Millimeter-wave high-efficiency multilayer parasitic microstrip antenna

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005 2101
Millimeter-Wave High-Efficiency Multilayer Parasitic
Microstrip Antenna Array on Teflon Substrate
Tomohiro Seki, Member, IEEE, Naoki Honma, Member, IEEE, Kenjiro Nishikawa, Member, IEEE, and
Kouichi Tsunekawa, Member, IEEE
Abstract—This paper proposes a highly efficient multilayer parasitic
microstrip antenna array that is constructed on a multilayer
Teflon substrate for millimeter-wave system-on-package modules.
The proposed antenna achieves a radiation efficiency of greater
than 91% and an associated antenna gain of 11.1 dBi at 60 GHz.
The antenna size is only 10 mm 10 mm. Additionally, this paper
describes a 60-GHz-band prototype antenna employing a multilayer
Teflon substrate that is well suited to achieving high gain and
a wide bandwidth. The measured performance of the prototype antenna
is also presented.
Index Terms—Active integrated antenna, broad-band mobile
wireless access, millimeter-wave frequency band, multilayer
Teflon substrate, system-on-package.
I. INTRODUCTION
SYSTEM studies and hardware investigations on high-speed
wireless communications are being conducted at millimeter-wave
and quasi-millimeter-wave frequencies [1]–[3].
These applications require compact, high-performance, and
low-cost wireless equipment. A highly integrated RF module,
the so-called system-on-package module, which employs a
multilayer structure, is effective in achieving the above requirements
[4]–[7]. It is necessary to adopt active integrated
antenna technology to achieve a module with antennas that
are low-power consuming and have low-noise characteristics
[8]–[10]. Several approaches to achieve an RF module
that is integrated with antennas were reported. One approach
uses a semiconductor chip antenna such as a microstrip antenna
(MSA) that is integrated with RF circuits on the same
semiconductor substrate [4]. However, in this approach, it is
difficult to establish a high-gain antenna that employs an array
antenna configuration on a semiconductor substrate due to the
substrate size. Therefore, high-gain compact antennas have
not yet been integrated with monolithic microwave integrated
circuits (MMICs). A multichip-module approach was also
proposed to construct a module integrated with antennas [5],
[6]. In this module, antennas and MMICs are connected by
wire bonding or a ribbon, which results in high-connection
loss. This approach also requires a low-loss feeding circuit.
The dielectric lens antenna was adopted to achieve a high-gain
antenna [7]. However, the commonly used lens antenna is constructed
using an expensive crystal material and it is difficult
Manuscript received October 1, 2004; revised February 17, 2005.
The authors are with the Nippon Telegraph and Telephone (NTT) Network Innovation
Laboratories, NTT Corporation, Kanagawa 239-0847, Japan (e-mail:
seki.tomohiro@lab.ntt.co.jp; t.seki@ieee.org).
Digital Object Identifier 10.1109/TMTT.2005.848757
0018-9480/$20.00 © 2005 IEEE
to mount it on the MMIC package. Additionally, a dielectric
lens antenna constructed using resin was investigated as a
low-cost alternative. There are problems, however, regarding
mounting the antenna on the MMIC package and achieving
high efficiency. Moreover, it is difficult to construct an array
antenna substrate on a single layer due to the limitations in
the manufacturing process for the millimeter-wave frequency
band. The technique for improving the radiation efficiency by
arranging parasitic elements above the feeding MSA elements
is examined [11]–[15]. However, obtaining a sufficient absolute
gain is difficult in this study [11]–[14]. This paper investigates
the high-gain antenna using a low permittivity or the air layer
[15]. However, only the directional gain is described and there
is a problem in that it is difficult to support the parasitic element
substrates. In particular, it is difficult to adopt this antenna
in the system-on-package for the millimeter-wave frequency.
