Letters

Integrated Nanoprobe for Measuring
Microscopic Electronic Properties
Masao Nagase†
Abstract
Nanotools for measuring the microscopic electronic properties of conductive materials have beendeveloped. Nanoprobes were integrated on microcantilevers for scanning probe microscopy using
nanofabrication technologies. The four-point nanoprobe enables us to measure microscopic properties
without a complex lithographic process.

1. Introduction
Recent progress in the miniaturization of electronic
devices has achieved very small devices withnanometer-sized dimensions, called nanodevices. A
new measurement technology for the nanometerregion, called nanometrology, is now urgentlyrequired for the further development and improvement of nanodevices. For instance, micro- and nanoorder electrical conductance measurement techniques
are especially needed for the polycrystal-basedorganic semiconductor devices that are expected tobecome next-generation electronic devices. So far,however, only very limited solutions for this measurement problem have been proposed. This situation
is an obstacle to the development of future electronicdevices.
The four-point-probe method is widely used formeasuring the electrical conductance of conductivematerials. In a four-point-probe measurement, fourprobes, aligned with the same pitch, make contact
with a sample surface. The voltage between the two
inner probes is measured under the condition of constant current between the two outer probes. Since thecontact resistance between the probes and the sample
is negligible in the four-point-probe configuration,
the correct value of the sheet resistance of the sample
can be measured.
The conventional four-point probe is a microprobe† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: nagase@aecl.ntt.co.jp

48

whose pitch between the probes is 1 mm in almost allinstruments (Fig. 1). Since the minimum pitch ofrecently developed probes is 100 µm, it is impossible
to measure the microscopic properties of conductive
materials using current probes. Miniaturization of
conventional probes is limited by the precision constraints of machine technology. The other problem
with conventional probes is high load force. The surface area of the sample is destroyed by the contact
force of the probes. This is a serious problem for thevery thin films utilized in miniaturized electronicdevices.
A new type of miniaturized four-point-probe system for measuring micro- and nano-order electricalproperties is urgently required. Our group is developing various types of four-point nanoprobe systems,
based on state-of-the-art lithography and related technologies that have been studied in NTT Basic
Research Laboratories for a long time. We havealready developed a four-point nanoprobe systemwith 500-nm-pitch metal electrodes [1] and anotherwith 60-nm-pitch Si electrodes [2]. A four-pointprobe with carbon nanosprings [3] has also beendeveloped. In this article, we describe two types offour-point nanoprobes.
2. Integrated nanotool based on SPM technology
The basis for the four-point nanoprobe is a Si
microprobe for scanning probe microscopy (SPM),
as shown in Fig. 2. The conventional probe has only
one tip on a single cantilever. For the nanoprobe,
electrical and mechanical systems are integrated in a
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Letters

Four-point nanoprobefabricated by NTT

1 mmCo

Sa

nvm

en

Standardfour-point probe

ple

tiona

Pitch: 100 µmsize,

lpro

EB-lithographybased probe

probbeepitch

FIB-lithographybased probe
1 µm Pitch: 500 nm

Grath in sin iz
film e o

0.1 mn

100 nmPitch: 60 nm
100 nmfDev
in iceUL si
SI ze

0.1 mmTungsten carbide probe(D = 0.05 mm)
Smallest commercial four-pointprobe from NTT-AT
EB: electron beam
FIB: focused ion beam
CNT: carbon nanotube

10 nm(D Mol
1 nm
NA ec, C ula
NT r si, e zetc.)

NTT-AT: NTT Advanced Technology Corp.
ULSI: ultralarge-scale integration

Fig. 1. Miniaturization of four-point probes.

Conventional probe for SPMNanoprobe
Si microprobe

Electricalsystem

MicrosystemMechanicalsystem
Integration

Multiprobe

Nanomechanicalprobe

Circuit formeasurements

Circuit formechanical control

Nanoelectromechanical system

Fig. 2. Integrated nanotool.

microsystem to construct a nanoelectromechanicalsystem. Lithographic techniques, such as electron
beam lithography and ion beam lithography, are used
to fabricate the nanoprobe on this nanotool. This integrated nanotool enables us to make a tiny, but highly
functional measurement system.
Since the integrated nanotool is based on SPM
technology, it easily achieves high resolution in a
two-dimensional measurement. Furthermore, theVol. 4 No. 9 Sep. 2006

contact force between the probes and the sample is
low enough to avoid damaging the sample. These are
the advantages of the integrated nanotool.
2.1 Four-point nanoprobe fabricated using
nanolithographic technology
Figure 3 shows an example of an integrated nanotool [1] based on the concept shown in Fig. 2. This
cantilever system has two levers. One (left) is for49

