TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING
IEEJ Trans 2007; 2: 249–261
Published online in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/tee.20139
Review Paper
RF MEMS Technology
Hyeon Cheol Kim∗
Kukjin Chun∗a
Recently, to meet the demands of multimode/multiband functions for the next generation communication
systems, RF MEMS technology is being developed for the reconfigurable transceiver system. In this article,
RF MEMS devices such as switches, high-Q inductors, and high-Q resonators are reviewed for their operating
principles and device structures, as well as the reliability and commercial issues. Single pole single throw
(SPST)-type MEMS switches show characteristics superior to solid-state switches in the aspects of insertion
loss, isolation, and linearity. Single pole multithrow (SPMT) switches will enable ultra small-sized cellular
phones, but insertion loss and coupling between channels should be improved. For a high-Q inductor, out-ofplane inductors show improved results, and an inductor with a Q-factor of 75 at 1GHz has been fabricated by
using the internal stress of a MoCr film. The MEMS inductor improves the performance of a voltage controlled
oscillator (VCO), but proper hermetic packaging and standard libraries are also necessary for mass production.
The MEMS resonator can operate up to 1.4 GHz and can be used as an oscillator for the timing device as well as
component of the filter circuit. The SiTime Company recently has started delivery of a product with performance
similar to the quartz oscillator, in which they solved the reliability issues by modifying the annealing and vacuum
packaging.  2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.
Keywords: RF MEMS technology, RF MEMS switch, high-Q inductor, high-Q resonator
Received 11 January 2007
1. Introduction
In the digital convergence era, next generation wireless communication systems seek terminal and personal
mobility, as well as portability, with a view to realizing
universal personal communication [1,2].
Wireless mobile communication technology has gradually grown from the initial advanced mobile phone
service (AMPS) in the mid-1990s, the 2nd generation
CDMA/GSM and the 2.5th generation PCS to the 3rd
generation IMT-2000/WCDMA for multimedia communication services, as shown in Fig. 1. Owing to various users’ demands, services have expanded from voice
communication into various multimedia communications
such as music files, video images, and so on.
Furthermore, the next generation communication systems need global roaming technology in order to realize
the global society of the 21st century, in which people
communicate with each other using a single transceiver
a Correspondence to: Kukjin Chun. E-mail: kchun@mintlab.snu.ac.kr
∗ School of Electrical Engineering and Computer Science, Seoul
National University, San 56-1 Shinlim, Kwanak, Seoul, 151-742, Korea
system anywhere in the world regardless of the operator. Currently, different wireless communication systems
are implemented in America, Europe, and Japan. There
is also much interest in the next generation network,
with one terminal to cover multimode/multiband functions including cellular, WLAN, and WiMAX in order
to enjoy various communication services as shown in
Fig. 2.
To achieve these goals, it is necessary to develop the
communication devices in the radio frequency (RF) range
with the size reduction of the RF module at low cost.
The ubiquitous life of the near future demands very
small communication modules with low power consumption in order to detect and analyze the information from
the various health monitoring devices regardless of the
location. For newly emerging technology such as mobile
radio frequency identification (RFID) or ubiquitous sensor network (USN), small, hand-held devices should be
developed soon.
Microelectromechanical systems (MEMS) technology
has been available to make the devices small, intelligent,
and easy to be integrated. Since the 1990s, MEMS
technology has been used for a variety of industrial
 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.
H. C. KIM AND K. CHUN
Fig. 1 Trends in wireless communication systems
Fig. 2 Next generation communication systems
applications, such airbag sensors, inkjet heads, digital
mirror TV, and micro fluidic biochips.
Generally speaking, MEMS is the technology to
make 2D or 3D microstructures by basically using
the semiconductor fabrication processes. Therefore, it
is easy to modify and reuse an old VLSI fabrication
facility for MEMS manufacture. RF MEMS technology
enables the realization of small communication device
elements or modules with high performance. It also
provides lower insertion loss, higher isolation, and
better linearity than the semiconductor devices currently
used.
