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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

Production of liquid cluster ions and their application to surface etching
G.H. Takaoka *, H. Noguchi, Y. Hironaka
Ion Beam Engineering Experimental Laboratory, Kyoto University, Nishikyo, Kyoto 615-8510, Japan Available online

Abstract A new type of cluster ion beam system using organic liquid materials such as ethanol has been developed, and it has several advantages for surface etching and chemical modification based on the different properties of liquid cluster ions. Ethanol vapors were ejected through a nozzle into a high-vacuum region, and ethanol clusters were produced by an adiabatic expansion phenomenon at the vapor pressures larger than 1 atm. In another case of producing ethanol clusters at a lower vapor pressure, He gas was used to mix up with ethanol vapors, and the mixed gases were ejected into a high vacuum region. Even if a vapor pressure of ethanol was 0.1 atm, ethanol clusters were produced at the He gas pressure larger than 1 atm. The ethanol clusters produced were ionized by an electron bombardment method, and the cluster ions were accelerated toward a substrate by applying an acceleration voltage. For the case of ethanol cluster ion irradiation at an acceleration voltage of 9 kV, the sputtering yields for Al, Cu, Ag and Au films, which were used as a substrate, were about ten times larger than that by Ar monomer ion irradiation. In addition, the surface flatness of the metal films was improved by irradiation of ethanol cluster ions. ñ 2005 Elsevier B.V. All rights reserved.
PACS: 36.40.Wa; 41.85.-p; 81.65.Cf Keywords: Ethanol cluster; Cluster ion beam; Cluster size; Surface etching; Sputtering yield

1. Introduction The ion beam process is one of the basic technologies in nanostructure fabrications such as etching and implantation [1­3]. However, the ion beam process using liquid source materials has not been investigated from the point of view of engineering applications, because the ion current available was extremely low [4,5]. With regard to liquid materials, which include organic materials, one of their excellent features is the presence of various kinds of structures and chemical properties [6]. This feature is useful for the chemical modification of solid surfaces using various kinds of liquid materials. In addition, the inherent fluid property of liquid materials might be effective for smooth

*

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

surface formation, if liquid material ion beams are applied to the surface treatment. A cluster is an aggregate of a few tens to several thousands of atoms, and equivalently low-energy and high-current ion beams can be realized using cluster ion beams [7]. The cluster ion beam technique has several advantages, one of which is that high-energy-density deposition and the collective motions of the cluster atoms during impact play important roles in the surface process kinetics [8­10]. The liquid cluster ion beam technique, which has the features of liquid materials as well as the advantages of cluster ion beams, has a high potential for the surface treatment as engineering applications such as surface etching and chemical modification. In this paper, the development of a new type of cluster ion beam system using organic liquid materials such as ethanol is described, and the size separation of the ethanol cluster ions by a retarding potential method is discussed. Furthermore, the surface etching for

0168-583X/$ - see front matter ñ 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.006


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several kinds of metal films irradiated by the ethanol cluster ion beams is investigated in order to clarify the specific characteristics of the cluster ion beam process for surface treatment. 2. Experimental apparatus Fig. 1 shows a schematic of the cluster ion beam system using organic liquid materials. Liquid material such as ethanol is introduced into a source, and it can be heated up to 150 °C by resistive heating of the source. The vapor pressure for ethanol is 44 Torr at room temperature, and it increases with the increase in source temperature. For example, the vapor pressures measured at the source temperatures of 105 °C, 118 °C and 125 °C were 2000 Torr, 3000 Torr and 4000 Torr, respectively. The vapors of ethanol are ejected through a nozzle into a high vacuum region, and the ethanol clusters are produced by an adiabatic expansion. In another case of producing the ethanol clusters at a lower vapor pressure, helium (He) gas is used to mix up with ethanol vapors, and the mixed gases are ejected through the nozzle into a high vacuum region. He gas has an important role of cooling down the ethanol vapors through the collisions, which result in effective production of ethanol clusters. The nozzle is made of glass, and it is a converging­diverging supersonic nozzle. The diameter of the nozzle at the throat was 0.1 mm. The clusters produced pass through a skimmer and a collimator, and they enter an ionizer. In the ionizer, the neutral clusters are 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 are accelerated by applying an extraction voltage to the extraction electrode. The extraction voltage (Vext) was adjusted between 0 kV and 2 kV. The extracted cluster ion beams are size-separated by a retarding potential method. The size-separated cluster ion beams are accelerated toward a substrate, which is set on a substrate holder. The acceleration voltage (Va) was adjusted between 0 kV and 10 kV. The substrates used were metal films such as Al, Cu, Ag

