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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 5, OCTOBER 2009
A Liquid–Solid Direct Contact Low-Loss
RF Micro Switch
Prosenjit Sen and Chang-Jin Kim
Abstract—This paper reports the design, fabrication, and testing of a liquid-metal (LM) droplet-based radio-frequency microelectromechanical systems (RF MEMS) shunt switch with
dc-40 GHz performance. The switch demonstrates better than
0.3 dB insertion loss and 20 dB isolation up to 40 GHz, achieving
significant improvements over previous LM-based RF MEMS
switches. The improvement is attributed to use of electrowetting
on dielectric (EWOD) as a new actuation mechanism, which allows
design optimized for RF switching. A two-droplet design is devised to solve the biasing problem of the actuation electrode that
would otherwise limit the performance of a single-droplet design.
The switch design uses a microframe structure to accurately position the liquid–solid contact line while also absorbing variations
in deposited LM volumes. By sliding the liquid–solid contact
line electrostatically through EWOD, the switch demonstrates
bounceless switching, low switch-on time (60 μs), and low power
consumption (10 nJ per cycle).
[2008-0266]
Index Terms—Electrowetting on dielectric (EWOD), liquidmetal droplet, microswitch, radio-frequency microelectromechanical systems (RF MEMS).
I. I NTRODUCTION
T
HE CONTACT reliability of solid–solid contacts becomes more important for microelectromechanical systems (MEMS) switches as surface plays an increased role at
microscale. The contact degradation [1] due to arcing, welding,
and material transfer, which is aggravated by contact bounce,
limits the operational life of all electric switches and in a greater
degree for MEMS switches [2]. In order to solve the contact
problems and enhance reliability, a liquid was placed at the
contact point of a surface-micromachined switch as early as
1996 [3]. However, the high surface tension of liquid-metal
(LM) becomes dominant in the reduced scale, making the
development of microscale replicas of macroscale reed relays
difficult. Surface-micromachined switches based on low actuation force mechanisms like electrostatic found their operational
speed slowed down by the high surface forces between the
liquid and the solid [4]. Better performance was possible for
larger microswitches and through use of large force actuation
mechanisms like electrothermal, which require more energy per
actuation cycle [5].
Manuscript received October 29, 2008; revised July 9, 2009. First published September 11, 2009; current version published September 30, 2009.
This work was supported by the DARPA HERMIT program. Subject Editor
K. F. Bohringer.
The authors are with Mechanical and Aerospace Engineering Department,
University of California (UCLA), Los Angeles, CA 90095 USA (e-mail: senp@
ucla.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2009.2029170
A more elegant approach in the development of an LMbased microswitch is through actuation of the LM droplet to
achieve switching with no movable structures. LM droplets
have been actuated by electrothermal [6], [7], electrostatic
[8], [9], and electrowetting-on-dielectric (EWOD) [10]–[12]
methods to achieve switching. Collectively, LM microswitches
have demonstrated no contact bounce [6], [7], [12], low switchon time (60 μs) [12], fast signal rise/fall time (5 μs) [12],
low contact resistance [5], long life [7], and the capability
to handle large currents (1 A) [7]. A more comprehensive
review of LM microswitches is presented in [13]. Although
some of the aforementioned devices have been tested for radiofrequency (RF) performance, they have mostly been limited
to 20 GHz. Most of these switches also suffered from slow
actuation speeds, leading to slow switching with latencies on
the order of 1 ms.
