RF MEMS
Encyclopedia
The radio frequency
microelectromechanical system (RF MEMS
) acronym refers to electronic component
s of which moving sub-millimeter-sized parts provide RF functionality. RF functionality can be implemented using a variety of RF technologies. Besides RF MEMS technology, III-V compound semiconductor
(GaAs
, GaN
, InP, InSb), ferrite
, ferroelectric, silicon
-based semiconductor (RF CMOS
, SiC
and SiGe
), and vacuum tube
technology are available to the RF designer. Each of the RF technologies offers a distinct trade-off between cost, frequency
, gain
, large-scale integration, lifetime, linearity, noise figure
, packaging
, power handling, power consumption, reliability
, ruggedness, size, supply voltage
, switching time
and weight.
oscillators with small form factor and low phase noise
, RF MEMS tunable
inductor
s, and RF MEMS switches, switched capacitor
s and varactors.
and HEMT
switches (FET and HEMT transistors in common gate
configuration), and PIN
diodes. RF MEMS switches, switched capacitors and varactors are classified by actuation method (electrostatic, electrothermal, magnetostatic
, piezoelectric), by axis of deflection (lateral, vertical), by circuit configuration (series, shunt
), by clamp
configuration (cantilever
, fixed-fixed beam
), or by contact interface (capacitive, ohmic
). Electrostatically-actuated RF MEMS components offer low insertion loss
and high isolation, linearity, power handling and Q factor
, do not consume power, but require a high control voltage and hermetic
single-chip packaging (thin film
capping, LCP
or LTCC
packaging) or wafer-level packaging
(anodic or glass frit
wafer bonding).
RF MEMS switches were pioneered by IBM Research Laboratory
, San Jose
, CA
, Hughes Research Laboratories
, Malibu, CA, Northeastern University in cooperation with Analog Devices
, Boston
, MA
, Raytheon
, Dallas, TX
, and Rockwell
Science, Thousand Oaks, CA. A capacitive fixed-fixed beam RF MEMS switch, as shown in Fig. 1(a), is in essence a micro-machined capacitor with a moving top electrode, which is the beam. It is generally connected in shunt with the transmission line
and used in X- to W-band (77 GHz
and 94 GHz) RF MEMS components. An ohmic cantilever RF MEMS switch, as shown in Fig. 1(b), is capacitive in the up-state, but makes an ohmic contact in the down-state. It is generally connected in series with the transmission line and is used in DC
to the Ka-band
components.
From an electromechanical perspective, the components behave like a damped mass-spring system, actuated by an electrostatic force. The spring constant is a function of the dimensions of the beam, as well as the Young's modulus
, the residual stress
and the Poisson ratio of the beam material. The electrostatic force is a function of the capacitance and the bias voltage. Knowledge of the spring constant allows for hand calculation of the pull-in voltage, which is the bias voltage necessary to pull-in the beam, whereas knowledge of the spring constant and the mass allows for hand calculation of the switching time.
From an RF perspective, the components behave like a series RLC circuit with negligible resistance and inductance. The up- and down-state capacitance are in the order of 50 fF and 1.2 pF, which are functional values for millimeter-wave circuit design. Switches typically have a capacitance ratio of 30 or higher, while switched capacitors and varactors have a capacitance ratio of about 1.2 to 10. The loaded Q factor is between 20 and 50 in the X-, Ku
- and Ka-band.
RF MEMS switched capacitors are capacitive fixed-fixed beam switches with a low capacitance ratio. RF MEMS varactors are capacitive fixed-fixed beam switches which are biased below pull-in voltage. Other examples of RF MEMS switches are ohmic cantilever switches, and capacitive single pole N throw (SPNT) switches based on the axial gap wobble
motor
.
drive voltage, as shown in Fig. 2, in order to avoid dielectric charging
and to increase the lifetime of the device. Dielectric charges exert a permanent electrostatic force on the beam. The use of a bipolar NRZ drive voltage instead of a DC drive voltage avoids dielectric charging whereas the electrostatic force exerted on the beam is maintained, because the electrostatic force varies quadratically with the DC drive voltage. Electrostatic biasing implies no current flow, allowing high-resistivity bias lines to be used instead of RF chokes
.
sealing. A cavity is required to allow movement, whereas hermeticity is required to prevent cancellation of the spring force by the Van der Waals force
exerted by water
droplets and other contaminants on the beam. RF MEMS switches, switched capacitors and varactors can be packaged using wafer level packaging. Large monolithic RF MEMS filters, phase shifters, and tunable matching
networks require single chip packaging.
Wafer-level packaging is implemented before wafer dicing
, as shown in Fig. 3(a), and is based on anodic, metal diffusion, metal eutectic, glass frit, polymer
adhesive
, and silicon fusion wafer bonding. The selection of a wafer-level packaging technique is based on balancing the thermal expansion coefficients of the material layers of the RF MEMS component and those of the substrates to minimize the wafer bow
and the residual stress, as well as on alignment
and hermeticity requirements. Figures of merit for wafer-level packaging techniques are chip size, hermeticity, processing temperature
, (in)tolerance to alignment errors and surface roughness. Anodic and silicon fusion bonding do not require an intermediate layer, but do not tolerate surface roughness. Wafer-level packaging techniques based on a bonding technique with a conductive intermediate layer (conductive split ring) restrict the bandwidth and isolation of the RF MEMS component. The most common wafer-level packaging techniques are based on anodic and glass frit wafer bonding. Wafer-level packaging techniques, enhanced with vertical interconnects, offer the opportunity of three-dimensional integration.
