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Nuclear Instruments and Methods in Physics Research B 206 (2003) 866869 www.elsevier.com/locate/nimb

Titanium-dioxide film formation using gas cluster ion beam assisted deposition technique
O. Nakatsu
a c

a,b,* ,

J. Matsuo b, K. Omoto b, T. Seki b, G. Takaoka b, I. Yamada

c

Semiconductor Equipment Division, Shimadzu Corporation, 380-1, Horiyamashita, Hadano-city, Kanagawa 259-1304, Japan b Ion Beam Engineering Experimental Laboratory, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8317, Japan Laboratory of Advanced Science and Technology, Himeji Institute of Technology, 1-2, 3 chome, Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1205, Japan

Abstract Gas cluster ion beam (GCIB) assisted deposition technique has been applied to form titanium-dioxide films. When oxygen cluster ions collide on solid surfaces, oxygen molecules in the clusters enhance oxidation due to high density energy deposition. Metal titanium pellets were used as source material for EB evaporation, because evaporation with metal pellets is much stable than that of oxide pellets. Films were deposited on sapphire (0 0 0 1) substrates with various conditions. Characteristics of the films were examined by use of XRD, RBS and AFM. When film was deposited with the acceleration voltage of 7 kV at 473 K, the well c-oriented rutile TiO2 film was formed with average roughness of 0.4 nm. Without assistance of GCIB rough amorphous film was formed in an atmosphere of oxygen. Very smooth surface films with good crystallinity were formed by GCIB assisted deposition technique. с 2003 Elsevier Science B.V. All rights reserved.
PACS: 81.15.Jj; 68.55.Jk; 77.84.Bw Keywords: Cluster; Ion; Assist; Deposition; Titanium dioxide; Titania

1. Introduction Gas cluster ion beam process has been developed at Kyoto University [1]. Subsequently this technique was applied to deposition processes, namely gas cluster ion beam assisted deposition. Formation of diamond like carbon (DLC) film
* Corresponding author. Address: Ion Beam Engineering Experimental Laboratory, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8317, Japan. Tel.: +81-75-7534994; fax: +81-75-751-6774. E-mail address: nakatsu@nishiki.kuee.kyoto-u.ac.jp (O. Nakatsu).

and tin-doped indium-oxide (ITO) film using GCIB cluster beam was reported previously [2,3]. In this paper titanium dioxide (TiO2 ) film deposition is reported. Titanium dioxide (TiO2 ) has numerous crystalline structures, such as, rutile, anatase and brookite and so on. It is known that anatase and brookite structures transforms to rutile structure at a temperatures exceeding 1073 K. Because of large refractive index of TiO2 , it has being applied to optical interference films. TiO2 film deposition was reported using various deposition techniques [48]. During gas cluster ion beam assisted deposition, gas cluster ion collides with the film deposited at the substrate, where

0168-583X/03/$ - see front matter с 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00880-2

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source material is evaporated from electron beam evaporator continuously. Multicollision effects, high density of energy deposition lead to the enhancement of oxidization. In addition to these effects, smooth surface morphology is also realised by lateral sputtering, which is peculiar to GCIB process [9].

deposition. Deposition rate is monitored by the crystal rate monitor. A heater is equipped in a substrate holder whose temperature during deposition is monitored by a thermo couple and controlled during deposition from room temperature to 673 K.

3. Crystal structure of the film 2. Gas cluster ion beam assisted deposition equipment Gas cluster beam assisted deposition system is equipped with a gas cluster ion beam source and an electron beam evaporator. The electron beam evaporation technique is widely used in industry, because of its wide range of control of the evaporation rate. The cluster source chamber is evacuated by 333 l/s mechanical booster pump and the deposition chamber is evacuated by a 1500 l/s turbo molecular pump down to 2 б 10ю5 Pa. The Gas cluster source chamber includes a nozzle in which the gas clusters are generated. By introducing the mixture of O2 gas and He gas into vacuum through the nozzle, neutral O2 gas cluster is generated by the effect of adiabatic expansion. Center of the neutral cluster beam flow is cut out by using a skimmer and introduced into an ionizer. A neutral O2 cluster is ionized by electron bombardment with ionization voltage of 300 V. The cluster size is distributed up to several thousands and the mean cluster size is about 2000 O2 molecules. Gas cluster ions are accelerated with the appropriate voltage, focused by lens placed at the top of the cluster ion source and transported to the substrate as O2 cluster ion beam. A needle valve is also equipped to the deposition chamber to realize high ambient pressure during deposition, if it is necessary. Source materials are evaporated by use of the electron beam evaporator and deposited on the substrates. For oxide film formation, metalic materials can be used as a source material for the EB evaporation, because oxgen cluster ions have great capability to oxydize metals. Moreover, evaporation from metal pellets were much stabler than that from oxide pellet. The ion current density of gas cluster ion is monitored by Faraday cup before Metal titanium pellet was used as source material. Deposition rate is set to 0.1 nm/s. Gas cluster ion beam current was set to 2.8 lA/cm2 . The amount of the oxygen cluster ions is high enough to oxidize the Ti atoms at this deposition rate, because the mean oxygen cluster size is about 2000. On the other hand oxygen partial pressure during deposition (5 б 10ю4 Pa) is insufficient to oxidize the film without GCIB irradiation, the film deposited without GCIB is opaque. Therefore transparent titania film is obtained only at the GCIB irradiation spot. The size of this transparent area is controlled to about 20 mm in diameter by using the einzel lens. This is large enough to estimate the specific character of the film. Sapphire (0 0 0 1) substrates were used and substrate temperature was set to 473 K. Acceleration voltage was varied. Film thickness was 185 nm. First, the crystal structure of the film is examined by conventional X-ray diffractometry (XRD). The films were formed at 3, 5, 7 and 9 kV of acceleration voltages. The XRD spectra of the films formed with the acceleration energy more than 5 kV show a rutile (2 0 0) peak. In the case of the film formed with 3 kV of acceleration voltage, two weak peaks appear, which are assigned to the peaks of rutile (2 0 0) and anataze (1 1 2). The crystallinity of these films was estimated by the full width at half maximum (FWHM) at the rutile (2 0 0) peak using the high resolution XRD system with monochromatic incident X-ray. All the samples show rutile (2 0 0) peak except for the one of the substrate. Fig. 1 shows the acceleration voltage dependence of FWHM of the peak. The FWHM of single crystal is shown in the same figure as a line and is 0.007°. FWHM decreases from 0.25° at 3 kV to 0.070° as the acceleration