and Au films. The film thickness was about 500 nm. The ion dose to the substrates is determined based on the ion current. When the desired ion dose is attained, the shutter is closed to terminate ion irradiation. The background pressure around the substrate was 6 · 10þ7 Torr, which was attained using a diffusion pump. 3. Results and discussion The cluster size was measured by a retarding potential method. The mass resolution in this method is low, but it is the most simple and the easiest way to measure the approximate cluster size distribution. Fig. 2 shows (a) a retarding spectrum for an ethanol ion beam and (b) a cluster size distribution as a parameter of vapor pressure without He gas. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 200 mA. The extraction voltage (Vext) was 1 kV, and the acceleration voltage (Va) was 5 kV. As shown in Fig. 2(a), a cluster ion beam contains many monomer ions, and it decreases rapidly at a retarding voltage of 0 V. In addition, the cluster ion current measured at positive retarding
12 10
Ve = 200 V, Ie = 200 mA, V ext = 1 kV, Va = 5 kV Ethanol (4 atm) Ethanol (3 atm) Ethanol (2 atm) Ethanol (1 atm)

Beam Current (µA)

8 6 4 2 0 -200 -100

a
1. 0

0 1 00 200 300 Retarding Voltage (V)

400

500

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Substrate Nozzle Ionizer

Intensity (arb.units)

0. 6

Ve = 200 V, Ie = 200 mA, V ext = 1 kV, Va = 5 kV Ethanol (4 atm) Ethanol (3 atm) Ethanol (2 atm) Ethanol (1 atm)

0. 4

Liquid Cluster Skimmer Source Collimator Pump

Size-Separator

0. 2

0. 0 0 300

Pump Pump
Fig. 1. Schematic of liquid cluster ion beam system.

b

600 900 1200 Cluster Size (atoms)

1500

Fig. 2. (a) Retarding spectrum for an ethanol ion beam and (b) ethanol cluster size distribution measured at ionization conditions of Ve = 200 V and Ie = 200 mA as a parameter of ethanol vapor pressure.


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voltages increases with the increase in vapor pressure, and an ion current of a few hundreds of nano Ampere is obtained. Based on the retarding spectrum at the positive retarding voltages, the energy can be converted to cluster size using the assumption that each ethanol molecule has an energy of 284 meV. As shown in Fig. 2(b), the cluster size is distributed between a few hundreds and a few thousands, and the intensity of ethanol clusters increases with the increase in vapor pressure. With regard to the mass resolution, which is defined principally by the uniformity of the potential at the ionizing point and the retarding electrode, the energy resolution of a typical retarding electrode is about 10 eV, and it corresponds to a size of 35 molecules per cluster. Although the mass resolution is quite low in this method, ethanol cluster ions with a size larger than a few hundreds are obtained. In another case of producing the ethanol clusters, He gas was used to mix up with ethanol vapors. Fig. 3 shows a cluster size distribution as a parameter of He gas pressure. The heated temperature of ethanol was 28 °C, which corresponded to the vapor pressure of 0.1 atm. The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 200 mA. The extraction voltage (Vext) was 300 V, and the acceleration voltage (Va) was 6 kV. As show in Fig. 3, the intensity of ethanol clusters increases with the increase of He gas pressure, even if the vapor pressure of ethanol was 0.1 atm. The cluster size is distributed between 100 and 1000 at the He pressure of 4 atm. The ethanol vapors are cooled down by the collision with the He gas, which results in the effective formation of ethanol clusters even at lower vapor pressure. The surface etching by irradiation of ethanol cluster ion beams for the metal films was investigated. Considering the etched depth and the ion dose, the sputtering yield was calculated by estimating the density of the metal films. Fig. 4 shows the sputtering yield for Al, Cu, Ag and Au films at an acceleration voltage of 9 kV. The cluster size was larger than 95 molecules per cluster. In the figure, the sputtering yield by irradiation of argon (Ar) ion beam at an accelera1. 0

