Knut Beekmann, Technologist & Saleem Shabbir, Applications Engineer, Oxford Instruments Plasma Technology
Introduction
Recent years have seen a dramatic growth in interest in transparent conductive oxides or TCOs. These materials are wide band gap semiconductors and have properties that are particularly suited for various high tech applications. They are found in solar cells, liquid crystal displays, and have found uses in gas sensors and for EMC and anti static shielding.
Indium Tin Oxide or ITO is one such TCO material with a band gap of ~3.7eV. ITO is of particular interest and is becoming a very important material in the manufacture of solar cells. Solar energy development and production is a high growth area in the technology sector. This is a direct result of the growing need to produce more energy from renewable sources in order to reduce world dependence on fossil fuels and the rising costs of extraction.
Sputter deposition
Sputtering or physical vapour deposition (PVD) is the favoured method for depositing thin layers of ITO. Although films can be produced by reactive sputtering of an indium tin alloy target, such a process is difficult to control and suffers from poor repeatability. Oxford Instruments Plasma Technology has developed a DC agnetron PVD process to deposit ITO films using a typical commercially available ceramic oxide target. In this case, process parameters can be adjusted and therefore provide films with optimum properties such as low resistivity and high optical transmission in the visible and near infra red spectrum.
Film Property Value
Film Thickness 100 nm
Refractive Index 2.03
Deposition Rate 33 nm min-1
Transmission minimum (400-1200nm) > 82% (minimum @400nm)
Transmission average (400-1200nm) 89%
Resistivity ~3 x 10-4 Ohm cm
Roughness – Mean (Ra) 1.95nm
Roughness – Max height (Rmax) 14.65nm
Important film properties such as the film crystal structure and composition determine the fundamental properties of ITO. The crystal structure is mainly dependent on the deposition temperature. Generally films deposited below 100°C are amorphous. Figure1 shows XRD data for a sample ITO film deposited at a temperature of 200°C indicating the presence of both (222) and (400) orientations.
Summary
Oxford Instruments Plasma Technology has developed a DC magnetron sputter deposition process that will be useful in various applications requiring good optical transmission and low resistivity. The process can be easily controlled by varying several process parameters in order to achieve optimum film properties.