Analog Integrated Circuits and Signal Processing, 32, 37 46, 2002
© 2002 Kluwer Academic Publishers. Manufactured in The Netherlands.
Simulations Based Design for a Large Displacement Electrostatically
Actuated Microrelay
GOOI BOON CHONG,1 KAM SEE HOON,1 IJAZ H. JAFRI2" AND DANIEL J. KEATING2
1
Nanyang Polytechnic School of Engineering, 80 Ang Mo Kio Ave 8, Singapore 569830
2
Corning Intellisense, 36 Jonspin Road, Wilmington, MA 01887, USA Tel.: [phone], Fax: [phone]
E-mail: GOOI Boon [e-mail]; KAM See [e-mail]; [e-mail]; [e-mail]
Received May 18, 2001; Accepted September 14, 2001
Abstract. An electrostatically actuated microrelay with large displacement, small actuation voltage and limited
plate surface dimensions is designed to meet stringent telecommunication switching requirements. Fabrication
feasibility and performance characteristics of the device are evaluated using a commercial CAD for MEMS tool.
Simulation results of the device performance including pull-in voltages for different suspension stiffness variations,
natural frequencies, stresses and restoring forces are presented.
Key Words: microrelay, microswitch, electrostatic actuation, MEMS, CAD simulation
1. Introduction of 0.15 VAC. Lifetimes of these devices have demon-
strated in excess of 109 operations. Other versions of
A microelectromechanical system (MEMS) or Mi- electrostatic microrelays have also been demonstrated
crosystem technology (MST) based relay manifests and have shown lower contact resistance through the
the combined attributes of a solid-state relay (i.e., fast use of metallic contact materials [7 14]. Another de-
switching time, small size, batch fabrication, low cost, vice has been reported with actuation voltages from
etc.) and a traditional electromechanical relay (i.e., 30 to 400 V [15], and a switched current of 10 mA.
smaller on-state resistance, higher off-state resistance). One electrostatically actuated device has reported life-
Three major types of actuation mechanisms extensively times in excess of 108 operations [8]. A recent micro-
investigated in the past include electrostatic, magnetic relay [16] displayed the ability to actuate with less than
and thermal. 24 V and was able to switch currents up to 200 mA.
Electrostatic actuation has been a mechanism of Various patents have been issued by the United States
choice if low displacements are required. Electrostati- Patents and Trademarks Office (USPTO) [17] for elec-
cally actuated microrelays have been reported in liter- trostatic relays and switching. Electrostatic actuation in
ature in as far back as 1979 [1]. The actuation principle MEMS devices is commonly used because of its sim-
and theory is well documented [2 4], and is based on plicity in design, fabrication and operation. However,
the principle of charge attraction. Various types of elec- for devices having a large gap between the electrostatic
trostatically actuated microrelays have been demon- plates, the voltage required for actuation is usually too
strated successfully on complimentary metal-oxide- large for common applications. The range of travel is
semiconductor (CMOS) circuitry [5,6]. These devices also limited by tilting instability. One of the approaches
had a carry current of approximately 10 mA. It was to overcome this limited travel is to use a series capac-
demonstrated that with a constant bias voltage applied itor to provide stabilizing negative feedback [18].
to the microrelay, they devices could be actuated with Electromagnetic actuation is best suited when low
a net driving voltage of 1 10 V. The offset voltage was voltages and high currents are used in switching.
approximately 32 V. Switching was achieved at op- Applications for electromagnetic actuation are in in-
eration frequencies of 100 kHz, with contact voltage tegrated circuit test equipment or automotive environ-
ment where low noise is required. Previously manu-
"
