RF/Microwave MEMS Devices and Fabrication Technologies for Enhanced-Functionality Wireless Communications
101
RF/Microwave MEMS Devices and Fabrication Technologies for
Enhanced-Functionality Wireless Communications
Vinod Kumar Khanna
MEMS & Microsensors, Solid-State Devices Division, CSIR-Central Electronics
Engineering Research Institute, Pilani-333031 (Rajasthan), India
E-mail: vkk_ceeri@yahoo.com
ABSTRACT: Applications of microelectromechanical systems (MEMS) in RF (radio-frequency) and microwave electronics
are poised to revolutionize wireless communications. RF/microwave MEMS encompass switches, tunable capacitors,
inductors, resonators, filters, phase shifters, couplers, antennas, oscillators, transceivers, etc. Unlike microelectronic
technology-based planar and stationary components, MEMS switches have mobile parts; varactor capacitances are controlled
by altering the gaps between electrodes and on-chip spiral inductors stand off the substrates, reducing parasitic effects.
RF MEMS, also called ‘Microwave MEMS’ or ‘Wireless MEMS’ has bestowed all types of communication systems: radar,
handsets, base stations, satellites, radio, instrumentation and test equipment, the key properties of low weight, volume,
power consumption, form factor and cost, together with increased frequency of operation, component integration and
reconfigurability, enabling overall superior functionality and performance. This paper surveys the state of the art in this
field, discusses the present scenario and highlights the challenges ahead. Attention is also drawn towards the shortfalls of
RF MEMS components. Packaging and reliability issues are addressed.
Keywords: RF/Microwave devices, MEMS devices, wireless communications, tunable filters, phase shifters, antennas.
1. INTRODUCTION
The ever-expanding demand for high-efficiency
microwave communication systems has boosted the
research and development endeavours for multifunctional, adaptive, low-power systems, bringing in its
wake the challenges of reconfigurability, spectrum
efficiency, device size miniaturization and cost minimization. MEMS technology for fabricating miniature
devices by combining mechanical parts and electronic
circuits, typically on a semiconductor chip, with dimensions varying from around 1 µm to several millimeters,
represents the foremost enabling route for realization of
such microwave systems [1]. RF MEMS incorporate
mechanical devices such as diaphragms, cantilever
beams, gears, springs, etc., with integrated circuits, for
different types of functions, as comprehensively reviewed in [2-4]. The objective of this review article is to
provide perspectives of microwave MEMS devices,
particularly in switching and tuning functions and in
the light of recent advancements, to researchers in the
field. Further, it aims to acquaint end-users with an
overall device picture and current status, enabling them
to envision their performance capabilities and understand their limitations. Present problem areas in this field
are also identified.
In this paper, Sec. 2 briefly introduces the vital
performance parameters of RF/microwave devices and
essential manufacturing processes used in MEMS
realization. A top-down treatment of the subject is
followed, in which Sec. 3 describes the applications of
RF/microwave MEMS, which have boosted research
efforts; these are MEMS technology-based tunable filters,
phase shifters, couplers, antennas, voltage-controlled
oscillators, etc. Subsequently from Sec. 4, elaboration of
the building units of RF/microwave MEMS commences
with transmission lines and membrane switches. Necessity
of MEMS switches is emphasized. Different types of
MEMS switches are defined. The equations and methods
for MEMS switch design are outlined, followed by their
fabrication steps and packaging techniques. Problems
faced with MEMS switches are indicated. Reliability
issues are dealt with in Sec. 5. From here onwards, focus
shifts from switches towards other RF MEMS components,
tracking the metamorphosis to tunable capacitors
(Sec. 6). Inductors are cursorily dealt with in Sec. 7. The
ensuing section 8 deals with MEMS resonators. Sec. 9
touches upon integration of MEMS devices with
microelectronics. The paper concludes with summarizing
remarks, bottlenecks and hurdles faced, and future
trends in Sec 10.
2. ORGANIZATION AND TERMINOLOGY OF
RF/MICROWAVE MEMS, AND KEY
MEMS PROCESSES
Fig. 1 shows how the RF/microwave MEMS has evolved
from basic building blocks to application-oriented units.
The common terminology of RF MEMS devices [2] is
enlisted in Table 1.
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Vinod Kumar Khanna
Figure 1: RF/Microwave MEMS: Structural Components and Applications.
Table 1
RF MEMS Device Parameters
Sl. No.
Parameter
Related Device
Definition
1.
Insertion loss
Switch
RF power lost in the device.
2.
Return loss
Switch
RF power reflected back by the device.
3.
Isolation
Switch
RF isolation between the input and output.
4.
Linearity
Switch
Independence of device impedance from the input RF signal power.
5.
Series resistance
RF component
6.
Quality factor
Equivalent resistance, R, present in a reactive component that is
being modeled as a simple series configuration between R and
pure imaginary impedance, Z im , where Z im = 1/(jωC) for a
capacitor, and Zim = jωL for an inductor.
Electrical or mechanical Ratio of the energy stored in a device to the energy dissipated per
component
cycle of resonance.
7.
Mechanical resonant
frequency
Any device
Particular frequency at which the stored kinetic and potential
energy resonates.
Figure 2: Silicon Micromachining Processes.
Commonly used processes in MEMS fabrication
include surface and bulk micromachining (see Fig. 2),
micromoulding, wafer-to-wafer/wafer-to-glass bonding
and LIGA [5]. Structurally, the RF MEMS devices are
divided into surface- and bulk-micromachined categories. On
the basis of functionalities, two types of devices are disting-
uished, namely, active and passive components. In the
microwave area, the term “active device” is usually
reserved to devices giving net RF/microwave power to
the circuit, such as amplifiers. In this sense, a MEMS
device is not truly active.
International Journal of Mechanical Engineering • July-December 2011 • Volume 4 • Issue 2
RF/Microwave MEMS Devices and Fabrication Technologies for Enhanced-Functionality Wireless Communications
2.1. Surface Micromachining
It is the removal of thin skinny layers of material on
silicon by etching. In surface-micromachined MEMS
devices, 3-D structures are constructed by the controlled
addition and removal of a sequence of thin-film layers
to/from the wafer surface called structural and sacrificial
layers, respectively. The substrate is used as a mechanical
support upon which the micromechanical elements are
fabricated. PolySi, SiO2, Si3N4, metallization layers or
photoresist coatings at/near the surfaces of devices, are
deposited and etched away to release overlying
mechanical structures. Sometimes the resulting freestanding structures are anchored at one or more
locations. In other cases, they are free to rotate about pin
joints. Fig. 2(a) illustrates the use of surface micromachining technique for the fabrication of a polysilicon
cantilever using silicon dioxide as the sacrificial material.
It may be noted that RF-MEMS discussed in this paper
employ metal surface micromachining, not polysilicon.
103
without any adhesives (Fig. 3). Depending on the glass
type, silicon and glass wafers are heated to temperatures
in the range of 300-500ºC. The components are brought
into contact and a high voltage ~ 1000 V, is applied across
the combination, whereby glass becomes bonded to
silicon with a permanent chemical bond. Eutectic Bonding
exploits the property of the eutectic temperature of SiAu, which is around 368 °C. The melting temperature of
the two surfaces in contact is lowered appreciably when
they are in touch. Thermo-compression bonding involves
placing the two mirror-finished surfaces of the silicon
in intimate contact at a high temperature.
2.2. Bulk Micromachining
It is the removal of large, deep sections of material. Bulk
micromachining is a method for sculpting 3-D structures
in a wafer by exploiting the anisotropic etching rates of
the different atomic crystallographic planes in the wafer.
