RADIO FREQUENCY MICRO-ELECTRO-MECHANICAL

SYSTEMS (RF-MEMS): A TECHNOLOGICAL ASPECT4.1: 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-a-

chip 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 RF-

MEMS. 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 guided-

wave 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 lumped-

elements), 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

Non-MEMS

True RF-

MEMS

Different

Micro-

Machining

Techniques

Surface (etching of the dielectric

layers; sacrificial layers are used)

Inductors, rectangular

waveguides

Variable

capacitor,

switch

Bulk (etching of the substrate;

etch-stop and sacrificial layers are

used)

Waveguides, cavity resonator

filters, membrane-supported

transmission lines.

-----------

Wafer bonding (bulk-

micromachined substrates are

bonded together)

Micro shield transmission lines,

coupled cavities, waveguides,

antennas, filters.

Variable

capacitor

LIGA (thick photoresist exposed to

x-rays, molded; plated to form 3-D

structure; no etching & layer is

used)

Microstrip filters, patch antennas V-

antennas,

filters.

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 electro-

static, 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, inter-

modulation 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 RF-

MEMS 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 (2-

4 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 co-

fired 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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(b)

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.

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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(a) (b)

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.

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 five-

meander 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 cross-

sectional 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 capacitance-

tuning 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.

The distinguished example of existing and upcoming applications needing these

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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.

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

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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 self-

supporting 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.

4.7: Summary

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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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