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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009
Microscale Liquid-Metal Switches—A Review
Prosenjit Sen and Chang-Jin “CJ” Kim, Member, IEEE
Abstract—Microelectromechanical systems (MEMS) have constituted an active R&D area over the last two to three decades, with
one of the earliest application topics being microswitches. Typical
designs involve actuation of microscale flexural elements (e.g.,
beams and membranes) to make a short or an opening in the transmission (signal) line. However, the problem of reliability of these
switches persisted due to the presence of a solid–solid contact.
Inspired by the regular mercury switches that use liquid–solid contact to solve the problems, several researchers have been exploring
the use of liquid metal (LM) in developing microscale switches.
Over time, the following two different approaches have evolved:
LM-wetted microswitches and LM-actuated microswitches. In this
paper, we summarize the progress of both approaches over the last
decade by reporting a series of LM microswitches, each with the
mechanism, fabrication, and performance. In addition, the properties of various LMs and LM alloys and the issues of fabrication
and packaging involving LM are presented to help understand the
reported developments as well as to assist in designing future LM
microswitches.
Index Terms—Liquid metal (LM), mercury switch, microelectromechanical systems (MEMS) switch, microswitch, reliability of microswitch.
I. I NTRODUCTION
A. Mercury in Regular (Macroscale) Switches
A
N ELECTRICAL switch is a device which can change the
flow of current in an electrical circuit. This conceptually
simple device finds its use in many electrical appliances in one
form or another. Its diverse application has led to the development of several switching technologies, including mechanical
switches, electromechanical relays, and transistors. One such
technology of our interest in this paper is the electromechanical
relay, where an electrical signal is used to actuate a mechanical
element to achieve switching. Despite being one of the earliest
technologies, the electromechanical relays have several attractive properties when compared to semiconductor switches: a
very high on–off impedance ratio with OFF-state impedance
on the order of 1010 –1014 Ω [1]. They also have low contact
resistance (< 50 mΩ [1]), can handle large currents (power),
and are relatively insensitive to environmental conditions (e.g.,
temperature, humidity, and radiation). However, they are large
and slow (greater than 1 ms switching delay), and the presence
of a solid–solid mechanical contact leads to contact bounce,
making them unsuitable for small-signal switching. Solid–solid
mechanical contact also leads to contact arcing and welding,
causing surface damage and material transfer at the contact.
This contact degradation limits the reliability and operational
life of these devices.
In contrast, semiconductor switches (e.g., transistors) are
fast, with nanosecond switching times. The absence of solid–
solid mechanical contact means that no problems are related
to contact bounce or contact degradation, leading to very long
operational lives. They are also very small in size, integrate well
with other circuit elements, and hence are cheaper. However,
they have a high ON-state resistance of 2–6 Ω, low open-state
impedance on the order of 105 –107 Ω [2], and low powerhandling capabilities. Also, their sensitivity to temperature and
radiation limits their range of operating environments.
In order to obtain longer life and lower switching noise while
retaining other benefits of electromechanical relays, mercurywetted relays were first developed at Bell Labs in the 1940s.
After several years of research and improvement, these switches
consisted of a movable and a fixed metallic contact in a sealed
glass envelope containing a pool of doped mercury. Doping
with Sn and Cu was used to protect against failure during longterm usage caused by the formation of amalgams, particularly
NiHg4 [3]. The solid contacts were wetted by mercury, which
was drawn in the capillary formed by the closing gap between
the moving and the static contacts when the switch was actuated magnetically. The switch was hermetically packaged in a
hydrogen environment to prevent arcing and mercury oxidation.
These switches demonstrated bounce-free operation, low contact resistance (< 50 mΩ), long life (over 109 cycles [4]), fast
rise time (on the order of 10 ps [5]), and, in some cases, high
isolation voltage over 5 kV [6]. Mercury-wetted relays were
heavily commercialized and extensively used in telephony and
other low-signal high-bandwidth applications [7]. However,
these switches were slow, sensitive to gravity, and posed environmental hazard, as the quantity of mercury contained in each
switch was significant. To solve the gravitational sensitivity
problem, mercury-film switches were developed [2], [8]. In
these switches, contact surfaces were permanently wetted in a
thin film of mercury, and the mercury pool was not required.
Despite several advantages, the risk of pollution due to spill
or improper disposal caused this technology to be replaced by
others over the last two decades.
B. MEMS Switches
Manuscript received May 21, 2008; revised September 5, 2008. First published October 31, 2008; current version published April 1, 2009. This work
was supported by the DARPA HERMIT program.
The authors are with the University of California, Los Angeles, CA 90095
USA (e-mail: senp@ucla.edu; cjkim@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/TIE.2008.2006954
Microelectromechanical systems (MEMS), although the
name was coined in the late 1980s, has been an active area of
research and development since the 1970s for several sensors
and actuators, with microswitches being one of the first applications. One of the earliest switch implementations, a SiO2 based reed relay fabricated using MEMS surface and bulk
0278-0046/$25.00 © 2009 IEEE
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
micromachining techniques, was demonstrated by Petersen in
as early as late 1970s [9], [10]. Like their macroscale ancestors,
most of the MEMS switches have a moveable mechanical
element actuated by various actuation mechanisms to achieve
a short or an open in the signal line. Electrostatic actuation
(e.g., [11] and [12]) leads to a simple design and requires
almost negligible power. These switches are fast, with typical
switching speed on the order of tens of microseconds. However,
this technique generally requires higher voltage (only specific
designs demonstrating less than 20 VDC ) and generates smaller
actuation force in the range of tens to hundreds of micronewtons, depending on the switch design and actuation voltage.
Microswitches operated by the electrothermal [13]–[15]
method, on the other hand, generate more closing force (in
the range of millinewtons [15]) and thus lead to low contactresistance values (in the milliohm range [16]). Although these
switches use low actuation voltage (less than 8 VDC ), highcurrent requirement makes them power hungry (20–40 mW to
maintain ON-state [15]). Bistable designs have been demonstrated such that power is consumed during switching but not to
maintain state [13]. The typical reaction time is slow and usually larger than several hundreds of microseconds. Electromagnetic actuation provides a large closing force and can be fast
[17] but requires complex designs to facilitate coils or magnets.
Micromechanical switches enjoy the same benefits over the
solid-state switches as was enjoyed by their macroscale mechanical switches. Reduced in size by MEMS technologies,
the miniature mechanical switches are now becoming a more
viable substitute of the solid-state switches. Despite significant
progress in the MEMS switch technology (with some switches
demonstrating more that 100 billion cycles [18], [19]), there
are still doubts about their long-term reliability under deployed
conditions. Two common failure mechanisms observed are
dielectric charging [20] and contact degradation [16]. The
contact failure mechanism is similar to that observed in the
macroscale electromechanical switches and is caused due to
arcing and welding at the solid–solid contact, leading to surface
damage and material transfer. This problem at contact has
led to the creation of many switch designs that avoid hot
switching. High-power operation accelerates the contact wear
[19], thus severely limiting the power-handling capabilities of
the MEMS switches. Large contact force is required to achieve
a proper metal–metal contact and low contact resistance but
unfortunately accelerates the contact wear [16], resulting in a
tradeoff between the switch reliability and its contact resistance.
To improve switch reliability against contact degradation, some
switches use prescribed actuation waveforms that minimize the
impact at the contact [21]. Taking a hint from their macroscale
predecessors in addressing the contact issue, some researchers
have explored the use of liquid metal (LM) for the development
of high-reliability microswitches.
C. The Review
The aim of this paper is to investigate the development of
LM-based microscale switches. In the next section, we will
discuss the available choices of LMs and LM alloys for the
development of the microscale switches. Their physical and
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chemical properties will be discussed in brief. In Section III, we
will discuss the various fabrication technologies developed to
realize LM microswitches. Owing to their high reactivity with
other metals, LMs pose serious material compatibility issues.
Innovative packaging technologies are also required to prevent
LM oxidation. The deposition of microscale LM droplets is a
difficult process owing to their high surface tension; several
deposition techniques suitable for different LMs are discussed.
Next, we will discuss micro LM-switch devices developed
for electrical applications. Two distinct design approaches will
be reviewed. In Section IV, we will study LM-wetted microswitches, where LM simply wets the contact surface of the
switching element. Finally, in Section V, we will look into LMactuated microswitches, where LM not only moves directly as
the switching element but also provides a wetting contact.
II. LMs AND LM A LLOYS
A. Mercury
Mercury is one of the five elements (others include gallium,
cesium, francium, and bromine) that are liquid at near room
temperature. Its earliest use was in ointments and cosmetics,
and its use in amalgamation with other metals was discovered
around 500 B.C. Its symbol Hg is derived from its Greek name
Hydrargyrum, which means “watery silver.” It is also known as
Argentum vivum in Latin, meaning “quicksilver.” So strong is
its history that mercury is the only metal that is still known by
its alchemical planetary name.
Mercury is extracted from its naturally occurring mineral
called cinnabar (HgS) by heating it to 800 ◦ C in a flow of O2 . Its
unique properties caught the interest of early scientists, leading
to the development of several scientific apparatuses such as
the thermometer, barometer, coulometer, and diffusion pump.
Gaseous mercury is used in mercury-vapor lamps. Mercury is
sometimes used as a coolant in nuclear reactors, although
sodium is preferred, as the high density of mercury makes
its pumping an inefficient process. Mercury forms alloys with
almost all metals, and the alloys are called amalgams. Its
amalgamation with gold or silver has been used by historic metallurgist for extraction and purification of the precious metals.