Single-chip packaging, as shown in Fig. 3(b), is implemented after wafer dicing, using pre-fabricated ceramic
or organic
packages, such as LCP injection molded packages or LTCC packages. Pre-fabricated packages require hermetic cavity sealing through clogging, shedding
, soldering
or welding
. Figures of merit for single-chip packaging techniques are chip size, hermeticity, and processing temperature.
resistors (TFR), metal-air-metal (MAM) capacitors, metal-insulator-metal (MIM) capacitors, and RF MEMS components. An RF MEMS fabrication process can be realized on a variety of wafers: III-V compound semi-insulating
, borosilicate glass, fused silica (quartz
), LCP, sapphire
, and passivated
silicon wafers. As shown in Fig. 4, RF MEMS components can be fabricated in class 100 clean rooms using 6 to 8 optical lithography steps with a 5 μm contact alignment error, whereas state-of-the-art MMIC
and RFIC fabrication processes require 13 to 25 lithography steps.
As outlined in Fig. 4, the essential microfabrication
steps are:
With the exception of the removal of the sacrificial spacer, which requires critical point drying, the fabrication steps are similar to CMOS fabrication process steps. RF MEMS fabrication processes, unlike BST or PZT
ferroelectric and MMIC fabrication processes, do not require electron beam lithography
, MBE
, or MOCVD.
(phase shifters
) and software-defined radio
s (reconfigurable antennas, tunable band-pass filter
s).
reconfigurability, and frequency tunability, are usually achieved by incorporation of III-V semiconductor components, such as SPST switches or varactor diodes. However, these components can be readily replaced by RF MEMS switches and varactors in order to take advantage of the low insertion loss and high Q factor offered by RF MEMS technology. In addition, RF MEMS components can be integrated monolithically on low-loss dielectric substrates, such as borosilicate glass, fused silica or LCP, whereas III-V compound semi-insulating and passivated silicon substrates are generally lossier and have a higher dielectric constant
. A low loss tangent
and low dielectric constant are of importance for the efficiency
and the bandwidth of the antenna.
The prior art includes an RF MEMS frequency tunable fractal antenna
for the 0.1–6 GHz frequency range, and the actual integration of RF MEMS switches on a self-similar Sierpinski gasket antenna to increase its number of resonant frequencies, extending its range to 8 GHz, 14 GHz and 25 GHz, an RF MEMS radiation pattern reconfigurable spiral antenna
for 6 and 10 GHz, an RF MEMS radiation pattern reconfigurable spiral antenna for the 6–7 GHz frequency band based on packaged Radant MEMS SPST-RMSW100 switches, an RF MEMS multiband
Sierpinski fractal antenna
, again with integrated RF MEMS switches, functioning at different bands from 2.4 to 18 GHz, and a 2-bit Ka-band RF MEMS frequency tunable slot antenna
.
rejection, in case the antenna fails to provide sufficient selectivity. Out-of-band rejection eases the dynamic range
requirement on the LNA and the mixer
in the light of interference
. Off-chip RF bandpass filters based on lumped bulk acoustic
wave (BAW), ceramic
, SAW
, quartz crystal, and FBAR
resonators have superseded distributed RF bandpass filters based on transmission line resonators, printed on substrates with low loss tangent, or based on waveguide cavities.
Tunable RF bandpass filters offer a significant size reduction over switched RF bandpass filter bank
s. They can be implemented using III-V semiconducting varactors, BST or PZT ferroelectric and RF MEMS resonators and switches, switched capacitors and varactors, and YIG
ferrites. RF MEMS resonators offer the potential of on-chip
integration of high-Q resonators and low-loss bandpass filters. The Q factor of RF MEMS resonators is in the order of 100-1000. RF MEMS switch, switched capacitor and varactor technology, offers the tunable filter designer a compelling trade-off between insertion loss, linearity, power consumption, power handling, size, and switching time.
. The statement is illustrated with examples in Fig. 6: assume a one-by-eight passive subarray is used for transmit as well as receive, with following characteristics: f = 38 GHz, Gr = Gt = 10 dBi
, BW = 2 GHz, Pt = 4 W
. The low loss (6.75 ps
/dB) and good power handling (500 mW) of the RF MEMS phase shifters allow an EIRP of 40 W and a Gr/T of 0.036 1/K. EIRP, also referred to as the power-aperture product, is the product of the transmit gain, Gt, and the transmit power, Pt. Gr/T is the quotient of the receive gain and the antenna noise temperature. A high EIRP and Gr/T are a prerequisite for long-range detection. The EIRP and Gr/T are a function of the number of antenna elements per subarray and of the maximum scanning angle. The number of antenna elements per subarray should be chosen in order to optimize the EIRP or the EIRP x Gr/T product, as shown in Fig. 7 and Fig. 8. The radar range equation can be used to calculate the maximum range for which targets can be detected with 10 dB of SNR
at the input of the receiver.
in which kB is the Boltzmann constant, λ is the free-space wavelength, and σ is the RCS
of the target. Range values are tabulated in Table 1 for following targets: a sphere
with a radius, a, of 10 cm (σ = π a2), a dihedral corner reflector with facet size, a, of 10 cm (σ = 12 a4/λ2), the rear of a car (σ = 20 m2) and for a non-evasive fighter jet (σ = 400 m2).
