CURRENT STATUS OF RF MEMS DEVICES FOR

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Nov 21, 2013 (3 years and 7 months ago)

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143


CURRENT STATUS OF RF MEMS DEVICES FOR
WIRELESS COMMUNICATION SYSTEMS

8.1: INTRODUCTION


Recently the focus of RF
-
MEMS research has
changed its direction towards
system integration, reliability and the development of RF
-
MEMS devices with novel
configurations to meet needs of wireless communication systems. These

systems have
gained worldwide

attention in

recent years, particularly due
to the proliferation in MEMS
technology which has facilitated the development of radio frequency micro
-
electro
-
mechanical (RF MEMS) devices with novel configurations.

RF
-
MEMS has emerged as a potential technology for wireless, mobile and
satellite
communication and defense applications. Extensive research has been carried out
to identify and overcome the limitations of RF MEMS technology for replacing PIN or
FET based switches for low
-

loss applications. The key benefit of this technology is that
th
e devices can be manufactured by processes as of VLSI that has helped in the
realization of many sub millimeter
-

sized parts to provide RF functionality. RF
-
MEMS
components include resonators, oscillators, tunable filters, switches, switched capacitors,
va
ractors and inductors. Among these the RF
-
MEMS switch has emerged as a favorite
for its high RF performance, ultra
-
low
-
power dissipation
, very high isolation, very low
insertion loss, very low cost

and large
-
scale integration.

The evolution of RF
-
MEMS devices is needed for the expanding broadband
wireless radio communication. Radio frequency, semiconductors technologies and IC
-
compatible MEMS technologies are improving day by day and have an important role in
the fast growing
market of

wireless com
munication systems. This
ch
ap
t
er

aims to

prese
nt

the current status of the RF
-
MEMS devices briefly for the wireless and the satellite
communication systems. This
ch
ap
t
er briefly discusses
novel
RF
-
MEMS Switch
es
,
vibrating

micromechanical diamond disc reson
ator, RF
-
MEMS variable capacitors and
MEMS tunable inductor.

Wireless communication systems are utilizing wireless sense and control
technology to bridge the gap between the physical world of humans and virtual world of
physics and electronics. The dream
is to automatically monitor and respond to forest fire

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avalanches, faults in satellites, traffics, hospitals

etc.
Novel RF
-
MEMS devices have
better high frequency performance extending beyond 100 GHZ and high Q factor solve
many problems of high
-
frequency
technology for wireless communications.

RF
-
MEMS
finds application in phased arrays and reconfigurable apertures switching networks
, phase
shifters

and single
-
pole N
-
throw switches.

Now there exist many different cellular standards, ultra wide band,
wireless
sensor networks and many private systems representing Bluetooth, public mobile
services, WLAN etc. Also there is coming up 4G system requiring new frequency
spectrum. It will be multi network system and will be carried out by the unification of
di
fferent networks to enable the ubiquitous connectivity. RF
-
MEMS is strong contender
as a complimentary technology allowing “More than Moore
” where

we can find
out the
expectation in
F
igure
8
.1

as below
.


Figure
8
.
1
:

More
than

Moore’s Law

RF
-
MEMS are manufactured using conventional 3D structuring technologies, like
bulk micro
-
machining ,surface micro machining , fusion bonding ,LIGA etc. The
material used includes Silicon, GaAs, SiC or SOI substrates. RF
-
MEMS have great
potential for integr
ation and miniaturization. They provide lower weight, lesser power
consumption, lower insertion
losses, improved

linearity, superior performance and higher

145


quality factors than conventional communication components. RF
-
MEMS are no longer
laboratory toys.

The MEMS technology has improved significantly many RF
-
MEMS devices like
micro
-
switches, micro
-
machined inductors tunable capacitors, resonators, oscillators,
micro
-
transmission lines, filters, surface acoustic wave (SAW) devices etc. These RF
-
MEMS devices

are worldwide used recently in mobiles, communication and satellite
systems. The literature and several books have demonstrated and discussed a large
number of RF
-
MEMS devices for

wireless communication
systems.

Now, focus of RF
-
MEMS research is to develo
p reliable RF
-
MEMS devices with new configurations.

We
are here presenting some of them with new configurations like RF
-
MEMS switch,
vibrating micromechanical diamond disc resonator, RF
-
MEMS variable capacitor and
MEMS tunable inductor.

