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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 decade and it has emerged as a potential technology for wireless, mobile and satellite communication and defence applications. RF MEMS provides an opportunity to revolutionize the wireless communication. This paper describes the Performance of low loss FixedFixed RF MEMS capacitive switch . The RF MEMS capacitive Fixed-Fixed switch exhibit lower losses, better reliability, and good performance at higher frequencies. RF MEMS switches can be classified based on their actuation mechanisms into categories such as electrostatic, electromagnetic and thermal. Most of the RF-MEMS switches reported to date have used electrostatic actuation , which normally requires high actuation voltages. In this paper a fixed-fixed RF MEMS capacitive switch is designed to achieve low actuation voltage and to analyse their performance parameters. Keywords: Capacitive, electrostatic actuation, pullin voltage, RF MEMS switch.

I.

INTRODUCTION

Wireless communication has made an explosive growth of emerging consumer markets, as well as in military applications of RF, microwave, and millimetre-wave circuits and systems. These include wireless personal communication systems, wireless local area networks, satellite communications, automotive electronics, etc. In these systems, the RF switch is one of the essential components to handle RF signals [1,2]. RF MEMS is an emerging technology that promises the potential of revolutionizing RF and microwave system implementation for the next generation of telecommunication applications [3]. Its low power, better RF performance, large tuning range, and integration capability are the key characteristics enabling system implementation with potential improvements in size, cost, and increased functionality. The term RF MEMS refers to the design and fabrication of MEMS for RF integratedcircuits. It should not be interpreted as the traditional MEMS devices operating at RF frequencies [4].MEMS devices in RF MEMS are used for actuation or

adjustment of a separate RF device or component, such as variable capacitors, switches, and filters.There has been great research effort on Radio Frequency Micro-Electro- Mechanical Systems (RF MEMS) switches because they have many advantages over p-i-n diode or field effect transistor (FET) switches [5]. RF MEMS switches show attractive electrical performance characteristics that are critically needed in the next generation RF switches with high isolation, very low insertion loss, wide bandwidth operation and excellent linearity [6, 7 and 8]. This makes it ideal to enable a plethora of wireless appliances operating in the home/ground, mobile, and space spheres such as handsets, base stations, and satellites. The main existing challenge in use of RF MEMS switches is high value of actuation voltage. As the high actuation voltage requires high voltage drive circuits which degrades life time and induces malfunction by charge trapping problem. So, in this paper we have focused in the reduction of actuation voltage by studying the various parameters which effect the actuation voltage. In this paper proposed RF MEMS capacitive switch based on fixed-fixed beam structure which shows an improvement in characteristics at higher frequencies. Here, we propose a switch which uses fixed-fixed shape beam and its parameters are analyzed. It has wide potential with multiband support for different applications like K and Ka band which is to be sight for different satellite communication. It is also supposed to support next generation mobile terminal applications.

II.

RF MEMS SWITCH

Switch is the basic element that connect or disconnect the electrical connection. There are two basic switches used in RF to millimeter-wave circuit design: the shunt switch and the series switch. The series MEMS switch is excellent for RF-40 GHz applications with a typical isolation of 50 dB at 1 GHz, and 30 dB at 10 GHz [9]. The shunt design is excellent at 10-100 GHz applications, with a typical isolation of 17 dB at 10 GHz and 35-40 dB at 30-40 GHz for a capacitance of 4 pF . From a mechanical point of view, MEMS switches can be a thin metal cantilever, air bridge, or diaphragm, from RF circuit configuration point of view, it can be series connected or parallel connected with an RF

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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 transmission line [4,10]. The contact condition can be capacitive (metal–insulator–metal) or resistive (metal-to-metal), polar ceramics such as (Ba,Sr)TiO3 - BST and designed to open the line or shunt it to ground upon actuation of the MEMS switch. Each type of switch has certain advantages in performance or manufacturability. Main mechanical operations of RF MEMS switches depends mainly on spring constant of material used i.e. k. We always require to have less k i.e. less stiff material because the deflection of beam depends on spring constant k and we need more deflection with given force for an given RF MEMS Switch. In this paper we have used Fixed-Fixed type beam shape with holes to lower the k value [10]. Calculation for spring constant for Fixed-Fixed shaped beam is given below. Fixed-Fixed flexure đ?‘Ą 3 đ?‘˜ = 4đ??¸đ?‘¤ (1) đ?‘™ Where k is a spring constant, E is a Young’s modulus, l is the length of the beam, t is the thickness of the beam. In many MEMS switches, small diameter holes (3–8 mm) are defined in the beam to reduce the squeeze film damping and increase the switching speed of the MEMS switch. The hole area can be up to 60% of the total surface area of the MEMS structure. The holes also result in a lower mass of the beam, which in turn yields a higher mechanical resonant frequency[10].

