Fullerene ion irradiation to silicon

cm

Nuclear Instruments

and Methods in Physics Research B 121 (1997) 480-483

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Beam lnteactions with Materials 8 Atoms

!!!Z ELSEVIER

Fullerene ion irradiation to silicon M. Tanomura, D. Takeuchi, J. Matsuo, G.H. Takaoka, I. Yamada Kyoro University Ion Beum Engineering Experimentul Luborutory. Sukyo, Kyoto 606-01, Jupan

Abstract Silicon has been irradiated by singly charged C,, fullerene ions with an ion dose of up to 1 X lOI ions/cm* at room temperature in order to study the damage formation of cluster ion bombardment on solid surfaces. Singly and doubly charged fullerene ions and some daughter ions of fullerene were observed. Mass separation was accomplished by a 90” sector magnet. The maximum current of the mass-separated singly charged C,, fullerene ion beam was about 10 nA. RBS (Rutherford backscattering spectrometry) channeling measurement of the Si(100) bombarded by 300 keV C,, ions (i.e. 5.0 keV per carbon atom) shows a remarkable increase in the surface peak of the defects even at the low atomic dose of 6 x lOI atoms/cm*. The C,, fullerene ion beam irradiation produced many defects. This is one of the typical non-linear effects of cluster bombardment.

1. Introduction

2. Experiments

Fullerene is a large molecule and a cluster of carbon. Typical fullerenes are C,, and C,,, which have 60 and 70 carbon atoms, respectively. Cluster ion beams have various unique characteristics as follows: high yield sputtering, multiple-collisions effects between the target atoms and the incident cluster atoms, and high-density energy deposition within a local surface region [I-4]. Experiments with cluster ion bombardment are of great interest, but it is difficult to obtain a cluster ion beam. Fullerene is a useful molecule in order to generate a carbon cluster ion beam, because fullerene can be easily evaporated and ionized as a cluster by electron bombardment. There have been various studies of the interaction between solid and carbon clusters [5-81. A. Hallen et al. reported that the vacancies produced by C,, ions at an energy of 400 keV per carbon atom were much shallow than those created by C, ions at the same velocity calculated with TRIM (TRansport of Ions in Matter) [7]. Because of the low C, ion current, the vacancy profiles were measured with DLTS (Deep Level Transient Spectroscopy) for surfaces bombarded by C,, ions with an ion dose up to IO’ ions/cm2. Dijbeli et al. reported that C, irradiation at the energy of 800 keV per carbon atom produced many more defects than C, with the same velocity [8]. Non-linear effects for large carbon cluster bombardment are expected. In this paper the channeling spectra of Si(lO0) substrates irradiated by singly charged C6,-, fullerene ions were measured in order to study irradiation effects of fullerene on solid surfaces.

Si(lO0) substrates were irradiated by singly charged C,, fullerene ions with ion doses up to I X lOI ions/cm*. The total number of carbon atoms is 60 times as many as the ion dose, because one ion is able to transport 60 carbon atoms. The energy of C,, ions used in this experiment was 300 keV (i.e. 5.0 keV per carbon atom in C,, fullerene). During the irradiation, the Si substrates were kept at room temperature. The pressure in the target chamber, evacuated by a TMP (Turbo Molecule Pump), was kept at 5 X IO-’ Torr. A schematic of the ion beam implantation system is shown in Fig. I. The fullerene ions were produced in a hot hollow cathode ion source. Fullerene powder, which contained the C,, and C,, fullerene, was evaporated in an oven close to the anode of the ion source into the ionizing region. The ionizing chamber pressure rose to 1 X 10s5 Torr during fullerene ion generation. Fullerene is easily destroyed in a high density plasma. The cracking of fullerene in the ion source gives a conducting carbon film on the boron nitride (BN) insulators used in the ion source. Once a carbon film is deposited on the BN insulators, the operation lifetime of the ion source is short. The fullerene ions were extracted with 10 kV and then mass-separated by a 90” sector magnet. After mass-separation, the C,, fullerene ions were electrostatically accelerated to energies as high as 380 keV. Subsequently the fullerene ion beam was scanned in order to produce a uniform beam and deflected at 3“ in order to remove any neutral beams. Finally the C,, ions reached the target. The ion currents were measured by a Faraday cup detector with a secondary

