RADIO FREQUENCY MICRO-ELECTRO-MECHANICAL
SYSTEMS (RF-MEMS): A TECHNOLOGICAL ASPECT
4.1: Introduction
Micro electro mechanical system (MEMS) is a technology that enables the batch
fabrication of miniature mechanical structures, devices, and systems. The technology
takes many benefits of existing integrated circuit (IC) fabrication technologies. Most
significant advantages are cost reduction through batch fabrication, device-to-device
consistency from lithography and etching techniques, and increase in performance from
miniaturization that leads to a fantastic size and weight reduction. In addition, by using
silicon and fabrication techniques well-suited with IC technology, MEMS mechanical
components can be integrated with electronics, producing a complete smart system-on-achip that interacts with the surroundings, and communicates with other systems.
MEMS technology is most active area in research and development now-a-days.
It is an extension of the photolithographic techniques used in electronic integrated
circuits. Electronic components have their performance determined by physical
parameters like metal-to-metal contact resistance, dielectric constants, size of electrodes
and their spacing, etc. In addition to these factors, overall size relative to wavelength is
another significant factor. It involves effects like skin depth, parasitic capacitances and
inductances, transmission line behavior and radiation. In fact, bad affects of these
frequency dependent factors diminish with the size reduction.
RF-MEMS technology is new buzzword in the communication field. It has its
impact on wireless communication, commercial, and electronic applications. Frost and
Sullivan Growth Partnership Services provide up-to-date knowledge and analysis of path
breaking developments in RF- MEMS technology. RF-MEMS technology is on the verge
of revolutionizing RF and microwave applications. The needs of modern RF systems for
lightweight, smaller volume, lower power consumption and cost with enhanced
functionality, operating frequency and component integration are initiating the growth of
RF-MEMS technology.
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4.2: Comparison of Various Technologies
RF-MEMS is relatively a new technology, but has already shown wonderful
characteristics like performance and reduction in overall cost. RF-MEMS technologies
are just started to step out of R&D laboratories and into commercial MEMS foundries.
The first paper on RF-MEMS was published thirty years ago. This paper was published
on electro-statically actuated cantilever-type capacitive membrane switches. However,
RF-MEMS technology is still in its infancy, many interesting components and systems
have been demonstrated over the last decade.
The current circuit designs use many gallium arsenide (GaAs) FETs (field effect
transistors), PIN diodes, and varactor diodes to attain the vital switching, filtering and
tuning functions. The units made using these components are characterized by high
power consumption, poor RF performances, low reliability, and high manufacturing cost.
Comparison of various technologies is given in Table 4.1.
Table 4.1: Comparison of various technologies
RF-MEMS technology diminishes many shortcomings and gives better
performance. In many cases, a single MEMS component replacement outperforms an
entire solid state circuit. Clearly, these are the general motivations for the growth of RFMEMS. Mostly RF-MEMS are fabricated using traditional 3D structure technologies like
bulk and surface micromachining but LIGA and SCREAM are also used for the higher
aspect ratio structures.
Early products of MEMS technology are inkjet heads, DLP chips, pressure and
inertial sensors. RF-MEMS development did not make its mark until 2002. Now key
breakthrough is about to happen in the telecommunication industry in terms of improved
performance, ease of reconfiguration and miniaturization. Major RF devices in which
such a breakthrough has been achieved are: micro-switches, tunable capacitors, micro-
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transmission lines, micromachined inductors, micromachined antennas and resonators,
including micro-mechanical resonators, bulk acoustic wave resonator (BAW), and cavity
resonators.
In the perspective of RF-MEMS, RF refers to radio frequencies beyond DC to
sub-millimeter wavelengths. It separates itself from optical MEMS technologies that
include the mid-infrared to ultra-violet part of the frequency spectrum. With RF-MEMS
technology, lumped-element and distributed-element transmission line components are
normally used. An RF-MEMS technology roadmap is shown in Figure 4.1. It shows main
RF-MEMS technologies reported in the literature worldwide.
Figure 4.1: Roadmap showing RF-MEMS technologies.
The primary aspect to be considered is fabrication technologies (surface and bulk
micromachining). For example, surface micromachining is used to realize 3-D planar
inductors, self-assembled inductors, antennas and sliding planar back-short impedance