RF MEMS phase shifters enable wide-angle passive electronically scanned array
s, such as lens arrays
, reflect arrays
, subarrays and switched beamforming
networks, with high EIRP and high Gr/T. The prior art in passive electronically scanned arrays, includes an X-band continuous transverse stub (CTS) array fed by a line source synthesized by sixteen 5-bit reflect-type RF MEMS phase shifters based on ohmic cantilever RF MEMS switches, an X-band 2-D lens array consisting of parallel-plate waveguides
and featuring 25,000 ohmic cantilever RF MEMS switches, and a W-band switched beamforming network based on an RF MEMS SP4T switch and a Rotman lens focal plane scanner.
The usage of true-time-delay TTD phase shifters instead of RF MEMS phase shifters allows UWB
radar
waveform
s with associated high range resolution, and avoids beam squinting or frequency scanning. TTD phase shifters are designed using the switched-line principle or the distributed loaded-line principle. Switched-line TTD phase shifters outperform distributed loaded-line TTD phase shifters in terms of time delay per decibel NF
, especially at frequencies up to X-band, but are inherently digital and require low-loss and high-isolation SPNT switches. Distributed loaded-line TTD phase shifters, however, can be realized analogously or digitally, and in smaller form factors, which is important at the subarray level. Analog phase shifters are biased through a single bias line, whereas multibit digital phase shifters require a parallel bus along with complex routing schemes at the subarray level.
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
microelectromechanical system (RF MEMS
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
) acronym refers to electronic component
Electronic component
An electronic component is a basic electronic element and may be available in a discrete form having two or more electrical terminals . These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function...
s of which moving sub-millimeter-sized parts provide RF functionality. RF functionality can be implemented using a variety of RF technologies. Besides RF MEMS technology, III-V compound semiconductor
Compound semiconductor
A compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table . These semiconductors typically form in groups 13-16 ,...
(GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
, GaN
Gan
Gan may refer to:-Computing and telecommunications:*.gan, the file extension for documents created by GanttProject*Generic Access Network formerly known as Unlicensed Mobile Access *Global Area Network- Mythology :...
, InP, InSb), ferrite
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
, ferroelectric, silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
-based semiconductor (RF CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
, SiC
SIC
Sic is a Latin word that means "thus" or, in writing, "it was thus in the source material".Sic may also refer to:* Sic, Cluj, a commune in Romania* SiC, Silicon carbide, a semiconducting material, also used to make metalworking tools from...
and SiGe
SiGe
SiGe , or silicon-germanium, is a general term for the alloy Si1−xGex which consists of any molar ratio of silicon and germanium. It is commonly used as a semiconductor material in integrated circuits for heterojunction bipolar transistors or as a strain-inducing layer for CMOS transistors...
), and vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
technology are available to the RF designer. Each of the RF technologies offers a distinct trade-off between cost, frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
, large-scale integration, lifetime, linearity, noise figure
Noise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
, packaging
Electronic packaging
Electronic packaging is a major discipline within the field of electronic engineering, and includes a wide variety of technologies. It refers to enclosures and protective features built into the product itself, and not to shipping containers...
, power handling, power consumption, reliability
Circuit reliability
Circuit reliability is the percentage of time an electronic circuit was available for use in a specified period of scheduled availability. Circuit reliability is given by where T o is the circuit total outage time, Ts is the circuit total scheduled time, and T a is the circuit total available time...
, ruggedness, size, supply voltage
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
, switching time
Switching time
For a frequency synthesizer, the switching time or more colloquially the switching speed is the amount of time from when the command for the next frequency is requested until the time that the synthesizer's output becomes usable and meets the specified requirements. Such requirements will vary...
and weight.
Components
There are various types of RF MEMS components, such as CMOS integrable RF MEMS resonators and self-sustainedSelf-sustainability
A system is self-sustaining if it can maintain itself by independent effort. The system self-sustainability is:# the degree at which the system can sustain itself without external support...
oscillators with small form factor and low phase noise
Phase noise
Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities...
, RF MEMS tunable
Electronic tuner
The term electronic tuner can refer to a number of different things, depending which discipline you wish to study.In the Discipline of radio frequency electronics an electronic tuner is a device which tunes across a part of the radio frequency spectrum by the application of a voltage or appropriate...
inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s, and RF MEMS switches, switched capacitor
Switched capacitor
A switched capacitor is an electronic circuit element used for discrete time signal processing. It works by moving charges into and out of capacitors when switches are opened and closed. Usually, non-overlapping signals are used to control the switches, so that not all switches are closed...
s and varactors.