Corresponding author. factured electromagnetic microrelays include ones that
38 Chong et al.
do not have fully integrated coils or magnetic compo- increase of 90 K was required. The other parameters
nents [19 21]. These devices used either an external included an operation time of 5 ms, a force of 19.6 mN
electromagnet [19,20] to actuate a movable member or (2 gF) at 25 µm defection, 27 V voltage and 25 mW
an integrated heating element [21] to demagnetize a power. This indicates that reasonable deflections could
portion of magnetic circuit, thereby changing state to- be achieved with a thermal actuation mechanism. A
wards another magnetized region and pulling the con- thermally actuated relay that uses mercury contacts to
tacting elements apart. However, the use of external reduce contact wear and arcing effects has also been re-
coils in these devices requires additional assembly and ported [29]. The reported contact resistance is less than
reduces the benefits of batch fabrication as the coils are 1 with a maximum carry current of 20 mA. Other
wound using standard wiring techniques. This type of reported thermally actuated devices include a temper-
device (based on external coils) has shown to achieve ature sensor [30] that uses the temperature sensitivity
contact resistances between 100 m to 150 m [22]. of micromechanical beams and switches. The switches
In the case of thermally controlled magnetic actua- close when heated, or if they are pre-latched with a
tion, the forces are relatively large, but it does tend to microscope probe, they pull apart with decreased tem-
increase the switching time and induce noise volt- perature. Tomonari et al. [31] designed a thermally ac-
ages because of thermal voltage generation effects (i.e., tuated bimetal relay. This relay uses silicon bimetal ma-
Seebeck effect). Using LIGA (a MEMS fabrication terials to provide Type-A relay contact. The device has
technique), a fully integrated device was designed that physical dimensions of 2 mm × 3 mm, control power
was able to switch 1 mA current between the contacts of only 100 mW, contact forces of 3.4 mN and can
with an estimated 250 mN of force when applying 1 A achieve a breakdown capability of 500 V. The displace-
coil current [23]. Other reported work includes the use ments achieved in this device are up to 30 microns,
of a planar spiral electromagnet as the driving element with switching times of 26 77 milliseconds. TiNi
of the micro relay [24]. Movements of about 40 mi- Alloy company [32] has also reported a shape mem-
crons at an applied current of 1 2 A were achieved. ory microribbon based relay that is claimed to provide
Yet another technique uses a spiral electromagnet that low-ohmic contact. This device is still in the develop-
actuates a cantilever beam created by combined bulk ment stages. Carlen et al. [33] designed a high actua-
and surface micromachining techniques [25]. This tion power, thermally activated paraffin microactuator.
device was reported to generate up to 200 µN forces at This actuator uses the phase change property of paraffin
80 mA coil current. Taylor [26] has designed and fab- wax to generate a volumetric change and corresponding
ricated fully integrated magnetically actuated micro- pressure increase on the silicon diaphragm. This actu-
machined relays. Two different electromagnet designs ation force could be harnessed for use as an actuation
were investigated (planar spiral and planar meander mechanism. The volumetric expansion is feasible when
electromagnets). The reported values for planar mean- the device sizes are large so that the small displacement,
der microrelays were a minimum contact resistance of large force could be converted to large displacement,
30 m , maximum switched current of 1.2 A, minimum small force. Various patents include Field et al. [34] us-
switching power of 33 mW and lifetime in excess of ing a thermally actuated element to make contact with
850 operations. Numerous patents have been issued on another element and Dhuler et al. [35] using arched
the electromagnetic actuation mechanism [27]. These microelectromechanical beams, which are actuated by
patents cover a broad area of magnetic actuation mech- providing heating from separate heating elements. The
anisms. However, none of these devices have been com- arched beams get radiatively heated to provide the
mercially viable so far, for various reasons such as fab- necessary displacement required for actuation. Ther-
rication costs compared to the mechanically actuated mal actuation can provide large forces and displace-
relays available in the market. ments. However, thermal cycling issues, response time
Thermal actuation has been used in a variety of and heat dissipation requirements must be considered
MEMS applications. Previously reported relays in- carefully when using this mechanism. Also, for a ther-
clude a thermally actuated beam that uses a polysilicon mally actuated device, the high temperature required
heater on top of a SiO2-Si-SiO2 clamped beam [28]. to achieve high displacements also limits the choice of
Deflections above 40 µm were achieved using an in- materials.
put voltage of approximately 45 V. The researchers This research study focuses on the design of a rela-
indicated that for 15 µm displacement, a temperature tively low voltage (50 volts) electrostatically actuated
Large Displacement Electrostatically Actuated Microrelay 39
microrelay having a large air gap of 100 µm with and small surface area is detrimental to the strength
1 mm× 1 mm plate surface dimensions. In the de- of the electrostatic force present at its initial position.
sign analysis, research is focused on two main con- High current safety requires that the device withstand
siderations. The first is to maintain plate parallelism 3 Amps current for one second for the contact part
during the entire 100 µm travel, as this is critical in of the relay. The temperature increase due to this cur-
achieving the maximum electrostatic force required rent flow is targeted not to exceed 200ć%C. The coupled
for the displacement. The second is that the stiffness requirements of low actuation voltage, large displace-
of the suspension system for the moving electrostatic ment gap for voltage surge protection, current and
plate must be of an optimal value within the con- heat dissipation, size and cost make this an interesting
fines of a 1 mm× 1 mm footprint. A quad-supported, design challenge.
