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| {{About||bioMEMS|bioMEMS|MEMS accelerometers|accelerometer|MEMS displays|interferometric modulator display|MEMS gyroscopes|gyroscope|MEMS microphones|microphone|MEMS pressure sensors|accelerometer|MOEMS|MOEMS}}
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| {{technical|date=February 2012}}
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| The '''[[radio frequency]] [[electromechanical|microelectromechanical]] system''' ('''RF [[Microelectromechanical systems|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]], [[indium phosphide|InP]], [[InSb]]), [[Ferrite (magnet)|ferrite]], [[ferroelectric]], [[silicon]]-based semiconductor ([[CMOS|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#LSI|large-scale integration]], lifetime, [[linearity]], [[noise figure]], [[Electronic packaging|packaging]], [[power handling]], [[power consumption]], [[Circuit reliability|reliability]], ruggedness, size, [[Power supply|supply voltage]], [[switching time]] and weight.
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| == Components ==
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| There are various types of RF MEMS components, such as CMOS integrable RF MEMS [[resonators]] and [[Self-sustainability|self-sustained]] [[Microelectromechanical system oscillator|oscillators]] with small form factor and low [[phase noise]], RF MEMS [[Electronic tuner|tunable]] [[inductor]]s, and RF MEMS [[switches]], [[switched capacitor]]s and [[varactor]]s.
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| === Switches, switched capacitors and varactors ===
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| The components discussed in this article are based on RF MEMS switches, switched capacitors and varactors. These components can be used instead of [[FET]] and [[HEMT]] switches (FET and HEMT transistors in [[common gate]] configuration), and [[PIN diode|PIN]] diodes. RF MEMS switches, switched capacitors and varactors are classified by actuation method ([[electrostatic]], electrothermal, [[magnetostatics|magnetostatic]], [[piezoelectric]]), by axis of deflection (lateral, vertical), by circuit configuration ([[Series circuits|series]], [[Shunt (electrical)|shunt]]), by [[Clamp (tool)|clamp]] configuration ([[cantilever]], fixed-fixed [[Beam (structure)|beam]]), or by contact interface ([[capacitive]], [[Ohmic contact|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 seal|hermetic]] single-chip packaging ([[thin film]] capping, [[liquid crystal polymer|LCP]] or [[low temperature co-fired ceramic|LTCC]] packaging) or [[Wafer-level Packaging|wafer-level packaging]] ([[anodic]] or glass [[frit]] wafer bonding).
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| RF MEMS switches were pioneered by [[IBM Research|IBM Research Laboratory]], [[San Jose, California|San Jose]], [[California|CA]],<ref>K. E. Petersen: "Micro-Mechanical Membrane Switches on Silicon," IBM J. Res. & Dev., vol. 23, no. 4, pp. 376-385, Jul. 1979</ref><ref>K. E. Petersen: "Silicon as a Mechanical Material," Proc. of the IEEE, vol. 70, no. 5, pp. 420-457, May 1982</ref> [[Hughes Research Laboratories]], [[Malibu, California|Malibu]], CA,<ref>L. E. Larson: “Micro-Machined Switch and Method of Fabrication,” U.S. Patent 5,121,089, Nov. 1, 1990</ref> [[Northeastern University]] in cooperation with [[Analog Devices]], [[Boston]], [[Massachusetts|MA]],<ref>P. M. Zavracky, S. Majumder, and N. E. McGruer: "Micromechanical Switches Fabricated Using Nickel Surface Micromachining," J. Microelectromech. Syst., vol. 6, no. 1, pp. 3-9, Mar. 1997</ref> [[Raytheon]], [[Dallas]], [[Texas|TX]],<ref>C. L. Goldsmith, B. M. Kanack, T. Lin, B. R. Norvell, L. Y. Pang, B. Powers, C. Rhoads, D. Seymour: "Micromechanical Microwave Switching". U.S. Patent 5,619,061, Oct. 31, 1994</ref><ref>C. L. Goldsmith, Z. Yao, S. Eshelman, and D. Denniston: "Performance of Low-Loss RF MEMS Capacitive Switches," IEEE Microwave Wireless Compon. Lett., vol. 8, no. 8, pp. 269-271, Aug. 1998</ref> and [[Rockwell International|Rockwell]] Science, [[Thousand Oaks]], CA.<ref name="autogenerated305">J. B. Hacker, R. E. Mihailovich, M. Kim, and J. F. DeNatale: “A Ka-band 3-Bit RF MEMS True-Time-Delay Network,” IEEE Trans. Microw. Theory Tech., vol. 51, no. 1, pp. 305–308, Jan. 2003</ref> 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-band|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 [[Direct current|DC]] to the [[Ka band|Ka-band]] components.
