WE1B-3
A Single-Pole Double-Throw (SPDT) Circuit Using Deep Etching
Lateral Metal-Contact Switches
M. Tang1, W. Palei1, W. L. Goh1, A. Agarwal2, L. C. Law1 and A. Q. Liu1†
1
2
School of Electrical & Electronic Engineering, Nanyang Technological University
Nanyang Avenue, Singapore 639798
Institute of Microelectronics, 11 Science Park Road, Science Park II, Singapore 117685
(†Corresponding Author: A. Q. Liu, Email: eaqliu@ntu.edu.sg; Tel: (65) 6790-4336; Fax: (65) 6792-0415)
Abstract — In this paper, a single-pole double-throw
(SPDT) switching circuit that employs lateral metal-contact
micromachined switches fabricated on silicon-on-insulator
(SOI) wafer is demonstrated to operate from DC to 6 GHz.
The size of the fabricated SPDT switch is about 1.2 mm × 1.5
mm. The lateral metal-contact micromachined switches are
formed on the quasi-finite ground coplanar waveguide
(FGCPW) transmission lines and actuated by electrostatic
force. The fabricated single-pole single-throw (SPST) lateral
micromachined switch has an insertion loss of 0.2 dB and a
return loss of 24 dB at 15 GHz. The isolation is 23 dB at 15
GHz. As for the fabricated SPDT switch, the measured
insertion loss is below 0.75 dB and the return loss is higher
than 19 dB at 5 GHz. The isolation at 5 GHz is above 33 dB.
The threshold voltage of these switches is 22.5 volts, and
these SOI switches are fabricated using deep reactive ion
etching (DRIE) and shadow mask technology.
Index Terms — SPDT switch, CPW, RF MEMS, Lateral
switch.
band, K-band and Ka-band applications [1-3]. In the
second design, the direct contact series switches were
used in the Ku-band, Q-band and V-band applications [47]. Lastly, toggle switches were employed to work in the
Ku band [8-9]. Very few MEMS SPDT switches have
been developed for applications that are below 6 GHz.
In this paper, a DC to 6 GHz SPDT switch is
presented. It makes use of lateral metal-contact switches
that are implemented in a quasi-FGCPW transmission line
and actuated by electrostatic force. The mechanical
structures consist of single-crystal silicon that are wrapped
around by aluminum (Al) and fabricated using the SOI
DRIE and the shadow mask technology. The experimental
results show the insertion loss of our proposed SPDT
switch is below 1 dB and the isolation obtained is above
31 dB at 0.45 GHz to 6 GHz.
I. INTRODUCTION
II. DESIGN OF SPDT AND SPST MEMS SWITCHES
The single-pole double-throw (SPDT) switches have
been extensively employed in the microwave and
millimeter wave communication systems, such as the
signal routing in transmit and receive applications, the
switched-line phase shifters in phased array antennas and
the wide-band tuning networks. Traditionally, GaAs
MESFETs and PIN diodes are integrated in the SPDT
switching circuits, which show good performances at low
frequencies, but deteriorate at high frequency range over
the gigahertz scale. As an alternative, the recently
developed radio frequency (RF) microelectromechanical
system (MEMS) switches are receiving more and more
attentions due to their low insertion loss, high isolation,
negligible power consumptions and good linearity. Until
now, only three different SPDT switches were
implemented using MEMS technology for replacing the
conventional solid state semiconductor switches. The first
design involved shunt capacitive switches and made use
of quarter wavelength transmission line sections for the X-
In Fig. 1, the MEMS SPDT switch is illustrated. The
circuit consists of a T-junction with a MEMS lateral
switch located at each of the output arms. The signal can
therefore be routed to the two different output ports with
one switched off and the other switched on. Both the
lateral switches are equipped with fixed connections at the
two different output ports and come into contact with the
contact bar at the T-junction upon turning on the switch
during the application of biasing voltage between the
signal line and the ground line from the output port. The
size of the SPDT switch is about 1.2 mm × 1.5 mm.
Either of the two MEMS SPST lateral switches that
are placed at the output branches of the SPDT switch
consists of a quasi-FGCPW transmission line and an
electrostatic actuator. The transmission line is formed
from three parallel waveguides, which are realized by
forming each waveguide on a 35 µm thick single-crystalsilicon plate that has been coated with a thin layer of
evaporated Al. Therefore the RF signal can propagate not
only along the metal on the top surface, but also along on
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0-7803-8331-1/04/$20.00 © 2004 IEEE
2004 IEEE MTT-S Digest
beam is attracted toward the fixed electrode by the
electrostatic force until its free-end touches the contact
bar, resulting in the on-state of the switch. Once the
biasing voltage is removed, the mechanical stresses in the
beam will overcome the stiction forces to pull the beam
away, hence switching the device off.