It is performed isotropically or anisotropically (preferentially in certain directions). In bulk-micromachined
devices, the mechanical structures are formed by etching
the supporting substrates. Fabrication of micromechanical
devices by bulk micromachining through etching deeply
into the silicon wafer is shown in Fig. 2(b).
2.3. Micromoulding
Here, the structures are fabricated using moulds to define
the deposition of the structural layer only in those areas
that constitute the micro-device structure, in contrast to
the previous approaches. Afterwards, the mould is
dissolved using an etchant that obviously does not attack
the structural material.
2.4. Bonding Techniques
Anodic bonding is a method of hermetically and
permanently joining a glass substrate with a silicon wafer
Figure 3: Glass-Si Anodic Bonding.
2.5. LIGA Process
The term ‘LIGA’ is an acronym representing the main
process steps involved, i.e., deep X-ray lithography,
electroforming, and plastic moulding (in German, Lithographie, Galvanoformung, Abformung). In this process,
structures of lateral design with high aspect ratios are
produced, i.e., with heights up to 1000 µm and lateral
resolution down to 0.2 µm, by using deep X-ray lithography.
3. RF/MICROWAVE MEMS IN WIRELESS
COMMUNICATIONS
3.1. MEMS Tunable Filters
A critical element enabled by MEMS technology is
tunable filter, of which several examples are given in
Table 2 [6-12]. The Table includes several different
techniques (acoustic, printed, etc.). Poor and good examples
are mixed to provide a perspective of the true scenario.
Table 2
Tunable Filters
Sl. No.
Name of the filter
and reference
Operational
mechanism
Frequency
(GHz)
1.
Two-pole monolithic
switched filter [6]
Surface-micromachined capacitors
provided a variable capacitance to
a coupled coplanar strip filter.
2.
Three-pole tunable
end-coupled filter [7]
3.
Wide-band tunable
filter [8]
Insertion loss
(dB)
Switching filter centre
frequency 37% between
10.7 GHz and 15.5 GHz
<2
Distributed loading structures were
switched with MEMS capacitive switches.
6 –10
3.3 – 3.8
Coplanar-waveguide filter was fabricated
on a glass substrate. Used loaded resonators
12–18
4.5 and 6.8 dB at 17.8
and 12.2 GHz for the
Contd. Table 2
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Vinod Kumar Khanna
Contd. Table
with MEMS capacitive switches, resulting
in a tuning range of 40% with ultrafine
resolution.
bias line resistance
of 20 kΩ/sq.
4.
Tunable stop-band
filter [9]
Coplanar waveguide structure was loaded
in its central strip with complementary
split-ring resonators (CSRRs) entrenching
MEMS variable capacitors.
39-48
(Q-band)
5.
Three-pole combline
filter [10]
Used MEMS capacitive switches on the
end of each resonator to choose fixed
metal-insulator-metal (MIM) capacitors.
8.2 – 11.3
4.4 to 6 dB
6.
Single-chip
multiple-frequency
AlN filter [11]
Contour-mode aluminum nitride
piezoelectric micromechanical resonators
were electrically cascaded in a ladder
structure to form low insertion
loss, bandpass filter.
Up to
236 MHz
4 dB at 93 MHz
7.
High-Q
evanescent-mode
tunable filter [12]
Evanescent-mode cavity was utilized
as a high-Q resonator and a high-Q
cantilever-switch capacitance network
was introduced as a tuning network.
Centre frequency
= 4.19 – 6.59 GHz,
Q = 535 – 845
2.46 – 1.28
A glance at the table reveals the different directions
pursued by researchers. For MEMS-based filters, two
types of frequency-tuning methods are distinguished,
viz., analog and digital tuning. Analog tuning is
comparatively easy with MEMS variable capacitors.
Although it provides continuous frequency variation, it
suffers from the shortcoming that the tuning range is
limited between 5–15%. On the other hand, in digitaltype tuning, a capacitor is switched in and out of the
circuit, resulting in discrete centre frequencies and
provision of broader tuning ranges (20% – 60%). Several
designs focus on 0.1 – 10 GHz frequency range. The vital
benefit of digital designs is their lower sensitivity to bias
and Brownian noise. Also, the center frequency is
accurately known, and susceptible marginally to drifting
with thermal variations.
One filter example (Fig. 4), (Nordquist et al.) [6],
consisted of a low-loss RF MEMS switched capacitor,
optimized for low loss and high-Q at microwave
frequencies.
Figure 4: RF-MEMS Switched Filter [6].
A stop-band filter (Gil et al.) [9] based on metamaterial transmission lines (artificial lines consisting of
a host line loaded with reactive elements) is shown in
Fig. 5. Resonant type metamaterial transmission lines
employ two principal strategies, namely, split-ring
Very small insertion
loss in allowed band;
rejection in stop band
< – 40 dB
resonators (SRRs) or complementary split-ring resonators
(CSRRs) as loading elements. Stop-band behaviour is
observed for transmission lines loaded solely by these
electrically small resonators. The explanation for its
origination in circuit theory is the presence of a transmission zero. The reason in terms of continuous media,
is the extreme value of the effective permeability (for
SRR-loaded lines) or permittivity (for CSRR-loaded
lines) in the neighbourhood of the resonance frequency
(highly positive and negative).
Figure 5: Stop-Band Metamaterial-Based Filter [9].
Here rectangular-shaped complementary split-ring
resonators (CSRRs) were etched in the central signal strip
of a 50 Ω coplanar waveguide (CPW) transmission line
with RF-MEMS bridges on top of them [9]. These RFMEMS bridges provided tunability to the structure,
yielding a tunable stop-band filter with 20% tuning range
operating at Q-band. The structure consisted of a fourstage periodic device wherein the distance between
adjoining CSRRs was kept as 220 µm. This example has
fairly high insertion loss (5 dB) out of band, which is
disadvantageous as a tunable RF-MEMS component.
For realizing bandpass filters using electromechanical resonators, there are two modalities. The first
style uses electrically coupled filters. Here an array of
resonators is coupled together exclusively by electrical
signals. The second style uses mechanically coupled filters,
wherein an array of resonators is coupled together by
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105
purely mechanical linkages; it deals with mechanical
MEMS resonators, operating on a completely different
mode from the other ones. This is the work of G.L. Piazza
et al. [11] that is completely different from others. A
comparison of the two modalities is interesting. It reveals
that electrical coupling of resonators offers implementational simplicity. Furthermore, external mechanical
links, which generally pose manufacturing problems,
particularly at high operational frequencies, are not
necessary. Also, it may be remarked that the link size is
generally a fractional part of the resonator size, around
0.1 times. At 1 GHz frequency, it is a critical dimension
< 0.5 micron. Additionally, rapid realization of complicated designs is easily achieved through electrical
cascading of resonators with zeros (termed as attenuation
poles) in the filter transfer function. On the contrary, the
beneficial aspects of mechanically coupled resonators
include the capability to adjust the bandwidth of the
filter, quite independently of material properties. This
is supplemented with the ability to improve out-of-band
rejection as an additional advantage. In the work of
Piazza et al. [11], electrically coupled resonators were
chosen for filter construction, primarily because of their
virtues of ease of design and fabrication.
resonators to a few ohms and provide the mandatory
broad bandwidths for filtering applications. Piazza et
al. [11] realized monolithically integrated multiplefrequency aluminium nitride (AlN) band-pass filters,
which represented a major breakthrough towards the
goal of highly integrated, single-chip, multi-band
solutions. Based on a new offshoot of MEMS resonator
technology supported on the excitation of contour modes
of vibration in AlN microstructures, bandpass filters at
93 and 236 MHz were demonstrated by these workers
by electrically cascading up to eight resonators in a
ladder topology. These filters displayed very promising
performances, because they produced low insertion
losses (4 dB at 93 MHz), gave large close-in and out-ofband rejection (40 and 27dB, respectively, for a 93-MHz
filter) and fairly sharp roll-off with a 20-dB shape factor
of 2.2. Further, the filters were about 20 times smaller
than existing surface acoustic wave (SAW) device-based
filters. This novel technology can have a revolutionary
impact on wireless communication systems by allowing
simultaneous fabrication of multiple frequency filters on
the same chip, which will in turn, lower the form factors
and manufacturing costs.