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| [[File:RF MEMS.png|thumb|640px|'''Fig. 1''': (a) A capacitive fixed-fixed beam RF MEMS switch, connected in shunt to a CPW line. (b) An ohmic cantilever RF MEMS
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| switch, connected in series to a microstrip line.]]
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| From an electromechanical perspective, the components behave like a [[Damped spring-mass system|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 [[Biasing|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.
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| 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 [[Femtofarad|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 band|Ku]]- and Ka-band.
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| 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 [[wikt:wobble|wobble]] [[Engine|motor]].<ref>S. Pranonsatit, A. S. Holmes, I. D. Robertson and S. Lucyszyn: "Single-Pole Eight-Throw RF MEMS Rotary Switch," IEEE/ASME J. Microelectromech. Syst., vol. 15, no. 6, pp. 1735-1744, Dec. 2006</ref>
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| == Biasing ==
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| RF MEMS components are biased electrostatically using a bipolar [[Non-return-to-zero|NRZ]] drive voltage, as shown in Fig. 2, in order to avoid [[Failure modes of electronics|dielectric charging]]<ref>J. R. Reid and R. T. Webster: "Measurements of Charging in Capacitive Microelectromechanical Switches," Electronics Letters, vol. 38, no. 24, pp. 1544-1545, Nov. 2002</ref> 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 [[choking|chokes]].
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| [[File:RF MEMS BIASING.png|thumb|320px|'''Fig. 2''': Electrostatic biasing of a capacitive fixed-fixed beam RF MEMS switch, switched capacitor or varactor.]]
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| == Packaging ==
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| RF MEMS components are fragile and require wafer level packaging or single chip packaging which allow for hermetic [[Microwave cavity|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]] exerted by [[water]] [[droplet]]s and other [[contaminant]]s 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 [[Impedance matching|matching]] networks require single chip packaging.
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| 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 coefficient]]s of the material layers of the RF MEMS component and those of the substrates to minimize the wafer [[Bending|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 (signal processing)|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.
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| Single-chip packaging, as shown in Fig. 3(b), is implemented after wafer dicing, using pre-fabricated [[ceramic]] or [[organic compound|organic]] packages, such as LCP injection molded packages or LTCC packages. Pre-fabricated packages require hermetic cavity sealing through clogging, [[Moult|shedding]], [[soldering]] or [[welding]]. Figures of merit for single-chip packaging techniques are chip size, hermeticity, and processing temperature.
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| [[File:RF MEMS PACKAGING.png|thumb|320px|'''Fig. 3''': (a) Wafer-level packaging. (b) Single chip packaging of an ohmic cantilever RF MEMS switch.]]
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| == Microfabrication ==
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| An RF MEMS fabrication process is based on surface micromachining techniques, and allows for integration of SiCr or [[Tantalum nitride|TaN]] [[thin film]] 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: [[compound semiconductor|III-V compound semi-insulating]], borosilicate glass, [[fused silica]] ([[quartz]]), LCP, [[sapphire]], and [[Passivation (chemistry)|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 [[monolithic microwave integrated circuit|MMIC]] and [[Radio Frequency Integrated Circuit|RFIC]] fabrication processes require 13 to 25 lithography steps.
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| [[File:RF MEMS FABRICATION PROCESS.png|thumb|320px|'''Fig. 4''': RF MEMS switch, switched capacitor, or varactor fabrication process]]
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| As outlined in Fig. 4, the essential [[microfabrication]] steps are:
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| * Deposition of the bias lines (Fig. 4, step 1)
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| * Deposition of the electrode layer (Fig. 4, step 2)
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| * Deposition of the [[dielectric]] layer (Fig. 4, step 3)
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| * Deposition of the [[sacrificial]] spacer (Fig. 4, step 4)
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| * Deposition of seed layer and subsequent [[electroplating]] (Fig. 4, step 5)
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| * Beam [[Photolithography|patterning]], release and [[critical point drying]] (Fig. 4, step 6)
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| 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 [[Barium strontium titanate|BST]] or [[lead zirconate titanate|PZT]] ferroelectric and MMIC fabrication processes, do not require [[electron beam lithography]], [[molecular beam epitaxy|MBE]], or [[metal organic chemical vapor deposition|MOCVD]].