Why Is Mercury Liquid?: It is interesting to note that even
though gold and mercury are neighbors in the periodic table,
there is no other consecutive pair of metals with such huge
difference in physical and chemical properties. The differences
in melting points (−38.84 ◦ C for Hg versus 1064.18 ◦ C for Au)
and in densities (13 550 kg/m3 for Hg versus 19 340 kg/m3 for
Au) are greater than anywhere else. While Au is an excellent
thermal and electrical conductor, Hg is only a fair conductor.
The clue lies in understanding the phenomenon which leads to
weaker Hg–Hg bonds. Relativistic effects lead to contraction of
the outermost 6s orbital. Relativistically contracted 6s orbital
is filled with two electrons in Hg and hence cannot contribute
significantly in the formation of metal–metal bonds, thus behaving more like noble gases. It is thought that Hg–Hg bonding
is mostly due to van der Waals forces and the weak interaction
of 6p orbital, leading to a weaker bond and low melting point.
A more complete explanation about the topic is discussed
in [22].
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009
Physical and Chemical Properties [23]: The atomic number
for Hg is 80, and its atomic weight is 200.59. Mercury freezes
at −38.84 ◦ C and boils at 357 ◦ C. Its density at 20 ◦ C is
13 550 kg/m3 and is a function of temperature. Denser than several solid metals (iron, copper, lead, etc.), mercury is the highest
density liquid known at room temperature. The vapor pressure
varies from 0.16 Pa at 0 ◦ C to 36.38 Pa at 100 ◦ C. Mercury is
also known to have the highest surface tension of 485 mN/m
for a room-temperature liquid, with the next being 73 mN/m
for water. The specific heat of mercury is 1390 J/kg · K, and
its thermal conductivity varies from 7.82 W/m · K at 0 ◦ C to
8.30 W/m · K at 20 ◦ C and to 9.47 W/m · K at 100 ◦ C. Its electrical resistivity is 0.96 μΩ · m, which is significantly higher
than that of gold (0.02 μΩ · m). The volume thermal expansion of mercury is given by V (t) = V ∗ (1 + 1.82 × 10−4 t +
7.8 × 10−9 t2 ), where t is in degrees Celsius and V is the
volume at 0 ◦ C.
Toxicity: Over the past few decades, the use of mercury
has been restricted significantly due to concerns about environmental hazards, although some doubt the risks estimated
for common appliances. In recognition of the potential toxic
effects, the permissible exposure limit (PEL) has been set by the
Occupational Safety and Health Administration at 0.01 mg/m3 .
Even though the PEL is indeed very low, it has been demonstrated that vapors produced by small spills are not hazardous.
Vapors emitted from small droplets diffuse in air quickly over
large distances, and the increase in concentration in the immediate vicinity is insignificant [24]. Mercury enters the body
most critically through the lungs, with up to 90% of inhaled
mercury being absorbed and significantly less through the skin
and the digestive system [25]. Contrary to the common belief,
mercury is not a cumulative toxin, having a half-life of three
days in the blood. However, mercury chemically bound to the
tissue can easily have a half-life of 90 days. Kidney takes an
active part in the excretion of absorbed mercury, but excessive
quantities can lead to renal failure. Mercury and several of its
compounds are insoluble and are less harmful when compared
to slightly soluble dimethyl mercury. Dimethyl mercury is
known to severely attack the central nervous system and causes
the often-fatal Minamata disease.
B. Gallium
With a melting point of 29.77 ◦ C, gallium melts at temperatures slightly higher than room temperature, and indeed,
body heat is sufficient to melt it. Gallium was discovered
spectroscopically and extracted by Lecoq de Boisbaudran in
1875. Its name is derived from the Latin gallus, which means
“a cock” (a translation of Lecoq). It is found in trace amounts
in diaspore, sphalerite, germanite, bauxite, and coal. Metallic
gallium finds its use in high-temperature thermometers. When
painted on glass, it wets the surface and forms a brilliant mirror.
Gallium also finds extensive use in doping of semiconductors.
Gallium arsenide can convert electrical energy to light directly,
finding application in LEDs.
Physical and Chemical Properties [23], [26]: The atomic
number of gallium is 31, and its atomic weight is 69.72. With
a melting point of 29.77 ◦ C and a boiling point of 2205 ◦ C,
it is one of the metals with the largest liquid range. It has
a very low vapor pressure (9.31 × 10−36 Pa at 29.9 ◦ C) even
at elevated temperatures (1 kPa at 1565 ◦ C). A lower vapor
pressure not only implies greater safety from accidental vapor
inhalation but also favors miniaturization. As liquid droplets
become smaller in volume, their rate of evaporation per volume
increases dramatically due to scaling laws. An inherent low
vapor pressure keeps the evaporation rate low. There is a strong
tendency to supercool, i.e., remain liquid below its freezing
point. Gallium expands approximately 3% on solidification,
with a specific gravity of 5.904 (29.6 ◦ C) in its solid phase and
a specific gravity of 6.095 (29.6 ◦ C) in its liquid phase. Pure
gallium is silvery in appearance. It does not crystallize to any
simple crystal structure and hence exhibits a conchoidal fracture similar to glass. The stable phase under normal conditions
is orthorhombic. There are many stable and metastable phases
as a function of temperature and pressure.
The surface tension of gallium measured at its melting point
in a hydrogen environment is 680 mN/m. The thermal conductivity of solid gallium is highly anisotropic and varies from
88.4 W/m · K (parallel to the b-axis) to 16.0 W/m · K (parallel
to the c-axis). Liquid gallium has a thermal conductivity of
28.1 W/m · K at 302.93 K and 32.8 W/m · K at 373.2 K. Electrical resistivity of gallium is 0.13 μΩ · m (at 273 K). Gallium
aggressively attacks nearly all metals (except tungsten and tantalum) at all temperatures, making it difficult to use and limiting
its application. However, this enhanced reactivity gives rise to
the several low-melting-temperature alloys discussed next.
C. Galinstan
In order to create an effective LM switch, the conductive
medium should remain liquid at temperatures well below 0 ◦ C.
There are several gallium-based alloys with low freezing points
formed using a technique called “freezing-point depression”
[27]. Freezing-point depression works on the principle that the
dissolved metal impurities, having a different crystal structure
and atomic size, in the gallium matrix inhibit crystallization
of the alloy. One of the most commercialized gallium alloys
having a low freezing point is galinstan (from Geratherm Medical) as a eutectic alloy of 68.5% gallium, 21.5% indium, and
10% tin [28]. Its name is derived from its constituents, with
stannum being the Latin for tin. Galinstan is being widely
used for commercial thermometers as a safe replacement for
mercury. Due to its better reflectivity and lower density, it can
be used as a substitute of mercury in liquid-mirror telescopes.
Due to the nontoxic nature of its constituent materials and its attractive properties, we consider galinstan a promising LM alloy,
which will replace its toxic counterparts in the coming future.
Physical and Chemical Properties [29]: Galinstan has a
melting point of −19 ◦ C and a boiling point greater than
1300 ◦ C. It has a very low vapor pressure even at elevated
temperatures (less than 10−6 Pa at 500 ◦ C) and is much lighter
than mercury with a density of 6440 kg/m3 . It has a thermal
conductivity of 16.5 W/m · K, which is several times higher
than that of mercury, and an electrical resistivity of 0.43 μΩ · m.
Although these properties are attractive, a major problem in the
development of practical applications arises from the fact that
galinstan very easily wets and adheres well to most surfaces.
In development of commercial thermometers, the inner wall of
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
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the tube is coated with gallium oxide to prevent the wetting of
galinstan.
III. M ICROFABRICATION AND P ACKAGING T ECHNOLOGIES
IN THE P RESENCE OF LMs
A. LM Material Compatibility
It is known that LMs can help in reducing the contact resistance and the contact wear of solid electrodes. They are also
known to improve the device operational life and remove many
unwanted characteristics of the usual solid-contact electromechanical relays including contact bounce. However, both
gallium and mercury have serious material compatibility issues.
While designing LM devices, one has to take into consideration
the long-term effect due to material compatibility. Both gallium
and mercury aggressively react with and dissolve most of
the metals, thus forming alloys [30]–[32]. The solubility of
different metals in mercury and gallium are shown in Fig. 1.
From Fig. 1, it is evident that only Cr, Ni, and Ti are resistant
to mercury. Pt also has lower solubility when compared to Au,
Ag, and Cu. However, Cr and Ti have a strong tendency to
form surface oxide, resulting in a very high contact resistance
and making their use impractical. Mercury does not react or
wet common dielectrics used in MEMS (e.g., silicon nitride or
silicon dioxide) and polymers. Compatibility of gallium with
common MEMS materials is given in Table I, compiled from
[33] and [34]. It can be easily seen that only a limited number of
materials are available for the design of MEMS switches using
gallium or gallium alloys. At room temperature, only titanium
and tungsten are compatible with gallium.
Another problem in the use of LMs arises due to their strong
tendency to oxidize in air. When exposed to air, their surface
almost instantaneously oxidizes, forming a thin membrane
around the droplet. One consequence of oxide formation for
gallium is that the oxide wets many materials that gallium does
not. As a result, the gallium droplet becomes more resistant
to motion due to its increased effective viscosity, slowing the
droplet motion. Dissolution of another metal in the LM matrix
may also increase the viscosity. Finally, these surface oxides
also contribute to the increase of contact resistance. All these
problems make it necessary to assemble and package LM
devices in an inert environment. Hermetic packaging is also
required for long-term operation.