Switches, switched capacitors and varactors
The components discussed in this article are based on RF MEMS switches, switched capacitors and varactors. These components can be used instead of FETFet
Fet is a municipality in Akershus county, Norway. It is part of the Romerike traditional region. The administrative centre of the municipality is the village of Fetsund.Fet was established as a municipality on 1 January 1838...
and HEMT
HEMT
High electron mobility transistor , also known as heterostructure FET or modulation-doped FET , is a field effect transistor incorporating a junction between two materials with different band gaps as the channel instead of a doped region, as is generally the case for MOSFET...
switches (FET and HEMT transistors in common gate
Common gate
In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a current buffer or voltage amplifier...
configuration), and PIN
PIN diode
A PIN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts....
diodes. RF MEMS switches, switched capacitors and varactors are classified by actuation method (electrostatic, electrothermal, magnetostatic
Magnetostatics
Magnetostatics is the study of magnetic fields in systems where the currents are steady . It is the magnetic analogue of electrostatics, where the charges are stationary. The magnetization need not be static; the equations of magnetostatics can be used to predict fast magnetic switching events that...
, piezoelectric), by axis of deflection (lateral, vertical), by circuit configuration (series, shunt
Shunt (electrical)
In electronics, a shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage.-Defective...
), by clamp
Clamp (tool)
A clamp is a fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure...
configuration (cantilever
Cantilever
A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...
, fixed-fixed beam
Beam (structure)
A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...
), or by contact interface (capacitive, ohmic
Ohmic contact
An ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage curve of the device is linear and symmetric. If the I-V characteristic is non-linear and asymmetric, the contact is not ohmic, but is a blocking or Schottky contact...
). Electrostatically-actuated RF MEMS components offer low insertion loss
Insertion loss
In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels ....
and high isolation, linearity, power handling and Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
, do not consume power, but require a high control voltage and hermetic
Hermetic seal
A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method and conditions of usage.-Etymology :...
single-chip packaging (thin film
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
capping, LCP
Liquid crystal polymer
Liquid-crystal polymers are a class of aromatic polyester polymers. They are extremely unreactive and inert, and highly resistant to fire.-Background:...
or LTCC
Low temperature co-fired ceramic
Low temperature co-fired ceramic devices are monolithic, ceramic microelectronic devices. In this context ‘co-fired ceramic’ means that the ceramic support structure and any conductive, resistive, and dielectric materials are fired in a kiln at the same time, and ‘Low temperature’ means that the...
packaging) or wafer-level packaging
Wafer-level Packaging
Wafer-level packaging is the technology of packaging an integrated circuit at wafer level. WLP is essentially a true chip scale package technology, since the resulting package is practically of the same size as the die....
(anodic or glass frit
Frit
Frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble...
wafer bonding).
RF MEMS switches were pioneered by IBM Research Laboratory
IBM Research
IBM Research, a division of IBM, is a research and advanced development organization and currently consists of eight locations throughout the world and hundreds of projects....
, San Jose
San Jose, California
San Jose is the third-largest city in California, the tenth-largest in the U.S., and the county seat of Santa Clara County which is located at the southern end of San Francisco Bay...
, CA
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, Hughes Research Laboratories
Hughes Research Laboratories
HRL Laboratories , was the research arm of the Hughes Aircraft Company. Its dedicated research center was established in 1960 in Malibu...
, Malibu, CA, Northeastern University in cooperation with Analog Devices
Analog Devices
Analog Devices, Inc. , known as ADI, is an American multinational semiconductor company specializing in data conversion and signal conditioning technology, headquartered in Norwood, Massachusetts...
, Boston
Boston
Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...
, MA
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
, Raytheon
Raytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...
, Dallas, TX
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
, and Rockwell
Rockwell
- People :* Dick Rockwell, an American comic strip and comic book artist, nephew of Norman Rockwell* Francis W. Rockwell, a United States Congressman from Massachusetts* Francis W...
Science, Thousand Oaks, CA. A capacitive fixed-fixed beam RF MEMS switch, as shown in Fig. 1(a), is in essence a micro-machined capacitor with a moving top electrode, which is the beam. It is generally connected in shunt with the transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
and used in X- to W-band (77 GHz
GHZ
GHZ or GHz may refer to:# Gigahertz .# Greenberger-Horne-Zeilinger state — a quantum entanglement of three particles.# Galactic Habitable Zone — the region of a galaxy that is favorable to the formation of life....
and 94 GHz) RF MEMS components. An ohmic cantilever RF MEMS switch, as shown in Fig. 1(b), is capacitive in the up-state, but makes an ohmic contact in the down-state. It is generally connected in series with the transmission line and is used in DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
to the Ka-band
Ka band
The Ka band covers the frequencies of 26.5–40 GHz. The Ka band is part of the K band of the microwave band of the electromagnetic spectrum. This symbol refers to "K-above" — in other words, the band directly above the K-band...
components.
From an electromechanical perspective, the components behave like a damped mass-spring system, actuated by an electrostatic force. The spring constant is a function of the dimensions of the beam, as well as the Young's modulus
Young's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
, the residual stress
Residual stress
Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...
and the Poisson ratio of the beam material. The electrostatic force is a function of the capacitance and the bias voltage. Knowledge of the spring constant allows for hand calculation of the pull-in voltage, which is the bias voltage necessary to pull-in the beam, whereas knowledge of the spring constant and the mass allows for hand calculation of the switching time.