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RF MEMS TECHNOLOGY
Table I. Comparison of commercially available solid-state switches and the RF MEMS switches at
2.4 GHz frequency with 3W power handling
Insertion loss (dB)
Isolation (dB)
Switching time
Switching voltage/current
Contact resistance ()
Return loss (dB)
Linearity (IP3) (dBm)
EMR [9]
Reed [10]
PIN diode [11]
FET [12]
MEMS [13]
0.3
25
4 ms
5 V/5 mA
0.15
>15
>50
0.25
15
250 µs
5 V/3 mA
0.13
15
>50
0.6
25
<1 µs
5 V/10 mA
<1
20
<40
0.5
15
50 ns
5 V/1 µA
<1
>15
<40
0.15
35
70 µs
68 V/0.2 µA
0.15
20
>65
(a)
(b)
Fig. 3 Configuration of (a) shunt-type and (b) series-type switches
As RF MEMS technology uses almost the same fabrication processes as in semiconductor processing, it is
easy to employ batch processes and to integrate the
device with an electronic circuit on the same substrate.
Besides these advantages, reconfigurable communication
modules can be achieved with the addition of mechanical movements. Typical research topics in RF MEMS
include switches, high-Q inductors, tunable filters, and
high-Q resonators [3].
Many research teams are now developing RF MEMS
devices, such as the ones at Hughes, the University of
Michigan, Rockwell, UC Berkeley, MIT, and Analog
Devices. Since the year of 2000, many companies
including start-ups have been directing their efforts
to commercialize devices with enhanced reliability. At
present, switches, resonators, and filters are available on
the market.
In this paper, important RF MEMS devices such
as switches, high-Q inductors, and high-Q resonators
are reviewed with regard to their operating principles
and device structures, as well as the reliability and
commercial issues.
Nowadays solid-state switches available are of the
FET, PIN diode, or Reed types, but they suffer from
the problems of high insertion loss, low isolation, high
power consumption, and bad linearity, which make RF
MEMS switches more promising. Table I compares the
commercially available solid-state switches and the RF
MEMS switches at 2.4 GHz frequency with 3W power
handling.
MEMS switches show characteristics superior to the
existing solid-state switches in the aspects of insertion
loss, isolation, and linearity, but have the disadvantage
of a high operating voltage. These MEMS switches are
usable for frequency band selection or mode selection to
transmit the signal to an antenna or to receive the signal
from an antenna connected to the terminal.
RF switches can be categorized into two groups, the
shunt type and the series type, depending on the operating
principle, as shown in Fig. 3.
A shunt-type switch normally remains in the ‘ON’
state and changes to the ‘OFF’ state when a control signal
is applied. On the contrary, a series-type switch normally
remains in the ‘OFF’ state, and changes to the ‘ON’ state
when a control signal is applied.
RF switches can also be categorized as capacitive and
resistive types depending on the method to transmit the
signal. The resistive type is realized by metal-to-metal
direct contact, while in case of the capacitive type, a
dielectric layer is formed between two metals. When
the control signal is applied, the moving upper electrode
comes into contact with the dielectric layer, resulting in
an impedance difference according to the ON/OFF states
as shown in Fig. 4.
2. RF Switches
An RF MEMS switch is a typical MEMS device that
is expected to be used in various application areas such
as a tunable filter, a gain controllable power amplifier
(PA) or low-noise amplifier (LNA), a dual-band voltage
controlled oscillator (VCO), and a dual-band antenna
[4–8].
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H. C. KIM AND K. CHUN
(a)
(b)
Fig. 4 ‘ON’–‘OFF’ operation of (a) resistive and (b) capacitive switches
In the case of the resistive switch, the contact resistance between the two metals is important to determine
the characteristic of the switch when in the ‘ON’ state. To
lower the contact resistance, gold is usually used. Gold
has good conductivity and good resistance to oxidation.
However, during prolonged operation, gold is damaged
by the stress due to the collision shock when the switch
is mechanically turned on, or by the microwelding phenomenon when a high-power signal flows through the
contact area. To overcome these disadvantages, the gold
composites such as gold-palladium and gold-platinum are
used [14].
