www.elsevier.com/locate/nimb

Nuclear Instruments and Methods in Physics Research B 257 (2007) 297–300

NIMBBeam Interactions

with Materials & Atoms

Nano-scale surface modification of materials with slow, highlycharged ion beams

M. Sakurai a,*, M. Tona b, S. Takahashi b, H. Watanabe b, N. Nakamura b, N. Yoshiyasu c,C. Yamada c, S. Ohtani b, H.A. Sakaue d, Y. Kawase a,

K. Mitsumori a, T. Terui e, S. Mashiko e

a Department of Physics, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japanb Institute for Laser Science, University of Electro-communications, Chofu 182-8585, Japan

c Department of Applied Physics and Chemistry, University of Electro-communications, Chofu 182-8585, Japand Coordination Research Center, National Institute for Fusion Science, Oroshi-cho, Toki 509-5292, Japan

e Kobe Advanced ICT Research Center, National Institute of Information and Communications Technology, Kobe 651-2492, Japan

Available online 17 January 2007

Abstract

Some results on surface modification of Si and graphite with highly charged ions (HCIs) are presented. Modified surfaces wereobserved using scanning tunneling microscopy. Crater-like structure with a diameter in nm region is formed on a Si(111)-(7 · 7) surfaceby the incidence of a single HCI. The protrusion structure is formed on a highly oriented pyrolytic graphite surface on the other hand,and the structure becomes an active site for molecular adsorption. A new, intense HCI source and an experimental apparatus are underdevelopment in order to process and observe aligned nanostructures created by the impact of collimated HCI beam.� 2007 Elsevier B.V. All rights reserved.

PACS: 81.16.�c; 61.80.Jh; 79.20.Rf

Keywords: Highly charged ion (HCI); Scanning tunneling microscopy (STM); Electron beam ion source (EBIS)

1. Introduction

The interaction of slow, highly charged ion (HCI) withsolid surfaces induces emission of X-rays, hundreds of sec-ondary electrons and various kinds of particles, and tracesof HCI impact in nanometer scale are created as a conse-quence of the deposition of large potential energy in HCIs.We have measured secondary ions from the surfaces ofclean and hydrogen-terminated Si, and SiO2 thin film onSi substrate, and also observed surface structure of thesesamples after HCI irradiation. Mass spectra of secondaryions emitted by the irradiation of HCIs exhibited uniquecharacteristics of HCI interaction; secondary ion yieldincreases with the charge state approaching almost unity

0168-583X/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2007.01.060

* Corresponding author. Tel./fax: +81 78 803 5648.E-mail address: msakurai@kobe-u.ac.jp (M. Sakurai).

for highest charge states (e.g. I50+), and the yield of Siand O ions as a function of the potential energy of incidentHCI obeys the Coulomb explosion model [1]. The second-ary electron yield associated with single HCI injection is sohuge that the injection event could be recognized by mon-itoring the emission of secondary electrons without excep-tion. STM observations and Raman spectroscopy in themodified structure of highly oriented pyrolytic graphite(HOPG) surfaces by the injection of HCIs have been per-formed [2]. We have also demonstrated the advantage ofHCI over the application for single-ion implantation tech-nique [3].

In the present paper, surface structures of Si(11 1)-(7 · 7) and HOPG irradiated with iodine HCIs wereobserved by a scanning tunneling microscope (STM). Anewly developed experimental apparatus for nano-fabrica-tion using HCI is also briefly described.

Fig. 1. A 7-axes goniometer for positioning a screening mask of incidentions and a sample holder independently with respect to the incident HCIbeam. (The mask is not mounted in the picture.) The base stage supportingthe mask and sample is movable for XYZ directions and rotatable aroundZ-axis. The XY stage mounted on the base stage enables translation of themask.

298 M. Sakurai et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 297–300

2. Experimental

2.1. Ion sources

Electron beam ion trap (EBIT) or electron beam ionsource (EBIS) is a typical ion source for producing HCIsin very high charge states. An EBIS/EBIT comprises anelectron gun that emits electron current in the order of100 mA, drift tubes for the storage of HCIs, an electroncollector, and superconducting magnet that compresseselectron beam at the central drift tube. Using electronbeam with the acceleration voltage of 10–100 kV and cur-rent density of the order of 1000 A/cm2, atoms are ionizedrepeatedly by an electron impact inside drift tubes.Repeated electron impact also increases the kinetic energyof HCIs produced and stored inside the drift tubes, thenthe HCIs with excess kinetic energy surmount potentialbarrier and are extracted with the extraction voltage of2–3 kV.

We have two ion sources for production of HCIs. Onehas an electron-beam ion trap (EBIT) arrangement [4]and installed at the University of Electro-Communica-tions (UEC). It has design parameters such as maximumelectron beam energy of 300 keV and electron current of300 mA. It produces 105 ions/s of e.g. I53+ or Bi83+. Thedetail of design and performance is described elsewhere[5]. The other is an electron beam ion source (EBIS)recently developed at Kobe University, which is designedfor the application of ion beam processing of materialsso that it is constructed and operated easily withoutdemanding expertise of EBIS using a separate, commer-cially available super-conducting (SC) magnet [6]. Maxi-mum values of electron beam energy and electroncurrent are 40 keV and 300 mA, respectively. The lengthof drift tube region is 20 cm, which is an intermediatevalue between traditional EBIS (�1 m) and EBIT (3 cm)devices.

