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Nuclear Instruments and Methods in Physics Research B 206 (2003) 838­841 www.elsevier.com/locate/nimb

Atomistic study of cluster collision on solid surfaces
Jiro Matsuo
a

a,*

, Toshio Seki

a,b

, Takaaki Aoki

a,b

, Isao Yamada

b,c

Ion Beam Engineering Experimental Laboratory, Kyoto University, Ysoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan b Collaborative Research Center for Cluster Ion Beam Process Technology, Japan c Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, CAST, 3-1-2 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1205, Japan

Abstract New surface modification processes have been demonstrated using gas cluster ion irradiations, because of the unique interaction between cluster ions and surface atoms, atomistic mechanisms of cluster ion bombardment must be understood for the further developments of this technology. Variable temperature scanning tunneling microscope in ultra high vacuum allows us to study ion bombardment effects on surfaces and nucleation growth at various temperatures. In the STM image of cluster-irradiated surface, large craters with diameter of about 10 nm were observed on Si(1 1 1)7 á 7 surfaces. The structure and size of the traces agree well with theoretical calculation. When the sample irradiated with Ar cluster was annealed at 600 °C, the hole remained, but the outer rim of the crater disappeared and the surface structure was reconstructed at the site of the rim. The depth of damage region in the target became shallower with decrease of the impact energy. These results indicate that low damage and useful surface modification can be realized using the cluster ion beam. ñ 2003 Elsevier Science B.V. All rights reserved.
PACS: 61.80.Lj; 36.40.)c; 61.72.Ji Keywords: Atom and molecule irradiation effects; Atomic and molecular clusters; Point defects and defect clusters

1. Introduction Clusters, aggregates of atoms or molecules, are interesting not only as a new state of matter but also as a new beam process for material modifications. When a cluster with the size of 1000 is accelerated with energy of 10 keV, each constituent atom has only 10 eV and these 1000 atoms collide with the surface within an area of several

Corresponding author. Tel.: +81-75-753-5953; fax: +81-75751-6774. E-mail address: jmatsuo@kuee.kyoto-u.ac.jp (J. Matsuo).

*

nm2 . Thus, clusters impact a surface with a low equivalent energy (low velocity) but with extremely high energy and particle density. Cluster ion beam process has opened to novel surface modification, such as surface smoothing, highquality thin film formation, high rate sputtering, nano-etching and very shallow implantation [1­6]. Collision of energetic cluster at the atomistic scale is still disputed, because of high-density energy deposition and multiple-collisions on surfaces. Traces of single ion impact on surfaces have been observed for both monomer ions and cluster ions [6­8]. Crater formation with cluster ions is one of the clear evidence of cluster effects (peculiarity of

0168-583X/03/$ - see front matter ñ 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00874-7


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clusters). In this paper, we have examined crater formation on Si(1 1 1)7 á 7 surfaces with a with variable temperature scanning tunneling microscope (VT-STM) and compared with experiments and theoretical calculations with molecular dynamics (MD) simulations. Cluster size effects of damage formation are discussed.

2. Experiments and simulations A VT-STM combined with ultra high vacuum (UHV) cluster ion irradiation system, has been used to investigate cluster ion bombardment effects [6]. Si(1 1 1)7 á 7 surfaces were obtained by flash heating above 1200 °C in UHV and irradiated with both monomer ions and Ar cluster ions. After irradiation samples were transferred into the STM chamber without being exposed to air and observed with VT-STM at the temperature of room up to 800 °C in situ. The conditions used for the observation were: tunneling current ­ 0.5 nA, bias voltage ­ +1.5V and the STM chamber pressure was kept lower than 5 á 10þ10 Torr. In order to examine the damage formation process by cluster impacts on silicon surfaces, the MD simulations with various size of Ar clusters were performed. The Stillinger­Weber potential model [9] was applied to the inter-atomic potential

of Si­Si and the Ziegler, Biersack and Littmark (ZBL) model [10] was applied to Ar­Si potentials. A Si(0 0 1) substrate was prepared as a target material, which consists of 32 768 atoms with a cube side of about 90 A and the periodic boundary conditions were applied on this target. Size of cluster ions used in this paper, was measured with a time of flight system. The sizes ranged from a few hundred to several thousands atoms and mean size of the clusters was about 1000. Si(1 1 1)7 á 7 surfaces were irradiated with Ar cluster ions with various accelerated energies. The mean size of the Ar cluster was about 1000. In order to observe impact traces individually, the ion dose of the clusters was set to about 1 á 1011 ions/ cm2 . According to the calculation with this ion dose, 200 traces are expected to found in this STM image area.

