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Interactions of liquid cluster ion beams with metal surfaces
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 258 (2007) 209–212 www.elsevier.com/locate/nimb
Interactions of liquid cluster ion beams with metal surfaces G.H. Takaoka *, M. Kawashita, K. Nakayama, T. Okada Ion Beam Engineering Experimental Laboratory, Kyoto University, Nishikyo, Kyoto 615-8510, Japan Available online 3 January 2007
Abstract In order to investigate the interactions of ethanol and water cluster ion beams with metal surfaces, titanium (Ti) substrates were irradiated at different acceleration voltages. The sputtered depth increased with increase of the acceleration voltage. The sputtering yield of Ti at an acceleration voltage of 9 kV for the ethanol cluster ions was approximately 100 times larger than that by argon (Ar) ion beams. On the other hand, for the case of the water cluster ion irradiation, the sputtering yield was a few tens times larger than that for Ar monomer ion irradiation. In addition, the AFM observation showed that the sputtered surfaces by the ethanol and water cluster ion beams had an average roughness of less than 1 nm. For other metal surfaces such as Al, Ni, Cu, Ag and Au surfaces, the sputtering yield by the liquid cluster ion beams was also much larger than that by Ar monomer ion beams at the same acceleration voltage. Furthermore, the wettability of the Ti surfaces was changed drastically by the species of the cluster ions as well as the acceleration voltage. For the Ti surfaces irradiated by the ethanol cluster ion beams, the contact angle decreased with increase of the acceleration voltage, and it became about 10 at higher acceleration voltages. However, for the case of the water cluster ion irradiation, the contact angle was almost the same at various acceleration voltages, and it was about 80 which was larger than that of unirradiated surface. 2006 Elsevier B.V. All rights reserved. PACS: 36.40.Wa; 61.46.Bc; 61.80.Jh; 68.47.Fg; 68.49.Fg; 81.65.Cf; 82.80.Rt Keywords: Cluster ion beam; Ethanol cluster; Water cluster; Sputtering; Wettability
1. Introduction Cluster ion beams are useful tools for the investigation of the fundamentals of solid-state physics, chemistry and related materials science [1]. The impact of accelerated cluster ion beams on the solid surfaces exhibits unique characteristics such as high-energy density irradiaition and low-velocity irradiation effects, which are not obtained by monomer ion beams. As a consequence, the properties of the bombarded surface are significantly altered through deposition and sputtering by the cluster ion beams [2,3]. Liquid-source cluster ion beams have several advantages for surface treatment, and they can be used in physical sputtering and chemical etching of the substrate material, for making surfaces smooth, and for cluster-activated
*
Corresponding author. Tel.: +81 75 383 2329; fax: +81 75 383 2343. E-mail address: [email protected] (G.H. Takaoka).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.128
chemical modification of the substrate surfaces [4,5]. The deposition of liquid cluster ions on the solid surface does not normally lead to the thick film, but the cluster material itself is deposited on the surface by forming extremely thin layer. This feature is useful for the chemical modification of solid surface using various kinds of liquid materials such as water, alcohol and paraffin. In addition, the interactions of cluster ions with surface atoms are occurred in small area and short time at nano-level, and the increase of incident energy for the cluster ions enhances the temperature of the bombarded surface [6]. Therefore, the chemical reaction between liquid clusters and surface atoms, which is occurred at non-equilibrium state, has many possibilities for various kinds of state transitions depending on the activation energy. This is much different from the chemical reaction at thermal equilibrium state in conventional wet processes. We have been focusing on the investigation of the fundamentals of solid-state physics and chemistry by using the ethanol and water cluster ion beams. In this