From an RF perspective, the components behave like a series RLC circuit with negligible resistance and inductance. The up- and down-state capacitance are in the order of 50 fF and 1.2 pF, which are functional values for millimeter-wave circuit design. Switches typically have a capacitance ratio of 30 or higher, while switched capacitors and varactors have a capacitance ratio of about 1.2 to 10. The loaded Q factor is between 20 and 50 in the X-, Ku
Ku band
The Kμ band is a portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to —in other words, the band directly below the K-band...
- and Ka-band.
RF MEMS switched capacitors are capacitive fixed-fixed beam switches with a low capacitance ratio. RF MEMS varactors are capacitive fixed-fixed beam switches which are biased below pull-in voltage. Other examples of RF MEMS switches are ohmic cantilever switches, and capacitive single pole N throw (SPNT) switches based on the axial gap wobble
Wobble
Wobble or wobbles can mean:* Wobble, to shake, or wiggle.* Wobble or wobbling effect, an artifact in video known as screen tearing* Wobble base pair, a type of basepair in genetics translation...
motor
Motor
Motor is a device that creates motion. It usually refers to an engine of some kind. It may also specifically refer to:*Electric motor, a machine that converts electricity into a mechanical motion...
.
Biasing
RF MEMS components are biased electrostatically using a bipolar NRZNon-return-to-zero
In telecommunication, a non-return-to-zero line code is a binary code in which 1's are represented by one significant condition and 0's are represented by some other significant condition , with no other neutral or rest condition. The pulses have more energy than a RZ code...
drive voltage, as shown in Fig. 2, in order to avoid dielectric charging
Failure modes of electronics
Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes...
and to increase the lifetime of the device. Dielectric charges exert a permanent electrostatic force on the beam. The use of a bipolar NRZ drive voltage instead of a DC drive voltage avoids dielectric charging whereas the electrostatic force exerted on the beam is maintained, because the electrostatic force varies quadratically with the DC drive voltage. Electrostatic biasing implies no current flow, allowing high-resistivity bias lines to be used instead of RF chokes
Choking
Choking is the mechanical obstruction of the flow of air from the environment into the lungs. Choking prevents breathing, and can be partial or complete, with partial choking allowing some, although inadequate, flow of air into the lungs. Prolonged or complete choking results in asphyxia which...
.
Packaging
RF MEMS components are fragile and require wafer level packaging or single chip packaging which allow for hermetic cavityCavity
A cavity is a hole. It may refer to:*Dental cavity, damage to the structure of teeth*Body cavity, a fluid filled space in many animals where organs typically develop*Cavity wall, a wall consisting of two skins with a cavity....
sealing. A cavity is required to allow movement, whereas hermeticity is required to prevent cancellation of the spring force by the Van der Waals force
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
exerted by water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
droplets and other contaminants on the beam. RF MEMS switches, switched capacitors and varactors can be packaged using wafer level packaging. Large monolithic RF MEMS filters, phase shifters, and tunable matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
networks require single chip packaging.
Wafer-level packaging is implemented before wafer dicing
Dicing
Dicing is a culinary knife cut in which the food item is cut into small blocks or dice. This may be done for aesthetic reasons or to create uniformly sized pieces to ensure even cooking...
, as shown in Fig. 3(a), and is based on anodic, metal diffusion, metal eutectic, glass frit, polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
, and silicon fusion wafer bonding. The selection of a wafer-level packaging technique is based on balancing the thermal expansion coefficients of the material layers of the RF MEMS component and those of the substrates to minimize the wafer bow
Bow
Bow may refer to:* Bow , an archery weapon that uses elasticity to propel arrows* Bowing , to lower the head or the upper body* Bow , the foremost point of the hull of a ship or boat...
and the residual stress, as well as on alignment
Alignment
Alignment may refer to:* Alignment , secondary evidence used to associate features such as postholes* Alignment , in Israel from 1965 to 1992...
and hermeticity requirements. Figures of merit for wafer-level packaging techniques are chip size, hermeticity, processing temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, (in)tolerance to alignment errors and surface roughness. Anodic and silicon fusion bonding do not require an intermediate layer, but do not tolerate surface roughness. Wafer-level packaging techniques based on a bonding technique with a conductive intermediate layer (conductive split ring) restrict the bandwidth and isolation of the RF MEMS component. The most common wafer-level packaging techniques are based on anodic and glass frit wafer bonding. Wafer-level packaging techniques, enhanced with vertical interconnects, offer the opportunity of three-dimensional integration.
Single-chip packaging, as shown in Fig. 3(b), is implemented after wafer dicing, using pre-fabricated ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
or organic
Organic
Organic may refer to:* Of or relating to an organism, a living entity* Of or relating to an organ- Chemistry :* Organic matter, matter that has come from a once-living organism, is capable of decay or the product of decay, or is composed of organic compound* Organic chemistry, chemistry involving...
packages, such as LCP injection molded packages or LTCC packages. Pre-fabricated packages require hermetic cavity sealing through clogging, shedding
Moult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...
, soldering
Soldering
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
or welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
. Figures of merit for single-chip packaging techniques are chip size, hermeticity, and processing temperature.
