REVIEW OF SCIENTIFIC INSTRUMENTS 81, 123708 (2010)

Microwave atomic force microscopy imaging for nanometer-scale electricalproperty characterization
Lan Zhang, Yang Ju,a) Atsushi Hosoi, and Akifumi Fujimoto
Department of Mechanical Science and Engineering, Nagoya University, Furo-Cho, Chikusa-ku, Nagoya
4648603, Japan(Received 4 September 2010; accepted 15 November 2010; published online 30 December 2010)
We introduce a new type of microscopy which is capable of investigating surface topography andelectrical property of conductive and dielectric materials simultaneously on a nanometer scale. The
microwave atomic force microscopy is a combination of the principles of the scanning probe microscope and the microwave-measurement technique. As a result, under the noncontact AFM working conditions, we successfully generated a microwave image of a 200-nm Au film coating on a
glass wafer substrate with a spatial resolution of 120 nm and a measured voltage difference of
19.2 mV between the two materials. © 2010 American Institute of Physics. [doi:10.1063/1.3525058]
I. INTRODUCTION

Electrical properties are the most significant intrinsic
characteristics of substances and strongly affect the work
functions of different materials, especially in nanometer-scalematerials and devices. Thus, the evaluation of electrical properties is a very important issue when investigating the physicalproperties of nanometer-scale materials and devices. Atomic
force microscopy1, 2 (AFM) is a well known and powerful
technique, which has an irreplaceable role in nano science andtechnology, because of its unique ability to image and manipulate surface structures on the nanometer scale.3–6 To characterize the electrical features of materials on the nanometer scale, extended techniques based on AFM, such as Kelvinprobe force microscopy (KFM),7, 8 scanning capacitance microscopy (SCM),9, 10 conducting atomic force microscopy (CAFM),11, 12 and electrostatic force microscopy (EFM),13, 14
have been developed over time. Although, KFM can measure the surface electrical potential of materials by detecting the electrostatic force between the probe tip and the sample, electrostatic forces, including van der Waals forces andchemical bonding forces, are also included in the measured
data; hence, sample surface chemistry and atmospheric conditions greatly impact the measured potential. Scanning capacitance microscopy can characterize electrical information
by measuring the capacitance between the tip of the probe
and the sample; however, it suffers from a limited spatial resolution and is sensitive to specimen thickness. Electrostaticforce microscopy can probe local charge distributions on surfaces with a satisfactory spatial resolution; however, the tip
must scan the sample surface twice: one time for the surface
topography measurement and the second for electrical information. The accuracy of the dual-pass scanning technique isgreatly impacted by probe drifts during the second scan. On
the other hand, microwave measurement has been of a greatinterest to many researchers because microwaves can propagate well in air and the sample’s response is directly rea) Author to whom correspondence should be addressed. Electronic mail:ju@mech.nagoya-u.ac.jp. Tel.: +81-52-789-4672. Fax: +81-52-789-3109.

0034-6748/2010/81(12)/123708/4/$30.00

lated to the electrical properties of the materials.15, 16 Thus,
to obtain microscopic electrical information, a variety of microwave measurement-based microscopic methods have beendeveloped, more specifically, scanning evanescent microwave
microscopy (SEMM),17, 18 and near-field scanning microwave
microscope (NSMM).19–21 Among these techniques, it is important to note that the measured signals will be affected by
the standoff distance between the probe tip and the sample,
thus, obtaining the absolute electrical characteristic information is difficult. To summarize, these AFM-based methodologies and microwave microscopy can only image relativeelectrical properties, and they are unable to measure the absolute values of the intrinsic electrical properties, such as conductivity, permittivity, and permeability. Microscopy that can
provide a simultaneous measurement of topography and electrical properties on a nanometer scale is still being developed.
Recently, to solve the problem that microwave signals are affected by the standoff distance, the technique tocombine the microwave microscopy with AFM has beenstudied.22–26 Weide group combined NSMM with a commercial AFM together,23, 24 in order to obtain the AFM
topography and microwave image of a sample simultaneously. Shen and co-workers invented a microwave impedance
microscope25, 26 (MIM) which fed a microwave signal to a
traditional AFM probe. It can also be used to sense theelectronic properties of measured samples. However, it isnoted that NSMM-AFM and MIM do not use matched probe
and cantilever as the microwave-guide connected with thesource of microwave signals. Thus, the microwave signals
may not propagate along the probe and emit from the tip apexof the probe. Therefore, these techniques can only measure
the impedances changes of the probe-sample system but not
the intrinsic electrical properties, such as conductivity, permittivity, and permeability. In this paper, we demonstrate anovel microwave atomic force microscopy (M-AFM) technique that maintains a constant standoff distance in an AFM
measurement and quantitatively evaluates the electrical properties of materials with a microwave instrument. The M-AFM
probe is operated in air in a noncontact mode. By combining
the advantages of AFM with microwave-based measurement,