8.2: RF
-
MEMS
Drawb
ack
s and
Novel

Research So
lutions

RF
-
MEMS
technology has

its

own share of problems.
Besides

the drawbacks like

high actuation voltage and slow switching speed
,

there are two main problems associated

with standard MEMS capacitive devices which are

temperature sensitivity of the movable
membrane, and d
ielectric charging problems in the isolator layers

(leading to

stiction).
Possible improvements in the micromechanical aspect (such as in actuation
voltage, in switching speed ), improvement in the diele
ctric layer, improvement in the
power handling capability and improvement in the RF performance by reduction in
parasitic are presented below.

1. Improvements in the micromechanical aspect:

(a) Improvement in actuation voltage:

In practice,
low

spring constant designs like meandering suspensions or thin
springs
(
to achieve lower actuation voltage
)

are used
. But such
designs have issues like
the reliability of the device and

the switching speed. The
use of push
-
pull concepts
requires

a relatively

high actuation voltage. Also, the use of low
-

height

bridges has been
reported to lower the actuation voltage

but
at the cost of

a reduction in the capacitance
ratio.

To

lower the actuation voltage

the use of new materials like AlSi
0.04
or Pt as
membrane for the MEMS switch, use of electromagnetic actuation along with

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electrostatic forces and exploiting buckling and bending effects due to residual stress has
been reported
.

But,
AlSi
0.04
has a much higher RF transmission

loss.

In recent ti
mes,

a
totally free flexible membrane supported over

three pillars has been

proposed
to lower
the actuation voltage
but it
needs

a double sacrificial layer system

and
causes low

switching speed.

(b) Improvement in switching speed:

H
igh

switching speed

of
RF
-
MEMS devices

i
s a ma
in limitation, and much

work
has

not

been done to improve the speed
. Usually, the increase in the switching speed of a
RF
-
MEMS switch affects badly the switching voltage. A
quicker

switching can be
obtained with
improved

stiffness, b
ut it
certainly leads to an

increase
in
the actuation
voltage.

Mercier et al.

proposed

the miniaturization of the switches to

obtain high speed
and reliability. Further,
they
al
so

demonstrated sub
-
microsecond switching

times using
dielectric membrane
switches with built in

tensile stress
.

Lacroix et al. have
repor
ted

that

the spring constant of the beam increases by

adding simple bent sides on miniaturized
beam edges

that

increase
s

sub
-
microsecond switching time.

2. Improvement in the dielectric layer
:

The surface roughness of the dielectric layer

bad
ly affects the capacitance ratio of
switches

and
precise

explanation

of the roughness
using

the

statistical approach is
reported
.

Various

models

are proposed to
know

how contact resistance
responds to the

variations

in the contact area, the number of

asperities in contact and the temperature and
the

resistivity profiles at the contact
locations
.

The

RF
-
MEMS

reliability chief
ly depend
s

on the dielectric charging
phenomenon.
There are many reports on

impact o
f the dielectric material,

distributed
dielectric charging and the modeling of

dielectric charging
.

Many materials
like

ZnO or
Al
2
O
3
alloys, PZT, PZT or HfO
2
multi layers, polymer
-
ceramic composites, BST, TiO
2
,
amorphous diamond, etc.
are

now

under consid
eration as
possible

replacements

o
f

SiO
2
/Si
3
N
4

as the dielectric material for RF
-
MEMS

technology.

The

required properties of dielectric

material as
an

insulation layer
points

that the
key

for selecting
a substitute of

SiO
2
/Si
3
N
4

dielectric are
:

(a)

Dielectric

constant,


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(b)
Dielectric

strength,

(c)
Resistivity
,


(d)
Leakage

current,

(e)
Surface

roughness,

(f)

Ferroelectric

properties and

(g)
Charge

trapping density.

V
ery few materials are
capable

with respect to all of

these guidelines.
M
uch
research

is still required to find

any material
that can

substitute

SiO
2
/ Si
3
N
4
as the
dielectric
.


3. Improvement in the power handling capability:

Power handling capability of the RF
-
MEMS
devices

is

primarily

restricted

by two
factors

like
:


(a)
Joule heating for high power
that
results in

melting

and welding of the contact
,



(b
) self

biasing and RF latching.