membrane. At (2/3g0), the increase in the electrostatic force is greater than the increase in the restoring force, resulting in the beam position becoming unstable and collapse of the beam to the down-state position. The pull-down (also called pull-in) voltage is found to be đ?‘‰đ?‘? đ?‘‰ = đ?‘‰

2đ?‘”đ?‘œ = 3

=

8đ?‘˜ đ?‘”đ?‘œ 3 27đ?œ–0 đ?‘Š. đ?‘¤ 8đ?‘˜ đ?‘”đ?‘œ 3 27đ?œ–0 đ??´

(2)

where V is the voltage applied between the beam and electrode, A= Ww is the electrode area, g0 is the zero-bias bridge height, ∈0 is the permittivity of air. As shown in Eq. (2), the pull down voltage depends on the spring constant of beam structure, and, beam gap g0 and electrode area A [12]. There are two approaches to reduce the actuation voltage: A first approach in lowering the actuation voltage is to increase the actuation area. Increasing the area is not a practical solution because the compactness is the prevailing issue and adoption of MEMS technology is to achieve the miniaturization. The second alternative, which offers the maximum design flexibility for a low-to-moderate actuation voltage, is to lower the switch spring constant, hence, designing a compliant switch. To reduce the actuation voltage, the key is beam structure of low spring constant k.

III.

RESULTS

A.

Figure1: Fixed-Fixed beam based RF-MEMS switch 1)

Electrostatic Actuation: When the voltage is applied between a fixed-fixed beam and the pull down electrode, an electrostatic force is induced on the beam. The electrostatic force applied to the beam is found by considering the power delivered to a time-dependent capacitance. This electrostatic force is approximated as being distributed evenly across the beam section above the electrode. As this electrostatic force is applied to the beam, the beam membrane starts to deflect downward, decreasing the gap g and increasing the electrostatic pressure on the

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, pulldown voltages, Young’s modulus, and other vector values could are obtained. Figure 3 shows capacitance matrix which shows self-capacitance terms (located on the diagonal of the capacitive matrix) should be positive and mutual-capacitance terms (off-diagonal elements) should be negative according to the ConventorWare’s convention. A Capacitance Matrix dialog that deviates from this rule is an indication that the mesh needs to be refined. Figure 4 shows pull-in voltage ranges for fixed-fixed beam based RF MEMS switch. The graph in figure 5 shows charge produced on a beam with different values of voltages of a capacitive MEMS switch.

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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 applied to many other devices, including tunable filters, other antenna geometries, signal splitters or military applications.

REFERENCES Figure 2: Voltage and Charge values for capacitive MEMS switch

Figure 3: Capacitance matrix for capacitive MEMS switch

Figure 4: Pull-in voltage for capacitive MEMS switch

1. Il-Joo Cho, Taeksang Song, Sang-Hyun Baek, and Euisik Yoon, “Low-Voltage and Low-Power RF MEMS Series and Shunt Switches Actuated 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. 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. 4. Vijay K. Varadan, K. J. Vinoy, K. A. Jose, “ RF 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.

Figure 5: Voltage versus capacitive MEMS switch.

IV.

Charge graph for

CONCLUSIONS

In this paper, fixed-fixed based RF MEMS switch is designed and simulated for a multiplefrequency antenna We have designed a fixed-fixed beam switch with pull-in voltage ranges from 3.18V to 3.5V with beam gap 1µm having a good RF characteristics with a lower actuation voltage. As by using MEMS switches, the losses are kept to a minimum which is very important factor to obtain high reconfigurability .This technology can be

Goldsmith C, Lin T H, Powers B. Micromechanical Membrane Switches for Microwave Applications [C]. In: IEEE MTT-S Int. Microwave Symp. Dig, 1995, pp. 91-96.

8. C. L. Dai, H. J. Peng, M. C. Liu, C. C. Wu and L.J. Yang. Design and Fabrication of RF MEMS Switch by the CMOS Process [J]. Tamkang Journal of Science and Engineering, Vol. 8, No 3 (2005), pp. 197- 202. 9. Jeremy B. Muldavin, Gabriel M. Rebeiz, “HighIsolation Inductively-Tuned X-Band MEMS Shunt Switches” , 2000 IEEE MTT-S Digest.

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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 10.

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. Gabriel M. Rebeiz, “RF-MEMS Switches: Status Of The Technology”, IEEE, 2003.

14.

15.

16.

Richard Chan, Robert Lesnick, David Becher, “Low- Actuation Voltage RF MEMS Shunt Switch With Cold Switching Lifetime Of Seven Billion Cycles”, IEEE, 2003. 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).

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