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ARTICLE IN PRESS

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

Energy distributions of high current cluster ion beams
T. Seki
a

a,b,*

, J. Matsuo

a

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

Abstract The cluster ion beam processes has been shown to produce new surface modification effects, such as surface smoothing, high rate sputtering and very shallow implantation. Now high current is needed to increase the productivity of cluster processing. However, the chamber pressure increases with cluster beam intensity. Energy distributions of the cluster ion beams show that both size and energy of cluster ion decrease by numerous collisions with residual gas. This indicates that it is necessary to reduce the chamber pressure for effective transport of cluster ion beams. ñ 2005 Elsevier B.V. All rights reserved.
PACS: 36.40.Wa; 36.40.Qv; 41.75.þI; 39.10.+j Keywords: Cluster; Ion beam; Energy distribution; Ion transportation; Residual gas

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
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).
*

modification processes, such as surface smoothing [1­3], shallow implantation [4,5] and high rate sputtering [6], have been demonstrated using gas cluster ions. In order to increase the productivity of these cluster processes, high throughput and large area irradiation must be provided. And also in order to realize the nano-level smoothing of hard materials, high ion dose is needed. Therefore, large current cluster ion beam is required for effective processing. In order to get the high current cluster ion beam, the cluster generator, ionizer and ion extraction have been studied [7]. However, it was determined that the chamber pressure increased with cluster beam intensity and that the

0168-583X/$ - see front matter ñ 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.109


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T. Seki, J. Matsuo / Nucl. Instr. and Meth. in Phys. Res. B xxx (2005) xxx­xxx

collision frequency of the cluster with residual gas increased as well. In this paper, in order to investigate the influence of the collisions with residual gas on cluster beams, the dependence of the cluster ion energy on chamber pressure was studied.

2. Experimental Fig. 1 shows a schematic diagram of the cluster ion beam irradiation system. 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 electron bombardment. The ionized clusters were accelerated and transported to a Faraday cup. Monomer ions were eliminated by a magnetic field. The energy distribution was measured with the electrostatic retarding potential method. A retarding grid between two grounding grids was located in front of the Faraday cup as shown in Fig. 1. The chamber pressure was controlled by introducing Ar gas into the chamber through a gas port.

Fig. 2. Energy distributions of the cluster ion beams.

3. Results and discussion Fig. 2 shows the energy distributions of the cluster ion beams. The energy distributions were obtained from the derivative of the curves of the retarding voltage dependence of beam current.

The acceleration energy of the cluster ions (E) was 20 keV, the mean cluster size (N) was about 2800 atoms and the chamber pressure (P) was changed from 1.1 · 10þ5 Torr to 2.0 · 10þ4 Torr. The energy distribution at 1.1 · 10þ5 Torr had a peak at 17 keV and the peak energy was a little lower than the initial acceleration energy of 20 keV. The peak energy decreased with the chamber pressure increasing. Fig. 3 shows the chamber pressure dependence of the peak energy and collision frequency between

Fig. 1. Schematic diagram of the cluster ion beam irradiation system.


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T. Seki, J. Matsuo / Nucl. Instr. and Meth. in Phys. Res. B xxx (2005) xxx­xxx

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size (N), the mean free path of cluster (Lc) can be calculated from the atomic radius of Ar (r) and the density of residual gas (ni) with the following formula: Lc ¼ 1 pÏr × rc ÷ n
2 i

¼

1 r pÏ1 × N
2 1=3 2

÷n

.
i

Ï2÷

Fig. 3. Chamber pressure dependence of peak energy and collision frequency between cluster ion and residual gas.

The peak energy has linearly decreased with chamber pressure and the collision frequency was also proportional to the chamber pressure. These results showed that the ion energy decreases by numerous collisions with residual gas molecules. Fig. 4 shows a model of energy loss in a collision with residual gas. When a cluster ion with velocity (v) collides with a residual gas atom, some atoms in the cluster ion are scattered. If the velocity of the cluster ion is preserved after collision, the energy loss of cluster (EL) can be calculated with the following formula: EL ¼ E0 N s; N0 Ï3÷

cluster ion and residual gas. The collision frequency (n) was calculated from the transportation length (L) and the mean free path of cluster (Lc) with the following formula: n¼ L ; Lc Ï 1÷

where L was 0.38 m. When Ar cluster ions are generated and accelerated to targets at 20 keV, the species of residual gas can be regarded as Ar and the velocity of cluster ion is sufficiently high to ignore the velocity of residual gas. If the cluster diameter (rc) is proportional to the 1/3 power of

where Ns is number of scattered atoms in a collision, E0 is the initial energy of cluster ion and N0 is the initial size of the cluster ion. This formula shows that the energy loss is proportional to the size loss in a collision. Therefore, the number of scattered atoms by a collision can be calculated with the following formula: Ns ¼ N0 N 0 dE N ¼ EL ¼ E0 E0 dn E
dE 0 dP dn 0 dP

;

Ï4÷

Fig. 4. Model of energy loss in a collision with residual gas.


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T. Seki, J. Matsuo / Nucl. Instr. and Meth. in Phys. Res. B xxx (2005) xxx­xxx

Fig. 5. Velocity dependence of the number of scattered atoms.

Fig. 6. Chamber pressure dependence of available transportation length.

dn where dE and dP are the slopes of chamber pressure dP dependence of peak energy and collision frequency, respectively. Fig. 5 shows the velocity dependence of the number of scattered atoms in a collision. The acceleration energy of the cluster ions was 10 keV or 20 keV. The mean cluster size was changed from 2000 atoms to 8400 atoms. This figure shows that the number of scattered atoms is proportional to the velocity of the cluster ion. Therefore, the number of scattered atoms can be calculated from the velocity of the particular cluster ion. Fig. 6 shows the chamber pressure dependence of available transportation length. When the energy loss of the cluster ion was below 20% of initial energy after transportation, the transportation length was regarded as the available transportation length. The acceleration energy was 20 keV. This figure shows that it is difficult to transport a cluster with the size of 100 atoms. Because such small clusters have high velocity and the number of scattered atoms in one collision is large, the cluster size decreases rapidly. If the transportation length is 0.5 m and the cluster size is 1000 atoms, it is necessary to keep the chamber pressure under 3 · 10þ5 Torr. This result indicates that it is necessary to reduce the chamber pressure for effective transport of the cluster ion beam.

4. Conclusion The chamber pressure dependence of cluster ion energy was investigated. The results indicated that both energy and size of cluster ion decrease by numerous collisions with residual gas. In order to keep the energy of cluster ion after transportation, it is necessary to reduce the chamber pressure. Acknowledgment This work is supported by New Energy and Industrial Technology Development Organization (NEDO). 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, in: 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.


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[7] T. Seki, J. Matsuo, G.H. Takaoka, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 902. [8] O.F. Hagena, W. Obert, J. Chem. Phys. 56 (5) (1972) 1793.