LITERATURE REVIEW
2.1: Introduction
The promising field of MEMS technology has been evolved from the integrated
circuit industry. The most intrinsic characteristics are miniaturization, microelectronics
integration, and accurate mass production. MEMS technology facilitates it possible to
manufacture electromechanical and microelectronics component in a single tiny device
ranging from 1µm to 1mm. The decrease in dimension of electromechanical systems
provides benefits like more reliability, more resonant frequency, and low thermal mass,
and hence, there exist significant reduction in power consumption.
In MEMS, while the electronics are manufactured by employing integrated circuit
processes, the micromechanical components are constructed using micromachining
processes that selectively etch away silicon wafer parts or make more structural layers to
form the electromechanical devices. The mechanical sensors and actuators, and electronic
controllers and microprocessors have been demonstrated to be fabricated on a single
substrate. Lithography guarantees the accurate dimension and position. The batch based
fabrication processes have the possibility to get large volume fabrication and therefore,
there is reduction in expenditure and considerable enhancement in the yield and
reliability.
With the development of the micromachining techniques, many multifaceted
devices have been produced and some of them have been already in the market. For
example, Analog Devices Inc. created the AD-XL50 micro-accelerometer for application
in automobile airbag deployment systems using surface micromachining. MEMS
technology has also gained the concentration of the wireless communication industry due
to its benefits such as low expenditure, more performance, compact size and weight, and
increased reliability. Therefore, a wonderful research is focused on the progress of
MEMS based components in the wireless and microwave technology area.
RF Switches and Relays CoventorWare offers a widespread environment for
designing and analyzing RF switches and relays, and comes with an extensive discussion
to design RF switches. ARCHITECT is the place to start for switches and relays that are
actuated with electrostatic or piezoelectric effects. It is only one of its kinds in offering a
single environment where you can perform all phases of design and analysis: high-level
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concept exploration, detailed geometry design, coupling with control circuitry, and
evaluation of S-parameters. With its complete electromechanical component library, one
can swiftly assemble a design with full specifications.
Nowadays, numerous MEMS switches, capacitors, inductors are being reported to
be integrated to make various RF devices. Devices such as phase shifters, switching and
reconfigurable networks, low power oscillators and varactors are being manufactured for
wireless communication industries. A large number of various RF-MEMS are being
patented, reported, and demonstrated now.
2.2: History of RF-MEMS
“There is plenty of room at the Bottom” – R. Feynman gives a milestone
presentation at California Institute of Technology in 1959. The first silicon pressure was
demonstrated in 1961. In 1967, there occurs invention of surface micromachining, a
fabrication technique. The first silicon accelerometer was demonstrated in 1970. Since
1970s, MEMS based sensors like pressure sensors, temperature sensors, accelerometers,
gas chromatographs have been developed. In 1982, LIGA process was invented.
The first critical review of silicon as mechanical material, in addition to electronic
material, was published by Kurt Petersen in 1982. Howe first proved the evolution of a
very useful method to build micromechanical elements using technologies that were
developed first to build microelectronic devices for the integrated circuits in influential
work done in 1982. R. T. Howe reported techniques to fabricate micro-beams from polysilicon layers. With the motivation of this demonstration, Howe constructed the first trial
product poly-silicon MEMS, a chemical vapor sensor that was a totally integrated microelectromechanical system.
First MEMS conference was organized in 1988. After four years, MCNC started
the multi-user MEMS processes (MUMPS) sponsored by Defense Advanced Research
Projects Agency (DARPA). Then, the MEMS technology is integrated with RF circuitry
fabrication that results in ground-breaking development in miniaturization, improved
performance, and lower expenditure of fabrication. These better performance RF-MEMS
devices made their ways in widespread areas of commercial, aerospace, and defense
fields, wireless communication systems, radar systems and satellite communication
systems.
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RF-MEMS have observed wonderful progress in the past 10 years due to its huge
commercial and defense potential. However, there was incredible progress in GaAs
HEMT devices (high-electron mobility transistor) and in silicon CMOS (complementary
metal-oxide-semiconductor) transistors; there was hardly any progress in semiconductor
switching diodes from 1985 to 2000. In 1980, the cut-off frequency of silicon CMOS
transistors was around 500 MHz and is currently around 100 GHz. Also, in 1980, the
cutoff frequency of GaAs HEMT devices was 10–20 GHz and is now above 800 GHz.
On the other hand, the cutoff frequency of GaAs or InP p-i-n diodes enhanced from
around 500 GHz in 1985 to about 2000 GHz in 2001. Clearly, a fundamental new
technology was needed to push the cutoff frequency of switching devices to 40,000 GHz
for low-loss applications, and this can happen with RF-MEMS technologies.
A survey of RF-MEMS research leads to four distinct areas as illustrated in the
Figure 2.1 below: -(i) RF-MEMS switches, varactors, and inductors that have been reported from
DC-120 GHz and are now a comparatively grown-up technology. Apart from the micromachined inductors, MEMS switches and varactors, when actuated, reallocate several
micrometers.
(ii) Micro-machined transmission lines, high-Q resonators, filters, and antennas
that are appropriate for 12–200 GHz. They are usually integrated with bulk
micromachining of silicon or, on thin dielectric membranes, but are motionless.
(iii) Thin film bulk acoustic resonators (FBARs) and filters that use acoustic
vibrations in thin films and that have reported exceptional performance up to 3 GHz with
very high Q > 2000. In recent times, FBAR technology leads to small low-loss filters for
wireless communication.
