International Journal of Advances in Engineering Sciences Vol.3, Issue 4, October, 2013
RF-MEMS Devices’ Taxonomy
Dr. Tejinder Pal Singh (T. P. Singh)
A. P., Applied Sciences Department
RPIIT
Bastara, Karnal, Haryana (INDIA)
tps5675@gmail.com
Abstract—The instrumentation and controls in the fields like
defense, aerospace, electronics and communication
continually needs for novel technologies and developments.
Radio frequency microelectromechanical systems (RFMEMS) technology shows remarkable potential over the
years. It has already made its charisma felt significantly by
providing replacement in electronics and communication
systems with large quality factors and defined tunability.
The RF-MEMS devices and components have emerged as
potential candidates for modern applications in the different
fields. The center theme of this paper is to classify the
different RF-MEMS devices under the different domains.
This research paper also highlights the fact that the
limitations faced by the current RF devices can be overcome
by the flexibility and better device performance
characteristics of RF-MEMS components. This paper
reviews the importance of RF-MEMS, their salient features,
and possible applications.
Keywords-RF-MEMS Devices; Reliability; Taxonomy; RF
systems; Application
I.
INTRODUCTION
At present, Micro-Electro Mechanical systems
(MEMS) are attaining much attractiveness from fields
(such as electronics and communication, defense, and
space) because of the benefits of size reduction without
any reduction in the performance of a system. In the
modern communication field, there is a need to have a
technology that can push the operating frequencies higher,
offer broader bandwidths, and manage multiple broad
band signals, all within a single device. Today, radio
frequency microelectromechanical system (RF-MEMS)
technology is rapidly growing, and trying to solve these
challenges. The key benefits include low loss, high
isolation, near-perfect linearity, and large instantaneous
bandwidth that traditional semiconductor technologies
cannot provide [1] [2].
The fundamental blocks of RF-MEMS systems are the
micromechanical components like tunable capacitors,
high
quality
inductors,
micro-resonators,
high
performance filters and switches which have the potential
to substitute the traditional distinct components. These
devices are fabricated using MEMS structures like
cantilevers, air bridges, membranes and inter-digital
structures. This paper here briefly reviews the significance
of RF-MEMS devices, their key features, and potential
applications.
Print-ISSN: 2231-2013 e-ISSN: 2231-0347
© RG Education Society (INDIA)
RF-MEMS technology has the applications in the field
like defense, communication, phased array radars,
reconfigurable antennas, automotive and cellular phone.
The potential applications of RF-MEMS devices are
shown in Fig. 1. The RF-MEMS provide components with
reduced size and weight, very low loss, low power
consumption, wide bandwidth, higher linearity, lower
phase noise, better phase stability and high isolation.
Wherever the application needs such features, MEMS can
offer solutions to substitute either components or circuits
or the subsystems using the components [3].
There are numerous RF-MEMS components which are
either used directly for replacement or integrated to form a
microsystem along with other semiconductor devices.
POTENTIAL APPLICATIONS OF RF
MEMS MASS APPLICATIONS
MOBILES, GPS, RFID, WLAN,
CONSUMER & IT
TELECOM
INFRASTRUCTURE
BASE STATIONS
MICROWAVE COM
TEST
RF
INSTRUMENTATIO
N
RF
MEMS
AUTOMOTIVE
ANTICOLLISI
ON RADAR
ROOF
ANTENNA
HIGH VALUE
APPLICATIONS
DEFENSE
RADIO
DEFENSE
COMMUNICATI
ON SYSTEM
MISSILES
SATELLITES
Figure 1. Potential Applications of RF-MEMS devices.
The components can also go along with silicon
technology or GaAs technology and the MEMS
components can be incorporated to give a system solution.
The restrictions of the usual RF integrated devices can be
conquered by the flexibility and improved device
performance properties of RF-MEMS components, which
finally propagate the device level advantages to the
system to achieve the unprecedented levels of
performance. The component level to system level growth
of a characteristic communication system using RFMEMS devices is shown in Fig. 2.
International Journal of Advances in Engineering Sciences Vol.3, Issue 4, October, 2013
using the high resistivity substrate like quartz. The tuning
range, nominal capacitance, tuning voltage, quality factor,
and self-resonant frequency are the factors to be
considered during the design taking into account specific
requirement. Different micromachined tunable capacitors
are shown in Fig. 3.