B. LM Deposition Techniques
A critical technique in the fabrication of LM microswitches
is disposition of a target volume of LM on desired positions.
Deposition through nozzles is not easy if the openings are in
microscale because of the high surface tension of LM droplets.
Manual positioning of such small LM droplets (tens of micrometers) is also quite challenging.
Mercury: In 1996, Saffer et al. [35] developed an innovative
technique to deposit microscale mercury droplets using selective condensation of mercury vapor on micropatterned nucleation sites. The nucleation sites are gold dots micropatterned
using the standard photolithography process. Mercury vapor
reacts with the very thin layer of gold and forms a liquid
Fig. 1. Solubility of different metals. (Top) In mercury at 298 K. (Bottom) In
gallium at 873 K [31], [32].
amalgam. Further condensation preferentially takes place at
the LM droplet, and the droplet grows on the lithographically
defined positions.
Fig. 2 shows the mercury deposition setup of Latorre et al.
[36], evolved from the original setup by Saffer et al. [35].
It consists of a deposition chamber, a mercury source pool,
and a sample holder. The deposition process starts by placing
the die on the sample holder. The shutter is closed, and the
mercury pool is heated to 140 ◦ C. The sample is heated to
100 ◦ C. The shutter is then opened, and the sample is moved
into the deposition chamber while the sample heater is turned
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TABLE I
COMPATIBILITY OF GALLIUM WITH METALS USED IN MEMS
Fig. 4.
Fig. 2. Schematic of the mercury droplet deposition setup using the selective
condensation technique [36].
Screen printing of gallium alloy as LM [26].
make gallium thermally stable and extremely difficult to evaporate. At room temperature, the vapor pressure of gallium is
8 × 10−39 torr, making it almost impossible to evaporate. As
noted by Truong [26] for gallium deposition through evaporation, gallium must be heated to at least 750 ◦ C in a vacuum
of approximately 7.6 × 10−6 torr. Despite these difficulties,
very recently, in 2007, Cao et al. [39] successfully deposited
galinstan using thermal deposition techniques.
Truong [26] developed in 2000 a screen printing technology
for the deposition of liquid gallium alloys, as shown in Fig. 4.
The screen was fabricated using KOH bulk-etched silicon
substrates. The idea of the whole process is to have selective
wetting regions on the substrate. After alignment of the
printhead to the substrate, the liquid alloy is bulged out of the
screen window by pressuring the printhead and makes contact
with the gold wetting pads on the substrate. When the screen is
separated from the substrate, the wetting pads retain the liquid
alloy with enough adhesion force that the liquid breaks from
the pool to form droplets. Truong was able to demonstrate the
formation of droplets with screen opening sizes of 40–70 μm at
a pressure of 5 lbf/in2 . There has been no report of depositing
microscale droplets of galinstan so far, which has become
available only recently.
IV. LM-W ETTED M ICROSWITCHES
Fig. 3. Dependence of droplet size on the duration for which the chips are
exposed to the mercury vapor [36].
off. The size of the droplet can be controlled by the exposure
time and the temperature. However, excessive exposure leads to
unwanted nucleation and deposition at random positions. Fig. 3
shows the average droplet diameter as a function of exposure
time. The setup was described in detail by Kim [37].
More recently (2006), a commercial liquid jetting system
(JetLab from MicroFab Technologies, Inc.) has been used for
the deposition of mercury droplets by Wan et al. [38]. The
system jets small droplets, whose diameter mostly depends on
the jetting orifice diameter. Another parameter affecting the
droplet diameter is the jetting voltage waveform.
Gallium and Gallium Alloys: Although it is a liquid at near
room temperature, gallium has one of the highest boiling points
and the lowest vapor pressures among metals. These properties
One of the two approaches to implement a liquid–solid contact, which truly mimics the regular (macroscale) wetted reed
relays, is to coat the contacts of the preexisting MEMS switches
with LM. Several implementations have been developed and
demonstrated over the last decade. Overall, scaling implies that
surface tension becomes dominant at micrometer scales over
structural forces. This explains why electrostatically actuated
microbeams, capable of generating small restoring force only,
were quite slow when detaching from a mercury droplet [40].
Better performance was demonstrated using an electrothermal actuation capable of generating larger force to actuate
thick beams, which command a large restoring force [41]. In
current developments, an LM-coated self-healing contact for
an electrostatically actuated membrane is being explored by
Honeywell Inc. for RF switch applications. We will discuss all
these developments in detail next.
A. Electrostatically Driven Microcantilever Relays With a
Stationary LM Contact
One of the earliest attempts to integrate LM into microswitches was by Saffer et al. [35] in 1996. Their approach
was simple yet unique. As shown in Fig. 5, laterally actuated
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
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Fig. 5. Schematic of the electrostatically driven microcantilever-based
mercury contact switch [35].
Fig. 7. Schematic of the electrostatic comb-drive-based mercury contact
switch [41].
Fig. 6. Fabricated cantilever device with a mercury droplet deposited at the
contact point on signal electrode [35].
polysilicon cantilevers of 2 or 3 μm widths and 300–500 μm
lengths were used. The driving electrodes were designed with
a curved shape to achieve large actuation with a reasonable
driving voltage in a fashion often referred to as “zipper actuation” [42]. Bumpers were designed to prevent the shorting
of the actuated cantilever with the driving electrode. An LM
droplet with a diameter of ∼10 μm was deposited on the
signal electrode using the condensation technique discussed in
Section III. A potential applied between the driving electrode
and the cantilever causes the cantilever to bend and make
contact with the deposited mercury droplet. The high surface
tension of mercury prevents the droplet from rolling even when
the actuated cantilever makes contact with the droplet.
Fabrication: The polysilicon surface-micromachined device
was fabricated using the MCNC Multi-User MEMS Process
(MUMP). The cantilever (2 μm wide) and the driving electrode
were made using 2 μm thick Poly1 layer, and the signal
electrode was formed from 0.5 μm thick Poly0 layer. The
2 μm thick sacrificial oxide layer was etched in HF to free
the beams using various releasing methods. A 10 μm2 metal
(Cr/Au) patch on signal electrode was used as the preferential
condensation site for the mercury droplet. As seen in Fig. 6, an
LM droplet was formed at the contact.
Results: The fabricated devices achieved tip deflections of
30 μm with a driving voltage of 60 VDC [35]. The measured
OFF-state resistance was greater than 200 MΩ. For contactresistance measurements, the total resistance was measured
with the actuated cantilever. The obtained resistance values
were corrected for the device (interconnect) resistance by
subtracting resistance values measured from reference devices
fabricated with the actual devices on the same chip for this
purpose. The obtained poly-mercury-poly contact resistance
was in the range of 800–1000 Ω. The high contact resistance
was attributed to the poor wetting of the polysilicon surface
by the mercury droplet [41]. However, this switch was not
reliable because the cantilever was too flexible and significantly
affected by the stiction with the droplet. Limited by the microfabrication technology at the time, the cantilever was only
Fig. 8. Fabricated comb-drive devices with mercury droplet at the contact
point on signal electrode [41].
as thick (2 μm) as it was wide (2–3 μm), lacking the vertical
rigidity needed against the out-of-plane bending [40].
B. Electrostatic Comb-Drive Microrelays With a Stationary
LM Contact
To solve the stiction problem faced by the cantilever-based
relays earlier, Simon et al. (1998) designed mercury contact
microswitches using a folded beam driven by comb-drive actuators [41], as shown in Fig. 7. The folded-beam structure
gave a higher vertical rigidity than the single cantilever. To
switch, the comb drive was actuated, and a central portion (i.e.,
more stable) of the structure made contact with the mercury
droplet placed on the signal line. The widths of the beams
were designed to be 2 or 3 μm, while the lengths varied
from 150 to 240 μm. The comb had 20 μm long fingers
(26–56 counts, depending on the design) with a gap of 2 μm
between the stationary and the movable fingers. Devices were
fabricated using the MUMP technology, as described previously. In postprocessing, the sacrificial layer was etched in HF
to release the structures, and an LM droplet was placed using
the condensation technique of Section III.
Results: A fabricated device is shown in Fig. 8. Switching
voltage of 35 VDC was sufficient to achieve the required 4 μm
travel to close the contact. The maximum driving frequency
of the device was 4 Hz. At higher frequencies, however, the
deflection was not enough to obtain switching. Up to 11 mA
switching was reported [41]. Larger spring constant and, hence,
the larger restoring force of the folded-beam structure were
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Fig. 10. LM-coated switches based on the electrothermal electromagnetic actuation. (a) Single-pole double-throw switch. (b) Bistable switch by
Cao et al. [39].
Fig. 9. Schematic description of the bidirectional electrothermal electromagnetic actuator by Cao et al. [43].
cited as the reasons for the ability of these switches to detach
from the mercury droplet reliably.
C. Electrothermally Actuated LM-Wetted Bistable Relays
Recently, in 2005, Cao et al. developed a bidirectional electrothermal electromagnetic actuator [43]. It has a simple design,
with a beam clamped at both ends to bonding pads (see Fig. 9).