Although micro resonator devices have a high
Q-factor, they have the disadvantage of high motional
resistance. A large motional resistance value translates
either into a bulky matching element necessity or
enormous insertion losses. Then it is not possible to
integrate these resonators with existing 50-ohm systems.
Piezoelectric materials such as aluminum nitride or
quartz offer larger electromechanical coupling coefficients
that substantially reduce the motional resistance of the
3.2. MEMS Phase Shifters
They employ switched line and distributed transmission
line approaches. Various topologies are available for
shifting phase. Switched-line and loaded-line designs
require distributed line lengths and tend to be large at
X-band. Alternatively, phase shift is accomplished by
switching low-pass and high-pass filter elements, which
typically requires large monolithic inductors. Table 3
presents some examples from the literature [13-16].
Table 3
MEMS Phase Shifters
Sl. No.
Name of the phase
shifter and reference
Principle of
operation
Bits
Frequency
(GHz)
Phase
shift
Insertion
loss (dB)
1.
Resonant switched
transmission line
filter [13]
Capacitive switches were
employed to perform two
quarter-wave transformations
enabling switching between
different delay paths,
thereby shifting the phase.
4-bit
34 GHz
0º – 337.5º
with 22.5º
steps
2.25 dB for
4-bit and 1.7 dB
for 3-bit phase
shifter
2.
Microstrip distributed
phase shifter [14]
Was a distributed MEMS transmission line (DMTL) phase
shifter comprising a high
impedance line (> 50 Ω) capacitively loaded by MEMS bridges
and microstrip radial stubs.
2 and
4-bit
DC to
18 GHz
262º (maximum)
at 16 GHz for
2-bit
–2.8 dB for 2- bit
6-bit
DC to
10-GHz
393.75-ps (Total
time delay)
1.8 ± 0.6 dB
of loss at 10 GHz
-
24 GHz
5.4°/mm
differential
phase shift
at 24 GHz
0.1 dB/mm
3.
Time delay
circuit [15]
Used metal contacting MEMS
switches to obtain series-shunt
SP4T switching networks.
4.
DTML phase shifter
with non-galvanic
electromagnetic
coupling [16]
Was a DMTL phase shifter
fabricated in Si-bulk micromachined technology, enabling
to commonly suspend all capacitive
loads on one movable plate and
allowing full-range analog and
homogeneous gap variation.
333° (maximum) –3.0 dB for 4-bit
at 16 GHz for 4-bit
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Vinod Kumar Khanna
A distributed MEMS transmission line (DMTL)
phase shifter (Fig. 6), Voigt et al. [16], consists of a high
impedance transmission line, periodically loaded with
lumped adjustable capacitors, which influence the line
impedance and phase velocity. DMTL phase shifters
show superior performance characteristics than their
semiconductor competitors in terms of loss and power
consumption. However, capability of analog phase shift
by semiconductor devices is difficult to realize with
individual MEMS bridges. Digital phase shifters
containing sections of capacitive loads serve to achieve
different states of phase shift.
Figure 6: (a) Top View and (b) Cross-Section of Phase
Shifter [16].
A conventional figure of merit for phase shifters is
the degrees of phase shift per dB of loss, along with the
operating frequency. For instance, example 2 in the Table
shifts signals by 262/2.8 = 93.57° for one dB of loss at 16
GHz, which is comparable to good MMIC phase shifters,
and not very impressive. For example 4, from the data
given in the table, the phase shift is = 5.4/0.1 = 54° per
dB, at 24 GHz, which is not very good for a phase shifter,
MEMS or non-MEMS. Further, RF-MEMS phase shifters
are not competitive for low-power applications, and lowpower RF-MEMS phase shifters cannot be considered
as an active area of research.
3.3. Micromachined Couplers
These devices couple a secondary system only to a wave
travelling in a particular direction in a primary transmission system.
Sung-Chan et al. [17] fabricated a hybrid ring coupler
on a GaAs substrate using surface micromachining
techniques. It had dielectric-supported air-gapped
microstrip line (DAML) structure. Coupling loss was 3.57
± 0.22 dB and the transmission loss was 3.80 ± 0.08 dB
across the frequency range of 85 to 105 GHz. The
isolation characteristics and output phase differences
were -34 dB and 180 ± 1°, at 94 GHz, respectively. Yusuke
et al. [18] demonstrated a micromachined coplanar
waveguide (CPW) 3-dB hybrid coupler. They employed
a micromachined enhanced coupling structure at the
middle of coupled transmission lines to obtain high directivity. The coupling structure was composed of alternately
overlapping plated conductors with micro-machined airgap structures. They fabricated the CPW hybrid coupler
with the enhanced coupling structure on a silicon
substrate. The return loss was 28 dB, the insertion loss
was 0.7 dB, and the isolation was 25 dB at 13 GHz.
3.4. Reconfigurable MEMS Antennas
Lately, tunable antennas have aroused considerable
interest because of the large number of international
wireless standards in vicinity to one another. A single
tunable antenna caters to various frequency bands,
getting rid of multiple antennas. A MEMS-switched
reconfigurable multi-band antenna is dynamically
reconfigured within a few microseconds to serve different
applications at drastically different frequency bands,
such as communications at L-band (1-2 GHz) and X-band
(8–12.5 GHz). Case studies are cited in Table 4 [19-22].
Table 4
MEMS Antennas
Sl. No.
Name of the antenna
and reference
Underlying
principle
1.
Coplanar patch
antenna [19]
Operated by electrostatically tuning
the resonant frequency of the antenna
by applying a DC bias voltage between
the MEMS varactor and the actuation pad
on the antenna.
2.
Sierpinski
antenna [20]
Employed three sets of RF MEMS switches
with different actuation voltages to sequentially
activate/deactivate parts of a multiband
Sierpinski fractal antenna.
3.
Microstrip
rectangular loop
antenna [21]
Worked by electronically changing the
loop physical perimeter using RF MEMS
switches.
4.
Planar inverted-F
antenna (PIFA) [22]
Had a tuning line with electrical path to
the ground plate through a MEMS switch
and loading capacitor. By altering the switch
status, the resonant frequency of the PIFA was
tuned with the loading capacitance value.
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Frequency
Return loss
5.185 to 5.545 GHz
Better than –40 dB
Between 2.4 and
18 GHz
Maximum loss
< –35 dB
1.16 to
2.08 GHz
Better than –60 dB
(simulated)
842 to 762 MHz
–22 dB at the
resonant
frequency
RF/Microwave MEMS Devices and Fabrication Technologies for Enhanced-Functionality Wireless Communications
107
from the DC source to the signal frequency. The main
component of the MEMS parametric up-converter [24]
was a time-varying capacitor, which consisted of a thin
diaphragm-type top electrode suspended above a
bottom electrode. The magnitude of the pump signal
voltage was decreased by making the structure resonant
at the pump frequency; this helped in minimizing the
gap between the diaphragm and the bottom electrode,
and evacuating the backside air between electrodes for
avoiding damping.
Figure 7: Coplanar Patch Antenna [19].