two-dimensional coil-spring suspension design was
investigated and the device performances were char-
3. Design and Simulation
acterized using IntelliSuite"! [36]. A detailed CAD
simulation study is conducted for this microrelay us-
Considering two parallel plates with an applied voltage
ing a simulated environment in the process simulation
(V ); the capacitance (C) between the plates is given by
modules of IntelliSuite"!. Using standard fabrication
(neglecting fringe field effects):
techniques, a three dimensional solid model is created
A
and automatically meshed. A mesh convergence study C = µoµr (1)
g
was conducted to ensure result accuracy. The results
where µo and µr are the free-space and relative permit-
obtained were compared against theoretical calcula-
tivities respectively, A is the area of the parallel plates
tions and were found to be in good agreement. Results
and g is the distance between the plates. When a volt-
of the device performance including pull-in voltages
age is applied between the two plates, the magnitude
for different suspension stiffness variation, natural fre-
of the potential energy is given by [38]:
quencies, stresses and restoring forces are presented.
This design study demonstrated that with the proper
1 µoµr AV2
2
We = CV = (2)
suspension system, electrostatic actuation is able to
2 2g
provide the required displacement over a large gap with
Then the force generated between the two plates may
low actuation voltages applied to relatively small plate
be calculated by taking the derivative of the energy in
dimensions.
the direction of the motion. Hence for z direction:
"We µoµr AV2
Fe = = (3)
"z 2g2
2. Device Specifications and Design Challenges
Figure 1 shows the theoretical plot of the above equa-
This microrelay application is targeted for the telecom- tion for actuation voltages of 40 volts to 55 volts. The
theoretical electrostatic force existing between the par-
munication industry, in which miniaturization is
allel plates at an instantaneous displacement from its
highly desirable without sacrificing performance while
initial 100 µm gap position can be determined from the
achieving lower cost. Functioning as a switch in
respective constant voltage lines.
telecommunication equipment, a large gap of at least
Superimposed in Fig. 1 is a line whose gradient rep-
100 µm (operation in vacuum/nitrogen) between the
electrostatic plates is required to prevent arcing dur- resents the stiffness, Kz, of a suspension system for
the moving electrostatic plate. Assuming a linear rela-
ing a possible high voltage surge on the order of 2000
tionship, the mechanical restoring force, Fm, from the
volts [37]. This safety requirement poses a fundamental
suspension system is related to it displacement, Z, by
challenge to the electrostatic actuation principle that is
the equation;
usually applied to gaps in the region of few microns. In
addition, the voltage for electrostatic actuation is lim-
Fm = Kz Z (4)
ited to 50 volts. Furthermore, in order to be economi-
and the instantaneous gap between the two electrostatic
cally viable, the design constraint of maximum device
plates is related to the displacement of the suspension
size of 1 mm × 1 mm is imposed. This size constraint is
system by;
based on economics of manufacturing. The combina-
tion of this large gap, relatively low actuation voltage g = (100 - Z) (5)
40 Chong et al.
Fig. 1. Electrostatic and mechanical restoring forces at different displacement of the moving electrostatic plate.
This line shown in Fig. 1 has a gradient of Kz = Using the same analogy, the suspension stiffness
0.075 µN/µm and is tangent to the constant actuation required for different pull in voltages can be deter-
voltage line of 50 V. This represents the minimum pull mined. To fulfill these stiffness requirements within a
in voltage required for the electrostatic plates to snap confined footprint of 1 mm × 1 mm, a quad-supported,
across the 100 µm gap. If an actuation voltage of 40 V is two-dimensional coil spring concept was implemented.
applied, this suspension stiffness would result in a me- Figure 2 shows this suspension system in its actuated
chanical restoring force that equals to the electrostatic mode. Each spring consists of 27 segments, each with
force at a displacement of 12 µm or a gap of 88 µm. lengths varying from 45 µmto 490 µm and width of
Fig. 2. A quad-support, two-dimensional coil spring suspension system.
Large Displacement Electrostatically Actuated Microrelay 41
Fig. 3. Process simulation window for device fabrication.