(iv) RF micromechanical resonators and filters that employ the mechanical
vibrations of very tiny beams to achieve high-Q resonance for 0.01-200 MHz in vacuum.
The mechanical movements are of the order of tens of angstroms. Very high-Q resonators
(>8000) have been made employing this technology up to 200 MHz, but two-pole filters
have only been reported up to 10 MHz applications. This technology still requires
frequent labor before it can be used in commercial applications in tiny 0.1-3 GHz filters.
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(a)
(b)
(c)
(d)
Figure 2.1 (a): Agilent film bulk acoustic resonator,
(b): University of Michigan micro-machined membrane filter,
(c): SEM of Radant MEMS switch, and
(d): A two-pole 7-MHz filter based on micromechanical resonators.
Coventor has worked closely with leading semiconductor companies, specialty
RF component suppliers, and research organizations to support their development of RF
MEMS. Our commercial and university customers have used CoventorWare to design an
amazing variety of RF switches, relays, resonators and varactors.
2.3: RF-MEMS Switch
Most common and basic circuit component, demonstrated and developed is RFMEMS switch. RF-MEMS technology has matured in fields like design, fabrication, and
packaging of the device. As a result, devices with high reliability, much better
performance, good isolation, lower insertion loss and better power handling were
developed. In 1991, first MEMS switch and the varactor was developed by L. Larson at
Hughes Research Labs in Malibu under the support of DARPA. It had shown elegant
performance up to 50 GHz that is much superior to GaAs devices.
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During 1995, Rockwell Science Center demonstrated a metal-to-metal contact
switch that is proper for DC-60GHz applications. Texas Instruments demonstrated a
contact shunt switch, also called Raytheon shunt switch that is proper for 10-120 GHz
applications. By 1998, Nguyen and Pacheco research groups at University of Michigan
developed low voltage capacitive shunt switches employing low spring constant beam.
Some examples of RF-MEMS switches are shown in Figure 2.2.
Northeastern University, MIT Lincoln Labs, Columbia University, Analog
Devices, Northop Grumman, and many more, were energetically reporting RF-MEMS
devices by1999. By 2001, more than 30 companies working in this field including the
giant consumer electronics firms like Motorola, Analog Device, Samsung, Omron, NEC
and ST-Microelectronics. Several new switch architectures have also been reported,
including the switches with air-bridge structure.
(a)
(b)
Figure 2.2(a): Schematic of the capacitive shunt switch using PZT/HFO2
dielectrics.
(b): A micrograph of a capacitive coupling MEMS RF switch (from
Raytheon/Texas Instruments (TI)).
OMRON, Japan demonstrated an electrostatically actuated mono-crystalline
silicon membrane switch that has good performance from DC to a few GHz. Korea
Advanced Institute of Science and Technology (KAIST) presented another switch with
very low actuation voltage. Samsung internal research institute of Technology (SAIT)
reported an electrostatically actuated switch operating on very low actuation voltages (~ 4
25
volts). The electrostatically actuated cantilever switch with good RF performance, and
operating at 30-60 volts, was demonstrated by Motorola, AZ, USA.
The first commercially qualified MEMS switch was announced by TeraVicta
Technologies in 2005. This switch based on high force disk actuator (HFDA) has
characteristic mean cycle before failure (MCBF) of approximately 200 million cycles, 20
times higher than the best electromechanical relays. Radant switch for US department of
defense had surpassed 200 billion cycles mark. The future of MEMS switches in the
coming years will be driven by three key factors: substantial improvement in reliability,
significant reductions in size and cost, and a wide variety of products for diverse
applications.
H. Kawai developed an RF-MEMS switch that includes movable electrodes
disposed with a space provided there in the direction of RF signal conduction of an RF
signal-conducting unit. A displacing unit displaces all the movable electrodes at the same
time in the same direction towards or away from the RF signal-conducting unit. The
electrical length of this conducting unit sandwiched between the movable electrodes is set
such that the amplitude of a combined signal including signals reflected in positions of
the RF signal-conducting unit facing the movable electrodes is less than the amplitude of
each of reflected signals reflected in positions of the signal-conducting unit.
R. A. Gilbert reported an RF-actuated MEMS switching element for use with
switchable RF structures like antennas and reflectors. An antenna within each MEMS
switch is coupled to a circuit that provides a trigger voltage based on an RF control signal
received at the antenna. The trigger voltage output of the circuit is used to control the
switch. This allows arrays of switches to be actuated by remotely generated RF signals
thus alleviating the need for running metallic conductors or optical fibers to each switch.
Frequency response characteristics, phasing, reflectivity, and directionality characteristics
may be altered in real-time.
J. H. Schaffner et al. reported an RF-MEMS switch with integrated impedance
matching structure. An impedance matching structure for an RF-MEMS switch having at
least one closeable RF contact in an RF line, the impedance matching structure
comprising a protuberance in the RF line immediately adjacent the RF contact that forms
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one element of a capacitor, the other element of which is formed by the switch's ground
plane.
J. Y. Park et al. disclosed an RF-MEMS switch and a fabrication method thereof.
According to an embodiment this RF-MEMS switch is actuated with a low voltage and a
low consumption power by using a piezoelectric capacitor actuated by being converted to
mechanical energy from electric energy when an electric field is applied to the
piezoelectric capacitor. A cap substrate can be formed by using an etching method, a
chemical mechanical polishing method, an electroplating method, etc., and the RFMEMS switch has a high reliability and a high yield.
