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Physica E 24 (2004) 6–9

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doi:10.1016

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Improvement of electrical contact at carbon nanotube/Pt byselective electron irradiation

Atsushi Andoa,*, Tetsuo Shimizub, Hidekazu Abeb, Yoshikazu Nakayamac,Hiroshi Tokumotod

aNanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),

1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, JapanbNanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Japan

cDepartment of Physics and Electronics, Osaka Prefecture University, Osaka, JapandNanotechnology Research Center, Research Institute for Electronic Science, Hokkaido University, Hokkaido, Japan

Abstract

We have demonstrated the fabrication process to obtain good electrical contact between carbon nanotube and

Pt-coated cantilever for scanning probe microscopy. The selective electron beam irradiation under DC-biased condition

in FE-SEM is effective in reducing contact resistance by orders of magnitudes. The current–voltage characteristic after

the irradiation suggests that the fabricated contact is highly conductive and of ohmic character.

r 2004 Elsevier B.V. All rights reserved.

PACS: 85.20.S; 73.40; 61.16.B

Keywords: Carbon nanotube; Electrical contact; Carbon nanotube SPM probe; Electron beam irradiation

1. Introduction

Electrical property is one of the importantinformation expressing nature of materials. Inorder to clarify electrical properties on a nan-ometer scale, the scanning probe microscopy(SPM)-based techniques are interesting becauseof their high lateral resolution. In these techniques,a conductive probe is a crucial part of theirexperimental setup, because it detects the mea-sured electrical properties from the sample surface.

onding author. Fax: +81-29-861-5584.

ddress: atsushi-ando@aist.go.jp (A. Ando).

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.physe.2004.04.014

The use of carbon nanotube (CNT) for theconductive SPM probes is interesting because ofits mechanical, electrical and chemical properties.The formation of conductive CNT probes requiresthe method of connecting a CNT with a baseprobe such as a cantilever.As the preparation method of CNT probes, we

have demonstrated the microprocess of the CNTattachment under scanning electron microscope(SEM) observation [1–3]. This method has thefollowing advantages. The probes can be fabri-cated in precise control of effective length and thedirection of CNT. Complicated structure such asthe probe equipped with 2 or 4 CNT tips can be

d.

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CNTcartridge

Stage1

Stage 2

Computer

Heater2

Power supply

Heater1FE-SEM

Pt-coated cantilever

CNT

Keithley236

Power supply

Fig. 1. Schematic drawing of experimental setup for fabrication

and electrical characterization of CNT probes in the nanoma-

nipulator.

A. Ando et al. / Physica E 24 (2004) 6–9 7

fabricated. Furthermore, in situ characterizationson structural, mechanical and electrical propertiesare possible. However, from the point of view of apreparation for good conductive probes, furtherimprovement is necessary. To fix a CNT on a baseprobe of cantilever, we use the deposition of acarbonaceous film caused by the electron-beamdissociation of contaminants in a SEM chamber.The properties of the deposited film depend on thedeposition conditions. Therefore, the electricalproperties of fabricated CNT probes are various.Moreover, non-ohmic current–voltage (I–V) char-acteristics are sometimes obtained. These charac-teristics suggest the existence of a barrier at theCNT–cantilever contact. Thus, for good conduc-tive CNT probes, it is important to establish thefabrication process of good electrical contactwithout such a barrier.In this paper, we focus on the improvement of

electrical contact between a CNT and a baseprobe of cantilever, and demonstrate a techniquefor the improvement by the selective electronbeam irradiation under DC-biased condition inFE-SEM.

2. Experimental

Fig. 1 is an experimental setup for fabricationof conductive CNT probes and evaluation ofcurrent–voltage (I–V) characteristics. To manip-ulate a CNT precisely, we used ‘nanomanipulator’,which is the FE-SEM equipped with two samplestages [3]. Each stage can move in X, Y and Z

directions independently. The stage for the CNTcartridge has a piezoelectric stage and can controlprecisely the length, the direction and the attach-ing place of a CNT. The length can be alsocontrolled by cutting usage of application of thepulse voltage between two stages.Multiwalled CNTs were prepared by a conven-

tional direct current arc discharge method [4]. Theaveraged diameter of the CNTs was 20 nm. Weused a CNT extracted from a CNT cartridgeprepared by an alternating current electrophoresistechnique [5]. Metal (Pt 30 nm/Ti 3 nm)-coatedSi cantilevers were used for the base of the CNTprobes.