To overcome the above problems, we proposed a multilayer
parasitic microstrip antenna array (MPMAA) structure based
on a parasitic antenna configuration [11] using a low-temperature
co-fired ceramic substrate (LTCC) suited to packaging
the MMIC chip [16]. However, since the LTCC substrate has a
high electric constant, it is difficult to achieve wide-band and
high-efficiency characteristics.
In this paper, we propose a highly efficient MPMAA structure
constructed on a Teflon substrate for a system-on-package
at millimeter-wave frequency bands [17]. At 60 GHz, the new
antenna achieves high radiation efficiency of greater than 91%
and high gain that is greater than 11.1 dBi. The remainder of
this paper is as follows. In Section II, we first describe the
system-on-package module and the design of the proposed
antenna. In Section III, we present the effect of the alignment
precision due to the manufacturing process. Moreover, in Section
IV, we describe the manufactured prototype antenna and
the tests conducted at 60 GHz. We also present characteristics
of the prototype antenna and clarify the design method.
II. DESIGN OF PARASITIC ELEMENT ARRANGEMENT
CONSTRUCTED WITH TEFLON SUBSTRATE
A. Millimeter-Wave System-on-Package Image
The concept of a system-on-package module integrated with
an antenna is shown in Fig. 1. The system-on-package module
has a highly integrated transceiver MMIC mounted on a multilayer
structure and the proposed MPMAA. The multilayer structure
for the MMIC chips and the MPMAA is vertically stacked.
Two types of feeding methods for the MPMAA are considered.
In the first method, a feeding element is constructed on the

2102 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005
Fig. 1. Novel structure of system-on-package integrated with an antenna.
Fig. 2. Antenna model.
MMIC chip and the MPMAA is fed by electromagnetic coupling
without a feeding line. In the other, a feeding element is
constructed where the multilayer substrate of the MPMAA and
the element is connected to the terminal of the MMIC using
flip-chip technology. As indicated in Fig. 1, the MPMAA employs
two parasitic layers. Four parasitic elements are arranged
on the first parasitic layer, and nine parasitic elements are arranged
on the second parasitic layer.
B. High-Efficiency Antenna Design
This proposed antenna achieves high gain because it increases
the antenna aperture using the coupling between the layers of
the multilayer substrate. The MPMAA design for the 60-GHz
band is described. We use the moment method as the calculation
method and assume that the ground plane is infinite. We adopt
the multilayer Teflon substrate ( at
10 GHz) as the antenna substrate due to its superior characteristics
such as a high gain and wider bandwidth. The simulation
model is shown in Fig. 2. Here, we use the via-fed method for
the feeding method. The size of the feeding element is
and the feeding point is away from the center of
the patch. The size of the parasitic elements mounted on the
first parasitic layer is and the patch size of the
second parasitic layer is . The four parasitic
elements mounted on the first parasitic layer are arranged such
Fig. 3. Relationship between I and P that achieves the maximum absolute
gain.
Fig. 4. Absolute gain and radiation efficiency characteristics versus P.
that they are equidistant from the center of the feeding MSA.
For convenience, the width (horizontal direction) is expressed
such that the width of the first parasitic layer and that of the
second parasitic layer are and , respectively, as shown in
Fig. 2. In addition, and represent the substrate thicknesses
of the first and second parasitic layers, respectively. The calculated
maximum absolute gain and the relationship between
and that achieve the maximum value when is varied are
shown in Fig. 3. Here, and are set to 0.25 and 0.55 mm, respectively.
In this figure, it is clear that is proportional to .
The relationship of the maximum absolute gain and the radiation
efficiency versus is shown in Fig. 4. Here, the substrate
thickness parameters are the same. In this figure, it is clear that
the maximum absolute gain is 11.1 dBi and that the radiation
efficiency is 91%, when is and is . The antenna
achieves the maximum radiation efficiency of 97% when
is . Here, we believe that the peak of the absolute gain
is obtained by increasing the antenna aperture according to the
increase in the number of parasitic elements and the degradation
in the efficiency due to the decrease in the coupling between the
feed element and the parasitic elements. Additionally, it is necessary
to clarify the effect of the variation in the substrate thickness
for the parasitic elements. The dependency of the absolute

SEKI et al.: MILLIMETER-WAVE HIGH-EFFICIENCY MPMAA ON TEFLON SUBSTRATE 2103
Fig. 5. Absolute gain characteristics versus substrate thicknesses I and P.