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| == Reliability ==
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| 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 [[milliwatt|mW]] of RF input power.<ref>H. S. Newman, J. L. Ebel, D. Judy, and J. Maciel: "Lifetime Measurements on a High-Reliability RF MEMS Contact Switch," IEEE Microwave Wireless Compon. Lett., vol. 18, no. 2, pp. 100-102, Feb. 2008</ref><ref>C. Goldsmith, J. Maciel, and J. McKillop: "Demonstrating reliability," IEEE Microwave Magazine, vol. 8, no. 6, pp. 56-60, Dec. 2007</ref> Reliability issues pertaining to high-power operation are discussed in the limiter section.
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| [[File:RF MEMS RELIABILITY.png|thumb|240px|'''Fig. 5''': (a) Dielectric charging induced beam stiction. (b) Humidity induced beam stiction.]]
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| == Applications ==
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| RF MEMS resonators are applied in filters and reference oscillators.<ref name="autogenerated251">C. Nguyen: “MEMS Technology for Timing and Frequency Control,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 54, no. 2, pp. 251–270, Feb. 2007</ref> RF MEMS switches, switched capacitors and varactors are applied in [[phased array|electronically scanned (sub)arrays]] ([[Phase shift module|phase shifters]]) and [[software-defined radio]]s (reconfigurable [[antennas]], tunable [[band-pass filter]]s).<ref>G. M. Rebeiz: "RF MEMS, Theory, Design and Technology," John Wiley & Sons, 2003</ref>
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| === Antennas ===
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| [[Polarization (waves)|Polarization]] and [[radiation pattern]] reconfigurability, and frequency tunability, are usually achieved by incorporation of III-V semiconductor components, such as [[Changeover switch|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 [[Antenna efficiency|efficiency]] and the bandwidth of the antenna.
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| The prior art includes an RF MEMS frequency tunable [[fractal antenna]] for the 0.1–6 GHz frequency range,<ref>D. E. Anagnostou et al. "Fractal Antennas with RF-MEMS Switches for Multiple Frequency Applications", in IEEE APS/URSI International Symposium, San Antonio, TX, June 2002, vol. 2, pp.22-25</ref> and the actual integration of RF MEMS switches on a self-similar [[Sierpinski gasket]] antenna to increase its number of [[Resonant frequency|resonant frequencies]], extending its range to 8 GHz, 14 GHz and 25 GHz,<ref>D. E. Anagnostou, G. Zheng, M. Chryssomallis, J. Lyke, G. Ponchak, J. Papapolymerou, and C. G. Christodoulou, "Design, Fabrication and Measurements of an RF-MEMS-Based Self-Similar Re-configurable Antenna", IEEE Transactions on Antennas & Propagation, Special Issue on Multifunction Antennas and Antenna Systems, Vol. 54, Issue 2, Part 1, Feb. 2006, pp.422 – 432</ref><ref>D. E. Anagnostou, G. Zheng, J. Papapolymerou and C. G. Christodoulou, U.S. Patent 7,589,674, "Reconfigurable multifrequency antenna with RF-MEMS switches", Sept. 15, 2009.</ref> an RF MEMS radiation pattern reconfigurable [[spiral antenna]] for 6 and 10 GHz,<ref>C. Jung, M. Lee, G. P. Li, and F. D. Flaviis: “Reconfigurable Scan-Beam Single-Arm Spiral Antenna Integrated with RF MEMS Switches,” IEEE Trans. Antennas Propag., vol. 54, no. 2, pp. 455–463, Feb. 2006</ref> an RF MEMS radiation pattern reconfigurable spiral antenna for the 6–7 GHz [[frequency band]] based on packaged Radant MEMS SPST-RMSW100 switches,<ref>G. H. Huff and J. T. Bernhard: “Integration of Packaged RF MEMS Switches with Radiation Pattern Reconfigurable Square Spiral Microstrip Antennas,” IEEE Trans. Antennas Propag., vol. 54, no. 2, pp. 464–469, Feb. 2006</ref> an RF MEMS [[Multi-band|multiband]] Sierpinski [[fractal antenna]], again with integrated RF MEMS switches, functioning at different bands from 2.4 to 18 GHz,<ref>N. Kingsley, D. E. Anagnostou, M. Tentzeris, and J. Papapolymerou: “RF MEMS Sequentially Reconfigurable Sierpinski Antenna on a Flexible Organic Substrate with Novel DC-Biasing Technique,” IEEE/ASME J. Microelectromech. Syst., vol. 16, no. 5, pp. 1185–1192, Oct. 2007</ref> and a 2-bit Ka-band RF MEMS frequency tunable [[slot antenna]].<ref>K. Van Caekenberghe and K. Sarabandi: "A 2-Bit Ka-Band RF MEMS Frequency Tunable Slot Antenna," IEEE Antennas and Wireless Propagation Letters, vol. 7, pp. 179-182, 2008</ref>
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| The [[Samsung Omnia W]] was the first smart phone to include a RF MEMS antenna.<ref>[http://www.theregister.co.uk/2012/11/07/wtf_is_rf_mems/ "WTF is... RF-MEMS?"]</ref>
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| === Filters ===
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| RF [[bandpass filter]]s can be used to increase [[out-of-band]] rejection, in case the antenna fails to provide sufficient [[Selectivity (electronic)|selectivity]]. Out-of-band rejection eases the [[dynamic range]] requirement on the [[low noise amplifier|LNA]] and the [[Frequency mixer|mixer]] in the light of [[Interference (communication)|interference]]. Off-chip RF bandpass filters based on lumped bulk [[Acoustics|acoustic]] wave (BAW), [[ceramic]], [[surface acoustic wave|SAW]], quartz crystal, and [[thin film bulk acoustic resonator|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.