tuners. While bulk micromachining is used to realize 3-D planar inductors and guidedwave structures like resonators, transmission lines, cavities and antennas. These
micromachined components cannot be considered as true MEMS components because
reconfigurable actuator is not used in converting a control voltage or current into
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mechanical movement. All the MEMS components are micromachined components, but
vice versa are not true.
The true RF-MEMS components are the switch, variable capacitor and antenna.
Clearly, the most significant RF-MEMS component is the switch because it can be used
to apply high performance and digitally-controlled components (R, L and C lumpedelements), circuits (impedance tuners, phase shifters, filters and antennas) and
subsystems (signal routing, T/R modules and antenna arrays). As the RF-MEMS switch
offers a superior performance over the PIN diode, the variable capacitor is better than
varactor diode for tuning, linearity, and RF power handling. The RF-MEMS variable
capacitor can be applied in high-performance switches and analogue controlled circuits
(phase shifters, impedance tuners, filters). The antenna (RF-MEMS component) has its
radiating elements that work by using any type of actuation mechanism.
It is now appropriate to connect all the RF-MEMS components and circuits to the
very important RF microsystems packaging issue. With the proper choice of packaging
solution, this is very much dependent on many external factors like economy, fabrication
technologies, reliability, and RF performance. RF-MEMS components and circuits are
now integrated into subsystems that are presenting a greater RF performance and
improved functionality.
4.3: RF-MEMS Fabrication Micromachining Technologies Outline
The toolbox of RF-MEMS technology is the techniques and processes used to
fabricate RF-MEMS. The fabrication technologies used in their manufacture are briefed
here in Table 4.2.
RF micro-systems have emerged due to frequent advances in
numerous manufacturing technologies that have fused together, leading to novel
characteristic features. Basically, surface micromachining technology has evolved from
multilayer micro-fabrication. But sacrificial layers are used in surface micromachining.
Here, micromachining is usually not applied to the substrate material, but on the
structural layers above it.
Bulk micromachining uses discriminatory crystallographic etching techniques of
silicon wafer substrates. Here, unlike with silicon, crystallographic etching techniques
cannot be applied and so chemical etching is a choice, but this provides inferior precision
and profile definition. For producing high aspect ratio microstructures, LIGA was
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developed. This technology offers microstructures several hundred microns thick, with a
minimum feature size of only a few microns. Practically, LIGA combines extremely
thick-film resists (often >1mm thick) and high energy x-ray lithography (~1GeV), that
can pattern thick resists with high reliability and also results in vertical sidewalls. LIGA
needs of high energy x-ray source which is very expensive and rare. Use of particular
epoxy-resin-based optical resist, called SU-8, is a cheap alternative to LIGA. It can be
spun in thick layers (>500 µm), patterned with lithography tools and get vertical
sidewalls.
Table 4.2: Roadmap of fabrication micromachining technologies for RF-MEMS
Category
Manufacturing Technologies
RF components demonstrated
True RF-
Non-MEMS
MEMS
Surface (etching of the dielectric
Inductors,
rectangular
layers; sacrificial layers are used)
waveguides
Variable
capacitor,
switch
Different
Micro-
Bulk (etching of the substrate;
Waveguides,
etch-stop and sacrificial layers are
filters,
used)
transmission lines.
Wafer
Machining
Techniques
bonding
micromachined
substrates
(bulkare
cavity
resonator
membrane-supported
-----------
Micro shield transmission lines,
Variable
coupled
capacitor
cavities,
waveguides,
bonded together)
antennas, filters.
LIGA (thick photoresist exposed to
Microstrip filters, patch antennas
V-
x-rays, molded; plated to form 3-D
antennas,
structure; no etching & layer is
filters.
used)
4.4: Actuation Mechanisms
For a true RF-MEMS component, in addition to the RF element, an
electromechanical actuator is required. Choice of the actuator depends on the available
fabrication technology. Certainly, the most common actuation mechanism is electrostatic, followed by piezoelectric, magnetic and electro-thermal. With these standard
mechanisms, the scratch-drive actuator is becoming more fashionable. The scratch-drive
is also based on some type of piezoelectric, electro-thermal or magnetic actuation. During
the design process of RF-MEMS components and circuits, there exist many confusing
needs and constraints that must be considered early on. Main considerations are:
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
Actuation mechanism (e.g. electro-static, electro-thermal, magnetic and
piezoelectric);

