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Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.391-394 Analysis of RF MEMS Capacitive Switch based on a Fixed-Fixed Beam Structure Divya Verma*, Ajay Kaushik** *,**MMEC, Maharishi Markandeshwar University, Mullana, Haryana(India),ABSTRACT RF MEMS has evolved over the past adjustment of a separate RF device or component,decade and it has emerged as a potential such as variable capacitors, switches, andtechnology for wireless, mobile and satellite filters.There has been great research effort on Radiocommunication and defence applications. RF Frequency Micro-Electro- Mechanical Systems (RFMEMS provides an opportunity to revolutionize MEMS) switches because they have manythe wireless communication. This paper advantages over p-i-n diode or field effect transistordescribes the Performance of low loss Fixed- (FET) switches . RF MEMS switches showFixed RF MEMS capacitive switch . The RF attractive electrical performance characteristics thatMEMS capacitive Fixed-Fixed switch exhibit are critically needed in the next generation RFlower losses, better reliability, and good switches with high isolation, very low insertion loss,performance at higher frequencies. RF MEMS wide bandwidth operation and excellent linearity [6,switches can be classified based on their 7 and 8]. This makes it ideal to enable a plethora ofactuation mechanisms into categories such as wireless appliances operating in the home/ground,electrostatic, electromagnetic and thermal. Most mobile, and space spheres such as handsets, baseof the RF-MEMS switches reported to date have stations, and satellites.used electrostatic actuation , which normallyrequires high actuation voltages. In this paper a The main existing challenge in use of RF MEMSfixed-fixed RF MEMS capacitive switch is switches is high value of actuation voltage. As thedesigned to achieve low actuation voltage and to high actuation voltage requires high voltage driveanalyse their performance parameters. circuits which degrades life time and induces malfunction by charge trapping problem. So, in thisKeywords: Capacitive, electrostatic actuation, pull- paper we have focused in the reduction of actuationin voltage, RF MEMS switch. voltage by studying the various parameters which effect the actuation voltage. In this paper proposed I. INTRODUCTION RF MEMS capacitive switch based on fixed-fixed Wireless communication has made an beam structure which shows an improvement inexplosive growth of emerging consumer markets, as characteristics at higher frequencies. Here, wewell as in military applications of RF, microwave, propose a switch which uses fixed-fixed shape beamand millimetre-wave circuits and systems. These and its parameters are analyzed. It has wideinclude wireless personal communication systems, potential with multiband support for differentwireless local area networks, satellite applications like K and Ka band which is to be sightcommunications, automotive electronics, etc. In for different satellite communication. It is alsothese systems, the RF switch is one of the essential supposed to support next generation mobile terminalcomponents to handle RF signals [1,2]. RF MEMS applications.is an emerging technology that promises thepotential of revolutionizing RF and microwave II. RF MEMS SWITCHsystem implementation for the next generation of Switch is the basic element that connecttelecommunication applications . Its low power, or disconnect the electrical connection. There arebetter RF performance, large tuning range, and two basic switches used in RF to millimeter-waveintegration capability are the key characteristics circuit design: the shunt switch and the seriesenabling system implementation with potential switch. The series MEMS switch is excellent forimprovements in size, cost, and increased RF-40 GHz applications with a typical isolation offunctionality. 50 dB at 1 GHz, and 30 dB at 10 GHz . The shunt design is excellent at 10-100 GHz applications, withThe term RF MEMS refers to the design and a typical isolation of 17 dB at 10 GHz and 35-40 dBfabrication of MEMS for RF integratedcircuits. It at 30-40 GHz for a capacitance of 4 pF . From ashould not be interpreted as the traditional MEMS mechanical point of view, MEMS switches can be adevices operating at RF frequencies .MEMS thin metal cantilever, air bridge, or diaphragm, fromdevices in RF MEMS are used for actuation or RF circuit configuration point of view, it can be series connected or parallel connected with an RF 391 | P a g e
Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.391-394transmission line [4,10]. The contact condition can membrane. At (2/3g0), the increase in thebe capacitive (metal–insulator–metal) or resistive electrostatic force is greater than the increase in the(metal-to-metal), polar ceramics such as restoring force, resulting in the beam position(Ba,Sr)TiO3 - BST and designed to open the line or becoming unstable and collapse of the beam to theshunt it to ground upon actuation of the MEMS down-state position. The pull-down (also calledswitch. Each type of switch has certain advantages pull-in) voltage is found to bein performance or manufacturability. Main 8𝑘mechanical operations of RF MEMS switches 2𝑔 𝑜 𝑔𝑜 3depends mainly on spring constant of material used 𝑉𝑝 𝑉 = 𝑉 = 27𝜖0 𝑊. 𝑤 3i.e. k. We always require to have less k i.e. less stiffmaterial because the deflection of beam depends on 8𝑘spring constant k and we need more deflection with 𝑔𝑜 3given force for an given RF MEMS Switch. In this = 27𝜖0 𝐴 (2)paper we have used Fixed-Fixed type beam shapewith holes to lower the k value . where V is the voltage applied between the beamCalculation for spring constant for Fixed-Fixed and electrode, A= Ww is the electrode area, g0 is theshaped beam is given below. zero-bias bridge height, ∈0 is the permittivity of air.Fixed-Fixed flexure As shown in Eq. (2), the pull down voltage depends 𝑡 3 on the spring constant of beam structure, and, beam 𝑘 = 4𝐸𝑤 (1) 𝑙 gap g0 and electrode area A . There are twoWhere k is a spring constant, E is a Young’s approaches to reduce the actuation voltage: A firstmodulus, l is the length of the beam, t is the approach in lowering the actuation voltage is tothickness of the beam. In many MEMS switches, increase the actuation area. Increasing the area is notsmall diameter holes (3–8 mm) are defined in the a practical solution because the compactness is thebeam to reduce the squeeze film damping and prevailing issue and adoption of MEMS technologyincrease the switching speed of the MEMS switch. is to achieve the miniaturization. The secondThe hole area can be up to 60% of the total surface alternative, which offers the maximum designarea of the MEMS structure. The holes also result in flexibility for a low-to-moderate actuation voltage,a lower mass of the beam, which in turn yields a is to lower the switch spring constant, hence,higher mechanical resonant frequency. designing a compliant switch. To reduce the actuation voltage, the key is beam structure of low spring constant k. III. RESULTS A. RF MEMS Design and Analysis Figure 2 shows the voltage and charge values on conductor calculated and measured for fixed-fixed based RF MEMS switch. Since Coventorware software could synthesize the multiply factors, such as electrostatic-forces, pull- down voltages, Young’s modulus, and otherFigure1: Fixed-Fixed beam based RF-MEMS switch vector values could are obtained. Figure 3 shows capacitance matrix which shows self-capacitance1) Electrostatic Actuation: terms (located on the diagonal of the capacitive When the voltage is applied between a matrix) should be positive and mutual-capacitancefixed-fixed beam and the pull down electrode, an terms (off-diagonal elements) should be negativeelectrostatic force is induced on the beam. The according to the ConventorWare’s convention. Aelectrostatic force applied to the beam is found by Capacitance Matrix dialog that deviates from thisconsidering the power delivered to a time-dependent rule is an indication that the mesh needs to becapacitance. This electrostatic force is approximated refined. Figure 4 shows pull-in voltage ranges foras being distributed evenly across the beam section fixed-fixed beam based RF MEMS switch. Theabove the electrode. As this electrostatic force is graph in figure 5 shows charge produced on aapplied to the beam, the beam membrane starts to beam with different values of voltages of adeflect downward, decreasing the gap g and capacitive MEMS switch.increasing the electrostatic pressure on the 392 | P a g e
Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.391-394 applied to many other devices, including tunable filters, other antenna geometries, signal splitters or military applications. REFERENCES 1. Il-Joo Cho, Taeksang Song, Sang-Hyun Baek, and Euisik Yoon, “Low-Voltage and Low-PowerFigure 2: Voltage and Charge values for capacitive RF MEMS Series and Shunt Switches ActuatedMEMS switch by Combination of Electromagnetic and Electrostatic Forces”, IEEE Transactions On Microwave Theory And Techniques, Vol. 53, No. 7, July 2005. 2. H. A. C. Tilmans, W. D. Raedt, and E. Beyne, “MEMS for wireless communications,” J. Micromech. Microeng., vol. 13, pp. 139–163, Jun. 2003.Figure 3: Capacitance matrix for capacitive MEMSswitch 3. Hung-Pin Chang, Jiangyuan Qian, Bedri A. Cetiner, F. De Flaviis, Mark Bachman, and G. P. Li, “Low Cost RF MEMS Switches Fabricated on Microwave Laminate Printed Circuit Boards.” Department of Electrical and Computer Engineering, University of California at Irvine, USA. Figure 4: Pull-in voltage for capacitive MEMS 4. Vijay K. Varadan, K. J. Vinoy, K. A. Jose, “ RF switch MEMS and Their Applications” John Wiley & Sons, Inc., 2003. 5. Mingxin Song, Jinghua Yin, Xunjun He, Yue Wang, “Design and Analysis of a Novel Low Actuation Voltage of Capacitive RF MEMS Switches”, Proceedings of the 3rd IEEE Int. Conf. on Nano/Micro Engineered and Molecular Systems January 6-9, 2008, Sanya, China. 6. Reines I. C., Goldsmith C. L., Nordquist C. D., Dyck C. W., Kraus G. M., Plut T. A., Finnegan P. S., Austin F. and Sullivan C. T. A low loss RF MEMS Ku-band integrated switched filter bank [J]. IEEE Microwave & Wireless Components Letters, vol.15, No.2 (2005), pp.74-76. 7. Goldsmith C, Lin T H, Powers B. Micromechanical Membrane Switches for Figure 5: Voltage versus Charge graph for Microwave Applications [C]. In: IEEE MTT-S capacitive MEMS switch. Int. Microwave Symp. Dig, 1995, pp. 91-96. IV. CONCLUSIONS 8. C. L. Dai, H. J. Peng, M. C. Liu, C. C. Wu and In this paper, fixed-fixed based RF MEMS L.J. Yang. Design and Fabrication of RF MEMS switch is designed and simulated for a multiple- Switch by the CMOS Process [J]. Tamkang frequency antenna We have designed a fixed-fixed Journal of Science and Engineering, Vol. 8, No 3 beam switch with pull-in voltage ranges from (2005), pp. 197- 202. 3.18V to 3.5V with beam gap 1µm having a good RF characteristics with a lower actuation voltage. 9. Jeremy B. Muldavin, Gabriel M. Rebeiz, “High- As by using MEMS switches, the losses are kept to Isolation Inductively-Tuned X-Band MEMS a minimum which is very important factor to obtain Shunt Switches” , 2000 IEEE MTT-S Digest. high reconfigurability .This technology can be 393 | P a g e
Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.391-39410. G. M. Rebeiz, RF MEMS Theory, design, and technology, New Jersey: John Wiley & Sons, Inc., 2003.11. S. P. Pacheco, D. Peroulis, L.P.B. Katehi, “MEMS single-pole double-throw (SPDT) X and K-band switching circuits,” Microwave Theory Tech -S Int. Microwave Symp., vol. 1, pp. 165–168, 2001.12. C. Goldsmith, J. Randall, S. Eshelman, T.H. Lin, D. Denniston, S. Clhen, B. Norvell, “Characteristics Of Micromachined Switches At Microwave Frequencies.” , 1996 IEEE MTT-S Digest.13. Dimitrios Peroulis, Sergio P. Pacheco, Kamal Sarabandi, Linda P.B Katehi, “ Electromechanical Considerations In Developing Low- Voltage RF MEMS Switches”, IEEE, 2003.14. Gabriel M. Rebeiz, “RF-MEMS Switches: Status Of The Technology”, IEEE, 2003.15. Richard Chan, Robert Lesnick, David Becher, “Low- Actuation Voltage RF MEMS Shunt Switch With Cold Switching Lifetime Of Seven Billion Cycles”, IEEE, 2003.16. F.M Guo, Z.Q. Zhu, Y.F. Long, G.Q. Yang, “Study On Low Voltage Actuated RF MEMS Capacitive Switches”, www.sciencedirect.com, sensors and actuators A 108(2003). 394 | P a g e