Ivanov et al. invented an integrated MEMS switch that provides a switch that is
formed on a semiconductor substrate. The semiconductor substrate has an insulated
handle wafer and a device layer over the insulator layer. Active semiconductor devices,
like transistors and diodes, are formed in device layer. The switch is formed over the
device layer during fabrication of the semiconductor device. Additional layers, such as
connecting layers, passivation layers, and dielectric layers, are inserted among these
layers. The present invention avoids the need to fabricate switches apart from the devices
that contain associated circuitry, and so, to mount the switches to modules that circuitry.
C. B. Freidhoff et al. invented a MEMS piezoelectric switch that has an
articulated unimorph bridge attached to a substrate. The bridge includes a passive layer of
Zirconia and at least one silicon-based material, an active layer of a piezo-electric
material that has a high piezoelectric coefficient, at least one pair of interdigitated
electrodes, disposed on the top surface of the active layer and across which the bias
voltage is applied, and a top contact electrode. A bottom contact electrode is provided on
the substrate, and signals flow through the switch when the top and bottom contact
electrodes contact one another.
D. Peroulis et al. reported a low cost process-independent RF-MEMS switch. The
switch can be fabricated with very high yield despite the high variability of the
manufacturing process parameters. It is fabricated with mono-crystalline material, e.g.,
silicon, as the moving portion. Its fabrication process is compatible with CMOS
electronics fabricated on Silicon-on-Insulator (SOI) substrates. It comprises a movable
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portion having conductive portion selectively positioned with a bias voltage to
conductively bridge a gap in a signal line.
Song et al. invented an RF-MEMS switch using semiconductor that includes two
electrodes coupled to two terminals of the power source. A semiconductor layer
combined with upper surface of the first electrode forms a potential barrier to become
insulated when a bias signal is applied from the power source. A second electrode
disposed at a predetermined distance away from the semiconductor layer, and it contacts
the semiconductor layer when a bias signal is applied from the power source. Therefore,
although the bias signal may not be cut off, free electrons and holes are recombined in the
semiconductor layer, whereby charge buildup and sticking can be prevented.
2.4: Tunable (Variable) Capacitors
MEMS technology is used for RF applications in the area of variable capacitors,
as a replacement for varactor diodes as tuners. In 1996, Young and Boser published a
gap-tuning capacitor using a surface micromachining technique. One electrode is
stationary on top of the substrate, and the other electrode, supported by micromachined
springs, is movable in the vertical direction normal to the substrate. The gap between the
movable and stationary electrodes is electrostatically adjusted by applying a tuning
voltage between the electrodes.
Dec and Suyama proposed a three-plate parallel plate tunable capacitor in 1997 to
increase this theoretical MTR (maximum tuning range) limit from 50% to 100%. Figure
2.3(a) shows a top-view image of this tunable capacitor. The three-plate parallel plate
system has two fixed plates; one suspended over the other. A third movable plate is made
in between the two fixed plates, with air gaps on its two sides.
Young et al., in 1998, demonstrated a tunable capacitor planned for monolithic
low-noise voltage controlled oscillators (VCOS). In theory, a tuning range of up to 50%
can be attained. However, experimentally the above capacitor illustrates that the tuning
range is only 16%. Yao et al (at the RSC) in 1998 published a MEMS area-tuning
capacitor based on suspended, massively parallel, inter-digitated comb structures
fabricated using a deep reactive ion etch (DRIE) of single-crystal silicon which is later
coated with metal thin film. One set of the comb structures is stationary and the other
movable.
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Z. Feng et al. reported a two-plate parallel plate tunable capacitor using an
electro-thermal actuator instead of the electrostatic actuator. Figure 2.3(b) shows a topview image of their tunable capacitor.
T. Y. Hsu et al., in 2008, developed a continuously tunable RF-MEMS capacitor
with ultra wide tuning range.
A method is provided of continuously varying the
capacitance of a MEMS varactor having a cantilever assembly mounted on a base
portion, the cantilever assembly having a first capacitance plate and a dielectric element
mounted thereon, and the base portion having a second capacitance plate mounted
thereon. The method includes applying a first actuation voltage to deform the cantilever
assembly until the dielectric element contacts the second capacitance plate leaving a gap
there, and applying a second actuation voltage larger than the first actuation voltage to
further deform the cantilever assembly to reduce the gap between the dielectric element
and the second capacitance plate.
(a)
(b)
Figure 2.3(a): A top-view image of the three-plate gap-tuning variable capacitor
developed by Dec and Suyama.
(b): A top-view image of the University of Colorado gap-tuning
variable capacitor using an electro-thermal actuator.
P. G. Steeneken et al. disclosed tunable capacitors using fluid dielectrics. The
dielectric medium change results in a change in the total dielectric constant of the
material between the electrodes (thus changing the capacitance of the capacitors.
Transporting or phase changing the dielectric fluids into and out of the electric field of
the capacitor, changes the effective dielectric constant and the capacitance of the
capacitor.