SYSTEM
(Phased Array Antenna, Switch Matrix, Cell Phone,
Pager)
CIRCUIT
(Phase Shifter, Transceiver, Filter, Oscillator)
DEVICE
(Switch, Inductor, Resonators, Varactor)
Figure 2. RF-MEMSs Components and Subsystems.
The possible substitutes that are better over the
traditional components in terms of high quality, low
power consumption and wider operational frequency
range are the tunable capacitors, high Q inductors, high
performance filters, and oscillators. These components
can be made using pre-CMOS or post- CMOS approaches
to integrate the MEMS components on the same wafer.
The chief benefit given by MEMS is the exact tunability
and the tuning ratio which the traditional components
cannot offer [4]. This provides the ability to fine tune the
circuits to the best optimum level and achieve the highest
performance. Re-configurability is another required
feature used to reconfigure the same antennas for different
frequencies without shifting from one antenna to another.
This helps in faster scanning and better directivity of the
antennas. The switches, which are characterized by very
low loss, low actuation voltages and better reliability, are
employed for reconfiguring the antennas, routing
networks, tunable filters, etc. Due to high linearity, RFMEMS devices are brilliant candidates for broad band
communication systems.
II.
(a) Three-plate gap-tuning variable capacitor [18].
(b) Gap-tuning variable capacitor using an electrothermal actuator[19].
Figure 3. Top view of micromachined tunable capacitors.
B. RF-MEMS Switches
The RF-MEMS switches have huge potential in
consumer electronics, defense, communication, and
aerospace. The applications include phase shifters,
switchable capacitors, tunable filters, switch matrix for
routing and phased array antennas[7][8][9]. Ideally,
switches are used to perform signal routing without
causing any power loss to signal. The desirable
parameters in RF switches are low insertion loss and
return loss in the closed state, high isolation in the open
state, high linearity, high power handling capability, low
operating voltages, high reliability, small size, and low
cost.
RF-MEMS DEVICES
A. Tunable Capacitors
The IC technology gives a fixed capacitance with a
sandwiched dielectric layer between two conductive
electrodes. Here, the parasitic capacitance and the series
resistance will cause losses and reduced quality factor. A
large chip area is also needed for very high capacitance.
The micromachined tunable capacitors meet the
requirements to a great extent. The tunable capacitors can
be realized either by surface micromachining or by bulk
micromachining. But, the surface micromachined
capacitors are straightforward and implemented by having
a bottom electrode on the substrate and a suspended
electrode on the top with an air gap [5] [6].
The tunability is achieved by the displacement of the
top membrane by applying an electrostatic force between
the two plates. For very high capacitance values, a suitable
dielectric with high dielectric constant can be employed in
the place of air between the two plates. The parasitic
capacitance can be reduced and Q can be increased by
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International Journal of Advances in Engineering Sciences Vol.3, Issue 4, October, 2013
DIELECTRIC
PULL-DOWN
ELECTRODE
MEMBRANE
ANCHOR
(a)
Z0
Z0
(a) Suspended Spiral type of Inductor [21].
C
L
Top conductor
Magnetic core
RS
SWITCH IMPEDANCE Zs = Rs + jωL + 1 / jωC
RESONANT FREQUENCY = f0 = (1/2T2) * (1/ (LC) 1/2)
Pad
(b)
Figure 4. (a) Top view the RF MEMS switch and (b) equivalent circuit
[20].
(b) Solenoid type of Inductor [22].
There exist various configurations in RF-MEMS
switches depending on the application. Usually a series
ohmic switch is used while transmitting the RF signals in
the frequencies dc to few GHz. For higher frequencies, a
shunt switch is used which provides very good isolation
[10]. The key parameters considered while designing a
switch are the losses, switching speeds, actuation
voltages, RF power handling, and life cycles. The widely
used actuation mechanism in switches is the electrostatic
actuation where the potential is applied between the two
electrodes to bring in the displacement of the free beam.
Top view and its equivalent circuit are shown in the Fig.
4.
Figure 5. Planar on-chip inductors.
D. MEMS Resonators
Q factor for normal electrical RLC circuits is limited
up to 100 due to the parasitic resistive losses in the circuit.
This also increases the insertion losses that cause
unwanted signal attenuation. The micromachined
resonators with Q factors much above 1000 over a wide
range of tunable frequencies when used in combination
with the integrated electronics will result in highly stable
communication system.
There are two types of MEMS resonators: beam
resonator and comb drive resonator. Both these suffer
from the limitation of operational frequency which can be
extended only up to few hundreds of MHz only. The bulk
cavity resonators exhibit a very high Q exceeding 10,000.
