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ARTICLE IN PRESS

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

Irradiation of silicon surface by Ar cluster ion beam: Cluster size effects
Y. Nakayama
a,* ,

S. Houzumi a, N. Toyoda b, K. Mochji a, T. Mitamura a, I. Yamada b

b

a Graduate School of Engineering, University of Hyogo, 2167 Syosya, Himeji, Hyogo 671-2201, Japan Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2 Kouto, Kamigori, Ako-gun, Hyogo 678-1205, Japan

Available online

Abstract The influence of cluster size on damage layer thickness produced by Ar gas cluster ion beam (GCIB) irradiation is presented. The GCIB has a wide cluster size distribution which has been difficult to control. To obtain a narrow size distribution of the GCIB, the mass filter containing a strong permanent magnet is developed. In this study, the sizeselected Ar-GCIB irradiates the Si substrates. The cluster size is varied between 500 and 20,000 atoms/cluster. After irradiation, the damage layer thickness on Si substrates is measured by ellipsometry. The results of size-selected ArGCIB irradiation on Si substrates at acceleration energy of 5 keV show that the damage decreases very quickly, from 7.9 nm to 0.3 nm with increasing the cluster size from 500 to 5000 atoms/cluster. This tendency is in agreement with the results calculated by molecular dynamics simulation. These results suggest that the GCIB process is promising for low damage processing of semiconductor material. ñ 2005 Elsevier B.V. All rights reserved.
PACS: 25.70.þz; 25.75.þq Keywords: GCIB; Cluster size; Damage formation

1. Introduction Clusters generated by GCIB consists of several thousands atoms, and the energy per constituent
Corresponding author. Tel.: +81 791 58 1419; fax: +81 791 58 0242. E-mail address: naka@lasti.u-hyogo.ac.jp (Y. Nakayama).
*

atom of cluster is the total acceleration energy divided by its cluster size [1]. For example, when the acceleration energy and cluster size are 10 keV and 1000 atoms, respectively, each atom has energy of 10 eV. These low-energy ions are useful for various applications, for instance, the important application is sputtering with very low substrate damage [2]. It has been reported from molecular dynamics

0168-583X/$ - see front matter ñ 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.084


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simulation that the cluster size is important factor for damage formation on the surface [3­5]. However, it has been difficult to control the cluster size of ions experimentally, because the average size of the cluster ions was controlled by varying the ionization condition or inlet gas pressure. As a result, the cluster size distribution was very wide and typically it was ranged from several thousands to several tens of thousands atoms. Therefore, a little of experimental results have been obtained concerning the effect of cluster size on GCIB processing. In this work, size-selected GCIB irradiates the Si substrates, and the effects of cluster size on damage formations are studied.

2. Experiment Fig. 1 shows a schematic diagram of the size-selected GCIB irradiation system. To obtain a narrow cluster size distribution of GCIB, this system has a strong permanent magnet for mass separation of the cluster ion beam. The magnetic field is 1.2 T at the beam center. The GCIB depends on both cluster size and the acceleration energy by passing through the magnetic field. This system has a sample holder placed on a linear motion rail at the end of the target part. The selected size of cluster ions can be obtained by adjusting position of the sample holder. The sample holder is driven from 0 mm (beam center of X position) to 560 mm with a pulse motor.

To confirm the cluster size distribution of this system, a time-of-flight (TOF) system is mounted on the linear motion rail. By using it, the TOF spectra can be measured without breaking the vacuum. Fig. 2 shows the cluster size distribution at 300 mm of sample holder position. When the acceleration energy is 5 keV, this position corresponds to the cluster size of 1000 atoms/cluster. The size distribution of non-selected Ar-GCIB is very wide, and full-width of half maximum (FWHM) corresponds to 9000 atoms/cluster. On the other hand, FWHM of the distribution of the size-selected Ar-GCIB is 400 atoms/cluster, which is 1/20 of that measured without mass separation. To study the cluster size effects on damage formation, Si substrates are used as targets. The total acceleration energy is varied from 5 to 30 keV, and the cluster size is varied from 500 to 20,000 atoms/ cluster. The ion dose is 1 · 1013 ions/cm2. Ionization electron energy is 400 eV. The irradiation damage of Si substrates is measured by ellipsometry [6,7] using two-layer model. The irradiation area on Si substrates is 10 · 10 mm, where three different points are examined. In this model, the silicon oxide and amorphous layers are formed on Si substrates by Ar-GCIB irradiations. The intensity ratio (W) and phase difference (D) of p and s waves are measured [6,7]. The increase of W means increase of the amorphous layer thickness (Ta), and the decrease of D means increase of the oxide layer thickness (To). The total

Fig. 1. Schematic diagram of the size-selected GCIB irradiation system.


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1.0
25

3

Damage layer thickness [nm]

Ar-GCIB Va=5keV X position:300mm

0.8

20

30 keV

Ar-GCIB Ion dose:1e13 ions/cm2

20 keV
15

0.6

0.4

non-selected GCIB

10 keV
10

0.2

size-selected GCIB

5

5 keV

0.0 0

2000 4000 6000 8000 10000 12000 Cluster Size [atoms/cluster]

0 1000 10000

Fig. 2. TOF mass spectra for the size-selected and non sizeselected GCIB. The sample holder stage position is 300 mm. The acceleration energy is 5 keV.