Fig. 6. Calculated frequency characteristics of the absolute gain and ƒII.
gain on the substrate thicknesses and is shown in Fig. 5. In
this figure, when and are 0.25 and 0.55 mm, respectively,
the absolute gain is maximum at 11.1 dBi. Additionally, it is
clear that the manufacturing margins of and are approximately
150 m and 100 m, respectively, and the gain reduction
is less than 0.5 dB. Furthermore, it is clear that the precision
of the substrate thickness of the conventional multilayer Teflon
substrate process is sufficient to construct the proposed antenna.
Fig. 6 shows the frequency characteristics of the proposed antenna.
In this figure, it is clear that the bandwidth is 2.6%, when
the characteristics are less than 10 dB. To consider the
mechanism of this antenna, the results of the current distribution
calculated using the moment-method analysis are shown in
Fig. 7. Here, Fig. 7(a) shows the front view and Fig. 7(b) shows
the rear view. From Fig. 7(a) and (b), we understand that, in the
feeding element, the current is distributed to the first layer parasitic
elements, and the current is distributed in the second-layer
parasitic elements via the first-layer parasitic elements. The RF
signal can be effectively separated from the feeding element and
passed to the second-layer parasitic elements without using a
power divider circuit, and the array operation can been achieved.
III. EFFECT OF ALIGNMENT PRECISION
This antenna constructed on the multilayered Teflon substrate
that binds the substrates using the bonding film is sensitive to
the effect of the alignment precision compared to when using
Fig. 7. Current distribution of the proposed antenna. (a) Front view. (b) Rear
view.
Fig. 8. Absolute gain reduction versus alignment precision of the - axis
direction.
other substrates such as the alumina–ceramic substrate. It is
necessary to clarify the effect of the alignment error derived
from the binding substrates. Here, we use the moment method
for the calculation method. The size of the feeding element is
and the feeding point is away from the
center of the patch. The size of the parasitic elements mounted
on the first parasitic layer is and the patch
size of the second parasitic layer is . Furthermore,
the position of the first parasitic layer is
and the position of the second parasitic layer is
. The calculated results of the absolute
gain reduction are shown in Fig. 8 when alignment error occurs

2104 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005
Fig. 9. Absolute gain reduction versus alignment precision of -axis direction.
in the direction of the -axis. The calculated results of the absolute
gain reduction are shown in Fig. 9 when alignment error
occurs in the direction of the -axis. In these figures, it is clear
that the effect of the alignment error in the direction of the -axis
is greater than that in the direction of the -axis. We believe that
the reason for this is that there is a standing wave in the direction
of the -axis of each patch. Additionally, it is clear that the
alignment margin of the first layer parasitic substrate and that
of the second layer parasitic substrate are approximately 100
and 50 m, respectively, and the gain reduction is less than
0.5 dB. Furthermore, it is clear that the alignment precision of
the conventional multilayer Teflon substrate process is sufficient
to construct the proposed antenna.
IV. MEASURED PERFORMANCE
A. Measurement System
In the millimeter-wave frequency range, the measurement deviation
that occurs due to soldering error and the difference in
the connecting torque is an important issue. To eliminate the
deviation, we developed a novel antenna-probing fixture based
on an RF on-wafer probing system. As a result, the same accurate
measurement as the MMIC chip measurement becomes
possible in the radiation pattern measurement of the miniaturized
antenna in the millimeter-wave frequency band. Here, the
millimeter-wave radiation pattern measuring system is shown
in Fig. 10. The configuration of the measuring system using an
RF probe and a photograph of the system are shown in Figs. 11
and 12, respectively. These on-wafer probes can be directly connected
to an antenna terminal formed on the planar substrate.