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Another new tunable capacitor with one suspended top plate and two fixed bottom
plates has been proposed by Jun Zou et al. Out of the two fixed plates and the top plate
constitute a variable capacitor, whereas the other fixed plate and the top plate are used to
supply electrostatic actuation for capacitance tuning. The convenient tuning range of 68%
is achieved practically. Goldsmith et al reported a similar device using bistable MEMS
membrane capacitor. By appropriately designing the device and the thickness of the
dielectric film, a high tuning ratio of 22:1 was achieved between the two stable states of
the membrane capacitor.
A. T. Hunt et al. demonstrated a tunable MEMS capacitor that comprises of two
capacitor electrodes out of which, one is movable by a switch to change the capacitor
dielectric spacing, and so, tune the capacitance. A tunable dielectric material and a nontunable dielectric material are in series between the electrodes. A third electrode
electrically controls tunable dielectric material. A controller is used to vary the
capacitance dielectric spacing for adjustment of the capacitance of the MEMS capacitor,
and to tune the dielectric material for adjustment of the capacitance, thereby to provide a
continuous analogue range of adjustment including the first and second ranges. This
arrangement provides independent control of the MEMS function and the dielectric
tuning function, and enables a continuous adjustability.
T. Shimanouchi et al. reported a variable capacitor which is appropriate for
suppressing fluctuation in driving voltage characteristic and for achieving a larger
variation ratio of static capacitance. The variable capacitor includes a fixed electrode and
a movable electrode. The fixed electrode includes a first opposing face, while the
movable electrode includes a second opposing face that faces the first opposing face. The
movable electrode further includes a curved portion that protrudes toward the fixed
electrode. The variable capacitor also includes a dielectric pattern provided on the first
opposing face.
Y. Furukawa et al. invented a tunable capacitor that comprises two electrodes and
a dielectric arranged between electrodes. The dielectric comprises dielectric material
having a value of a relative dielectric constant varying at least within the operation
temperature range. The invention relies on the idea of varying temperature to vary a
capacitance.
Advantageous are high-tuning ratio, small device area, and stable
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capacitance value in case the temperature is well controlled. The invention further relates
to a semiconductor device comprising the electronic device, and to a method of
manufacturing such electronic device.
2.5: Film Bulk Acoustic Resonators (FBARs)
Resonators are the fundamental building block for more complex components like
duplexers, filters, and oscillators (frequency or timing sources). MEMS resonators for
duplexers and filters typically exploit Bulk Acoustic Wave (BAW) resonance modes in a
suspended or otherwise isolated volume of material. The MEMMech solver in
ANALYZER is ideal for simulating BAW-based resonators, including those
characterized as Film Bulk Acoustic Resonators (FBARs). Oscillators tend to rely on
mechanical resonance modes (bending modes in beam- or plate-like parts, possibly in
combination with mass effects).
A quartz crystal resonator is a famous example of traditional bulk acoustic wave
(BAW) resonator. Film Bulk Acoustic Resonators (FBARs) are the micro-machined or
MEMS version of traditional BAWs. The progress of FBARs already started over twenty
years ago. The rapid development of wireless communication systems has yielded
improved interest in the development of FBARs for use in RF oscillators and microwave
filters.
A 30 GHz cavity resonator was demonstrated by Kim et al. with a quality factor
of 120. At lower frequencies, cavity resonators become impractical due to their excessive
large dimensions. Discera, Silicon Clocks and SiTime are some of the companies going
in production of these resonators. These resonators are very promising as a replacement
for conventional off chip quartz reference oscillators. However this technology is new
and will take time before rapid deployment.
J. D. Larson III et al., invented a temperature-compensated film bulk acoustic
resonator (FBAR) device comprises an FBAR stack that comprises an FBAR
characterized by a resonant frequency having a temperature coefficient and a
temperature-compensating layer comprising doped silicon dioxide. The FBAR comprises
opposed planar electrodes and a piezoelectric element between the electrodes. The
piezoelectric element has a temperature coefficient on which the temperature coefficient
of the resonant frequency of the FBAR depends at least in part.
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Figure 2.4(a):Cross-sections of a membrane supported film bulk-acoustic
resonator (FBAR).
(b):Cross-sectional view of a solidly mounted bulk-acoustic
resonator (SMR).
Figure 2.4(a) shows a membrane supported FBAR which is published by S.V.
Krishnaswamy et al. Membrane materials like silicon nitride, silicon oxide or p+-silicon,
are used. Such membrane supported FBARs using AlN films have been demonstrated to
operate at resonant frequencies in the low-GHz range with a few hundreds of quality
factor. Figure 2.4(b) illustrates a solidly mounted resonator which is reported by K.M.
Lakin, et al.
Y. K. Park et al. created an air-gap type film bulk acoustic resonator (FBAR) by
securing two substrate parts, one providing a resonance structure and the other providing
a separation structure, i.e., a cavity. When the two substrate parts are secured, the
resonance structure is over the cavity, forming an air gap isolating the resonant structure
from the support substrate. The FBAR may be used to form a duplexer, which includes a
plurality of resonance structures, a corresponding plurality of cavities, and an isolation
part formed between the cavities. The separate creation of the resonance structures and
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the cavities both simplifies processing and allows additional elements to be readily
integrated in the cavities.
J. D. Larson III et al. reported a film bulk acoustic resonator package and method
of fabricating same. A micro-fabricated device has a first substrate, a second substrate, a
film bulk acoustic resonator (FBAR device, and a circuit. The second substrate is bonded
to the first substrate to define a chamber. The FBAR device is located on a surface of the
first substrate and inside the chamber. The circuit is located on a surface of the second
substrate and inside the chamber. An electrical connection connects the circuit and the
FBAR device.