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| 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 [[yttrium iron garnet|YIG]] ferrites. RF MEMS resonators offer the potential of [[Radio-on-a-chip|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.<ref name="autogenerated251"/> 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.<ref>R. M. Young, J. D. Adam, C. R. Vale, T. T. Braggins, S. V. Krishnaswamy, C. E. Milton, D. W. Bever, L. G. Chorosinski, Li-Shu Chen, D. E. Crockett, C. B. Freidhoff, S. H. Talisa, E. Capelle, R. Tranchini, J. R. Fende, J. M. Lorthioir, A. R. Tories: “Low-Loss Bandpass RF Filter Using MEMS Capacitance Switches to Achieve a One-Octave Tuning Range and Independently Variable Bandwidth,” IEEE MTT-S International Microwave Symposium Digest, vol. 3, pp. 1781-1784, Jun. 2003</ref>
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| === Phase shifters ===
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| Passive subarrays based on RF MEMS phase shifters may be used to lower the amount of T/R modules in an [[active electronically scanned array]]. 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, G<sub>r</sub> = G<sub>t</sub> = 10 [[dBi]], BW = 2 GHz, P<sub>t</sub> = 4 [[Watt|W]]. The low loss (6.75 [[picosecond|ps]]/dB) and good power handling (500 mW) of the RF MEMS phase shifters allow an EIRP of 40 W and a G<sub>r</sub>/T of 0.036 1/K. EIRP, also referred to as the power-aperture product, is the product of the transmit gain, G<sub>t</sub>, and the transmit power, P<sub>t</sub>. G<sub>r</sub>/T is the quotient of the receive gain and the antenna noise temperature. A high EIRP and G<sub>r</sub>/T are a prerequisite for long-range detection. The EIRP and G<sub>r</sub>/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 G<sub>r</sub>/T product, as shown in Fig. 7 and Fig. 8. The [[Radar#Radar equation|radar range equation]] can be used to calculate the maximum range for which targets can be detected with 10 dB of [[Signal-to-noise ratio|SNR]] at the input of the receiver.
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| <p align=center><math>{\mathrm{R = \sqrt[4]{\frac{\displaystyle {\mathrm{\lambda^2 \, EIRP \, G_R/T \, \sigma}}}{{\mathrm{\displaystyle 64 \, \pi^3 \, k_B \, BW \, SNR}}}}}}</math> | |
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| in which k<sub>B</sub> is the [[Boltzmann constant]], λ is the free-space wavelength, and σ is the [[Radar cross-section|RCS]] of the target. Range values are tabulated in Table 1 for following targets: a [[Mie theory|sphere]] with a radius, a, of 10 cm (σ = π a<sup>2</sup>), a [[Dihedral (aircraft)|dihedral]] corner reflector with facet size, a, of 10 cm (σ = 12 a<sup>4</sup>/λ<sup>2</sup>), the rear of a car (σ = 20 m<sup>2</sup>) and for a non-evasive fighter jet (σ = 400 m<sup>2</sup>).
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| {| class="wikitable"
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| |+ '''Table 1''': Maximum Detectable Range <br />(SNR = 10 dB)
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| |-
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| !
