NANO SPATIAL RESOLUTION WITH 60 GHz NEAR-FIELD
SCANNING MILLIMETER-WAVE MICROSCOPE
Myungsik Kim1, Hyun Kim1, Jooyoung Kim1, Barry Friedman2, and Kiejin Lee1
Department of Physics, Sogang University, C.P.O. 1142, Seoul 121-742, Korea
Department of Physics, Sam Houston State University, Huntsville, Texas 77341
ABSTRACT. We report an imaging technique for various samples using a near-field scanning
millimeter-wave microscope (NSMM) by using a standard tunable rectangular waveguide at
operation frequency /= 60GHz. By monitoring the change of g-factor in the near field zone as
the probe scanned over the object, we obtain quantitative images of the sample. By proper
tuning process and using proper probe-tips, we could improve sensitivity and a spatial resolution
to better than 500 nm for the patterned YBa2Cu3Oy thin films on MgO substrates.
INTRODUCTION
High spatial resolution and high sensitivity imaging techniques have
become increasingly important and attractive subjects. Over the years, various
scanning techniques, such as atomic force microscope (AFM), scanning near-field
optical microscopy (NSOM), scanning tunneling microscopy (STM), scanning
electron microscope (SEM) and various microwave near-field microscope have been
developed to image local variation of properties and structure of materials [1-7].
Recently, many groups have demonstrated a high spatial resolution imaging technique
for conducting materials using a near-field scanning millimeter-wave microscope
(NSMM) by the several different design with the various type of probe and resonator.
For the millimeter-wave range, several designs were used, for example, a narrow
resonant slit [8-11] and the spatial resolution did not exceed 30-100 jim. Thus, instead
of a slit probe, we use a conventional metallic probe coupled to a wave-guide
resonator [11].
In this paper, instead of the wide range of millimeter-wave source and fixed
length of wave-guide, we used a standard rectangular wave-guide as a resonator
coupled to a sharpened probe tip. The metallic probe tip is mounted to the wave-guide
to couple energy into and out of the resonant wave-guide probe. By monitoring the
change of quality factor between probe tip and surface of sample in the near field zone
as scanned over the object, we can scan the near-field image of the sample. The
principal of operation can be understood by perturbation theory of the resonant cavity
considering the radius of the probe tip and the change distribution to depend on the
sample tip distance. To achieve the largest possible sensitivity, we control the length
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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of probe tip to couple the energy to the probe tip. By using a tunable A/4 movable
plunger, we could observe dramatically improved near-filed scanning millimeterwave microscope images of the YBaiCusOy thin film on the MgO substrate with the
spatial resolution of better than 500 nm.
EXPERIMENTAL
The experimental set-up of our near-field millimeter-wave scanning
microscope with an operating frequency / =60 GHz is shown in Figure 1. Tunable
rectangular wave-guide coupled to the metallic probe is used as a resonant cavity. A
fundamental TEioi mode was excited in the wave-guide resonator. The directional
coupler coupled to a Gunn oscillator and a crystal detector. The probe tip was
mounted near the movable A/4 short with a tuning screw. After the sample is brought
close to the sample, by proper tuning the movable A/4 short we could improve the
sensitivity and the spatial resolution
(a)
Fine-tuning
A/4 plunger
Resonant wave-guide
Detector
*RFin
Tip
(b)
x-y-z stage
Resonant
waveguide
Isolator
Movable
Coupler
7J4 plunger
Detector
FIGURE 1. (a) Resonant wave-guide probe with probe-tip and (b) schematic diagram of experimental
configuration of the near-field scanning millimeter microscope with a resonant waveguide.
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The set-up was mounted on a motorized x-y-z translation stage with a
resolution of 0.1 jim driven by a computer-controlled microstepping motor. A
computer was also used to record the data from the lock-in amplifier and detector.
Figure 1 (b) shows the schematic diagram of the experimental configuration. The
metallic probe tips were made by a iron needle with a chemical etching. The
sharpened metallic probe mounted to tune coupling energy from wave-guide to the
probe effectuated through the controlling as the distance from probe tip to wave
reflected point and the insertion length of probe inside wave-guide. Note that the
coupling methods are often used either for measurement of standing wave patterns or
to couple energy into or out of a resonator. To achieve the largest possible sensitivity
and improved spatial resolution, we chose the operation condition corresponding to
the maximum probe sensitivity by adjusting the length of A/4 plunger of the resonator.
The detailed experiment setup has been described in elsewhere [11,12].
RESULTS AND DISSCUSSIONS
In this experiment, an image of the sample is accepted by the change of the
quality factor by the interaction between the probe tip and surface of sample. Since
the sample presents a small perturbation to the resonator, we can use perturbation
theory to find the change in the resonant frequency [13]
f
where EQ and HQ , and 6b and //o are the electric and magnetic field, and dieclectric constant
and magnetic permeability before perturbation. The A refers to the changes caused by the
sample existence. These facts indicate that the resonant frequency changes depending on
where the perturbation is located in the resonant cavity. If the TEio mode is the dominant
propagation mode in the wave-guide, the mode carriers all of the average power P can be
written P=I02Rir/2.> where 70 is the terminal current, R^ is the input resistance into the
probe-tip. Thus, input resistance can be written by R^ = bZ\/a, where a and b are the
lengths of wave-guide. We apply this expression to the probe-tip, treating it as a probe of
input resistance, and observed that a probe-tip may be a current probe in a resonant
rectangular wave-guide. If the probe-tip is in the near field zone, then the millimeter-wave
is perturbed mostly from conducting surface. By measuring the amplitude and the resonant
frequency while scanning the surface, it is possible to map of electrical properties of the
sample surfaces.