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samples formed above 5 kV of acceleration voltage. In the case of 5 kV sample, the crystallinity near the surface is lower than that of the interface. This might be originated in an insufficient irradiation of cluster ions according to charge up. In the case above 7 kV, the crystallinity near the interface is as good as that of 5 keV. The crystallinity of the films near the surface is much better than that near the interface. vmin of the titanium atoms near the surface, which is defined as the ratio counts measured from the channeling direction at surface to the counts measured from random direction, is 0.18 and comparably small as the twice as much as the balk crystal.
Fig. 1. Acceleration voltage dependence of Ra and FWHM showing that the film formed with acceleration voltage has minimum values at 7 kV.

4. AFM observation of the film surfaces Surface roughness of the film is examined by AFM. The value of average roughness (Ra ) is measured. The acceleration voltage dependence of Ra is also shown in Fig. 1. The films formed with acceleration voltage above 5 kV have smooth surfaces. Fig. 3 shows the AFM image of the film formed at 7 kV, where Ra shows the smallest value of 0.4 nm. This value is less than the Ra of 0.50.7 nm, which is reported for a hetero epitaxial film [10]. Cluster ion beam density dependence of the

voltage increase to 7 kV. The films with good crystallinity are formed above 7 kV. The azimuthal orientation of the films is rutile [0 0 1]//sapphire Ѕ1010 and rutile [0 1 0]//sapphire Ѕ12 10. Lattice mismatch between the film and the substrate is 3.36% and 7.76% respectively. Fig. 2 shows the Rutherford backscattering spectroscopy (RBS) spectra of the films whose XRD spectrum shows that they have rutile (0 0 1) structure. Solid line shows the random spectrum. The other lines show the channeling spectra of the

Fig. 2. GCIB acceleration voltage dependence on RBS spectrum. The ratio of disordered atoms has the smallest value (0.35) at 9 kV acceleration voltage.

Fig. 3. AFM image of TiO2 film formed with 7 kV irradiation of gas cluster ion beam has ultra smooth surface (Ra ј 0:4 nm).

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surface roughness is also examined for samples deposited at current densities varying from 0.5 to 2.5 lA/cm2 . Films formed above 0.5 lA/cm2 have smooth surfaces. On the other hand, by using EB evaporation in the oxygen ambient without cluster ions, rough amorphous film was formed (Ra ј 2:7 nm). The ultra-smooth surface films with good crystallinity were formed by the effect of GCIB assisted deposition.

Energy and Industrial Technology Development Organization (NEDO).

References
[1] I. Yamada, J. Matsuo, Mater. Sci. Semicond. Process. 1 (1998) 27. [2] J. Matsuo, W. Qin, M. Akizuki, T. Yodoshi, I. Yamada, in: Mat. Res. Soc. Symp. Proc., Vol. 504, Material Research Society, 1998, p. 87. [3] T. Kitagawa, 16th International Conference on the Application of Accelerators in Research and Industry, University of North Texas, Department of Physics, Denton, Texas, USA, 2000/11/1-4. [4] J. Rodriguez, M. Gomez, J. Ederth, G.A. Nkikasson, C.G. Nkikasson, C.G. Granqvist, Thin Solid Films 365 (2000) 119. [5] S. Chen, M.G. Mason, H.J. Gysling, G.R. Paz-Pujalt, J. Vac. Sci. Technol. A 11 (5) (1993) 2419. [6] Q. Chen, Y. Qian, Z. Chen, Appl. Phys. Lett. 66 (13) (1995) 1608. [7] F. Imai, K. Kunimori, T. Manabe, T. Kumagai, Thin Solid Films 310 (1997) 184. [8] Q. Guo, W.S. Oh, D.W. Goodman, Surf. Sci. 437 (1997) 49. [9] N. Toyoda, H. Kitani, N. Hagiwara, T. Aoki, J. Matsuo, I. Yamada, Mater. Chem. Phys. 54 (1998) 262. [10] P.A. Morris Hotsenpiller, G.A. Wilson, A. Roshko, J.B. Rothman, G.S. Rohrer, J. Cryst. Growth 166 (1996) 779.

5. Conclusions TiO2 films with well c-oriented structure and ultra-smooth surface were formed using gas cluster ion beam technique with the acceleration voltage higher than 5 kV. The Ra of the film formed with the acceleration voltage 7 kV is 0.4 nm. The ultrasmooth surface films with good crystallinity were formed by the effect of GCIB assisted deposition.

Acknowledgements We thank Dr. Yoshida for his support of RBS measurement. This work is supported by New