Abstract The manipulation of the plasma of a d.c. planar magnetron may be achieved easily by adjusting the magnetic field of the magnetron, in conjunction, with the placing of the anode. Such an arrangement of an “unbalanced” magnetron has been studied with regard to the bias voltage that appears on the substrate and the resultant ion and electron currents that flow to it. These are compared with the heat load experienced by it, which is related to the energy dissipation of the magnetron system. The magnetron source is considered from the point of view of providing energy bombardment of the substrate, and the growing film, and the efficacy of this bombardment in initiating structural and chemical reactions of the surface, as well as giving a heat load. A typical unbalanced magnetron, made by us, gave an insulated substrate a bias voltage of 25–30 V with an ion current of 3.4 mA cm−2. The heat load was 100 mW cm−2. Of the energy supplied to the magnetron 82% went into the cooling water and 3% into the substrate; the rest was dissipated by the plasma.

Fluorine doping of IN2O3 films employing ion-plating techniques

Abstract Fluorine can be introduced as a gaseous dopant in In2O3 films by evaporating the metal in an O2-CF4 r.f. discharge. A study of the electronic and optical properties of the films showed marked dependences of the sheet resistance, optical transmittance and optical reflectance edge on the relative fluorocarbon (CF4) partial pressure. Auger analysis proved that the incorporation of 2.3 at.% F in the films induced a sheet resistance of 40 Ω/□ and a transmittance of 80% in the visible region. Hall effect measurements of the as-deposited and the annealed films indicated that large values (i.e. 13 cm2 V-1 s-1 for N = 7 × 1026m-3) of the Hall mobility can be achieved with fluorine doping. Optical transmittance and reflectance measurements showed that fluorine-doped In2O3 films deposited onto room temperature glass substrates exhibited a plasma edge in the near IR and transmittances ranging between 60% and 80% in the visible part of the spectrum.

Design and use of a vacuum system for high rate reactive sputtering of TiO2/TiN/TiO2 solar control films

Abstract The compound metal TiN shows solar selective properties both in bulk and thin film form. It has previously been suggested that thin films of TiN, when anti-reflected by TiO 2 layers, would form a hard and durable solar selective coating. We have made such multi-layer films and report on their properties. This paper also describes a new sputter coater and gives details of its design and performance. Features of this coater include a high pumping rate to give stable reactive sputtering and a load lock system to provide a high productivity of samples.