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Rev. Sci. Instrum. 81, 123708 (2010)

FIG. 1. (Color online) Schematic diagram of the M-AFM probe that wasused to measure the electrical properties of materials in this study.

topographical imaging with a high spatial resolution and thein situ measurement of nanometer-scale electrical properties
can be realized. As can be observed in Fig. 1, the M-AFM
probe consists of a homogeneous parallel-plate (the metal
films are located on the top and bottom surfaces of the probe)
and a nano-slit that is across the tip apex. These special structures allow microwave signals to propagate through the probe
and emit from the tip of the M-AFM probe. When the MAFM probe is located above a sample surface, the nano-slit on
the tip acts as a source of microwave signals that transmit into
the surface of the sample and as a receiver of the microwave
signals that reflect from the measured sample. By analyzing
the measured reflection signals, the electrical information of
the measured materials can be determined.
In previous studies, M-AFM probes have enabled us tomeasure the topography of a grating sample with high resolution in air in the noncontact AFM mode.27 These results
demonstrate that the M-AFM probe has a similar ability toimage the surface topography of materials in comparison witha conventional Si AFM probe. To confirm the emission of microwaves at the tip of the M-AFM probe, a network analyzer
was connected to the M-AFM probe by a coaxial line.28 Whenthe test samples approached the tip of probe, the reflected
microwave signals were measured by the network analyzer,
which indicates that the nano-slit at the tip of probe can act
as a sensor to emit and receive microwave signals. Moreover,the effects of using Al and Au as waveguide coating materials for the propagation of microwave signals in the M-AFM
probe has also been studied.29 However, because it is difficult
to synchronize the network analyzer that is used to measure
the microwave signals with the AFM scanner that is used tocreate the AFM image, a microwave AFM image has not yet
been achieved. Therefore, in this study a compact microwave
instrument was constructed which can be appropriately synchronized with an AFM scanner. Consequently, a microwave
AFM image was realized.
II. EXPERIMENTAL SETUP

Figure 2 schematically depicts the integrated test systemof the M-AFM. In our M-AFM system, the initial microwave

FIG. 2. Diagram of the M-AFM system.

signals, which are working at a frequency f = 16.66 GHz, aregenerated by a microwave generator. Next, the frequency ofthe microwave signals is extended by a six-frequency multiplier, which results in a stable testing frequency f = 94 GHz.
The microwave signals propagate through an isolator and acirculator and then propagate into the M-AFM probe. The
transmission line that connects the circulator and the probechanges from a rectangular waveguide into a coaxial line,
which then changes into the parallel-plate waveguide (in the
M-AFM probe). A detector is connected to the circulator, tomeasure the microwave signals that are received by the tipof the probe and indicate the voltage data that are converted
from the reflected microwave signals. The measured signals
are synchronized with positional information that is obtained
from the AFM scanner, which is then used to create a microwave image. At the same time, by evaluating the output
voltage data, the electrical properties of the measured materials can be determined.
To reduce the attenuation of microwaves that propagate in
the M-AFM probe and obtain a desired structure, we elected
to use an undoped GaAs wafer as the substrate of the M-AFM
probe.30, 31 Wet etching was used to fabricate the probe because, unlike dry etching techniques, side etching occurs under the resist pattern during the wet etching, which permits
the realization of a microfabricated pyramidal GaAs tip. To
facilitate proper microwave signal propagating through theprobe, a parallel-plate waveguide was formed by evaporating Au films onto the top and bottom surfaces of the M-AFM
probe. The Au films on both sides are connected at the endof the probe cantilever, and there are no Au films on the sidesof the cantilever and probe body. To make microwave signals emit from the apex of the probe tip, a nano-slit at the tipof the probe was fabricated via a focused ion beam process(FIB) to short-circuit the Au films on the two surfaces of theprobe. Consequently, a homogeneous parallel-plate waveguide was formed, and microwave signals were able to propagate along the probe and emit from the tip apex of the M-AFM
probe. It should be noted that the dimensions of the GaAs substrate and the Au films on the M-AFM probe determine theAuthor complimentary copy. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp

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Rev. Sci. Instrum. 81, 123708 (2010)

FIG. 4. SEM image of measured sample.