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faraday cup

deflector

accelerator

target chamber

extracti

hot hollow cathode ion source Fig. I. Schematic

diagram of the ion beam implantation

electron suppresser biased at - 300 V and having an input aperture area of 1 cm*. We used a 2 MeV 4Het beam for the RBS measurements. The defects of Si produced by fullerene ion irradiation were measured by channeling spectra.

3. Fullerene ion beam An example of a fullerene ion beam spectrum with an energy of 300 keV is shown in Fig. 2. The oven current was 15 A, and the temperature was about 450°C. Fig. 3 shows singly charged C,,, C , and C, ion beam currents as a function of oven current. C, and C, ions were due to cracked fullerene. At over 20 A of oven current, the C,,

system consisting

of ion source.

ion beam current decreased, while those of C, and C2 increased. This is because fullerene was thermal dissociated. An appropriate oven temperature was about 450°C. where the vapor pressure of C,, was about 3 X IO- ’ Torr [9]. The peaks of the fullerene ion beam were found as shown in Fig. 2. The peaks at the carbon number of 60 and 70 correspond to single charged C& and C& fullerene ions, respectively. Cracking ions of fullerene with even number were observed in the spectra. Since C& and C.& ions are particularly unstable carbon cluster, the peaks at the number of 30 and 35 probably correspond to doubly charged C,&- and C:i ions. For example, Horak et al. produced about 4 nA of a C& fullerene ion beam and I nA of a Gil fullerene ion beam [s]. Because of the low resolution of their mass separator, they could not separate

doubly/charged cracking fullerene

60

65

70

Carbon Number Fig.

2. Fullerene

ion beam spectra with an energy of 300 keV. Note that cracking

ions of fullerene with even number were found in the

spectra.

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Energy 3OOkeV

a

6-

Energy

‘;t

a

300keV

i v) 600 4-

g

I

z 0

:

:

400 r

UNIRRADIATED 200

220

function of oven current.

240

260

280

300

CHANNEL

Fig. 3. Ion beam current of singly charged C,,, C, and Cz as a

Fig. 5. Channeling spectra of Si(100) substrates irradiated by C,, ions. The energy was 300 keV and the ion dose ranged I X lOI*

from I X lOI ions/cm*. C& and C& in the fullerene ion beam. The fullerene ion beam used in this experiment had 5 nA of singly charged C,,. This beam was a high current of high quality because of an ability to resolve C& and cracked fullerene C& in the beam.

4. Channeling

RBS spectrum

Random and channeling RBS spectra from Si(lO0) substrates, irradiated by singly charged C,, fullerene ions, are shown in Fig. 4. The energy was 300 keV (5.0 keV per carbon atom in C60 fullerene) and the ion dose was

I

6ooo-

1

4ooo-

I_____\

F3’J@J-

random spectrum

5

LO-

channdlng spectrum C,+ 300 keV 1x12’* [ions/cm*]

