RF MEMS

The MEMS acronym stands for Micro-Electromechanical System and is used to refer to components of which sub-millimeter-sized parts need to move for the components to have electronic functionality. RF MEMS passives, such as capacitors, inductors,
resonators and switches, offer low loss, high Q factor, high linearity and good power handling. They can be fabricated in class 100 clean rooms using 5 to 8 lithography steps, whereas state-of-the-art III-V MMIC fabrication processes require 13 to 25 lithography steps. There are various types of RF MEMS switches, switched capacitors and varactors, and they are classified by actuation method (electrostatic, magnetic, piezoelectric, thermal), by contact mechanism (capacitive, metal-to-metal), or by anchor mechanism (cantilever, fixed-fixed beam) [Rebeiz, G. M.: "RF MEMS, Theory, Design and Technology". John Wiley & Sons, 2003] . The component shown in Fig. 1., is a capacitive fixed-fixed beam RF MEMS switch, developed and patentedby Raytheon in 1993 [Goldsmith, C. L.; e.a.: "Micromechanical Microwave Switching". U.S. Patent 5,619,061, Oct. 31, 1993] [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.] . Capacitive RF MEMS switches, switched capacitors and varactors are in essence micro-machined capacitors with a moving top electrode - i.e. the beam.

Theory

From an electromechanical perspective, the components behave like a mass-spring system, actuated by an electrostatic force. The spring constant is a function of the dimensions of the beam, of the Young's modulus, of the residual stress and of the Poisson ratio of its material. The electrostatic force is a function of the capacitance and the bias voltage. Knowledge of spring constant and mass allows for calculation of the pull-in voltage, which is the bias voltage necessary to pull-in the beam, and of the switching time.

From an RF perspective, the components behave like a series RLC circuit with negligible resistance and inductance. Theup- and down-state capacitance are in the order of 50 fF and 1.2 pF, which are functional values for millimeter-wave circuit design targeted toward the 35 and 94 GHz propagation windows. 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.

Applications

Applications for RF MEMS technology in a radar sensor include T/R modules and passive subarrays. Within a T/R module, RF MEMS based tunable matching networks [K. J. Herrick, G. Jerinic, R. P. Molfino, S. M. Lardizabal, and B. Pillans: "S-Ku Band Intelligent Amplifier Microsystem," Proceedings of the SPIE, vol. 6232, May 2006] [T. Vaha-Heikkila, K. V. Caekenberghe, J. Varis, J. Tuovinen, and G. M. Rebeiz: "RF MEMS Impedance Tuners for 6-24 GHz Applications," International Journal of RF and Microwave Computer-Aided Engineering, vol. 17, no. 3, pp. 265-278, May 2007] and true time delay (TTD) phase shifters can be used to load-pull the power amplifier and time delay the RF signal. To date, RF MEMS based duplexers - i.e. single pole double throw (SPDT) switches, are too slow to be used in T/R modules of high-resolution pulsed radar sensors, despite their low-loss, high linearity and good power handling.

Passive subarrays, in which each antenna is driven by a low-loss passive TTD phase shifter, are an option for lowering the cost of an active electronically scanned array (AESA) when transmit power and noise figure requirements are less stringent. A variety of technologies, such as III-V MMIC, silicon germanium RFIC technogoly and microwave photonics technology, has been used to fabricate TTD phase shifters and demonstrate ESAs based on TTD beamformers, as shown in Fig. 2. Of these technologies, RF MEMS allows for the realization of the lowest loss TTD phase shifters (257°/dB at 50 GHz) with the best power handling (500 mW) currently available [N. S. Barker and G. M. Rebeiz: "Optimization of Distributed MEMS Transmission-Line Phase Shifters - U-Band and W-Band Design," IEEE Trans. Microwave Theory Tech.,vol. 48, no. 11, pp. 1957-1966, Nov. 2000] [B. Pillans, S. Eshelman, A. Malczewski, J. Ehmke, and C. Goldsmith: "Ka-Band RFMEMS Phase Shifters," IEEE Microwave Wireless Compon. Lett., vol. 9, no. 12, pp. 520-522, Dec. 1999] [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. Microwave Theory Tech., vol. 54, no. 1, pp. 383-392, Jan. 2006.] [B. Lakshminarayanan and T. M. Weller: "Design and Modeling of 4-Bit Slow-Wave MEMS Phase Shifters," IEEE Trans. Microwave Theory Tech., vol. 54, no. 1, pp. 120-127, Jan. 2006] [B. Lakshminarayanan and T. M. Weller: "Optimization and Implementation of Impedance-Matched True-Time-Delay Phase Shifters on Quartz Substrate," IEEE Trans. Microwave Theory Tech., vol. 55, no. 2, pp.335-342, Feb. 2007] . Loss in MMICs and RFICs is due to conductor and substrate loss, while loss in photonic TTD beamformers is due to up and down conversion from RF to the optical domain and vice versa, and it prevents both technologies from being implemented in a passive radar sensor.