Microfabrication
An RF MEMS fabrication process is based on surface micromachining techniques, and allows for integration of SiCr or TaN thin filmThin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
resistors (TFR), metal-air-metal (MAM) capacitors, metal-insulator-metal (MIM) capacitors, and RF MEMS components. An RF MEMS fabrication process can be realized on a variety of wafers: III-V compound semi-insulating
Compound semiconductor
A compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table . These semiconductors typically form in groups 13-16 ,...
, borosilicate glass, fused silica (quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
), LCP, sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
, and passivated
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
silicon wafers. As shown in Fig. 4, RF MEMS components can be fabricated in class 100 clean rooms using 6 to 8 optical lithography steps with a 5 μm contact alignment error, whereas state-of-the-art MMIC
Monolithic Microwave Integrated Circuit
A Monolithic Microwave Integrated Circuit, or MMIC , is a type of integrated circuit device that operates at microwave frequencies . These devices typically perform functions such as microwave mixing, power amplification, low noise amplification, and high frequency switching...
and RFIC fabrication processes require 13 to 25 lithography steps.
As outlined in Fig. 4, the essential microfabrication
Microfabrication
Microfabrication is the term that describes processes of fabrication of miniature structures, of micrometre sizes and smaller. Historically the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device...
steps are:
- Deposition of the bias lines (Fig. 4, step 1)
- Deposition of the electrode layer (Fig. 4, step 2)
- Deposition of the dielectricDielectricA dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
layer (Fig. 4, step 3) - Deposition of the sacrificial spacer (Fig. 4, step 4)
- Deposition of seed layer and subsequent electroplatingElectroplatingElectroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
(Fig. 4, step 5) - Beam patterningPatterningPatterning may refer to:*Photolithography*Pattern formation...
, release and critical point drying (Fig. 4, step 6)
With the exception of the removal of the sacrificial spacer, which requires critical point drying, the fabrication steps are similar to CMOS fabrication process steps. RF MEMS fabrication processes, unlike BST or PZT
Lead zirconate titanate
Lead zirconate titanate , also called PZT, is a ceramic perovskite material that shows a marked piezoelectric effect. PZT-based compounds are composed of the chemical elements lead and zirconium and the chemical compound titanate which are combined under extremely high temperatures. A filter is...
ferroelectric and MMIC fabrication processes, do not require electron beam lithography
Electron beam lithography
Electron beam lithography is the practice of emitting a beam of electrons in a patterned fashion across a surface covered with a film , and of selectively removing either exposed or non-exposed regions of the resist...
, MBE
Molecular beam epitaxy
Molecular beam epitaxy is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...
, or MOCVD.
Reliability
Contact interface degradation poses a reliability issue for ohmic cantilever RF MEMS switches, whereas dielectric charging beam stiction, as shown in Fig. 5(a), and humidity induced beam stiction, as shown in Fig. 5(b), pose a reliability issue for capacitive fixed-fixed beam RF MEMS switches. Stiction is the inability of the beam to release after removal of the drive voltage. A high contact pressure assures a low-ohmic contact or alleviates dielectric charging induced beam stiction. Commercially-available ohmic cantilever RF MEMS switches and capacitive fixed-fixed beam RF MEMS switches have demonstrated lifetimes in excess of 100 billion cycles at 100 mW of RF input power. Reliability issues pertaining to high-power operation are discussed in the limiter section.Applications
RF MEMS resonators are applied in filters and reference oscillators. RF MEMS switches, switched capacitors and varactors are applied in electronically scanned (sub)arraysPhased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
(phase shifters
Phase shift module
thumb|right|A microwave Phase Shifter and Frequency Translator. Picture courtesy of A phase shifter is a microwave network which provides a controllable phase shift of the RF signal...
) and software-defined radio
Software-defined radio
A software-defined radio system, or SDR, is a radio communication system where components that have been typically implemented in hardware are instead implemented by means of software on a personal computer or embedded computing devices...
s (reconfigurable antennas, tunable band-pass filter
Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
s).
Antennas
Polarization and radiation patternRadiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
reconfigurability, and frequency tunability, are usually achieved by incorporation of III-V semiconductor components, such as SPST switches or varactor diodes. However, these components can be readily replaced by RF MEMS switches and varactors in order to take advantage of the low insertion loss and high Q factor offered by RF MEMS technology. In addition, RF MEMS components can be integrated monolithically on low-loss dielectric substrates, such as borosilicate glass, fused silica or LCP, whereas III-V compound semi-insulating and passivated silicon substrates are generally lossier and have a higher dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
. A low loss tangent
Loss tangent
The loss tangent is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy. The term refers to the tangent of the angle in a complex plane between the resistive component of an electromagnetic field and its reactive component.-Electromagnetic...
and low dielectric constant are of importance for the efficiency
Antenna efficiency
In electromagnetics, antenna efficiency or radiation efficiency is a figure of merit for an antenna. It measures the electrical losses that occur throughout the antenna while it is operating at a given frequency, or averaged over its operation across a frequency band...
and the bandwidth of the antenna.