The first known implementation of an LM-based microswitch handling RF signals reported an RF performance of
40 dB isolation and 0.1 dB insertion loss up to 2 GHz [6]. In
this implementation, an LM droplet in a microchannel filled
with deionized water was toggled by the fluidic pulse generated
by thermal bubble expansion. However, the presence of water
in the OFF-state would have severely degraded the isolation
at higher frequencies. The switch-on latency was ∼10 ms,
and required 100 mW of power. Another thermally actuated
switch used thermal expansion of air to break and move LM
droplets [7]. Use of air as a working fluid improved the RF
performance significantly. Better than 1 dB insertion loss and
20 dB isolation were reported up to 18 GHz. In addition,
reported were ∼0.92 ms switching latency and 10 μJ energy
required per cycle. Recently, an electrostatically actuated LM
capacitive switch was reported with a 0.6 dB insertion loss of
up to 20 GHz [14]. The isolation, however, degraded to 10 dB
at 15 GHz and was attributed to the slotline mode arising due
to the asymmetrical switch design. The switch-on time was
reported to be ∼1 ms. To avoid the toxicity, nonmercury liquids
have been explored, although, so far, with little success. An
LM alloy Galinstan was immersed in a Teflon solution for a
reflective switch [15], and water was used for a reflective and
absorptive switch [16]. However, the LM alloy or water was
manually pumped to achieve switching, thus restricting its use
as a microswitch. Nevertheless, better than a 1.3 dB insertion
loss and a 20 dB isolation were demonstrated up to 100 GHz.
Pumping of fluid also limited the expected switching time to
10 ms [16].
In this paper, we design, fabricate, and evaluate an LM-based
low-loss RF switch for dc-40 GHz operation range. First, the
feasibility of LM to function in a microswitch at RF frequencies
1057-7157/$26.00 © 2009 IEEE
SEN AND KIM: LIQUID–SOLID DIRECT CONTACT LOW-LOSS RF MICROSWITCH
Fig. 1. Schematic of the experiment to study LM (mercury) behavior at high
frequency. Manually placed LM droplet acts as a capacitive RF short. Contact
angle and droplet curvature lead to a liquid–solid contact area smaller than the
SU-8 frame dimension. The switch width is 600 μm.
is assessed. Then, the LM actuation mechanism adopted is
described. After illustrating the problems associated with a
single-droplet switch design, a two-droplet design is proposed.
After discussing the device fabrication, we report experimental
and simulation results.
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Fig. 2. Measured insertion loss (without LM) and isolation (with LM) from
test devices shown in Fig. 1. Device capacitance is calculated to be 25 pF from
curve fitting. Series resistance is 0.225 Ω. A good low-frequency fit is obtained
even when inductance is ignored.
the liquid–solid contact line in Fig. 1 than the inner area of the
microframe.
III. EWOD-BASED D ROPLET ACTUATION FOR S WITCHING
II. T EST OF LM D ROPLET FOR H IGH -F REQUENCY RF
For an LM droplet to perform as the RF shunt-switching
element, it should be able to provide sufficient isolation when
shorting the signal plane to the ground plane. Despite the new
possibilities (e.g., Galinstan), mercury is the only viable option
for an actuated submillimeter LM droplet at room temperatures.
Even though mercury is not among the best conductors, with its
resistivity ∼40 times that of gold, its conductivity is not considered a main issue. For example, a 100 μm wide droplet should
provide ∼40 dB isolation at 40 GHz. Since most of the previous
LM-based RF switch work has been limited to 20 GHz, we
conducted an experimental study to reveal any unexpected behavior and assess the suitability of mercury as an RF switching
element up to 40 GHz. As schematically shown in Fig. 1, the
test device consisted of a 60–460–60 μm coplanar waveguide
(CPW), fabricated using 8000 Å gold liftoff. A thin dielectric
layer (1500 Å silicon nitride) over the patterned electrodes
protected the gold CPW from mercury. A 100 μm tall SU-8
microframe defined the switch width (600 μm) and held the
LM droplet in position, while the experiment was conducted.
An HP 8510C network analyzer was used to measure the
scattering parameters. The device insertion loss was measured
without the LM droplet. An LM droplet was placed manually,
and the device isolation was measured, as shown in Fig. 2.
