A compact V-band 2-bit reflection-type MEMS phase shifter

Abstract

Air-gap overlay CPW couplers and low-loss series metal-to-metal contact microelectromechanical system (MEMS) switches have been employed to reduce the loss of reflection-type MEMS phase shifters at V-band. Phase shift is obtained by changing the lengths of the open-ended stubs using series MEMS switches. A 2-bit [135] reflection-type MEMS phase shifter showed an average insertion loss of 4 dB with return loss better than 11.7 dB from 50 to 70 GHz. The chip is very compact with a chip size as small as 1.5 mm /spl times/ 2.1 mm.

Full text

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324IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 12, NO. 9, SEPTEMBER 2002
A Compact V-Band 2-Bit Reflection-Type
MEMS Phase Shifter
Hong-Teuk Kim, Student Member, IEEE, Jae-Hyoung Park, Student Member, IEEE, Jounghwa Yim,
Yong-Kweon Kim, Member, IEEE, and Youngwoo Kwon, Member, IEEE
Abstract—Air-gap overlay CPW couplers and low-loss series
metal-to-metal contact microelectromechanical system (MEMS)
switches have been employed to reduce the loss of reflection-type
MEMS phase shifters at V-band. Phase shift is obtained by
changing the lengths of the open-ended stubs using series MEMS
switches. A 2-bit (135 ) reflection-type MEMS phase shifter
showed an average insertion loss of 4 dB with return loss better
than 11.7 dB from 50 to 70 GHz. The chip is very compact with a
chip size as small as 1.5 mm
2.1 mm.
Index Terms—MEMS switch, reflection-type phase shifter.
I. INTRODUCTION
R
the small size compared with the distributed phase shifters.
However, their millimeter-wave application has so far been
limited due to the increased losses of the couplers and switches
at high frequencies. Microelectromechanical system (MEMS)
technology brings low-loss possibility, and can thus be ef-
fectively employed to reduce the loss of the reflective phase
shifters at millimeter-wave frequencies.
The switch benefits most from the MEMS technology as can
be judged by the reported low-loss characteristics of the mi-
cromachined switches [1]–[3]. When the loss of the switch is
minimized with the help of MEMS technology, the loss of the
coupler becomes major loss contributor in the reflection-type
switches. For example, a 4-bit X-band reflection-type MEMS
phase shifter using microstrip Lange couplers and capacitive
shunt MEMS switches showed a low average insertion loss of
1.4 dB at 8 GHz, out of which 1 dB was attributed to the loss of
the cascaded Lange couplers [4]. The problem becomes more
serious for CPW couplers. A 3-dB MMIC CPW Lange coupler
with a 5 m gap between the coupled lines on a GaAs substrate
showedinsertionlosseshigherthan1dB atthecenterfrequency
of 20 GHz due to field crowding at the edges [5].
Recently, the authors developed a low-loss air-gap overlay
CPW MMIC 3-dB coupler fabricated with MEMS technology
[6]. The air-gap offset broadside coupling between two lines
offers tight coupling and reduces the conductor loss by redis-
EFLECTION-TYPE phase shifters using switches and
couplers are suitable for monolithic applications due to
Manuscript received December 7, 2001; revised March 19, 2002. This work
was supported by the Korean Ministry of Science and Technology through the
Creative Research Initiative Program. The review of this letter was arranged by
Associate Editor Dr. Arvind Sharma.
The authors are with the Center for 3-D Millimeter-Wave Integrated Sys-
tems,SchoolofElectricalEngineering,SeoulNationalUniversity,Seoul,Korea
(e-mail: htkim@snu.ac.kr; ykwon@snu.ac.kr).
Publisher Item Identifier 10.1109/LMWC.2002.803198.
Fig. 1.
MEMS phase shifter.
Schematic and photograph of 2-bit (135 at 60 GHz) reflection-type
tributing currents over broad surfaces. Similar approaches to re-
duce the conductor loss in the uniplanar transmission lines have
also been demonstrated in the authors’ previous work [7], [8].
In this work, MEMS technology is applied to reduce the loss
of the millimeter-wave reflection-type phase shifters. Metal-to-
metal contact series MEMS switches are employed for low-loss
switching ofthelinelengths,and theair-gapoverlayCPW 3-dB
couplers are used to minimize the loss of the couplers.
II. DESIGN
A 2-bit (0 –45 –90 –135 ) reflection-type phase shifter
was realized using the air-gap overlay CPW 3-dB coupler and
metal-to-metal contact series MEMS switches. Fig. 1 shows the
schematic and the photograph of the phase shifter. For phase
shift, open-ended lines consist of three line sections, each with
an electrical length near 22.5 at 60 GHz, separated by series
MEMS switches. The signal propagates through the switchable
lines and reflects from the end of the open stub. If the phases
of the reflected waves from both arms are the same, the signals
add in phase and appear at the output of the air-gap overlay
coupler. By actuating each pair of metal contact switches along
the separated lines, the electrical length of each arm can be
changed by 22.5 , resulting in subsequent
of 45 at 60 GHz. All the line lengths of the both arms are
tuned for maximum relative phase shift at the center frequency
of 60 GHz, resulting in a wideband phase shift flatness [9].
The chip size is only 1.5 mm
kind of phase shifter is well suited to V-band compact array
antenna systems.
phase shift
2.1 mm, showing that this
1531-1309/02$17.00 © 2002 IEEE