Fig. 7 shows the layout of the coplanar patch antenna
of Maddela et al. [19]. This antenna had a coplanar
waveguide (CPW) feed at the bottom radiating edge and
a MEMS varactor at the top radiating edge. The three
constructional layers constituting the tunable coplanar
patch antenna were substrate, spacer and a polyimide
film. The spacer layer provided the required spacing
between the substrate and the polyimide layer. A flexible
Kapton E polyimide film (capable of withstanding
millions of mechanical flexing cycles) with 100-150 A°
NiCr seed layer and 3 µm thick Cu cladding was used
by these investigators.
3.5. Mixer-Filters, Amplifiers and VoltageControlled Oscillators (VCOs)
In the MEMS mixer-filter of Chen et al. [23], the nonlinearity of the electrostatic force with drive voltage on the
MEMS resonators was utilized, downconverting GHz
RF input signals to excite MHz mechanical resonance
for intermediate frequency (IF) filtering, which was
capacitively transduced into an electrical IF output. Thus
mixing and filtering functions were performed concurrently as the RF signals were traversing the resonators.
Variable MEMS capacitors with high-Q MEMS
inductors were used in the VCO circuit architecture
(Fig. 8) of Ramachandran et al. [25] for switching between
frequencies > 400 MHz apart. The CMOS and MEMS
inductor, both fabricated with a 5.8 GHz VCO [26],
showed that in comparison to the CMOS inductor, the
CMOS MEMS inductor succeeded in achieving a 5 dB
lower phase noise improvement at 1 MHz offset in this
5.8 GHz VCO.
4. UBIQUITOUS STRUCTURAL COMPONENTS
OF RF MEMS
From here onwards, the building blocks of RF MEMS
are pursued, beginning with two pervasive components,
viz., transmission lines and switches.
4.1. Micromachined Transmission Lines
Genesis of most of the transmission line limitations, such
as frequency dispersion and, to a certain degree, insertion
loss, lies in the properties of the substrate or media where
they are defined. MEMS technology has been successfully exploited to lessen the influence of the substrate.
Figure 9: (a) A Membrane-Supported Microstrip Line, and
(b) Microshield Transmission Line.
Figure 8: Schematic Diagram of MEMS VCO [25].
Parametric devices work on transference of power
from the pump frequency to the signal frequency, in
contrast to standard amplifiers, which convey power
In the membrane-supported microstrip (Fig. 9a) [5], the
transmission line is defined on a thin membrane, with
dielectric constant close to unity, by bulk-etching the
substrate underneath the trace through processing from
backside. However, a drawback of the membranesupported microstrip line is the absence of an intrinsic
ground plane. This requires that the structure be placed
on top of another metallized substrate by soldering or
fusion bonding. An alternative is the microshield
transmission line (Fig.9b) [27] in which a central conductor
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Vinod Kumar Khanna
along with the ground planes is placed on the membrane.
The purpose of the metallized cavity is to avoid crosstalk between adjoining lines and radiation into parasitic
substrate modes. Other noteworthy transmission line
structures include top-etched coplanar waveguide, LIGA
micromachined coplanar transmission line, etc.
4.2. The Need of RF MEMS Switch Technology
In RF MEMS, the workhorse component is the MEMS
switch. To appreciate the need of MEMS technologies in
RF/microwave applications, Table 5 makes a comparative
pros and cons analysis of MEMS switches (resistive and
capacitive) with conventional switches [2, 28-30].
Table 5
Pros and Cons of MEMS Switches Over Existing Switches
(a) Relative Pros of MEMS Switches
Parameter (Unit)
Size (mm )
2
Series resistance (Ω)
Insertion loss at
1 GHz (dB)
Isolation at 1 GHz (dB)
Linearity (dBm)
Third order harmonics
Integration compatibility
Power consumption
MEMS switch
GaAs MESFET
P-I-N diode
EM relay
Remarks
< 0.1
1-5
0.1
Medium-Large
Consumes smaller space.
0.5
3-5
1
0.1 – 0.5 ohm
Lower than FET and P-I-N
diode but comparable to EM
relay.
0.1 – 0.2
0.5 – 1
0.5 – 1
0.1 – 0.4
Lower than other switches.
> 40
20-40
40
15-25
Higher than other switches.
35
35
30-45
50
Comparable to other
switches (~30 dBm like P-IN diode or FET switches)
but lower than relays.
Very good
Poor
Poor
Good-Very good
Superior/comparable to
other switches.
Excellent
Excellent
Excellent
Average-Difficult
Excellent.
1 µW
1 – 5 mW
1 – 5 mW
Medium-High
Control current
< 10 µA
< 10 µA
10 mA
3-5 mA
Upper frequency limit (Hz)
70 × 10
4 × 10
20 × 10
5-40 × 10
9
9
9
Lowest amongst all
switches (near zero).
Lowest amongst all switches.
9
Highest of all the switches.
(b) Relative Cons of MEMS Switches
Parameter
MEMS switch
GaAs MESFET
P-I-N diode
EM relay
Remarks
20-80
1-10
1-10
5-12
Very high as compared to
other switches.
10
(Continuous)
0.5
(Continuous)
5
(Continuous)
10-35
(Continuous)
Lower than other switches
except FET.
Breakdown voltage
Low
Low
High
High
Lower than other switches.
Switching time (µs)
0.3< t < 1 µs
2-10 ns
< 1 µs
250 µs- 1ms
Larger than FET and diode
but smaller than EM relay
switches.
Life cycle (million)
Up to 105
106
106
1
Smaller than FET and diode
but larger than EM relay.
5.6 x
x
1.8 x
2.6
Costliest of the swiztches,
mainly because of packaging
and the high-voltage drive
chip costs.
Actuation
voltage (Volts)
Switching power (W)
Cost ratio
The relative superiority of MEMS switches in terms
of isolation and upper frequency limit over other
switches is brought out in Fig.10 (a) and that of insertion
loss in Fig.10 (b). The switching speed of one type of
commercially available MEMS switches is between 300
nsec and 1 µsec. These MEMS switches have a
demonstrated life of 100 billion switching cycles.
Linearity of mechanical relays is comparable or better
than MEMS relays. Evidently, cycles make no sense for
GaAs or PIN switches.
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electrode under the beam. By applying a voltage to the
control electrode, the beam is pulled down or up to complete or discontinue the connection between two conductors.
(a)
Figure 11: Series Switch: (a) Connection, (b) 3-D view;
(c) OFF Sate, and (d) ON State.
(b)
Figure 10: Comparison of Important Parameters of Different
Switches: (a) Isolation, Linearity and Upper
Frequency; and (b) Insertion Loss.
4.3. Types of RF MEMS Switches
In a shunt switch, the power line is sandwiched
between two ground lines (Fig.12a). The switch turns
on to short the power on the signal line to the ground.
This prevents the power from going past the switch
(Fig. 12a). In the shunt switch (Figs.12b and c), the beam
is clamped at both the ends and the control plate pulls
down the beam when a potential is applied to it. This
ensures that the signal traverses the shorter path to the
ground and is not transmitted to the ensuing circuit.
RF-MEMS switches [31-38] are categorized by the circuit
configuration or the type of switching contacts (series,
resistive or ohmic and shunt), the positions of the armature
(inline or broadside), lateral or vertical structures, and the
actuation mechanisms. Typically the series and shunt
(or parallel) is referred to the topology, not the type of
contact. A series switch could be capacitive. As opposed to
this, shunt switches are not necessarily capacitive
switches since shunt DC-contact switches with isolated
actuation electrodes are feasible. Distinction must be
made as series versus shunt; metal contact versus capacitive.