When current is passed through the beam, Joule heating causes
the fixed–fixed beam to develop a compressive stress and buckle
once the stress passes the critical value. A NdFeB permanent
magnet is placed under the device to generate a constant
magnetic field of 0.2 T. The electromagnetic Lorentz force
experienced by the current-carrying beam controls the direction
of beam actuation. The direction of deflection is determined by
the direction of the current in the beam. The actuators were
able to generate greater than 100 μN forces for 2–3.5 μm thick
beams (fabricated using poly-MUMPs) and up to 20 mN for
50 μm thick beams (fabricated using SOI wafers). While the
thinner (2–3.5 μm) beams could be driven up to 4 kHz, the
thicker (50 μm) beams were capable of a few tens of hertz
operation only. In this thermal device, response time and, hence,
maximum driving frequency were determined by the cooling
rate of the beams, which explains the lower driving frequency of
the thicker beams. Based on the developed actuator, Cao et al.
demonstrated an LM-coated SPDT and bistable switches
(see Fig. 10) [39], [44]. The contacts were coated in galinstan.
Fabrication: A metal-MUMP process was first used for
device fabrication. This process has eight thin-film layers patterned using six lithography steps with a 20 μm electroplated
nickel serving as the structural layer. The actuation beams
were designed to be 950–1200 μm long, 10 μm wide, and
20 μm thick. Relay actuators were built on a 25 μm trench
to provide better thermal isolation. Electrical isolation between
the actuators and the contact was achieved using a nitride
bridge, as shown in Fig. 11. To achieve low contact resistance,
the 2 μm gold overcoat layer in the metal-MUMP process is
used to cover the top and sidewalls of the contacts.
For further reduction in contact resistance, they decided to
coat the contacts with LM. On a wetting contact surface, LM
fills and smoothens all the microscale surface roughness, thus
Fig. 11. Cross-sectional view and SEM of the LM-wetted microswitch by
Cal et al., illustrating nitride bridge [44].
providing a larger true contact area and hence helping reduce
the contact resistance [45], [46]. Considering the toxicity of
mercury, the authors decided to use galinstan. In postprocessing, the alloy was evaporated onto the metal-MUMP die using
a thermal evaporator. The highly directional nature of thermal
evaporation caused most of the LMs to end up on the top
substrate. However, by placing the die at an angle inside the
evaporator and subsequent rotation of the die during the evaporation process allowed the achievement of significant sidewall
coating so that a measurable difference in the contact resistance
was obtained. A good control of the coating process is required,
as excessive coating of the contact could cause device failure
due to liquid bridging.
Results: Bistable relays were designed and fabricated.
These relays, however, were not able to achieve bistability
due to residual stress in the nickel, which changed the shape
of the bistable beam such that they cannot achieve bistability.
Fabricating the devices on a 25 μm trench led to a reduction
in the heat loss to the substrate and hence improved the energy
efficiency of the devices by three times. The relays were
operated at 0.25–0.5 VDC , requiring 0.5–0.8 A of current. The
breakdown voltage was measured to be greater than 200 VDC .
The measured OFF-state resistance was greater than 100 MΩ.
To measure the contact resistance, the authors used a technique
similar to that of Simon et al. [7]. First, the total resistance
was measured, and then, both the probe tips were placed on
the same contact pad to obtain the contact resistance of the
probe tips. Subtracting the contact resistance of the probe tips
from the total resistance gave the total resistance for the device.
While interpreting the data, it is important to consider that such
techniques are susceptible to significant experimental errors.
Since the surface resistance of interconnects were insignificant
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
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to fully recover the evaporated LM during hot switching by
collecting the satellite droplets back to the desired positions.
Fig. 13 shows the SEM photographs of the fabricated microdevice. Honeywell uses micropatterned wetting features to
collect the small satellite droplets (see Fig. 14). The features
were designed such that surface tension causes the smaller
droplets to move toward the larger central droplet. For fabrication of the bridge, Honeywell uses tungsten, instead of the
commonly used gold, probably for two reasons. First, tungsten
is one of the very few metals that do not react with gallium.
Second, the high melting temperature and hardness of tungsten
provide extra contact reliability.
Fig. 12. Schematic of the electrostatically driven membrane switch with selfhealing gallium contact by Honeywell [47].
(0.003 Ω/), the authors assumed the measured device
resistance to be the contact resistance of the device. The
uncoated gold–gold contact resistance measured by the authors
was 0.3–0.4 Ω. In comparison, LM-coated devices had contact
resistance as low as 0.015 Ω. Even without LM, 50 VDC and
1 A of power were hot switched. However, during highpower hot switching, the contacts deteriorated fast and failed
catastrophically with a huge increase in the contact resistance
(several megaohms) within several hundreds to thousands of
cycles. In applications with large currents, the heat generated
at the contacts due to contact resistance was enough to actuate
the thermal actuators, limiting the current-carrying capacity
of the devices. For example, less than 0.5 W of power was
required to actuate the switch, limiting the current to 1.3 A
for uncoated contacts (0.3 Ω). In comparison, coated contacts
(0.02 Ω) could handle approximately 5 A without selfswitching. When passing very large currents, contact heating
deformed the actuating beams. One way to reduce this problem
was to make the fixed contacts very large, allowing excess heat
to be lost to the substrate. Another method used a spring-loaded
contact design, as shown in Fig. 10(a), which thermally isolates
the moving contact from the actuator. Without considering for
self-actuation, even the uncoated contacts could handle 4–5 A
of current, but at this point, the beams were bent out of shape.
For coated contacts, the test was limited to 4–5 A due to failure
of the probe tips.
D. Self-Healing RF MEMS Switch With Gallium Contacts
One of the recent works using LM-wetted contacts for microswitches was led by Honeywell for RF applications [47].
The schematic of the switch is shown in Fig. 12. Under highpower operation, dc-contact RF MEMS switches are known to
fail by surface erosion caused by localized welding and stiction.
Honeywell’s solution for this problem was to wet the contact
points of the switches with LM. However, during contact separation, necking of the LM takes place (i.e., formation of a very
thin LM bridge). Even though gallium has a high boiling point
(2205 ◦ C), high current density through this narrow neck region
causes high local resistive heating, leading to boiling, and
condensation of the evaporated droplets leads to the formation
of satellite droplets. One of the main goals of this project was
V. LM-A CTUATED M ICROSWITCHES
The other approach to implement a liquid–solid contact is to
actuate LM directly as the moving element in order to achieve
switching. This approach is different from the regular wetted
reed relays and most microswitches, where what move are solid
elements. To move or manipulate the LM for switching, several
actuation mechanisms have been developed, including thermal,
electrostatic, and electrowetting. Recently, electrowetting-ondielectric (EWOD) of LM droplets has also been demonstrated
for low-latency switching applications. In this section, we will
discuss these developments in detail.
A. Thermal-Vapor-Bubble-Actuated LM Microrelays
In the earliest attempts to develop LM-based microswitches,
Simon et al. [48], [49] actuated in 1996 an LM droplet in a
microchannel filled with a dielectric liquid using a pressure
generated by thermal bubbles. The schematic of the device
is shown in Fig. 15. The device design consisted of two
bulk-micromachined reservoirs connected through a V-shaped
channel (called “V-groove throat”). The reservoirs had suspended heater elements to eliminate heat loss to the substrate.
A signal electrode ran through the V-shaped channel but was
disconnected inside the throat, as seen in the figure. An LM
(mercury) droplet was deposited in the channel near the signal
electrodes. Actuating the LM droplet and positioning it over
the disconnected signal electrode caused the signal line to get
connected (i.e., switch on). Formation and expansion of thermal
bubbles in a reservoir by passing current through the heater
elements generated the pressure, inducing a momentary flow
of the dielectric liquid along the channel and moving the LM
droplet. It was noted that inclusion of an air cavity inside the
DI-water-filled device helped protect the seals against breaking
due to pressure buildup [49].
Theoretical estimation of the retarding force, which gives
the actuation pressure required to move an LM droplet, is
complicated, so a simple experiment was performed. Small LM
droplets of lengths ranging from 200 to 900 μm were introduced into glass tubes with 200 and 300 μm inner diameters.
Pressure was applied at one end of the tube, while the other
end was maintained at atmosphere. The aim of the experiment
was to measure the pressure required to initiate the droplet
motion. Pressure was measured using an electrical transducer
where the droplet motion was sensed using a CdS photodiode.
For example, 1.1–1.5 lbf/in2 was required to move a mercury
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009
Fig. 13. Fabricated devices. Courtesy of Youngner of Honeywell [47].
Fig. 14. (Top) Wetting structures with deposited gallium. (Bottom) Satellite droplets. Courtesy of Youngner of Honeywell [47].
certain locations [7]. This results in adhesion of the droplet to
the surface, and energy is required to put the droplet in motion.
The resulting retarding force on a droplet sitting on a substrate
is given by
Fγ = 2r × γlv × cos π − θadv − cos π − θrec
Fig. 15. Schematic of the thermal-bubble-actuated mercury droplet switch as
the earliest LM microswitch by Simon et al. [49].
column in a tube of 200 μm diameter. Despite a significant
scatter in the measured data, a general P ∝ 1/r relation was
correctly reported. This observed relation could be explained
by considering contact-angle hysteresis as the retarding force
against the droplet motion.