Isolation of better than 25 dB over the range of 5–40 GHz
proves the suitability of LM at high frequencies. Since the
isolation profile solely depends on switch capacitance at low
frequencies [17], a good low-frequency fit was obtained even
when switch inductance was ignored. The switch capacitance
and resistance extracted from the curve fitting were 25 pF and
0.225 Ω, respectively, while the switch capacitance calculated
using the microframe dimensions was 75 pF. The fitted capacitance was smaller due to the convex curvature of the nonwetting
droplet, leading to a smaller actual contact area as outlined by
Microswitches based on LM actuation have suffered mostly
due to their slow actuation speeds [13]. As the first bottleneck,
switching speed has been limited by the accuracy with which
the droplets have been deposited and positioned. We have
previously reported that the EWOD actuation of the contact line
of an LM droplet confined by a microframe, as shown in Fig. 3,
allowed a high-speed operation and demonstrated the mechanism for dc applications [12]. While a microframe structure, in
this case made of SU-8, held the droplet in position with a lithographic accuracy, high surface tension of the LM droplet ensured that the interface position was defined accurately within
the microframe. Furthermore, any variation in the deposited
LM volume was absorbed by the meniscus at the large back
opening rather than by the meniscus at the small front opening
(see Fig. 3). Any variation in the LM droplet volume due to
variation in ambient temperature will also be absorbed by the
back opening keeping the front meniscus practically unaffected.
By keeping the droplet meniscus at the important front opening
unaffected by the volume variation of the droplet, the droplet
contact line was positioned always accurately from the signal
electrode. This means high droplet placement accuracy is not
required and manual placement is sufficient. These features
allowed a switch design with very small switching gaps (e.g.,
10 μm for 600 μm diameter droplet), resulting in a fast
switching. Device failure by escape of LM droplet from
the microframe due to vibrational and horizontal shock has
been evaluated in [12]. For similar device design, ∼16 G
stability was reported. Vibrational stability will be improved
once device fabrication is advanced to further miniaturize the
devices.
The grounding electrode grounded the LM droplet. When a
potential (100 V) was applied to the actuation electrode, the
liquid–solid contact area spread and made contact with the signal electrode. The area of the signal electrode which the LM
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 5, OCTOBER 2009
Fig. 3. Schematic of an EWOD-actuated confined LM droplet switch. (Left and center) The switch is in OFF-state showing the liquid–solid contact line accurately
positioned (∼10 μm) away from the signal electrode. (Right) Application of an electrical potential to the actuation electrode causes the liquid–solid interface to
spread by EWOD, and the LM makes contact with the signal electrode (switch-on). Exemplary switch dimensions: back opening 400 μm, front opening 250 μm,
droplet height (i.e., gap between substrate and top plate) 400 μm.
Fig. 4. Schematic of the two-droplet LM RF switch. Switch design consists of two mirror-imaged microframe structures. Note that the LM droplets sit on the
ground planes instead of the signal electrode, eliminating signal leak path.
covers and makes contact is hereafter known as the contact
region. Using the initial regime of a fast (0.5 m/s) contact-line
motion of EWOD, 60 μs of switch-on time was demonstrated.
Signal rise and fall time was better than 5 μs with no contact
bounce, and the switch used 10 nJ of energy per cycle. It was
also demonstrated that dielectric charging had little ill effect
on the actuation mechanism for 105 switching cycles. We will
use the same actuation mechanism to design and demonstrate a
low-loss RF microswitch.
IV. T WO -D ROPLET RF S WITCH D ESIGN
A natural extension of the EWOD-based dc switch design
demonstrated in [12] for an RF switch would be to place a
single droplet on the signal line of a CPW. When actuated, the
droplet would spread and make contact with the ground plane,
shunting the RF signal. However, the required overlap between
the actuation electrode and the droplet contact area for EWOD
mechanism to work, as shown in Fig. 3, will create a signal leak
path at high frequencies as described in detail in [18]. Using
high-resistivity SiCr with surface resistivity of 1200 Ω/ has
been demonstrated as a solution for this kind of problems [19].
In order to simplify fabrication, having more freedom in selecting the cap material and achieve lower insertion loss, a twodroplet design was developed, as schematically shown in Fig. 4.
Based on 20–180–20 μm CPW on fused silica substrates, the
design uses two mirror-imaged microframes, each enclosing
an LM droplet placed on the ground planes of the CPW. The
droplets are grounded through a window etched in the dielectric
layer covering the CPW, as shown in Fig. 4. Placing the droplets
on the ground plane, as opposed to the case of one droplet on
the signal line, solves the problem of a signal leak through the
bias lines, because there is no LM above the signal electrode
when the droplet is not actuated. In the mean time, the actuation
electrode, which is capacitively coupled to the LM droplet, is
also grounded at RF frequencies. When the droplet is actuated,
the interface spreads over to the signal electrode to make
contact, and the signal is shunt to the ground plane via the
LM across the two direct contacts, i.e., signal-to-LM and LMto-ground. Grounding the LM droplets also provides a greater
freedom in selecting the package cap material and allows use of
grounded cap.
