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Nuclear Instruments and Methods in Physics Research B 206 (2003) 875­879 www.elsevier.com/locate/nimb

Stable optical thin film deposition with O2 cluster ion beam assisted deposition
N. Toyoda *, Y. Fujiwara, I. Yamada
Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, 3-1-2 Kouto, Kamigori, Hyogo 678-1205, Japan

Abstract O2 gas cluster ion beam (O2 -GCIB) assisted depositions were studied to deposit high-quality Ta2 O5 ,Nb2 O5 and SiO2 films. The optimum irradiation conditions of O2 -GCIB were the acceleration energy of 5­9 keV and the cluster ion current density over 0.5 lA/cm2 , respectively. Nb2 O5 /SiO2 films deposited with O2 -GCIB assist showed very uniform and dense structures without columnar or porous structures. Due to the significant surface smoothing effect of GCIB, the interface and top surface of Nb2 O5 /SiO2 multi-layer were quite flat. The interference filter deposited with O2 -GCIB assist deposition was very stable and there was no shift of wavelength before and after environmental tests. As O2 GCIB is equivalently very low-energy ion beam and is able to deposit flat and dense amorphous films, it is suited to deposit multi-layered films where low-energy assisting ions are required. с 2003 Elsevier Science B.V. All rights reserved.
PACS: 81.15.Jj; 81.65.Mq Keywords: Cluster; Ion assist deposition; Surface smoothing

1. Introduction With rapid progress of state-of-art information technology and electro-optical devices, it is getting very important to provide high-quality optical thin films. Films for optical communication requires precise control of uniformity, stability, flatness and stress. If the film is porous or low density, filter property will shift. Also, as the number of layers of these filters is a few hundreds, it is also preferable to deposit a flat film at low-temperature, low-en* Corresponding author. Tel.: +81-791-58-0428; fax: +81791-58-0242. E-mail address: ntoyoda@lasti.himeji-tech.ac.jp (N. Toyoda).

ergy and low-charge conditions. To improve film properties, various deposition technique employing energetic ions have been developed, however, there are still many problems to overcome such as radiation damage by high-energy ions, voids formation due to micro-discharge, surface roughness propagation to upper layer, and so on. We have been proposing the gas cluster ion beam (GCIB) process to replace conventional ion beams at low-energy region for surface smoothing [1,2], reactive cluster ion etching [3], surface cleaning 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 (several eV) ion beam can be easily realized.

0168-583X/03/$ - see front matter с 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00882-6


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Also as there are enhancements of chemical reactions near the surface due to dense energy deposition at local area [5], high-density films without heating the substrate are realized. Most interesting characteristic of cluster ion beams is that it exhibits strong surface smoothing effect because of its lateral sputtering [6]. Even though a rough surface is formed in a mid layer, the roughness does not propagate to the upper layer, which is advantageous for multi-layered film depositions [7]. So it is very interestingto employ gas cluster ion beams as assisting ions for thin film deposition because it will realize to deposit very smooth and high-density films at low temperature. In this study, high-quality optical films (Ta2 O5 , Nb2 O5 and SiO2 ) were deposited with the O2 gas cluster ion beam (O2 -GCIB) assisted depositions. Film properties and structures were studied under various deposition conditions and environmental tests were performed to study the stability of the interference filters.

Fig. 1. Schematic diagram of GCIB assisted deposition system.

