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N IM B
Beam Interactions with Materials & Atoms

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

Interactions of ethanol cluster ion beams with silicon surfaces
G.H. Takaoka *, H. Noguchi, M. Kawashita
Ion Beam Engineering Experimental Laboratory, Kyoto University, Nishikyo, Kyoto 615-8510, Japan Available online

Abstract The liquid cluster ion beam system was developed, in which cluster ions of organic molecules such as ethanol could be produced. In order to investigate the interactions of ethanol cluster ion beams with solid surfaces, SiO2 substrates and SiO2 films were irradiated at different acceleration voltages. The sputtered depth increased exponentially with the acceleration voltage. When the acceleration voltage was 9 kV, the sputtereddepths of Si and SiO2 at a dose of 1 · 1016 ions/cm2 were 344.6 nm and47.2 nm, respectively. The sputtering yield for the Si surface was approximately 100 times larger than that by Ar ion beams. With regards to the sputtering ratio of Si to SiO2, the ratio increased with decreasing acceleration voltage. This suggests that chemical reactions between Si and ethanol produced silicon hydride which was the dominant etching material for the Si surfaces. In addition, the AFM observation showed that the sputtered surface had an average roughness of less than 1 nm. With regards to the crystalline state of the sputtered surface, the RBS and ellipsometry measurements were performed, and they showed that the Si surface damage induced by ethanol cluster ion irradiation was less than that by Ar monomer ion irradiation. Thus, Si surfaces etched at high sputtering speed by ethanol cluster ion beams had a lower damage and an atomically flat surface. ñ 2005 Elsevier B.V. All rights reserved.
PACS: 36.40.Wa; 41.85.þp; 81.65.Cf Keywords: Cluster ion beam; Ethanol cluster; Sputtering yield; Dry process; Wet process

1. Introduction It is well known that the wet process using organic liquid materials has been applied to the surface treatment for solid surfaces. Usually, etching by organic liquid materials such as ethanol and acetone is not achieved even at elevated temperatures. Instead of them, acid materials such as hydrochloric acid, hydronitric acid and hydrofluoric acid are used for etching the solid surfaces. On the other hand, ion beam process as a dry process is also applied to the surface treatment [1­3]. It has several advantages, one of which is that surface and interface characteristics of materials can be controlled on an atomic scale. Another advantage is that the accelerating energy of the ion beams toward the solid surfaces can be applied to the enhance-

*

Corresponding author. Tel./fax: +81 75 383 2343. E-mail address: gtakaoka@kuee.kyoto-u.ac.jp (G.H. Takaoka).

ment of the chemical reaction between ions and surface atoms. When the liquid materials are used as the source material of ion beam process, it is expected that the interaction of the liquid material ions with the solid surfaces is much different from that of the wet process. Clusters are tiny particles, which are in a position to a link between atomic (or molecular) state and bulk state [4]. They constitute a new phase of matter, with significantly different properties than solids, liquids and gases. Furthermore, after the clusters are ionized and accelerated to solid surfaces, many atoms constituting a cluster ion impact instantaneously on a local area of the surfaces. Therefore, high-density energy deposition and multiple collisions are realized, which result in exhibiting the specific properties in ion beam interactions with solid surfaces [5­7]. In this paper, the interactions of ethanol cluster ion beams with silicon surfaces are investigated in order to clarify the difference between the wet and dry processes. Furthermore, the irradiation damage and surface etching process

0168-583X/$ - see front matter ñ 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.073


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are discussed based on the specific properties of the ethanol cluster ion beams.
Sputtered Depth (nm)

350 300 Vext=300V, Vd=27V 250 200 150 100 50 0 0 3 9 6 Acceleration Voltage (kV)
Ve=200V, Ie=200mA Dose:1.0â1016 ions/cm2 Si Substrate SiO2 Film

2. Experimental procedure The details of the liquid cluster ion source was described elsewhere [8]. The vapors of ethanol were ejected through a nozzle into a high-vacuum region, and the vaporized ethanol clusters were produced by an adiabatic expansion. The neutral clusters produced were ionized by electron bombardment. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was adjusted between 0 mA and 250 mA. The cluster ions were accelerated by applying an extraction voltage to an extraction electrode, and the extraction voltage (Vext) was 300 V. The extracted cluster ions were size-separated by a retarding potential method. The retarding voltage (Vd) was 27 V, and the cluster size used was larger than 95. The sizeseparated cluster ion beams were accelerated toward a substrate, which was set on a substrate holder. The acceleration voltage (Va) was adjusted between 0 kV and 10 kV. The substrates used were Si(1 0 0). To be compared with the cluster ion irradiation on the Si surface, SiO2 films, which were thermally grown on the Si substrates, were also irradiated by the ethanol cluster ion beams. The background pressure around the substrate was 6 · 10þ7 Torr, which was attained using a diffusion pump. For the evaluation of ion irradiation damage for the Si substrates, the Rutherford backscattering spectrometry (RBS) and ellipsometry measurements were performed. For the RBS channeling measurement, the surface peaks for the irradiated Si substrates were measured, and the number of displaced atoms was calculated from the area of the surface peak. For the ellipsometry measurement, the values of Delta and Psi were measured, and the thickness of oxide and damaged layers were calculated based on the values of Delta andPsi measured. In the calculation, the irradiated Si surface was assumed to form the oxide and damaged layers [9]. In the assumption, the top layer is the silicon oxide layer with a refractive index of 1.465. The second layer is the damaged layer, and the refractive and extinguished indexes are assumed to be 4.630 and þ0.76, respectively. In addition, the refractive and extinguished indexes of Si substrate surface are assumed to be 3.868 and þ0.024, respectively. 3. Results and discussion Fig. 1 shows the dependence of sputtered depth for Si(1 0 0) substrates and SiO2 films on acceleration voltage. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 200 mA. The cluster ion dose was 1 · 1016 ions/cm2. The Si substrates and the SiO2 films were at room temperature, and the SiO2 film thickness was 500 nm. The sputtered depth was measured by the step profiler (Veeco Instruments: DEKTAK3173933). As shown in the figure, the sputtered depth in-