Letters

1 µm

100 µmElectricalmeasurement
probe

Surfaceobservation
probe

Piezoresistors
Pt electrodeFour-point probe
electrodes

300 nmBimorph actuator
200 nm(a) Overview of dual probe system(optical micrograph)(b) Tip of electrical measurement
probe

Fig. 3. Multifunctional integrated four-point nanoprobe.

electrical measurement with the four-point probes
and the other (right) is for surface observation with aconventional SPM tip. The contact force between thesample and each probe is controlled independently
using piezoresistors (electrical system) on the cantilever base operating in self-sensitive detection
mode. The surface observation probe is removed in
the vertical direction by a bimorph actuator (mechanical system) when the electrical measurement probe
is in use. As shown in Fig. 3(b), the tip of the electrical measurement probe is a nanoprobe for four-pointprobe measurement. Four Pt nanoelectrodes 200-nm
wide and with 500-nm pitch were fabricated by ananolithographic technique using a focused ion beam
(FIB). This dual cantilever system can be used in acommercial SPM system.
This dual cantilever system enabled the resistivityof the selected position in the topographic image
taken by the surface observation lever to be measured
by the electrical-measurement cantilever with highresolution. The feasibility of the electrical measurement was confirmed by measuring the contact characteristics using a flat conductive sample.
2.2 Four-point probe with nano-springs
The conventional four-point probe consists of four
probes aligned with the same pitch (typically 1 mm).
Mechanical springs are mounted at the root of eachprobe to maintain electrical contact between theprobes and the sample. The springs compensate for50

the height differences of the contact point between
the probes and the sample surface. Because it is very
difficult to miniaturize the probes and springs using
conventional machining technology, it had not beenpossible to reduce the distance between the probes.
Since the relative heights of the electrodes on thefour-point probe described in the previous section
cannot be changed independently, this system can beused only for flat surfaces. We plan to use nanofabrication technology to create a structure similar to theconventional four-point probe for the nano world.
Recently, various three-dimensional structures withspatial resolution in the nanometer range have been
fabricated using the FIB deposition technique [4]. We
integrated four carbon probes with nanosprings [3]on a Si cantilever with Al electrodes using FIB deposition, as shown in Fig. 4. The diameter of the carbon
probe is 110 nm, and the diameter of the nanospringis 380 nm. The carbon probe made from diamondlike carbon (DLC) is electrically conductive andmechanically hard. These properties are desirable forthe electrical probe.
The mechanical properties of the carbon springwere investigated using a spring with a 1300-nm
diameter, as shown in Fig. 4(c). The length of thenanospring was expanded to 5.5 µm longer than its
initial length which was about 11 µm. No distortionof the spring shape was observed after the experiment. In another experiment, measurements foundthe stiffness of the spring to be almost the same as
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Diamond-like carbon
probe

Carbon probe

Probe diameter:
110 nmSpring diameter:
380 nmNanospring(b) Electron micrograph of probe tip

Nanospring
Al electrodeSi microprobe

Nanosprings compensate for height
differences between the probes
and sample surface.
(a) Schematic of four-point probe

BeforeStretchedAfterby 5.5 µm
Stiffness of nanospring is almost the same asthat of steel spring.
(c) Mechanical properties of nanospring

Fig. 4. Four-point probe with nanosprings.

that of the standard steel spring.
The probe heights in Fig. 4(b) are about 10 µm andthey vary over a 200-nm range. The nanosprings
compensate for the height differences. We confirmedthat electrical contact between all four probes and thesample was established. This result shows that thenanosprings on the Si cantilever were actually acting
as tiny mechanical devices.
3. Electrical measurement using the four-point
nanoprobe
In measurements using the conventional four-pointprobe, damage to the sample surface cannot be avoided because high pressure is applied to the sample.
The impact of this damage is not negligible for a verythin film or mechanically fragile film.
In our multiprobe on the SPM cantilever, the contact force between the sample and probe is controlled
at very low pressures by the SPM system. The feasibility of the multiprobe with nanosprings was confirmed for the measurement of the field-effect transistor (FET) characteristics of a very thin Si layer.
Three electrodes—gate, source, and drain—arerequired to measure the FET characteristics of semiVol. 4 No. 9 Sep. 2006