article, sputtering effects of the ethanol and water cluster ions on metal surfaces such as titanium (Ti) surface, etc. are investigated in order to clarify the interactions of the liquid cluster ion beams with solid surface atoms. Furthermore, the Ti surface state after sputtering is discussed based on atomic force microscope (AFM) and contact angle measurements. 2. Experiments Liquid materials such as ethanol and water were introduced into the source and heated up to 150 C. The vapors were ejected through a small nozzle into a vacuum region. When the vapor pressure was larger than 1 atm, the vaporized clusters were produced by an adiabatic expansion. The clusters were ionized by an electron bombardment method. The electron voltage for ionization (Ve) was adjusted between 0 V and 300 V, and the electron current for ionization (Ie) was adjusted between 0 mA and 250 mA. The cluster ions were extracted by applying an extraction voltage to the extraction electrode. The extracted cluster ions were size-separated by applying a retarding potential, which was 27 V for ethanol cluster and 14 V for water cluster, respectively. The cluster ions with the size larger than approximately 100 molecules per cluster were accelerated toward the substrates by applying an acceleration voltage. The acceleration voltage (Va) was adjusted between 0 kV and 10 kV. The substrates used were metal films with a thickness of approximately 500 nm, and the substrate temperature was room temperature. The background pressure around the substrate was 1 · 107 Torr, which was attained using a turbo-molecular pump. The cluster size was measured by a time-of-flight (TOF) method. In the cluster size measurement, it was assumed that the cluster ion was a singly charged ion. If the multiply charged cluster ions were produced, the dissociation of cluster ions were assumed to be occurred due to the Coulomb repulsion forces between the constituent atoms of a cluster ion [7,8]. The peak size was about 500–1000 molecules-per-cluster for the ethanol cluster ions and about 2500 molecules-per-cluster for the water cluster ions, respectively. The intensity of the cluster ions increased with increase of the vapor pressure. 3. Results and discussion The sputtering process by irradiation of liquid cluster ions on Ti surfaces was investigated. The sputtered depth was measured by the step profiler (Veeco Instruments: DEKTAK- 3173933). Fig. 1 shows the dependence of sputtered depth for Ti surfaces on acceleration voltage for the ethanol and water cluster ion irradiation. The cluster size was larger than 100 molecules-per-cluster, and the ion dose was 1.0 · 1016 ions/cm2. As shown in the figure, the sputtered depth increases with increase of the acceleration voltage. When the acceleration voltage is 9 kV, they are 160 nm for the ethanol cluster ion irradiation and 22.3 nm for the water
Sputtered depth (nm)
G.H. Takaoka et al. / Nucl. Instr. and Meth. in Phys. Res. B 258 (2007) 209–212 200 Ti Substrate 180 Ethanol clusterion Water clusterion 160 Dose = 1.0 x 1016 ions/cm2 140 Ve = 200 V, Ie = 200 mA 120 100 80 60 40 20 0 0 1 2 3 4 5 6 7 8 9 10
Acceleration voltage (kV) Fig. 1. Dependence of sputtered depth for Ti surfaces on acceleration voltage for the ethanol and water cluster ion irradiation.
cluster ion irradiation, respectively. Taking account of the sputtered depth and the ion dose, the sputtering yield was calculated by estimating the density of Ti such as 4.5 g/ cm3. The sputtering yield at an acceleration voltage of 9 kV was 90.1 atoms-per-ion for the ethanol cluster ion irradiation and 12.6 atoms-per-ion for water cluster ion irradiation. The sputtering yields for several kinds of metal surfaces irradiated by the ethanol and water cluster ion beams were investigated. Fig. 2 shows the sputtering yield for Al, Ti, Ni, Cu, Ag and Au surfaces at an acceleration voltage of 9 kV. In the figure, the sputtering yield by irradiation of argon (Ar) monomer ion beam at an acceleration voltage of 9 kV is also shown. In the case of water cluster ion irradiation, the sputtering yield is about ten times larger than that by Ar monomer ion irradiation, even if the incident energy of a water molecule as a constituent particle of a cluster ion is less than 90 eV. On the other hand, in the case of ethanol cluster ion irradiation, Al, Ti and Ni surfaces are sputtered more effectively, and the sputtering yield is approximately 100 times larger than that by Ar monomer ion irradiation. This is considered to be due to the enhancement of chemical sputtering, in which alkyl compound of metal is formed as a volatile product. Alkyl radicals
Sputtering yield (atoms/ion)
210
Ethanol cluster ion 9 keV Water cluster ion 9 keV Ar monomer ion 9 keV
1000
100
10
1
0.1
Al
Ti
Ni Cu
Ag Au
Fig. 2. Sputtering yield for Al, Ti, Ni, Cu, Ag and Au surfaces by irradiation of ethanol cluster, water cluster and Ar monomer ion beams at an acceleration voltage of 9 kV.