E
lectro
-
thermal models
have been reported
to predict

the power handling
capability of RF
-

MEMS
devices.

The use of

two bri
dge level

topology has

been
investigated till 8W of RF power.

4. Improvement in the RF performance by reduction in

parasitic:

Various reports show

that the parasitic
has

a

significant

role in the
quality

factor
of the device
s
.
Due to its attenuation for RF

signals, t
he CMOS

grade

low
-
resistivity
silicon substrate is not
appropriate

for

high
-
frequency applications. For the high
resistivity substrates, a frequency of

10 GHz is
big

sufficient

to drive the silicon substrate
into

its dissipative mode

of dielectr
ic
.

Hence, the use

of substrates

and a suitable passivation layer
have

a

v
ita
l

role in
the RF performance of the device.
At 40 GHz, b
y

using polymers
like

Kapton,
polyimide
resin, BCB resin
etc

as the passivation

layer on the low resistivity
silicon

substrate, the
insertion loss

could be
decreas
ed to 3dB/cm
.

8.3:

NOVEL
RF
-
MEMS
SWITCH
ES

RF
-
MEMS switches play an important role in the application of the multiband
and multichannel wireless system for
reconfiguration

integrated circuits
.
Fig
ure

8.3

illustrates the RF micromechanical switch
by
Z
.

J.

Y
a
o

e
t

a
l
.

This particular RF device

148


consists of a beam fixed at both ends and suspended over a metal electrode. T
his could be
the central conductor of a coplanar waveguide. When sufficient amount of
voltage is
applied between beam and its underlying electrode, shorting the ensuing electrostatic
force

pulls the beam down to the electrode.


Figure
8.
2
:

R
adio
F
requency

Micro
-
Mechanical
Switches
.

As a result shorting between beam and electrode takes place (for the case of a
direct contact switch), or the electrode to beam capacitance gets increased which affects
an AC short (in case of capacitive switch, where a dielectric film atop the electrode).
In
both the cases the switch is effectively closed. In case of electrostatic actuation the
voltage levels greater than 20volts are generally used for radar applications. Although,
this actuation level is very high for the integrated transistor circuits use
d in wireless
handset applications. Therefore, either the actuation voltage level is reduced or
accommodated at the system
-
level via charge pumping etc.

RF micro
-
mechanical switches with insertion losses about 0.1dB and low switch
power consumption
of ord
er pico
-
watts have been reported. Hence these can easily
outperform semi
-
conductor switches (FETs and diodes) in antenna and filter switching
applications.

Their micro mechanical structure allows the use of metal materials with
much lower resistivity than
semiconductors. These RF micro
-
switches with microsecond

149


switching speeds are now reported. The microsecond speeds are useful for many

switching applications in wireless systems where low loss and high linearity are
important.

A
nother

novel RF
-
MEMS switch

with two movable electrodes is here mentioned
that is proposed by S K Lahiri et al
,
in
2009

by

using

bulk

micromachining

the substrate
below the CPW central

line, typically low
-
resistivity silicon, under the membrane

area
selectively. It
facilitates

both the CPW central line and

the shunt membrane to move in
opposite direction

at the same time
.

RF
-
MEMS switches with two movable electrodes
have also been reported by Babaei et al., but they mainly deal with the process
technologies to fabricate the swi
tch. The two movable electrodes result in the reduction in
actuation voltage and switching time and the removal of the silicon from beneath the
CPW central line causes a reduction in the parasitic.

Two wafers oxidized on both sides are used in the fabrica
tion of this switch. A
thin Cr/Au layer is deposited on the top surface of bottom high resistivity silicon wafer
and on the lower surface of top wafer. On bottom wafer, CPW structure is defined by
photolithography. The central conductor is covered by a fit
ting protecting layer. A thin
eutectic gold layer is selectively deposited on the outer conductors. Then, a thin low
-
loss
dielectric film (0.15μm) is deposited to cover the central conductor over the regions
where capacitors will be formed. Windows etched
on the oxide film in the backside (after
oxidation) are used for selective removal of silicon by bulk micromachining to release the
central conductor.