Fig. 1. Dependence of sputtered depth for Si(1 0 0) substrates and SiO2 films on the acceleration voltage for ethanol cluster ions. Irradiation conditions for the Si substrates and SiO2 films are Ve = 200 V, Ie = 200 mA, Vd = 27 V and an ion dose of 1.0 · 1016 ions/cm2.

creases exponentially with the acceleration voltage. When the acceleration voltage is 9 kV, the sputtered depths of Si and SiO2 are 344.6 nm and 47.2 nm, respectively. The effective sputtering of SiO2 with ethanol cluster ion beams can be applied to a surface cleaning process for Si substrates. Furthermore, based on the consideration of the sputtered depth and the ion dose, the sputtering yield was calculated by estimating the density of Si. The sputtering yield of Si at an acceleration voltage of 9 kV was 178 atoms/ion, which is approximately 100 times larger than that by argon (Ar) ion beam sputtering. In addition, even when the acceleration voltage is 3 kV, Si is sputtered to a depth of 21.3 nm. In this case, the energy per molecule for ethanol cluster ions is less than 32 eV. Therefore, chemical reactions between Si and ethanol occurred, which resulted in the dominant sputtering process at low energy. With regards to the sputtering ratio of Si to SiO2, the ratio increases with decreasing acceleration voltage. The selectivity arises from the volatility of the reaction products and the difference in binding energy among the materials. The sputtered depth of SiO2 at an acceleration voltage of 3 kV is less than the detection level of the surface profiler, and the sputtering ratio at 3 kV is higher than a few tens. This suggests that chemical reactions between Si and ethanol produce silicon hydride which is the dominant etching material for the Si surfaces. Fig. 2 shows the dependence of the number of displaced Si atoms on (a) the ion dose and (b) the acceleration voltage. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 250 mA. The cluster size was larger than 95. The acceleration voltage (Va) in Fig. 2(a) was kept at 6 kV, and the ion dose in Fig. 2(b) was at 1 · 1015 ions/cm2. As shown in Fig. 2(a), the number of displaced atoms increases with increase in the ion dose, and it is saturated at ion doses larger than 1 · 1014 ions/cm2. To be compared with Ar monomer ion irradiation, the number of displaced atoms is much lower at high doses. With regard to the acceleration voltage dependence, as shown in Fig. 2(b), the number of displaced


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Number of Displaced Atoms (x1016 atoms/cm2)

10 8 6 4 2

Ve=200V, Ie=250mA Vext=300V, Vd=27V Va=6kV Ethanol cluster ion Ar monomer ion

195 190 185 180
Td (å)

40
1015

80

0
Unirradiated 1014 13 10 40 1014 1015

Delta (degree)

175 170 165 160 155 150 145

Unirradiated
0 10
12

10

13

10

14

10

15

10

16

80
Tox (å)

1016ions/cm2

(a)

Dose (x1015 ions/cm2)
10 8 6 4 2 Unirradiated 0 0123456 78 9 10 Ve=200V, Ie=250mA Vext=300V, Vd=27V Dose:1.0x1015 ions/cm Ethanol cluster ion Ar monomer ion

120
9 10 11 12

Number of Displaced Atoms (x1016 atoms/cm2)

140
2

Va=6k V Ethanol cluster ion Ar monomer ion
13 14 15 16 17

Psi (degree)
Fig. 3. Values of Delta and Psi measured by ellipsometry for Si surfaces irradiated by ethanol cluster ion and Ar monomer ion at different ion doses. Irradiation conditions for the Si substrates are Ve = 200 V, Ie = 250 mA, Vd = 27 V and Va = 6 kV.