Surface roughness, variation in surface chemical properties,
and surface contamination cause the droplet interface to prefer
(1)
where r is the radius of the liquid–solid contact line, γlv is
the surface tension of the droplet, and θadv and θrec are the
advancing and receding contact angles, respectively. As seen
in (1), the retarding force due to contact-angle hysteresis is
proportional to the circumference (radius) and, hence, the 1/r
relation with the pressure. Contact-angle hysteresis strongly
depends on the surface properties, which leads us to suspect
that the large scatter observed in their data was due to surface
oxidation of LM and poor control over the tube surface quality.
To have a better understanding of the driving mechanism, it
is important to realize that due to the nonwetting nature on SiO2
(contact angle ∼145◦ ), mercury does not fill the sharp corners
of the triangular cross section (see Fig. 16) of the microchannel.
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
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Fig. 16. Nonwetting droplet in a tube with different cross sections. (a) Round
(glass) tube. (b) V groove made in silicon [49].
Fig. 18. Oscilloscope trace showing bistable behavior and switching latency
of 10 ms [49].
Fig. 19. Thermally actuated LM switch by Kondoh et al. [50].
Fig. 17. SEM of the fabricated device (before LM droplet) [49].
The leaks hamper the pressure buildup, and the LM is driven
by the drag of the leaking medium. However, this leak was not
necessarily a bad thing because, along with the contact-angle
hysteresis and high surface tension of mercury, this leak made
the bistable operation of the switch possible.
Fabrication: The devices were fabricated on silicon wafers
[7], [49] (see Fig. 17). The V-shaped channels were formed
using KOH etching. A single metallization of chromium and
nickel was used to define the heaters, the signal lines, and the
contact pads. To seal the device, a new microfabrication technique was developed, in which Teflon was spin coated and patterned to form microgaskets. The reservoir (100–200 μm2 ) was
etched, and the heater elements were released in a single step
of XeF2 etching. A mercury droplet was deposited using the selective condensation technique of Section III. After wetting the
device with DI water and placing a cover glass on top, the gap
outside the gasket was dried and sealed with UV-cured epoxy.
Results: The authors were able to achieve bubble nucleation
and LM droplet actuation at 10–15 VDC and ∼100 mW of
power input to the heater. Although their design could generate
enough power, they had poor control over the bubble growth
and location. The switch lag time observed was 10 ms, giving
a maximum driving frequency of 100 Hz (see Fig. 18). When
on, the device could handle 22 mA of current through mercury.
Higher currents were not tested due to the fear of excessive
heating at the contacts in the presence of mercury. The contact
resistance measured for a Ni–Hg contact using a four-point
technique was 120 Ω/μm2 (2.5 Ω for a 48 μm2 contact area) [7].
Only a semibistable operation could be achieved. Once actuated, the device stayed on for several minutes before returning
to the original (initial) position (switch off). This was probably
due to the back pressure caused by the cooling liquid. The
switch could, however, be switched off by turning on the heater
in the other reservoir. The switch was able to achieve 40 dB
isolation and less than 0.1 dB insertion loss at 2 GHz.
B. Thermally Actuated LM Microswitch
Another thermally actuated LM switch for RF switching was
developed in the early 2000s at Agilent Laboratories in Japan by
Kondoh et al. [50]. The device had two reservoirs with heating
elements, as shown in Fig. 19. Although they used thermal
expansion like the one discussed in the previous section, in
this case, the expanding gas actuated mercury directly to cut
or merge mercury slug in a microchannel to switch. As shown
in Fig. 19, initially, contacts L and M were connected. When a
current is passed through a heating element in a reservoir filled
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009
Fig. 21. Switching latency and bistable operation. Source: Kondoh et al. [50].
Fig. 20. Device fabrication and assembly [50].
with gas, the expanding gas breaks the LM column and moves
the broken LM droplet to the other edge, thus connecting the R
and M contacts.
Fabrication: The 700 μm thick top glass had reservoirs,
subchannels, and metal elements. The subchannels and the
reservoirs were fabricated using sandblasting. The subchannels
were 70 μm wide and 50 μm deep, and the reservoirs were
1.3 × 0.6 mm in size and about 100 μm deep. The metal pads
were evaporated thin films of chromium, platinum, and gold.
The intermediate glass was 100 μm thick and had sandblasted
through-holes to define the reservoirs and the main channel.
Cytop was spin coated for adhesion bonding and sealing. The
bottom ceramic substrate had sputtered TaN resistive elements
serving as heater elements. A 0.8 mg of mercury was put on the
contacts before bonding the plates together. Mercury wets the
contact pads and prevents gas leakage during switch operation.
However, small leakage present helps in equalizing pressure
in several tens of microseconds for true bistable operation,
somewhat similar to the leakage discussion in Fig. 16. The
fabricated device is shown in Fig. 20.
Results: A four-point technique was used to measure the
contact resistance. A resistance of 55–59 mΩ was measured
as the combined value for mercury and the contact resistance.
Kondoh et al. selected 0.8 ms pulse of 22.5 VDC as the standard
operating condition requiring less than 10 μJ. The switching
signal is shown in Fig. 21, where switching latency of 0.92 ms
is observed. A high-speed camera was used to confirm the
switching latency observed. The insertion loss was better than
1 dB, and isolation was better than 20 dB from dc to 18 GHz.
Only a few-milliohm contact-resistance variation was observed
for operation up to 3 × 105 cycles. Switching operation was
confirmed to over 108 cycles. The switch was able to handle 1 A
of dc current.
C. Electrostatically Actuated LM Droplet Switch in Cavity
Kim et al. [51] demonstrated in 2002 a microswitch with an
LM droplet sliding by electrostatic attraction from a sidewall
of a cavity. For switch design, it was necessary to have an
estimate of the actuation forces required to move the droplet. In
microscale, a liquid droplet resting on a surface takes the shape
of a truncated sphere because of the reduced inertial effects at
Fig. 22. (a) Contact-angle definition. (b) Force equilibrium of a sliding
droplet [51].
Fig. 23. Experimental setup to measure contact-angle hysteresis with electrostatic attraction by Latorre et al. [36].
microscale, as shown in Fig. 22(a). The contact angle is the
result of an equilibrium of the interfacial energies (γsl for solid
to liquid, γsv for solid to vapor, and γlv for liquid to vapor) and
is described mathematically by Young’s equation [52]
γlv cos θ + γsl = γsv .
(2)
When a force is applied to the droplet parallel to the substrate
surface, the droplet deforms, and the contact angle differs
between the leading (advancing) and tailing (receding) edges
[see Fig. 22(b)]. The static equilibrium is expressed by (1).
This mechanical equilibrium is maintained until a critical force
required to slide the droplet is applied. The difference in the
advancing and receding contact angles at this critical moment
is known as the contact-angle hysteresis and characterizes the
minimum force required to move the droplet.
Fig. 23 shows the experimental setup used by Latorre et al.
[36] to measure contact-angle hysteresis with electrostatic attraction. A mercury droplet of known radius was placed on
a SiO2 surface. A 5 μm wide nickel line was used to bias
the mercury droplet. The gap between the droplet and the
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
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Fig. 25. Optical photograph of a device under test [51].
Fig. 24. Schematic of the electrostatically actuated LM microswitch in cavity
by Kim et al. [51].
actuation electrode was first adjusted to a desired value. Then,
the potential was increased until the LM droplet slides and
snaps to the actuation electrode. They reported contact-angle
hysteresis measured from these experiments to be ∼ 6◦ , which
implied a driving force of 6.7 μN for a 300 μm diameter droplet.
Fabrication: The schematic of the devices designed by
Kim et al. [51] is shown in Fig. 24. It consists of a 100 μm deep
pit to contain the LM (mercury) droplet. In order to pattern
the driving and signal electrodes with less than 30 μm width,
a new shadow masking process was developed. The shadow
mask was formed using very thin (∼40 μm thick) silicon
wafers. The thin wafers were bonded to Borofloat glass wafers
(∼500 μm thick) using anodic bonding to simplify handling.
After the desired pattern was transferred to the thin silicon
wafers using photolithography and deep reactive ion etching
(RIE), the carrier Borofloat wafer was dissolved in concentrated HF.
Silicon wafer was etched in KOH to form the device cavity.
Thermal oxide was grown as the passivation layer. A shadow
mask was aligned to the wafer using a custom setup and
temporarily bonded using photoresist. Metal was evaporated
through the shadow mask to form the driving electrodes. A
5000 Å silicon dioxide was deposited by plasma-enhanced
chemical vapor deposition (PECVD) as the passivation layer.
Another shadow mask was used to deposit the signal electrodes.
Finally, the mercury droplet was placed manually or formed by
the selective condensation method of Section III. The device
was packaged in air or with a dielectric liquid.
Results: The fabricated microswitch (see Fig. 25) was tested
after packaging it with silicone oil to ease the droplet motion by
reducing contact-angle hysteresis. The larger dielectric constant
of the oil compared with that of air somewhat increased the
attraction force applied by the actuation voltage, which ranged
from 100 to 150 VDC . This variation was probably due to
the varying gap between the droplet surface and the actuation
electrodes on the sidewall. Unfortunately, using oil had a speed
penalty on the device; a switching frequency of only 1 Hz was
demonstrated. However, it was an acceptable speed for its target
application of reconfigurable circuits.
Fig. 26. Experiments to characterize effects of microstructures on adhesion
force by Shen et al. [57]. (a) Testing samples. A: pitch and B: line width.
(b) Sliding force. (c) Detaching force.