The gold film on top wafer is patterned by photolithography to shape the stripes bridging
between the
ground electrodes on either side. Then, a thin gold eutectic film is deposited
and patterned as required for the eutectic bonding. Windows are opened (after oxidation)
on the bottom surface such that the stripes become free to move after bulk
micromachinin
g. The backside alignment should be perfect. Finally, bulk
micromachining is performed in a fitting anisotropic etchant like EDP or TMAH with
oxide mask on both sides of the selected bonded wafers. The eutectic coated surfaces of
the two wafers are held to
gether face to face, aligned correctly and bonded at temperature
about 300
o
C at a adequate contact pressure. Fit anti
-
stiction structures and treatment

150


should be included.




Figure
8.
3: SEM picture of the fabricated MEMS shunt switch without top
substrate
.

A
n

SEM picture of the fabricated MEMS shunt switch without top substrate is
shown in f
igure
8.
3. The

thickness of gold electrodes in the diagram is about 1μm

and the
gap between the electrodes is estimated to 2.5μm.

The maximum and minimum value
s of
capacitance are

found to be 3.5pf and 30fF respectively.

These reported novel designs
have been mentioned by keeping in mind the limitations of the RF
-
MEMS technology
discussed in section 8.2. These have benefits of reducing the switching time, parasi
tic,
and actuation voltage at the same time.

In the early days, use of RF
-
MEMS

switches was difficult due to reliability
issues. Lifetime of early switches was of the order of only 10 million cycles.

Now, due
to
combination

of
contact engineering, adequat
e packaging and fabrication control, RF
-
MEMS switches with switching around 100 billion cycles are available. This clears a way
for more use of these switches into wireless communication systems.

RF
-
MEMS switches could have applications in band switching a
nd
filter
switching because future wireless receivers will need to operate at several bands covering
wide range of frequencies including 1
-
5 GHz.

Still the research issues for RF
-
MEMS
Switches include their reliability, their switching speeds, their switching
voltages etc.



151


8.4:

VIBRATING MICRO
-
MECHANICAL DIAMOND DISC RESONATOR

As

vibrating

micro
-
mechanical

resonators produces much higher quality fa
ctor
than their electrical counterparts, hence, these are essential components in
communication systems. With improving MEMS technology, these devices can be
designed to oscillate over a very wide

operating

frequency range, varying from <1 KHz
to >1GHz. Th
is much frequency range makes them ideal for ultra stable oscillation and
low loss
filter functions.


A lot of vibrating micro
-
mechanical resonators had been reported. For example,
clamped
-
clamped beam resonator, free
-
free beam resonator, wine glass disc r
esonator,
counter
-
mode disc resonator, hollow disc ring resonator etc.
Many researches had been
p
e
rformed

on the VHF range resonator to use the reference oscillator of the
wireless
communication systems
.

A 1.51GHz nano
-
crystalline diamond micro
-
mechanical
disc resonator had been
reported from U C Berkeley.

It can achieve a quality factor up to 11,555 in vacuum and
10,100 in air (i.e. at atmospheric pressure).Its quality factor is very impressive as
compared to
others. The

resonator consists of a 2
µm

thick
diamond disc having diameter
of 20
µm
. Its

diamond disc has been fabricated by using the CVD process.
The disc is
suspended by a doped
-
polysilicon stem self aligned to be exactly at it center. Th
e
n all this
is surrounded by dop
ed
-
polysilicon electrodes. Th
e spacing between elec
trodes and disc
perimeter is 80
nanometer.

When vibrating in its radial contour mode, the disc expands and contracts around
its
perimeter. This

amounts to a high stiffness and high kinetic energy. As disc center
corresponds to a nod
e

loc
ation for the radial contour vibration mode

shape, anchor losses
through the supporting stem are very much prevented.

In this way
,

this design can retain a very high quality factor even at UHF
frequency
. Also
, the

high stiffness
of
its radial contour
mode gives this resonator a
very
large total kinetic energy during vibration.
Due to this, the energy losses arising from
viscous gas damping are very much reduced in this resonator. Therefore, this helps the
resonator in maintaining quality factors greate
r than 10,000 even at atmospheric pressure.

This single

resonator attains a frequency applicable to the RF front terminals of
many commercial wireless devices. Since its quality factor is 10100 in air. Therefore, to

152


achieve high quality factor, this design

removes the requirement for vacuum which
makes it economical
.


