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Nuclear Instruments and Methods in Physics Research B 237 (2005) 240244 www.elsevier.com/locate/nimb

Development of polyatomic ion beam system using liquid organic materials
G.H. Takaoka *, Y. Nishida, T. Yamamoto, M. Kawashita
Ion Beam Engineering Experimental Laboratory, Kyoto University, Nishikyo, Kyoto 615-8510, Japan Available online 20 June 2005

Abstract We have developed a new type of polyatomic ion beam system using liquid organic materials such as octane and ethanol, which consists of a capillary type of nozzle, an ionizer, a mass-separator and a substrate holder. Ion current extracted after ionization was 430 lA for octane and 200 lA for ethanol, respectively. The mass-analysis was realized using a compact E · B mass filter, and the mass-analyzed ion beams were transferred toward the substrate. The ion current density at the substrate was a few lA/cm2 for the mass-separated ion species. Interactions of polyatomic ion beams with silicon (Si) surfaces were investigated by utilizing the ellipsometry measurement. It was found that the damaged layer thickness irradiated by the polyatomic ions with a mass number of about 40 was smaller than that by Ar ion irradiation at the same incident energy and ion fluence. The result indicated that the rupture of polyatomic ions occurred upon its impact on the Si surface with an incident energy larger than a few keV. In addition, the chemical modification of Si surfaces such as wettability could be achieved by adjusting the incident energy for the ethanol ions, which included all the fragment ions. с 2005 Elsevier B.V. All rights reserved.
PACS: 39.10.+j; 41.75.юi; 41.85.юp; 82.65.+r Keywords: Ion beam; Polyatomic ion; Chemical modification; Ethanol; Octane

1. Introduction Ion beam processes have been applied to surface treatment as a nanotechnology [13], in which
Corresponding author. Tel./fax: +81 75 383 2343. E-mail address: gtakaoka@kuee.kyoto-u.ac.jp (G.H. Takaoka).
*

surface and interface characteristics of materials can be controlled on an atomic scale. The predominant properties of the ion beam technology are based on the ability to control the ion kinetic energy by adjusting an acceleration voltage. In addition, atomic, molecular and polyatomic ions including cluster ions are available, and the interaction of these ions with solid surfaces is different

0168-583X/$ - see front matter с 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.138

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depending on the ion species used [46]. Compared to atomic ions, polyatomic ions have several advantages, such as the ability to control the size and structure of polyatomic ions by adjusting the ionization conditions. However, ion beam processes using liquid source materials have not been widely investigated from the point of view of engineering applications, although increasing interest in liquid polyatomic ions has been shown in mass spectroscopic studies [79]. With regard to the liquid materials, which include organic materials, one of their excellent features is the presence of various kinds of structures. For example, there are many polyatomic fragments present in the organic materials such as octane and ethanol, which have different structures and chemical properties. The variety of these ions is very important for surface modification of materials such as hydrophilic or hydrophobic surface modification. In addition, the inherent fluid property of liquid materials is useful for providing a continuous supply of ions, which is advantageous in the engineering applications of ion beam processes. In this article, a liquid polyatomic ion beam system is developed, and the configuration and characteristic of the system are described. By using the system, octane and ethanol ion beams are produced, and the interaction of the ion beams with silicon (Si) surfaces is investigated. Furthermore, the surface modification such as wettability using ethanol ion beams is discussed to be compared with argon ion beams.

Fig. 1. Schematic illustration of the polyatomic ion beam system.

2. Experimental apparatus Fig. 1 shows a schematic illustration of the developed polyatomic ion beam system. The system consists of mainly a capillary type of nozzle, an ionizer, a mass-separator and a substrate holder. Vapors of liquid materials such as octane and ethanol are introduced into the system through a stainless steel pipe. The vapors are ejected through the nozzle into a vacuum chamber with a pressure of about 10ю5 Torr, which is evacuated by a diffusion pump. They pass through a collimator and enter the ionizer. In the ionizer,

the neutral vapors are ionized by electron bombardment. 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 100 mA. Several fragment ions can be produced by dissociative ionization. These ions are accelerated by applying a voltage to the extraction electrode. The extraction voltage (Vext) was adjusted between 0 kV and 6 kV. The extracted ions are focused by an einzel lens, and deflected by applying an voltage to the deflection electrode. The deflected ions enter an E · B filter (Wien filter), in which ion beam axis, electric field (E) and magnetic field (B) are all mutually perpendicular. The mass-separated ion beams by adjusting the electric field in the E · B filter 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 Si(1 0 0) substrates. The fluence of ions to the substrates is determined based on the collected ion current by a Faraday cup. When the desired ion fluence is attained, the shutter is closed to terminate ion irradiation. The background pressure around the substrate is 8 · 10ю7 Torr, which is attained using a turbomolecular pump.

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3. Results and discussion 3.1. Ion beam characteristics Fig. 2 shows the relationship between the ion current and the extraction voltage for (a) octane and (b) ethanol ions. The ionization condition was at Ie = 100 mA and Ve = 100300 V. As shown in the figure, the ion current increases with increasing extraction voltage, and the saturated currents of about 430 lA for octane and about 200 lA for ethanol are obtained at an extraction voltage of 6 kV. The ion current obtained is stable, and it can be kept constant during ion irradiation. On the other hand, the ion current decreases with increasing the electron voltage for ionization (Ve). This is considered to be due to the fact that the optimum ionization potential is a few tens eV for

the case of octane and ethanol, and the ionization efficiency decreases with an increase of Ve ranging from 100 V to 300 V. An electron bombardment method produces many fragments. Dissociative ionization in octane and ethanol molecules was studied using an E · B mass filter. Fig. 3 shows the mass spectra of (a) octane and (b) ethanol ion beams. The electron voltage for ionization was 100 V, and the electron current for ionization was 100 mA. The extraction voltage was 2 kV. As shown in Fig. 3(a), the highest peak is C3H7 with a mass number of 43, and the second highest peak is C2H5 with a mass number of 29. Although the peak of C8H18 ions did not appear, fragment ions of C8H18 molecules are observed. For the case of ethanol ions, the mass number of the highest peak is 31, which corresponds to CH2OH ions. The second highest peak