Assuming that the electrostatic force is applied at the
midpoint of the electrode part of the cantilever beam, the
equivalent stiffness, k, of the cantilever beam can be
derived using the following expression:
12 EI1 I 2
k=
(1)
5 3
3
2
2
( 4 L1 + 9 L2 L1 + 6 L1L2 ) I 2 + L2 I1
4
1 3
1 3
(2)
where
I1 =
w1 t
I2 =
w2 t
12
12
where w1 and L1 are the width and length of the narrow
part of the cantilever beam, w2 and L2 are the width and
length of the electrode part of the beam, t is the thickness
of the beam and E is the Young’s modulus.
The threshold voltage is given by
the sidewall of the transmission lines. To avoid short
circuit between the signal line and the ground lines after
metal evaporation, the handle Si under the gap between
the signal line and the ground lines are etched through
from the backside of the wafer using DRIE, as seen in Fig.
3(d). With the help of commercial 3D FEM simulation
software – Ansoft’s high-frequency structure simulator
(HFSS), a 50-Ω transmission line can be obtained by
simply adjusting the width of the CPW signal line, S; the
width of the gap between the signal line and the ground
line, W; and the width of the ground line, G. In this
switch, parameters S, W and G at three ports were
designed to be 132 µm, 34 µm and 300 µm respectively to
accommodate the 150 µm-pitch ground-signal-ground
coplanar probes. A cantilever beam in the direction of the
signal line acts as the movable electrode of the actuator, as
well as forms part of the signal line. The ground lines
beside the cantilever beam are extended toward the
cantilever beam and the width of the gaps between the
cantilever beam and the ground lines are 33 µm wide. At
the free-end of the cantilever beam, one ground line
protuberates toward the cantilever beam further to serve as
a fixed electrode.
Vth =
Port 3
(Output2)
Lateral
switch
Through-wafer
trenches
Lateral
switch
III. FABRICATION PROCESS
All the components of the switches were fabricated
on a SOI wafer, which includes a 35 µm low resistivity
(LR) device active silicon (Si) layer (<0.1Ω-cm), 2 µm
buried thermal silicon dioxide (SiO2) layer and 500 µm
high resistivity (HR) handle silicon layer (>4000Ω-cm).
The process flow at the cross sections of A-A’ and B-B’ is
summarized in Fig. 3.
The process began with a SiO2 of 2.0 µm deposition
on a SOI substrate using PECVD. Upon patterning of
SiO2 using RIE, the HR Si was etched through via DRIE
from the backside using 10 µm photoresist as mask
material, as seen in Fig. 3(a). After that, another DRIE
step was employed to etch the LR Si to buried SiO2 layer
using the top SiO2 as the hard mask. The exposed SiO2
was removed by Buffered Oxide Etchant (BOE) (Fig.
3(b)). Following that, the SOI wafer was temporarily
bonded to a shadow mask [10] using photoresist as
intermediate material. A 1.5 µm thick Al film was
deposited on both the surface and the sidewalls of the
switches through the shadow mask via evaporation (Fig.
3(c)). Finally, the shadow mask is de-bonded and
separated from the SOI substrate (Fig. 3(d)). Due to nature
of the evaporation process, the Al coated at the sidewall is
B′
A
G
S
G
A′
Port1 (Input)
Fig. 1 SEM micrograph of the MEMS SPDT switch.
L1
L2
Cantilever beam
w1
Anchor
V
(3)
where g is the gap distance between the two electrodes.
In our design, L1 = 275 µm, L2 = 162 µm, w1 = 2.5
µm, w2 = 5 µm, g = 6 µm, d = 4 µm and t = 35 µm,
therefore Vth is 23.6 V.
B
Port 2
(Output1)
8kg 3
27ε 0 L2 t
w2
g
d
Fixed electrode
Contact bar
Fig. 2 Illustrated top view of the electrostatic actuator.
Fig. 2 shows the illustrated top view of the
electrostatic actuator that is indicated using a dashed circle
in Fig. 1. When a biasing voltage is applied between the
cantilever beam and the fixed electrode, the cantilever
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IV. RESULTS AND DISCUSSION
thinner than that coated at the surface. Fig. 4 is a cross
section SEM Micrograph illustrating step coverage of a 10
µm trench. The thickness of the Al layer at the surface is
about 1.5 µm, while that at the sidewalls is about 6250 Ǻ.
