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Nuclear Instruments and Methods in Physics Research B 237 (2005) 455458 www.elsevier.com/locate/nimb

Development of 1 mA cluster ion beam source
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 14 July 2005

Abstract High ion dose is needed to realize the nano-level smoothing and etching of hard materials using cluster ion beam. Large current is needed to increase the productivity of processing. In order to get the large current cluster ion beam, the cluster generator, ionizer and ion extraction has been studied. The intensity of neutral beams generated from various shapes of nozzles was measured and the orifice diameter of skimmer was adjusted. As a result, the 10 times stronger neutral beams could be generated and a maximum beam current of 2.4 mA was achieved at the acceleration voltage of 45 kV with the source gas pressure of 15,000 Torr. The ratio of monomer ion in the beam was 58%. This result indicates that the beam current of cluster ion except monomer ion is about 1 mA. With this beam current, 12-in. wafers can be treated with 2 · 1015 ions/cm2 in about 4 min. The process speed is high enough so that the cluster beam is available for next generation processes. с 2005 Elsevier B.V. All rights reserved.
PACS: 36.40.Wa; 41.75.юI; 39.10.+j; 07.75.+h; 82.80.Rt Keywords: Cluster; Ion beam; Time-of-flight (TOF); Nozzle

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
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).
*

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. In order to increase the productivity of these cluster processes, high throughput and large area irradiation must be provided. In addition high ion dose is needed to realize the nanolevel smoothing of hard materials. Therefore, large

0168-583X/$ - see front matter с 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.05.021

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T. Seki, J. Matsuo / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 455458

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

current cluster ion beam is required for effective processing. Fig. 1 shows a schematic diagram of the cluster ion beam irradiation system. Adiabatic expansion of high-pressure gas through a nozzle is utilized for the formation of Ar gas cluster beams [9]. The cluster beam was introduced into high vacuum through a skimmer. The neutral clusters were ionized by electron bombardment. The ionized clusters were accelerated and transported to targets. In order to get the high current cluster ion beam, the cluster generator, ionizer and ion extraction have been studied [7,8]. It was reported that neutral beam intensity from the large size of metal nozzle was larger than that from usual glass nozzle. In this paper, new metal nozzles were constructed and the orifice diameter of skimmer was adjusted to beam diameter.

skimmer was adjusted to beam diameter. The mass distributions were measured with a compact timeof-flight (TOF) system. The system can be set in the cluster irradiation machines and used for the cluster size monitor.

3. Results and discussion Fig. 2 shows the skimmer diameter dependence of neutral beam intensity from the metal nozzles B and C. The source gas was Ar. The pressure measured by an ion gauge on the beam line in process chamber was regarded as the neutral beam intensity. The orifice diameter of skimmer (Ds) was changed from 1.2 mm to 5.0 mm. The neutral beam intensity increased with the orifice diameter of skimmer. The neutral beam intensity at the skimmer diameter of 2.0 mm was about two times larger than that at the skimmer diameter of 1.2 mm, but the neutral beam intensity from metal nozzle B at the skimmer diameter of 3.0 mm was almost similar to that at the skimmer diameter of 2.0 mm. This result indicates that the diameter of neutral beam from nozzle B is about 2 mm and the diameter of neutral beam from nozzle C is larger than 3 mm. When the orifice diameter of skimmer was 5.0 mm, the neutral beam intensity from nozzle C was 10 times larger than that from usual glass nozzle A.

2. Experimental In order to get high intensity of neutral cluster beam, two kinds of nozzle B and C were made from metal. These were conical nozzles and each orifice diameter was 0.1 mm. Nozzle C was larger than nozzle B. The neutral beams from the metal nozzles were introduced into high vacuum through various orifice diameters of skimmers. The skimmer orifice diameter dependence of neutral beam intensity was measured and orifice diameter of

T. Seki, J. Matsuo / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 455458

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Fig. 2. Skimmer diameter dependence of neutral beam intensity.

In order to confirm that the beam generated from the metal nozzle is cluster beam, the mass distribution was measured with a time-of-flight (TOF). Fig. 4 shows the Ar cluster size distributions of the beams generated from the glass nozzle A and metal nozzle C. The source gas pressure of nozzle A and C were 4000 Torr and 15,000 Torr, respectively. The ionization energy was 500 eV and the emission current was 300 mA. This ionization condition is typically used for irradiations. The size to charge distributions from nozzle A

Fig. 3. Acceleration energy dependence of cluster beam current.

Fig. 3 shows the acceleration energy dependence of cluster beam current. Metal nozzle C was used to generate cluster beam. In order to extract cluster ions from the ionizer efficiently, the maximum acceleration voltage was raised to 45 kV and the electric intensity for extraction increased. As a result, when the source gas pressure was 15,000 Torr and the acceleration energy was 45 keV, the beam current reached 2.4 mA.

Fig. 4. Ar cluster size distributions.

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T. Seki, J. Matsuo / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 455458

4. Conclusion In order to get high intensity of neutral cluster beam, new metal nozzles were constructed and orifice diameter of skimmer was adjusted to beam diameter. The new metal nozzle can realize the 10 times larger beam intensity with almost same size distribution as compared with usual glass nozzle. As a result, the maximum beam current of 2.4 mA was achieved at the acceleration voltage of 45 kV with the source gas pressure of 15,000 Torr. The fraction of monomer ions in the beam was 58%. These results indicate that the beam current of cluster ion except monomer ion is about 1 mA.

Fig. 5. Emission current dependence of ratio of monomer ion.

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

and C have peaks at size of 1800 atoms and 2200 atoms, respectively. These results indicate that the metal nozzle can realize the 10 times larger beam intensity with almost same size distribution as compared with usual glass nozzle. Fig. 5 shows emission current dependence of ratio of monomer ion in the beams generated from metal nozzle C. The acceleration energy was 20 keV and the ionization energy was 500 eV. The ratio of monomer ion increased with the emission current. When the source gas pressure was 15,000 Torr and the emission current was 300 mA, it reached 58%. Because 2.4 mA of beam current can be gotten at this ionization concision, this result indicates that the beam current of cluster ions only is about 1 mA, if the monomer ions can be eliminated. With this beam current, 12-in. wafers can be treated with 2 · 1015 ions/cm2 in about 4 min. The process speed is enough high and the cluster beam is available for next generation processes.

References
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