G.H. Takaoka et al. / Nucl. Instr. and Meth. in Phys. Res. B 258 (2007) 209–212
consisting of the ethanol molecule, which are produced after impact of the ethanol cluster ions on the metal surfaces, have an important role in chemical reaction for these metal surfaces. The rate of chemical reaction (m) with various kinds of reaction channels is described as follows; [9] n kT X Q v/N exp i ; ð1Þ h i¼1 kT where N is the number density of liquid molecules such as ethanol molecule and water molecule, h is Planck constant, k is Boltzmann constant, T is the temperature of liquid molecules after impact, and Qi (i = 1, 2, . . ., n) is the activation energy for a reaction channel (i). The equation indicates that the rate of chemical reaction increases with increasing temperature. When the liquid cluster ions impact on the metal surfaces, the cluster ions are broken up, and the multiple collisions between liquid molecules and metal atoms are occurred. As a result, many metal atoms are displaced and vibrated, and the incident energy of the cluster ions is used for heating an impact region of the metal surfaces. The molecular dynamic (MD) simulations showed that the temperature of the cluster impact region was a few tens of thousands degrees, although the substrate temperature was at room temperature [6]. Therefore, the thermal vibration of the liquid molecules, which is expressed by kT/h, becomes extremely high, and the barrier height for the chemical reaction, which is expressed by Qi/ kT, becomes comparably low. For the case of the ethanol cluster ion irradiation, the high temperature at the impact region enhances several kinds of chemical reactions available between the alkyl radicals of the ethanol molecule and the metal surface atoms, and the volatile materials such as several kinds of metal organic compounds are produced. On the other hand, for the case of the water cluster ion irradiation, hydroxyl (OH) radicals are produced after impact on the surfaces, and the oxidation occurs through
211
the chemical reaction between the OH radicals and the metal surface atoms except for Au atoms. Furthermore, during the impact, the physical sputtering of the metal oxide surfaces also occurs due to the high-energy density deposition by the water cluster ion beams. With regards to the oxide layer formation, it was confirmed by the XPS measurement. As a result, the metal surfaces such as Al, Ti, Ni, etc are sputtered by the water cluster ion irradiation, although the oxide layer is remained on the metal surface. The sputtered Ti surfaces were measured by using an atomic force microscope (AFM). Fig. 3 shows the surface morphologies (a) for unirradiated surface and for sputtered surfaces by (b) ethanol and (c) water cluster ion beams, respectively. The acceleration voltage was 6 kV, and the ion dose was 1.0 · 1015 ions/cm2. As shown in the figure, the surface roughness for the unirradiated substrates is 0.68 nm. On the other hand, the sputtered surfaces become rough, and the surface roughness is 0.84 nm for ethanol cluster ion irradiation and 0.74 nm for water cluster ion irradiation, respectively. Smooth surface with a roughness less than 1 nm is obtained even after sputtering. The lateral sputtering effect is responsible for the smooth surface formation by the cluster ion irradiation. The liquid cluster ion beam process has unique characteristics suitable for surface treatment such as high sputtering yield and smooth surface formation at an atomic level, which are not achieved by the conventional wet process. The wettability of the Ti surfaces irradiated by ethanol and water cluster ion beams were investigated by measuring the contact angles for water droplet, which was put on the Ti surfaces immediately after their removal from the vacuum chamber. Fig. 4 shows the dependence of the contact angle on the acceleration voltage for the cluster ion irradiation. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 200 mA. The ion dose was 1.0 · 1015 ions/cm2. The cluster size was larger than 100 molecules-per-cluster.
Fig. 3. AFM images (a) for unirradiated Ti-surface and for sputtered Ti-surfaces by (b) ethanol and (c) water cluster ion beams, respectively.