SEN AND KIM: LIQUID–SOLID DIRECT CONTACT LOW-LOSS RF MICROSWITCH
Insertion loss of a switch is due to a variation of the switch
impedance from the characteristic impedance. The presence of
actuation electrodes causes a change in the gap between the
signal line and the ground planes. At first sight, it may seem that
this discontinuity will lead to a large insertion loss. However,
the actuation electrode, which is capacitively coupled to the
ground plane through the LM droplet, acts as an extension
of the ground plane at RF frequencies. To reduce losses, the
signal line of the CPW is tapered in accord with the actuation
electrode to maintain 50 Ω impedance. This tapering minimizes
the impedance-mismatched section, as shown in Fig. 4, to
the pointed extension of signal line, where the LM droplet
makes contact with the signal line when actuated. Thus, the
insertion loss in this design is from the capacitance due to the
impedance-mismatched contact region and the LM droplet. To
avoid formation of dielectric bridges and hence minimize their
effect on insertion loss, microframe structures were positioned
away from the gaps of the CPW.
The design of the microframe and its positioning with respect
to the contact region was described in [12]. In the current device
(Fig. 4), the microframe is designed to be 400 μm high with the
opening at the back and front of 400 and 250 μm, respectively.
The actuation electrode length “w” is 250 μm. At the ends
of the actuation electrode (location B, as shown in Fig. 4),
the gap between the actuation electrode and the signal line is
20 μm, forming a 20–180–20 μm CPW. Prior to the contact
region (location C, as shown in Fig. 4), the gap between the
actuation electrode and the signal line is 17 μm, and the signal
linewidth is tapered to 146 μm to obtain a 50 Ω 17–146–17 μm
CPW. The contact region is 100 μm long. However, the gap
between the actuation electrode and the signal line is 5 μm.
Using Maxwell SV from Ansoft Corporation, the capacitance
due to each contact region is approximately calculated as 6.2 fF.
The 3-D shape of the droplet interface makes the calculation of
the capacitance between the signal line and the droplet difficult.
The minimum gap between the signal line and the LM interface
is 15 μm. Although the surface area of the 3-D LM droplet
is larger than the contact region, most of the LM surface is
significantly farther away from the signal line. Thus, a smaller
contribution to the overall switch capacitance is expected from
the LM droplets.
V. D EVICE FABRICATION
Device fabrication, as shown in Fig. 5, starts with a 700 μmthick fused silica substrate. One-thousand-ångström chromium
is evaporated on the substrate and patterned lithographically
using a wet etchant. Eight-thousand-ångström silicon oxide,
which isolates the bias lines (same as actuation electrode) from
the CPW, is deposited using plasma-enhanced chemical vapor
deposition (PECVD) and etched using reactive ion etching
(RIE). CPW is formed by liftoff of 8000 Å-thick gold using
200 Å Cr as adhesion layer. LOR-20B from MicroChem is used
with AZ5214 from Clariant to obtain a clean liftoff of such a
thick metal. For the dielectric separating the actuation electrode
from the LM, 3500 Å nitride is deposited using PECVD and
etched by RIE. Since most LMs including mercury attack most
metals, a protective layer is required at the contact regions.
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Fig. 5. Process flow for device fabrication. Section AA from Fig. 4 is
depicted.
Several metals (e.g., chromium, iron, platinum, nickel, and
tungsten) are known to be compatible with mercury. A layer of
2000 Å Cr/Ni is deposited at the contact regions using liftoff.
