CURRENT STATUS OF RF MEMS DEVICES FOR
WIRELESS COMMUNICATION SYSTEMS
8.1: INTRODUCTION
Recently the focus of RF-MEMS research has changed its direction towards
system integration, reliability and the development of RF-MEMS devices with novel
configurations to meet needs of wireless communication systems. These systems have
gained worldwide attention in recent years, particularly due to the proliferation in MEMS
technology which has facilitated the development of radio frequency micro-electromechanical (RF MEMS) devices with novel configurations.
RF-MEMS has emerged as a potential technology for wireless, mobile and
satellite communication and defense applications. Extensive research has been carried out
to identify and overcome the limitations of RF MEMS technology for replacing PIN or
FET based switches for low- loss applications. The key benefit of this technology is that
the devices can be manufactured by processes as of VLSI that has helped in the
realization of many sub millimeter- sized parts to provide RF functionality. RF-MEMS
components include resonators, oscillators, tunable filters, switches, switched capacitors,
varactors and inductors. Among these the RF-MEMS switch has emerged as a favorite
for its high RF performance, ultra-low-power dissipation, very high isolation, very low
insertion loss, very low cost and large-scale integration.
The evolution of RF-MEMS devices is needed for the expanding broadband
wireless radio communication. Radio frequency, semiconductors technologies and ICcompatible MEMS technologies are improving day by day and have an important role in
the fast growing market of wireless communication systems. This chapter aims to present
the current status of the RF-MEMS devices briefly for the wireless and the satellite
communication systems. This chapter briefly discusses novel RF-MEMS Switches,
vibrating micromechanical diamond disc resonator, RF-MEMS variable capacitors and
MEMS tunable inductor.
Wireless communication systems are utilizing wireless sense and control
technology to bridge the gap between the physical world of humans and virtual world of
physics and electronics. The dream is to automatically monitor and respond to forest fire
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avalanches, faults in satellites, traffics, hospitals etc. Novel RF-MEMS devices have
better high frequency performance extending beyond 100 GHZ and high Q factor solve
many problems of high-frequency technology for wireless communications. RF-MEMS
finds application in phased arrays and reconfigurable apertures switching networks, phase
shifters and single-pole N-throw switches.
Now there exist many different cellular standards, ultra wide band, wireless
sensor networks and many private systems representing Bluetooth, public mobile
services, WLAN etc. Also there is coming up 4G system requiring new frequency
spectrum. It will be multi network system and will be carried out by the unification of
different networks to enable the ubiquitous connectivity. RF-MEMS is strong contender
as a complimentary technology allowing “More than Moore” where we can find out the
expectation in Figure 8.1 as below.
Figure 8.1: More than Moore’s Law
RF-MEMS are manufactured using conventional 3D structuring technologies, like
bulk micro-machining ,surface micro machining , fusion bonding ,LIGA etc. The
material used includes Silicon, GaAs, SiC or SOI substrates. RF-MEMS have great
potential for integration and miniaturization. They provide lower weight, lesser power
consumption, lower insertion losses, improved linearity, superior performance and higher
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quality factors than conventional communication components. RF-MEMS are no longer
laboratory toys.
The MEMS technology has improved significantly many RF-MEMS devices like
micro-switches, micro-machined inductors tunable capacitors, resonators, oscillators,
micro-transmission lines, filters, surface acoustic wave (SAW) devices etc. These RFMEMS devices are worldwide used recently in mobiles, communication and satellite
systems. The literature and several books have demonstrated and discussed a large
number of RF-MEMS devices for wireless communication systems. Now, focus of RFMEMS research is to develop reliable RF-MEMS devices with new configurations. We
are here presenting some of them with new configurations like RF-MEMS switch,
vibrating micromechanical diamond disc resonator, RF-MEMS variable capacitor and
MEMS tunable inductor.
8.2: RF-MEMS Drawbacks and Novel Research Solutions
RF-MEMS technology has its own share of problems. Besides the drawbacks like
high actuation voltage and slow switching speed, there are two main problems associated
with standard MEMS capacitive devices which are temperature sensitivity of the movable
membrane, and dielectric charging problems in the isolator layers (leading to
stiction).Possible improvements in the micromechanical aspect (such as in actuation
voltage, in switching speed ), improvement in the dielectric layer, improvement in the
power handling capability and improvement in the RF performance by reduction in
parasitic are presented below.
1. Improvements in the micromechanical aspect:
(a) Improvement in actuation voltage:
In practice, low spring constant designs like meandering suspensions or thin
springs (to achieve lower actuation voltage) are used. But such designs have issues like
the reliability of the device and the switching speed. The use of push-pull concepts
requires a relatively high actuation voltage. Also, the use of low- height bridges has been
reported to lower the actuation voltage but at the cost of a reduction in the capacitance
ratio.
