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N IM B
Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B xxx (2005) xxx­xxx www.elsevier.com/locate/nimb

Cluster size dependence of sputtering yield by cluster ion beam irradiation
T. Seki
a

a,b,*

, T. Murase a, J. Matsuo

a

Quantum Science and Engineering Center, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan b Osaka Science and Technology Center, Osaka 550-0004, Japan Available online

Abstract In order to investigate the dependence of the sputtering yield on cluster size, surfaces were irradiated with cluster ion beams with various size distributions, and the sputtering yields were measured. It was found that there was a threshold energy for sputtering with cluster ion irradiation. When the cluster size was constant, the sputtering yield was proportional to acceleration energy. These results suggested an empirical formula to calculate sputtering yield from the ion energy and the size distribution of the cluster. The empirical formula indicates that the sputtering yield is proportional to the 1.1 power of the size, when the acceleration energy per atom is constant. ñ 2005 Elsevier B.V. All rights reserved.
PACS: 36.40.Wa; 41.75.þi; 82.80.Rt; 79.20.Am Keywords: Cluster; Ion beam; Sputtering yield; Size distribution; Empirical formula

1. Introduction A cluster is an aggregate of a few to several thousands atoms. When many atoms constituting a cluster ion bombard a local area, high-density energy deposition and multiple-collision processes are realized. Because of the unique interaction between cluster ions and surface atoms, new surface modification processes, such as surface smoothing [1­3], shallow implantation [4,5] and high rate sputtering [6], have been demonstrated using gas cluster ions. These nonlinear effects are assumed to be due to the fact that the cluster is an aggregate of a certain number of atoms. The size dependence of the damage created by Ar cluster ion irradiation has been already studied with Rutherford backscattering spectrometry (RBS) [7]. However, the cluster size dependence of the sputtering yield is not yet clearly understood. In this paper, the cluster size dependence of
* Corresponding author. Address: Quantum Science and Engineering Center, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan. Tel.: +81 774 383977; fax: +81 774 383978. E-mail address: seki@sakura.nucleng.kyoto-u.ac.jp (T. Seki).

sputtering yield was studied and an empirical formula to calculate sputtering yield from the ion energy and size of the cluster was suggested. 2. Experimental Adiabatic expansion of a high-pressure gas through a nozzle is utilized for the formation of Ar gas cluster beams [8]. The neutral clusters were ionized by the electron bombardment method. The size distributions of Ar cluster ion beams can be measured using the time of flight (TOF) method. In a previous work it was reported that the size distributions can be controlled by the source gas pressure and the ionization condition [9]. Fig. 1 shows the cluster size distributions at various values of source gas pressure (Ps), ionization voltage (Ve) and emission current (Ie). The peak cluster size is about 2000 at the condition of Ps = 4000 Torr, Ve = 500 V and Ie = 400 mA, about 3000 at the condition of Ps = 6000 Torr, Ve = 300 V and Ie = 300 mA, about 5000 at the condition of Ps = 6000 Torr, Ve = 100 V and Ie = 100 mA, and about 10,000 at the condition of Ps = 6000 Torr, Ve = 50 V and

0168-583X/$ - see front matter ñ 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.023


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yield with cluster is proportional to the acceleration energy and that there is a threshold energy for sputtering at about 3 eV/atom. From this data, a formula is proposed to calculate the yield with cluster (Y(N)) from cluster size (N) and total ion energy (E) as follows: pE Y ÏN ; E÷ ¼ kN þ Eth ; Ï 1÷ N where Eth is a threshold energy for sputtering, p is a coefficient of size effects and k is a constant. The threshold energy Eth can be correlated to the surface binding energy of the targets. The surface binding energies for Si and Au are 4.7 eV and 3.8 eV, respectively. Fig. 3 shows the size dependence of the sputtering yield of Si and Au with Ar cluster. The lines with square dots show experimental data. Other lines show the data calculated from the intensity of size distribution (I(N)) at each source and ionization conditions in Fig. 1 with the following formula, R Y ÏN ÷I ÏN ÷dN Y¼ R . Ï 2÷ I ÏN ÷dN When I(N) was less than 5% of the peak intensity, the TOF signal was regarded as noise and I(N) was taken as 0. The parameter k was estimated from the slope of the energy dependence of the sputtering yield in Fig. 2. The coefficient of size effects p changed from 1.05 to 1.2. The calculated line at p = 1.1 was fitted to the experimental line in both cases of Si and Au. At that time, the values of parameter k for Si and Au were 0.0013 eVþ1 and 0.0016 eVþ1, respectively. Because the beam current was low at large sizes and the TOF measurement data had some error, the sputtering yields for the peak size of 10,000 were larger than the calculated value. As a result, the parameters of the empirical

Fig. 1. Cluster size distributions at various source gas pressure (Ps), ionization voltage (Ve) and emission current (Ie).