4.3.1 Series and Shunt Switches
As the name implies, a series switch is connected in series
with the power line (Fig. 11). It either opens or closes
the line, turning it OFF or ON. The contacting surface is
usually located at the end of a singly supported cantilever
beam. For switching ON or OFF, there is a control
Figure 12: Shunt Switch: (a) Connection, (b) Up (ON) State
and (c) Down (OFF) State.
Series and shunt switch structures are compared in
Table 6.
Table 6
Series Versus Shunt MEMS Switch
Sl. No.
Series MEMS switch
Shunt MEMS switch
1.
Very low ON-state insertion loss.
Higher insertion loss but independent of the contact force,
relaxing the requirements of the actuation mechanism.
2.
Very high OFF state isolation
A trade-off between insertion loss and isolation exists.
3.
Very susceptible to stiction, corrosion
and microscopic bonding of the metal
surfaces of contact electrodes.
Not prone to such effects.
4.
Usually requires considerable force to
create a good metal-to-metal contact.
Less force needed. It has a longer lifetime.
5.
Suitable for low and medium frequencies.
Unsuitable for low frequencies but suitable for very high frequencies.
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Table 7
Frequency Ranges of Applications of MEMS Switches
DC
LF
HF
UHF
Microwave
Series switch
Shunt switch
Consumer electronics (Mobile telephones, GPS, etc.)
Automotives (Low power relays, navigation systems, radars)
Instrumentation and measurements (Test equipment)
Telecommunications Network (Satellites)
Homeland security (Satellites)
Various applications of MEMS switches are
described in Table 7.
4.3.2 Inline and Broadside Series/Shunt Switches
Sub-classification of both the above varieties of switches
is based on the actuation plane (Fig.13). In the inline
structure (Fig.13a), the actuation plane is co-linear with
the transmission line whereas in the broadside structure,
the actuation plane is orthogonal to it. The main
dissimilarity between the two designs is that the RF
signal passes through the entire inline switch. As a result,
inline switches must be fabricated using a thick metal
layer (Au, Al, Pt, etc.). On the other hand, only the contact
portion of the broadside switch (Fig.13b), needs to be
fabricated using a metal layer, and the actuation portion
is composed of either a dielectric or a dielectric/metal
cantilever.
Figure 13: (a) Inline Series Switch and (b) Broadside
Series Switch.
4.3.3 Lateral and Vertical Switch Structures
The two categories of switches are compared in Table 8
[39-40].
Table 8
Lateral and Vertical Switches
Sl. No.
Lateral switch
Vertical switch
1.
The actuator, conductor pads, support
structures and contacts are made in a single
step of photolithography.
These require several lithographic operations.
2.
Dynamic behaviour is superior to many of the
vertical contacting switches.
Inferior dynamic behaviour of many switches.
3.
Consume larger area.
Require smaller area.
4.
Difficulty in metal deposition over a vertical side wall.
Easy metal deposition on a horizontal surface.
5.
Contact mechanisms are very inferior to that of a vertical
contacting switch, because of roughness in etched
side-surfaces and contact materials.
Provide superior contact mechanisms.
4.3.4 Electrostatically-, Electromagnetically-,
Piezoelectrically- and Thermally - Actuated
Switches
Electrostatically-actuated switches work on the Coulombic
force of attraction between two oppositely-charged
plates. They are the simplest of all the switches because
they do not require any special processing steps, which
are not supported by normal CMOS technology. In
piezoelectrically-actuated switches [41-44], a piezoelectric
actuator attached to the switch membrane provides the
necessary force. Thermal actuation entails the usage of two
materials with different expansion coefficients. On
heating the materials, the beam bends away from the
material with the higher thermal expansion coefficient.
Another method in this class employs shape memory
alloys. Clearly, these thermal methods have not been
popular and commonplace due to their high power
consumption. Electromagnetic methods of actuation work
on alignment of a magnetic material in a magnetic field
[45]. By changing the alignment direction, the switch is
turned ON or OFF. This is a novel method and is
advantageous compared to other methods but requires
special processing steps involving magnetic materials.
See Table 9 for a comparison of switch actuation
mechanisms at a glance.
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Table 9
Comparison of RF MEMS Switches Actuated by
Different Mechanisms
Sl. No.
Actuation
mechanism
Actuation
voltage
Actuation
speed
Power
requirement
1.
2.
3.
4.
Electrostatic
High
High
Low
Simple and robust.
Piezoelectric
Lower than
electrostatic
High
Low
Prone to parasitic thermal
expansion of layers.
Electrothermal
Low
Low
High
Bulky.
Electromagnetic
Low
Low
High
Provides high contact force.
where Vs is the applied voltage. Resonant frequency
is [29, 33]
4.4. Design Parameters and Equations of MEMS
Switches
The electrical model of a MEMS series switch is a series
capacitance in the up-state position and a small resistance in the downstate position [29, 33, 46-49]. The
isolation of a series switch in the up-state position is
written as [29, 33]
|S21|2 = 4ω2 Cu2 Z20 for 2ω CuZ0 << 1
(1)
where Cu is the up-state capacitance and Z0 is the
transmission-line impedance. Isolation depends on the
spacing between the input and output ports or the OFF
state capacitance of the switch.
Factors contributing to loss in the switch called
insertion loss are contact resistance and resistance of the
wave guide [29, 33]:
|S21| =
R
1– s
Z0
The principal factors influencing the magnitude of
the actuation voltage are electrode spacing, area of
actuation electrode and dielectric material separating the
two electrodes. Pull-down voltage is given by [29, 33].
8 kd 3
27 ε 0 A
(3)
where the spring constant
k=
0.25EWt 3
l3
(4)
d is the air gap, ε0 is permittivity of free space, A is
the area of membrane, and E is Young’s modulus of the
material. W, t and l are the width, thickness and length
of the beam respectively.
The power handling capacity of the switch is mainly
determined by the properties of the transmission line.
The switching time [33]
t=
3.67Vp
Vs ω 0
k
m
ω0 =
(5)
(6)
where m is the mass of the cantilever/bridge. For a
shunt-capacitive switch, the up-state reflection coefficient
is obtained from the equation [29, 33]
 1
|S11|2 = 0.25ω2 Cu2 Z02 for   ω CuZ0 << 1 (7)
 2
The downstate isolation is [29, 33]
|S21|2 ≅
4
, f<<f0 ;
ω 2 C d2 Z02
=
4 Rs2
, f ≅ f0 ;
Z02
≅
4ω 2 L2
, f>>f0
Z02
(2)
where Rs is the contact resistance of the switch.
Vp =
Structural
features
(8)
where L is the inductance in the lumped CLR model
of the switch, Cd is the down-state capacitance of the
switch and f0 is the down-state resonant frequency [29,
33]
f0 =
0.5
π LC d
(9)
CON/COFF ratio is a key parameter for the capacitive
coupling shunt switch as it is a determining factor for
both insertion loss and isolation. A large CON, required
to maintain high isolation, requires an intimate contact
between the membrane and the dielectric film over the
bottom electrode in blocking or down state of the switch.
A small COFF, required for maintaining low insertion loss,
calls for a large gap between the membrane and the
bottom electrode, which is a tradeoff with achieving a
low pull-down voltage.