To lower the actuation voltage the use of new materials like AlSi0.04 or Pt as
membrane for the MEMS switch, use of electromagnetic actuation along with
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electrostatic forces and exploiting buckling and bending effects due to residual stress has
been reported. But, AlSi0.04 has a much higher RF transmission loss. In recent times, a
totally free flexible membrane supported over three pillars has been proposed to lower
the actuation voltage but it needs a double sacrificial layer system and causes low
switching speed.
(b) Improvement in switching speed:
High switching speed of RF-MEMS devices is a main limitation, and much work
has not been done to improve the speed. Usually, the increase in the switching speed of a
RF-MEMS switch affects badly the switching voltage. A quicker switching can be
obtained with improved stiffness, but it certainly leads to an increase in the actuation
voltage.
Mercier et al. proposed the miniaturization of the switches to obtain high speed
and reliability. Further, they also demonstrated sub-microsecond switching times using
dielectric membrane switches with built in tensile stress. Lacroix et al. have reported that
the spring constant of the beam increases by adding simple bent sides on miniaturized
beam edges that increases sub-microsecond switching time.
2. Improvement in the dielectric layer:
The surface roughness of the dielectric layer badly affects the capacitance ratio of
switches and precise explanation of the roughness using the statistical approach is
reported. Various models are proposed to know how contact resistance responds to the
variations in the contact area, the number of asperities in contact and the temperature and
the resistivity profiles at the contact locations.
The RF-MEMS reliability chiefly depends on the dielectric charging
phenomenon. There are many reports on impact of the dielectric material, distributed
dielectric charging and the modeling of dielectric charging. Many materials like ZnO or
Al2O3 alloys, PZT, PZT or HfO2 multi layers, polymer-ceramic composites, BST, TiO2,
amorphous diamond, etc. are now under consideration as possible replacements of
SiO2/Si3N4 as the dielectric material for RF-MEMS technology.
The required properties of dielectric material as an insulation layer points that the
key for selecting a substitute of SiO2/Si3N4 dielectric are:
(a) Dielectric constant,
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(b) Dielectric strength,
(c) Resistivity,
(d) Leakage current,
(e) Surface roughness,
(f) Ferroelectric properties and
(g) Charge trapping density.
Very few materials are capable with respect to all of these guidelines. Much
research is still required to find any material that can substitute SiO2/ Si3N4 as the
dielectric.
3. Improvement in the power handling capability:
Power handling capability of the RF-MEMS devices is primarily restricted by two
factors like:
(a) Joule heating for high power that results in melting and welding of the contact,
(b) self biasing and RF latching.
Electro-thermal models have been reported to predict the power handling
capability of RF- MEMS devices. The use of two bridge level topology has been
investigated till 8W of RF power.
4. Improvement in the RF performance by reduction in parasitic:
Various reports show that the parasitic has a significant role in the quality factor
of the devices. Due to its attenuation for RF signals, the CMOS grade low-resistivity
silicon substrate is not appropriate for high-frequency applications. For the high
resistivity substrates, a frequency of 10 GHz is big sufficient to drive the silicon substrate
into its dissipative mode of dielectric.
Hence, the use of substrates and a suitable passivation layer have a vital role in
the RF performance of the device. At 40 GHz, by using polymers like Kapton, polyimide
resin, BCB resin etc as the passivation layer on the low resistivity silicon substrate, the
insertion loss could be decreased to 3dB/cm.
8.3: NOVEL RF-MEMS SWITCHES
RF-MEMS switches play an important role in the application of the multiband
and multichannel wireless system for reconfiguration integrated circuits. Figure 8.3
illustrates the RF micromechanical switch by Z. J. Yao et al. This particular RF device
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consists of a beam fixed at both ends and suspended over a metal electrode. This could be
the central conductor of a coplanar waveguide. When sufficient amount of voltage is
applied between beam and its underlying electrode, shorting the ensuing electrostatic
force pulls the beam down to the electrode.
Figure 8.2: Radio Frequency Micro-Mechanical Switches.
As a result shorting between beam and electrode takes place (for the case of a
direct contact switch), or the electrode to beam capacitance gets increased which affects
an AC short (in case of capacitive switch, where a dielectric film atop the electrode).In
both the cases the switch is effectively closed. In case of electrostatic actuation the
voltage levels greater than 20volts are generally used for radar applications. Although,
this actuation level is very high for the integrated transistor circuits used in wireless
handset applications. Therefore, either the actuation voltage level is reduced or
accommodated at the system-level via charge pumping etc.