However, dimensions are very large especially in the low
frequency range up to few hundreds of MHz. The
micromachined resonators can be realized in dimensions
that are order of magnitude smaller than cavity resonators,
at the same time, meeting the high quality requirements in
addition to low loss and high linearity[11] [12].
Both configurations cannot get higher frequencies. The
film bulk acoustic resonators (FBAR) are better for
applications requiring higher frequencies (up to 2 GHz)
with Q > 1000.
C. MEMS Variable Inductors
MEMS variable inductors find wide applications in the
RF-MEMS-based communication systems. In normal
planar inductor, the resistive metal lines and the dielectric
losses in the substrate, degrade the Q factor and cause
parasitic capacitances. The micromachined inductors with
suspended metal structures give very high Q factor
resulting in high frequency performance of the systems.
Micromachined inductors are used in low-noise
oscillators, high-gain amplifiers, and matching networks.
These are characterized with inductances of few nH
and the Q factors of 20-30 using MEMS inductors. There
are two configurations of inductors: solenoid type and
suspended spiral type as shown in Fig. 5. The inductor
consists of a planar spiral made in one layer of metal and
a connection to the center of the spiral in another layer of
metal [10].
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International Journal of Advances in Engineering Sciences Vol.3, Issue 4, October, 2013
E. MEMS Resonators
A phase shifter is a two port network, in which the
phase difference between output and input signal can be
managed by a control (dc bias) signal. Phase shifters with
low cost, insertion loss, drive power, and continuous
tunability result in light weight phased array antennas.
The phase shifters find application in telecommunication
and military radars. The phase shifters are extensively
used in phased array antennas in defense for applications
like smart munitions, missile communication, drones,
missile radar seekers, aircraft and helicopters etc.
applications in wireless communication, navigation,
sensor systems. They could be used in switches, phase
shifters, signal routings, impedance matching networks,
exciters, transmitters, filters, RF receivers. RF-MEMS
devices can be grouped as active devices and passive
devices.
 Active MEMS devices: switches, varactors, and
tuners.
 Passive MEMS devices: bulk micromachined
transmission lines, filters, couplers.
However, it is still premature for a taxonomy of RFMEMS devices, yet the progress till date tends to put them
into different classes depending on whether one takes an
RF or MEMS viewpoint. From the RF viewpoint, the
MEMS devices are simply grouped by the RF-circuit
component they consists of, be it reactive elements,
switches, filters, or something else. From the MEMS
viewpoint, these are put into three separate classes based
on where and how the MEMS actuation is carried out
relative to the RF circuit. The three classes are mentioned
below:
There are two configurations of the MEMS phase
shifters: one with switched line and the other with
distributed MEMS transmission line (DMTL). The phase
shifter with switched line approach has MEMS switches
in the configuration which when selectively actuated
results in desired phase shift while the DMTL phase
shifter can be optimized to get low insertion loss over a
broad frequency band. The phase shifters based on RFMEMS switches causes reduction in losses than their solid
state counter parts. The decrease in losses reduces the
need for the MW power and so reduces the size but give
better performance [13] [14]. MEMS switches in phase
shifter design result in lower loss phase shifters at any
frequency particularly from 8-100 GHz. Figure 6 shows a
three dimensional view of DMTL phase shifter at RCI.
A. RF Interinic
These are the devices in which the MEMS structure is
positioned inside the RF circuit and has the dual roles of
both the actuation and RF-circuit function. In this group,
one may regard as conventional cantilever and diaphragm
type MEMS that can be employed as electrostatic
microswitch and comb-type capacitors. With the discovery
of electro-active polymers, multifunctional elegant
polymers and micro-stereo lithography, these RF-MEMS
can be easily used with polymer based polymers. These are
stable, flexible, and lifelong. In addition, these can be
integrated with the organic thin film transistor. Shunt
electrostatic micro switch, inductors and comb capacitors
are the examples that are put in the RF-intrinsic class.
B. RF Reactive
In this group of RF-MEMS devices, the MEMS
structure is positioned inside, where it has role of RF
function that is attached to the attenuation. The examples
of this class are capacitively coupled tunable filters and
micromechanical resonators. These devices facilitate the
required RF functions in the associated circuit. Millimeter
wave and microwave planar filters on thin dielectric
membrane exhibit low losses, and are suitable for low
price, compact, high performance millimeter wave onechip integrated circuits.