100

Sputtering Yield (atoms/ion)

10

Cu
1

Ag

Au

Al
Ethanol cluster ion : 9 keV Armonomer ion : 500 eV

0.1 0 10 20 30 40 50 60 70 80

Atomic Number
Fig. 4. Sputtering yield for Al, Cu, Ag and Au films irradiated by ethanol cluster ions and Ar monomer ions.

Intensity (arb.units)

0. 8

0. 6

0. 4

Ethanol 28°C (0.1atm) Ve = 200 V, Ie = 200 mA V ext = 300 V, Va = 6 kV He (1 atm) He (2 atm) He (3 atm) He (4 atm) Without He

0. 2

0. 0

0

100 200 300 400 5 00 6 00 700

Cluster Size (atoms)
Fig. 3. Ethanol cluster size distribution measured at ionization conditions of Ve = 200 V and Ie = 200 mA as a parameter of He gas pressure. Ethanol vapor pressure is 0.1 atm. Fig. 5. AFM images of Au film surfaces (a) unirradiated and (b) irradiated by ethanol cluster ions at an acceleration voltage of 9 kV and an ion dose of 1 · 1015 ions/cm2.


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tion voltage of 500 V is also shown. In the case of the ethanol cluster ion irradiation, the sputtering yield is about ten times larger than that by Ar monomer ion irradiation, even if the incident energy of an ethanol molecule as an constituent particle of a cluster ion is less than 100 eV. Furthermore, the Al films, which have the small sputtering yield for the case of physical sputtering by Ar ion beams, are sputtered more effectively than the Cu, Ag and Au films. This is considered to be due to the enhancement of chemical sputtering, in which alkyl compound of Al is formed as a volatile product. Alkyl radicals consisting of the ethanol molecule, which are produced after impact of the ethanol cluster ions on the film surfaces, have an important role of chemical sputtering. The sputtered surfaces for the metal films were observed using an atomic force microscope (AFM), and the surface roughness was measured. Fig. 5 shows AFM images of Au films with and without ethanol cluster ion irradiation. The acceleration voltage for the cluster was 9 kV, and ion dose was 1 · 1015 ions/cm2. The surface flatness is improved with the increase in the ion dose, and the surface roughness decreases from 12 nm to 6 nm. This is due to the lateral sputtering effect of cluster ion irradiation as well as the inherent fluid property of liquid materials. The high rate sputtering and very flat surface formation can be achieved by the ethanol cluster ion irradiation. These results are explained by the unique characteristics of the cluster ion irradiation effects such as the high density irradiation effect and multiple collision effect, which are not achieved by the conventional ion beam technology. 4. Conclusions The liquid cluster ion beam system was developed, in which cluster ions of organic molecules such as ethanol

were produced by an adiabatic expansion. The ethanol cluster formation was also achieved by using He gas as a mixture gas. The size separation of the ethanol cluster ions was performed by a retarding potential method, and the cluster size was distributed between a few hundreds and about 1000. The cluster ion beams were irradiated on Al, Cu, Ag and Au films, and the sputtering yield at an acceleration voltage of 9 kV was approximately 10 times larger than that by Ar ion beams. In addition, the surface of the metal films was improved by the cluster ion irradiation. These results are ascribed to the unique characteristics of the cluster ion irradiation effects such as the high density irradiation and multiple collision effects. Thus, ethanol cluster ion beams can be applied to the surface treatment such as surface etching with high sputtering yield and smooth surface formation at an atomic level.

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