In order to develop a new structure microwave probe, the fabrication of AFM probe on the GaAs wafer was studied. A waveguide was introduced by evaporating Au film on the top and bottom surfaces of the GaAs AFM probe. A tip having 8 μm high, and curvature radius about 50 nm was formed. The dimensions of the cantilever are 250×30×15 μm. The open structure of the waveguide at the tip of the probe was introduced by using FIB fabrication. AFM topography of a grating sample was measured by using the fabricated GaAs microwave probe. The fabricated probe was found having nanometer scale resolution, and microwave emission was detected successfully at the tip of the probe by approaching Cr-V steel and Au wire samples

Hamada, M.

Ju, Y.

Kobayashi, T.

Soyama, H.

I-Revues

9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
A Nanostructual Microwave Probe Used for 
 Atomic Force Microscope 
 
Y. Ju*, M. Hamada*, T. Kobayashi**, H. Soyama** 
* Department of Mechanical Science and Engineering, Nagoya University 
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 
** Department of Nanomechanics, Tohoku University 
Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan 
 
 
 
Abstract- In order to develop a new structure microwave 
probe, the fabrication of AFM probe on the GaAs wafer was 
studied.  A waveguide was introduced by evaporating Au film on 
the top and bottom surfaces of the GaAs AFM probe.  A tip 
having 8 µm high, and curvature radius about 50 nm was 
formed.  The dimensions of the cantilever are 250×30×15 µm.  
The open structure of the waveguide at the tip of the probe was 
introduced by using FIB fabrication.  AFM topography of a 
grating sample was measured by using the fabricated GaAs 
microwave probe.  The fabricated probe was found having 
nanometer scale resolution, and microwave emission was 
detected successfully at the tip of the probe by approaching 
Cr-V steel and Au wire samples. 
 
I.  INTRODUCTION 
With the development of nanotechnology, the 
measurement of electrical properties in local area of materials 
and devices has become a great need.  Although a lot kinds of 
scanning probe microscopes have been developed for 
satisfying the requirement of nanotechnology, a microscope 
technique which can determine electrical properties in local 
area is still under developing.  Recently, microwave 
microscope has been an interest to many researchers [1-4], 
due to its potential in the evaluation of electrical properties of 
materials and devices.  The advance of microwave is that the 
response of materials is directly relative to the 
electromagnetic properties of materials.  Recently, we have 
proposed a microwave atomic force microscope (M-AFM) 
which is expected to be able to realize the evaluation of 
electrical properties as well as the measurement of 
topography of materials in nanoscale orders [5].  This 
technique combined the characteristic of the microwave 
microscope and the atomic force microscope which has 
nanoscale spatial resolution and can keep standoff distance 
constantly by the atomic force between the tip and sample.  
The details to fabricate the M-AFM probe using wet etching 
technique have been described in Ju et al. [5], and the 
evaluation of the capability to sense the topography of 
materials by using the M-AFM probe was demonstrated in Ju 
et al. [6].  In this paper, the propagation of microwave signal 
in the probe and the emission of the signal at the tip of the 
probe were carried out.  
II. PROBE FABRICATION 
In this research, no doped GaAs wafer was used as the 
substrate of the probe in order to restrain the attenuation of 
microwave propagating in the probe.  To obtain the desired 
structure, wet etching was used to fabricate the probe.  In 
contrast to dry etching, a side etching occurs under the etching 
mask.  Utilizing this property, a cone-shaped microtip can be 
obtained.  Early studies suggested that a square resist pattern 
having 14 µm sides and one side at 45° to the <011> direction 
is the most suitable mask for etching the tip of the probe [7].  
By considering the chemical activities at different crystalline 
planes, the length direction of the etching mask for forming 
the beam of the cantilever was patterned along the <011> 
direction.  Consequently, side-etching occurs under the resist 
mask, and mesa-type planes appear at both sides of the beam 
(45° inclined plane).  On the other hand, an inverse-mesa type 
plane is formed at the end of the beam (60°–75° inclined 
plane) [6].  Under the same conditions as the beam fabrication 
process, the body of the probe was formed by backside 
etching.  The etching mask was patterned on the bottom 
surface, and etching was carried out until the substrate was 
penetrated.  After that, Au film of 50 nm thickness was 
deposited on the top and bottom surfaces of the probe to 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1. SEM photograph of the tip of the GaAs AFM probe. 
50 nm of curvature 
radius 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2. The cantilever of the probe having dimensions of 
250×30×15 µm. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3. SEM photograph of the microslit of the microwave AFM probe  
introduced by FIB fabrication. 
 