A collection of these devices is shown in the RFMEMS technology diagram of figure 7. The richest class is
clearly the RF-intrinsic, which already boasts three
promising devices. Here, we have tunable capacitors and
inductors that are expected to operate up to at least a few
GHz in frequency, and we have RF-embedded switches
that operate well from a few GHz up to at least 100 GHz.
Figure 6. A three dimensional view of designed DMTL phase shifter at
RCI [13].
F. RF-MEMS Transceivers
These find applications in wireless systems. The need
of portable wireless systems with more functionality and
rapid growth in the technology create novel devices with
greater functional density and with more integratability
into the system [15]. RF-MEMS have made their
influence with their potential in transceivers associated
with very low power consumption, high quality factor,
and size reduction. In fact, applications like personal
communication systems (PCS), wireless networking, and
radars are being completely exploited by MEMS to
decrease the parts count, power consumption, and lighter
weight but with superior RF performance.
III.
TAXONOMY-MEMS VIEWPOINT
RF-MEMS and microwave industry is reaping the
benefits of MEMS technology. The continuous advance in
MEMS technology attracted researchers towards the
development of MEMS devices for RF applications. RFMEMS devices have a wide range of potential
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International Journal of Advances in Engineering Sciences Vol.3, Issue 4, October, 2013
TABLE I.
Class
I
II
III
Micromachined
Structures
Movable Parts
Yes
Yes
Yes
No
Yes
Yes
Impact
Examples of
RF-MEMS
Devices
Figure 7. Three different RF-MEMS device categories [23].
IV.
TAXONOMY-RELIABILITY VIEWPOINT
The taxonomy of RF-MEMS devices [16] [17] is
recently becomes a hot issue in such a way that it has
tendency to include any device which is made with at
least one step of micro-machining technology. So, it has
become necessary to make a division of various RFMEMS devices in such a way that it becomes significant
for studies regarding reliability. This is important to make
some common criteria for the accelerated tests and ageing
models. Three different classes of reliability of RFMEMS devices are briefed in the table 1. This taxonomy
of RF-MEMS devices has been done in accordance with
the level of mechanical complexity and boundary
conditions.
The class-I has all the passive components that have
been designed for diminished losses through micromachining fabrication. Mechanical movements of any part
of the structure of this class of RF-MEMS devices are not
required during the functioning and working. However,
some deformations might take place during various
processes involved in fabrication. Reliability and stability
of this class of RF-MEMS devices in the long term do not
alter significantly from those of conventional RF passive
components.
Stability problems of the structures of these devices
might take place to thin dielectric membranes. Often,
these dielectric membranes are used for high quality
factor passive components fabricated by using micromachining. When heat diffusion of the bulk material is not
good, then the membranes also have tendency to expose
thermal problems. In addition to these problems, the
devices which are under repeated temperature cycles
during assembly and packaging process can be prone to
structural deformations that cause failure of the device.
The second class of RF-MEMS devices demands
mechanical movement of some part during the working of
the devices. This class of RF-MEMS devices consists of
devices having micro-machined structure and moveable
parts. Notable examples of this class are very high quality
factor micro-electro- mechanical resonators and
continuously tuning capacitors. Due to repeated
mechanical movements and vibrations, novel stress
mechanisms are introduced on the constituted parts of
these devices.
TAXONOMY OF RF-MEMS DEVICES [16]
No
No
Yes
High-Q
Suspended
Inductors:
spiral,
self
assembled
coils; low-loss
RFMembranes;
RF-CMOS
substrate
removal postprocessing
Very High-Q
micro-electromechanical
resonators;
continuously
tuning
capacitors.
Ohmic contacts
RFMEMS
relays;
switched
capacitors;
capacitive
coupling RFMEMS
switches and
multiplexers.
Plastic deformations, mechanical relaxations, fatigue,
creep etc can disturb the stability of electro-mechanical
behavior of these devices. All these failure and
degradation mechanisms cause the mechanical failure of
second class of RF-MEMS devices. In addition to this,
oxidation and absorption like surface effects can cause
stresses in moving and oscillating part. As a result
complex stability problems are introduced that help in the
failure of device.
The third class of RF-MEMS devices comprises of all
the devices demanding two distinguished mechanical
moveable parts to attain and keep contact during a definite
time of cycle of the operation. Novel problems related to
reliability are caused due to the presence of mechanical
contact between the moving parts of device. These
reliability problems may be of mechanical type and
electrical type.