G.H. Takaoka et al. / Nucl. Instr. and Meth. in Phys. Res. B 258 (2007) 209–212
Contact angle (degree)
212 100 90 80 70 60 50 40 30 20 10 0
Ti substrate Dose = 1.0x1015 ions/cm2 Water cluster ion Ethanol cluster ion (Unirradiated)
0 1 2 3 4 5 6 7 8 9 10
Acceleration voltage (kV) Fig. 4. Dependence of the contact angle for the Ti surface on the acceleration voltage for the ethanol and water cluster ion irradiation.
The contact angle for the unirradiated surfaces was also measured, and it was about 30. As shown in the figure, the contact angle for the ethanol cluster ion irradiation decreases with increase of the acceleration voltage, and it is less than 10 at an acceleration voltage larger than 6 kV. According to the Q-mass spectroscopy measurement, fragmentation of the ethanol molecules occurred after impact of the ethanol cluster ions, and the main fragment was CH2OH radical. Therefore, it is considered that the CH2OH radical has a bond with the Ti surface atoms through Ti-CH2 bonding, and the top radical of this bond, that is OH radical, enhances the wettability of the Ti surface. On the other hand, the contact angle for the water cluster ion irradiation becomes large at various acceleration voltages, and it is about 80. The Ti surface becomes hydrophobic by the irradiation of the water cluster ions. This is ascribed to the chemical modification of the Ti surfaces by the OH radicals, which are produced after impact of the water cluster ions on the surfaces. The dangling bonds produced on the Ti surfaces have a bond with the OH radicals, which result in the titanium oxide layer formation on the surfaces. 4. Conclusion In order to investigate the interactions of ethanol and water cluster ion beams with metal surfaces, titanium (Ti) substrates were irradiated at different acceleration voltages. The sputtered depth increased with increase of the acceler-
ation voltage. The sputtering yield of Ti at an acceleration voltage of 9 kV for the ethanol cluster ions was approximately 100 times larger than that by argon (Ar) ion beams. This is ascribed to the enhancement of the chemical sputtering by the ethanol cluster ion beams. On the other hand, for the case of the water cluster ion irradiation, the sputtering yield was a few tens times larger than that for Ar monomer ion irradiation. This is due to the physical sputtering by the water cluster ion beams. In addition, the AFM observation showed that the sputtered surfaces by the ethanol and water cluster ion beams had an average roughness of less than 1 nm. For other metal surfaces such as Al, Ni, Cu, Ag and Au surfaces, the sputtering yield by the ethanol and water cluster ion beams was also much larger than that by Ar monomer ion beams at the same acceleration voltage. Furthermore, the wettability of the Ti surfaces irradiated by the ethanol and water cluster ion beams were investigated by measuring the contact angles for water droplet. The contact angle for the unirradiated surface was about 30. For the surfaces irradiated by the ethanol cluster ion beams, the contact angle decreased with increase of the acceleration voltage, and it became about 10 at higher acceleration voltages. However, for the case of the water cluster ion irradiation, the contact angle became large at various acceleration voltages, and it was about 80. Thus, the wettability of the Ti surfaces was changed drastically by the species of the cluster ions as well as the acceleration voltage. References [1] M.E. Mack, Nucl. Instr. and Meth. B 237 (2005) 235. [2] I. Yamada, G.H. Takaoka, Jpn. J. Appl. Phys. 32 (1993) 2121. [3] G.H. Takaoka, M. Kawashita, K. Omoto, T. Terada, Nucl. Instr. and Meth. B 232 (2005) 200. [4] G.H. Takaoka, H. Noguchi, T. Yamamoto, T. Seki, Jpn. J. Appl. Phys. 42 (2003) L1032. [5] G.H. Takaoka, H. Noguchi, Y. Hironaka, Nucl. Instr. and Meth. B 242 (2006) 100. [6] Z. Insepov, I. Yamada, Surf. Rev. Lett. 3 (1996) 1023. [7] K. Sattler, J. Muhlbach, O. Echt, P. Pfan, E. Rechnagel, Phys. Rev. Lett. 47 (1981) 160. [8] J.G. Gay, B.J. Berne, Phys. Rev. Lett. 49 (1982) 194. [9] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, John Wiley & Sons Inc., New York, 1976 (Chapter 9).