25 µm. The gap between the segments is 10 µm. The tion steps (after gold electroplating) to low temperature
different stiffnesses are achieved through different de- processes such as PECVD processes.
position thicknesses, thus capitalizing on the advan- Subsequent to the fabrication simulation, a three-
tages associated to using a common mask. dimensional solid model was created. Figure 4 shows
The fabrication feasibility of this design was eval- the device solid model before removal of sacrificial
uated in a detailed CAD simulation study using the layers. This model is then automatically meshed with
process simulation modules of IntelliSuite"! [36,39]. mechanical and electrical meshes. Coupled electrome-
Figure 3 shows the key fabrication steps as shown in chanical analysis was carried out using the Electrome-
IntelliFab"! process window. These key steps include chanical Analysis module of IntelliSuite"!. A mesh
thermal oxide growth on silicon substrate (a quartz sub- convergence study was also conducted to ensure result
strate could be used if required, e.g., for various RF accuracy.
applications) and addition of silicon nitride passiva-
tion layer. Then a doped polysilicon layer is deposited
using LPCVD process for the lower stationary elec- 4. Results and Discussions
trostatic plate (polysilicon). This is followed by a low
temperature sacrificial oxide deposition. Next a polysil- Results from detailed CAD simulation studies revealed
icon layer is used for upper plate, and finally another that device snapping is possible at different pull-in volt-
polysilicon layer is used for the quad springs. A dif- ages by varying the suspension stiffness. The quad-
ferent conductor layer can be used for top plate if an suspension system is able to maintain plate paral-
etch stop is needed for creation of polysilicon springs. lelism during the entire 100 µm travel. The electrostatic
The electrostatic plates are doped to attain a resistiv- pressure on the plate surfaces is uniformly distributed
ity of 0.03 ohm-cm to accommodate the heat dissipa- throughout the entire surfaces and no tilting instability
tion required during 3 Amps current surge for one sec- was manifested.
ond. For contact materials, gold or gold contacts can be Figure 5 shows the pull-in voltage of the microre-
used, however, it does restrict the subsequent fabrica- lay with different suspension stiffnesses. The results
42 Chong et al.
Fig. 4. Solid model showing various fabrication layers (sacrificial layer not removed). Side and top views.
Fig. 5. Simulation results of pull-in voltages for different coil spring stiffnesses.
are in good agreement with theoretical predictions, the 50 volts pull-in voltage stiffness design resulted in a
having a deviation of about 1%. The natural frequen- first mode natural frequency of about 460 Hz. The out-
cies of the device using different suspension stiffnesses of-plane frequencies range is from 410 Hz to 570 Hz
are depicted in Fig. 6. The first three modes of reso- for different spring stiffness. The slight difference of
nance were analyzed. The first mode represents Z-axis about 2% between the second and third mode frequency
planar displacement of the moving electrostatic plate. is due to the orientation of the coil spring anchor points.
The second and third mode represents the out-of-plane From the functional aspect of the microrelay, these fre-
rotation of the plate about its axes of symmetry. quencies are acceptable for its intended application.
First mode resonance occurs at about 370 Hz to However, considerable emphasis must be placed on the
500 Hz for suspension stiffnesses ranging from 0.0475 package vibration isolation requirements if the relay is
to 0.0900 µN/µm, corresponding to a pull-in voltage required to operate in an external environment having
range from 40 volts to 55 volts. Of particular interest, excitation frequency close to this range.
Large Displacement Electrostatically Actuated Microrelay 43
Fig. 6. Natural frequencies for different coil spring stiffnesses.
Stress analysis shows that the maximum Von-Mises It should be noted that in this design, the mechan-
stress occurs near the turning point of each spring seg- ical restoring force is the only mechanism to retrieve
ment. These stresses with the corresponding restoring the moving plate back to its initial position. This type
force of the coil spring suspension system are depicted of relay (Type A-SPST) will work in condition when
in Fig. 7. For the 50 volts pull-in voltage design, the low contact force is acceptable. If the environment has
Von-Mises stress is about 40% of the yield strength of moisture and propagates stiction, it will be difficult to
polysilicon (Fig. 8). These stress values could be fur- release the relay due to microweld formation in high
ther reduced by reducing the stress concentration effect current switching applications. Various analytical mod-
due to the sharp corners of the spring design through els for stiction/adhesion (depending on type) are avail-
filleting. able in literature. However, no numerical solution or
Fig. 7. Maximum Von Mises stresses and restoring forces of the coil springs.