In case of the capacitive switch, the impedance
difference determines the characteristics of the switch,
such as the insertion loss, the isolation, switching voltage,
and so on. When the switch is in the ‘ON’ state,
the impedance is determined by the dielectric constant
of the material, which is lowered as the dielectric
constant becomes higher. Therefore high-k materials
such as silicon nitride are used to implement the switch
with a low insertion loss. But, the problem of stiction
occurs when the charges are accumulated during the
operation and the dielectric layer gets damaged owing
to the selective etching of the sacrificial layer during the
fabrication process.
Figure 5 displays a cross-section of a capacitive
MEMS switch for RF applications. The actuation is
achieved using an electrostatic force between the top and
the bottom electrodes, which is given by:
F =
QE
CV E
CV 2
εAV 2
= =
= 2
2
2
2 g + εtdr
2 g + εtdr
Fig. 5 Cross-section of a capacitative MEMS switch
where g0 is the initial height of the membrane, and k
is the spring constant of the membrane. Equilibrium is
achieved when the two forces are equal. The solution of
this cubic equation in g results in a stable position up to
approximately g0 /3 and then a complete collapse of the
switch occurs to the down-state position. The voltage that
causes this collapse is called the pull-down voltage, Vp ,
and is
Vp ∼
=
εAV 2
2 = k(g − g0 )
2 g + εtdr
(3)
This pull-down voltage can be reduced by making g0
narrower, but the isolation characteristic will be worse
because of low impedance at the ‘OFF’ state.
According to the operating principles, the switches are
categorized as electrostatic, magnetostatic, piezoelectric,
and thermo mechanical types and Table II shows the
difference among them.
Electrostatic switches consume no power and are fast
and of small size, but have a high operating voltage
compared to other devices.
On the basis of the moving structure of the contact
part, switches are categorized as cantilever, plate, rotary,
and lateral types. The published MEMS switches are
listed in Table III. Plate switches are available these days
owing to their high reliability, while cantilever switches
are simple to realize.
To receive and transmit signals at different frequencies
in one terminal in the multimode operation, as shown
in Fig. 6, single pole multithrow (SPMT) switches are
preferable to multiple single pole single throw (SPST)
switches. Considering the switching speed, relatively
(1)
where, V , g and C are the applied voltage, gap distance
and capacitance between the lower and the upper electrodes, respectively, and A, td and εr are the area, the
thickness and relative dielectric constant of the dielectric
material. On the other hand, there is a pull-up force due
to the spring constant of the switch:
F =
8kg03
27εA
(2)
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RF MEMS TECHNOLOGY
Table II. Comparison of MEMS switches with various operating principles
Electrostatic [15–20]
Magnetostatic [21–23]
Piezoelectric [24]
20–80
0
0
Small
1–200
50–1000
3–5
20–150
0–100
Medium
300–1000
50–200
3–20
0
0
Medium
50–500
50–200
Voltage (V)
Current (mA)
Power (mW)
Size
Switching time (µs)
Contact force (µN)
Thermo mechanical [25]
3–5
5–100
0–200
Large
300–1000
500–4000
Table III. Comparison of MEMS switches with various moving structures
Characteristics
Reference
Cantilever
Plate
Torsion
Lateral
SiO2 cantilever:
200 µm length
10 µm width
2 µm thick
Gold contact area:
400 µm2
Yao [15]
Single
crystalline
silicon spring
Driving electrode
area: 1mm2
Gap distance: 3 µm
Actuation voltage:
∼5 V
Push-pull operation
SiNx cantilever structure
Electrothermal
actuation
0.6 µm of LPCVD
silicon nitride Poly-Si
beam Au side contact
Komura [19]
Hah [20]
Wang et al. [25]
Filter Bank
Receiver
Transmitter
MEMS Switch
HEMT Switch
Fig. 6 Switched filter banks for wireless applications
Fig. 7 Diagram of eddy current generation due to a magnetic
field
slow MEMS switches would be useful for channel selection, and solid-state switches such as a high electron
mobility transistor (HEMT) would be used for a frequently changing signal.
Table IV shows recently the developed SPMT switches. SPMT types can realize smaller sizes by sharing the
ground plane than the SPST type, but cross-talk between
channels and insertion loss increase are problems.