Nuclear Instruments and Methods in Physics Research B 258 (2007) 209–212 www.elsevier.com/locate/nimb
Interactions of liquid cluster ion beams with metal surfaces G.H. Takaoka *, M. Kawashita, K. Nakayama, T. Okada Ion Beam Engineering Experimental Laboratory, Kyoto University, Nishikyo, Kyoto 615-8510, Japan Available online 3 January 2007
Abstract In order to investigate the interactions of ethanol and water cluster ion beams with metal surfaces, titanium (Ti) substrates were irradiated at different acceleration voltages. The sputtered depth increased with increase of the acceleration voltage. The sputtering yield of Ti at an acceleration voltage of 9 kV for the ethanol cluster ions was approximately 100 times larger than that by argon (Ar) ion beams. On the other hand, for the case of the water cluster ion irradiation, the sputtering yield was a few tens times larger than that for Ar monomer ion irradiation. In addition, the AFM observation showed that the sputtered surfaces by the ethanol and water cluster ion beams had an average roughness of less than 1 nm. For other metal surfaces such as Al, Ni, Cu, Ag and Au surfaces, the sputtering yield by the liquid cluster ion beams was also much larger than that by Ar monomer ion beams at the same acceleration voltage. Furthermore, the wettability of the Ti surfaces was changed drastically by the species of the cluster ions as well as the acceleration voltage. For the Ti surfaces irradiated by the ethanol cluster ion beams, the contact angle decreased with increase of the acceleration voltage, and it became about 10 at higher acceleration voltages. However, for the case of the water cluster ion irradiation, the contact angle was almost the same at various acceleration voltages, and it was about 80 which was larger than that of unirradiated surface. 2006 Elsevier B.V. All rights reserved. PACS: 36.40.Wa; 61.46.Bc; 61.80.Jh; 68.47.Fg; 68.49.Fg; 81.65.Cf; 82.80.Rt Keywords: Cluster ion beam; Ethanol cluster; Water cluster; Sputtering; Wettability
1. Introduction Cluster ion beams are useful tools for the investigation of the fundamentals of solid-state physics, chemistry and related materials science [1]. The impact of accelerated cluster ion beams on the solid surfaces exhibits unique characteristics such as high-energy density irradiaition and low-velocity irradiation effects, which are not obtained by monomer ion beams. As a consequence, the properties of the bombarded surface are significantly altered through deposition and sputtering by the cluster ion beams [2,3]. Liquid-source cluster ion beams have several advantages for surface treatment, and they can be used in physical sputtering and chemical etching of the substrate material, for making surfaces smooth, and for cluster-activated
*
Corresponding author. Tel.: +81 75 383 2329; fax: +81 75 383 2343. E-mail address: [email protected] (G.H. Takaoka).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.128
chemical modification of the substrate surfaces [4,5]. The deposition of liquid cluster ions on the solid surface does not normally lead to the thick film, but the cluster material itself is deposited on the surface by forming extremely thin layer. This feature is useful for the chemical modification of solid surface using various kinds of liquid materials such as water, alcohol and paraffin. In addition, the interactions of cluster ions with surface atoms are occurred in small area and short time at nano-level, and the increase of incident energy for the cluster ions enhances the temperature of the bombarded surface [6]. Therefore, the chemical reaction between liquid clusters and surface atoms, which is occurred at non-equilibrium state, has many possibilities for various kinds of state transitions depending on the activation energy. This is much different from the chemical reaction at thermal equilibrium state in conventional wet processes. We have been focusing on the investigation of the fundamentals of solid-state physics and chemistry by using the ethanol and water cluster ion beams. In this