Figure 2 shows the reflection coefficient S\\ of the resonator and the resonance
frequency dependences on the lengths of A/4 plunger from probe tip that can be controlled
by a tuning screw. As the length of the cavity decreased, the resonance frequencies were
shift from 56.25 GHz to 65.125 GHz. As can be seen in Figure 2, the transmitted power to
the probe tip depending on the position of A/4 plunger position and at the/= 60 GHz, we
could obtain the maximum Q-factor.
451
-10
CO
|.20
55
-30
56
58 60 62 64
frequency(GHz)
-40
55
60
65
70
75
Frequency(GHz)
FIGURE 2. Measured Su of tunable resonator depends on the cavity lengths from probe tip (a) / =
4.07 mm for/- 56.25 GHz, (b) / = 3.66 mm for/- 58 GHz, (c) / = 3.25 mm for/- 60 GHz, (d) / 2.84 mm for/- 62.25 GHz, (e) / - 2.43 mm for/- 65.125 GHz. The inset shows the resonance
frequency (fr) depends on the cavity length.
Insertion length of probe tip (h) is also related to the transmission power of the
probe tip. The relation of insertion length and tranmission power P can be written as
[14],
1.0
(a)
£? 0.8
o 0.6
1
0.4
00
H 0.2
0.005
0.01
0.015
0.02
Insertion length (mm)
FIGURE 3. Transmitted power depends on insertion length of probe tip at various modes; (a) TEi0
mode, (b) TE2o mode, and (c) TE30 mode. The dotted line represented the experiment data.
452
(2)
cos|,*O
—h -cos(h
— nn
\b
-sin—n- c )
2
M2-f-l
U J I'J
,
(3)
where a and Z> are the width and height of resonant waveguide, m and n are the mode
number, co is the frequency, and h is the insertion length of probe tip inside resonant
wave-guide. Figure 3 shows the plots of equation (2) for the various TEno modes and
the experiment data depends on the insertion length h. As can be seen Figure 3(a), the
TEio mode is dominant in resonant waveguide. As the insertion length of electric
probe tip was increased, the transmission power was increased. The power became the
maximum when the insertion length h equal to the height of waveguide (b). This fact
shows that the coupling power of probe tip depends on the insertion length of probe
tip.
Figure 4 shows an optical image of sample and two-dimensional scanning
images of the detector output at different cavity lengths of the resonator for a YBCO
film on a MgO substrate without and with tuning the A/4 plunger . The probe-sample
distance was 1 jLtm. By tuning the resonance cavity, we have different contrast
scanning images.
FIGURE 4. (a) Optical image of the sample and three-dimensional near-field scanning images of
YBCO thin film on MgO substrate (b) without and (c) with tuning process.
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(b)
-^a
o
DOQXXXHWfcoa I
10
20
30
Distance (|im)
40
50
FIGURE 5. (a) Two-dimensional NSMM image of YBCO thin film on MgO substrate and (b) onedimensional intensity variation along the line A-A' indicated in (a). The obtained near-field scanning
microwave image with a spatial resolution 500 nm at operating frequency of f= 60 GHz.
When the source impedance matched the resonator impedance, we could obtain not
only the spatial resolution of 2 jiim but also the high contrast image, as shown in
Figure 4(c). Note that, by tuning the resonance cavity, it shows the spatial resolution
could be dramatically improved. This indicates that the relation between the contrast
and the spatial resolution depends on the cavity length. Thus, if the perturbation by
the sample-tip interaction was not large, the operation frequency corresponding to the
maximum probe contrast can be chosen by properly adjusting the length A/4 of the
resonant cavity.
Figure 5 shows two-dimensional near-field image of YBCO thin film on
MgO substrate and one-dimensional intensity variation along the line A-A' indicated
in (a). The changes in spatial resolution and sensitivity as a function of the cavity
length were clearly modulated by tuning the resonance cavity. At maximum
sensitivity, we obtained near-field scanning microwave images with a spatial
resolution of 500 nm at an operating frequency of/=60 GHz.
In summary, we have demonstrated a near-field scanning microscope by
using a tunable rectangular waveguide at operation frequency / = 60 GHz. By the
tuning the length of the cavity of the resonant waveguide, we could demonstrate
improved sensitivity and spatial resolution. Under the tuning condition, we could
obtain images with the high spatial resolution of better than 500 nm. Our future efforts
include extending our measurement to the imaging of electro-magnetic characteristic
of the sample surface.
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ACKNOWLEDGEMENTS
This work was supported by the Electronic and Telecommunications Research
Institute and by grant No RO1-2001-000042-0 from the Korea Science & Engineering
Foundation.
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