Improved Spray Pyrolysis Deposition System for Polycrystalline Conductive SnO 2 Thin Films
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435 Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
In this paper we report the description of our apparatus and electrical and compositional properties of Bismuth doped Tin Oxide thin films, deposited at different temperatures in the range 290-350 'C, on Corning glass 7059. DEPOSITION PROCEDURE AND APPARATUS DESCRIPTION The apparatus is divided in two major parts, the aeroosol chamber and the deposition chamber of a new design, shown in Fig. 1. In the aerosol chamber a dilute solution of stannic chloride and bismuth chloride is properly atomized using nitrogen as the carrier gas. The diluted solution in isopropyl-alcohol is prepared in large quantities such as 500 ml. It is further diluted in DI water immediately before the start of the deposition run. The H2 0 molecule is directly involved in the chemical process. The Bismuth/Stannic chloride ratio has been kept constant during all the deposition runs. The deposition chamber consists of a flat rotating hot plate that can house substrates up to 18 cm in diameter and of a diffusor consisting in a pyrex glass ring surrounding the hot plate. The rotation speed of the plate is kept constant during the deposition at about 10 rpm. The aerosol is injected radially in the deposition chamber through the many small holes that are regularly arranged in the inner surface of the glass ring, close by the hot plate. The chamber is covered by a pyrex glass plate that enables the aerosol to saturate the volume over the hot plate, thus resulting in an improved uniformity in the film thickness over the substrate area. The diffusor has four equidistant inputs and it is linked to the aerosol chamber by means of 4 teflon tubes, which makes the vapours more uniform inside the ring. The copper plate is coated to prevent oxidation and outdiffusion effects from it. A graphite varnish silicate over nickel coated copper was the most stable solution found and is the coating barrier currently used. A deep groove is machined on the lateral surface of the plate and a thermocouple is always in contact with its inner side while the plate is rotating. This solution has consistently improved the temperature control of the plate such that, between 290-400 °C the temperature is maintained constant to within 4 'C. The glass substrate is carefully cleaned with a multistep process that ends with alcoholic potash followed by abundant rinses in DI water before its insertion in the deposition chamber. Temperature ramp up and down is performed in nitrogen flow at a rate of 5 °C/min.
AREOSOL IN
ROTATING PLATE
THERMOCOUPLE
Fig. 1) Cross-section of the deposition chamber. EXPERIMENTAL RESULTS The films were deposited on Corning 7059 glass substrates, keeping a constant deposition time equal to 20 min. Figure 2 is a typical plot of surface resistivity over one surface. A downward parabolic-like temperature dependence of the deposition rate was found, with a maximum around 310 'C. The thickness of the films deposited at 310 'C, that has been
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estimated from their light yellow colo
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