D. Electrostatically Actuated LM Droplet Switch on
Structured Surface
Realizing that the previously reported LM droplet switches
by Kim et al. [51] had to use high voltages and immerse the
droplet in oil because of the excessive adhesion in microscale,
Shen et al. [53], [54] took a new approach of structuring
the surface. If nonwetting is maintained, a structured surface
[Fig. 26(a)] reduces the liquid–solid contact area and, thus, the
overall adhesion forces [55], [56]. In order to have a quantitative
understanding of the reduction of adhesion forces due to physical surface modification, Shen et al. carried out some simple
experiments (see Fig. 26), using surfaces structured with microline patterns.
Pitch [Fig. 26(a)] was maintained constant at 10 μm, and the
line width was varied to get various contact ratios (i.e., ratio
of liquid–solid contact area with respect to a flat substrate)
between 0.3 and 1. Two different sets of electrostatic actuation
experiments were carried out. The first set was to determine the
horizontal sliding forces [see Fig. 26(b)], and the second set
was to determine the vertical detaching forces [see Fig. 26(c)].
In both experiments, at first, a known volume of the LM droplet
was deposited on the substrate, and then, the actuation electrode
was positioned to achieve a desired gap using a micropositioner.
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009
Fig. 28. Planar device postprocessed on CMOS circuit. The LM switch and
underlying circuit are integrated [53].
Fig. 27.
Electrostatic LM droplet switch of a planar design by Shen et al. [53].
Then, the actuation voltage was increased until the droplet slid
or detached from the surface. For the sliding force, there was
a decrease in the overall surface adhesion, and hence, lower
driving voltages were required with a decrease in the contact
ratio. For example, for a 550 μm diameter droplet with a gap
of 20 μm, at a contact ratio of 1 (i.e., flat surface), the sliding
voltage is 115 VDC ; at a contact ratio of 0.5, the sliding voltage
was 62 VDC ; and at a contact ratio of 0.3, the sliding voltage
was 42 VDC [57]. From these results, the authors claimed that
the surface adhesion can be controlled by physical surface modification. The results of droplet detachment were more complex
to analyze due to bridging of the LM and breakdown of the
air between the LM and the electrode. However, the authors
were able to confirm, as predicted by their analytical analysis,
that much higher voltages (greater than four times) and, hence,
an order of magnitude larger forces are required to detach in
comparison to slide [58]. This can be understood intuitively
by realizing the fact that detachment involves destruction and
creation of surfaces, thus requiring larger energy also over a
shorter period.
Encouraged by the reduced actuation voltage using a microstructured surface, Shen et al. developed a planar design,
i.e., with no cavity (see Fig. 27). In this design, the LM
droplet was always in contact with the common electrode,
which is grounded. When sufficient potential was applied to the
driving electrode, the electrostatic force caused the droplet to
roll and make contact with the signal electrode. There were
several advantages of a planar design. First, the LM droplet
motion was limited to sliding only, and droplet detachment
was avoided, significantly reducing the actuation force required
when compared to the droplet in the cavity design. Second, the
actuation was self-limited, even though there was no wall to
physically limit the droplet motion, as seen in Fig. 27(a). At the
beginning [see Fig. 27(b)], the signal electrode was electrically
floating and thus contributed to the electrostatic force applied
Fig. 29. Actuation and signal waveform of a planar switch with structured
surface integrated with the CMOS driving circuit [53].
to the LM droplet. However, when the droplet made contact
with the signal electrode [see Fig. 27(c)], the signal electrode
was grounded. The grounded signal electrode shielded the
fringing fields, which was responsible for the actuation force,
thus restricting any further droplet motion. Third, this design
allowed devices with much smaller switching gap (i.e., moving
distance required for switching) in comparison to the switch
design with a droplet in a bulk-etched cavity. Finally, the planar
design simplified the fabrication and made the integration of
the microswitch with the underlying CMOS IC circuit more
compatible, as demonstrated by the authors.
Fabrication: The planar microswitch was fabricated by
postprocessing on CMOS chips made using X-FAB Semiconductor Foundries. The driving electrodes and the passivation
dielectric were integrated in the CMOS fabrication. Postprocessing started by thinning the passivation dielectric using
RIE. In the next step, the dielectric was patterned to structure
the surface to reduce adhesion. Next, Cr/Ni was deposited for
the ground and signal electrodes and contact pads. A fabricated
device before LM deposition is shown in Fig. 28. Finally,
depending on the LM droplet size, the droplet is deposited
manually or using the selective condensation technique of
Section III.
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
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Fig. 30. Electrowetting phenomenon on electrode passivated with a dielectric
layer (EWOD) [60].
Fig. 31. Experimental setup to measure contact-line speed under EWOD
actuation [60].
Fig. 33. Switching profile of a low-latency switch showing bounce-free
operation [60].
a smooth surface. However, this stability deteriorates to 3 G
when the surface was structured to get an actuation voltage
of 15 VDC . The measured performance is shown in Fig. 29,
which shows switching latency of 1 ms and maximum driving
frequency of 50 Hz.
E. Fast LM Droplet Switch Using EWOD
Fig. 32. Schematic design and operation of the EWOD-driven fast LM droplet
switch [60].
Results: By integrating various techniques aimed at reduction of the driving voltages, the authors were able to achieve
actuation at as low as 15 VDC . However, this comes at a
price of the droplet stability. Making the surface rough reduced
the actuation voltage required, but it also reduced the device
stability against shock and vibration. For example, an actuation
voltage of 80 VDC and a stability of 80 G were reported on
Many types of LM microswitches having desirable characteristics in their target application have been discussed until
now. However, most of them have limited RF capabilities due
to their slow switching speeds on the order of milliseconds.
Sen and Kim explored the use of recently developed EWOD actuation of LM [59] for development of a low-latency switch in
2007 [60].
Fig. 30 shows the phenomenon of electrowetting of a droplet
on an electrode coated with thin dielectric layer or EWOD [61],
[62]. On application of an electric field between the conducting
liquid and the electrode, the liquid–solid interfacial energy decreases, and the droplet spreads with an accompanying change
in the contact angle, as shown in Fig. 30. The droplet beads
back to the initial shape when the electric field is removed. They
used this contact-line motion, instead of moving the droplet, to
explore a fast low-latency LM droplet switch.
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Fig. 34. LM-based micro RF switch demonstrated by Chen et al. [64]. (a) Side view. (b) Top view.
TABLE II
SUMMARIZED CHARACTERISTICS OF VARIOUS LM-BASED MICROSWITCHES
In order to design a fast switch, they measured the sliding
speed of the liquid–solid contact line when the droplet is spread
by EWOD. The experimental setup used is shown in Fig. 31.
A computer was used to generate step voltages of varying magnitudes, which were amplified using an amplifier to actuate the
droplet. A high-speed camera was used to capture the droplet
dynamics at 10 000 ft/s. The same computer was used to trigger
the camera and synchronize it with the actuation voltage. The
captured movies were analyzed to extract the evolution of the
droplet. For a droplet with a radius ∼1 mm, the contact angle
rapidly changed and stabilized to its final value within 500 μs,
while the contact line continued to move. For an actuation
voltage of 100 VDC , sliding speeds up to 50 cm/s were observed
during the initial 100–200 μs of the contact-line motion.
It is important to realize that fast actuation speed alone is not
enough to obtain fast switching. It is also necessary to obtain a
small droplet of a precise volume, hence a small and consistent
switching gap (the distance that the interface needs to move
to achieve switching). The goal was achieved by placing the
droplet in a lithographically defined microframe. The high surface tension of the LM droplet ensured that it is accurately positioned and allowed a design with much smaller switching gaps
than possible without the microframe. Switches were designed
for 300 μm droplets with switching gaps of 10 and 20 μm. The
frame also provided vibrational stability against up to ∼16 G.
The schematic of the design and switch operation is shown in
Fig. 32. When a potential is applied to the actuation electrode,
the droplet spreads and makes contact with the signal electrode.
Fabrication: Chromium is patterned and wet etched to form
the actuation electrodes. A 3500 Å silicon nitride was deposited
using PECVD to serve as the passivation layer. The dielectric
was patterned and etched using RIE to open the contact pads.
Lift-off Cr/Ni was used to form the ground and the signal
electrodes. The microframe was formed using a multicoat
SU-8 process to yield 500 μm thick structures and was spin
coated with a hydrophobic layer (Teflon AF). The hydrophobic
layer was then patterned and etched in O2 plasma to open the
signal electrodes. In the final step, a 600 μm diameter droplet
(mercury) was placed manually on the device.
Results: The fabricated LM EWOD microswitch, although
not in a sealed package, was tested. The best switch-on voltage
observed was 75 VDC , with the best switch-off voltage being
45 VDC . However, a device-to-device variation in the minimum
switch-on voltage was observed, which was attributed to the
uncertainty in contact-angle hysteresis. Due to this uncertainty
in the switch-on voltage, the switching tests were done at
100 VDC . The switching profile is shown in Fig. 33. Bouncefree operation is attained without any complicated circuit [63].
The measured switch-on latency was 60 μs, and the switchoff latency was 150 μs. The signal rise and fall times were
better than 5 μs. The contact resistance measured using a fourpoint technique was reported to be 2.35 Ω for a 50 × 50 μm
Ni–Hg contact. The large contact resistance was attributed to
LM oxidation due to device packaging in air. In the preliminary
test, 1 W of hot switching was reported for tens of cycles
without any visual damage.