Fabrication technologies (surface and bulk micromachining, LIGA, wafer
bonding etc);

Control parameters (e.g. voltage, current, power, energy and speed);

Intrinsic RF performance (e.g. quality factor, resonant frequency, insertion loss,
isolation, linearity, return losses);

Layout (e.g. area, topology and topography);

Packaging (e.g. hermetic packaging, standardization and extrinsic parasitic effect
on RF performance);

Subsystems integration (e.g. self-actuation and cost).
Although RF-MEMS technology presents superior RF performance, any of the
above requirements can degrade its performance. Due to this reason, RF-MEMS
components and circuits are subjected to very harsh practical trade-offs in their designs.
Hence, while designing, all of the above requirements are carefully considered so that a
few solutions remain for detailed CAD simulation (i.e. using electromagnetic, circuit,
mechanical and thermal simulators). In practice, after deciding the level of RF
performance of the MEMS component or circuit, suitable methods of actuation can be
tried to find out.
Electrostatic actuation is the most common, as it can produce small components
that are tough and relatively simple to fabricate. They are also relatively fast and tolerate
environmental variations. They consume negligible power and only during switching
between states, some residual energy is required to hold them in the actuated state. The
main disadvantage with electrostatic actuation is that it is difficult to combine a low
actuation voltage with good switch isolation, because of small spatial separation distances
between electrodes. Moreover, self-actuation by the RF signal being switched can be a
serious problem.
Piezoelectric actuation is based on a bimorph cantilever or membrane, where a
differential narrowing due to the piezoelectric effect causes the bending of structure.
With this mechanism, actuation can be done speedily. Unluckily, there occurs differential
thermal expansion of different layers that causes parasitic thermal actuation. This can be
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prevented by designing the structure symmetrical with respect to the thermal
characteristics of the layers. Integrating piezoelectric materials into a MEMS
environment is also very challenging, because films are difficult to mold and the high
crystallization temperatures are involved in the processing.
Both magnetic and electro-thermal actuation offers the advantages of low control
voltages and high contact force. However, unlike their electrostatic and piezoelectric
counterparts, they are slow, and need a relatively high electric current. They dissipate
significant levels of power in the actuated state. Also, magnetic actuators are relatively
large in size and difficult to manufactured, as they require a 3-diamentional coil with a
soft magnetic core.
4.5: RF-MEMS Components and Circuits
In the previous sections, main technological aspects (named fabrication
technologies, and actuation mechanisms) that are required to realize RF-MEMS
components have been briefed. The MEMS fabrication technologies are able to remove
the substrate below passive structures, can elevate them over the substrate, and can obtain
high-aspect ratio/large cross-sectional area structures. These techniques help designer to
fight with the limits of passive components. However, it is important to understand the
main requirements for each component or circuit before their realistic exhibitions. In this
section, notable examples of RF-MEMS components and circuits like variable capacitors,
the switches, mechanical resonators and filters are presented as case studies to show
advance of RF-MEMS technology.
Some of the main parameters are mentioned here to understand the technological
aspect of RF-MEMS devices in a clear manner. These parameters are significant in one
application or another. These parameters are used to express the performances of the RF
devices. The insertion loss is defined as the RF loss dissipated in the device between the
input and output of the device in its pass through state (the closed state). This loss is due
to skin depth effect, and resistance loss from signal lines and contact. The isolation loss is
the RF isolation between input and output of the device in its blocking state (the open
state). The key contributing factors are capacitive coupling and surface leakage. The
linearity of an RF component can be defined as the independence of the device
impendence from the input RF signal power in a two tone RF intermodulation
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measurement. The quality factor for an electrical and mechanical component is the ratio
of the energy stored in a device to the energy dissipated per cycle of resonance. For an
electrical component, Q is the ratio of imaginary part of impedance to its real part. The
resonance frequency of a device may be defined as the particular frequency at which the
stored kinetic energy and potential energy resonates.
4.5.1: Case Study of RF-MEMS Switches
RF-MEMS switches have better RF performance than PIN diodes within HMICs
and cold-FETs within RFIC/MMICs now. With both PIN diodes and cold- FETs, intermodulation distortion exhibits serious restrictions at higher RF-power levels.
Architectures of these systems can be greatly renowned by improving their performance
and functionality; and reducing complexity, size and cost. This can be done by using RFMEMS technology. The advantages of RF-MEMS switches over p-i-n-diode or FET
switches are:

Negligible Power Consumption: Electrostatic actuation requires 20-80 V but does
not consume any current, resulting low power dissipation to10-100 nJ per
switching cycle.