RF micro-mechanical switches with insertion losses about 0.1dB and low switch
power consumption of order pico-watts have been reported. Hence these can easily
outperform semi-conductor switches (FETs and diodes) in antenna and filter switching
applications. Their micro mechanical structure allows the use of metal materials with
much lower resistivity than semiconductors. These RF micro-switches with microsecond
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switching speeds are now reported. The microsecond speeds are useful for many
switching applications in wireless systems where low loss and high linearity are
important.
Another novel RF-MEMS switch with two movable electrodes is here mentioned
that is proposed by S K Lahiri et al, in 2009 by using bulk micromachining the substrate
below the CPW central line, typically low-resistivity silicon, under the membrane area
selectively. It facilitates both the CPW central line and the shunt membrane to move in
opposite direction at the same time. RF-MEMS switches with two movable electrodes
have also been reported by Babaei et al., but they mainly deal with the process
technologies to fabricate the switch. The two movable electrodes result in the reduction in
actuation voltage and switching time and the removal of the silicon from beneath the
CPW central line causes a reduction in the parasitic.
Two wafers oxidized on both sides are used in the fabrication of this switch. A
thin Cr/Au layer is deposited on the top surface of bottom high resistivity silicon wafer
and on the lower surface of top wafer. On bottom wafer, CPW structure is defined by
photolithography. The central conductor is covered by a fitting protecting layer. A thin
eutectic gold layer is selectively deposited on the outer conductors. Then, a thin low-loss
dielectric film (0.15μm) is deposited to cover the central conductor over the regions
where capacitors will be formed. Windows etched on the oxide film in the backside (after
oxidation) are used for selective removal of silicon by bulk micromachining to release the
central conductor.
The gold film on top wafer is patterned by photolithography to shape the stripes bridging
between the ground electrodes on either side. Then, a thin gold eutectic film is deposited
and patterned as required for the eutectic bonding. Windows are opened (after oxidation)
on the bottom surface such that the stripes become free to move after bulk
micromachining. The backside alignment should be perfect. Finally, bulk
micromachining is performed in a fitting anisotropic etchant like EDP or TMAH with
oxide mask on both sides of the selected bonded wafers. The eutectic coated surfaces of
the two wafers are held together face to face, aligned correctly and bonded at temperature
about 300oC at a adequate contact pressure. Fit anti-stiction structures and treatment
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should be included.
Figure 8.3: SEM picture of the fabricated MEMS shunt switch without top
substrate.
An SEM picture of the fabricated MEMS shunt switch without top substrate is
shown in figure 8.3. The thickness of gold electrodes in the diagram is about 1μm and the
gap between the electrodes is estimated to 2.5μm. The maximum and minimum values of
capacitance are found to be 3.5pf and 30fF respectively. These reported novel designs
have been mentioned by keeping in mind the limitations of the RF-MEMS technology
discussed in section 8.2. These have benefits of reducing the switching time, parasitic,
and actuation voltage at the same time.
In the early days, use of RF-MEMS switches was difficult due to reliability
issues. Lifetime of early switches was of the order of only 10 million cycles. Now, due to
combination of contact engineering, adequate packaging and fabrication control, RFMEMS switches with switching around 100 billion cycles are available. This clears a way
for more use of these switches into wireless communication systems.
RF-MEMS switches could have applications in band switching and filter
switching because future wireless receivers will need to operate at several bands covering
wide range of frequencies including 1-5 GHz. Still the research issues for RF-MEMS
Switches include their reliability, their switching speeds, their switching voltages etc.
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8.4: VIBRATING MICRO-MECHANICAL DIAMOND DISC RESONATOR
As vibrating micro-mechanical resonators produces much higher quality factor
than their electrical counterparts, hence, these are essential components in
communication systems. With improving MEMS technology, these devices can be
designed to oscillate over a very wide operating frequency range, varying from <1 KHz
to >1GHz. This much frequency range makes them ideal for ultra stable oscillation and
low loss filter functions.
A lot of vibrating micro-mechanical resonators had been reported. For example,
clamped-clamped beam resonator, free-free beam resonator, wine glass disc resonator,
counter-mode disc resonator, hollow disc ring resonator etc. Many researches had been
performed on the VHF range resonator to use the reference oscillator of the wireless
communication systems.