Shin et al. demonstrated a tunable resonator that includes a film bulk acoustic
resonator (FBAR) for performing a resonance, and at least one driver which is arranged
at a side of the FBAR and is deformed and brought into contact with the FBAR by an
external signal, thereby changing a resonance frequency of the FBAR. Accordingly, a
multiband integration and a one-chip manufacture can be implemented simply using a
micro electro mechanical system (MEMS) technology and a mass production is possible.
R. C. Ruby et al invented a manufacturing process for fabricating an acoustical
resonator on a substrate having a top surface. First, a depression in said top surface is
generated. Next, the depression is filled with a sacrificial material. The filled depression
has an upper surface level with said top surface of said substrate. Next, a first electrode is
deposited on said upper surface. Then, a layer of piezoelectric material is deposited on
said first electrode. A second electrode is deposited on the layer of piezoelectric material
using a mass load lift-off process.
J. D. Larson III et al. demonstrated FBAR devices with simplified packaging. The
encapsulated film bulk acoustic resonator device comprises a substrate, an FBAR stack
over it, an element for acoustically isolating the FBAR stack from the substrate,
encapsulant covering the FBAR stack, and an acoustic Bragg reflector between them. The
FBAR stack comprises an FBAR and has a top surface remote from the substrate. The
FBAR comprises opposed planar electrodes and a piezoelectric element between the
electrodes. The acoustic Bragg reflector comprises a metal Bragg layer and a plastic
Bragg layer juxtaposed with the metal Bragg layer. The large ratio between the acoustic
impedances of the metal of the metal Bragg layer and the plastic material of the plastic
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Bragg layer enables the acoustic Bragg reflector to provide sufficient acoustic isolation
between the FBAR and the encapsulant for the frequency response of the FBAR device.
2.6: High-Q inductors
Integrated inductors with high-performance are in more and more demand in
modern communications goods. Key parameters in integrated inductors are the quality
factor Q and the self-resonance frequency. Quality factor peaks as the device transforms
from inductive to capacitive characteristics. Abidi’s research group at UCLA presented a
paper that illustrates a spiral inductor made on a 2 μm CMOS silicon wafer with the
portion of silicon substrate directly beneath the inductor removed to reduce the substrate
parasitic effect. Suspended spiral inductor is developed by Chang et al.
T. Matsumoto et al. reported an inductor element and an integrated electronic
component that facilitate achieving a higher Q value are provided. The inductor element
includes a substrate, a coil unit spaced from the substrate, and a plurality of conductive
columns. The coil unit includes a plurality of spiral coils each constituted of a spiralshaped coil lead. The spiral coils are disposed such that their winding directions are the
same, and that the coil lead of each spiral coil includes a portion extending between the
coil lead of at least one of the other spiral coils. End portions of the spiral coils are fixed
to the substrate via the conductive columns. The integrated electronic component of the
invention includes such inductor element.
I. S. Song invented an inductor element having a high quality factor. The inductor
element includes an inductor helically formed on a semiconductor substrate and a
magnetic material film on a surface of the inductor for inducing magnetic flux generated
by the inductor. The magnetic material film includes a first magnetic material film
disposed on a lower surface of the inductor, between the substrate and the inductor, and a
second magnetic material film disposed on an upper surface of the inductor. The
magnetic material film may be patterned according to a direction along which the
magnetic flux flows radially. Since the magnetic flux proceeding toward the upper part
and lower part of the inductor is induced by the magnetic material film, the effect of the
magnetic flux generated in the inductor on external circuits may be reduced and the
efficiency of the inductor may be enhanced.
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Later, a gas-phase based isotropic etching method of the silicon substrate was
demonstrated that improves resonance frequency and quality factor of the inductor. Ziaie
et al. demonstrated another micro-machined inductor for RF applications in the
meantime. Yoon et al demonstrated a 3D spiral inductor with Cu metal lines on a glass
substrate, as shown in Figure 2.5 (a). In this 3D fabrication method, plated Ni is used as
the moulding material for Cu plating. Also in 1999, Young et al demonstrated a 3D coil
inductor with a low-loss alumina core that is illustrated in Figure 2.5 (b).
(a)
(b)
Figure 2.5 (a): Three Dimensional Cu spiral inductors made on a glass substrate.
(b): Three Dimensional Berkeley Cu coil inductor with a low-loss alumina core.
H. Ishikawa invented a variable inductor that includes a conductor and an electroconductive member. The conductor has a coil and a pair of terminals electrically
connected with the coil. The electro-conductive member is movable closer to and farther
away from the coil. The inductance between the terminals becomes smaller as the
distance between the coil and the electro-conductive member becomes shorter.
Conversely, the inductance between the terminals becomes larger as the distance between
the coil and the electro-conductive member becomes longer.
T. Weller et al. reported a tunable micro electromechanical inductor. This
invention provides a monolithic inductor developed using radio frequency micro
electromechanical techniques. In a particular embodiment of the present invention, a
tunable radio frequency micro-electromechanical inductor includes a coplanar waveguide
and at least one direct current actuate-able contact switch positioned to vary the effective
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width of a narrow inductive section of the center conductor of the CPW line upon
actuation the DC contact switch.
M. Hargrove et al. reported an integrated variable inductor that is achieved by
placing a second closed-loop inductor immediately above or below a primary inductor.
