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.
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 . 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 .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 . 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.
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 . 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 (metalâ€“insulatorâ€“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 . 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 Youngâ€™s modulus, l is the length of the beam, t is the thickness of the beam. In many MEMS switches, small diameter holes (3â€“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.
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??´
where V is the voltage applied between the beam and electrode, A= Ww is the electrode area, g0 is the zero-bias bridge height, âˆˆ0 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 . 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.
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, Youngâ€™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 ConventorWareâ€™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
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Figure 5: Voltage versus capacitive MEMS switch.
Charge graph for
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