B-B’
SOI
The DC and RF characteristics of both the SPST and
SPDT switches were measured. Fig. 5 presents the
displacement result of the cantilever beam as the applied
voltage increases. Due to the metal of about 6000 Ǻ
coated at the sidewalls of the cantilever beam and the
contact bar, the original distance between the cantilever
beam and the contact bar, d, had reduced from 4 µm to
about 2.8 µm. The distance decreased with the increase of
the applied voltage. When the biasing voltage increased to
about 22.5 V, the cantilever beam was attracted to touch
the contact bar rapidly from 1.4 µm away. Therefore, the
threshold voltage of the switch is about 22.5 V.
A-A’
LR Si
SiO2
HR Si
(a) SiO2 deposition and patterning on the surface of LR Si
and HR Si was etched from the backside of the SOI
substrate by DRIE.
3.0
2.5
2.0
Distance (µm)
(b) LR Si trench-etched by DRIE. SiO2 was etched using
BOE.
Shadow mask
1.5
1.0
0.5
0.0
0
(c) Al coating through a shadow mask.
5
10
15
20
25
30
Applied voltage (Volts)
Fig. 5 Relation between distance and applied voltages of
the cantilever beam used in the SPST and SPDT switches.
The RF response of the system was measured using
the HP 8510C Vector Network Analyzer with tungsten-tip
150 µm-pitch Cascade Microtech ground-signal-ground
coplanar probes. A short-open-load-through (SOLT)
calibration technique was employed.
The SPST lateral MEMS switch is the key component
of the proposed SPDT switch. The measured S-parameters
of the fabricated SPST lateral MEMS switch are given in
Fig. 6. The isolation of the switch at the off-state is 32 dB
at 5 GHz and 23 dB at 15 GHz respectively. The insertion
loss and the return loss of the switch at the on-state are
0.07 dB and 32 dB at 5 GHz, and 0.2 dB and 24 dB at 15
GHz respectively. The applied bias voltage is 30 V. Fig. 7
provides the S-Parameters of the fabricated SPDT switch.
The insertion loss and the return loss of the SPDT switch
are determined by S21 and S11 respectively through the
input and output branch that contains the actuated switch
while the switch in the other output branch is in the offstate. The isolation of the SPDT switch is characterized by
S21 along the signal line with the switch in the off-state,
while the input signal is routed to the branch containing
the actuated switch. As being seen in Fig. 7, the insertion
(d) Shadow mask de-bonded
Si
SiO2
Al
Fig. 3 Fabrication process flow of trenched CPW lines.
1.5 µm
6250 Ǻ
Fig. 4 Cross section SEM micrograph illustrating the step
coverage of a 10 µm trench in width.
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respectively. The isolation of the SPST switch is 23 dB at
15 GHz. The fabricated SPDT switch has an insertion loss
and a return loss that are below 0.75 dB and 19 dB
respectively at 5 GHz. The isolation of the SPDT switch is
33 dB at 5 GHz. The size of the fabricated SPDT switch is
about 1.2 mm × 1.5 mm. These results indicate that the
proposed SPDT switch is of small size and offers low loss
by using the lateral metal-contact MEMS switch, and can
be well applied in DC to 6 GHz applications.
S-Parameters (dB)
loss of the SPDT switch is below 0.75 dB, the return loss
is over 19 dB, and the isolation is higher than 33 dB from
DC to 5 GHz. The voltage applied is 30 V.
It should be noted that the loss due to individual
MEMS SPST switch is negligible up to 15 GHz. The
dominant force of the SPDT switch is the loss in the CPW
transmission lines since no air bridges were fabricated at
the T-junction to equalize the CPW ground-plane
potential. Therefore, power can be converted from the
desired CPW mode to the parasitic coupled slotline mode.
Modifying the fabrication process can help to solve this
problem.
0.0
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-0.4
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-0.6
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-0.8
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-1.0
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-1.2
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-1.4
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-1.8
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Insertion loss
Return loss
Isolation
-2.0
-2.2
-50
-55
-2.4
-60
0
5
10
15
20
25
Frequency (GHz)
Fig. 6 Measured S-parameters of a SPST switch.
0.00
0
-0.25
-5
-0.50
-10
S-Parameters (dB)
-0.75
-15
Insertion loss
-1.00
-20
-1.25
-25
Return loss
-1.50
-30
-1.75
-35
-2.00
-40
Isolation
-2.25
-45
-2.50
-50
-2.75
-55
-3.00
-60
0
1
2
3
4
5
6
Frequency (GHz)
Fig. 7 Measured S-parameters of a SPDT switch.
V. CONCLUSION
In this paper, a DC to 6 GHz SPDT switch using
lateral metal-contact MEMS switches was designed,
fabricated and measured. The measured threshold voltage
is 22.5 V. The insertion loss and the return loss of the
MEMS SPST switch at 15 GHz are 0.2 dB and 24 dB
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