Figure
8.4:

SEM Photograph of the C
VD Diamond Micromechanical Disk
Resonator
.

This resonator can operate at and beyond gigahertz frequencies when properly
scaled and do so while retaining
sufficiently large dimensions to maintain proper power
handling
. Fig
ure
8.4

shows the SEM photograph of the diamond disc resonator with self
aligned stem. Also the frequency temperature coefficient of this resonator is
-
12ppm/deg.C. It has found practical
applications as reference local
oscillator, VHF
-
S
-
Band Filter and RF channel select networks.

Micro
-
mechanical resonators are replacing conventional crystal oscillators used as
frequency
reference

or time source devices
. These resonators have more advanta
ges than
crystal oscillators in terms of money, size reduction, and better integration with silicon
etc.
T
here operating frequency could be increased into GHz range. Higher frequency
operation would also allow new filtering and mixing functions, and comple
tely novel
architectures.

8.5:

RF
-
MEMS VARIABLE TUNABLE CAPACITOR

Variable capacitor, with performance superior to varactor diodes in areas such as
non
-
linearity and losses, can be feasible with MEMS technology. Initially,
MEMS

parallel plate variable
capacitors with an electro
-
static actuator are

fabricated. The tuning

153


range of such type of MEMS capacitors is limited up to 50% due to the failure of
capacitor structure when the voltage becomes more than the pull
-
in
-
voltage.


Figure
8.5:

A Schematic Di
agram of MEMS Variable Capacitor
.

Then a MEMS variable capacitor with 100% tuning range was proposed in. In this
actuation electrodes are spaced differently from the capacitor plates. But, in practice, this
capacitor should operate over a smaller tuning ra
nge to avoid collapse of capacitor.
Therefore, MEMS variable capacitors having a much wider tuning range should be
designed without the collapse of the capacitor structure.

Another MEMS variable capacitor with novel configuration and design was
proposed in
. Figure 8.5 shows a schematic diagram of this capacitor. It consists of two
movable parallel plates with an insulating dielectric layer on top of the bottom plate.
Since both plates are flexible, both plates can attract each other. Hence, maximum
spacing
between two plates decreases before the pull
-
in
-
voltage comes into action. In
addition the capacitor has an extended tuning range even after the two plates touched
each other.

This capacitor is constructed using two structural layers, three sacrificial lay
er and
two insulating layers of Nitride. The top plate is made of nickel with a thickness of 24
µm. It is covered by a gold layer of thickness 2 µm. The bottom plate is made of poly
-
silicon which is covered by a nitride layer of a thickness of 0.35 µm. The

different layers

154


use to make this capacitor are prepared using MetalMUMPs process. The two
dimensional layers are generated using CoventorWare.


Figure
8.6:

An SEM image of the MEMS Variable Capacitor
.

An SEM picture of the fabricated MEMS variable capac
itor is shown in fig
ure
8.6
. A
t 1GHz the achievable tuning range of this capacitor is found equal to 280 %. This
value is very much greater than that of traditional parallel plate variable capacitor
s
.
Paired with medium quality factor inductors, this
capacitor can enhance the performance
of low noise voltage controlled oscillators (VCOs). Also
,

when paired with micro
-
mechanical inductor
s

it can allow implementation of low noise VOCs with much lower
power consumption than those using IC technology
.

8.6:

RF
-
MEMS TUNABLE INDUCTORS

Paired with inductors having quality factor greater than 20, tunable MEMS
capacitors will be useful in communication circuits.

Conventional IC technology can only
produce spiral inductors with quality factors less than 7 due to
excessive series resistance
and substrate losses.


Using MEMS technologies inductors with quality factor as high as
70 at 1GHz have been demonstrated.

T
his inductor with qua
lity factor about 70 when

paired with
a micro
-
mechanical
capacitor should enhanc
e performance of low noise VCOs with much lower power
consumption than those using traditional IC technology. Also, if inductors with quality
factor about 300 become possible, then tunable RF filters might be achievable that could
find use in RF transceive
rs circuits like filters
,

matching networks etc.
MEMS tunable
inductors can also be used to build filters with tunable bandwidth.