The closed-loop configuration of the second inductor may be broken on-chip by several
ways, including use of a transistor to selectively short together both ends of the second
inductor. If one wishes to alter the inherent inductance characteristics of the primary
inductor, the transistor coupling both ends of the second inductor is actuated. Thus, a
current applied to the primary inductor induces a current in the second inductor by
inductive coupling. The second current in the second inductor then alters the impedance
of the primary inductor by mutual inductance. Thus, the inductance value of the primary
inductor is altered.
T. Weller et al. also reported a tunable micro electromechanical inductor. This
invention provides a monolithic inductor developed using RF-MEMS techniques. In a
particular embodiment of the present invention, a tunable RF-MEM inductor includes a
coplanar waveguide and a direct current actuate-able contact switch positioned to vary
the effective width of a narrow inductive section of the center conductor of the CPW line
upon actuation the DC contact switch. In a specific embodiment of the present invention,
the direct current actuate-able contact switch is a diamond air-bridge integrated on an
alumina substrate to realize an RF switch in the CPW and micro-strip topology.
H. L. Stalford et al. reported a micro-electromechanical tunable inductor is
formed from a pair of substantially-identically-sized coils arranged side by side and
coiled up about a central axis which is parallel to a supporting substrate. An in-plane
stress gradient is responsible for coiling up the coils which. The inductance provided by
the tunable inductor can be electro-statically changed either continuously or in discrete
steps using electrodes on the substrate and on each coil. The tunable inductor can be
formed with processes which are compatible with conventional IC fabrication so that, in
some cases, the tunable inductor can be formed on a semiconductor substrate alongside or
on top of an IC.
S. Y. Jeon et al. demonstrated a variable inductor that includes a first lead having
both ends to receive a pair of difference signals, a second lead having both ends to
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receive a pair of the difference signals, and a switch selectively supplying a pair of the
difference signals to the second lead by turning on/off according to a control signal.
Accordingly, a variable inductor can be implemented that is compact and maximizes the
variation rate of inductance.
2.7: Antenna
SPST switches or varactor diodes can be eagerly replaced by RF-MEMS switches
and varactors in order to take benefits of the low insertion loss and high Q-factor
provided by RF-MEMS technology. In addition, RF-MEMS components can be
incorporated on low-loss dielectric substrates, (like LCP, borosilicate glass, or fused
silica), while III-V semi-conducting silicon substrates exhibit normally more losses and
have a higher dielectric constant.
Literature illustrates a number of important examples of antenna. For example, an
RF-MEMS frequency tunable antenna for the 0.1-6 GHz frequency range, an antenna
with integrating RF-MEMS switches on a gasket (Sierpinski) antenna for 8-25 GHz
frequency range, an RF-MEMS radiation pattern reconfigurable spiral antenna for the 67 GHz frequency range, and a two-bit Ka-band RF-MEMS frequency tunable slot
antenna. E. Erdil et al. developed reconfigurable rectangular slot antenna loaded with
MEMS capacitors (Figure 2.6 (a)). The same group also demonstrated the patch antenna
loaded with MEMS capacitors (Figure 2.6 (b)).
(a)
(b)
Figure 2.6 (a): The photograph of the rectangular slot antenna loaded with MEMS
capacitors.
(b): The photograph of the patch antenna loaded with MEMS capacitors.
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J. H. Schaffner et al demonstrated an antenna system including a Luneberg Lens
having a spherically shaped outer surface and a spherically shaped focal surface spaced
from its outer surface with a plurality of patch antenna elements disposed along the focal
surface of the Luneberg Lens; and a power combiner for combining signals received by
said plurality of patch antenna elements. The disclosed antenna system may be used a
part of a robust GPS system having a plurality of GPS satellites each transmitting a GPS
signal; a plurality of airborne GPS platforms, each GPS platform including a GPS
transmitter for transmitting its own GPS signal, the GPS signals being transmitted from
the plurality of airborne GPS platforms being differentiated from the GPS signals
transmitted by visible GPS satellites; and at least one terrestrially located GPS receiver
for receiving the GPS signals transmitted by visible ones of the GPS satellites and by
visible ones of said airborne GPS platforms.
W. K. Choi et al. reported a micro-strip patch antenna formed by using a micro+electro-mechanical system technology. The micro-strip patch antenna includes: a
substrate provided with a ground formed on a bottom surface of the substrate; a feeding
line formed on a top surface of the substrate for feeding an electric power; a coupling
stub formed on the top surface of the substrate and electrically connected to the feeding
line; a plurality of supporting posts erected on the top surface of the substrate; and a
radiating patch formed on the supporting posts, thereby forming an area of air between
the radiating patch and the top surface of the substrate.
A. Tran et al. developed a Micro-electromechanical (MEM) switch antenna. A
MEMS antenna is provided comprising a dielectric layer, and a conductive line radiator
formed overlying the dielectric layer including at least one selectively connectable
MEMS conductive section to vary the length of the radiator. The antenna includes a
plurality of selectively connectable MEMS conductive sections and a plurality of fixedlength conductive section. The MEMS conductive sections may be parallel aligned along
the radiator width, and/or parallel aligned along the radiator length.
J. P. Ebling et al demonstrated an antenna that comprises a dielectric material
having first and second surfaces, a discrete lens array operatively coupled to the first
surface, and at least one broadside feed antenna operatively coupled to the second
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surface. Figure 2.7 illustrates general view of the rectangular slot antenna loaded with six
MEMS cantilever type capacitors.