However, unlike in the case of a low-frequency
CMOS circuit, it appears to be difficult to adopt this
technology directly to high RF applications. Because
existing inductors are made by a conventional CMOS
process using Al or Cu metallization on a silicon
substrate, they do not provide high enough performance
for high-end RF ICs since the resistance increases owing
to the small metal thickness as the integration level of
the CMOS circuit is higher [29,30].
Besides, the substrate loss due to the eddy current
degrades the characteristics of the inductor, which may
be ignored in low-frequency applications. Eddy current
flows through the closed loop that is locally formed in
a conductor of spiral shape, and is generated by the
change of the magnetic flux passing through the inside
of the conductor. As shown in Fig. 7, a blocking current
develops to prevent the change of the magnetic flux when
3. Inductors
Complementary metal oxide semiconductor (CMOS)
is a key technology in the rapidly growing mobile
communication solution owing to its high productivity,
integration levels, and well-established infrastructure.
High-Q inductor is a key element in determining the
performance of RF circuit blocks such as VCO, LNA
and PA, which currently use the inductors fabricated by
monolithic CMOS technology.
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H. C. KIM AND K. CHUN
Table IV. SPMT MEMS switches
Research group
Feature
SPMT
Insertion Loss (dB)
Isolation (dB)
Operating voltage (V)
Switching time (µs)
Size (mm2 )
ETRI [26]
Michigan [27]
SP6T
0.43–0.7@6 GHz
40@6 GHz
27.5
8
1
SP4T
0.3–0.9@10 GHz
40@10 GHz
50
10
—
the magnetic flux increases along the axis perpendicular
to the conductor plate.
Recently there have been a number of studies to
overcome these problems by using MEMS technology.
Nanyang Tech. [28]
SP4T
1@6 GHz
30@6 GHz
30
—
—
The problem of resistive loss originating from a thin
metal can be solved by electroplating to make the metal
layer thicker up to 20 µm. The substrate loss due to the
eddy current can also be reduced when the inductors are
lifted up from the substrate.
Figure 8 shows the photograph and the measured
results of the inductor fabricated by the researchers at
Stanford University in 1998. They made a patterned
ground shield (PGS) on the substrate to prevent eddy
current generation in the substrate, which is patterned in
a direction orthogonal to the current flow [31,37].
The graph shows that the Q-factor of the inductor
measured at 2 GHz was 7.6, which was 20% improved
compared to the method without the PGS.
Figure 9 shows the research result of a solenoid-type
inductor presented by KAIST in 1998 [32]. As the
solenoid-type inductor occupies less area than the spiral
inductor, a circuit of smaller area can be fabricated.
Also, by using Cu, the Q-factor of the inductor can
be higher because of the low resistance of the Cu line
than the conventional Al. Because the magnetic flux is
formed parallel to the substrate, the substrate loss due
to the eddy current should be reduced. To fabricate a
thick metal structure, Cu electroplating was used. The
fabricated inductor shows a Q-factor of 19 at 5.5 GHz.
Figure 10 shows the solenoid inductor with an air gap
designed by Samsung Electronics [33]. As this inductor
is elevated with an air gap above the substrate, it can
additionally reduce the substrate loss that might occur
through the contact with the substrate. The basic concept
is that the substrate loss should be minimized because
the relative dielectric constant of air is very small.
This inductor was also fabricated using Cu electroplating, and its Q-factor was measured to be 58 at 7 GHz.
Figure 11 shows another MEMS inductor fabricated at
the Imperial College [34]. Using the reflow process of
the solder, the spiral inductor was made to be suspended
perpendicular to the substrate. The 90◦ tilted inductor has
(a) top view of the pattern ground shield
(b) Q factor vs. frequency
Fig. 8 Inductor with patterned ground shield [31]
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RF MEMS TECHNOLOGY
(a) Die photograph of the solenoid type inductor
(a) Photograph of the fabricated inductor
(lower right: concept diagram)
(b) Q factor vs. frequency
(b) Measured Q factor vs. frequency
Fig. 10 Solenoid-type inductor with air gap [33]
Fig. 9 Solenoid-type inductor made by Cu electroplating [32]
a Q-factor of 20 at 3 GHz, which is a few times higher
than the CMOS inductor which lies on the substrate.