Ie = 30 mA. In order to investigate the cluster size dependence of the sputtering yield, surfaces of Si substrates and of Au films were irradiated with Ar cluster ion beams at the four above-mentioned conditions, and the sputtering yields were measured. 3. Results and discussion Fig. 2 shows the dependence of the sputtering yield of Si and Au on the acceleration energy of the Ar cluster. The source gas pressure was 6000 Torr, ionization voltage was 300 V, and emission current was 300 mA. In the figure, the yield and energy were divided by the cluster size 2000, respectively. This result shows that the sputtering

Fig. 2. Acceleration energy dependence of the sputtering yield of Si and Au with Ar cluster.

Fig. 3. Size dependence of the sputtering yield of Si and Au with Ar cluster beam.


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tering by cluster beam, the cluster size must be controlled strictly. 4. Conclusion There is a threshold energy for sputtering with cluster ion irradiation. For a constant cluster size the sputtering yield is proportional to acceleration energy. These results suggest an empirical formula to calculate sputtering yield from ion energy and cluster size. The empirical formula indicates that sputtering yield is proportional to the 1.1 power of the size, when the acceleration energy per atom is constant. Acknowledgement This work is supported by New Energy and Industrial Technology Development Organization (NEDO).
Fig. 4. Size dependence of the sputtering yield of Si calculated from formula (1) at various acceleration energies (Ea).

References
[1] H. Kitani, N. Toyoda, J. Matsuo, I. Yamada, Nucl. Instr. and Meth. B 121 (1997) 489. [2] N. Toyoda, N. Hagiwara, J. Matsuo, I. Yamada, Nucl. Instr. and Meth. B 148 (1999) 639. [3] A. Nishiyama, M. Adachi, N. Toyoda, N. Hagiwara, J. Matsuo, I. Yamada, AIP Conference Proceedings (15th International Conference on Application of Accelerators in Research and Industry) vol. 475, 1998, p. 421. [4] D. Takeuchi, J. Matsuo, A. Kitai, I. Yamada, Mater. Sci. Eng. A 217­ 218 (1996) 74. [5] N. Shimada, T. Aoki, J. Matsuo, I. Yamada, K. Goto, T. Sugui, J. Mater. Chem. Phys. 54 (1998) 80. [6] I. Yamada, J. Matsuo, N. Toyoda, T. Aoki, E. Jones, Z. Insepov, Mater. Sci. Eng. A 253 (2000) 249. [7] T. Aoki, T. Seki, A. Nakai, J. Matsuo, G.H. Takaoka, Trans. Mater. Res. Soc. Jpn 28 (2003) 485. [8] O.F. Hagena, W. Obert, J. Chem. Phys. 56 (5) (1972) 1793. [9] T. Seki, J. Matsuo, G.H. Takaoka, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 902.

formula (1) used to calculate sputtering yield of Si and Au were Si : k ¼ 0:0013 eVþ1 ; Eth ¼ 4:7 eV; p ¼ 1:1; Au : k ¼ 0:0016 eVþ1 ; Eth ¼ 3:8 eV; p ¼ 1:1. Fig. 4 shows the size dependence of the sputtering yield calculated from formula (1) for Si. This figure indicates that the yield can reach 250 atoms/ion at the acceleration energy of 100 keV with the size of 2000, the most commonly used cluster ion beam. On the other hand, because at acceleration energy of 20 keV the sputtering yield is zero with cluster size larger than 4000, it was found that only the clusters with the size below 4000 are used for sputtering at the acceleration energy of 20 keV, in spite of wide size distributions of the cluster beam. Thus, for efficient sput-