To reduce hysteresis (i.e., static friction which restricts contactline motion) and have a reasonable actuation voltage, a thin
hydrophobic coating of Teflon is used. Teflon is spin coated
to obtain a 2000 Å film and baked at 320 ◦ C for 3 h. Further
processing of Teflon-coated wafer is difficult due to the low
surface energy of Teflon, which results in poor adhesion of
any film coated on it. To successfully coat photoresist (PR),
we add surfactant to the PR. The Teflon layer is patterned
lithographically and etched in oxygen plasma. After the PR is
removed in acetone, the patterned Teflon layer is baked again
at 320 ◦ C for 3 h. To allow the building of the microframe
in subsequent steps, the Teflon-covered area is minimized. To
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 5, OCTOBER 2009
Fig. 6. (Top) Optical micrograph showing hydrophobic layer was processes (i.e., coated and patterned) before microframe fabrication. CPW is 20–180–20 μm,
(green) actuation electrode is 260 μm long, and signal electrode is 100 μm long and 50 μm wide. (Bottom) SEM of a fabricated device before LM droplet
placement.
Fig. 7. Measured profile of a current generation device with very small difference of height between the actuation electrode and the contact region.
obtain 400 μm-thick microframe structures, SU-8 2150 from
MicroChem is deposited by a single spin and soft baked at
95 ◦ C for 3 h. Temperature is always ramped up or down with
a rate of 60 ◦ C/hr from 50 ◦ C. A 300 s step exposure with
each step consisting of a 30 s exposure and a 20 s delay is used
to help reduce surface hardening due to heat. After a 45 min
postexposure bake, the features are developed with agitation.
The Teflon layer is again baked in a nitrogen environment, but
this time at 200 ◦ C for 3 h. A lower temperature and nitrogen environment are used to prevent SU-8 burning. Finally, a
∼ 400 μm-diameter mercury droplet is placed in each microframe manually. SEM micrographs of a fabricated device before LM placement are shown in Fig. 6.
This process deviates from the usual processing practice of
coating and patterning a Teflon layer as the last step. Such
a conventional practice protects the Teflon from any further
chemical processing, which may degrade its quality. Following
this convention, however, is not possible for our case due
to the presence of the tall SU-8 microstructures. If Teflon is
coated over tall microstructures, the surfactant-mixed PR film
is destabilized during spin coating, leading to the dewetting of
the PR on the Teflon. Furthermore, the capillary force causes
PR to accumulate in the small spaces between the tall SU-8
microstructures. Teflon is baked after every step to recover the
film quality from any degradation during processing. Fig. 6
shows the final two steps, where the Teflon is processed first
and then the SU-8 microstructures are fabricated.
Another aspect of designing the process flow is to minimize
the surface topography; the moving contact line needs to slide
across. When an electric potential is applied to the actuation
electrode, the contact line spreads over it (see Fig. 3). The
section of contact line over the signal electrode, however, does
not see the electric field from the actuation electrode and
hence feels no actuation force. Instead, the contact line over
SEN AND KIM: LIQUID–SOLID DIRECT CONTACT LOW-LOSS RF MICROSWITCH
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Fig. 8. Optical micrograph of the experimental setup. (Left) Bias tees connection to the GSG probe tips and the dc actuation probe tip. (Right) Device under test
with two LM (mercury) droplets.
the actuation electrode pulls this forceless section toward the
signal electrode. With the local loss of actuation force over the
signal electrode, the contact line is prone to pinning by any
small surface bump as explained in detail in [18]. This problem
was identified when the previous generation devices gave a
capacitive shunt response instead of the expected dc shunt. The
current process, shown in Fig. 5, solves the problem of contactline pinning by lowering the contact region to the same level as
the actuation region, as shown in Fig. 7.
VI. S IMULATION AND T EST R ESULTS
Device simulations were carried out using high frequency
structure simulator (HFSS) from Ansoft Corporation. Resistivity of mercury was defined as 961 nΩ · m. SU-8 was defined
as a dielectric with dielectric constant of 3.25. The shape of
the LM droplet remains spherical and is easily defined when
not actuated. Determining the interface shape when actuated is,
however, not trivial. To solve this issue, we assume a simplified
polygonal shape approximately matching the actuation electrode shape for the droplet interface. Although an approximate
interface shape is used, important parameters (e.g., the overlap
area at the contact region and width of the bridge between
the signal line and the ground plane) will vary insignificantly
from the true situation. An experimental setup to test the RF
switching performance is shown in Fig. 8. HP 8510C or Agilent
E8361A network analyzer is used to measure the device performance. A dc signal from a National Instrument multifunctional
data acquisition amplified using a Trek amplifier is used to
actuate the switch. Ground-signal-ground (GSG) probe tips
from Picoprobe are used to contact the CPW. To calibrate the
setup on a wafer, thru-reflect-line calibration was performed.