propagate a microwave signal in the probe.  Both plane 
surfaces of the waveguide, which were made of evaporated 
Au film, were connected at the end of the beam.  However, 
there was no Au film on the sides of the beam, since the 
formed inclined planes at the beam sides were not facing the 
direction of evaporation.  By using focused ion beam (FIB) 
fabrication, a slit at the tip of the probe was formed to open 
the connection of the Au film on the two surfaces of the 
probe.  Consequently, the microwave will propagate along the 
probe and emit at the tip of the probe. 
Scanning electron microscopy (SEM) observation was 
carried out for the fabricated M-AFM probe.  As shown Fig. 
1, a sharp tip having high aspect ratio (2.0) was obtained.  The 
tip is the 8 µm high, and the curvature radius of the tip is about 
50 nm.  Fig. 2 shows the observation of the cantilever of the 
probe, where the tip is located near the front edge of the 
cantilever.  As shown in Fig. 3, a microslit was introduced 
cross the cantilever through the center of the tip by FIB  
fabrication.  The width of the microslit is small than 100 nm. 
III. EXPERIMENTAL RESULTS 
A.  AFM topography 
In order to confirm the spatial resolution of the fabricated 
M-AFM probe, the AFM topography of a grating sample 
having 2000 line/mm was measured by using the fabricated 
probe.  JSPM-5400 was used for measuring the sample in 
non-contact mode which is suitable for carrying out 
microwave measurement.  The scan area was 2 × 2 µm, scan 
speed was 3.0 µm/sec.  Fig. 4 shows the AFM topography of 
the grating sample obtained by the M-AFM probe.  Fig. 4a) is 
2-D image of the topography, and Fig. 4b) shows the 3-D 
image.  The white spots in these figures are due to micro-dusts 
on the sample surface.  It is observed that the fabricated 
M-AFM probe has the capability to catch the AFM 
topography and height information in nanometer orders. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 4. AFM surface topography of the grating sample 
obtained by the fabricated M-AFM probe: 
 (a) 2-D image, (b) 3-D image. 
Cantilever 
Tip 
Tip 
Microslit by FIB 
GaAs 
Au 
(a) 
(b) 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 5. The schematic diagram of the improved probe holder. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 6. The connection of the probe with the coaxial line. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 7. Photograph of microwave measurement system.  
B.  Propagation and detection of microwave 
The microwave signal was measured by a network analyzer 
where the working frequency is 75~110 GHz.  A coaxial line 
was used to connect the M-AFM probe with the network 
analyzer.  In order to realize such connection, the coaxial line 
having the diameter of 1 mm was fixed on the probe holder 
which is used to set up an AFM probe for AFM measurement. 
The schematic diagram of the improved probe holder is 
shown in Fig. 5, and the enlarged view of the probe 
connection is shown in Fig. 6.  The outer and inner conductors 
of the coaxial line are connected to the bottom and top 
surfaces of the M-AFM probe, respectively. Therefore, 
microwave transmission line changes form the coaxial line to 
the parallel plate waveguide (in the probe).  By using this 
holder, M-AFM probe can be set up repeatedly, and the 
measurement of microwave response and AFM surface 
profile can be realized simultaneously.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 8. The measurement result of the amplitude of reflection coefficient  
when the test sample is absent at the tip of the M-AFM probe. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 9. The measurement result of the amplitude of reflection coefficient  
when the Cr-V steel sample is present at the tip of the M-AFM probe. 
AFM 
Network analyzer 
Fabricated probe 
Coaxial cable 
Wire 
Coaxial Cable 
Wire 
M-AFM Probe 
PZT 
Network analyzer 
Microwave probe 
Probe holder 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 10. The measurement result of the amplitude of reflection coefficient  
when the Au wire is present at the tip of the M-AFM probe. 
 
