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Nuclear Instruments and Methods in Physics Research B 206 (2003) 902­906 www.elsevier.com/locate/nimb

Generation of the large current cluster ion beam
T. Seki
a c

a,b,*

, J. Matsuo a, G.H. Takaoka a, I. Yamada

b,c

Ion Beam Engineering Experimental Laboratory, Kyoto University, Sakyo, Kyoto 606-8501, Japan b Collaborative Research Center for Cluster Ion Beam Process Technology, Japan Laboratory of Advanced Science and Technology, Himeji Institute of Technology, Ako, Hyogo 678-1205, Japan

Abstract High ion dose is needed to realize the nano-level smoothing and etching of hard materials with increasing the productivity of processing using cluster ion beam. In order to achieve large current cluster ion beam, the cluster generation, ionization and ion transportation were studied. The intensity of neutral cluster beam increased with source gas pressure. The efficient ionization and extraction were realized by structural improvement of the ionizer. As a result, when the gas pressure was 15,000 Torr and the electron emission current is 300 mA, the beam current reached 500 lA. The size distributions of large current Ar cluster ion beam were measured using time-of-flight method. The distributions prove that the neutral beams include clusters with the size up to 160,000 atoms and that the distributions can be controlled by the source gas pressure and ionization condition. с 2003 Elsevier Science B.V. All rights reserved.
PACS: 82.80.Rt; 41.75.)i; 9.25.Ni; 79.70.+q Keywords: Cluster; Ion beam; Time-of-flight; Nozzle

1. Introduction A cluster is an aggregate of a few to several thousands atoms. Because many atoms constituting a cluster ion bombard a local area, high-density energy deposition and multiple-collision processes are realized. Because of the interactions, cluster ion beam processes can produce unusual new surface modification effects, such as surface smoothing, high rate sputtering and very shallow

* Corresponding author. Address: Ion Beam Engineering Experimental Laboratory, Kyoto University, Sakyo, Kyoto 606-8501, Japan. Tel.: +81-75-753-4994; fax: +81-75-751-6774. E-mail address: seki@kuee.kyoto-u.ac.jp (T. Seki).

implantation [1­4]. Various outstanding applications of the cluster ion beam have included so far: high quality tin doped indium oxide films obtained by O2 cluster ion assisted deposition at room temperature [5], smoothing of diamond films by Ar cluster beam [6,7], formation of an ultrashallow junction by using B10 H14 ion implantation [8]. It is necessary to develop the technology of large current cluster ion beam, because of increasing the productivity of processing using cluster ion beam. In order to generate large current cluster ion beam, it is important to generate many neutral clusters, ionize the clusters efficiently and transport the cluster ions without loss. Therefore, optimization of cluster generation conditions, optimization

0168-583X/03/$ - see front matter с 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00896-6


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of ionization conditions and improvement of extraction method were studied.

discharge. The acceleration voltage can be increased to 25 kV.

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 [9]. When a supersonic flow ejects from the nozzle, shockwaves are generated [10]. These shockwaves disturb the generation of neutral cluster beams. To avoid formation of such shockwaves, the skimmer was developed. The skimmer extracts the core of the supersonic flow and the cluster beam is introduced into high vacuum. The neutral clusters are ionized by the electron bombardment method. The ionizer consists of filaments and anode. Electrons ejected from hot filaments are accelerated toward the neutral cluster beam and ionize clusters. The ionized clusters are extracted and accelerated towards targets. When the intensity of neutral beam increase, a lot of gas flows into the process chamber where an ionizer and targets are set. It is necessary to evacuate the process chamber with high-speed pumps for keeping high vacuum during irradiation. The process chamber is evacuated with two cryopumps. The pumping speed of each cryopumps is 2500 `/s. The pressure in the process chamber is kept less than 1 б 10ю4 Torr during Ar cluster irradiation. The pressure is low enough to generate cluster ion beams without any vacuum

3. Results and discussion Neutral beams were generated from a Laval nozzle made from glass. The diameter of the throat of nozzle is 0.1 mm. Fig. 2 shows the source gas pressure dependence of neutral beam intensity. The pressure measured by an ion gauge on the beam line in process chamber was regarded as the neutral beam intensity. The neutral beam intensity increased with the source gas pressure.

Fig. 2. The source gas pressure dependence of neutral beam intensity.