The measured insertion loss is better than 0.3 dB up to
40 GHz, as shown in Fig. 9 along with the simulation results,
showing a good match. The return loss is given by
|S11 | =
ωCu Z0
2
(1)
where Cu is the switch capacitance. The switch capacitance
calculated by curve fitting the return loss is 14 fF, while switch
capacitance calculated from HFSS simulation is 18 fF. This
difference is due to uncertainty in the contact-line position.
Contact-angle hysteresis leads to an uncertainty in the static
contact angle, resulting in a variation of the contact line position. A 5◦ variation in the contact angle will lead to a variation
of ∼ 6 μm in the contact-line position. This variation of the LM
Fig. 9. Device insertion loss and return loss.
Fig. 10. Device isolation.
interface position will lead to a variation in the contribution of
the LM to the switch capacitance. With approximately 12.4 fF
contribution from the contact regions (calculated above), the
LM contribution is approximately 1.6 fF measured or 5.6 fF
simulated.
The switch is actuated using 100 V, and isolation is measured, as shown in Fig. 10. The switch isolation is given by
√
R2 + ω 2 L2
(2)
|S21 | = (R + 0.5Z0 )2 + ω 2 L2
where R and L are the switch resistance and inductance,
respectively. The isolation measured is better than 20 dB up to
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 5, OCTOBER 2009
40 GHz. The 5.2 pH switch inductance is obtained from the
curve fitting the simulation data. It is not possible to obtain
a good match for the resistance from the simulation, as the
software does not account for the contact resistance, which is
the major source of the switch resistance. The switch resistance
is obtained as 1.32 Ω by fitting to the measured isolation. The
extracted resistance of two contacts in parallel is in good accord
with the previously reported 2.35 Ω for a single 50 μm × 50 μm
contact [12].
VII. S UMMARY AND C ONCLUSION
In this paper, a low-loss RF MEMS switch based on
liquid–solid direct contact using two LM droplets has been
presented. With ∼ 600 μm in size (not including the CPW extension required for testing), this switch is comparable to other
beam-based RF MEMS switches, which are usually hundreds
of micrometers in length [2]. The basic design was based on
the previously reported fast EWOD actuation of an LM droplet
in the microframe, which led to bounceless operation with
60 μs switch-on time, less than 5 μs signal rise/fall time, and
nanojoules of energy consumption per actuation cycle. After
identifying a problem of signal leakage at RF frequencies, a
two-droplet design has been developed to solve the biasing
problem that otherwise would degrade the RF performance and
require deposition of a high-resistivity SiCr layer. Switch design was optimized with the aim of improving RF performance.
Device fabrication required special care to prevent contact-line
pinning, which led to poor liquid–solid contact. The insertion
loss was measured to be better than 0.3 dB and isolation better
than 20 dB both up to 40 GHz. The fitted switch characteristics
included a 14 fF up-state capacitance and a 5.2 pH down-state
inductance. The switch contact resistance extracted from curve
fitting was 1.32 Ω, similar to the value reported previously for
a dc switch.
With a liquid–solid contact, the reported switching technology is expected to lead to high-reliability RF MEMS switches
including contact switches. It is further expected that the use
of LM droplets will allow the development of high-power hot
switching devices. There are, however, several key issues that
need to be solved before the technology matures to deploy
the devices. With contact reliability not a primary issue for
this switch, the major expected failure mechanism is through
dielectric charging. Even though dielectric charging has been
demonstrated to have little effect on the actuation mechanism
up to 105 cycles [12], reliability demonstrations up to 109
cycles, or more would require hermetic packaging. Hermetic
packaging of these devices in an inert environment is a significant challenge due to the low boiling point of mercury. Gallium
and its alloys (e.g., Galinstan) would allow packaging at higher
temperatures, although instead they are more susceptible to surface oxidation. Detailed results related to hermetic packaging
of liquid metal droplets and long-term device reliability will be
presented elsewhere after completion of the ongoing study.