conductor materials. In the experiment, two probes
on the Si cantilever were used as the source and drainelectrodes of a metal-oxide-semiconductor (MOS)
FET, as shown in Fig. 5(a). The buried oxide layer inthe silicon-on-insulator (SOI) substrate acted as agate oxide. The MOS-FET characteristics of theultra-thin (10 nm) Si layer on SOI were successfullymeasured using the four-point nanoprobe on a Si cantilever [5]. As shown in Fig. 5(b), the conductance of
the Si layer was controlled by the gate voltage applied
to the Si substrate. Complicated procedures with anumber of lithographic processes are required tomeasure FET characteristics in conventional methods. The four-point nanoprobe eliminates the needfor time-consuming processes such as a lithographic
process in order to measure the FET characteristics ofa semiconductor thin film on an insulator layer.
Optimization of the deposition conditions for thinfilm transistors, such as polysilicon and organic semiconductors, can be more easily achieved than with theconventional method. If we can make a probe smaller than a grain of a polycrystal material, we will beable to measure the conductance within one grain andthe conductance across a grain boundary.

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0.3
VdSource

DrainCarbonnanoprobes

Al electrodes

10 (µm)1µm

1µm

Nanosprings

Drain current Ids (nA)

Vd = 1 VProbe
Si layer
0.2IdsBuried oxidelayer
Si substrateVg

0.1

Si cantilever
0–4(a) Electron micrograph of FET probe–3–2–1Gate voltage Vg (V)0(b) Measured FET characteristics

Fig. 5. FET probe for semiconductor film.

4. Conclusion
Multiprobe systems are required for measuring ofsheet resistance and electrical transport characteristics. A conventional SPM system with a single probe
can measure only the contact characteristics between
the probe and a sample. In this article, we described
two kinds of four-point nanoprobes integrated on an
SPM cantilever using a nanofabrication process,along with their application for electrical measurements. The first one is the four-point nanoprobe forelectrical measurement with piezoresistors integrated
on the dual cantilever system. Four Pt nanoelectrodes, 200 nm wide, were successfully fabricated on
the tip of the cantilever by FIB nanolithography. The
second is a set of carbon probes with nanosprings
integrated on the Si cantilever. The height differencesamong the probes are compensated for by thenanosprings. The four-point probe with nanosprings
can be used to measure the FET characteristics of avery thin Si film.
Four-point nanoprobes integrated on an SPM cantilever using a nanofabrication process will be useful
in developing new semiconductor materials for future
electronic devices. In the near future, nanoprobe distances will be reduced to less than 100 nm, which is
the typical grain size of polycrystal films. Our final
goal is to make a sub-10-nm gap electrode for measuring molecular devices.

52

References
[1]

[2]

[3]

[4]

[5]

M. Nagase, H. Takahashi, Y. Shirakawabe, and H. Namatsu, “Nanofour-point probes on microcantilever system fabricated by focusedion beam,” Jpn. J. Appl. Phys., Vol. 42, pp. 4856-4860, 2003.
M. Nagase and H. Namatsu, “A Method for Assembling NanoElectromechanical Devices on Microcantilevers Using Focused Ion
Beam Technology,” Jpn. J. Appl. Phys., Vol. 43, pp. 4624-4628, 2004
M. Nagase, K. Nakamatsu, S. Matsui, and H. Namatsu, “Carbon Multiprobes with Nanosprings Integrated on Si Cantilever Using
Focused-Ion-Beam Technology,” Jpn. J. Appl. Phys., Vol. 44, pp.
5409-5412, 2005.
S. Matsui, T. Kaitio, J. Fujita, M. Komuro, K. Kanda, and Y. Haruyama, “Three-Dimensional Nanostructure Fabrication by Focused-IonBeam CVD,” J. Vac. Sci. & Technol., B 18, pp. 3181-3184, 2000.
M. Nagase, K. Nakamatsu, S. Matsui, H. Namatsu, and H. Yamaguchi, “Carbon Multiprobe on a Si Cantilever for Pseudo-MetalOxide-Semiconductor Field-Effect-Transistor,” Jpn. J. Appl. Phys.,Vol. 45, pp. 2009-2013, 2006.

Masao Nagase
Nanostructure Technology Research Group,Physical Science Laboratory, NTT Basic
Research Laboratories.
He received the B.E., M.E., and Dr.Eng.
degrees in nanometrology for nanodevices fromWaseda University, Tokyo, in 1982, 1984, and1997, respectively. In 1984, he joined the LSILaboratories of Nippon Telegraph and Telephone Public Corporation (now NTT), where heworked on the R&D of fabrication processes for
sub-micrometer VLSIs. In 1996, he moved toNTT Basic Research Laboratories, where he isresearching fabrication processes for mesoscopic
devices, such as single-electron and nanoelectromechanical devices. He is a member of the Japan
Society of Applied Physics.

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