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


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Fig. 3. Cluster size distributions of the beam generated from a Laval nozzle. Fig. 4. The electron emission current dependence of ionization efficiency.

In order to confirm that the neutral beam includes clusters, the mass distributions were measured with a time-of-flight (TOF). Fig. 3 shows Ar cluster size distributions of the beams generated from the Laval nozzle. The source gas pressure was changed from 4000 to 15,000 Torr. The ionization energy was 25 eV and the emission current was 10 mA. Because both ionization energy and emission current were low, the size reduction by ionization could be ignored and the size distributions represented the initial size distributions of neutral beams. The size distributions proved that the neutral beams included clusters with the size up to 160,000 atoms and the size of cluster became larger with the source gas pressure. These results indicate that both beam intensity and cluster size increase with the source gas pressure. The neutral clusters were ionized by the electron bombardment method. Fig. 4 shows the electron emission current dependence of ionization efficiency. The ionization energy was 300 eV and the source gas pressure was 4000 Torr. The ionization efficiency was calculated from the difference between neutral beam intensity during ionization and non-ionization. This method is based on the fact that ionized particles disperse rapidly and cannot reach the ion gauge at no acceleration. The ionization efficiency increased with the emission

current. When the emission current was 100 mA, the ionization efficiency reached about 80%. This result shows that the emission current of 100 mA is necessary for ionization of the cluster beam.

Fig. 5. The source gas pressure dependence of cluster beam current just after ionizer.


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Fig. 6. The ionization condition dependence of cluster size distribution: (a) emission current dependence, (b) ionization energy dependence.

In order to extract cluster ions from the ionizer efficiently, the anode length was shortened. The structural improvements of ionizer suppressed the ion expansion in the anode and realized the extraction of large current cluster ion beams. Fig. 5 shows the source gas pressure dependence of cluster beam current just after ionizer. The beam current increased with source gas pressure. When the source gas pressure was 15,000 Torr, the ionization energy was 300 eV and the emission current was 300 mA, the beam current reached 500 lA. With this beam current, 6 in. wafers can be treated with 1 б 1016 ions/cm2 for about 10 min. The cluster size is a unique and important parameter for the efficient surface modification with cluster ions. It is important to measure and control the size distributions of cluster beams. The size distributions of large current Ar cluster ion beam were measured using TOF method. Fig. 6 shows the ionization condition dependence of cluster size distribution. The cluster size got smaller with the emission current and ionization energy. Clusters were destroyed or separated to several small clusters by electron bombardment and the size was reduced. When the source gas pressure was 4000 Torr, the ionization energy was 300 eV and the

emission current was 300 mA, the mean cluster size was about 2000 atoms. This result indicates that the cluster size can be controlled by the ionization condition.

4. Conclusion In order to generate large current cluster ion beam, cluster generation, cluster ionization and improvement of extraction method were studied. When the gas pressure was 15,000 Torr and the electron emission current is 300 mA, the beam current reached 500 lA. This value is highest ion current among the reports of gas cluster ion beam generation. The cluster ion beams include clusters with the size up to 160,000 atoms and the size distributions can be controlled by the source gas pressure and ionization condition.

Acknowledgements This work is supported by New Energy and Industrial Technology Development Organization (NEDO).


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T. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 902­906 [5] W. Qin, R.P. Howson, M. Akizuki, J. Matsuo, G. Takaoka, I. Yamada, Mater. Chem. Phys. 54 (1­3) (1998) 258. [6] N. Toyoda, N. Hagiwara, J. Matsuo, I. Yamada, Nucl. Instr. and Meth. B 148 (1999) 639. [7] A. Nishiyama, M. Adachi, N. Toyoda, N. Hagiwara, J. Matsuo, I. Yamada, AIP Conf. Proc. (15th International Conference on Application of Accelerators in Research and Industry) 475 (1998) 421. [8] N. Shimada, T. Aoki, J. Matsuo, I. Yamada, K. Goto, T. Sugui, J. Mat. Chem. Phys. 54 (1998) 80. [9] O.F. Hagena, W. Obert, J. Chem. Phys. 56 (5) (1972) 1793. [10] H.W. Liepmann, A. Roshko, Element of Gas Dynamics, John Wiley and Sons Inc., New York, 1960.

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