A 1.51GHz nano-crystalline diamond micro-mechanical disc resonator had been
reported from U C Berkeley. It can achieve a quality factor up to 11,555 in vacuum and
10,100 in air (i.e. at atmospheric pressure).Its quality factor is very impressive as
compared to others. The resonator consists of a 2 µm thick diamond disc having diameter
of 20 µm. Its diamond disc has been fabricated by using the CVD process. The disc is
suspended by a doped-polysilicon stem self aligned to be exactly at it center. Then all this
is surrounded by doped-polysilicon electrodes. The spacing between electrodes and disc
perimeter is 80nanometer.
When vibrating in its radial contour mode, the disc expands and contracts around
its perimeter. This amounts to a high stiffness and high kinetic energy. As disc center
corresponds to a node location for the radial contour vibration mode shape, anchor losses
through the supporting stem are very much prevented.
In this way, this design can retain a very high quality factor even at UHF
frequency. Also, the high stiffness of its radial contour mode gives this resonator a very
large total kinetic energy during vibration. Due to this, the energy losses arising from
viscous gas damping are very much reduced in this resonator. Therefore, this helps the
resonator in maintaining quality factors greater than 10,000 even at atmospheric pressure.
This single resonator attains a frequency applicable to the RF front terminals of
many commercial wireless devices. Since its quality factor is 10100 in air. Therefore, to
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achieve high quality factor, this design removes the requirement for vacuum which
makes it economical.
Figure 8.4: SEM Photograph of the CVD Diamond Micromechanical Disk
Resonator.
This resonator can operate at and beyond gigahertz frequencies when properly
scaled and do so while retaining sufficiently large dimensions to maintain proper power
handling. Figure 8.4 shows the SEM photograph of the diamond disc resonator with self
aligned stem. Also the frequency temperature coefficient of this resonator is 12ppm/deg.C. It has found practical applications as reference local oscillator, VHF-SBand Filter and RF channel select networks.
Micro-mechanical resonators are replacing conventional crystal oscillators used as
frequency reference or time source devices. These resonators have more advantages than
crystal oscillators in terms of money, size reduction, and better integration with silicon
etc. There operating frequency could be increased into GHz range. Higher frequency
operation would also allow new filtering and mixing functions, and completely novel
architectures.
8.5: RF-MEMS VARIABLE TUNABLE CAPACITOR
Variable capacitor, with performance superior to varactor diodes in areas such as
non-linearity and losses, can be feasible with MEMS technology. Initially, MEMS
parallel plate variable capacitors with an electro-static actuator are fabricated. The tuning
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range of such type of MEMS capacitors is limited up to 50% due to the failure of
capacitor structure when the voltage becomes more than the pull-in-voltage.
Figure 8.5: A Schematic Diagram of MEMS Variable Capacitor.
Then a MEMS variable capacitor with 100% tuning range was proposed in. In this
actuation electrodes are spaced differently from the capacitor plates. But, in practice, this
capacitor should operate over a smaller tuning range to avoid collapse of capacitor.
Therefore, MEMS variable capacitors having a much wider tuning range should be
designed without the collapse of the capacitor structure.
Another MEMS variable capacitor with novel configuration and design was
proposed in. Figure 8.5 shows a schematic diagram of this capacitor. It consists of two
movable parallel plates with an insulating dielectric layer on top of the bottom plate.
Since both plates are flexible, both plates can attract each other. Hence, maximum
spacing between two plates decreases before the pull-in-voltage comes into action. In
addition the capacitor has an extended tuning range even after the two plates touched
each other.
This capacitor is constructed using two structural layers, three sacrificial layer and
two insulating layers of Nitride. The top plate is made of nickel with a thickness of 24
µm. It is covered by a gold layer of thickness 2 µm. The bottom plate is made of polysilicon which is covered by a nitride layer of a thickness of 0.35 µm. The different layers
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use to make this capacitor are prepared using MetalMUMPs process. The two
dimensional layers are generated using CoventorWare.
Figure 8.6: An SEM image of the MEMS Variable Capacitor.
An SEM picture of the fabricated MEMS variable capacitor is shown in figure
8.6. At 1GHz the achievable tuning range of this capacitor is found equal to 280 %. This
value is very much greater than that of traditional parallel plate variable capacitors.
Paired with medium quality factor inductors, this capacitor can enhance the performance
of low noise voltage controlled oscillators (VCOs). Also, when paired with micromechanical inductors it can allow implementation of low noise VOCs with much lower
power consumption than those using IC technology.
8.6: RF-MEMS TUNABLE INDUCTORS
Paired with inductors having quality factor greater than 20, tunable MEMS
capacitors will be useful in communication circuits. Conventional IC technology can only
produce spiral inductors with quality factors less than 7 due to excessive series resistance
and substrate losses. Using MEMS technologies inductors with quality factor as high as
70 at 1GHz have been demonstrated.