The main advantage of passive subarrays based on RF MEMS TTD phase shifters is the high effective isotropically radiated power(EIRP). A high EIRP, or equivalently a high power-aperture product, is a prerequisite for long-range detection.

$\{mathrm\{EIRP\; =\; G\_T\; P\_T$

where G_T is the transmit gain and P_T is the transmit power. Due to the low loss and the good power handling of RF MEMS TTD phase shifters, the EIRP and G_r/T of passive subarrays based on RF MEMS technology is unrivalled by contemporary competing passive technologies and give sensors a decisive edge in detection and engagement of targets at a fraction of the cost of the AESA solution. G_r/T is the quotient of the receive gain and the antenna noise temperature. The statement is illustrated with examples in Fig. 3: Assume a one-by-eight passive subarray [K. Van Caekenberghe, et all: "Ka-BandRF MEMS TTD Passive Electronically Scanned Array," in IEEE AP-S Dig., Jul. 2006.] is used for transmit as well as receive, what is the maximum range for which targets can be detected with 10 dB of receiver SNR? Assume following characteristics: G₀ = 10 dBi, BW = 2 GHz, P_T = 4W. The low loss (6.75 ps/dB) and good power handling (500 mW) of the RF MEMS TTD phase shifters allow for high EIRP of 40 W and G_r/T of 0.036 1/K. Values are calculated using the radar range equation

$\{mathrm\{R\; =\; sqrt\; [4]\; \{frac\{displaystyle\; \{mathrm\{lambda^2\; ,\; EIRP\; ,\; G\_r/T\; ,\; sigma\}$mathrm{displaystyle 64 , pi^3 , k_B , BW , SNR

and are tabulated in Table 1 for 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 contemporary non-evasive fighter jet (σ = 400 m²). A Ka-band hybrid ESAs capable of detecting a car 100 m in front and engaging a fighter jet at 10 km can be realized using 2.5 and 422 passive subarrays (and T/R modules), respectively. The EIRP and G_r/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 to optimize the EIRP or the EIRP x G_r/T product, as shown in Fig. 4 and Fig. 5.

Passive subarrays based on RF MEMS TTD phase shifter technology also outperform switched beam forming networks (BFNs) based onsingle-pole N-throw (SPNT) switches and lenses, such as the Archer, Luneberg and Rotman lens in terms of EIRP. Note that themaximum transmit power of a switched BFN is limited by the power handling of the SPNT switch [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 ofPhased Array Antenna Systems with RF MEMS for Automotive Radar Applications," IEEE Trans. Microwave Theory Tech., vol. 53, no. 6, pp. 1968-1975, Jun. 2005] , as shown in Fig. 2.

The prior art of passive sensors based on RF MEMS phase shifters and TTD phase shifters, as shown in Fig. 2., 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 excited in unison and based on metal contact switches developed by HRL Laboratories, has been demonstrated by Raytheon in 2002. The ESA scans 90° in the H-plane, but the reflect-type phase shifter makes the design dispersive and narrow-band [J. J. Lee, C. Quan, R. Allison, A. Reinehr, B. Pierce, R. Y. Yoo, and J. Schaffner: "Array Antennas Using Low-Loss MEMS Phase Shifters," in IEEE AP-S Dig., Jul. 2002] . A 2-D lens consisting of parallel-plate waveguides and featuring 250,000 RF MEMS switches was recently demonstrated by Radant MEMS. The lens is illuminated with a TEM wave and TTD phase shifting is achieved by periodically loading the parallel-plate waveguides with tunable capacitive diaphragms consisting of liquid crystal polymer (LCP) hybrids with hermetically packaged RF MEMS switches wire bonded to them. The Radant RF MEMS DC contact switch has demonstrated a lifetime of more than 250,000 billion cycles [J. Maciel, J. Slocum, J. Smith, and J. Turtle: "MEMS Electronically Steerable Antennas for Fire Control Radars," in IEEE Radar Conf. Dig., Apr. 2007] . Both radar sensors work at X-band and are based on metal-to-metal contact RF MEMS switches. Other ventures currently working on RF MEMS based ESAs include MEMTronics, Thin-Kom (CTS-based) and X-Com Wireless.

References

External links

* [http://www.youtube.com/watch?v=2xiX6_wbb-U Video of RF MEMS Switch]
* [http://www.youtube.com/watch?v=dsmedmwtgfw Video of RF MEMS CPW TTD Phase Shifter]
* [http://www.amicom.info/ European Network of Excellence (NoE) AMICOM]
* [http://www.memtronics.com/ MEMTronics]
* [http://www.research.philips.com/technologies/light_dev_microsys/mems/index.html Philips Research]
* [http://www.radantmems.com/ Radant MEMS]
* [http://www.thin-kom.com/ Thin-Kom]
* [http://www.wtc-consult.de/arrro.html RF MEMS Roadmap]
* [http://www.delfmems.com/ DelfMEMS]
* [http://www.teravicta.com/ RF MEMS Switches: TeraVicta Technologies]
* [http://www.cmac.com/products-and-applications/LTCC.php C-MAC MicroTechnology LTCC packaging for RF MEMs]