500 450 400 350 300 250 200 150 100 50 0 0 1 2 3 4 5 6

Ion Current [ A]

Ion Current [ A]

(a) Octane Ie=100mA Ve=100V Ve=200V Ve=300V

300

(b) Ethanol 250 Ie=100mA Ve=100V Ve=200V 200 Ve=300V 150
100 50 0 0 1 2 3 4 5 6

Extraction Voltage [kV]

Extraction Voltage [kV]

Fig. 2. Relationship between the ion current and the extraction voltage for (a) octane and (b) ethanol ions as a parameter of electron voltage for ionization (Ve).

Fig. 3. Mass spectra of (a) octane and (b) ethanol ion beams.

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corresponds to CH3CHOH ions with a mass number of 45. Although the mass resolution is not sufficient to clearly separate each ion type, other fragment ions such as C2H5 and CH3 are also observed. 3.2. Ion irradiation effect The irradiation effect of octane and ethanol ions on Si(1 0 0) surfaces was investigated using ellipsometry. Fig. 4 shows the values of Delta and Psi measured for C3H7, CH3CHOH and Ar ion irradiated samples. The incident ion energy was varied from 2 keV to 6 keV for the fluence of 1 · 1015 ions/cm2. In the figure, an initial value corresponds to the unirradiated sample. The dotted lines shown in the figure correspond to the thickness of oxide and damaged layers, which were calculated based on the values of Delta and Psi measured. In the calculation, the irradiated Si surface is assumed to form the oxide and damaged layers as the top and second layers, respectively [10]. As shown in the figure, the damaged thickness (Td) for C3H7 and CH3CHOH ion irradiation increase with increasing incident ion energy, but it is smaller than that for Ar ion irradiation. This indicates that the fragmentation of the C3H7 and CH3CHOH ions occurred upon impact and that the constituent atoms were implanted into the substrate with
190 180 Td (е)
0 0 20 20

lower energy than the atomic Ar ions. Furthermore, the oxide layer thickness (Tox) for CH3CHOH ion irradiation is larger than that for C3H7 ion irradiation. This is ascribed to the inclusion of oxygen atom into the Si surface for the CH3CHOH ion irradiation, which results in the oxide layer formation on the Si surface. Fig. 5 shows the change of contact angles of water droplets for the Si surface irradiated with ethanol ion beams as a function of incident ion energy, which was varied from 0.5 keV to 3 keV. The fluence of ions is 1 · 1016 ions/cm2. The contact angle was measured using a contact angle meter (Kyowa Interface Science, CA-D Type). As shown in the figure, the contact angle for Ar ion irradiation is similar to an initial value of the Si surface, and the damage layer formed by Ar ion irradiation does not influence the wettability. On the other hand, compared with the irradiation effect of Ar ions, the contact angle for ethanol ion irradiation is lower than the initial value for the unirradiated substrate, and it decreases with decreasing incident ion energy. In particular, the contact angle of 0.5 keV and 1.0 keV ion irradiation is less than 10°, and the irradiated Si surface exhibits superhydrophilicity. Improvement of the wettability of the Si surfaces is ascribed to the formation of hydrophilic bonds such as OH bonds. When CH2OH and CH3CHOH ions with low incident energy are dissociated upon their impact on the

40 60 80 100 120140 4keV 6ke V 160 2keV

90 Contact Angle [degree] 80 70 60 50 40 30 20 10 0 0 1 2 Incident Energy [keV] 3
Substrate:Si Dose: 1x10 ions/cm
16 2

Delta (degree)

170 160 150 140 130 9

Tox (е)

40 2ke V
60 80 100 + Ar
C3 H
+ 7 +

4ke V

6ke V

Ethanol Ar+ initial

C2 H4 OH

Substrate:Si

initial

Dose:1x10

15

ions/cm

2

10 11 12 13 14 15 16 17 18 19 Psi (degree)

Fig. 4. Values of Delta and Psi measured for the Si surfaces irradiated at different incident ion energies by C3H7, CH3CHOH and Ar ions.

Fig. 5. Contact angles of water droplets for the Si surfaces irradiated with ethanol and Ar ion beams as a function of incident ion energy.

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Si surface, the hydroxyl groups produced could be bonded with the surface atoms. Thus, the hydrophilic bond formation by various ion type irradiations influences the wettability, and it is found that the surface modification of the Si substrates by ethanol ion beams could be controlled using the chemical properties of hydroxyl group.

than 10°. By utilizing the different ion species and adjusting the incident energy for the ethanol ions, which included the fragment ions, the chemical modification of Si surfaces such as wettability could be achieved.

References 4. Conclusion A liquid polyatomic ion beam system was developed, in which fragment ions of octane and ethanol molecules with different mass numbers and structures were produced by electron bombardment ionization. The interaction of the mass-separated ion beams with the Si surfaces was studied. The ellipsometry measurement showed that the fragmentation of polyatomic ions with higher incident energy occurred upon its impact on the surface. In addition, the ethanol ion beams including the different fragment ions were irradiated on the Si surfaces. It was found that the contact angle of water droplets for the Si surface could be controlled by incident energy, and that the contact angle of 0.5 keV and 1.0 keV ion irradiation was less
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