This inductor with quality factor about 70 when paired with a micro-mechanical
capacitor should enhance performance of low noise VCOs with much lower power
consumption than those using traditional IC technology. Also, if inductors with quality
factor about 300 become possible, then tunable RF filters might be achievable that could
find use in RF transceivers circuits like filters, matching networks etc. MEMS tunable
inductors can also be used to build filters with tunable bandwidth.
By coupling a drive coil to the RF inductor, a few variable inductors have become
feasible. The tunability is attained by varying mutual inductance between the two
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inductors. Based upon the relative phase of the current in the two coils, the mutual
inductance component can be increased or decreased continuously. By this technique, a
100% tuning range has been attained. But an additional driver circuit is used in the drive
coil in this technique.
The tunable inductor circuit proposed in consists of variable capacitors, two fixed
capacitors and two inductors. The parallel plate variable capacitor is shown in the Figure
8.7. It is fabricated as follows: the upper plate is made of poly 2 and lower plate is made
of poly 1. Then a layer of gold is deposited on upper plate. The air gap between these
plates is of 0.75 µm. To ensure the etching of the oxide between these two plates, holes
are made in the plates of the capacitor. Capacitance value can be changed by voltage
applied to the variable capacitor. When top plate moves towards fixed lower plate, the
distance between plates changes. This changing distance changes the value of
capacitance.
Figure 8.7: MEMS Tunable Inductor Chip
The tunable inductor was constructed using Multi-User MEMS processes
(MUMPs) surface ploy silicon micro-machining technology. The MUMPs process has
three layers of poly silicon (poly0, poly1, and poly2), two layers of oxide and one layer
of gold (poly2). The gold layer is deposited on the top ploy silicon layer. The thicknesses
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of gold layer, poly 2 layer , second oxide layer, poly 1 layer, first oxide layer and ploy0
layer are 0.5 µm, 1.5 µm, 0.75 µm, 2.0 µm, 2.0 µm, 2.0 µm and 0.5 µm respectively.
The design of fixed capacitors is same except that voltage is not applied to the
plates of the fixed capacitors by R. R. Mansour et al. This design uses eight pads: six
pads for the coplanar RF input and output signals and two pads for grounds and a DC
voltage. Here the pads are made of poly 2 layer and gold layers. To ensure the trapping of
oxide and protection from the HF etch, anchors are provided around the edges of the
pads. The following Figure 8.7 shows a fabricated MEMS tunable inductor chip.
8.7
Summary
In recent years, the focus of RF-MEMS research has been shifted to system
integration, reliability and the development of RF-MEMS devices with novel
configurations to meet needs of wireless communication systems. The evolution of RFMEMS needed for expanding the broadband wireless communication systems. It should
be progressed in parallel with the miniaturization of the CMOS following diversification
technologies of “More than Moore”. Micromechanical devices attained via MEMS
technologies have been described that can play potentially a key role in removing the
board- level packaging requirements that currently limit the size of communication
transceivers etc.
RF-MEMS devices are the strong candidates that could meet future wireless
communication
systems
requirements
in
terms
of
low
power
consumption,
reconfiguration, reliability and miniaturization. As there are many different kinds of
technologies, modeling, theories, processes and simulation techniques for the RF-MEMS,
there would be no anxiety for the future because many researchers, groups etc have
extremely high motivation for the RF-MEMS devices.
The literature and several books have demonstrated and discussed numerous RFMEMS devices for wireless communication systems. Now, RF-MEMS research is
focusing to develop reliable RF-MEMS devices with new configurations. This very
chapter, here, presents some of them with novel configurations such as RF-MEMS
switches, vibrating micromechanical diamond disc resonator, RF-MEMS variable
capacitor and MEMS tunable inductor. Possible improvements in the micromechanical
aspect (such as in actuation voltage, in switching speed), improvements in the dielectric
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layer, improvements in the power handling capability and improvements in the RF
performance by reduction in parasitic are also presented in this chapter.
During the last years, RF-MEMS have experienced a tremendous progress in
terms of technology development and RF-applications. RF-MEMS circuits have been
demonstrated with such exceptional properties that they potentially can be part of the
essential building blocks of next generation emerging wireless communication and RFsensing applications. RF-MEMS technology has reached a level where MEMS switches
can be successfully integrated into practical RF systems with proven long term reliability.
The integration of MEMS switches and Monolithic Microwave Integrated Circuits
(MMICs) is thus considered as a next logical step in MMIC process growth and
commercialization of RF-MEMS technology.
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