article, sputtering effects of the ethanol and water cluster ions on metal surfaces such as titanium (Ti) surface, etc. are investigated in order to clarify the interactions of the liquid cluster ion beams with solid surface atoms. Furthermore, the Ti surface state after sputtering is discussed based on atomic force microscope (AFM) and contact angle measurements. 2. Experiments Liquid materials such as ethanol and water were introduced into the source and heated up to 150 C. The vapors were ejected through a small nozzle into a vacuum region. When the vapor pressure was larger than 1 atm, the vaporized clusters were produced by an adiabatic expansion. The clusters were ionized by an electron bombardment method. The electron voltage for ionization (Ve) was adjusted between 0 V and 300 V, and the electron current for ionization (Ie) was adjusted between 0 mA and 250 mA. The cluster ions were extracted by applying an extraction voltage to the extraction electrode. The extracted cluster ions were size-separated by applying a retarding potential, which was 27 V for ethanol cluster and 14 V for water cluster, respectively. The cluster ions with the size larger than approximately 100 molecules per cluster were accelerated toward the substrates by applying an acceleration voltage. The acceleration voltage (Va) was adjusted between 0 kV and 10 kV. The substrates used were metal films with a thickness of approximately 500 nm, and the substrate temperature was room temperature. The background pressure around the substrate was 1 · 107 Torr, which was attained using a turbo-molecular pump. The cluster size was measured by a time-of-flight (TOF) method. In the cluster size measurement, it was assumed that the cluster ion was a singly charged ion. If the multiply charged cluster ions were produced, the dissociation of cluster ions were assumed to be occurred due to the Coulomb repulsion forces between the constituent atoms of a cluster ion [7,8]. The peak size was about 500–1000 molecules-per-cluster for the ethanol cluster ions and about 2500 molecules-per-cluster for the water cluster ions, respectively. The intensity of the cluster ions increased with increase of the vapor pressure. 3. Results and discussion The sputtering process by irradiation of liquid cluster ions on Ti surfaces was investigated. The sputtered depth was measured by the step profiler (Veeco Instruments: DEKTAK- 3173933). Fig. 1 shows the dependence of sputtered depth for Ti surfaces on acceleration voltage for the ethanol and water cluster ion irradiation. The cluster size was larger than 100 molecules-per-cluster, and the ion dose was 1.0 · 1016 ions/cm2. As shown in the figure, the sputtered depth increases with increase of the acceleration voltage. When the acceleration voltage is 9 kV, they are 160 nm for the ethanol cluster ion irradiation and 22.3 nm for the water
Sputtered depth (nm)
G.H. Takaoka et al. / Nucl. Instr. and Meth. in Phys. Res. B 258 (2007) 209–212 200 Ti Substrate 180 Ethanol clusterion Water clusterion 160 Dose = 1.0 x 1016 ions/cm2 140 Ve = 200 V, Ie = 200 mA 120 100 80 60 40 20 0 0 1 2 3 4 5 6 7 8 9 10
Acceleration voltage (kV) Fig. 1. Dependence of sputtered depth for Ti surfaces on acceleration voltage for the ethanol and water cluster ion irradiation.
cluster ion irradiation, respectively. Taking account of the sputtered depth and the ion dose, the sputtering yield was calculated by estimating the density of Ti such as 4.5 g/ cm3. The sputtering yield at an acceleration voltage of 9 kV was 90.1 atoms-per-ion for the ethanol cluster ion irradiation and 12.6 atoms-per-ion for water cluster ion irradiation. The sputtering yields for several kinds of metal surfaces irradiated by the ethanol and water cluster ion beams were investigated. Fig. 2 shows the sputtering yield for Al, Ti, Ni, Cu, Ag and Au surfaces at an acceleration voltage of 9 kV. In the figure, the sputtering yield by irradiation of argon (Ar) monomer ion beam at an acceleration voltage of 9 kV is also shown. In the case of water cluster ion irradiation, the sputtering yield is about ten times larger than that by Ar monomer ion irradiation, even if the incident energy of a water molecule as a constituent particle of a cluster ion is less than 90 eV. On the other hand, in the case of ethanol cluster ion irradiation, Al, Ti and Ni surfaces are sputtered more effectively, and the sputtering yield is approximately 100 times larger than that by Ar monomer ion irradiation. This is considered to be due to the enhancement of chemical sputtering, in which alkyl compound of metal is formed as a volatile product. Alkyl radicals
Sputtering yield (atoms/ion)
210
Ethanol cluster ion 9 keV Water cluster ion 9 keV Ar monomer ion 9 keV
1000
100
10
1
0.1
Al
Ti
Ni Cu
Ag Au
Fig. 2. Sputtering yield for Al, Ti, Ni, Cu, Ag and Au surfaces by irradiation of ethanol cluster, water cluster and Ar monomer ion beams at an acceleration voltage of 9 kV.