Figure 12 shows the out-of-plane inductor fabricated
by PARC (Palo Alto Research Center, Inc.), which used
the internal stress of a MoCr film [35]. After depositing
the MoCr film on the sacrificial layer, the film was
removed by wet etching. The released MoCr film was
curled because of the residual stress, and both ends of the
released film meet each other to create a curl as designed.
The measured result shows a Q-factor of 75 at 1 GHz,
but this inductor has the drawback that the self-resonance
frequency is too low to be applied at high frequency.
So far, the inductors with air gap were investigated.
Though these inductors have a high Q-factor, they require
very good packaging to be adaptable to commercial
products. Inductors to satisfy both high performance
and stability of structures by packaging require more
development.
To alleviate this problem, inductors are fabricated on
a low-k dielectric material instead of being elevated
from the substrate [36]. Figure 13 shows the SEM
photograph and the test result of the inductor made
on benzocyclobutene (BCB) dielectric material, which
Fig. 11 Inductor suspended perpendicular to the substrate [34]
is often used as the low-k dielectric material in highfrequency applications, since BCB is a thermally and
chemically stable material and its relative dielectric
constant is only 2.6. The inductor on BCB is structurally
stable, but has shown a Q-factor of about 25 at 2 GHz
so far, which is slightly lower than that of the inductor
with an air gap.
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H. C. KIM AND K. CHUN
(a) SEM photograph of the inductors on BCB
(a) SEM picture of the curl type MoCr inductor
(b) Q factor vs. frequency
Fig. 13 An inductor on a BCB substrate [36]
(b) Q factor vs. frequency
The MEMS resonator is trying to replace its position.
Although a number of resonators including ceramic,
silicon, and passive elements have been studied in the
past 60 years, none of them could replace the quartz
resonator as a basic timing device which shows high
temperature stability and good phase noise. Today, more
than one billion quartz oscillators are estimated to be used
in almost all electronic devices including mobile phones,
broadband terminals, entertainment devices, industrial
equipment, digital cameras, and even in automobiles.
However, quartz resonators cannot be integrated with
a circuit on a silicon wafer, and have the disadvantages of
high cost of miniaturization, nonstandard fabrication and
packaging processes, and high sensitivity to temperature,
vibration, and shock. Therefore, the electronic industry has been seeking new technology to overcome these
weaknesses of the oscillator without sacrificing its performance.
Nathanson and Newell published a report on a surfacemicromachined resonant gate transistor made with a
metal wire in 1965 [39]. In 1982, Peterson demonstrated
silicon’s properties to be used as a resonator [40]. Since
the 1990s, MEMS resonators have been developed by
using silicon as the resonant material.
Fig. 12 Curl-type out-of-plane inductor [35]
So far, various MEMS inductors have been reviewed,
and the MEMS inductor shows better performance than
the existing spiral inductor. The results are summarized
in Table V.
An example of the use of the MEMS inductor to
RF IC is shown in Fig. 14 [38]. MEMS inductors
were integrated on SiGe RF IC to build a 5.2-GHz
VCO circuit. The circuit elements except inductors were
fabricated using an IBM SiGe process. MEMS inductors
were fabricated to be suspended over the substrate at a
height of 20 µm. The Cu-electroplated MEMS inductor
shows a Q-factor over 20 and an inductance of 0.8 nH.
The VCO with MEMS inductor shows 5 dBc better phase
noise characteristics and 6 dBm better output power than
that with a CMOS inductor.
4. Silicon Resonators
The quartz resonators has been chosen as a basic
timing device since the middle 1940s owing to its
excellent mechanical and piezoelectric characteristics.
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Table V. Summary of inductors
Research group
Stanford Univ. [31]
Mikro Systeme Int’l AG [37]
KAIST [32]
Samsung Electronics [33]
Imperial college [34]
PARC [35]
Hong Kong Univ. [36]
Inductance
(nH)
Quality
Factor
f(Qmax )
(GHz)
Type
7.4
1.8
2.5
2
2
13
1
6.76
6
19
58
20
85
25
2
3.5
5.5
7
3
1
2
Patterned ground shield
n+ blocking structure
Solenoid inductor
Air-suspended solenoid inductor
Out-of-plane inductor
Out-of-plane inductor
Low-k dielectric
(a)
(b)
(c)
(d)
Published
1998
1998
1998
1998
2001
2002
2002
Fig. 14 5.2 GHz VCO chip with MEMS inductor: (a) Circuit block diagram; (b) A test board for measurement;
(c) SEM photograph of the fabricated MEMS VCO; (d) Enlarged SEM photograph of the integrated MEMS inductor
on VCO
operating circuit can be integrated on a silicon wafer with
CMOS circuitry.