Given the main parameters of interest to the circuit
designer, a typical design procedure for a MEMS switch,
e.g. a capacitive switch, provides the circuit designer
with a simple set of parameters, along with an accurate
RF model of the device. Design procedure starts with
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identification of the type of implementation. Ideally, the
devices will be processed on the die containing the active
devices after fabrication of these devices. This may not
be often practicable due to die handling limitations,
particularly for prototype designs. For concept demonstration, the MEMS switches may be located on a
separate die, and later wire bonded to the active device
die. However, the effect of the bond wire inductance
must be included, and this may constrain circuit
topologies. The size ratios between the two capacitances
must be determined in conjunction with the circuit
design. Although the typical capacitance ratio for a
MEMS switch is in the 30-40: 1 range, this may be too
high. In cases where a smaller ratio is adequate, the
switch dimensions are adjusted or a fixed capacitance is
added in parallel with the switch. Fixation of capacitor
size and chip area allocation are done. The circuit
designer must determine how much chip area can be
devoted to the switch.
layer (silicon nitride) (Fig. 14d). A contact dimple is
formed. Finally, the top metal film (Au) is deposited and
patterned (Fig. 14e). The top metal is released by etching
the sacrificial layer (Fig. 14f). Sometimes PECVD oxide
is used as a structural material with polyimide as a
sacrificial layer. There are several variations of this basic
process.
It must be clearly pointed out here [50] that the
RF-power handling capability is a fundamental performance parameter of MEMS switching devices. Besides
excessive heat dissipation, the power handling capability
is constrained by the self-biasing and/or RF-latching
phenomena consequential upon the induction of a nonzero electrostatic pulling force on the suspended
structure by the available RF power from the source.
Therefore, the hitherto implicitly postulated perfect
matching of the device to the network in the ON-state
(i.e. absence of reflection) and thus a fixed DC-equivalent
RMS voltage on the capacitor, in the study of self-biasing
of RF-MEMS switches, becomes untenable. For RF power
values exceeding a critical value, pull-in or self-biasing
takes place. Practically, however, the assumption of the
perfect match is invalidated because of the switch capacitance increase with rising RF power, thereby causing a
variation in the reflected signal and thus a fall in the
DC-equivalent voltage source. In practice, an RF-MEMS
shunt switch ideally matches for one and only one
accurate gap height, in case of MEMS capacitance
compensation by a local appropriate design of the CPW
line. No sooner than the electrostatic force begins to come
into picture, bridge movement occurs and the matching
alters.
4.5. MEMS Switch Fabrication
Fig. 14 shows the process flow diagram for series switch
realization, and Fig. 15 the same for shunt switch [51].
In Fig. 14 for a series switch, starting from a Si wafer,
first lowermost metal layer (e.g., Au) is deposited and
its pattern defined by photolithography and etching (Fig.
14a). Then the sacrificial layer [Plasma-Enhanced
Chemical Vapour Deposition (PECVD) silicon dioxide]
is deposited and patterned by etching (Fig. 14b) followed
by deposition (Fig. 14c) and selective etching of dielectric
Figure 14: (a)-(f) Process Sequence for Series Switch
Fabrication. (g) Actuation.
In Fig. 15 for a shunt switch, the first step (Fig. 15a)
involves deposition and definition of bottom film to
make actuation electrodes and RF lines. The next step is
dielectric film deposition and definition (Fig. 15b). Then
the sacrificial film is deposited and delineated (Fig. 15c).
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Subsequently, the top metal film is deposited and defined
(Fig. 15d). The final step is releasing the top electrode
by etching the sacrificial film (Fig. 15e).
113
5. RELIABILITY OF RF MEMS SWITCHES
5.1. Packaging Issues
A critical step in MEMS switch manufacturing is the
packaging technique [2]. In conventional practice, the
MEMS switches are packaged with extreme precaution
in a clean-room environment using established hermetic
packaging methods, but this is an exorbitantly expensive
approach, representing the costliest process in the switch
production chain and constituting a significant chunk
of the final switch price. There is currently intensification
of research to develop wafer-scale packaging processes,
which are compatible with aforesaid MEMS switch
fabrication sequences. These are based on processes such
as low-temperature hermetic glass bonding, minimal out
gassing, Au-to-Au bonding, etc. Contemporarily, the
reliability and packaging of MEMS switches are areas
demanding frantic research efforts globally.
One approach for the zero-level package [2] is to bond
a recessed capping chip on the MEMS device wafer.
Needless to say that this bonding operation is performed
at temperatures below 400 °C so that the metallization
and other materials of the RF-MEMS switching device
do not undergo any degradation, resulting in impairment
of device functionality.
Figure 15: (a)-(e) Fabrication Steps of Shunt Switch.
(f) Actuated Device.
4.6. Single-Pole Multiple Throw (SPMT) Switches
SPMT switches containing thin metallic films are not
mechanically reliable due to film deformation caused
by heat or film stress effects during fabrication process
steps [52-54]. Even if best possible conditions are maintained, insertion loss is high. The reasons are substrate
loss and open-stub effects from multi-path fading. Moreover, high drive voltage is necessary to create the large
contact force crucial for low insertion loss. Single-pole
four-throw (SP4T) RF MEMS switch for band selection
in a multi-band, multi-mode front-end module of a
wireless transceiver system [55], was driven by a double
stop comb drive, with a lateral resistive contact, and
composed of monocrystalline silicon on glass. A large
contact force at a low-drive voltage was obtained from
electrostatic actuation of the double-stop comb drive.
In the wafer level micro-encapsulation (WlµE)
technique (Fig. 16) [56-58], instead of releasing the
membrane at the time of etching the sacrificial layer of a
MEMS switch, an additional cage sacrificial layer was
applied over the unreleased switch membrane, followed
by the dielectric cage deposition. The intent of this cage
sacrificial layer was to produce the required gap between
the membrane and packaging cage. Holes were etched
into the cage and the sacrificial layers were etched by
plasma process, resulting in a released switch with a
packaging superstructure stationed upon it. After
release, a liquid encapsulant, such as spin-on-glass (SOG)
or Cyclotene series 4000 benzocyclobutene (BCB), was
applied over the wafer in a dry N2 ambient, jacketing
the cage structure but unable to drip through the cage
holes due to surface tension. Curing of the SOG or BCB
was done at 250°C, forming a closed seal over the switch.
The packages were reported to provide <0.1 dB package
insertion losses up to 110 GHz.
Figure 16: Cross-Sectional View of Packaged MEMS
Switch [56-58].
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5.2. Problems with MEMS Switches
Main mechanisms responsible for malfunctioning of
MEMS switching devices are [59-62]: (i) Material
cracking, creep and fatigue, chiefly in the armature and
the hinged supports; (ii) Deterioration of ohmic contacts
caused by cratering or wear, breakdown of the dielectric
layer(s), and other reasons; and (iii) Stiction (or adherence)
of the switching contacts or of the actuation electrodes.
The underlying causative factors are electrostatic forces
and interfacial forces, like capillary forces and Van der
Waals forces, at the dimensional scale of microstructures.
Currently, there are several problems that need to be
resolved, notably lack of high-power RF MEMS switches,
long-term reliability, etc. Packaging issues, high fabrication
cost, high actuation voltage levels and phenomena like
stiction, need to be seriously addressed in the near future
[63-64]. These factors make RF MEMS switches a
formidably challenging research field.
5.3. Novel Approaches for RF MEMS Switch
Improvement
Gold has been the material of choice for both the fixed
electrodes and the suspended microbridges in MEMS
switches. But Au microbridges are deficient in the high
spring constant (linked to Young‘s modulus) needed to
surmount the stiction and electrostatic charging effects.
For MEMS switches, SiC is appealing due to its chemical
inertness, anti-stiction properties and mechanical
stiffness [63, 65]. Crystalline SiC has a large Young's
modulus (>350 GPa) yielding a high spring constant.