This system can measure the antenna radiation without the need
to mount RF connectors on the antenna substrate. This method
yields higher accuracy and higher efficiency. This fixture can
employ two probe heads. One probe is used to feed the RF signal
to the antenna terminal and the other is used to feed the control
signal and dc power to the devices mounted on the antenna substrate.
B. Antenna Performance
We manufactured a prototype antenna that is constructed
using the Teflon substrate to confirm the design in the 60-GHz
Fig. 10. Antenna pattern measuring system.
Fig. 11. Configuration of measuring system using RF probe.
Fig. 12. Measuring system using RF probe.
band. We use a microstrip line and a via-hole to feed the
MSA, which demonstrates the operation of the feeding element
shown in Fig. 2. Two parasitic layers are arranged such
that the size and location of the MSA and parasitic elements,
substrate thickness, and location of the feeding points achieve

SEKI et al.: MILLIMETER-WAVE HIGH-EFFICIENCY MPMAA ON TEFLON SUBSTRATE 2105
Fig. 13. Prototype antenna.
Fig. 14. Measured radiation pattern of i-plane.
the maximum absolute gain, as described in Section IV-B.
A photograph of the prototype antenna is shown in Fig. 13.
The figure on the left-hand side shows the side view of the
antenna element and the figure on the right-hand side shows
the feeding line. The fabricated prototype antenna chip size is
10.0 mm 10.0 mm 1.1 mm. The - and -plane radiation
patterns are shown in Figs. 14 and 15, respectively. In these
figures, the measured and calculated radiation patterns are
fairly equal regarding the main lobe. Next, we describe the
bandwidth of this antenna. The frequency characteristics of
the measured maximum gain and the characteristics are
shown in Fig. 16. In this figure, the frequency bandwidth for
the gain that is 3 dB lower than the peak gain for this antenna
is from 59.3–62.6 GHz. Therefore, the bandwidth compared to
the center frequency is 5.5%. The frequency characteristics
and the gain are in good agreement with the calculated results
shown in Fig. 6. Additionally, we clarify that the estimated
absolute gain is greater than 8.5 dBi because the measured
absolute gain of this antenna is greater than 7.4 dBi including
the RF probe loss, which is approximately 1.1 dB at 60 GHz.
The dispersion between the calculated and measured gain is
approximately 3 dB. Here, three causes are considered for this
gain reduction. The first cause is the loss in the microstrip
line/coplanar line transition. The second cause originates from
a discrepancy in the bonding film thickness in the design
compared to the actual due to the manufacturing process, and
Fig. 15. Measured radiation pattern of r-plane.
Fig. 16. Frequency characteristics of the absolute gain and ƒII.
the third cause is due to the production accuracy in the antenna
manufacturing. More specifically, the gain reduction because
of the change in the bonding film thickness is 1 dB or more.
In addition, the reduction in the gain in that case is 1 dB or
more, although the alignment accuracy when the antenna is
manufactured is approximately 100 m. As a research topic
in the future, we must conduct research on the bonding film
material that has electric characteristics that are similar to the
Teflon substrate. Furthermore, it seems that improving the
alignment binding accuracy of the multilayer Teflon substrate
to approximately 50 m is necessary.
V. CONCLUSION
This paper has proposed a highly efficient MPMAA constructed
on a multilayer Teflon substrate for millimeter-wave
system-on-package modules. The design and performance of
the proposed array antenna has been described. The proposed
prototype antenna employs a multilayer Teflon substrate that
is well suited to achieving high gain and a wide frequency
bandwidth. This paper has also presented the prototype antenna
performance such as the return-loss characteristics and
the radiation pattern characteristics in the 60-GHz band. The
radiation efficiency of the proposed antenna is greater than
91%. Moreover, we demonstrated a prototype antenna for the
60-GHz band.