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| ! RCS (m<sup>2</sup>)
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| ! Range (m)
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| |-
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| | Sphere
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| | 0.0314
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| | 10
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| |-
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| | Rear of Car
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| | 20
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| | 51
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| |-
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| | Dihedral Corner Reflector
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| | 60.9
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| | 67
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| |-
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| | Fighter Jet
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| | 400
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| | 107
| |
| |}
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| [[File:RF MEMS EIRP TIMES GT VERSUS N 1.png|thumb|640px|'''Fig. 6''': EIRP x G<sub>r</sub>/T]]
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| [[File:RF MEMS EIRP VERSUS N.png|thumb|240px|'''Fig. 7''': EIRP versus number of antenna elements in a passive subarray.]]
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| [[File:RF MEMS EIRP TIMES GT VERSUS N 2.png|thumb|240px|'''Fig. 8''': EIRP x G<sub>r</sub>/T versus number of antenna elements in a passive subarray.]]
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| RF MEMS phase shifters enable wide-angle [[passive electronically scanned array]]s, such as [[Lens (optics)|lens arrays]], [[Reflective array antenna|reflect arrays]], subarrays and switched [[beamforming]] networks, with high [[effective isotropically radiated power|EIRP]] and high G<sub>r</sub>/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,<ref>J. J. Lee, C. Quan, and B. M. Pierce: “Low-Cost 2-D Electronically Scanned Array with Compact CTS Feed and MEMS Phase Shifters,” U.S. Patent 6 677 899, Jan. 13, 2004</ref><ref>C. Quan, J. J. Lee, B. M. Pierce, and R. C. Allison: “Wideband 2-D Electronically Scanned Array with Compact CTS Feed and MEMS Phase Shifters,” U.S. Patent 6 822 615, Nov. 23, 2004</ref> an X-band 2-D lens array consisting of parallel-plate [[Waveguide (electromagnetism)|waveguides]] and featuring 25,000 ohmic cantilever RF MEMS switches,<ref>J. J. Maciel, J. F. Slocum, J. K. Smith, and J. Turtle: “MEMS Electronically Steerable Antennas for Fire Control Radars,” IEEE Aerosp. Electron. Syst. Mag, pp. 17–20, Nov. 2007</ref> and a W-band switched beamforming network based on an RF MEMS SP4T switch and a Rotman lens [[Focal plane#Focal points and planes|focal plane]] scanner.<ref>J. Schoebel, T. Buck, M. Reimann, M. Ulm, M. Schneider, A. Jourdain, G. J. Carchon, and H. A. C. Tilmans: "Design Considerations and Technology Assessment of Phased Array Antenna Systems with RF MEMS for Automotive Radar Applications," IEEE Trans. Microwave Theory Tech., vol. 53, no. 6, pp. 1968-1975, Jun. 2005</ref>
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| The usage of true-time-delay TTD phase shifters instead of RF MEMS phase shifters allows [[Ultra-wideband|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<ref name="autogenerated305"/><ref>G. L. Tan, R. E. Mihailovich, J. B. Hacker, J. F. DeNatale, and G. M. Rebeiz: “Low-loss 2- and 4-Bit TTD MEMS Phase Shifters Based on SP4T Switches,” IEEE Trans. Microw. Theory Tech., vol. 51, no. 1, pp. 297–304, Jan. 2003</ref><ref>C. D. Nordquist, C. W. Dyck, G. M. Kraus, I. C. Reines, C. L. Goldsmith, W. D. Cowan, T. A. Plut, F. Austin, P. S. Finnegan, M. H. Ballance, and C. T. Sullivan: “A DC to 10 GHz 6-Bit RF MEMS Time Delay Circuit,” IEEE Microw. Wireless Compon. Lett., vol. 16, no. 5, pp. 305–307, May 2006</ref> or the distributed loaded-line principle.<ref>N. S. Barker and G. M. Rebeiz, “Optimization of distributed MEMS phase shifters,” in IEEE MTT-S Int. Microw. Symp. Dig., pp. 299–302, 1999</ref><ref>A. S. Nagra and R. A. York, “Distributed Analog Phase Shifters with Low Insertion Loss: ” IEEE Trans. Microw. Theory Tech., vol. 47, no. 9, pp. 1705–1711, Sep. 1999</ref><ref>J. Perruisseau-Carrier, R. Fritschi, P. Crespo-Valero, and A. K. Skrivervik: “Modeling of Periodic Distributed MEMS Application to the Design of Variable True-Time-Delay Lines,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 1, pp. 383–392, Jan. 2006</ref><ref>B. Lakshminarayanan and T. M. Weller: “Design and Modeling of 4-Bit Slow-Wave MEMS Phase Shifters,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 1, pp. 120–127, Jan. 2006</ref><ref>B. Lakshminarayanan and T. M. Weller: “Optimization and Implementation of Impedance-Matched True-Time-Delay Phase Shifters on Quartz Substrate,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 2, pp. 335–342, Feb. 2007</ref><ref>K. Van Caekenberghe and T. Vaha-Heikkila: "An Analog RF MEMS Slotline True-Time-Delay Phase Shifter," IEEE Trans. Microw. Theory Tech., vol. 56, no. 9, pp. 2151-2159, Sep. 2008</ref> Switched-line TTD phase shifters outperform distributed loaded-line TTD phase shifters in terms of time delay per decibel [[Noise figure|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.
