Mist deposition technique as a green chemical route for synthesizing oxide and organic thin films
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Mist deposition technique as a green chemical route for synthesizing oxide and organic thin films Shizuo Fujita1, Kentaro Kaneko1,2, Yutaka Fukui1,2, Hiroyuki Nishinaka1,2, Takumi Ikenoue1,2, and Taichi Nomura1,2 1 Photonics and Electronics Science and Engineering Canter, Kyoto University 2 Department of Electronic Science and Engineering, Kyoto University
ABSTRACT Ultrasonic spray-assisted mist deposition techniques have been developed as a costeffective and environmental friendly deposition method for oxide and organic thin films. The chemical vapor deposition (CVD) of a variety of oxide thin films having unique functions, such as Cr2O3, Cu2O, Fe3O4, and Al2O3 thin films, has been demonstrated as well as high-quality ZnO and Ga2O3 films ever reported. In addition to the films deposition by the CVD process, the deposition of organic material thin films from the source solution has also been achieved; as examples we have shown the patterned deposition of water-soluble fluorescent polymers with a metal mask. This may substitute the spin-coating technique and contribute to increase the source consumption efficiency in the thin film deposition. We appeal that the mist deposition is a unique and promising technique as a green chemical route for film deposition. INTRODUCTION Vapor-deposition has been an essential technique for the growth of highly functional semiconductor materials. For this purpose the use of dangerous and toxic sources of organometallics has been a solution but nobody believes that this technique is environmentalfriendly. The purpose of using organometallics is to supply the constituent elements in the gasphase, because many organometallic compounds have high vapor pressures. However, we can propose another technique, which is far inexpensive and environmental-friendly compared to the use of organometallic compounds, for the growth of various thin films. What we can propose is the mist deposition technique. Applying ultrasonic power with a transducer to a liquid solution, the solution is atomized to form mist particles consisted with precursors diluted in the solution. This technique is similar to the pressured-spray deposition, but the important merit of the ultrasonic atomization lies in the smaller particle size and more uniform distribution of the particle size. The diameter of mist particles d atomized at the frequency of f can be empirically given by the Lang's equation [1] d = 0.34 (8πσ /ρ f 2)
1/3
[cm],
where σ is the surface tension and ρ is the liquid density. The experimental results showed the measured diameter of 1.8 ± 0.2 µm for the ultrasonic atomization at the frequency of 3 MHz, while the broad size distribution of 1-10 µm for the pressure-splayed particles [2]. The drop velocity of particles can be calculated from the force-balance between the gravity and the friction in air of the particles. The results are given in figure 1. For the particles formed by the ultrasonic atomization, the drop velocity is around 10-2 cm/s, that is, the particles can nearly float
Figur
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