Microwave propagation should perform accurately in the 
fabricated M-AFM probe in order to evaluate electrical 
property of materials.  To confirm that, the response of 
microwave reflection was examined by approaching a Cr-V 
steel sample and an Au wire (50 µm diameter) at the tip of the 
probe.  The microwave measurement system used in the 
experiment is shown in Fig. 7.  The experiment was carried 
out as follows: microwave signal generated in network 
analyzer was feed to the M-AFM probe; the test samples were 
approached to the tip of the probe; the amplitude-frequency 
response was measured.  The reflection signal will change 
due to the difference between characteristic impedance of the 
samples and air when microwave is propagating in the probe 
and emitting at the tip.  It means that microwave emission at 
the tip of the M-AFM can be evaluated by measuring the 
response of the microwave signal. 
Fig. 8 and Fig. 9 show the measurement results of the 
microwave response.  The amplitude of the reflection 
coefficient was measured by sweeping the working frequency. 
Here, the swept frequency range is 2 MHz and the center 
frequency is 94.44 GHz.  Fig. 8 is the result that the test 
sample is absent at the tip of the M-AFM probe.  Fig. 9 is the 
result that the Cr-V steel sample is present at the tip of the 
probe.  When the test sample is made to be approached at the 
tip of the probe, as shown in Fig. 9, the peak changing in 
amplitude is observed clearly.  This result indicates that the 
microwave emitted at the tip of the probe.  The amount of the 
variation of the amplitude is about 0.2 dB.  Fig. 10 is the result 
that the Au wire is present at the tip of the probe.  It is 
confirmed that the amount of variation of the amplitude is 
about 0.15 dB.  Here, since the Au wire has the diameter of 50 
µm which is great small than the dimensions of Cr-V steel 
sample, a smaller signal was obtained.   However, it should be 
noted that the measured signal is sensitive enough for the 
evaluation of electrical property in the microwave AFM 
measurement. 
 
IV. CONCLUSION 
M-AFM probes were fabricated on the GaAs wafer by using 
wet etching process.  A waveguide was introducing on the 
probe by evaporating Au film on the both surfaces of the probe.  
SEM observation shows that a tip having 8 µm high and 
curvature radius about 50 nm were formed.  The open 
structure of the waveguide at the tip of the probe was obtained 
by using FIB fabrication.   AFM measurement was performed.  
It is indicated that GaAs microwave probe has a capability to 
catch AFM topography of materials in nanometer scale.  
Microwave emission was detected at the tip of the probe 
which is large enough for the evaluation of electrical 
properties in the microwave AFM measurement. 
ACKNOWLEDGMENT 
This work was supported by the Japan Society for the 
Promotion of Science under Grant-in-Aid for Scientific 
Research (S) 18106003 and (A) 17206011; Ministry of 
Education, Culture, Sports, Science and Technology of Japan 
under Grant-in-Aid Exploratory Research 18656034. 
REFERENCES 
[1]  Steinhauer, D. E., Vlahacos, C. P. Wellstood, F. C., Anlage, S. M., 
Canedy, C., Ramesh, R., Stanishevsky, A. and Melngailis, J., 
“Imaging of microwave permittivity, tenability, and damage 
recovery in (Ba, Sr) TiO3 thin films,” Applied Physics Letters, Vol. 
75, No. 20, 1999, pp. 3180-3182. 
[2]  Duewer, F., Gao, C., Takeuchi, I. and Xiang X.-D., “Tip-sample 
distance feedback control in a scanning evanescent microwave 
microscope,” Applied Physics Letters, Vol. 74, No. 18, 1999, pp. 
2696-2698. 
[3]  Tabib-Azar, M., Akinwande, D., “Real-time imaging of 
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Instruments, Vol. 71, No. 3, 2000, pp. 1460-1465. 
[4]  Ju, Y., Saka, M. and Abé, H., “NDI of delamination in IC packages 
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[5]    Ju, Y., Kobayashi, T., and Soyama, H., “Fabrication of a GaAs 
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[6]   Ju, Y., Kobayashi, T., and Soyama, H., “Development of a 
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[7]     Ju, Y., Sato, H., and Soyama, H., “Fabrication of the tip of GaAs 
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A Nanostructual Microwave Probe Used for Atomic Force Microscope

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A Nanostructual Microwave Probe Used for Atomic Force Microscope

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