By coupling a drive coil to the RF inductor, a few variable inductors have become
feasible. The tunability is attained by varying mutual inductance between the two

155


inductors. Based upon the relative phase of the current in the two coils, the mutual
inducta
nce component can be increased or decreased continuously. By this technique, a
100% tuning range has been attained. But an additional driver circuit is used in the drive
coil in this technique.

The tunable inductor circuit
proposed in

consists of variable
capacitors, two fixed
capacitors and two inductors. The parallel plate variable capacitor is shown in the Fig
ure
8.7
. It is fabricated as follows: the upper plate is made of poly 2 and lower plate is made
of poly 1. Then a layer of gold is deposited on upp
er plate. The air gap between these
plates is of 0.75
µm
. To ensure the etching of the oxide between these two plates, holes
are made in the plates of the capacitor. Capacitance value can be changed by voltage
applied to the variable capacitor. When top pl
ate moves towards fixed lower plate, the
distance between plates changes. This changing distance changes the value of
capacitance.


Figure
8.7:

MEMS Tunable Inductor Chip

The tunable inductor was constructed using Multi
-
User MEMS processes
(MUMPs) surface ploy silicon micro
-
machining technology. The MUMPs process has
three layers of poly silicon (poly0, poly1, and poly2), two layers of oxide and one layer
of gold (poly2). T
he gold layer is deposited on the top ploy silicon layer. The thicknesses

156


of gold layer, poly 2 layer , second oxide layer, poly 1 layer, first oxide layer and ploy0
layer are 0.5 µm, 1.5 µm, 0.75 µm, 2.0 µm, 2.0 µm, 2.0 µm and 0.5 µm respectively.

The de
sign of fixed capacitors is same except that voltage is not applied to the
plates of the fixed capacitors by
R. R. Mansour et al.

This design uses eight pads: six
pads for the coplanar RF input and output signals and two pads for grounds and a DC
voltage.
Here the pads are made of poly 2 layer and gold layers. To ensure the trapping of
oxide and protection from the HF etch, anchors are provided around the edges of the
pads. The following Figure 8.7 shows a fabricated MEMS tunable inductor chip.

8.7

Summary

In recent years, the focus of RF
-
MEMS research has been shifted to system
integration, reliability and the development of RF
-
MEMS devices with novel
configurations to meet needs of wireless communication systems.
The evolution of RF
-
MEMS needed for expandi
ng the broadband wireless communication systems. It should
be progressed in parallel with the miniaturization of the CMOS following diversification
technologies of “More than Moore”. Micromechanical devices attained via MEMS
technologies have been describe
d that can play potentially a

key role in removing the
board
-

level packaging requirements th
a
t currently limit the size of communication
transceivers etc.

RF
-
MEMS devices are the strong candidates that could meet future wireless
communication systems req
uirements in terms of low power consumption
,
reconfiguration
, reliability and miniaturization
. As there are many different kinds of
technologies, modeling, theories, processes and simulation techniques for the RF
-
MEMS,
there would be no anxiety for the fut
ure because many researchers, groups etc have
extremely high motivation for the RF
-
MEMS devices.

The literature and several books have demonstrated and discussed numerous RF
-
MEMS devices for wireless communication systems. Now, RF
-
MEMS research is
focusing

to develop reliable RF
-
MEMS devices with new configurations. This very
chapter, here, presents some of them with novel configurations such as RF
-
MEMS
switches, vibrating micromechanical diamond disc resonator, RF
-
MEMS variable
capacitor and MEMS tunable i
nductor.
Possible improvements in the micromechanical
aspect (such as in actuation voltage, in switching speed), improvements in the dielectric

157


layer, improvements in the power handling capability and improvements in the RF
performance by reduction in para
sitic are also presented in this chapter.

During the last years, RF
-
MEMS have experienced a tremendous progress in

terms of technology development and RF
-
applications.

RF
-
MEMS circuits have been
demonstrated with such

exceptional

properties that they potentially can be part of the

essential building blocks of next generation emerging wireless

communication and RF
-
sensing applications. RF
-
MEMS

technology has reached

a level where MEMS switches
can

be successfully

integrated into pr
actical RF systems with proven long term

reliability.
The integration of MEMS switches and

Monolithic Microwave Integrated Circuits
(MMICs) is

thus

considered as a next logical step in MMIC process

growth
and

commercialization

of RF
-
MEMS technology.