The selective electron beam irradiation [6,7] wasperformed using 15 keV electrons in the FE-SEM.Keithley 236 source measure unit was used forevaluating the electrical properties of the CNTprobes during and after the fabrication process.

3. Results

Fig. 2 is a typical FE-SEM image during thefabrication process of a conductive CNT probe.Just after attachment by the van der Waals force,the I–V curves were nonlinear and the resistancewas very high. For fixing of the CNT andimprovement of the CNT–cantilever contact, theselective electron beam irradiation was performedin the area shown in Fig. 2. In our case, applyingDC voltage at the contact was necessary for theimprovement.Fig. 3 shows time evolution of the bias voltage

at the CNT cartridge and the current through aCNT during the fabrication. First, we selected aCNT from the cartridge and attached it to the tipof the cantilever. Then the voltage of the cartridgewas biased at 4V. The current was very smalland two-terminal resistance was very high

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1

0

-1

-2

-3

-4

-5

-6

Bia

s vo

ltag

e (V

)

0 400300200100

Time (s)

Cur

rent

(µA

)

-20

5

0

-5

-10

-15

-25

Selectiveelectronirradiation

Fig. 3. Time evolution of the bias voltage and the current

through a CNT during the fabrication of a CNT probe.

Time (s)

0 100 200 300 500400

Selectiveelectronirradiation

-20

-10

5 0

-5

-15

-25

Cur

rent

(µA

)

-4

-2

0

4

2

Bia

s vo

ltag

e (V

)

Fig. 4. Time evolution of the bias voltage and the current

through a CNT during the fabrication of a CNT probe.

Fig. 2. FE-SEM image during fabrication process of a CNT

probe. A square indicates the area of electron beam irradiation.

A. Ando et al. / Physica E 24 (2004) 6–98

(X10GO), which is dominated by a barrier atCNT–cantilever contact. Although the irradiationat the area of the contact was performed, noimprovement was observed and the resistance wasunchanged at the bias voltage of 4V. To improvethe contact resistance, more negative bias condi-tions were required. Finally, the current increaseddramatically by the irradiation at 5V andreached 20 mA of the current limit. The contactresistance was reduced to less than 250 kO.This improvement also depends on the polarity

of the bias voltage. Fig. 4 shows time evolution ofthe bias voltage at the cartridge and the currentthrough a CNT during the fabrication. At +3V,an increase in the current was saturated with+3 mA. On the other hand, the current increasedby the irradiation at 3V and reached 20 mA ofthe current limit.

These results shown in Figs. 3 and 4 can beexplained as follows. After the attachment of aCNT to the cantilever, the CNT has two differentcontacts at the both ends: One is the CNT–cantilever contact and the other is CNT–thecartridge contact. The electrons supplied by theirradiation tend to flow to the positive contact.Therefore, in case of applying a positive biasvoltage to the cartridge, the electrons flow out intothe external circuit through the cartridge, notthrough the CNT–cantilever contact. This factmay result in the dependence on the polarity of thebias voltage. Since the irradiated electrons flowthrough the CNT–cantilever contact at a negativebias voltage of the cartridge, the improvement ismore effective. The bias voltage assists electronflow through the insulated layer at the CNT–cantilever contact and flowing electrons modifythe layer. Although the detailed process has notbeen studied, we assume that the modification isgraphitization of the carbonaceous film at theCNT–cantilever contact.Fig. 5 shows a typical example of the I–V

characteristics of the fabricated CNT probes afterthe selective electron beam irradiation. Althoughthe resistance of the CNT probes depends on theintrinsic electrical properties of attached CNT,good conductive (o100 kO typical) CNT probescan be fabricated by selective electron beamirradiation under DC biased condition.

4. Summary

Fabrication and electrical characterization ofgood conductive CNT probes by using the

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Res

ista

nce

(kΩ

)14

13

12

11

15

10

Bias voltage (V)0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2

0

-20

-40

-60

-80

-100

Cur

rent

(µA

)

Fig. 5. Current–voltage characteristic of the fabricated CNT

probe after the selective electron beam irradiation.

A. Ando et al. / Physica E 24 (2004) 6–9 9

nanomanipulator have been demonstrated. Theelectrical contact at a CNT/a Pt-coated cantileverwas improved by selective electron beam irradia-tion under DC biased condition. In this technique,flowing electrons lead to the graphitization of thecarbonaceous film at the contact. The bias voltageassists electron flow through the insulated film atthe contact. So, the improvement has a threshold

voltage. The fabricated CNT probes are highconductive and nanomanupilator is a powerfultool to fabricate and characterize conductive CNTprobes.

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