Also, SiC surfaces are hydrophobic, making it less prone
to stiction. But the high thermal processing temperatures
required to manufacture single crystalline and polycrystalline-SiC growth (>900 °C) are unfavourable for
SiC microbridge-based RF MEMS switches, primarily
because in such structures, the microbridge spans an Au
element which cannot forebear such high processing
temperatures. Plasma-enhanced chemical vapor
deposition (PECVD) is used to produce SiC at much
lower deposition temperatures than conventional CVD,
typically below 400 °C but SiC films deposited by
PECVD are typically amorphous in microstructure. The
amorphous microstructure generally corresponds to a
reduced Young's modulus than found in crystalline SiC
but much higher than Au. Scardelletti et al. [59] and Parro
et al. [65] fabricated PECVD-based amorphous-silicon
carbide switches. They observed that the Young's
modulus of the a-SiC films was insensitive to film
thickness. Upon metallization, it decreased slightly. In
contrast, metallizing the 300 nm-thick a-SiC film resulted
in a significant increase in residual stress. Their findings
suggest that the residual stress of the metal layer must
be considered when designing microbridge-based
switches using submicron-thick structural layers.
Novel approaches have been explored to improve
the reliability of RF MEMS switches. Ke et al. [66] have
proposed a wafer-level packaged switch with a
corrugated diaphragm for residual stress reduction. They
noticed that pull-in voltage of the switch was drastically
reduced from 105 V for a flat diaphragm to 51 V for
two-corrugation diaphragm and to 42 V for fourcorrugation diaphragm. Kim et al. [67] reported a
technique for decreasing the bending of the membrane
in a switch caused by internal stress gradient. They
fabricated a thick and stiff membrane switch in which
the membrane consisted of a flexible spring allowing an
UP-DOWN actuation mode at low voltage and a pivot
under the membrane itself facilitating a seesaw mode
ON-OFF operation. The minimum actuation voltage was
~ 10–12 V.
6. MEMS TUNABLE CAPACITORS
6.1. Figures of Merit of MEMS Capacitors
These are unbiased base capacitance (values ranging
typically from tens of picoFarads for very-high-frequency
(VHF) applications to about 0.1pF for applications
approaching the X-band); tuning ratio (varying from
about tens of per cent to excess of 2:1); equivalent series
resistance or quality factor, Q; associated inductance; and
device linearity in response to RF power.
6.2. Comparison with Capacitive Semiconductor
Counterparts
In Table 10 and Fig. 17, comparisons of MEMS tunable
capacitors with their traditional semiconductor varactor
counterparts are drawn out [2, 4, 5, 68-75].
Table 10
Semiconductor Varactors Versus Tunable MEMS Capacitors
Semiconductor varactors
Semiconductor on-chip varactors suffer from excessive
series resistive losses with associated low unloaded
Q-factors ~5-20 [76-77]. Off-chip discrete varactors have
a higher unloaded Q-factor of at least 40. But the most
important shortcoming of a semiconductor varactor is
the inherent dependence of the capacitance on the RF
signal power, making the component behave highly
nonlinearly.
RF-MEMS capacitors
RF-MEMS tunable capacitors offer substantial improvement
over on-chip varactors and comparable with off-chip varactors
~ 20-60 at 1 GHz and 100 at 0.4 GHz [78-81]. Also, MEMS
capacitors offer excellent linearity. These capacitors further
promise low noise and the ability to keep the signal circuit separate
from the control circuit, simplifying the bias circuitry.
They provide integration capability with high-Q inductors,
generally not achievable in semiconductor technology.
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7. MEMS HIGH-Q INDUCTORS
Table 11 and Fig. 19 portray the Q-factor advantage of
MEMS tunable inductors [2, 4, 5, 82-93] over other types
of inductors. Two types of inductor structures are
illustrated in Fig. 20: planar and solenoidal. Solenoid-like
inductors, raised above the substrate, aimed at decoupling the inductor properties from those of the substrate,
are fabricated by surface micromachining. Here, the
designer has a greater flexibility to increase the number
of turns for inductance maximization or use a larger
conductor track width for series resistance minimization.
Figure 17: Q-Factors for Various Capacitor Approaches.
MEMS capacitors have been fabricated in variablearea and variable-gap structural configurations. Fig. 18
(a) and (b) show a tunable capacitor in which plates move
to readjust overlapping area for capacitance changes.
Fig. 18 (c) illustrates the operation of a variable gap RF
MEMS capacitor.
Figure 19: Q-Factor Chart of Inductors.
(a)
(b)
Figure 20: (a) Suspended Planar Spiral Inductor
Over Cavity, and (b) 3D Inductor.
(c)
Figure 18: (a) Fixed and Movable Plate Arrangement in an
Interdigitated-Plate Variable-Area Tunable Capacitor.
(b) Two Pairs of Plates in a Practical Implementation.
(c) Principle of Parallel Plate Gap-Tunable MEMS Capacitor.
It is observed that amputation of the silicon substrate
or construction of the inductor resembling a coil in air
provides the advantage that the parasitic capacitances
are annihilated, increasing the Q-factor. Induced eddy
currents in the substrate and accompanying energy
losses due to Joule heating effect are also brought down.
Traditional integrated circuit planar spiral inductors
reside on their host low-resistivity substrate, and
consequently, are afflicted with undesirable effects such
as low self-resonant frequency, low Q and limited
operating bandwidth.
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Vinod Kumar Khanna
Table 11
Q-Factors of Inductors in Si(Bi) CMOS, GaAs MMIC and MEMS Technologies
Si (Bi)CMOS or bipolar technologies
In standard 10 ohm-cm silicon wafer
technologies, the Q-factor of the onchip (spiral) inductors is < 10 and
self-resonance frequencies are 5-20
GHz [94-95]. The low Q-factor
evolves from losses in the conductive
substrate combined with resistive
losses in the Al (alloy) metallization.
GaAs MMIC technology
MEMS technology
The maximum attainable Q-factors
are on the order of tens. Improved
techniques, such as Au, Cu or thick
(3-5 µm) Al, high-resistivity Si
substrates, copper damascene
interconnection technology, thick
passivation layers below the inductor
or special 3D designs assist in raising
the Q-factor up to 20-30 for Si-based
technologies [96-102].
8. MEMS RESONATORS
Based upon their operating principle, resonators [4, 107115] are broadly classified into two groups: (a)
Electromagnetic (EM) wave resonators, and (b) Electromechanical or (Electro) acoustic wave (AW) resonators. Notable
examples of EM wave resonators are the lumped element
LC-type resonators, transmission line resonators, cavity
resonators and dielectric resonators. Among acoustic
wave (AW) resonators, mention may be made of mechanical resonators, bulk acoustic wave (BAW) resonators and
surface acoustic wave (SAW) resonators [4]; the latter
two classes are distinguished from the first on the basis
of their fabrication method. Some important resonators
are described below:
(i) LC resonators: A MEMS capacitor is implemented to tune the resonant frequency of the LC
1
tank, given by
.
LC
(ii) T(Transmission)-line resonators: Here, the MEMS
technology is utilized to remove the substrate
underneath the microstrip line, thus diminishing
the losses incurred due to the substrate. For
tuning T-line resonators, they are loaded with
RF-MEMS tunable capacitors.
Figure 21: A Cavity Resonator.