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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW
F. LM-Based RF Capacitive Shunt Switches
Based on the actuation mechanism demonstrated by
Shen et al. [53], [54], an RF switch was demonstrated by
Chen et al. [64]. The schematic of the switch is shown in
Fig. 34. Using electrostatic actuation, the LM droplet is moved
on or away from a coplanar waveguide (CPW). When the
LM droplet is moved on the CPW, the RF signal is capacitively shorted to ground, and the switch turns off. Moving
the LM droplet away from the CPW leads to a passage of the
RF signal with low loss, and the switch turns on. The switch
demonstrated an insertion loss of 0.6 dB up to 20 GHz and an
isolation of 21.1 dB at 20 GHz [64].
Chen et al. [65] also explored the galinstan droplet to replace
mercury. Since galinstan reacts with most metals, the LM
droplet was encapsulated in a microchannel and immersed
in a Teflon solution. However, no actuation mechanism was
demonstrated, and the device was tested by moving the droplet
on and off manually.
VI. S UMMARY AND C ONCLUSION
We started this paper with a study of the various LM and
LM alloys available for use in LM microswitches. A good
understanding of their properties was essential to design the
microswitches and develop appropriate fabrication techniques,
as was discussed in Section III. Mercury, although easy to
use in comparison to gallium, has restricted application due
to its toxicity. Gallium, on other hand, has serious material
compatibility issues. It is also more difficult to integrate with
existing MEMS fabrication technologies. We have reviewed
a majority of the LM-based microswitches reported over the
last ten years, explaining the concept, fabrication, and results,
and summarized their characteristics in Table II. LM-based
switches using mercury have demonstrated very low contactresistance values and demonstrated up to 1 A of hot switching,
an order of magnitude larger than that of solid-based MEMS
switches. Even though many LM-based microswitches have
been explored over the past decade, the field is still embryonic.
Application-specific optimization (e.g., RF MEMS and highpower switching) of the LM-based microswitches will require
further research and development. There are several key technologies that need to be developed in order to be able to design
reliable devices and commercialize LM-based MEMS switches.
One such technology is a low-temperature hermetic packaging
to seal devices laden with LM droplets in an inert environment. Another such required technology is accurate droplet
deposition. Although several deposition techniques have been
developed, improvement in deposition technology is required
to achieve the accuracy demanded by commercial products.
Development of new fabrication technology incorporating materials compatible with other LMs or LM alloys will allow
development of switches using other LMs.
ACKNOWLEDGMENT
The authors would like to thank Prof. L. Lin, Dr. A. Cao,
Dr. W. Shen, and Dr. D. Youngner for their valuable comments.
The authors would also like to thank A. Lee for her help in
improving the readability of this paper.
1329
R EFERENCES
[1] J. Breickner, “A mercury relay which operates in any plane,” in Proc.
17th Annu. Nat. Relay Conf., Stillwater, OK, Apr. 1969, pp. 23.1–23.9.
[2] W. A. Schilling, “Have you tried switching with mercury films?”
Microwaves, vol. 11, no. 2, pp. 46–48, Feb. 1972.
[3] J. E. Bennett, M. P. van der Wielen, W. E. Asbell, and M. R. Pinnel,
“Prevention of bridging failure in mercury switches,” IEEE Trans. Parts,
Hybrids, Packag., vol. PHP-12, no. 4, pp. 380–387, Dec. 1976.
[4] B. Mitchell, Ed., “Low cost mercury relay means more applications,”
in Electronic Engineering, London, U.K.: Morgan-Grampian Publishers
Limited, Aug. 1972.
[5] J. Andrews, “Random sampling oscilloscope for the observation of
mercury switch closure transition times,” IEEE Trans. Instrum. Meas.,
vol. IM-22, no. 4, pp. 375–381, Dec. 1973.
[6] K. Anderson, Ed., “Squeeze works for mercury switches,” in Electronics,
New York: McGraw-Hill, Nov. 1974.
[7] J. Simon, “A liquid filled microrelay with a moving mercury micro-drop,”
Ph.D. dissertation, Univ. California (UCLA), Los Angeles, CA, 1997.
[8] J. Fletcher, “Developments in mercury film switching,” Electron.
Compon. (GB), vol. 12, no. 2, pp. 46–48, Feb. 1972.
[9] K. Petersen, “Silicon as a mechanical material,” Proc. IEEE, vol. 70, no. 5,
pp. 420–457, May 1982.
[10] K. E. Petersen, “Micromechanical membrane switches on silicon,”
IBM J. Res. Develop., vol. 23, no. 4, pp. 376–385, 1979.
[11] R. Chan, R. Lesnick, D. Becher, and M. Feng, “Low-actuation voltage RF
MEMS shunt switch with cold switching lifetime of seven billion cycles,”
J. Microelectromech. Syst., vol. 12, no. 5, pp. 713–719, Oct. 2003.
[12] G. M. Rebeiz, “RF MEMS switches: Status of the technology,” in
Proc. Int. Conf. Solid-State Sens., Actuators, Microsyst., Boston, MA,
Jun. 2003, pp. 1726–1729.
[13] J. Qui, “An electrothermally-actuated bistable MEMS relay for power
applications,” Ph.D. dissertation, MIT, Cambridge, MA, 2003.
[14] Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, “A micromachined
RF microrelay with electrothermal actuation,” Sens. Actuators A, Phys.,
vol. 103, no. 1/2, pp. 231–236, Jan. 2003.
[15] Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, “Low-voltage lateralcontact microrelays for RF applications,” in Proc. IEEE Int. Conf. Micro
Electro Mech. Syst., Las Vegas, NV, Jan. 2002, pp. 645–648.
[16] D. Hyman and M. Mehregany, “Contact physics of gold microcontacts for
MEMS switches,” IEEE Trans. Compon. Packag. Technol., vol. 22, no. 3,
pp. 357–364, Sep. 1999.
[17] W. P. Taylor, O. Brand, and M. G. Allen, “Fully integrated magnetically
actuated micromachined relays,” J. Microelectromech. Syst., vol. 7, no. 2,
pp. 181–191, Jun. 1998.
[18] J. Maciel, S. Majumder, R. Morrison, and J. Lampen, “Lifetime characteristics of ohmic MEMS switches,” in Proc. SPIE, Bellingham, WA, 2004,
pp. 9–14.
[19] G. M. Rebeiz, RF MEMS Theory, Design, and Technology, 1st ed.
Hoboken, NJ: Wiley Interscience, 2003.
[20] S. Melle, F. Flourens, D. Dubuc, K. Grenier, P. Pons, J. L. Muraro,
Y. Segui, and R. Plana, “Investigation of dielectric degradation of microwave capacitive microswitches,” in Proc. IEEE Conf. Micro Electro
Mech. Syst., Maastricht, The Netherlands, Jan. 2004, pp. 141–144.
[21] D. A. Czaplewski, C. W. Dyck, H. Sumali, J. E. Massad, J. D. Kuppers,
I. Reines, W. D. Cowan, and C. P. Tigges, “A soft-landing waveform
for actuation of a single-pole single-throw ohmic RF MEMS switch,”
J. Microelectromech. Syst., vol. 15, no. 6, pp. 1586–1594, Dec. 2006.
[22] J. L. Norrby, “Why is mercury liquid? Or, why do relativistic effects not
get into chemistry textbooks?” J. Chem. Educ., vol. 68, pp. 110–114,
1991.
[23] D. R. Ledi, CRC Handbook of Chemistry and Physics, 88th ed. Boca
Raton, FL: CRC Press, 2007–2008.
[24] T. G. Winter, “The evaporation of a drop of mercury,” Amer. J. Phys.,
vol. 71, no. 8, pp. 783–786, Feb. 2003.
[25] A. K. Furr, CRC Handbook of Laboratory Safety, 5th ed. Boca Raton,
FL: CRC Press, 2000.
[26] T. D. Truong, “Selective deposition of micro scale liquid gallium alloy droplets,” M.S. thesis, Univ. California, (UCLA), Los Angeles, CA,
2000.
[27] L. T. Taylor, J. Rancourt, and C. V. Perry, “Electrical switches and sensors
which use a non-toxic liquid metal composition,” U.S. Patent 5 478 978,
Dec. 26, 1995.
[28] J. S. Hsu, “Two-stage eutectic metal brushes,” United States
20070152533, Jul. 5, 2007.
[29] Material Safety Data Sheet for Galinstan, Geratherm Medical,
Geschwenda, Germany, 2004.
Authorized licensed use limited to: IEEE Xplore. Downloaded on April 13, 2009 at 16:26 from IEEE Xplore. Restrictions apply.
1330
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009
[30] C. Guminski, Z. Galus, and C. Hirayama, Solubility Data Series-Metals
in Mercury, 1st ed. Oxford, U.K.: Pergamon, 1986.
[31] C. Guminski, “Selected properties of simple amalgams,” J. Mater. Sci.,
vol. 24, no. 8, pp. 2661–2676, Aug. 1989.
[32] C. Guminski, “Solubility of metals in liquid low-melting metals,”
Zeitschrift Fuer Metallkunde, vol. 81, no. 2, pp. 105–110, 1990.
[33] R. N. Lyon, Liquid Metals Handbook. Washington, D.C.: Atomic
Energy Comm., Dept. Navy, 1952.
[34] I. A. Sheka, I. S. Chaus, and T. T. Mityureva, The Chemistry of Gallium.