3. Results and discussion Fig. 1 shows STM images of irradiated Si(1 1 1)7 á 7 surfaces. Atomic steps of silicon surface were clearly observed. The white spots in these images are the traces created by cluster ion impacts. When the impact energy of the cluster ions was 8 keV, the number of traces was quite

Fig. 1. STM images of Si(1 1 1)7 á 7 surfaces irradiated with Ar cluster ions of various impact energies.


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close to the number of incident ions. However, when the impact energy was 1 keV, almost no damage was found on this STM image. In case of monomer ions, several surface vacancies formation was reported after irradiation of Xe ions with the energy of 1 keV. Many vacancies are generated with monomer ion irradiation not only on the surface but also underneath the surface. In the case of cluster ion with the energy of 1 keV [6], there is no vacancy both on the irradiated surface and underneath the surface. Fig. 2 shows a trace created with Ar cluster ion at the energy of 8 keV on Si(1 1 1)7 á 7 surfaces. White small dots are ad-atoms of Si(1 1 1)7 á 7 surface. The size of the trace is about 5 nm. Rim and hole were clearly observed in the trace. Height of the rim is about 1 atomic step. Si(1 1 1)7 á 7 structure is kept very near the crater. This indicates that the impact of energetic clusters creates very shallow and local defects. High temperature STM shows that the atoms in the rim can be removed with low temperature annealing at 600 °C and the hole of the crater can be removed at 800 °C [6]. Surface atoms, which were located under-

Fig. 3. Damage formation calculated with MD simulation. Si(1 0 0) surfaces were irradiated with Ar688 cluster ions at various impact energy. The dark points are displaced Si atoms.

Fig. 2. Single trace on Si(1 1 1)7 á 7 surface irradiated with Ar cluster ions.

neath the rim, were well order in the 7 á 7 structure. The atoms in the rim are able to diffuse on the surface at 600 °C, because diffusivity of the adatoms on the surfaces is quite high. Therefore, the ad-atoms are able to migrate on the surface at low temperature and may stick at the step-edge. In order to compare with theoretical calculations, MD simulation was carried out. In the MD simulation, Si(1 0 0) surfaces were irradiated with Ar2000 cluster at various impact energies. The snap shots of MD simulations were shown in Fig. 3. Displaced Si atoms and Ar atoms are depicted as a dark and white circles, respectively. The hemispherical damage was formed in the target. At the impact energy of 6 keV, the center hole and outer rim were formed by cluster ion impacts. The size and shape of trace agrees well with the result of the STM observation shown in Fig. 2. No damage is found on the surface at the energy of 4 keV. However, 60% of the incident energy was deposited on the surface without defect formation. High energy density deposition without damage formation is quite useful for surface processing with cluster ions. Not only energy of cluster ion, but also size of cluster ion is a key parameter of damage formation. Threshold energy of damage formation with cluster ions increases with the size of the cluster. MD calculation shows that no


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damage process is realized with 20 keV Ar cluster ions with the size of 20,000 [11]. 4. Summary Si(1 1 1)7 á 7 surfaces were irradiated by Ar cluster ion. Large craters with diameter of about 10 nm were clearly observed in the STM images of cluster-irradiated surfaces. The structure and size of the traces agree well to MD simulation result. When energy of cluster ions is 8 keV, the number of traces of ion impact is quit close to the ion dose. Few traces are found on the surfaces irradiated with 1 keV Ar cluster ions. The depth of damage region in the target became shallower with decreasing impact energy. When the sample irradiated with Ar cluster was annealed at 600 °C, the hole remained, but the outer rim of the crater disappeared and the surface structure was reconstructed at the site of the rim. MD calculation shows that no damage is found on the surface at the energy of 4 keV. However, 60% of the incident energy was deposited on the surface without defect formation. High energy density deposition without damage formation is quite useful for surface processing with cluster ions.

Acknowledgements The authors thankfully acknowledge NEDO (New energy and industrial technology development organization) in Japan for supporting this work.

References
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