44 Chong et al.
Fig. 8. Von Mises stresses for the device at 50 volts (maxima circled).
model exists that can explain or accurately predict stic- that through proper design, relatively large displace-
tion during fabrication/release and stiction during op- ments can be achieved using electrostatic actuation
eration. A device operated in a vacuum will have a mechanism. More research involving other practical
lower moisture content and will be less susceptible to aspects, material issues, and fabrication requirements
stiction. However, in the interest of limiting the scope can be done in the areas mentioned above.
of study, no research was performed on stiction issues.
Transient analyses (including determination of switch-
ing times and squeeze film damping effects) have not 5. Conclusions
yet been performed on the device. This design process
was dedicated to obtaining solutions to the steady-state This simulation based design study has demonstrated
aspects of the design (e.g., spring constants, plate par- that with a carefully designed suspension system, it is
allelism, pull-in voltage, etc.). The design parameters
possible to achieve large displacement via electrostatic
are limited by the solutions to these problems, and by
actuation using a relatively small actuation voltage
varying these parameters (within the usable domain
over small surface dimensions. Its relevance has been
determined during this process), the designer can opti- demonstrated on a microrelay with device specifica-
mize the transient results. The settling time for the relay tions meeting stringent telecommunication equipment
after it breaks the contact has not been investigated. If requirements. The unique quad-supported, two-
required, a physical breaking mechanism can be used dimensional coil spring suspension system is the key
to avoid the oscillatory motion that may result after the principle of achieving the low stiffness required for the
switch/contact breaks. Alternatively, a third electrode device to function over tight footprint size. The device
placed above the spring suspension can be used to at- mechanical characteristics are able to meet the device
tract the plate to its equilibrium position after the first
functional expectations. Improvement of the mechani-
break occurs. The contact materials (usually gold or
cal characteristics can be achieved through refining the
gold-alloys), which significantly affect device opera- surface geometry of the coil spring. A circular spiral
tion and lifetime, require investigations. This study has
coil spring design instead of a square coil spring would
focused on a design and its variations and has shown certainly improve the mechanical characteristics.
Large Displacement Electrostatically Actuated Microrelay 45
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39. He, Y., Marchetti, J. and Maseeh, F.,  MEMS computer-aided Technology (1993). He received his Masters of Science
design, in Proc. European Design & Test Conference and
(1994) and Ph.D. (1999) also in mechanical engineer-
Exhibition on Microfabrication, Paris, France, 1997.
ing from the State University of New York at Stony
Brook. Since 1996, while working at GT Equipment
Technologies, Inc., he conducted research for develop-
ing new systems and technologies for III V compound
semiconductors, bulk polysilicon growth, purification
of metallurgical grade silicon for photovoltaic indus-
try, critical point drying for MEMS, and supercriti-
cal fluid based photoresist removal for semiconductor
industry. After joining Corning Intellisense in 2000,
he has been conducting applied research, design and
development in MEMS technology. He holds various
Gooi Boon Chong received the B. Eng (Hons)
patents in MEMS and semiconductor technologies. His
degree in mechanical engineering from the National
publications and research interests in MEMS design
University of Singapore in 1989 and the M. Eng degree
and process include thermally and electrostatically ac-
in fluid mechanics from the Nanyang Technological
tuated devices, micro hot plate sensors, microrelays,
University, Singapore in 1992. He is presently an aca-
microswitches, micromirrors, stiction in MEMS, and
demic staff with the School of Engineering, Nanyang
microfluidics applications.
Polytechnic, Singapore. His current research interest
is in the design of MEMS devices for bio-medical
applications.
Kam See Hoon received the B. Eng (Hons) degree
in electrical engineering from the National University
of Singapore in 1990. He is presently an academic staff
with the School of Engineering, Nanyang Polytechnic,
Singapore. His current research interest is in the design
of MEMS devices used in photonics applications.
Daniel J. Keating received a Master of Science
degree in mechanical engineering from M.I.T. While
taking classes in Product Design and Development,
Inventions and Patents, Finite Element Analysis, and
Engineering Mathematics, he performed research in
the field of numerical model accuracy and verification
for the Engineering Analysis group (ESA-EA) at the
Los Alamos National Laboratory in New Mexico. He
also did his undergraduate studies at M.I.T., receiving a
Bachelor of Science degree in mechanical engineering.
Ijaz H. Jafri received his Bachelor of Science in He is presently Senior applications engineer at Corning
mechanical engineering from New York Institute of Intellisense.