Very High Isolation: RF-MEMS switches have very low off-state capacitances (24 fF) leading to outstanding isolation at 0.1-40 GHz.

Very Low Insertion Loss: RF MEMS switches have an insertion loss of -0.1 Db to
40 GHz.

Low Cost: RF-MEMS switches are built on quartz, Pyrex; low-temperature cofired ceramic (LTCC), mechanical-grade high-resistivity silicon, or GaAs
substrates using surface micromachining techniques.
However, along with these wonderful advantages, RF-MEMS switches also have
their limitations and drawbacks like:

Low Speed: The switching speed of most MEMS switches is only 2-40 μs.
Certain communication and radar systems require much faster switches.

Poor Power Handling: RF-MEMS switches cannot handle more than 20-50 mW.

High-Voltage Drive: RF-MEMS switches need 20-80 V for operation, and this
needs a voltage up-converter chip for telecommunication systems.
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
Poor Reliability: The reliability of RF-MEMS switches is 0.1-10 billion cycles.
But, many systems require switches with 20-200 billion cycles.

Packaging: MEMS switches need to be packaged in inert atmospheres (nitrogen,
argon, etc.) and in very low humidity, resulting in hermetic or near-hermetic
seals. Packaging cost is high, and the packaging technique itself may affect the
reliability of the RF-MEMS switch.

