CLUSTER SIZE DEPENDENCE OF SPUTTERING YIELD BY CLUSTER ION BEAM IRRADIATION T. Seki 1,2), T. Murase 1), J. Matsuo 1) 1) Quantum Science and Engineering Center, Kyoto University 2) Collaborative Research Center for Advanced Quantum Beam Process Technology Abstract Cluster ion beam can realize high rate and high accuracy etching because of the multiple collision effect and the high-density energy deposition effect on cluster impacted surfaces. These nonlinear effects are assumed to be due to the fact that cluster is an aggregate of a certain number of atoms. However, cluster size dependence of sputtering yield is not understood clearly. In order to investigate the cluster size dependence of sputtering yield, surfaces were irradiated with cluster ion beams, which had various size distributions, and the sputtering yields were measured. Cluster ion beams with different size distributions were generated with controlling the ionization conditions of clusters. Size distributions were measured using Time-of-Flight (TOF) method. When the acceleration energy was constant, the sputtering yield decreased with cluster size increasing. When the cluster size is large enough, the energy of one atom in the cluster is sufficient low and sputtering is hard to occur. There is a threshold energy for sputtering with cluster ion irradiation. When the cluster size was constant, the sputtering yield was proportional to acceleration energy. These results suggested empirical formula to calculate sputtering yield from ion energy and size distribution of cluster. The empirical formula indicates that sputtering yield is proportional to the 1.1 power of size, when the acceleration energy per atom is constant. 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 modification processes, such as surface smoothing 1-3), shallow implantation 4,5) and high rate sputtering 6), have been demonstrated using gas cluster ions. These nonlinear effects are assumed to be due to the fact that cluster is an aggregate of a certain number of atoms. Size dependence of damage created by Ar cluster ion irradiation was already studied with Rutherford backscattering spectrometry (RBS) 7). It was found that the amount of damage is different depending on cluster size distribution. However, cluster size dependence of sputtering yield is not understood clearly. In this paper, the cluster size dependence of
Figure 1: Schematic diagram of the cluster ion beam irradiation system. sputtering yield was studied and empirical formula to calculate sputtering yield from ion energy and size of cluster was suggested. Experimental Figure 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 the electron bombardment method. The size distributions of Ar cluster ion beams can be measured using Time of Flight (TOF) method. Previous work reported that the size distributions can be controlled by the source gas pressure and ionization condition 9). Figure 2 shows the cluster size distributions at various source gas pressure (Ps), ionization voltages (Ve), and emission current (Ie). The peak cluster sizes at four kinds of ionization condition A, B, C, and D Figure 2: Cluster size distributions at various source gas were about 2000, 3000, 5000, and pressure (Ps), ionization voltages (Ve), and emission 10000, respectively. In order to current (Ie).
investigate the cluster size dependence of sputtering yield, surfaces of Si substrates and Au films were irradiated with the four kinds of Ar cluster ion beams, and the sputtering yields were measured. Results and discussion Figure 3 shows the acceleration energy dependence of the sputtering yield of Si or Au with Ar cluster. Because ionization condition was A, the yield and energy were divided by cluster size 2000, respectively. This result shows that the yield with cluster is proportional to the acceleration energy and there is a threshold energy for sputtering at about 4 ev/atom. From this data, a formula is proposed to calculate the yield with cluster (Y(N)) from cluster size (N) and total ion energy (E) as follows, p E Y ( N, E) = kn ( Eth ), (1) N where E th is a threshold energy for sputtering, p is an index of size effects, and k is a constant. The threshold energy E th was regarded as the surface binding energy of targets. The surface binding energies for Si and Au were 4.7 ev and 3.8 ev, respectively. Figure 4 shows the size dependence of the sputtering yield of Si or Au with Ar cluster. The ionization conditions were A, B, C, and D. The lines with square dots show experimental data. Another lines show the data calculated from intensity of size distribution (I(N)) at each ionization conditions in figure 2 with follows formula, Y = Y ( N ) I( N ) dn. (2) I( N ) dn When the I(N) was less than 5 % of the peak intensity, the signal of TOF was regarded Figure 3 : Acceleration energy dependence of the sputtering yield of Si or Au with Ar cluster. Figure 4 : Size dependence of the sputtering yield of Si or Au with Ar cluster beam.
(a) Energy dependence (b) Size dependence Figure 5 : Size dependence of the sputtering yield of Si calculated from formula (1). as noise, so the I(N) was taken 0. The parameter k was estimated from the slope of energy dependence of the sputtering yield in figure 3. The index of size effects p was changed from 1.05 to 1.2. The calculated line at p = 1.1 was fitted to the experimental line in both cases of Si and Au. At that time, the parameter k for Si and Au were 0.0013 ev -1 and 0.0016 ev -1, respectively. As a result, the parameters of empirical formula (1) to calculate sputtering yield of Si and Au were, Si : k=0.0013 ev -1, E th =4.7 ev, p=1.1, Au : k=0.0016 ev -1, E th =3.8 ev, p=1.1. Figure 5 shows the energy and size dependence of the sputtering yield of Si calculated from formula (1). The cluster ion beam with the size of 2000 at the acceleration energy of 20 kev is used typically for sputtering. Figure 5(a) shows that highest yield can be got with the size of 500 at the energy. If the energy is kept at 20 kev, the cluster size may decrease for high speed sputtering. Figure 5(b) shows that the yield can reach 250 atoms/ion at the acceleration energy of 100 kev with the size of 2000. The result indicates that extremely high sputtering can be realized with large Figure 6 : Energy dependence of the sputtering yield size cluster irradiation at high energy. per Ar atom calculated with formula (1) and TRIM. Figure 6 shows the energy
dependence of the sputtering yield per Ar atom calculated with formula (1) and TRIM 10). The Target is Si substrate. This figure shows that cluster can etch substrates even if the energy per atom is less than the threshold energy for sputtering with monomer ion irradiation. The fast is the most remarkable difference between cluster irradiation and monomer irradiation for sputtering. The low threshold energy of cluster causes for high sputtering yield with cluster ion irradiation at low energy per atom. This result indicates that the surface modification with very low damage can be done with cluster ion beam. Summary There is a threshold energy for sputtering with cluster ion irradiation. When the cluster size is constant, the sputtering yield is proportional to acceleration energy. These results suggest empirical formula to calculate sputtering yield from ion energy and size of cluster. The empirical formula indicates that sputtering yield is proportional to the 1.1 power of size, when the acceleration energy per atom is constant. Acknowledgment This work is supported by New Energy and Industrial Technology Development Organization (NEDO). References 1) H.Kitani, N.Toyoda, J.Matsuo and I.Yamada, Nucl. Instr. and Meth. B121 (1997) 489. 2) N.Toyoda, N.Hagiwara, J.Matsuo and I.Yamada, Nucl. Instr. and Meth. B148 (1999) 639. 3) A.Nishiyama, M.Adachi, N.Toyoda, N.Hagiwara, J.Matsuo and I.Yamada, AIP conference proceedings (15-th International Conference on Application of Accelerators in Research and Industry) 475 (1998) 421. 4) D.Takeuchi, J.Matsuo, A.Kitai and I.Yamada, Mat. Sci. and Eng. A217/218 (1996) 74 5) N.Shimada, T.Aoki, J.Matsuo, I.Yamada, K.Goto and T.Sugui, J. Mat. Chem. and Phys. 54 (1998) 80. 6) I.Yamada, J.Matsuo, N.Toyoda, T.Aoki, E.Jones and Z.Insepov, Mat. Sci. and Eng. A253 (2000) 249.
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