NEAR-FIELD SCANNING MICROWAVE MICROSCOPE USING A
DIELECTRIC RESONATOR
Jooyoung Kim1, Hyun Kim1, Myungsik 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
2
ABSTRACT. We describe a near-field scanning microwave microscope which uses a high-quality
dielectric resonator with a tunable screw. The operating frequency isf= 4.5 GHz. The probe tip is
mounted in a cylindrical resonant cavity coupled to a dielectric resonator. By tuning the tunable screw
coming through the top cover, we could improve sensitivity, signal-to-noise ratio, and spatial
resolution to better than 1.5 Jim. To demonstrate the ability of local microwave characterization, the
surface resistance of metallic thin films has been mapped.
INTRODUCTION
The near-field scanning microwave microscope (NSMM) with high spatial
resolution has become increasingly important. Recently, a variety of scanning probe
techniques have been developed for microwave- and millimeter-wave ranges [1-13]. In
order to achieve the largest possible sensitivity and spatial resolution of the NSMM, both
the quality of the resonator and the sensitivity of the probe must be maximized [1, 13]. A
high quality factor of the resonator permits measurement of relatively small variation in
electrical properties of samples. High spatial resolution and contrast in near-field
microscope images can be achieved by using a sensitive probe tip coupled to a resonator.
Thus, a high quality resonator is essential. Recently, dielectric materials with a dielectric
constant about 30-40 with good temperature stability have become available. In this
context, we adopted a relatively high quality dielectric resonator coupled to a sharpened
metallic probe tip. The main advantage of this system is its high Q factor (104-10 ) and
sensitivity, which result in a high spatial resolution. Another advantage is that the
dielectric resonator has a tuning screw for mechanical frequency tuning over a range of
500 MHz. Our device is extremely simple, small and compact with an effective shielding
structure from the far-field components. The exact resonant frequencies of certain resonant
modes can be easily computed by numerical simulations. In this work we report a nearfield scanning microwave probe with a dielectric resonator coupled to a sharp probe tip at
operating frequency / = 4.5 GHz. We demonstrate improved sensitivity and spatial
resolution better than 1.5 jim of the near-field images of metal Cr films on a glass
substrate.
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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Network Analyzer
Dielectric Resonator
RFon
MW Source
F=10MHz-20GHz
_L
Computer
FIGURE 1. Diagram of the experimental configuration of near-field scanning microwave microscope using
a dielectric resonator.
EXPERIMENTAL
The experimental setup of our near-field scanning microscope with a dielectric
resonator is shown in Fig. 1. We measured a reflectivity Sn when we calibrated and
characterized our near-field microwave microscope system. However, to map a near-field
microscope image, we measured the transmitted power of the NSMM at the operation
frequency. The Ba(ZrTa)C>3 dielectric cylindrical resonator with dielectric constant c = 29
has inner and outer diameters of 2 mm and 14 mm, respectively, and a height of 5.8 mm.
The diameter of the metal circular cylindrical cavity is 32 mm and the height is 14 mm.
The dielectric resonator has a fixed resonance frequency with its frequency determined by
the material, dimensions, and the shielding cavity conditions.14 The resonance frequency
of a given mode of TEoi is calculated by using Ansoft/HFSS software and measured by a
network analyzer (Agilent 8753ES). The unloaded Q factor was 24 000 and the
temperature coefficient of the resonance frequency was 0.0- 0.1 ppm/°C. The operation
frequency and impedance of the NSMM can be precisely tuned over a range of 500 MHz.
The resonance frequency shift was highly sensitive to the tuning screw that is mounted on
a top cover as shown in Fig. 1. As shown in Fig. 1, a stabilized rf frequency/= 10 MHz-20
GHz (HP83620A) was input into the dielectric resonance cavity and the cavity output was
connected to a power meter (HP 437B) and a spectrum analyzer (Advantest R4131D). The
spectrum analyzer and network analyzer were used in the tuning process of the dielectric
resonance cavity.
RESULTS AND DISCUSSION
There are four important parameters of the NSMM: the sensitivity of the probe (Sd),
the quality of the resonator (Sx), spatial resolution (AR), and signal-to-noise ratio. Note that,
3d can be determined by the physical interaction between the evanescent field and the
sample. S& can be written as Sf=£oAcf/JCod , where Aeff is the effective area of probe tip, Co
is the capacitance of the resonator, fr is the resonance frequency and d is the distance
between the probe tip and the sample. Sx is directly related to the quality factor (Q) of the
dielectric resonator and can be written as ASn/Af. Note that, Sx can be estimated from the
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slope of the reflectivity Sn of the resonator and the slope increased as Q increased. Thus, a
higher Q causes a larger slope of the reflectivity Sn and, thus, increasing the Q of resonator
will increase Sx. The minimum detectable detector output power APmi of the NSMM can be
written as [13,16],
&Pout=MSxSdPin
(1)
zJPom is directly correlated with the sensitivity of the NSMM. Thus, to achieve the largest
possible sensitivity, both Sd and Sx must be maximized. In order to improve the signal-tonoise ratio the input power Pin also must be maximized. By getting the maximum coupling
to the NSMM probe system with the input microwave source one can achieve the
maximum sign-to-noise ratio of the NSMM. Thus, an impedance matching process for the
NSMM is essential. In this experiment, we choose a resonance cavity tuning process to
match the impedance 50 Q at the resonance frequency. Note that, when the source
impedance matched the impedance by tuning the resonance cavity, the reflection
coefficient F can be made to be zero. At this time, the power Pm delivered into the
resonance cavity is a maximum. Thus, Sd, Sx, Q and the input power Pin must be
maximized, finally resulties in an improved spatial resolution.