We note on improvements performed on the designstructure shown in the earlier publications [6–8] in orderto achieve target specifications. We found some errors inthe calculation of magnetic field distribution, whichrevealed magnetic shields at electron gun and collectordid not function sufficiently, then the electron gun wasmoved away from the SC magnet by 9 cm and the magneticshield at collector was reconstructed with thicker shieldstructure and stronger ceramic supports. The aperture sizesof the suppressor and extractor electrodes were choked inorder to suppress the return current of electron beamtoward the electron gun. Maximum electron-currentincreased from 20 mA to 180 mA by these improvements.The intensity of HCIs produced by the EBIS is in the orderof 108 ions/s, however, the charge state distribution ispeaked at lower charge states (e.g. Ar11+) due to relativelyhigh base pressure during operation in the drift tube region(�1 · 10�7 Pa). We are planning to cool the drift tube elec-trodes by a closed-cycle refrigerator to improve the charge-state distribution.

2.2. Experimental apparatus using HCI beams

The experimental results shown in the present paper areobtained using the EBIT at the UEC. The HCIs of iodinewere extracted with the leaky mode operation from theEBIT and their charge states were selected by using a sectormagnet. The kinetic energy of HCIs (Iq+) is 3 · q keV forthe present experiments. A collision chamber with basepressure of 2 · 10�8 Pa was connected to an observationchamber equipped with an STM apparatus [9,10]. Thesample irradiated with HCIs was transported betweenthe chambers in the vacuum. Before HCI-bombardments,the well-defined surfaces of the samples were prepared withstandard cleaning procedures, and confirmed by the STMimages. For HOPG surfaces after irradiation, dye mole-cules (Bis-MSB) were deposited using a Knudsen-Cell.The average thickness of deposited layers was 1 nm asmonitored with a quartz microbalance. The sample wasannealed at 100 �C after the deposition. All of the STMimages were observed at room temperature.

Formation of periodic structure created by the irradia-tion of single HCI would be intelligible demonstration inorder to display the characteristics of HCI on the applica-tion for nano-processes. We have developed an apparatus

M. Sakurai et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 297–300 299

to process a sample with periodically arranged traces ofHCI irradiation at Kobe University. The periodic structurewill be formed using a mask with small holes (the diameteris less than 100 nm) made of a thin film of SiO2. The appa-ratus comprises an electrostatic deceleration lens system, asample preparation chamber and a UHV-STM. The prep-aration chamber has LEED optics, a sputter ion gun and a7-axes goniometer, as shown in Fig. 1, with which theposition of the base plate that supports the mask and sam-ple is adjustable with respect to the incident HCI beamwith degrees of freedom of XYZ translation and rotationaround Z-axis. The goniometer also equips an XY stagedriven by piezoelectric motors (Nanomotion Ltd.) for pre-cise translation of the mask against the sample with the res-olution of 50–100 nm. The sample is also verticallytransferable over 20 cm between the LEED observationlevel and HCI beam level.

3. Results and discussion

Fig. 2 shows a STM image and depth profile of the cra-ter-like structure created by the impact of a single I50+ HCIon the Si(1 11)-(7 · 7) surface. The dark domain indicatesthe removal of topmost layers, and it has an area of5.5 nm2 and the maximum depth is �0.35 nm. Similar mea-surements of removed volume for different charge states

0 10 20 30-0.4

-0.2

0

0.2

Distance [nm]

Hei

ght

[nm

]

Fig. 2. A STM image (a) and depth profile (b) of crater-like structurecreated by the impact of a single I50+ HCI on the Si(111)-(7 · 7) surface.

(30 6 q 6 50) resulted in the outstanding q-dependence;the volume sharply increases with the charge of incidention. In other words, the size of crater is controllable withthe incident charge q. The mechanism of sputtering of Siatoms could be discussed by an analysis on ion chargedependence of sputtered volume and quantity of emittedsecondary ions. The detailed description of the discussionwill appear in a separate publication [11].

An example for selective adsorption of molecules at theimpact sites is exhibited in Fig. 3. Fig. 3(a) is a STM imageof HOPG surface after the irradiation of I30+ HCIs show-ing many dots with protruding structure randomly sprin-kled over the surface. The size of dots is few nm. A STMimage after the deposition of Bis-MSB molecules over theHCI-irradiated surface is shown in Fig. 3(b). The size ofsprinkled islands is increased to �10 nm compared toFig. 3(a), that means the Bis-MSB molecules condense atthe traces of HCI injection, since a Bis-MSB moleculehas elongated structure with the length of �2 nm. The pres-ent results mean that the irradiation of HCI creates activenano-spots on an inert substrate, that is, HCI is utilized forthe production of functional material with nanometer size.The irradiation of singly charged ions may be able to form

Fig. 3. (a) A STM image of HOPG after the irradiation of I30+ HCIs. (b)A STM image after vapor deposition of Bis-MSB molecules over thesurface HOPG after the irradiation of HCIs.

300 M. Sakurai et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 297–300

active spots, however, production efficiency estimated fromRaman peak intensity for certain fluence is much smallerthan those of HCI impact [2], and we cannot definitelymonitor the incidence of singly charged ion by detectingsecondary electrons. The advantage of HCI is that the inci-dence is recognized certainly due to the large secondaryelectron yield [3].

Acknowledgments

This work was performed under the auspices of theCREST program, ‘‘Creation of Ultrafast, Ultralow Power,Super-performance Nanodevices and Systems’’ of the JapanScience and Technology Agency and also the contractresearch programs of National Institute of Informationand Communications Technology with Kobe University.

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