However, there are still technical problems, such as
drift due to packaging contamination, aging due to
fatigue, and frequency shift due to temperature variation
of the silicon. The MEMS resonator is too small to
respond sensitively to mass variation such as surface
contamination [41].
The MEMS resonator is different from a quartz resonator in the aspects of mechanical/electrical characteristics, the fabrication method, and the operating circuitry.
A MEMS resonator requires a simpler fabrication process for a smaller size than the quartz resonator. Furthermore, by using batch fabrication, silicon resonators
can be made much cheaper than quartz resonators. The
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H. C. KIM AND K. CHUN
Table VI summarizes the types of silicon resonators.
In the case of the vertical resonator, the resonating
beam follows Newtonian’s mechanics and the dynamic
response is given by:
mg + bg + kg = Fe
oxide filling, polysilicon deposition, vent holes, sacrificial layer release, 1000 ◦ C baking, and thick polysilicon
capping layer formation on a 10 − µm thick (active silicon) SOI wafer. Figure 16 shows the top view and the
cross-section of the resonator developed by SiTime Co.
There are three major issues of frequency errors to be
controlled for commercial use, that is, initial frequency
offset, temperature coefficient, and aging. In the case
of SiTime’s resonator, the high-temperature-annealed
silicon resonator shows a resonance frequency drift of
less than 1 ppm in 1 year and hysteresis characteristics
of less than 0.2 ppm in cycling tests of over 300 cycles
at −40 to 80 ◦ C, which is a result of the stability of the
resonant material due to the high-temperature annealing
and hermetic vacuum sealing [47]. Also, the hermetic
vacuum sealing process also protects the resonator from
particulate contamination [48].
Table VII shows a summary of the two types of
resonators. Their characteristics are quite similar to those
of quartz resonators.
(4)
where m, b, and k are the mass, damping coefficient and
spring constant of the resonator, respectively, and Fe is
the electrical force. This is a second-order system with a
resonant frequency of f0
1
f0 =
2π
kr
mr
At the early stage of the resonator development, the
comb drive-type of silicon resonator was used, and then
to obtain a high Q-factor, the fine gap resonator with a
high aspect ratio was developed. To make the resonator
operate at higher frequencies, the contour resonator
was developed. Other substrates with higher Young’s
modulus are being investigated by using SiC or diamond
as shown in Fig. 15. Figure 15 shows the trend in the
development of the frequency and Q-factor of silicon
resonators developed so far. The maximum operating
frequency achieved from these resonators is 1.4 GHz as
reported by the group at the University of Michigan.
Silicon resonators have been commercially developed
by SiTime Co. and Discera Co. They improved the reliability issues by reducing the fatigue of the beams by
removing the microcracks and scallops by annealing.
The packaged resonator of the SiTime Company was
fabricated with the processes of 0.4 µm trench etching,
5. Conclusions
For high-speed mobile Internet as well as cellular
phones, people are asking for more complicated functions
in different frequency bands. Reconfigurable transceiver
systems are adequate for the demands, but they require
a much smaller size with very low power dissipation.
RF MEMS technology will be a good solution to the
next generation of communication systems. In this paper,
RF MEMS devices such as switches, inductors, and
resonators were investigated for these applications.
Table VI. Silicon resonators
Type
Vertical resonator [42]
Lateral resonator [43]
Comb resonator [44]
Contour resonator [45]
Figures
Q-factor
Resonance frequencya
f0 =
1
2π
kr
mr
0 = √L
Leff =
meff
η2
eff Ceff
, Ceff =
∂C η = VDC
a
0 =
1
η2
ksys
k
m
f0 =
α
R
E
ρ
,
∂x
E, Young’s modulus; ρ, Density of the disk; α, a factor based on the modal shape and Poisson’s ratio; R, radius of the disk.