Cluster size [atoms/cluster]
Fig. 3. Dependence of the damage layer thickness on cluster size obtained for various acceleration energies. Ion dose is 1 · 1013 ions/cm2 for all the samples.

damaged layer thickness (Td) is defined as Ta + ToþTno, where Tno is native oxide thickness (2.6 nm is used throughout this study). In this paper, the cluster size effects on irradiation damage in Si are studied using Td.

3. Results and discussion Fig. 3 shows the dependence of damage layer thickness Td on cluster size. Total acceleration energy and ion dose are 5, 10, 20 and 30 keV and 1 · 1013 ions/cm2, respectively. The cluster sizes of the Ar-GCIB are 500, 1000, 2000, 5000, 10,000 and 20,000 atoms/cluster. Fig. 3 shows that, when the acceleration energy is 5 keV, the damage layer thickness decreases monotonically from 7.9 nm to 0.3 nm with increasing the cluster size from 500 to 5000 atoms/cluster. When acceleration energy is 10 keV, the damage layer thickness decreases from 14.3 nm to 7.3 nm with increasing the cluster size from 1000 to 20,000 atoms/cluster. In the case of acceleration energy of 20 keV, the damage layer thickness is not much changed with increasing cluster size between 500 and 10,000 atoms. However, when the cluster size is over 10,000 atoms, the damage layer thickness decreases from 19.4 nm to 13.8 nm. On the other

hand, when acceleration energy is 30 keV, the difference of damage layer thicknesses is not significant for cluster size variation from 500 to 20,000 atoms/cluster. The damage layer thickness dependence on energy per atom is replotted in Fig. 4. When acceleration energy is 5 keV, the damage layer thickness decreases below 10 eV/ atom. In the case of 10 and 20 keV, the absolute damage thickness is different from that at the acceleration energy of 5 keV, however, the damage layer thickness started decreasing below 10 eV. These results show that the damage layer thickness sharply decreases below 10 eV/atom. These experimental results are compared with the results of molecular dynamics (MD) simulations obtained for Ar-gas cluster ion impact on Si surface [3­5]. Fig. 5 shows MD simulation of the number of displaced Si atoms as a function of cluster size (the case of 5 keV Ar cluster impact). In addition, the experimental dependence of damaged layer thickness on cluster size obtained for Si target is plotted in Fig. 5. The MD simulation shows that the number of displaced Si atoms decreased with increasing the cluster size from 1000 to 3000 atoms/cluster. Both dependences demonstrate almost the same tendency. In MD simulations, it was shown that 1/3 of the total


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30 keV
20

acceleration energy was deposited on Si without damages. It is expected that non-damage process in Si is possible with GCIB irradiation.

Damage layer thickness [nm]

15

20 keV 10 keV

4. Conclusions As compared with non size-selected GCIB, the cluster size distribution is a more narrow function in the case of size separation, and the ratio of FWHMs is about 1/20. Using the ellipsometry, we have measured that the damage layer thickness on Si substrates decreases below 10 eV/atom with increasing the cluster size (total acceleration energies of Ar-GCIB are 5, 10 and 20 keV). This tendency shows good agreement with the MD simulation. Our results demonstrate that, low-damage processing with GCIB irradiation can be possible by controlling both the total acceleration energy and the cluster size.

10

5 keV
5

0 0.1 1

Ar-GCIB Ion dose:1e13 ions/cm2
10

Energy per Atom [eV/atom]
Fig. 4. Damage layer thickness as a function of energy per atom for various acceleration energies. Ion dose is 1 · 1013 ions/ cm2 for all the samples.

1800 1600

10

References
[1] K. Goto, J. Matuo, Y. Tada, T. Tanaka, Y. Momiyama, T. Sugii, I. Ymada, Int. Electron Device Meet. Tech. Dig. (1997) 471. [2] I. Yamada, J. Matsuo, Z. Insepov, T. Aoki, T. Seki, N. Toyoda, Nucl. Instr. and Meth. B 164­165 (2000) 944. [3] T. Aoki, T. Seki, J. Matsuo, Z. Insepov, I. Yamada, Nucl. Instr. and Meth. B 153 (1999) 269. [4] T. Aoki, J. Matsuo, G. Takaoka, Nucl. Intr. and Meth. B 202 (2003) 278. [5] T. Aoki, J. Matsuo, Nucl. Instr. and Meth. B 216 (2004) 185. [6] K. Riedling, Ellipsometry for Industrial Applications, Springer-Verlag, New York, 1988. [7] H.G. Tompkins, A Userós Guide to Ellipsometry, Academic Press, London, 1993.

MD simulations Experiments

9 8 7 6

Number of total displaced atoms

Damage layer thickness [nm]

1400 1200 1000

5 800 4 600 400 200 0 3 2

Ar-GCIB Va:5keV
1000 10000

1 0

Cluster size [atoms/cluster]

Fig. 5. Number of displaced Si atoms as a function of cluster size for the MD simulation of 5 keV Ar cluster bombardment, and the damage layer thickness on Si substrates measured for the 5 keV Ar-GCIB irradiation.