Growth of TiO 2 thin films on Si(001) and SiO 2 by reactive high power impulse magnetron sputtering
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Growth of TiO2 thin films on Si(001) and SiO2 by reactive high power impulse magnetron sputtering F. Magnus1, B. Agnarsson1, A. S. Ingason1,2, K. Leosson1, S. Olafsson1, and J. T. Gudmundsson1,3 1
Science Institute, University of Iceland, Dunhaga 3, IS-107, Reykjavik, Iceland Thin Film Physics, Department of Physics (IFM), Linköping University, Linköping SE-581 83, Sweden 3 UM-SJTU Joint Institute, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai, 200240, China 2
ABSTRACT Thin TiO2 films were grown on Si(001) and SiO2 substrates by reactive dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS) at temperatures ranging from 300 to 700 °C. Both dcMS and HiPIMS produce polycrystalline rutile TiO2 grains, embedded in an amorphous matrix, despite no postannealing taking place. HiPIMS results in significantly larger grains, approaching 50% of the film thickness at 700 °C. In addition, the surface roughness of HiPIMS-grown films is below 1 nm rms in the temperature range 300–500 °C which is an order of magnitude lower than that of dcMS-grown films. The results show that smooth, rutile TiO2 films can be obtained by HiPIMS at relatively low growth temperatures, without postannealing. INTRODUCTION TiO2 finds application in a variety of electrical and optical devices due to its high refractive index and good thermal stability. In bulk form, TiO2 is known to exist in three crystalline structures; two tetragonal structures, the anatase phase and the rutile phase; and an orthorhombic structure, the brookite phase. In its thin film form, only anatase and rutile are observed [1, 2]. When TiO2 is grown at low temperature it tends to be amorphous [2]. At higher temperatures (up to 600 °C) the anatase phase is favored whereas above 600 °C the rutile phase starts to appear. By postannealing above 800 °C rutile phase dominated films are obtained but this transformation also strongly depends on the deposition temperature, the ion energy and the energy flux during growth [1-5]. In its rutile phase, the transparency and high refractive index make TiO2 attractive for the glass coating industry, where it is used in low emissivity and antireflective coatings [1]. In microelectronic applications, the high dielectric constant (κ = 80) is also of interest [1, 6]. TiO2 coatings in the anatase phase are used as photocatalysts, as self cleaning and antibacterial surfaces [7, 8], and for coating of biomedical surfaces [9]. TiO2 is also hard and chemically resistant which can be useful in many applications. High power impulse magnetron sputtering (HiPIMS) is a novel ionized physical vapor deposition (IPVD) technique that has shown great promise in materials processing [10]. By pulsing the target with short unipolar voltage pulses at a low frequency and low duty cycle, a high electron density is achieved which leads to a high ionization fraction of the sputtered vapor [11]. Davis et al. [12] have demonstrated growth of TiO2 thin films by reactive HiPIMS from a
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Ti target. The resulting films exhibited a
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