Overall Cost: No doubt, their manufacturing cost is low but packaging cost is
very large.
There is development of several series switches by the companies, like Motorola,
Hughes Research Labs, U.S. Air Force Research Labs, University of California,
Berkeley, Samsung, NEC, and Thompson-CSF. All have very low capacitances and low
contact resistance, but none have attained the maturity of the Rockwell or the Analog
Devices switches.
(a)
(b)
Figure 4.2: (a) Broadside MEMS-series switches with one electrode, and
(b) Broadside MEMS-series switches with two electrodes.
The RF-MEMS switches developed today, obey the fundamental mechanical laws
established centuries back. But, the scale and the forces involved in the switches have
much different status at the micro-scale. Surface forces and viscous air damping
dominate over inertial and gravitational forces. The switches are either fabricated using a
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fixed-fixed membrane or a floating cantilever (diving-board design) and are modeled as
mechanical springs with an equivalent spring constant as shown in the Figure 4.2 and
Figure 4.3.
(a)
(b)
Figure 4.3: (a) Low-height high-spring-constant gold MEMS switch and
(b) Low spring-constant MEMS switch.
Raytheon developed the first practical MEMS capacitive shunt switch as
illustrated in the Figure 4.4. The switch is based on a fixed-fixed metal (Al or Au) beam
design. The anchors are coupled to the coplanar-waveguide ground plane, and the
membrane is grounded. In a microstrip finishing, the switch anchors are either connected
to the ground plane using via holes or using an l/4 radial stub. In the Raytheon design, a
center pull-down electrode is used. A silicon-nitride layer (thickness of 1000-2000Å) is
used to separate the metal membrane from the pull-down electrode.
Pacheco et al. demonstrated a 9V electrostatically actuated switch, having a fivemeander arm at each of the four corners of the capacitive membrane bridge. Here, the
capacitance ratio=2.5 pF/47fF=43; insertion loss is 0.16dB at 40GHz; isolation=26dB at
40GHz; and self-actuation occurs with a mean RF power of 6.6W. A microphotograph of
a capacitive membrane switch is shown in Figure 4.5.
The capacitive switch as shown in the figure 4.5 is outstanding for 10-120 GHz,
but does not facilitate ample capacitance for 0.1-20 GHz applications. Using two pull-
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down electrodes on either side of the center area of the switch, a dc-contact shunt switch
can be built by eliminating the dielectric layer in the middle of the switch. The dc-contact
shunt switch leads to high isolation at 0.1-20 GHz, which is practical for wireless
applications. The performance of a dc-contact shunt switch depends on the contact
resistance and the ground inductance.
Figure 4.4: Raytheon MEMS capacitive shunt switch: cross-section view and electrical
CLR model.
There occurs failure due to stiction when the stiction force is greater than the
restoring force of the spring in the ‘down’ position. Prediction of the stiction force is
difficult as this depends on the surface quality of the electrodes and on the environmental
conditions (humidity and surface contamination of the electrodes). With low actuation
voltage switches, similar to as shown in Figure 4.5, stiction can be a severe problem. So,
manufacturing companies have to cope with issues relating to reliability and packaging.
RF-MEMS technology has been used to implement magnetically actuated and
electrothermally actuated switches. A micromachined magnetic latching switch has been
demonstrated by Ruan et al. These operate from DC to 20GHz with insertion loss of
1.25dB and an isolation of 46dB.The device is based on selective magnetization of a
perm-alloy cantilever in a permanent external magnetic field. Switching is caused by a
short current pulse through an integrated coil below the cantilever.
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Figure 4.5: The University of Michigan capacitive membrane switch.
4.5.2: Case Study of Variable capacitors
Variable capacitors are very useful in phase shifters and provide frequency
control of tuners, filters and antennas. For all these applications, enhancing the
capacitor’s quality factor is of vital for minimizing loss and maximizing noise
performance. Till this time, varactor diodes could only provide voltage control of
capacitance. They can exhibit relatively low Q-factors and also useful for frequency agile
applications. These are also sensitive to medium RF power levels and do not exhibit
linear frequency tuning characteristics. RF-MEMS capacitors can overcome some of the
limits of varactor diodes, but these have much slower control speeds.
A bulk machined electrostatically actuated RF-MEMS variable capacitor, having
interdigitated fingers, was demonstrated by Rockwell Science Center. Here, large crosssectional area structure/ high aspect ratio silicon was micromachined using 25µm deep
reactive ion etching (DRIE), as shown in Figure 4.6. This capacitor provides high tuning
linearity, a small part count (making it less prone to failure) and is small in size. At a
tuning voltage of 5.3V, the maximum capacitance was 6 pF, with a 4:1 capacitancetuning ratio, and the unloaded quality factor was ~265 at 500MHz. After that, Feng et al.
reported electrothermally actuated RF-MEMS variable capacitors.
Chiao et al. reported surface machined variable capacitor. Since, sometimes
linearity of tuning is of more importance than dynamic range; circular parallel plate
variable capacitors were realized. The top plate, which is electrically isolated from the
fixed bottom plate, is physically attached to two circular scratch drive actuators. These
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actuators move in opposite directions, which permits the top plate to rotate by ±90°. The
gap between the plates is 2µm and the overlapping area can be changed by an amount
equivalent to a 30 minute increment in angular rotation.