Figure 2 shows the Smith chart and the reflectivity S\\ for a Cr thin film on a glass
substrate (A) in the absence of the sample, (B) upon approaching the sample with sample-
(A)
(B)
STOP 4 889.890 080 MHz
4.55
4.6
4.65
Resonance Frequency (GHz)
FIGURE 2. Measured Sn of tunable resonator viewed on a Smith Chart: (A) in the absence of the sample,
(B) with d= 1 mm without tuning, and (C) proper tuning and (D) measured Sn: (a) chart (A), (b) chart (B),
and chart (C).
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tip distance d = 1 Jim, and (C) with a tuning process for the NSMM. We measured film
reflectivity Su using a network analyzer with the experimental probe as shown in Fig.
2(D). As shown in (a), the resonant frequency without perturbation was/ r = 4.509 GHz and
the unloaded Q factor was 24 000. Note that, the method of measuring the Q of the cavity
was to measure the bandwidth of the cavity from its half-power points. The observed
resonance frequency coincided with the frequency calculated by using the HFSS software.
The impedance at this resonance frequency was 50 Q as shown in Fig. 2 (A). When a Cr
film is placed at d = 1 |im, both the resonance frequency /r and the Q factor decreased by
perturbation due to the interaction between the tip and the sample. The resonance
frequency/r and the Q decreased to 4.494 GHz and 980, respectively, and the impedance
increased up to 70.2 Q, as shown in (b). Thus, in order to achieve high sensitivity again,
one needs to tune the Q factor and the impedance of the resonator. This requires proper
matching by using a tuning screw. By properly tuning the resonance cavity we solved the
matching problems, resulting in a highly sensitive NSMM, as shown by curve (c) and Fig.
2(C). The tuned loaded Q factor was improved up to 22 000 again. The new resonant
frequency and the reflectivity at the minimum were/= 4.46 GHz and 55 dB, respectively.
Figure 3 shows (a) three-dimensional near-field scanning images for a Cr film on a
glass substrate without a tuning process and (b) with a tuning
FIGURE 3. Three-dimensional near-field scanning images of a Cr thin film (a) without and (b) with proper
tuning of the resonance cavity.
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process match to 50 £1. We measured the transmission power at an operation frequency/=
4.46 GHz and mapped the scanning images of the detector output power with the
experimental setup as shown in Fig. 1. By properly adjusting the tuning screw of the
resonance cavity, it shows the spatial resolution could be improved from 3 jam to 1.5 jum.
We emphasize, to increase the spatial resolution, both the sensitivity of probe and the Q of
resonator should be maximized.
To characterize the sensitivity of our NSMM we used metal thin films of Au, Ag, Co,
and Cr with the same thickness of 300 nm on a glass substrate. The film thickness is larger
than the skin depth at the operating frequency. We measured film reflectivity S\\ using a
network analyzer at the same probe tip-sample separation with d = 1 jtim with the
experimental probe as shown in Fig. 1. Figure 4 shows the resonator's reflection coefficient
S\\ changes for metallic samples. Both the resonance frequency and the quality factor of the
dielectric resonator are affected by the sample resistance. In addition, the amount of change
in the reflection coefficient depends on the sample resistance. The dependence of the
reflectivity S\\ on the sample resistance can be derived by using standard transmission line
theory [8,15] and is given by
(2)
R-.+Z.
where Zp is the effective impedance of the probe and RKS is the resistance of sample. The
4.47
4.48
4.49
4.51
4.52
Frequency (GHz)
FIGURE 4. . Measured reflectivity S\\ of different metal thin film samples with the same thickness of 300
nm: (a) air, (b) Cr, (c) Co, (d) Al, and (e) Au. The inset shows the measured reflectivity Su plotted against
thin film resistances of Cr (0), Co (V), Al (A), and Au (D ) taken from a material handbook. A dot line shows
a fit to Eq. (2) with Zp = 50 Q.
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insert shows the measured and fitted reflectivity of S\\ as a function of surface resistances
for different samples taken from material handbooks [17]. A fit to Eq. (2) yields Zp = 50 Q.
As expected we also found a linear relation between the variation of l/Q and the surface
resistance [8],
In summary, we report a near-field scanning microwave microscope by using a high
quality dielectric resonator at an operating frequency o f / = 4.5 GHz. The probe tip is
mounted in the cylindrical resonant cavity coupled to the dielectric resonator. By tuning
the tunable screw, we could improve sensitivity and spatial resolution to better than 1.5
Jim. To demonstrate the ability of local microwave characterization, the surface resistance
of metallic thin films has been mapped. Our future efforts include extending our
measurements to millimeter-wave and to lower temperature.
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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