G.H. Takaoka et al. / Nucl. Instr. and Meth. in Phys. Res. B 258 (2007) 209–212
consisting of the ethanol molecule, which are produced after impact of the ethanol cluster ions on the metal surfaces, have an important role in chemical reaction for these metal surfaces. The rate of chemical reaction (m) with various kinds of reaction channels is described as follows; [9] n kT X Q v/N exp i ; ð1Þ h i¼1 kT where N is the number density of liquid molecules such as ethanol molecule and water molecule, h is Planck constant, k is Boltzmann constant, T is the temperature of liquid molecules after impact, and Qi (i = 1, 2, . . ., n) is the activation energy for a reaction channel (i). The equation indicates that the rate of chemical reaction increases with increasing temperature. When the liquid cluster ions impact on the metal surfaces, the cluster ions are broken up, and the multiple collisions between liquid molecules and metal atoms are occurred. As a result, many metal atoms are displaced and vibrated, and the incident energy of the cluster ions is used for heating an impact region of the metal surfaces. The molecular dynamic (MD) simulations showed that the temperature of the cluster impact region was a few tens of thousands degrees, although the substrate temperature was at room temperature [6]. Therefore, the thermal vibration of the liquid molecules, which is expressed by kT/h, becomes extremely high, and the barrier height for the chemical reaction, which is expressed by Qi/ kT, becomes comparably low. For the case of the ethanol cluster ion irradiation, the high temperature at the impact region enhances several kinds of chemical reactions available between the alkyl radicals of the ethanol molecule and the metal surface atoms, and the volatile materials such as several kinds of metal organic compounds are produced. On the other hand, for the case of the water cluster ion irradiation, hydroxyl (OH) radicals are produced after impact on the surfaces, and the oxidation occurs through
211
the chemical reaction between the OH radicals and the metal surface atoms except for Au atoms. Furthermore, during the impact, the physical sputtering of the metal oxide surfaces also occurs due to the high-energy density deposition by the water cluster ion beams. With regards to the oxide layer formation, it was confirmed by the XPS measurement. As a result, the metal surfaces such as Al, Ti, Ni, etc are sputtered by the water cluster ion irradiation, although the oxide layer is remained on the metal surface. The sputtered Ti surfaces were measured by using an atomic force microscope (AFM). Fig. 3 shows the surface morphologies (a) for unirradiated surface and for sputtered surfaces by (b) ethanol and (c) water cluster ion beams, respectively. The acceleration voltage was 6 kV, and the ion dose was 1.0 · 1015 ions/cm2. As shown in the figure, the surface roughness for the unirradiated substrates is 0.68 nm. On the other hand, the sputtered surfaces become rough, and the surface roughness is 0.84 nm for ethanol cluster ion irradiation and 0.74 nm for water cluster ion irradiation, respectively. Smooth surface with a roughness less than 1 nm is obtained even after sputtering. The lateral sputtering effect is responsible for the smooth surface formation by the cluster ion irradiation. The liquid cluster ion beam process has unique characteristics suitable for surface treatment such as high sputtering yield and smooth surface formation at an atomic level, which are not achieved by the conventional wet process. The wettability of the Ti surfaces irradiated by ethanol and water cluster ion beams were investigated by measuring the contact angles for water droplet, which was put on the Ti surfaces immediately after their removal from the vacuum chamber. Fig. 4 shows the dependence of the contact angle on the acceleration voltage for the cluster ion irradiation. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 200 mA. The ion dose was 1.0 · 1015 ions/cm2. The cluster size was larger than 100 molecules-per-cluster.
Fig. 3. AFM images (a) for unirradiated Ti-surface and for sputtered Ti-surfaces by (b) ethanol and (c) water cluster ion beams, respectively.