The major effect that diminishes the working of
devices is the stiction of mechanical parts that keep the
mechanical contact. Due to stiction of mechanical parts
restoration of resting position becomes almost impossible
even after the removal of actuation force. The stiction can
happen due to many factors like redistribution and
accumulation of electric charge in dielectric slabs,
capillary effects due to humid environment, micro –
welding of metals due DC or RF power etc.
Examples of this class of devices are ohmic-contact
RF- MEMS relays, switched capacitor, capacitive coupling
RF-MEMS switches and multiplexers. Electrical ohmic
contacts between two metallic surfaces may be affected
from stability problems that arise due to number of cycles,
variation in resistance of ohmic contacts, transfer and
erosion of material, surface contaminations and other
surface effects like absorption and oxidation.
V.
TAXONOMY-APPLICATION VIEWPOINT
RF-MEMS include several distinct types of devices,
such as RF-MEMS switches and relays, tunable inductors,
resonators, varactors (variable capacitors), antennas,
transceivers and phase shifters. Applications of RFMEMS include all types of wireless communications,
radar, satellites, Missiles, instrumentation, WLAN, GPS,
RFID and test equipment. Compared to conventional RF
35
International Journal of Advances in Engineering Sciences Vol.3, Issue 4, October, 2013
components, RF-MEMS offer significant benefits, like
lower power consumption, lower insertion loss, and lower
cost. Another possible application of RF-MEMS is their
implementation in transceivers in wireless systems. The
table 2 shows the taxonomy of RF-MEMS devices as per
the application viewpoint.
The MEMS technology has the potential of replacing
many traditional Radio Frequency (RF) components used
in now-a-days mobile, communication and satellite
systems. In many cases, such RF-MEMS components
would not only reduce substantially the size, weight and
power consumption but also promise superior
performance in comparison with current technologies.
These days MEMS and RF-MEMS can be found in
many different applications across multiple markets. RFMEMS experts believe market forces are enabling a
second wave of applications, limited to some selected but
very large industries in which MEMS components have
clear advantages over traditional electronic components.
In particular, the telecommunications industry is ripe for
MEMS technology. RF-MEMS are mainly used in the
fields such as automotive, electronics, space, defense,
medical and communications.
TABLE II.
CONCLUSION
The RF-MEMS technology is on the edge of bringing
out a noteworthy change in the fields of communication
radar communication, instrumentation and aerospace in
the coming years. An incredible amount of work is going
on the earth due to their potential for tactical applications
in defense field. In this paper, a brief summary of RFMEMS devices (micromechanical resonators, MEMS
variable inductors, RF-MEMS switch, tunable capacitors,
phase shifters, and MEMS transceivers) is given.
This paper reviews the importance of RF-MEMS, their
salient features, and possible applications in brief. This
paper also classifies the RF-MEMS devices on the basis of
different viewpoints such as reliability, application, and
MEMS. Three different classes of reliability of RF-MEMS
devices are briefed in the table 1.This taxonomy of RFMEMS devices has been done in accordance with the level
of mechanical complexity and boundary conditions. From
the MEMS viewpoint; these are put into three separate
classes based on where and how the MEMS actuation is
carried out relative to the RF circuit. The three classes are
RF-extrinsic, RF-intrinsic, and RF-reactive.
TAXONOMY OF RF-MEMS DEVICES AS PER
APPLICATION DOMAIN
REFERENCES
[1]
H J De Los Santos. Introduction to microelectromechanical
(MEM) microwave
systems. Artech House, Norwood, MA,
1999.
large
[2]
J J Yao. RF MEMS from a device perspective. J. Micromech.
Microeng. 2000; 10, R9-R38.
----
----
[3]
large
large
very
large
S Lucyszyn. Review of radio frequency microelectromechanical
systems technology. In: Proceedings of IEEE Sci. Meas. Techn.
2004, 151(2), p. 93-103.
[4]
Tunable
Capacitors
----
large
large
A Dec and K Suyama. Micromachined electromechanically
tunable capacitors and their application to ICs. IEEE Trans.
Microwave Theo. Techn. 1998; 46(12), 2587-605.
[5]
5
Resonators
----
6
7
Sr.
No
.
VCOs
Memtenna
Devices
Requiremen
t
very
large
-------
D J Young and B E Boser. A micromachined variable capacitor
for monolithic low Noise VCOs. In: Proceedings of Solid-state
Sensors and Actuator Workshop, Technical Digest, Hilton Head,
NC. 1996, p. 86-9.