FIG. 3. SEM images: (a) the fabricated M-AFM probes; (b) the cantileverof the M-AFM probe; (c) the 100-nm-wide-FIB-fabricated nano-slit thatis across the cantilever and through the center of the tip; and (d) highmagnification image of the tip.

characteristic impedance of the waveguide. To match thecharacteristic impedance of the associated transmission coaxial line, the waveguide is expected to have a characteristic
impedance of 50 . Thus, the cantilever and body of the MAFM probe are designed with the dimensions of 250 × 30
× 15 μm and 2740 × 720 × 340 μm, respectively.
The SEM images of the fabricated M-AFM probes are
depicted in Fig. 3(a). Figure 3(b) shows the as-fabricated cantilever of the M-AFM probe. The dimensions of the M-AFM
probe depend on several small variations of experimental parameters, including the developing time of the resist pattern,
the wet etching rate, and the electron beam (EB) evaporation rate. The average dimensions of the cantilever and the
body of the M-AFM probes are typically 252 × 31 × 14 μm
and 2742 × 723 × 339 μm, respectively. Thus, the characteristic impedance of the M-AFM probes is, on average, 49.3 .
Figure 3(c) depicts an SEM photograph of the FIB-fabricatednano-slit that has been patterned across the cantilever throughthe center of the probe tip. The observed tip is located near thefront edge of the cantilever. As can be observed in Fig. 3(d),the tip is approximately 7 μm high, and the nano-slit is approximately 100 nm in width.
We prepared a sample for the scanning test of surface topography and microwave imaging. At first, a resist mask was
patterned onto the glass substrate wafer by lithography. After developing the resist pattern, a 200-nm thick Au layer was
deposited on the glass substrate by EB evaporation. Finally,
the unexposed photoresists were lifted off in acetone. The resulting Au and glass step structure is depicted in SEM image
shown in Fig. 4.

III. RESULTS AND DISCUSSION

Figure 5 depicts the M-AFM scanning results of the sample at the step area between the Au coating film and the glass
wafer substrate. The M-AFM worked in noncontact mode,
wherein the scanning speed was fixed to 5 μm/s, scanningarea was 10 μm × 10 μm, environmental temperature was
24.5 ◦ C, and the relative humidity was 38.4%. Figure 5(a) depicts the surface topography of the M-AFM-measured sample. In this image, the left side represents the Au film, whereas
the right side is the glass substrate. As can be seen in thescanning profile depicted in Fig. 5(a), the thickness of theAu film was approximately 200 nm on average. In addition,
Fig. 5(b) depicts the three-dimensional image with height

FIG. 5. (Color online) The scanning results that were obtained by the MAFM: (a) AFM topography image of the Au/glass step sample; (b) a threedimensional image that corresponds to (a); (c) microwave image of the output
voltage that was converted from the measured microwave signals; and (d) athree-dimensional image that corresponds to (c).

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123708-4

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information that corresponds to Fig. 5(a). Figure 5(c) depicts
the microwave image of the voltage that was converted fromthe measured microwave signals, which were simultaneously
acquired by the M-AFM probe at the corresponding position
depicted in Fig. 5(a). Figure 5(d) depicts a three-dimensional
image with electrical information that corresponds to that depicted in Fig. 5(c). This experimental result demonstrates thatthe microwave image has two spatial phases. Because thestandoff distance between the tip of the M-AFM probe andthe surfaces of the Au film and glass substrate is constant andcontrolled by the atomic force, thus, the response of the microwave signals were observed to change based on the different electrical characteristics of the measured materials. As per
Fig. 5(c), the output voltage over the glass area is larger thanthat over the Au area, because the scanning started from the
Au area with the initial offset from the nulling operation.
An analysis of the scanning profile depicted in Fig. 5(c)demonstrates that the spatial resolution is higher than
120 nm, and that the output voltages measured over the Au
and glass areas were 262.5 and 281.7 mV, respectively. The
difference between the measured voltages between the Au
and glass areas is 19.2 mV. Since, the stability of the measurement is high, this value is large enough for evaluating theelectrical properties of other materials having the conductivity
between Au and glass. Based on the evaluation method of microwave measurement,32 taking two different samples as thereference, the M-AFM should allow us to scan the electricalconductivities of other materials on a nanometer scale. In addition, based on the same principle, it can also be used to measure the permittivities of dielectric materials on a nanometerscale.

IV. CONCLUSIONS

We have created an M-AFM-obtained microwave image
using a compact microwave instrument that was optimallysynchronized with an AFM scanner. The distinguishing features of M-AFM are its ability to maintain a constant standoff
distance between the probe tip and the sample surface andto measure the microwave signal interacted with the sample.
Therein, both the topography and electrical-property imagesof the sample can be simultaneously characterized. Therefore,
M-AFM is able to measure, in situ, the distribution of electrical properties on a nanometer scale. As shown in the experimental results, we successfully generated a microwave image of a 200-nm Au film coating on a glass wafer substratewith a spatial resolution of 120 nm, and, moreover, we measured the voltage difference between these two materials to be
19.2 mV. We believe that the high spatial resolution and simultaneous measurement capability of this M-AFM system
will have important implications to nanotechnology characterization in the immediate future.

Rev. Sci. Instrum. 81, 123708 (2010)

ACKNOWLEDGMENTS

This work was supported by the Japan Society for the
Promotion of Science under Grants-in-Aid for Scientific Research (A) 20246028 and (S) 18106003.
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