(iii) Cavity resonators: Micromachining techniques
are here used to make minuscule cavities
(Fig. 21) and dielectric resonators offering the
In membrane-supported spiral inductors
formed by etching away the silicon
substrate, Q values range from 6 to 28 at
6-18 GHz [103]. For the inductors suspended or levitated in air, the Q-factor is 17 at
3.5 GHz for a 1.5 nH inductor fabricated on
a 1-10 ohm-cm Si substrate [101]. For
integrated spiral inductors of 9 µm thick
Cu suspended over a glass substrate and
a low-resistivity Si substrate, inductances
vary from 15-40 nH and Q-factors
from 40 to 50 at frequencies of 0.9 – 2.5
GHz [104-106].
advantage of easy combination with monolithic
integrated circuits [107]. A disadvantage of an
air-filled cavity resonator is its excessively large
size (of the order of wavelength), even at mmwave frequencies.
(iv) Dielectric Resonators: By using a filler material
with a high relative dielectric constant εr, the
wavelength and hence the dimensions of the
cavity are lowered by a factor
ε r . This fact is
exploited in a dielectric resonator (DR), which
essentially consists of a piece of a high relative
permittivity (εr) insulating material.
(v) Mechanical Resonators: rely on the resonance of a
structural member, e.g., a beam or a disk. Very
high Q-factors ~ 1000–10,000 with peak values
as high as 105, have been obtained for vacuumencapsulated micromachined mechanical resonators. The piezoelectric layer is made of thin
films of aluminum nitride (AlN) or zinc oxide
(ZnO). The resonator sizes are as small as a few
tens to a few hundreds of µm on a side, representing typically a MEMS design.
(vi) BAW and SAW Resonators: differ in their
fabrication approach from the above resonators.
Fig. 22(a) shows the cross-section of a solidlymounted thin film BAW resonator consisting of a
piezoelectric AlN film sandwiched between Mo
electrodes. (100) Si substrates covered with a
polysilicon layer (in which an air gap was etched
approximately 8 µm under the surface) were
used [115]. Aluminium nitride is the favourite
piezoelectric material for high-frequency piezoacoustic devices because of its high acoustic
wave velocity and its large electromechanical
coupling coefficient, besides its high thermal
conductivity and the possibility to grow highly
c-axis oriented AlN films by room temperature
techniques such as sputtering, therefore assuring
International Journal of Mechanical Engineering • July-December 2011 • Volume 4 • Issue 2
RF/Microwave MEMS Devices and Fabrication Technologies for Enhanced-Functionality Wireless Communications
full compatibility with standard IC technology
[115]. In Fig. 22(b), a reflector-on-membrane-type
BAW resonator is depicted in which the membrane
as well as reflector materials are conductive. The
air gap is realized in a polysilicon supporting
layer; the reflector consists of a single pair of Ti/
W. Mo was chosen as electrode material since it
allows large effective coupling coefficient at high
frequencies. Additionally, it is characterized by
small acoustic wave attenuation, low electrical
resistivity and is selectively etched with respect
to AlN. Also, it promotes the growth of high
quality AlN layers. Membrane-type resonator
was realized by directly depositing AlN
piezoelectric layer embedded into Mo electrodes
on the poly-Si/Si substrate. Membrane-type
resonators generally present the advantages of
higher fabrication yield and, in some cases,
easier temperature coefficient of frequency
(TCF) compensation, as compared to solidlymounted resonators (SMRs); on the other hand,
they suffer from lower mechanical stability and
power handling capabilities in view of the fact
that membrane thickness needs to be reduced
to 1 µm or less to avoid the presence of many
undesired resonances, at the expense of robustness
and heat conductivity.
Fig. 22 (c) shows a SAW resonator consisting of an
input interdigitated electrode (IGT) pair and an output
IGT pair on a piezoelectric quartz substrate.
Figure 22: (a) Solidly-Mounted BAW Resonator,
(b) Membrane-Type BAW Resonator and (c) SAW Resonator.
9. INTEGRATION OF MEMS COMPONENTS
IN CIRCUITS AND SYSTEMS
The future of RF-MEMS is deep-rooted, not so much in
individual components as in integrated RF microsystems.
Understandably, monolithic integration of two or more
117
switches to yield multiple switch contacts is a viable next
step. An example is single-pole double-throw (SPDT)
switch. Although the integration of MEMS devices with
active circuitry has been demonstrated, integrated RF
MEMS are still in infancy and are gradually developing.
Morris et al. [116] developed a flexible generalpurpose RF MEMS manufacturing process based on a
multi-function process stack. Their process flow consists
of three parts: (i) The substrate connect layer used to
separate the upper layers from the substrate, provides
connections to underlying circuits and offers a
planarized surface on which subsequent processing is
done. (ii) The thick metal layer, used for passive devices
and interconnections, is made of Cu embedded in silica.
(iii) The thin metal layer comprises three layers of Au alloy
of 0.5 micron thickness, two layers of sacrificial material
and a silica mechanical layer. The above process has been
implemented in several foundries and different devices
have been fabricated. It thus paves the way to the
formation of a diversity of tunable and reconfigurable
RF passive circuits.
Kuwabara et al. [117] reported a novel structure and
fabrication process for integrated RF-MEMS technology.
For integration, an adaptable multilayer structure and
its fabrication process enabled realization of various
MEMS devices on the same substrate, whereas for
protection, a wafer-level encapsulation process created
small thin capsules to safeguard these devices. Each
capsule had walls, a roof, and a sealing film. While the
walls and roof were formed simultaneously as the
devices, the etching holes in the roof were sealed with
thin film by a selective sealing method. Encapsulated
MEMS devices, such as switches and varactors, were cofabricated on the same substrate, showing promising
results specially for fabricating RF MEMS transceivers.
Present RF transceivers contain many LSI chips and
abundant off-chip passive components. The escalating
number of available communication bands will multiply
the number of components and make RF transceivers
bulky. This will also increase power consumption and
curtail battery life. Therefore, reconfigurable RF MEMS
transceiver circuits, with alterable circuit configurations
according to the wireless standards used, will enable
multiband operation without enlarging the size and
raising power consumption.
Integration of capacitively-transduced MEMS
resonators with characteristic frequencies in the HF and
VHF bands has evolved as one of the futuristic key
options to overcome the limitation presented by discrete
passive elements in the downscaling to the chip level
for RF communications systems. Lopez et al. [118] have
described a strategy to design and fabricate a MEMS
resonator using a CMOS standard technology choosing
the optimal structural and sacrificial layers for the
resonators by means of a defined figure of merit.
International Journal of Mechanical Engineering • July-December 2011 • Volume 4 • Issue 2
118
Vinod Kumar Khanna
10. CONCLUSIONS AND OUTLOOK
MEMS technology has offered exciting possibilities for
realizing a new generation of high-performance, small,
low-power consuming, high-frequency components
such as switches, variable capacitors, tunable oscillators
and filters. Functional principles and technological
considerations of these components were discussed.
With evolving development trends, it is expected that
these RF-MEMS components will supplant their prevalent discrete counterparts in a wide range of RF,
microwave and millimeter-wave applications [119-122].
But this can be accomplished only if major technical
obstacles have been removed, more specifically, through
the development of an appropriate cost-effective
packaging technology and by solution of reliability
issues. Further, this must be achieved in large volume
production at a competitively low price. Although RF
MEMS devices offer attractive performance features,
practical problems on packaging, long-term reliability
and integration with microelectronics need to be
addressed. Hot topics on this technology include tuners
for cell phones, switches for AlN-based acoustic filters, etc.
Research on RF-MEMS is progressing towards perfection
of these technologies. The question “Are RF MEMS
devices commercially viable, or will they become so soon ?”
was addressed at a 2011 International Microwave Symposium
panel discussion titled “Commercial viability of RF MEMSa reality or a dream?”, and the petite answer was “Yes”,
RF MEMS devices are commercially viable [123].
ACKNOWLEDGEMENT
The author wishes to thank the Director, CSIR-CEERI,
Pilani for encouragement and guidance.
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