Figure 4.6: Electrostatically actuated bulk-micromachined silicon variable capacitor,
designed and fabricated at Rockwell Science Center
4.5.3: Case Study of Mechanical Resonators and Filters
RF-MEMS technology is continuously enhancing the properties of RF devices.
Researchers center on high-Q filter applications using mechanical resonators. The
mechanical filters transform electrical signals into mechanical energy, do a filtering
function, and then change the remaining mechanical energy back into an electrical
energy. Unlike other electromechanical filters (quartz-crystal filters, ceramic filters that
are composed of electrically coupled resonators, and surface acoustic wave filters), a
mechanical filter is coupled mechanically and allows bi-directional propagation within
the filter.
The design of a mechanical filter involves basic principles of physics,
electromechanical transducer concepts, vibration theories, and filter circuitry. Johnson
explains the design of mechanical filters in his book entitled ‘Mechanical Filters in
Electronics’. These macroscopic mechanical filters have a typical central frequency
below 600 kHz due to their size and manufacturing capability. The insertion losses are
about 2 dB. However, in modern communication applications, the central frequencies are
much higher and much lower insertion losses are required.
First mechanical filter was developed in 1946 by Robert Adler. Then the research
focused on the mechanical filter optimization and the development of new generation
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filters. It leads the mass production of these mechanical filters in the 1950s for telephone
applications rapidly. Figure 4.7 shows an actual mechanical filter with disk resonators
coupled with mechanical wires and the equivalent circuit of this mechanical filter. It was
made by Rockwell International for use in frequency division multiplex telephone
systems. The rapid development and adaptation of the mechanical filter is due to its
superior characteristics (like a large quality factor, good temperature stability, aging
properties). All these parameters are needed to get low-loss, narrow bandwidth, and high
stability filters.
Figure 4.7: A mechanical filter used in telephone systems by Rockwell International
RF-MEMS technology was applied to miniaturize size of mechanical resonators.
A mechanical filter based on these resonators was first published in 1992. Since then, a
number of papers have been published on the optimization of these mechanical filters,
and on MEMS-based resonators. The resonance frequency of a mechanical resonator is
increased by further downscaling size to nanometer scale. Using lithography and etching
techniques, fixed–fixed silicon beam resonators have been demonstrated with a
fundamental resonant frequency of 14 MHz and Q of 2500 at normal temperatures.
MEMS band pass filters based on the fixed–fixed beam resonator design were
first demonstrated in 1997. It uses a two-resonator design coupled electrically instead of
common mechanical coupling in a mechanical filter. The central frequency was up to
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14.5 MHz with a Q ~1000 at a pressure of 23 mTorr, a dc biasing voltage of 40 V and
the insertion loss about 13.4 dB. MEMS-based mechanical resonators and filters have
shown promising characteristics in achieving important filter parameters, like narrow
bandwidth, low loss, and good stability.
Figure 4.8 shows a resonator using a free–free beam structure. It is a torsional
resonator with two torsional beams supported at its flexural node points. The torsional
beams are anchored to substrate at each end, and are designed to have a length equal to a
1/4 of the wavelength, so that the free–free beam sees zero impedance into the supports.
This configuration decreases the clamping loss at the anchoring points of the beams. It
can exhibit a resonant frequency of 92.25 MHz with a Q about 8000.
Figure 4.8: A 70.95 MHz free–free torsional beam resonator (from University of
Michigan).
For technological point of view it must be kept in mind that, as the size decreases,
the signal power handling capability, parasitic and load reactance, the output electrical
impedance and the electromechanical coupling coefficient (energy stored in the
mechanical system to the total input energy) must be considered as important design
parameters. Hermeticity is another key parameter to get high-Q and long-term stability of
the resonators.
4.6: Promises and Challenges of RF-MEMS Technology
The constant call for more elastic and lightweight RF-MEMS systems with
negligible power consumption and reduced fabrication expenditure has increased the
demand of a technology that can improve operating frequency, reconfiguration, and
functionality, integration of constituent parts, reliability, battery life and RF performance.
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The distinguished example of existing and upcoming applications needing these
characteristics and parameters is wireless communication that includes wireless handsets
for messaging, internet services for e-commerce, and wireless data links like Bluetooth
and location services employing the global positioning system (GPS).
RF-MEMS technology is considered to boast the prospective to make possible
large operational frequency bandwidths, get rid of off-chip passive components, build
negligible interconnect losses, and generate just about faultless switches and resonators in
the situations of a fabrication method well-suited with existing IC and MMIC
approaches. RF-MEMS devices show prospective to be used for integrated voltage
controlled oscillators (VCOs) in global positioning systems (GPS) in the form of MEMS
inductors and tunable variable capacitors. RF-MEMS devices show prospective to be
used in the form of micro-switches for impedance networks in the company of power
amplifiers and to shrink the component density in multi-standard mobile phones.