2. Experiment Fig. 1 shows a schematic diagram of the GCIB assisted deposition system. Basically, the construction of this system is based on an ion beam assisted deposition, however, O2 cluster ions with very low energy per molecules are employed as assisting ions. O2 neutral clusters were formed by supersonic expansions of high-pressure O2 ­He mixing gas through a Laval nozzle into a vacuum chamber. The typical stagnation pressure in the nozzle was from 3000 to 7600 Torr. The average cluster size of O2 cluster ion beam measured with time of flight mass spectrometer was approximately 1000­3000 atoms/cluster. Subsequently, neutral O2 clusters were ionized by electron bombardments and were accelerated up to 10 keV. O2 cluster ion current density was 1.0 lA/cm2 at the target. Ta2 O5 , Nb2 O5 and SiO2 were evaporated from electron beam evaporator with deposition rate of 0.1 nm/s. Targets were quartz substrates 30 mm in diameter. Deposition rates were monitored by a quartz crystal monitor and thickness of the films were controlled with both quartz crystal

monitor and reflection type optical monitor. After deposition, transmittance spectra, surface morphologies, cross-sectional images were measured with a spectrometer, an atomic force microscope (AFM) and a secondary electron microscope (SEM). Stabilities of Nb2 O5 /SiO2 multi-layer were studied from changes of transmittance spectra after boilingtest (100 °C, 5 h) and high-humidity test (85 °C, 85%, 250 h).

3. Results and discussions In GCIB processes, energy of each constituent atom (energy/atom) defines the physical phenomena that occur at a bombarded area. Below several eV/atom, adequate energy deposition is expected without causing damage or sputtering. Fig. 2 shows acceleration energy dependence of refractive indexes and surface roughness of Ta2 O5 films deposited with O2 -GCIB. The refractive index was measured at wavelength of 633 nm. The thickness of Ta2 O5 films and ion current density were 180 nm and 160 nA/cm2 , respectively. The acceleration energy of 0 keV represents a neutral O2 cluster beam irradiation. The dotted line in Fig. 2 repre-


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Fig. 2. Acceleration energy dependence of refractive index and surface roughness of Ta2 O5 films with O2 -GCIB assist. Depo sition rate and thickness of the film were 1.0 A and 180 nm, A/s respectively.

Fig. 3. Cluster ion current density dependence of refractive index and surface roughness of Ta2 O5 and SiO2 films. The accelearation energy of O2 -GCIB and the thickness of films were 7 keV and 200 nm, respectively.

sents the data with electron beam depositions. With increasing acceleration energy, refractive index increased and had a maximum at 7 keV. From transmittance spectra, there was no absorption of light in Ta2 O5 films throughout the visible wavelength. Also, from Rutherford backscattering measurement, tantalum oxide film with O2 -GCIB was stoichiometric Ta2 O5 . Regarding to the surface roughness, it suddenly decreased at acceleration energy of 5 keV and showed saturated value of 0.5 nm below 11 keV. From these results, the optimum acceleration energy of O2 -GCIB is between 5 and 9 keV. Fig. 3 shows ion current density dependence of refractive indexes and surface roughness of Ta2 O5 films. The refractive index and the surface roughness were measured as the same way in Fig. 2. The acceleration energy of O2 -GCIB was fixed at 7 keV. Ion current density of 0 represents the result with neutral O2 cluster beams. In the case of neutral O2 cluster beam assist deposition, the refractive index of Ta2 O5 was the same as that of an electron beam

deposition. However, with increasing the cluster ion current density, the refractive index gradually increased and showed saturation value at 2.2 over ion current density of 0.8 lA/cm2 . In Fig. 3, the surface roughness suddenly decreased from 1.6 to 0.6 nm around the ion current density of 0.5 lA/ cm2 . In the case of SiO2 , the drop of the surface roughness was around 0.7 lA/cm2 . Same experiments were also performed for Nb2 O5 films with O2 -GCIB assist deposition, and similar tendencies were observed. From these experiments, it is found that the required O2 cluster ion current density is more than 0.5 lA/cm2 . To study the structure of films deposited with O2 -GCIB assist deposition, Nb2 O5 /SiO2 interference filters were formed. Fig. 4(a) and (b) shows cross-sectional SEM image of Nb2 O5 /SiO2 multilayer formed with or without O2 -GCIB irradiations. The magnification was 70,000. This interference filter had 13 layers of Nb2 O5 and SiO2 films designed to have a center wavelength at 1.5 lm. The thickness of Nb2 O5 and SiO2 were 174