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Fig. 15 Trend of the development of the frequency and Q-factor of the silicon resonator
developed [46]
Fig. 16 SiTime’s resonator: (a) Top view; (b) Cross-section [47]
loss. Henceforth, it should be possible to integrate an
inductor with CMOS circuitry with the same performance
and be packaged for good stability of the structures at low
cost. Standard libraries including those for inductors for
circuit designers are also necessary for mass production.
In the case of silicon resonators, it is expected to
replace the quartz oscillator in the market because of its
similar performance to the quartz oscillator, but smaller
size and a low cost. Currently, resonators can operate
up to 1.4 GHz, which is suitable for many applications
including filters above 1 GHz. Fine dimensional control
with a high aspect ratio or very sensitive readout circuitry
is required for higher frequency operations.
The need for high-performance passive and active
devices for high-frequency operation such as wireless
mobile transceiver is increasing. In order to meet both
high mobility and portability as well as the low cost
of production in the new communication environment,
such devices should be essentially integrated with VLSIs
RF MEMS switches are superior to solid-state switches
in the aspects of insertion loss, isolation, and linearity,
but are inferior because of the high operating voltage, low
switching speed, and poor reliability. To overcome these
problems, converters can be used for lower voltages, and
different materials are under evaluation for a long life
span. Despite the short history, some SPST-type switches
such as Teravicta’s TT712, Radantmems’s RMSW220D,
and Panasonic’s AMEX101 are commercially available.
SPMT switches will enable ultra small-sized cellular
phone, but insertion loss and coupling between channels
should be improved.
High-Q inductors have been developed to enhance the
circuit performance. To reduce the resistive loss of the
inductor, processes to realize 3D structures, to make the
metal line thicker, or to replace the metal with a higher
conductivity material such as Cu have been developed.
Alternate approaches to lift the inductor up above the
substrate have matured, resulting in minimum substrate
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H. C. KIM AND K. CHUN
Table VII. Comparison of quartz oscillator and SiTime’s resonator
Quartz crystal
Size
Frequency
Aging
Compensated temperature stability
Resonant Q
Shock/vibration Immunity
CMOS integration
Packaging
2–5 mm
1–80 MHz
3–5 ppm in the first year
1–10 ppm
100–200 K
Poor
No
Ceramic or metal
and their performance should be improved at higher
frequencies.
Furthermore, by using RF MEMS devices it is possible
to develop small, single-chip transceivers and to realize
portable IT terminals with the size of a wristwatch. It can
advance the development of the next generation handsets
that can communicate in multibands and multimodes.
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
400 µm
1–50 MHz
3 ppm in the first year
1–10 ppm
75–150 K
Good
Yes
Plastic
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Hyeon Cheol Kim received the B.S., M.S., and Ph.D.
degrees in Electronic Engineering
from Seoul National University in
1990, 1992, and 1998, respectively.
He worked as a research staff at Samsung Advanced Institute of Technology from 1998 to 2001 and as a senior
engineer at Chromux Technologies
Inc. from 2001 to 2003. He worked
as a BK21 Contract Assistant Professor in Seoul National University from
2004 to 2006. He is a Member of IEEE and a life member
of IEEK. His areas of research include micromachining,
semiconductor sensors, RF MEMS, integrated MEMS,
and packaging.
Kukjin Chun received the B.S. degree in Electronic
Engineering from Seoul National University in 1977 and the M.S. and
Ph.D. degrees in Electrical Engineering from the University of Michigan in 1981 and 1986, respectively.
He was an Assistant Professor in the
Department of Electrical and Computer Engineering at the Washington State University from 1986 to
1989. He joined the faculty of Seoul
National University in 1989, where he is currently a Professor in the School of Electrical Engineering. He is a
senior member of IEEE and a life member of IEEK.
He has been a director of the Center for Advanced
Transceiver System since 2000, which develops RF frontend solution for next-generation wireless communication
systems. His research interests include integrated sensors,
intelligent microsystems, and MEMS processing technologies.
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IEEJ Trans 2: 249–261 (2007)