G.H. Takaoka et al. / Nucl. Instr. and Meth. in Phys. Res. B 258 (2007) 209–212
Contact angle (degree)
212 100 90 80 70 60 50 40 30 20 10 0
Ti substrate Dose = 1.0x1015 ions/cm2 Water cluster ion Ethanol cluster ion (Unirradiated)
0 1 2 3 4 5 6 7 8 9 10
Acceleration voltage (kV) Fig. 4. Dependence of the contact angle for the Ti surface on the acceleration voltage for the ethanol and water cluster ion irradiation.
The contact angle for the unirradiated surfaces was also measured, and it was about 30. As shown in the figure, the contact angle for the ethanol cluster ion irradiation decreases with increase of the acceleration voltage, and it is less than 10 at an acceleration voltage larger than 6 kV. According to the Q-mass spectroscopy measurement, fragmentation of the ethanol molecules occurred after impact of the ethanol cluster ions, and the main fragment was CH2OH radical. Therefore, it is considered that the CH2OH radical has a bond with the Ti surface atoms through Ti-CH2 bonding, and the top radical of this bond, that is OH radical, enhances the wettability of the Ti surface. On the other hand, the contact angle for the water cluster ion irradiation becomes large at various acceleration voltages, and it is about 80. The Ti surface becomes hydrophobic by the irradiation of the water cluster ions. This is ascribed to the chemical modification of the Ti surfaces by the OH radicals, which are produced after impact of the water cluster ions on the surfaces. The dangling bonds produced on the Ti surfaces have a bond with the OH radicals, which result in the titanium oxide layer formation on the surfaces. 4. Conclusion In order to investigate the interactions of ethanol and water cluster ion beams with metal surfaces, titanium (Ti) substrates were irradiated at different acceleration voltages. The sputtered depth increased with increase of the acceler-
ation voltage. The sputtering yield of Ti at an acceleration voltage of 9 kV for the ethanol cluster ions was approximately 100 times larger than that by argon (Ar) ion beams. This is ascribed to the enhancement of the chemical sputtering by the ethanol cluster ion beams. On the other hand, for the case of the water cluster ion irradiation, the sputtering yield was a few tens times larger than that for Ar monomer ion irradiation. This is due to the physical sputtering by the water cluster ion beams. In addition, the AFM observation showed that the sputtered surfaces by the ethanol and water cluster ion beams had an average roughness of less than 1 nm. For other metal surfaces such as Al, Ni, Cu, Ag and Au surfaces, the sputtering yield by the ethanol and water cluster ion beams was also much larger than that by Ar monomer ion beams at the same acceleration voltage. Furthermore, the wettability of the Ti surfaces irradiated by the ethanol and water cluster ion beams were investigated by measuring the contact angles for water droplet. The contact angle for the unirradiated surface was about 30. For the surfaces irradiated by the ethanol cluster ion beams, the contact angle decreased with increase of the acceleration voltage, and it became about 10 at higher acceleration voltages. However, for the case of the water cluster ion irradiation, the contact angle became large at various acceleration voltages, and it was about 80. Thus, the wettability of the Ti surfaces was changed drastically by the species of the cluster ions as well as the acceleration voltage. References [1] M.E. Mack, Nucl. Instr. and Meth. B 237 (2005) 235. [2] I. Yamada, G.H. Takaoka, Jpn. J. Appl. Phys. 32 (1993) 2121. [3] G.H. Takaoka, M. Kawashita, K. Omoto, T. Terada, Nucl. Instr. and Meth. B 232 (2005) 200. [4] G.H. Takaoka, H. Noguchi, T. Yamamoto, T. Seki, Jpn. J. Appl. Phys. 42 (2003) L1032. [5] G.H. Takaoka, H. Noguchi, Y. Hironaka, Nucl. Instr. and Meth. B 242 (2006) 100. [6] Z. Insepov, I. Yamada, Surf. Rev. Lett. 3 (1996) 1023. [7] K. Sattler, J. Muhlbach, O. Echt, P. Pfan, E. Rechnagel, Phys. Rev. Lett. 47 (1981) 160. [8] J.G. Gay, B.J. Berne, Phys. Rev. Lett. 49 (1982) 194. [9] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, John Wiley & Sons Inc., New York, 1976 (Chapter 9).