Significant switch-to-switch contact resistance variation was
observed, which is attributed to oxidation of the LM in air. It is
also expected that cycle-to-cycle contact resistance degradation
will happen in air due to continuous oxidation of LM. This
problem will be solved once hermetic packaging in an inert
environment is developed for these switches. Even with switch
contact resistance of 1.32 Ω which is comparably higher than
the best MEMS switches [20], the LM-based switches are
expected to fare better due to no contact degradation from
arcing or welding. Simplified 2-D FEM simulation shows for
designs based on silicon substrates with adequate heat sinks our
devices should be able to handle ∼1 A before mercury boiling
at 357 ◦ C. These tests were not performed considering the toxic
nature of mercury vapor. Galinstan which boils at 1300 ◦ C
should be able to handle much larger currents.
ACKNOWLEDGMENT
The authors would like to thank J. Jenkins and T. Wu for
their discussions about the project, Dr. S. Mathai and the staff
of the Center for High Frequency Electronics, University of
California, Los Angeles, for their help with the RF measurements, and A. Lee for her help with this paper.
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[13] P. Sen and C.-J. Kim, “Microscale liquid-metal switches—A review,”
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[14] C.-H. Chen and D. Peroulis, “Electrostatic liquid-metal capacitive shunt
MEMS switch,” in IEEE MTT-S Int. Microw. Symp. Tech. Dig., Jun. 2006,
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[15] C.-H. Chen, J. Whalen, and D. Peroulis, “Non-toxic liquid-metal
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SEN AND KIM: LIQUID–SOLID DIRECT CONTACT LOW-LOSS RF MICROSWITCH
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[18] P. Sen, “Driving liquid-metal droplets for RF microswitching,” Ph.D.
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Prosenjit Sen was born in Calcutta, India, in 1978.
He received the B.Tech. degree in manufacturing
science and engineering from Indian Institute of
Technology, Kharagpur, India, in 2000 and the Ph.D.
degree in mechanical engineering from the University of California, Los Angeles (UCLA), in 2007.
He is currently with the Micro and Nano Manufacturing Laboratory, Department of Mechanical
and Aerospace Engineering, UCLA. His research interest includes microfludic systems, droplet dynamics, liquid-metal-based RF MEMS, and reliability of
electrowetting-on-dielectric devices.
Dr. Sen was the recipient of the Institute Silver Medal from the Indian
Institute of Technology.
997
Chang-Jin “CJ” Kim received the B.S. degree
from Seoul National University, Seoul, Korea, the
M.S. degree from Iowa State University, Ames, with
Graduate Research Excellence Award, and the Ph.D.
degree in mechanical engineering from the University of California, Berkeley, in 1991.
In 1993, he joined the faculty of the University
of California, Los Angeles (UCLA), where he has
developed several MEMS courses and established
a MEMS Ph.D. major field in the Department of
Mechanical and Aerospace Engineering. Directing
the Micro and Nano Manufacturing Laboratory, UCLA, he is also active in
the commercial sector as board member, scientific advisor, and consultant.
He currently serves as a Subject Editor for the IEEE/ASME J OURNAL OF
M ICROELECTROMECHANICAL S YSTEMS and on the Editorial Advisory Board
of the IEEJ Transactions on Electrical and Electronic Engineering. His research interests include MEMS and nanotechnology, including the design and
fabrication of micro/nanostructures, actuators, and systems, with a focus on the
use of surface tension.
Dr. Kim has served on numerous technical program committees, including
Transducers and the IEEE MEMS Conference, and on the U.S. Army Science
Board as a Consultant. He is currently chairing the Devices and Systems
Committee of the ASME Nanotechnology Institute and serving on the National
Academies Panel on Benchmarking the Research Competitiveness of the U.S.
in Mechanical Engineering. He was the recipient of a TRW Outstanding Young
Teacher Award, National Science Foundation CAREER Award, Association for
Laboratory Automation Achievement Award, Samueli Outstanding Teaching
Award.