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| {{clear}}
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| ==References==
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| {{reflist|2}}
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| == Reading ==
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| * [http://www.cambridge.org/gb/knowledge/isbn/item2710256/?site_locale=en_GB ''' S. Lucyszyn (Ed), "Advanced RF MEMS", Cambridge University Press, ISBN 978-0-521-89771-6, Aug. 2010''']
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| == External links ==
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| === Conferences ===
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| * [http://www.ieee-mems2011.org/ IEEE MEMS 2011]
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| * [http://www.memswave2010.unisalento.it/ MEMSWAVE 2010]
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| | |
| === Journals ===
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| * [http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=84 IEEE/ASME Journal of Microelectromechanical Systems]
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| * [http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=22 IEEE Transactions on Microwave Theory and Techniques]
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| * [http://www.iop.org/EJ/journal/JMM Journal of Micromechanics and Microengineering]
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| | |
| === Magazines ===
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| * [http://www.scoop.it/t/test-post-1 RF MEMS Mag - “Everything about MEMS for Telecommunications”]
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| * [http://www.eetimes.com/TechSearch/Search.jhtml?site_id=EE+Times&Site+ID=EE+Times&queryText=RF+MEMS&Search.x=0&Search.y=0&Search=Search EE Times]
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| * [http://www.electroiq.com/index/mems.html ElectroIQ]
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| * [http://www.smalltimes.com/ Small Times]
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| | |
| === Newsletters ===
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| * [http://www.isuppli.com/mems-and-sensors/ iSuppli]
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| * [http://www.memsinvestorjournal.com/ MEMS Investor Journal]
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| | |
| === R&D === | |
| * [http://www.darpa.mil/mto/programs/mems/index.html DARPA MEMS/NEMS Science & Technology Fundamentals (USA)]
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| * [ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-amicom_en.pdf EU FP6 AMICOM: The European NoE on RF MEMS and RF Microsystems (EU)]
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| * [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)]
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| * [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)]
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| * [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)]
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| * [http://www.mmc.or.jp/e/outline-e/activities.html#fineMEMS Fine MEMS (JP)]
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| | |
| === Software ===
| |
| * [http://www.comsol.com/products/mems/ COMSOL]
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| * [http://www.coventor.com Coventor]
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| * [http://www.intellisensesoftware.com/ IntelliSense]
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| * [http://rfmaxima.sourceforge.net/RF_MEMS.html rfMaxima (open-source)]
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| * [http://www.softmems.com/ SoftMEMS]
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| * [http://www-ee.eng.hawaii.edu/~garmire/sugar/ SUGAR (open-source)]
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| * [http://rfmems.sourceforge.net/ Verilog-AMS RF MEMS Model Library (open-source)]
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| * [https://nanohub.org/tools/cvgraph/index.php MEMSLab - Electromechanical actuator simulation suite]
| |
| | |
| === Videos ===
| |
| * [http://www.youtube.com/watch?v=UdMjqtfOeag A 2-Bit Ka-Band RF MEMS Frequency Tunable Slot Antenna]
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| * [http://www.youtube.com/watch?v=dsmedmwtgfw RF MEMS CPW TTD Phase Shifter]
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| * [http://www.youtube.com/watch?v=7EvFJSNdN44 RF MEMS Slotline TTD Phase Shifter]
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| * [http://www.wat.tv/video/delfmems-rf-mems-switch-technology-26heb_2jujb_.html RF MEMS Switch technology]
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| * [http://www.youtube.com/watch?v=2xiX6_wbb-U RF MEMS Switch]
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| [[Category:Microelectronic and microelectromechanical systems]]
| |