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KIM et al.: COMPACT V-BAND 2-BIT REFLECTION-TYPE MEMS PHASE SHIFTER 325
(a)
(b)
Fig. 2.
coupler.
(a) Schematic and (b) photograph of the air-gap overlay 3-dB CPW
III. KEY COMPONENTS AND FABRICATION
Overlay couplers and metal-to-metal contact series switches
are the key components for the reflection-type phase shifters.
Fig. 2 shows the simplified schematic and the photograph of
the air-gap overlay 3-dB CPW V-band coupler that is basically
scaled from the Ka-band coupler presented in [6]. The total
length of the coupler is 650
m. The offset coupling changes
side at the mid point of the coupler, allowing easy access to
balanced MMIC circuits. The structure is thus compatible with
Langecouplers.Forstrongersupportandpreventionofbending,
airbridgepostsareplacedateveryquarter-lengthpointalongthe
length of the coupler. A commercial EM simulator, IE3D, was
usedforthedesign.Theoptimizedparametervalues,suchasthe
overlap width (OW) of the coupled lines, are specified in Fig. 2.
The coupler is made up of the electroplated gold and is fab-
ricated on a 520- m-thick quartz substrate. The thickness of
the bottom metal is 3
m and that of the top metal is 2
The air gap between the conductors is 1.5
the measured insertion loss of the coupler used in the reflec-
tive phase shifter. The switchable lines were removedfrom both
arms, presenting open terminations. The input RF signal is, in
this way, reflected from the coupled and the through port of the
coupler and recombines in phase at the output port of the cou-
pler. The insertion losses are 0.38 and 0.66 dB and the return
losses are 17 and 18 dB at 40 and 60 GHz, respectively. This
demonstrates very low loss nature of the air-gap overlay CPW
coupler at V-band.
Fig.4showsthestructureandoperationofthemetal-to-metal
contact switch [3]. The process flow is summarized below.
First, 3- m-thick gold transmission lines are electroplated.
To avoid dc voltage short, a 0.3- m silicon nitride dielectric
layer is deposited on the part of CPW ground plate to be used
m.
m. Fig. 3 shows
Fig. 3.
CPW coupler with open termination.
Measured insertion loss and return loss of the air-gap overlay 3-dB
Fig. 4.Structure and photograph of the metal-to-metal contact switch.
Fig. 5.
switch.
Measured insertion loss and isolation of the metal-to-metal contact
as the driving electrode. Photoresist is used as the sacrificial
layer material. To increase the contact force in the actuation of
the switch, the sacrificial layer on the contact part is slightly
etched with anisotropic reactive ion etching, resulting in the
smaller gap from the signal line compared with that between
the driving electrode and ground plate. Next, contact metal is
electroplated with a 2- m-thick gold. A 0.5- m-thick silicon
nitride deposited using PECVD process is patterned as an
insulating layer connecting the contact part and driving plate.
The 2- m-thick nickel metal structures for driving the electrode
and the spring part are formed. Finally, the sacrificial layer is
removed with oxygen plasma process. When a threshold bias
is applied, the suspended driving electrode is pulled down so

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326IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 12, NO. 9, SEPTEMBER 2002
(a)
(b)
Fig. 6.
phase shifter. (a) Insertion loss and return loss; (b) phase shift.
Measured results of 2-bit (135
at 60 GHz) reflection-type MEMS
that the contact bar touches the signal line and the switch is
on state. Mechanical actuation occurs at 35–40 V. Measured
insertion loss and isolation of the fabricated switch are shown
in Fig. 5. Isolation in off state is 15 dB and the insertion loss
in on state is 1 dB at 60 GHz.
IV. MEASUREMENTS
On-wafer RF measurements were made using a CASCADE
probe station and an HP 8510XF network analyzer, which
covers a wide frequency range from 45 MHz to 110 GHz.
Line-reflect-reflect-match (LRRM) technique was employed
using on-wafer standards for calibration. Measured
of the fabricated 2-bit reflection-type phase shifter are
shown in Fig. 6(a). The return losses for all the switching states
are better than 11.7 dB from 50 to 70 GHz and the average
insertion loss is about 4.2 dB at 60 GHz. Fig. 6(b) illustrates
the differential phase shift as a function of frequency for all the
switching states. The measured average phase error for all the
switching states is 4.6% at 60 GHz. Details of the phase shift
errors and the losses at 60 GHz are listed as a function of the
switching states in Table I. The loss can be represented as
and
Loss (dB)
(1)
TABLE I
PHASE SHIFT ERROR AND THE LOSS OF THE 2–BIT REFLECTION
TYPE MEMS PHASE SHIFTER MEASURED AT EACH
SWITCHING STATE. THE DATA ARE MEASURED AT 60 GHz
where
loss (
to the phase unbalance from the couplers and the lines.
is the number of switches that are on-state. The large
dB) for some switching states at 50 GHz is attributed
V. CONCLUSION
The air-gap offset broadside coupling used in the overlay
CPW couplers resulted in a very low loss of 0.66 dB at 60 GHz.
MEMS series switches also showed a small insertion loss of
1 dB at 60 GHz. Combined effects of these two allowed 2-bit
(135 ) reflection-type phase shifters to show an average inser-
tionlossof4dBwithbetterthan11.7-dBreturnlossoverawide
frequencyrangefrom50to70GHz.Compactsize,lowloss,and
wide band characteristics make the phase shifters of this work
a promising candidate for multibit-controlled phase shifters for
V-band and above.
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