New York: Elsevier, 1966.
[35] S. Saffer, J. Simon, and C.-J. Kim, “Mercury-contact switching with gapclosing microcantilever,” in Proc. Micromachined Devices Compon. II,
Austin, TX, Oct. 1996, pp. 204–209.
[36] L. Latorre, J. Kim, J. Lee, P.-P. de Guzman, H. J. Lee, P. Nouet, and
C.-J. Kim, “Electrostatic actuation of microscale liquid-metal droplets,”
J. Microelectromech. Syst., vol. 11, no. 4, pp. 302–308, Aug. 2002.
[37] J. Kim, “Movement of liquid metal and aqueous solution on microand nano-engineered non-wetting surfaces,” Ph.D. dissertation, Univ.
California, Los Angeles, CA, 2003.
[38] Z. Wan, H. Zeng, and A. Feinerman, “Area-tunable micromirror based
on electrowetting actuation of liquid-metal droplets,” Appl. Phys. Lett.,
vol. 89, no. 20, p. 201 107, Nov. 2006.
[39] A. Cao, P. Yuen, and L. Lin, “Microrelays with bidirectional electrothermal electromagnetic actuators and liquid metal wetted contacts,”
J. Microelectromech. Syst., vol. 16, no. 3, pp. 700–708, Jun. 2007.
[40] S. M. Saffer, “Formation of mercury micro-drop arrays and polysilicon microrelays with a stationary mercury contact,” M.S. thesis, Univ.
California, Los Angeles, CA, 1997.
[41] J. Simon, S. Saffer, F. Sherman, and C.-J. Kim, “Lateral polysilicon microrelays with a mercury microdrop contact,” IEEE Trans. Ind. Electron.,
vol. 45, no. 6, pp. 854–860, Dec. 1998.
[42] R. Legtenberg, E. Berenschot, M. Elwenspoek, and J. Fluitman, “Electrostatic curved electrode actuators,” in Proc. IEEE Conf. Micro
Electro Mech. Syst., Amsterdam, The Netherlands, Jan./Feb. 1995,
pp. 37–42.
[43] A. Cao, J. B. Kim, and L. Lin, “Bi-directional electrothermal electromagnetic actuator,” J. Micromech. Microeng., vol. 17, no. 5, pp. 975–982,
May 2007.
[44] A. Cao, P. Yuen, and L. Lin, “Bi-directional micro relays with liquid-metal
wetted contacts,” in Proc. IEEE Conf. Micro Electro Mech. Syst., Miami,
FL, Jan. 2005, pp. 371–374.
[45] S. Majumder, N. E. McGruer, G. G. Adams, A. Zavracky, P. M. Zavracky,
R. H. Morrison, and J. Krim, “Study of contacts in an electrostatically actuated microswitch,” in Proc. IEEE Holm Conf. Elect. Contacts,
Arlington, VA, Oct. 1998, pp. 127–132.
[46] R. S. Timsit, “The true area of contact at a liquid metal–solid interface,”
Appl. Phys. Lett., vol. 40, no. 5, pp. 379–381, Mar. 1982.
[47] D. Youngner, Honeywell AES Technology Centers of Excellence, 2007,
private communication.
[48] J. Simon, S. Saffer, and C.-J. Kim, “A micromechanical relay with
thermally-driven mercury micro-drop,” in Proc. IEEE Conf. Micro Electro
Mech. Syst., San Diego, CA, 1996, pp. 515–520.
[49] J. Simon, S. Saffer, and C.-J. Kim, “A liquid-filled microrelay with a
moving mercury microdrop,” J. Microelectromech. Syst., vol. 6, no. 3,
pp. 208–216, Sep. 1997.
[50] Y. Kondoh, T. Takenaka, T. Hidaka, G. Tejima, Y. Kaneko, and
M. Saitoh, “High-reliability, high-performance RF micromachined switch
using liquid metal,” J. Microelectromech. Syst., vol. 14, no. 2, pp. 214–
220, Apr. 2005.
[51] J. Kim, W. Shen, L. Latorre, and C.-J. Kim, “A micromechanical switch
with electrostatically driven liquid-metal droplet,” Sens. Actuators A,
Phys., vol. 97/98, pp. 672–679, Apr. 2002.
[52] T. Young, “An essay on the cohesion of fluids,” Philos. Trans. Roy. Soc.
London, vol. 95, pp. 65–87, 1805.
[53] W. Shen, R. T. Edwards, and C.-J. Kim, “Electrostatically actuated metaldroplet microswitches integrated on CMOS chip,” J. Microelectromech.
Syst., vol. 15, no. 4, pp. 879–889, Aug. 2006.
[54] W. Shen, R. T. Edwards, and C.-J. Kim, “Mercury droplet microswitch for
reconfigurable circuit interconnect,” in Tech. Dig. Int. Conf. Solid-State
Sens. Actuators, Boston, MA, Jun. 2003, pp. 464–467.
[55] C.-H. Choi and C.-J. Kim, “Large slip of aqueous liquid flow over a nanoengineered superhydrophobic surface,” Phys. Rev. Lett., vol. 96, no. 6,
p. 066 001, Feb. 2006.
[56] J. Kim and C.-J. Kim, “Nanostructured surfaces for dramatic reduction
of flow resistance in droplet-based microfluidics,” in Proc. IEEE Conf.
Micro Electro Mech. Syst., Las Vegas, NV, Jan. 2002, pp. 479–482.
[57] W. Shen, J. Kim, and C.-J. Kim, “Controlling the adhesion force by physical surface modification for electrostatic actuation of microscale mercury
droplets,” in Proc. IEEE Conf. Micro Electro Mech. Syst., Las Vegas, NV,
Jan. 2002, pp. 52–55.
[58] W. Shen, J. Kim, and C.-J. Kim, “Resistance of liquid-metal droplets
against actuation of microstructured surfaces,” in Proc. Int. Mech. Eng.
Congr. Expo., IMECE2001/MEMS-23830, New York, Nov. 2001.
[59] P. Sen and C.-J. Kim, “Electrostatic fringe-field actuation for liquid-metal
droplets,” in Proc. 13th Int. Conf. Solid-State Sens., Actuators, Microsyst.,
Seoul, Korea, 2005, pp. 705–708.
[60] P. Sen and C.-J. Kim, “A fast liquid-metal droplet switch using EWOD,”
in Proc. IEEE 20th Int. Conf. MEMS, Kobe, Japan, 2007, pp. 767–770.
[61] J. Lee, H. Moon, J. Fowler, T. Schoellhammer, and C.-J. Kim, “Electrowetting and electrowetting-on-dielectric for microscale liquid handling,” Sens. Actuators A, Phys., vol. 95, no. 2/3, pp. 259–268, Jan. 2002.
[62] H. Moon, S.-K. Cho, R. L. Garrell, and C.-J. Kim, “Low voltage
electrowetting-on-dielectric,” J. Appl. Phys., vol. 92, no. 7, pp. 4080–
4087, Oct. 2002.
[63] P. M. S. D. de Moraes and A. J. Perin, “An electronic control unit for
reducing contact bounce in electromagnetic contactors,” IEEE Trans. Ind.
Electron., vol. 55, no. 2, pp. 861–870, Feb. 2008.
[64] C.-H. Chen and D. Peroulis, “Electrostatic liquid-metal capacitive shunt
MEMS switch,” in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2006,
pp. 263–266.
[65] C.-H. Chen, J. Whalen, and D. Peroulis, “Non-toxic liquid-metal
2–100 GHz MEMS switch,” in IEEE MTT-S Int. Microw. Symp. Dig.,
Jun. 2007, pp. 363–366.
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 Micromanufacturing
Laboratory, UCLA. His research interests include
microfluidic systems, droplet dynamics, liquidmetal-based RF microelectromechanical systems,
and reliability of electrowetting-on-dielectric devices.
Dr. Sen was the recipient of the Institute Silver Medal from the Indian
Institute of Technology.
Chang-Jin “CJ” Kim (S’89–M’91) received the
B.S. degree from Seoul National University, Seoul,
Korea, the M.S. degree from Iowa State University,
Ames, and the Ph.D. degree in mechanical engineering in 1991 from the University of California,
Berkeley.
Since joining the faculty of the University of
California, Los Angeles (UCLA), in 1993, he has
developed several microelectromechanical systems
(MEMS) courses and established a MEMS Ph.D.
major field in the Mechanical and Aerospace Engineering Department. Aside from directing the Micro and Nano Manufacturing
Laboratory, he is also an IRG Leader for the NASA-supported Institute for
Cell Mimetic Space Exploration and a Founding Member of the California
NanoSystems Institute, UCLA. His research includes MEMS and nanotechnology, including design and fabrication of micro/nano structures, actuators, and
systems, with a focus on the use of surface tension.
Prof. Kim was the recipient of the TRW Outstanding Young Teacher
Award, NSF CAREER Award, ALA Achievement Award, Samueli Outstanding
Teaching Award, and Graduate Research Excellence Award. He 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 as a Subject Editor for the JOURNAL
OF M ICROELECTROMECHANICAL S YSTEMS , on the Editorial Advisory Board
for the IEEJ Transactions on Electrical and Electronic Engineering, and on
the National Academies Panel on Benchmarking the Research Competitiveness
of the U.S. in Mechanical Engineering. He is also active in the commercial
sector as board member, scientific advisor, consultant, and cofounder of startup
companies.
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