The prior art includes an RF MEMS frequency tunable fractal antenna
Fractal antenna
A fractal antenna is an antenna that uses a fractal, self-similar design to maximize the length, or increase the perimeter , of material that can receive or transmit electromagnetic radiation within a given total surface area or volume.Such fractal antennas are also referred to as multilevel and...
for the 0.1–6 GHz frequency range, and the actual integration of RF MEMS switches on a self-similar Sierpinski gasket antenna to increase its number of resonant frequencies, extending its range to 8 GHz, 14 GHz and 25 GHz, an RF MEMS radiation pattern reconfigurable spiral antenna
Spiral antenna
In microwave systems, a spiral antenna is a type of RF antenna. It is shaped as a two-arm spiral, or more arms may be used. Spiral antennas were first described in 1956 . Spiral antennas belong to the class of frequency independent antennas which operate over a wide range of frequencies....
for 6 and 10 GHz, an RF MEMS radiation pattern reconfigurable spiral antenna for the 6–7 GHz frequency band based on packaged Radant MEMS SPST-RMSW100 switches, an RF MEMS multiband
Multi-band
In telecommunications, the terms multi-band, dual-band, tri-band, quad-band and penta-band refer to a device supporting multiple radio frequency bands used for communication...
Sierpinski fractal antenna
Fractal antenna
A fractal antenna is an antenna that uses a fractal, self-similar design to maximize the length, or increase the perimeter , of material that can receive or transmit electromagnetic radiation within a given total surface area or volume.Such fractal antennas are also referred to as multilevel and...
, again with integrated RF MEMS switches, functioning at different bands from 2.4 to 18 GHz, and a 2-bit Ka-band RF MEMS frequency tunable slot antenna
Slot antenna
A slot antenna consists of a metal surface, usually a flat plate, with a hole or slot cut out. When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves in similar way to a dipole antenna. The shape and size of the slot, as well as the driving frequency,...
.
Filters
RF bandpass filters can be used to increase out-of-bandOut-of-band
The term out-of-band has different uses in communications and telecommunication. In case of out-of-band control signaling, signaling bits are sent in special order in a dedicated signaling frame...
rejection, in case the antenna fails to provide sufficient selectivity. Out-of-band rejection eases the dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
requirement on the LNA and the mixer
Mixer
Mixer may refer to:An electronics device:* DJ mixer, a type of audio mixing console used by disc jockeys* Electronic mixer, a device for adding or multiplying signal voltages together...
in the light of interference
Interference (communication)
In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
. Off-chip RF bandpass filters based on lumped bulk acoustic
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
wave (BAW), ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
, SAW
Surface acoustic wave
]A surface acoustic wave is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate.-Discovery:...
, quartz crystal, and FBAR
Thin film bulk acoustic resonator
Thin Film Bulk Acoustic Resonator is a device consisting of a piezoelectric material sandwiched between two electrodes and acoustically isolated from the surrounding medium. FBAR devices using piezoelectric films with thicknesses ranging from several micrometres down to tenth of micrometres...
resonators have superseded distributed RF bandpass filters based on transmission line resonators, printed on substrates with low loss tangent, or based on waveguide cavities.
Tunable RF bandpass filters offer a significant size reduction over switched RF bandpass filter bank
Filter bank
In signal processing, a filter bank is an array of band-pass filters that separates the input signal into multiple components, each one carrying a single frequency subband of the original signal. One application of a filter bank is a graphic equalizer, which can attenuate the components...
s. They can be implemented using III-V semiconducting varactors, BST or PZT ferroelectric and RF MEMS resonators and switches, switched capacitors and varactors, and YIG
Yttrium iron garnet
Yttrium iron garnet is a kind of synthetic garnet, with chemical composition 323, or Y3Fe5O12. It is a ferrimagnetic material with a Curie temperature of 550 K....
ferrites. RF MEMS resonators offer the potential of on-chip
Radio-on-a-chip
Radio-on-a-chip or RoC systems are single-chip devices that incorporate a receiver, an amplifier, and power management into a single chip, which can thus be embedded into very small or even portable electronics....
integration of high-Q resonators and low-loss bandpass filters. The Q factor of RF MEMS resonators is in the order of 100-1000. RF MEMS switch, switched capacitor and varactor technology, offers the tunable filter designer a compelling trade-off between insertion loss, linearity, power consumption, power handling, size, and switching time.
Phase shifters
Passive subarrays based on RF MEMS phase shifters may be used to lower the amount of T/R modules in an active electronically scanned arrayActive Electronically Scanned Array
An Active Electronically Scanned Array , also known as active phased array radar is a type of phased array radar whose transmitter and receiver functions are composed of numerous small solid-state transmit/receive modules . AESAs aim their "beam" by broadcasting radio energy that interfere...
. The statement is illustrated with examples in Fig. 6: assume a one-by-eight passive subarray is used for transmit as well as receive, with following characteristics: f = 38 GHz, Gr = Gt = 10 dBi
DBI
DBI may refer to:*dBi, decibel isotropic*Perl DBI, a database interface for Perl*Diazepam binding inhibitor*DBi Technologies Inc., a provider of text mining, scheduling and UI design component software, services and computer programs...