2106 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005
ACKNOWLEDGMENT
The authors thank Dr. M. Umehira and Dr. I. Toyoda, both of
the Nippon Telegraph and Telephone (NTT) Corporation, Kanagawa,
Japan, for their constant encouragement.
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Tomohiro Seki (M’94) was born in Tokyo, Japan, in
1967. He received the B.E. and M.E. degrees in electrical
engineering from the Tokyo University of Science,
Tokyo, Japan, in 1991, and 1993, respectively.
In 1993, he joined the Nippon Telephone and
Telegraph (NTT) Corporation, Kanagawa, Japan,
where he has been engaged in research on planar
antennas and active integrated antennas for the millimeter-wave
and microwave bands. He is currently
a Research Engineer with the Antenna Research
Group, NTT Network Innovation Laboratories, NTT
Corporation.
Mr. Seki is a member of the Institute of Electronics, Information and Communication
Engineers (IEICE), Japan. He was the recipient of the 1999 Young
Engineer Award presented by the IEICE.
Naoki Honma (M’00) was born in Sendai, Japan, in
1973. He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from Tohoku University,
Sendai, Japan, in 1996, 1998, and 2005, respectively.
In 1998, he joined the Nippon Telegraph and
Telephone (NTT) Radio Communication Systems
Laboratories, NTT Corporation, Kanagawa, Japan.
He is currently with the NTT Network Innovation
Laboratories, NTT Corporation, Kanagawa, Japan.
His current research interest is planar antennas for
high-speed wireless communication systems.
Dr. Honma is a member of the Institute of Electronics, Information and Communication
Engineers (IEICE), Japan. He was the recipient of the 2003 Young
Engineers Award presented by the IEICE and the 2003 Asia–Pacific Microwave
Conference (APMC) Best Paper Award.
Kenjiro Nishikawa (A’93–M’00) was born in Nara,
Japan, in 1965. He received the B.E. and M.E. degrees
in welding engineering and Dr. Eng. degree in
communication engineering from Osaka University,
Suita, Japan, in 1989, 1991, and 2004, respectively.
In 1991, he joined the Nippon Telegraph and
Telephone (NTT) Radio Communication Systems
Laboratories (now the NTT Network Innovation
Laboratories), NTT Corporation, Kanagawa, Japan,
where he has been engaged in research and development
on three-dimensional (3-D) and uniplanar
monolithic microwave integrated circuits (MMICs) on Si and GaAs and
their applications. He is currently interested in millimeter-wave transceivers,
system-in-package technologies, active integrated antennas, and high-speed
communication systems.
Dr. Nishikawa is a Technical Program Committee member of the IEEE Compound
Semiconductor Integrated Circuit Symposium and the IEEE Radio and
Wireless Symposium. He is a member of the Institute of Electronics, Information
and Communication Engineers (IEICE), Japan. He was the recipient of the
1996 Young Engineer Award presented by the IEICE.
Kouichi Tsunekawa (M’89) was born in Niigata,
Japan, on January 21, 1958. He received the B.S.,
M.S., and Ph.D. degrees in science engineering from
Tsukuba University, Ibaraki, Japan, in 1981, 1983,
and 1992, respectively.
In 1983, he joined the Nippon Telegraph and
Telephone (NTT) Electrical Communications Laboratories
(ECL), NTT Corporation, Tokyo, Japan.
Since 1984, he has been engaged in the research
and development of portable telephone antennas in
land mobile communication systems. From 1993
to 2003, he was with NTT DoCoMo Incorporation. He is involved with
radio propagation research, intelligent antenna systems for wireless communications,
and the development of IMT-2000 antenna systems. He is
also currently engaged in the development of IMT-2000 antenna systems.
He is currently an Executive Research Engineer with the Wireless Systems
Innovation Laboratory, NTT Network Innovation Laboratories, NTT
Corporation, Kanagawa, Japan.