Improvement of surface roughness by ultra-thin film deposition with oxygen cluster ion beam assist deposition
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Improvement of surface roughness by ultra-thin film deposition with oxygen cluster ion beam assist deposition Noriaki Toyoda 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 Ta2O5 films were deposited on a rough surface (average roughness 1.3nm, peak-to-valley 14nm) and surface roughness evolutions and improvements by O2 gas cluster ion beam (O2-GCIB) assisted deposition was studied. The average roughness dramatically decreased from 1.3nm to 0.5nm after deposition of Ta2O5 films 20nm in thickness with 7 keV of O2 cluster ion beams. As there was no etching or sputtering of Ta2O5 film by 7keV O2-GCIB irradiations, O2-GCIB assist deposition realized significant improvement of surface roughness by additional deposition of Ta2O5 film whose thickness was close to the peak-to-valley of original surface. It is expected that morphological evolution of the film by GCIB assist deposition becomes completely different from conventional ion assist deposition due to energetic cluster ion impacts. 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. Thin films for optical communication require precise control of uniformity, stability, flatness and stress. Surface smoothness is one of the important parameter to reduce optical loss by scattering of light. We’ve been proposing the gas cluster ion beam (GCIB) 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 employing B10H14 ions [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. It is very difficult to transfer monomer ions in this low-energy regime due to space charge beam expansion. 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. Therefore high-density thin films can be obtained without heating the substrate. Most interesting feature of cluster ion beams is that it exhibits strong surface smoothing because of its lateral sputtering effects [6]. In the previous study, it was found that high quality and very stable optical films (Ta2O5, SiO2 and Nb2O5) can be deposited with O2-GCIB assist depositions when the acceleration energy of O2-GCIB was between 7 and 9 keV and the ion current density were over 0.5 µA/cm2, respectively [7]. In this study, surface roughness evolutions by sputtering with O2-GCIB or by additional Ta2O5 film deposition with O2-GCIB were explored. EXPERIMENT Figure 1 shows a schematic diagram of the gas cluster ion beam (GCIB) assisted depositio
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