High Quality Optical Film Formation with O 2 Cluster Ion Assisted Deposition
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High Quality Optical Film Formation with O2 Cluster Ion Assisted Deposition Noriaki Toyoda, Kenichi Shirai, Mititaka Terasawa, Shinji Matsui and Isao Yamada Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology 3-1-2 Kouto, Kamigori, Ako, Hyogo, 678-1205, JAPAN. ABSTRACT High-quality Ta2O5/SiO2 were deposited with oxygen gas cluster ion assisted deposition at low-temperature for advanced optical filters. With gas cluster ion assisted deposition, high refractive index and very smooth surface of Ta2O5 films were realized. The optimum cluster ion energy and cluster ion current density for Ta2O5 films were found to be 7keV and 0.5µA/cm2, respectively. The structure of film was very uniform and no porous or columnar structures were observed. The surface or interfaces of Ta2O5/SiO2 films were also very flat by surface smoothing effect of cluster ion beams. Very smooth surface can be realized even though the bottom surface was rough. There was no significant wavelength shift after an environmental test, which indicates that dense oxide films were formed at low-temperature with O2 cluster ion assisted deposition.
INTRODUCTION With the rapid progress of state-of-art information technology such as dense wavelength multiplexing (DWDM), it is getting very important to provide high-quality optical thin film for these precise filters. Optical film itself has very long history used for anti-reflection coating, dielectric mirror and so on. However, films for optical communication requires precise control of uniformity, stability, flatness and stress. If the film is porous or low-density, wavelength shift of optical filter occurs due to penetration of water into the films. Also, as the number of layers of these filters is a few hundreds, it is also preferable to deposit the film at low temperature to suppress film stresses after deposition. We’ve been proposing the gas cluster ion process to replace the conventional ion beam at low-energy region, such as surface smoothing [1,2], reactive cluster ion etching[3] and ultra shallow ion implantations [4]. Gas clusters are huge aggregates with several to thousands of atoms. As each atom in a cluster shares total acceleration energy, an ultra low energy ion beam with several eV/atom can be easily realized. For example, when the total acceleration energy is 10keV and the cluster size is 1000, each atom in the cluster has only 10eV/atom. Also, as the irradiated area with cluster ion becomes high-temperature and high-pressure conditions[5] due to dense energy deposition in local area, there are enhancements of chemical reactions near the surface and it will be possible to obtain high-density films without heating the substrate itself. Most interesting characteristics of cluster ion beams is that it exhibits strong surface smoothing because of its lateral sputtering effects[6]. So it is very interesting to employ gas cluster ion beams for thin film assist deposition because it will realize to deposit a very smooth and high-density films at low-temperature.
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