Figure 2.7: Schematic of the rectangular slot antenna loaded with six MEMS cantilever
type capacitors.
Qing Ma et al. invented slot antenna having a MEMS varactor for resonating
frequency tuning. Briefly, a slot antenna may include a primary slot and one or more
secondary slots. The size of the antenna may be reduced by adding one or more of the
secondary slots which may add additional inductance to the antenna. Furthermore, the
size of the antenna may be reduced by increasing the inductance of the secondary slots
via increasing the length of the slots or by changing the shape of the slots. The antenna
may include one or more MEMS varactors coupled to one or more of the secondary slots.
The resonant frequency of the slot antenna may be tuned to a desired frequency by
changing the capacitance value of one or more of the MEMS varactors.
D. Anagnostou et al. invented a reconfigurable multi-frequency antenna with RFMEMS switches. A self-similar multiband reconfigurable antenna includes a planar
antenna structure formed on a surface of a substrate, the antenna structure including
symmetrically opposed self-similar geometry antenna arms defining a self-similar or
Sierpinski gasket configuration for each arm of the antenna. MEMS type switches are
provided for operatively connecting adjacent antenna patches on each arm of the antenna
configuration, and a voltage source is provided for selectively actuating the switches.
Selective actuation of the switches enables up to four different antenna configurations
each having a different resonant frequency, and wherein each resonant frequency
demonstrates a similar radiation pattern.
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D. D. Haziza devised an apparatus and method for antenna RF feed. An RF feed
is provided which is structured as a curved reflector coupled to a sidewall of a waveguide
cavity. A radiation source is situated facing the curved reflector. The RF feed may be
coupled to a waveguide cavity having radiation elements coupled to top surface thereof,
to thereby feed an antenna array. When an antenna array is used, several curved reflector
RF feeds may be used, operating in the same or different frequencies.
A. M. Kinghorn et al. invented an antenna to decrease the complexity and cost of
antenna arrays. It provides an antenna array made up of a vertical stack of horizontal
linear structures each having several groups of neighboring array antenna elements with
variable numbers, each connected to a transmit/receive module . The advantages of this
configuration is that less communication modules (like transmit/receive modules) are
required to operate the antenna array, reducing the weight, power consumption and cost
of an antenna apparatus incorporating such an antenna array without significantly
limiting the capability and/or performance of a system.
2.8: Phase shifters
A phase shifter is a control circuit found in many microwave communication,
radar and measurement systems. Traditionally, one of the main reasons why conventional
MMIC technology came about was because of the need to miniaturize phase shifters so
that they could be easily integrated into compact phased antenna arrays. Theoretically,
phase shifters could be positioned directly between the antennas’ radiating elements and
their associated T/R modules, to create a fully distributed two dimensional phased array
antenna system. Low DC control power and repeatable batch processing are important
goals for phased array applications.
RF-MEMS digital delay lines were reported in many research papers in the last
decade. Pillians et al. reported a four-bit monolithic switched-line delay line with microstrip on high resistivity silicon. At this time, MEMS capacitive membrane switches were
used, with CON/COFF~100 and an actuation voltage of 45V. Rockwell Science Center has
demonstrated high performance DC to 40GHz 3-bit and 4-bit true time delay networks,
using SPDT switches on GaAs. This state-of-the-art RF-MEMS phase shifter is shown in
Figure 2.8.
A two-stage two-bit reflection-type delay line, with tapped delay line
reflection terminations was also reported by Malczewski et al. The same transmission
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line medium, substrate and MEMS switches were used with the earlier switched line
example.
Figure 2.8: Micrograph of a DC to 40 GHz four-bit true time delay network on
GaAs.
Mechanically or magnetically tuned phase shifters needs much power, gives low
tuning speed, and are bulky due to its macro-scale tuning mechanism. Electronically
tuned phase shifters are good candidates for highly integrated systems due to fast tuning
and compactness. On the other hand, due to losses and non-linearity linked with p-n
junctions and MOS structures and the complication in biasing, these may be deficient in
application to current wireless communication. These applications require less cost, tiny
size, small power consumption millimeter-wave devices and circuits.
Figure 2.9: The Four-bit miniature X-band phase shifter developed by the Univ. of
Michigan and Rockwell Scientific.
Figure 2.9 presents a four-bit miniature X-band phase shifter (of size
3.2×2.1mm2)
made by the University of Michigan and Rockwell Scientific. Passive
sub-arrays using RF-MEMS phase shifters are used to reduce the T/R modules in active
electronically scanned sub-arrays. RF-MEMS phase shifters facilitate passive
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electronically scanned sub-arrays, such as reflect array, lens array, switched beam
forming networks etc, with high radiated power. Early passive electronically scanned
arrays, includes a W-band switched beam forming network based on an RF-MEMS SP4T
switch, an X-band 2-D lens array having parallel-plate waveguides and 25,000 ohmic
cantilever RF-MEMS switches and an X-band continuous transverse stub (CTS) array
using sixteen 5-bit reflect-type RF-MEMS phase shifters.
A V-band 2-bit phase shifter is designed with three delay lines providing
90°/180°/270° phase shift in relation to the reference line at 60 GHz as shown in figure
2.10 (a). These four delay lines are connected to the input and output using two SP4T RFMEMS switches. While in operation, each phase delay path is chosen by actuating the
corresponding two throws. Figure 2.10 (b) illustrates a 35 GHz, 1-bit switched line phase
shifter (180°bit).