RF-MEMS technology pledges to make possible on-chip switches with negligible
reserve power consumption, switching power in nano-Joules and actuation voltage less
than 5V. In addition to it, it assures to possess top quality inductors, variable capacitors,
exceptionally stable oscillators and superior performance filters operating in the wide
frequency range lying between tens of MHz to many GHz. The ease of use of such high
quality RF components will present designers with the expectations that they have long
projected to generate novel, straightforward, and influential reconfigurable systems.
The guarantee of miniaturization using MEMS for radio frequency applications
seems closer to science imagination. MEMS technology is drawing great attention from
the moment it is being subjected to radio and microwave frequency applications.
However, it has not achieved much mass market implementation due to definite problems
and issues related to reliability and packaging. All the reliability and packaging issues are
not still found solutions. The key reliability issues are related to fabrication, packaging,
radiation, life degradation, breakdown and leakage of dielectrics, stiction problem,
temperature drift, metal-to-metal contact resistances, creep, surface contaminations,
electrical characterization etc. All these reliability and packaging problems are
obstructing the fast growth of RF-MEMS technology.
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RF-MEMS growth is obstructed because of the materials and fabrication methods
that are not much reliable. In addition to these, there exist a large number of challenges
involved in the packaging of RF-MEMS devices. Superior packaging approach is
indispensable for the triumphant performance of RF-MEMS and microwave components.
To eliminate the unnecessary resonances, electromagnetic interferences and coupling,
RF-MEMS packaging procedures center on checking moisture and particulates that may
badly affect the movement of self-supporting MEMS structures and numerous energy
losses (such as acoustic and thermal).
Furthermore, the arrangement, design and materials employed in the packaging of
RF-MEMS devices are decisive because these have much poor affect on the performance
of the system. The expenditure, out-gassing, stiction, dicing, environmental and
functional interfaces, and non-availability of a standard package, reliability, modeling,
and integration are the chief packaging problems. Currently, there exists no standard
packaging solution and an application dependent format is required for each RF-MEMS
device. Package is application dependent. Packaging is the very costly fabrication step
and generally makes up to 90% of the overall price of a RF-MEMS device.
The RF-MEMS devices comprise of both moving mechanical structures and selfsupporting components that must be protected during the processing and standby mode.
All the electronic components and mechanical parts must remain uncontaminated during
both the manufacturing process besides the working lifetime of the parts.
In spite of its numerous challenges, RF-MEMS technology has great prospective
to modernize wireless communication systems. Components based on this technology
exhibit superior RF performance and tunability over a much wide range of frequencies of
operation. An RF-MEMS switch provides much enhanced insertion loss, isolation, and
linearity. This technology may be utilized to conquer the limitations accomplished with
integrated RF devices. It also facilitates circuits to have top quality performance that is
not easily obtained by the other technologies. The key parameters of RF-MEMS passive
devices (like switches, variable capacitors, mechanical resonators, tuneable inductors and
transmission lines) are low power consumption and reconfigurability. These parameters
guarantee that omnipresent wireless connectivity and high volume applications will
become possible almost immediately.
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4.7:
Summary
Due to the persistent progress in technologies, the RF-MEMS foundry services
offered to designers will continue to increase. By now, MEMS technology has reported
its superior RF performance over traditional techniques. Many new components and
circuits have been demonstrated in the literature. However, the difficulty in matching the
future needs of the RF component designer with the limitations of commercial MEMS
foundry processes should not be underestimated. Moreover, there are inherent problems
associated with RF-MEMS technology. For example, at low microwave frequencies,
resonant structures are relatively large and so they can be difficult to move under
electromechanical actuation. Due to such problems, true RF MEMS antennas have been
difficult to implement and it is difficult yet to demonstrate variable inductors.
In addition to performance of RF-MEMS, their overall cost is the most important
factor from the industry’s point of view. Accordingly, it is possible that the switch will
remain the most vital RF-MEMS component. Because, future work will investigate its
volume production (e.g. hermetic packaging, reliability up to 10 billion switching cycles
and low cost) improved functionality (e.g. with multiple pole multiple throw topologies),
and subsystem integration (e.g. in signal routing applications). The focus of RF-MEMS
circuits has started to revolutionize. However, due to their ubiquitous role in wireless
systems, high-Q tuneable filters are receiving more interest. Technically, the air-filled
metal-pipe rectangular waveguide can get very low transmission losses in some
applications. A key breakthrough could be achieved in millimeter-wave filter technology,
if this 3-D guided-wave transmission line can be integrated with RF-MEMS tuning.
Unluckily, such technological advances are not likely to be reported in the short time and
may never become economically feasible.
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