SURFACE PROCESSING WITH HIGH-ENERGY GAS CLUSTER ION BEAMS Toshio Seki and Jiro Matsuo, Quantum Science and Engineering Center, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan Abstract Gas cluster ion beams can produce high rate sputtering with low damage compared with the more common monomer ion beam process. In order to realize high-speed surface processing, a high-energy gas cluster ion beam irradiation system was developed in which high-energy Ar cluster ion beams were generated. The mean size of the Ar cluster was about 1800 atoms, as measured using the time-of-flight (TOF) method. Si substrates were irradiated with the Ar cluster ions at acceleration energies between 20 80 kev. The sputtering yield increased with acceleration energy and reached about 230 atoms/ion at 80 kev, a value about 180 times higher than that of Ar monomer ions. Au films were also irradiated at acceleration energies between 20 80 kev and the surfaces were observed with Atomic Force Microscope (AFM). The high-energy cluster ion irradiation cause a decrease in surface roughness. These results indicate that high-speed smooth etching can be realized with high-energy cluster ion beams. This processing method can be applied to fabricate nano-devices. Introduction The gas cluster ion beam process has a high potential for material processing in nano-technology devices, such as photonic crystals and MEMS (Micro Electro Mechanical System). In order to fabricate the devices, one needs to etch targets in a high-speed, low-damage, and ultra-smooth process. A cluster is an aggregate of a few to several thousands atoms. When the many atoms constituting a cluster ion bombard a local area, high-density energy deposition and multiple-collision processes occur simultaneously. Because of the unique interactions between cluster ions and surface atoms, new surface modification processes could be developed, and surface smoothing [1-4], shallow implantation [5,6], high rate sputtering [7] and low damage surface processing [8] have been demonstrated using this technique. In order to realize high-speed surface processing with gas cluster ion beams, a high current and high-energy cluster ion beam is needed. In a previous work, a maximum beam current of 1 ma was achieved [9]. With this beam current, 12-inch wafers can be treated with 2x10 15 ions/cm 2 in about 4 minutes, and this is sufficiently high for next generation processes. On the other hand, etching yields by cluster ion beams are expected to increase with acceleration energy [10]. In this work, a high-energy gas cluster ion beam irradiation system was developed in which high-energy Ar cluster ion beams were generated. The effects of increasing accelerating energy on the sputtering yield with high-energy gas cluster ion beams and on surface roughness were investigated.
Figure 1. Schematic diagram of the high-energy gas cluster ion beam irradiation system. Experimental Figure 1 shows a schematic diagram of the high-energy gas cluster ion beam irradiation system. Adiabatic expansion of a high-pressure gas through a nozzle is utilized for the formation of the gas cluster beam. When a supersonic flow ejects from the nozzle, shockwaves are generated [11], which disturb the generation of neutral cluster beams. To avoid formation of such shockwaves, a skimmer is utilized to pass the core of the supersonic flow into high vacuum. The neutral clusters are then ionized by electron bombardment. The ionizer consists of a number of filaments and an anode. Electrons ejected from the hot filaments are accelerated towards the neutral cluster beam and ionize the clusters. The ionized clusters are extracted and accelerated to energies up to 80keV towards the target. Monomer ions are eliminated by a magnetic field. The mass distributions were measured with a compact time-of-flight (TOF) system. The system can be installed in cluster irradiation machines and used as a cluster size monitor. Figure 2 shows the Ar cluster size distribution when the source gas pressure was 0.53 MPa (4000 Torr), the ionization energy was 400 ev and the emission current was 200 ma. The mean cluster size was about 1800 atoms. The cluster beam has almost the same cluster size as other cluster irradiation systems [4]. In order Figure 2. Ar cluster size distribution when the source gas pressure was 0.53 MPa (4000 Torr), the ionization energy was 400 ev and the emission current was 200 ma; the mean cluster size was about 1800 atoms.
to investigate the energy dependence of the sputtering yield, Si substrates and Au films were irradiated with the Ar cluster ion beam at the above-mentioned conditions. The sputtering yields were measured with a contact profiler and the irradiated surfaces were measured with an Atomic Force Microscope (AFM; Shimadzu SPM9500J2). For average roughness the data was derived from 1 1 µm 2 surface images measured by AFM. The roughness of Si substrates was 0.057 nm before irradiation. Results and discussion Figure 3 shows the sputtering yields of Si and Au with Ar cluster and monomer as a function of acceleration energy. The sputtering yield of Si with Ar monomer, calculated using TRIM [12], did not increase with the acceleration energy. The sputtering yields of Si and Au with Ar cluster, however, have increased with acceleration energy. The sputtering yield of Si with Ar cluster reached about 230 atoms/ion at 80 kev, and this value is about 180 times higher than that obtained with Ar monomer ions. These results show that high-speed etching can be realized with high-energy cluster ion irradiation. In a previous work, a formula was proposed to calculate the yield with cluster (Y(N)) from the cluster size (N) and the 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 a coefficient of size effects, and k is a constant [10]. The parameters of the empirical formula (1) used to calculate the 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. The sputtering yields of Si and Au calculated from the empirical formula are described in Figure 3. The cluster size (N) was 1800 atoms. Although the empirical formula was based on yields at energies up to 20 kev, the sputtering yields calculated from it were similar to experimental data of yields at high-energy. This result shows that the empirical formula can apply to the high-energy range up to 80 kev. Because the empirical formula indicates that the yield is proportional to the acceleration energy, it is expected that the yield would increase if a higher Figure 3. Acceleration energy dependence of the sputtering yield of Si and Au with Ar cluster and Ar monomer; operating conditions as in Fig.2.