(a)
(b)
Figure 2.10 (a): A V-band two-bit phase shifter.
(b): A 35 GHz, one-bit switched line phase shifter (180°bit).
2.9: Filters
A significant size is reduced by using tunable RF band-pass filters instead of
switched RF filters. They can be integrated using switches, switched capacitors, III-V
semiconductor variable capacitors, PZT ferroelectric and RF-MEMS resonators. RFMEMS resonators provide the prospective of integration of high-Q resonators and lowloss band-pass filters. The Q factor of RF-MEMS resonators is in the order of 100-1000.
RF-MEMS technology provides the tunable filter designer a transaction among power
consumption, power handling, insertion loss, switching time, linearity, and size.
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Figure 2.11: Design of a UHF five-pole filter (die size is 3.5 mm into 14 mm).
Modern wireless or satellite communication, radar, electronic warfare, and
instrumentation, all demand tunable filters for flexible and adaptive operations over wide
frequency range. RF band-pass filter is employed to enhance band rejection, in case the
antenna fails to give sufficient selectivity. RF band-pass filters based on lumped bulk
acoustic wave, ceramic, SAW, quartz crystal, and thin film bulk acoustic resonators have
performed better than distributed RF filters based on waveguide cavities, and
transmission lines. The following Figure 2.11 shows a design of the UHF five-pole filter
(die size is 3.5 mm into 14 mm).
2.10: Transmission Lines
For high-power microwave applications, micro-strip transmission lines and filters
have been demonstrated on a fused quartz substrate using the LIGA process. Metal films
between 10 and 500 μm in thickness were plated to form the transmission lines with a
substrate thickness of 420 μm. This thick metal structure makes possible high-power
applications. Transmission lines, as well as low-pass and bandpass microwave filters,
have been fabricated using this process technology and characterized. Alternatively, for
high power applications, millimeter waves can be treated as quasi-optic beams, where
micro-machined apertures can be formed to steer and combine the propagating millimeter
waves.
Transmission lines have their own limitations, like frequency dispersion and
insertion loss; that originate in the properties of the substrate or media. MEMS
technology has been effectively exploited to reduce the influence of the substrate in
various types of transmission lines, as shown in Figure 2.12. They are the membrane
43
supported micro-strip, coplanar micro-shield transmission line, top-side etch coplanar
waveguide and micro-machined waveguide.
(a)
(b)
(c)
Figure 2.12: Various types of transmission lines.
In the membrane supported micro-strip, the transmission line is defined on a thin
dielectric membrane by bulk-etching the substrate below the trace via backside
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processing. But, micro-strip line possesses no intrinsic ground plane. The coplanar
micro-shield removes this limitation by including the ground planes defining the groundsignal-ground structure. The top-side-etch coplanar waveguide removes complications of
backside etching of the membrane and micro-shield lines. It relies on opening etch
windows through the top passivation layer to make a pit below the line. In micromachined waveguide, micromachining and wafer bonding techniques are pointed to
overcome the lower-dimension bound of traditional machining approaches.
Ground–signal transmission lines are fabricated using high-resistivity singlecrystal substrate silicon with a DRIE technique. Figure 2.13 shows a portion of such
transmission lines in a 30 GHz stepped-impedance low pass filter. These transmission
lines are attached to on-chip comb-drive actuators so that the transmission line spacing
can be electromechanically and continuously adjusted by applying a voltage on the combdrive actuators. This adjustment results in a change in the impedance of a short
transmission line that can be modeled as a lumped shunt capacitor, and can be used for
the phase shifting functionality. A maximum phase shift of 48◦ at a frequency of 48 GHz
was achieved using a drive voltage of 45 V. The insertion loss was less than 1.8 dB.
Figure 2.13: Metalized single-crystal silicon transmission lines (50 μm in
height) as a part of a 30 GHz stepped-impedance low pass filter.
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2.11: Summary
The most intrinsic characteristics of RF-MEMS technology are miniaturization,
microelectronics integration, and accurate mass production. With the development of the
micromachining techniques, many multifaceted devices have been produced and some of
them have been already emerged. A large number of various RF-MEMS such as
switches, filters, antennae, variable capacitors, tunable inductors resonators, etc are being
patented, reported, and demonstrated now. R. Feynman gave a milestone presentation at
California Institute of Technology in 1959. Since then miniaturization and rethinking
towards micro scale had begun. A brief history about progress of RF-MEMS devices and
technology has been given in the Section 2. 2.
Most widespread and fundamental circuit components, demonstrated and
developed, are RF-MEMS switches. By 2001, more than 30 companies working in this
field including the giant consumer electronics firms like Motorola, Analog Device,
Samsung, Omron, NEC and ST-Microelectronics. Several new switch architectures have
also been reported, including the air-bridge structure. MEMS technology is used for RF
applications in the area of variable capacitors, as a replacement for varactor diodes as
tuners. A large number of RF-MEMS switches, Film Bulk Acoustic Resonators (FBARs),
variable capacitors, resonators, high quality factor inductors, filters, and phase shifters
with high quality factors, antennae, transmission lines etc. has been invented, published,
demonstrated and reported in the history and literature. Some of them are reported and
presented in this chapter from section 2.3 to section 2.10.
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