lOOOr 0

50

100

150

200

250

300

CHANNEL NUMBER Fig. 4. Random strates irradiated

1 X IO” ions/cm’. By using TRIM, the damage depth was calculated at about 150 A for C , ions at an energy of 5.0 keV. The depth resolution of the RBS measurement is not sufficient to study the damage at a 150 .& depth. Fig. 5 shows the channeling spectra of Si(lOO1 substrates irradiated by C, ions. The energy was 300 keV and the ion dose ranged from 1 X IO’* to 1 X lOI ions/cm*. The area of the Si surface peak increased with ion dose. There was little difference of the FWHM (full width at half maximum) of the surface peak in RBS channeling spectra with various ion doses. The defects could be estimated from the area of surface peak, because the area of surface peak corresponded to the total number of defects [IO]. C,, ion irradiation at the low ion dose of 1 X lOI ions/cm’, which corresponded to 6 X lOI atoms/cm2, could generate many more defects. The number of disordered atoms produced by carbon monomer ions at an energy of 5.0 keV and ion dose of 6 X 10 I4 ions/cm* was about 1 X 10 ” atoms/cm*, according to the TRIM calculation. The low value of disordered atoms (1 X lOI atoms/cm*) was close to the number calculated from the surface peak in the channeling spectrum of unirradiated Si( 100) [IO]. If the number of vacancies per incident carbon atom produced by fullerene is equal to that by carbon monomer, no increase in the surface peak would be observed (see Fig. 51. The number of the defects produced by C,, seem to be several times more than that by carbon monomers at the same velocity. D&e-Ii et al. reported that the number of vacancies created by C, ions with the energy of 200 keV per atom when electronic energy loss is dominant is about 1.1 times as much as that by a C, ion with the same velocity. The number of vacancies by C, and C, ions, with 800 keV per

and channeling RBS spectra from Si(100) by singly charged C, fullerene ions.

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atom, was about 1.3 and 0.75 times as much as that by a C, ion with the same velocity [8]. In the energy region where electronic energy loss was dominant, a small size cluster would not produce many defects. In our experiment. the nuclear energy loss was predominant and the cluster size was large. At this low energy, the large size carbon cluster produced many more defects. This is one of the typical non-linear effects of cluster irradiation.

5. Conclusion Si( 100) substrates were irradiated by singly charged C,, fullerene ions in order to study the defects created by cluster irradiation on solid surfaces. The channeling spectra show that many defects were produced by C,, irradiation at the low atomic dose of 6 X lOI atoms/cm’. C,, fullerene ion beam irradiation produced many more defects than carbon monomer irradiation, The non-linear effects of cluster bombardment could be seen clearly, when fullerene ions were used.

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References

[I] 1. Yamada. G.H. Takaoka, M.1. Current, Y. Yamashita and M. Ishi, Nucl. Instr. and Meth. B 74 t 1993) 341. [2] M. Akizuki, M. Harada, Y. Miyai, A. Doi, T. Yamaguchi, J. Matsuo, G.H. Takaoka, C.E. Ascheron and I. Yamada, Nucl. Instr. and Meth. B 99 (1995) 229. [j] I. Yamada, J. Matsuo, 2. Insepov and M. Akizuki. Nucl. Instr. and Meth. B 106 (1995) 165. [4] J. Matsuo, A. Kitai, G.H. Takaoka and I. Yamada. IO be published. [5] P.D. Horak and U.J. Gibson, Appl. Phys. Lett. 65 (1994) 968. [6] H. Dammak, A. Dunlop, D. Lesueur, A. Bnmell. S. DellaNegra and Y. Le Beyec, Phys. Rev. Lett. 74 (1995) I 135. [7] A. Hallen, P. Hakanson, N. Keskitalo, J. Olsson. A. Brunelle. S. Della-Negra and Y. Le Beyec. Nucl. Instr. and Meth. B 106 (1995) 233. [8] M. DBbeli, F. Ames, R.M. Ender, M. Suter, H.A. Synal and D. Vetterli, Nucl. Instr. and Meth. B 106 (1995) 43. [9] J. Abrefah, D.R. Olander, M. Balooch and W.J. Siekhaus, Appl. Phys. Lett. 60 (1992) 1313. [IO] L.C. Feldman, R.L. Kaufman and P.J. Silverman, Phys. Rev. Lett. 39 t 1977) 38.

Acknowledgements The authors would like to thank Radiation Laboratory of Nuclear Engineering, Kyoto University, for using the RBS system.

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