(a) Si substrate (b) Au film Figure 4. Acceleration energy dependence of the roughness of Si and Au surfaces after Ar cluster irradiation; operating conditions as in Fig. 2. energy cluster ion beam were generated. Figure 4 shows the average roughness of Si and Au surfaces after Ar cluster irradiation as a function of the acceleration energy for an Ar cluster ion dose of 5 10 15 ions/cm 2. The roughness after irradiation increased with the acceleration energy. Because the initial Si surface was atomically flat, the Si substrates were roughened by Ar cluster irradiations. When a surface is bombarded with a cluster, a crater is formed on the surface [13], and its size increases with the acceleration energy [14]. It is proposed that the increase in surface roughness is caused by the crater formation. On the other hand, the initial roughness of Au films was 1.84 nm, and it decreased by cluster irradiation; the minimum value obtained was 0.66 nm at 20 kev. The roughness after irradiation increased with the acceleration energy, but the roughness at 80 kev was still lower than the initial roughness. The increase in surface roughness is attributed also to crater formation. These results indicate that the smoothing effect of the cluster ion beam was preserved in this high-energy range. Figure 5 shows a model of surface smoothing with Ar cluster irradiation. A crater is formed on the surface by single cluster ion impact. When a cluster bombards the surface at a low energy, the crater size is small and the peak-to-valley height is also small. However, when the cluster bombards the surface at higher energy, the crater size is larger and the peak-to-valley height is also larger. The surface roughness at high ion dose corresponds to the average peak-to-valley height of the craters. Because the peak-to-valley height of the crater increases with acceleration energy, the surface roughness at high ion dose also increases. Therefore, if an atomically flat surface, such as a Si surface, is irradiated with a cluster ion beam, the surface roughness increases with acceleration energy. However, the surface roughness caused by the crater formation is less than 2.0 nm at acceleration energies in the range 20 80 kev. When the initial surface is rougher than the surface roughness caused by the
Figure 5. A model of surface smoothing with Ar cluster irradiation. crater formation, the cluster ion beam can smooth the surface. This effect is a fundamental effect of the cluster ion beam and it is preserved (although somewhat reduced) at high energy. The surface can be smoothed until it reaches the roughness caused by the crater formation. Conclusions Si substrates and Au films were irradiated with Ar cluster ions at acceleration energies of between 20 80 kev. The Si sputtering yield increased with acceleration energy and reached about 230 atoms/ion at 80 kev, a value about 180 times higher than that of Ar monomer ions. The surface roughness of Au films decreased with the high-energy cluster ion irradiation. The surface can be smoothed until it reaches the roughness caused by the crater formation. These results indicate that high-speed and smooth surface processing can be realized with high-energy cluster ion beams. Acknowledgments This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. 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. T.Seki and J.Matsuo, Nucl. Instr. and Meth. B216 (2004) 191.
5. D.Takeuchi, J.Matsuo, A.Kitai and I.Yamada, Mat. Sci. and Eng. A217/218 (1996) 74 6. N.Shimada, T.Aoki, J.Matsuo, I.Yamada, K.Goto and T.Sugui, J. Mat. Chem. and Phys. 54 (1998) 80. 7. I.Yamada, J.Matsuo, N.Toyoda, T.Aoki, E.Jones and Z.Insepov, Mat. Sci. and Eng. A253 (1998) 249. 8. M.Akizuki, J.Matsuo, M.Harada, S.Ogasawara, A.Doi, K.Yoneda, T.Yamaguchi, G.H.Takaoka, C.E.Asheron and I.Yamada, Nucl. Instr. and Meth. B99 (1995) 229. 9. T.Seki and J.Matsuo, Nucl. Instr. and Meth. B237 (2005) 455. 10. T.Seki, T.Murase and J.Matsuo, Nucl. Instr. and Meth. B (2005). (In press) 11. H.W.Liepmann and A.Roshko, "Elements of Gas Dynamics" (John Wiley and Sons, Inc., New York, 1960). 12. J.P.Biersack and L.G.Haggmark, Nucl. Instr. and Meth., 174 (1980) 257. 13. T.Seki, T.Kaneko, D.Takeuchi, T.Aoki, J.Matsuo, Z.Insepov and I.Yamada, Nucl. Instr. and Meth., B121 (1997) 498. 14. D.Takeuchi, T.Seki, T.Aoki, J.Matsuo and I.Yamada, J. Mat. Chem. and Phys. 54 (1998) 76.