(b)

Acceleration Voltage (kV)

Fig. 2. Dependence of number of displaced atoms for Si surfaces irradiated by ethanol cluster ions and Ar ions on (a) the ion dose and (b) the acceleration voltage. Irradiation conditions for the Si substrates are Ve = 200 V, Ie = 250 mA, and Vd =27V.

atoms for the ethanol cluster ion irradiation is also less than that for the Ar ion irradiation at the same acceleration voltage. The mass number for ethanol molecule and Ar atom is 46 and 40, respectively, and both the particles have the similar mass. However, for the case of ethanol cluster ion irradiation, the incident energy of an ethanol molecule, which is the constituent particle of a cluster ion, is the accelerating energy divided by the cluster size, and it is less than 100 eV. Therefore, the irradiation damage for the ethanol cluster ions is less than that for Ar monomer ions. In addition, at an acceleration voltage of 1 kV, the number of displaced atoms by the ethanol cluster ion irradiation is the same as that of the unirradiated Si surface. Because the incident energy of an ethanol molecule is less than 10 eV, the damage-free surface is obtained by extremely low energy irradiation of the ethanol cluster ion beams. Fig. 3 shows the values of Delta and Psi measured for Si surfaces irradiated by ethanol cluster ions and Ar monomer ions at different ion doses. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 250 mA. The acceleration voltage (Va) was 6 kV. For the ethanol cluster ion irradiation, the thickness of oxide layer increases with the increase in the ion dose, and it is saturated at a dose of 1 · 1015 ions/cm2. With regards to the damage thickness, it is less than 1 nm and is almost the same at the ion doses between 1 · 1013 ions/ cm2 and 1 · 1016 ions/cm2. The total thickness of the oxide and damage thickness corresponds to the damage thickness measured by the RBS channeling. This indicates that the

damage layer induced by the ethanol cluster ion irradiation is very thin, and that the silicon oxide layer with larger thickness is formed on the Si surface irradiated. The oxygen as a constituent atom of the ethanol molecule might contribute to the oxide layer formation. On the other hand, for the case of Ar monomer ion irradiation, the damage layer thickness increases with the increase in the ion dose. At an ion dose of 1 · 1016 ions/cm2, both the damage and oxide layer thickness become very large. This is explained by the fact that high energy Ar ion irradiation forms the damage layer on the Si surfaces. At an ion dose of 1 · 1016 ions/cm2, more Ar atoms are incorporated near the Si surface, and some of the Ar atoms are replaced by oxygen atoms, which result in the increase of the oxide layer thickness. The sputtered Si surfaces were observed using an atomic force microscope (AFM), and the surface roughness was measured. The surface roughness for the unirradiated substrate was 0.18 nm. It increased with the increase in ion dose, and it was 0.65 nm at an ion dose of 1 · 1016 ions/ cm2. A smooth surface at an atomic level was obtained even after sputtering. This is ascribed to the inherent fluid property as well as the migration effect of the ethanol molecules broken up after impacts of the ethanol cluster ions on the Si surfaces. 4. Conclusions The liquid cluster ion beam system was developed, in which cluster ions of organic molecules such as ethanol were produced. The size separation of the cluster ions was performed by a retarding potential method. The ethanol cluster ions with a size larger than 95 were irradiated on Si(1 0 0) substrates, and the substrate surface was sputtered even at an acceleration voltage of a few kV. In addition, the sputtering yield at an acceleration voltage of 9 kV was


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4 G.H. Takaoka et al. / Nucl. Instr. and Meth. in Phys. Res. B xxx (2005) xxx­xxx [3] J. Fujita, M. Ishida, T. Ichihashi, Y. Ochiai, T. Kaito, S. Matsui, Nucl. Instr. and Meth. B 206 (2003) 472. [4] M.A. Duncan, D.H. Rouvray, Scientific America, December 1989, p. 110. [5] I. Yamada, G.H. Takaoka, Jpn. J. Appl. Phys. 32 (1993) 2121. [6] I. Yamada, Nucl. Instr. and Meth. B 148 (1999) 1. [7] H. Biederman, D. Slavinska, H. Boldyreva, H. Lehmberg, G. Takaoka, J. Matsuo, H. Kinpara, J. Zemek, J. Vac. Sci. Technol. B 19 (2001) 2050. [8] G.H. Takaoka, H. Noguchi, T. Yamamoto, T. Seki, Jpn. J. Appl. Phys. 42 (2003) L1032. [9] J.L. Buckner, D.J. Vitkavage, E.A. Irene, J. Appl. Phys. 63 (1988) 5288.

approximately 100 times larger than that by Ar ion beams. Furthermore, the sputtered surface had an average roughness of less than 1 nm. Thus, ethanol cluster ion beams have unique characteristics suitable for the Si surface treatment such as high sputtering yield and smooth surface formation at an atomic level, which are not achieved in the conventional wet process. Acknowledgment The authors are grateful to the Quantum Science and Engineering Center of Kyoto University for the RBS measurement. References
[1] J.E. Greene, Mater. Res. Bull. 26 (2001) 777. [2] G.H. Takaoka, K. Tsumura, T. Yamamoto, Jpn. J. Appl. Phys. 41 (2002) L660.