[6]
RFID
Rada
r
Missile
s
Switches
large
very
large
very
large
J J Yao, S Park and J Denatale. High tuning ratio MEMS based
tunable capacitors for RF communications applications. In:
Proceedings of Solid-state Sensors and Actuator Workshop,
Technical Digest, Hilton Head, SC. 1998, p.124- 7.
1
------Instrumentatio
n
very
large
very
large
-------
[7]
2
Phase
shifters
----
----
very
large
very
large
E R Brown. RF-MEMS switches for reconfigurable integrated
circuits. IEEE Trans. Microwave Theo. Techn. 1998; 46 (11),
1868-80.
MEMS
Inductors
[8]
3
----
----
----
----
J Danson, C Plett and N Tait. Design and characterization of a
capacitive switch for improved RF amplifier circuits. In:
Proceedings of IEEE Custom Integrated circuits. 2005, p. 251-4.
4
Tunable
Capacitors
----
large
very
large
very
large
[9]
5
Resonators
mediu
m
very
large
large
[10]
6
VCOs
very
large
very
large
B
Lacroix et al. Sub-microsecond RF MEMS switched
capacitors. IEEE Trans. Microwave Theo. Techn. 2007; 55(6),
1314-21.
M Maluf and K Williams. An introduction to
microelectromechanical systems engineering. Ed. 2. Artech
House, Inc, 2004.
7
MEMTENN
A
[11]
K Wang, A C Wong and C T C Nguyen. VHF free-free beam
high-Q micromechanical resonators. J. Microelectromechanical
Sys. 2000; 9(3), 347-60.
[12]
W T Hsu and C T C Nguyen. Stiff compensated temperature
insensitive micromechanical resonators. In: Proceedings of 15th
Sr.
No
.
Devices
Requiremen
t
Wireless
WLA
N
GPS
1
Switches
very
large
very
large
2
Phase
shifters
----
3
MEMS
Inductors
4
----
large
---mediu
m
very
large
large
36
International Journal of Advances in Engineering Sciences Vol.3, Issue 4, October, 2013
IEEE Conference on Micro Electro Mechanical Systems,
Technical Digest, Las Vegas, NV. 2002, p. 731-34.
[13]
[14]
[15]
[19]
G M Rabeiz, G L Tan and J S Hayden. RF MEMS phase shifters:
design and applications. IEEE Microwave. 2002; 3(2), 72-81.
Z Feng et al. Design and modeling of RF MEMS tunable
capacitors using electro-thermal actuators. Tech. Digest. IEEE
MTT-S Int. Microwave Symp. 1999; 1507–10.
[20]
N S Barker and G M Rabeiz. Optimisation of distributed MEMS
transmission-line phase shifters-U band and W band designs.
IEEE Trans. Microwave Theo. Techn. 2000; 48, 1957-66.
C Goldsmith et al. Micromechanical membrane switches for
microwave applications. Tech. Digest. IEEE Microwave Theory
and Techniques Symp.1995; 91–4.
[21]
J B Yoon et al. High-performance three-dimensional on-chip
inductors fabricated by novel micromachining technology for RF
MMIC. Tech. Digest. IEEE MTT-S Int. Microwave Symp. 1999;
1523–6.
[22]
Chong Lie et al. Fabrication of a solenoid-type inductor with Febased soft magnetic core. J. of Magnetism and Magnetic
Materials. 2007; 308 (2), 284-8.
[23]
E R Brown. RF-MEMS switches for reconfigurable integrated
circuits. IEEE Transactions on Microwave Theory and Tech.
1998; 46 (11), 1868-80.
T P Singh et al. Current status of RF-MEMS devices for wireless
communication systems. Int. J. of Appl. Engg. Res. 2011; 6(18),
2181-89.
[16] J Maciel. Recent Reliability Results in RF MEMS. In:
Proceedings of IEEE MTT-S Int. Microwave Symp. Workshop
Notes, WFE: Recent applications in RF MEMS (June 12-17),
Long Beach, CA. 2005, p. 269-72.
[17]
T P Singh et al. RF-MEMS devices: problems regarding
reliability and degradation mechanisms. Int. J. of Contemp.
Practices. 2011; 1(6), 40-9.
[18]
A Dec and K Suyama, Micromachined varactor with wide tuning
range. Electron. Lett. 1997; 33, 922–4.
37