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Fig. 4. SEM cross-sectional images of Nb2 O5 /SiO2 interferenc filters: (a) without O2 -GCIB assist and (b) with O2 -GCIB assist at acceleration energy of 9 keV.

and 266 nm, respectively. The acceleration energy and O2 cluster ion current density were 9 keV and 2.0 lA/cm2 , respectively. The deposition rate was 0.1 nm/s and the target was quartz substrate without anti-reflection coating on the backside. In Fig. 4, the bright layers represent Nb2 O5 films and the dark one represents SiO2 . In the case of depositions without O2 -GCIB assist as shown in Fig. 4(a), each layer was porous and there were many columnar structures. The interface between each layer was not flat and large grains and particles were observed. The average roughness of these films measured with AFM was about 1.5 nm. On the other hand, with O2 -GCIB assist deposition shown in Fig. 4(b), all the layers have very uniform structures and there was no porous or columnar structure. Also the interfaces and top surface were very flat with average roughness of 0.5 nm due to the surface smoothing

effect of gas cluster ion beams. From X-ray diffraction spectrum, these films had complete amorphous structures. Transmittance spectra for this interference filter formed with O2 -GCIB assist deposition showed 96% of transmittance at wavelength of 1.55 lm. As there was no anti-reflection coating on the backside of quartz substrates and there was typically 4% of loss due to the reflection at the backside, the transmittance of 96% indicates that there was almost no absorption in this filter. To verify stabilities of the film deposited with O2 -GCIB assisted deposition, environmental tests were performed for interference filters shown in Fig. 4(a) and (b). At first, filters were dipped in boiling water for 5 h, and subsequently highhumidity test were carried out by putting these filters in a chamber which was kept at 85 °C with humidity of 85% for 250 h. Fig. 5(a) shows transmittance spectra of the N2 O5 /SiO2 interfer-

Fig. 5. Transmittance spectra of Nb2 O5 /SiO2 interference filters before and after environmental test (100 °C for 5 h and 85 °C, 85% for 250 h): (a) without O2 -GCIB assist and (b) with O2 -GCIB assist at acceleration energy of 9 keV.


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ence filter deposited without O2 -GCIB assist. In Fig. 5(a), the transmittance spectra significantly moved almost 30 nm after a boiling test. The highhumidity test was also carried out after the boiling test, however, the film had many wrinkles on its surface and it was impossible to measure a transmittance spectrum. These wavelength shifts of spectra were caused by percolation and absorption of water inside of the films during these tests, which means that the film was coarse and very low density. However, in the case of O2 -GCIB assist deposition as shown in Fig. 5(b), there was no change of spectra even after boiling and highhumidity tests. These results indicate that O2 GCIB assist deposition can form high-density films so that it is stable even after these environmental tests.

was very stable and there was no shift of wavelength after environmental tests. As O2 -GCIB is equivalently very low-energy ion beam and is able to deposit flat and dense amorphous films, it is suited to deposit multi-layered films where lowenergy assisting ions are required.

Acknowledgements We gratefully acknowledge the support of the New Energy and Industrial Technology Development Organization (NEDO) under grant ``Cluster ion beam process technology''. We also thank M. Sato of Nikon Co. for the cooperation of experiments and valuable discussions.

4. Conclusion O2 cluster ion beam assisted depositions were applied to deposit high-quality optical films such as Ta2 O5 , Nb2 O5 and SiO2 . The optimum irradiation conditions of O2 -GCIB for Ta2 O5 and Nb2 O5 films were the acceleration energy of 5­9 keV and the ion current density over 0.5 lA/cm2 , respectively. Nb2 O5 /SiO2 multi-layer deposited with O2 -GCIB assist showed very uniform and dense structures. Also interface and top surface of Nb2 O5 /SiO2 multi-layer was quite flat with average roughness below 0.5 nm. This interference filter

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