, BW = 2 GHz, Pt = 4 W
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
. The low loss (6.75 ps
Picosecond
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
/dB) and good power handling (500 mW) of the RF MEMS phase shifters allow an EIRP of 40 W and a Gr/T of 0.036 1/K. EIRP, also referred to as the power-aperture product, is the product of the transmit gain, Gt, and the transmit power, Pt. Gr/T is the quotient of the receive gain and the antenna noise temperature. A high EIRP and Gr/T are a prerequisite for long-range detection. The EIRP and Gr/T are a function of the number of antenna elements per subarray and of the maximum scanning angle. The number of antenna elements per subarray should be chosen in order to optimize the EIRP or the EIRP x Gr/T product, as shown in Fig. 7 and Fig. 8. The radar range equation can be used to calculate the maximum range for which targets can be detected with 10 dB of SNR
Signal to Noise
Signal to Noise is a graphic novel written by Neil Gaiman and illustrated by Dave McKean. It was originally serialised in the UK style magazine The Face, beginning in 1989, and collected as a graphic novel in 1992, published by Victor Gollancz Ltd in the UK and by Dark Horse Comics in the US.The...
at the input of the receiver.
in which kB is the Boltzmann constant, λ is the free-space wavelength, and σ is the RCS
RCS
RCS may refer to:Clubs and societies*Racing Club de Strasbourg*Radcliffe Choral Society*Royal College of Science*Royal College of Surgeons*Royal Commonwealth SocietyTechnology*Radar cross-section*Radio Computing Services*Reaction control system...
of the target. Range values are tabulated in Table 1 for following targets: a sphere
Mie theory
The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...
with a radius, a, of 10 cm (σ = π a2), a dihedral corner reflector with facet size, a, of 10 cm (σ = 12 a4/λ2), the rear of a car (σ = 20 m2) and for a non-evasive fighter jet (σ = 400 m2).
| RCS (m2) | Range (m) | |
|---|---|---|
| Sphere | 0.0314 | 10 |
| Rear of Car | 20 | 51 |
| Dihedral Corner Reflector | 60.9 | 67 |
| Fighter Jet | 400 | 107 |
RF MEMS phase shifters enable wide-angle passive electronically scanned array
Passive electronically scanned array
A passive electronically scanned array , contrary to its active counterpart AESA, is a phased array which has a central radiofrequency source , sending energy into phase shift modules, which then send energy into the various emitting elements in the front of the antenna...
s, such as lens arrays
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
, reflect arrays
Reflective array antenna
In telecommunication, a reflective array antenna is an antenna in which multiple driven elements are mounted in front of a surface designed to reflect the radio waves in a desired direction....
, subarrays and switched beamforming
Beamforming
Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in the array in a way where signals at particular angles experience constructive interference and while others experience destructive...
networks, with high EIRP and high Gr/T. The prior art in passive electronically scanned arrays, includes an X-band continuous transverse stub (CTS) array fed by a line source synthesized by sixteen 5-bit reflect-type RF MEMS phase shifters based on ohmic cantilever RF MEMS switches, an X-band 2-D lens array consisting of parallel-plate waveguides
Waveguide (electromagnetism)
In electromagnetics and communications engineering, the term waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used to carry radio waves...
and featuring 25,000 ohmic cantilever RF MEMS switches, and a W-band switched beamforming network based on an RF MEMS SP4T switch and a Rotman lens focal plane scanner.
The usage of true-time-delay TTD phase shifters instead of RF MEMS phase shifters allows UWB
Ultra-wideband
Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...
radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
s with associated high range resolution, and avoids beam squinting or frequency scanning. TTD phase shifters are designed using the switched-line principle or the distributed loaded-line principle. Switched-line TTD phase shifters outperform distributed loaded-line TTD phase shifters in terms of time delay per decibel NF
Noise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
, especially at frequencies up to X-band, but are inherently digital and require low-loss and high-isolation SPNT switches. Distributed loaded-line TTD phase shifters, however, can be realized analogously or digitally, and in smaller form factors, which is important at the subarray level. Analog phase shifters are biased through a single bias line, whereas multibit digital phase shifters require a parallel bus along with complex routing schemes at the subarray level.
Reading
Conferences
Journals
- IEEE/ASME Journal of Microelectromechanical Systems
- IEEE Transactions on Microwave Theory and Techniques
- Journal of Micromechanics and Microengineering
Magazines
Newsletters
R&D
- DARPA MEMS/NEMS Science & Technology Fundamentals (USA)
- [ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-amicom_en.pdf EU FP7 AMICOM: The European NoE on RF MEMS and RF Microsystems (EU)]
- [ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-rf-mems-cluster-workshop-report_en.pdf EU FP7 MEMS-4-MMIC: Enabling MEMS-MMIC Technology for Cost-Effective Multifunctional RF-System Integration (EU)]
- [ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-memspack_en.pdf EU FP7 MEMSPACK: Zero- and First-level Packaging of RF MEMS (EU)]
- [ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-retina_en.pdf EU FP7 RETINA: Reliable, Tuneable and Inexpensive Antennas by Collective Fabrication Processes (EU)]
- Fine MEMS (JP)
Software
- COMSOL
- Coventor
- IntelliSense
- rfMaxima (open-source)
- SoftMEMS
- SUGAR (open-source)
- Verilog-AMS RF MEMS Model Library (open-source)