Design & Analysis of RF MEMS capacitive switches manufacturing process on the coplanar waveguide

Transcription

1 International Research Journal of Applied and Basic Sciences 2013 Available online at ISSN X / Vol, 4 (7): Science Explorer Publications Design & Analysis of RF MEMS capacitive switches manufacturing process on the coplanar waveguide Gholamhosein moloudian *1, Asghar ebrahimi 2 1. Sama Technical and vocational traning collage,islamic Azad Universiy, Kazeroun Branch, Kazeroun, Iran 2. Malek Ashtar University Of Technology *Corresponding Author gholamhosein_movludian@yahoo.com ABSTRACT: In this paper, we design and simulate RF MEMS switch in suspension bridge mode. We chose the transmission line of type CPW. First, by using TX LINE2003 software, we have designed and calculated the dimensions of transmission line for having characteristic impedance 50. We have chosen the substrate made of silicon, conductor of transmission line of made of cooper, dielectric layer made of SiO 2, and upper electrode (bridge) made of aluminum. We simulate the manufacturing process of the switch in Intellisuite software. We have chosen 3 forms for upper electrode (bridge) format and we performed thermo-electromechanical analysis for each case. In each case, we measure the pull-down voltage and compare them with each other. According to results, it is shown that form a,is more relevant for choosing the bridge. Key words: Electrostatic, substrate, micro-electromechanical switch, waveguide, excitation voltage INTRODUCTION Micro-electromechanical switches, or in brief RF MEMS switches, are widely used in telecommunications and satellite communications. Switches which are manufactured by utilizing MEMS technologies and with cost much less than other switches, are the best alternative for conventional switches such as field-effect transistors made of Gallium Arsenide, as well as p-i-n diodes for utilization in telecommunication systems in rather high radio frequencies. Some of their desirable characteristics are low loss, isolation, relatively good insulation when the switch is off, linear characteristic and low power consumption. Micro-electro mechanical capacitive switches are made of different metals such as Nickel, Aluminum and Gold, but the problem with them is the required voltage for performing switching and connecting electrodes, which may be reduced by using the inventions and designs used in this paper. Until about three decades ago, switches which were used in electrical systems were two types: mechanical switches and semiconductive switches, but from about 1980 on, micro-electro mechanical switches (MEMS) entered the electronics market which had significant advantages than the other two types and some disadvantages. Using MEMS switches in radio frequencies (RF MEMS Switch) began in 1990s. high frequency switches play an important role in systems which are used in microwave field (such as cell phones and wireless networks) and systems which work in millimeter waves field (such as radar, and satellite systems). In radar systems, these switches are used as phase shifters. MEMS switches are made by micro-machine technology and their basis is that with proper excitation of the switch, the mechanical part is excited and is put in two statuses of On and Off, and it may be used for generating a short circuit or open circuit in circuits, in particular, in RF transmission lines. The electrostatic actuation mechanism for the excitation has some merits such as fast response and simple drive electronics. A typical electro-statically actuated microbeam is indeed a straight microcantilever or microbridge having an intitial distance from a rigid substrate, actuated by a transverse distributed electrical force caused by the input voltage applied between the microbeam and substrate. The excitation voltage corresponding to the instability is called the static pull-in voltage. The pull-in instability is one of the important phenomena that should be considered in the design, analysis and simulation of micro electro-mechanical system(m. Rahaeifard et al, 2011). This phenomenon is experimentally investigated by some researcher such as (nathanson et al. and taylor, 1968). Many reaserchers have studied the static pull-in phenomenon. here some of these works are reviewed. (Rong et al., 2004) proposed a model to analytically calculate the pull-in voltage for multilayer clamped-clamped

2 microbridges uing an energy based approach. (Stolken and weans, 1998) performed some micro-bending tests to determine the length scale parameter of pure nickel. (Mojahedi et al., 2010) presented an analytical investigation for the static pull-in microcantilevers and microbridges using the homotopy perturbation method.( Degani and Nemirovaski, 2002) analyzed the pull-in of microcanilever modeled as two-degree of freedom system.( Zhang and Zhao, 2006) studied the static pull-in of microbeams utilizing taylor series expantion of the electrostatic force.( Chaterjee and Pohit, 2009) presented a model for the analysis of static and dynamic pull-in instability of microcantilevers considering the nonlinearities caused by large deformation of the microbeam. RF MEMS Switches MEMS Switches may be categorized in two categories,as the manufacturing combination point of view, which are: switch with anchor beam or Cantilever, and switch with constant-constant beam or suspension bridge.both of these groups may be divided into serial and parallel categories, in terms of method of placement across the transmission line. For excitation of mechanical part of RF MEMS Switches, four mechanisms may be used: electro-static, thermal, magnetic and piezoelectric (Haslina Jaafar et al, 2011). One of the Special Applications of RF MEMS Switches is in military and defense industries. MEMS Switches have components with high power handling, but are advantageous in RF and lower frequencies and have low switching speed. A new technology which has advantages of bothsolid state switch and electromechanical switch, is the MEMS technology (Hyman D.et al, 1999). Parallel RF MEMS Switches on same page waveguide (cpw) are the most applicable type of microelectromechanical switches. The schematic of this type of switch is shown in Fig.(1) which is composed of a very thin metal layer which is placed suspended on the central conductor (Katehi, L. P. B et al, 2002). a. side-view b. c. side-top Figure1. Schematic of RF MEMS Switch Without applying excitation voltage, the layer has a greater distance with central conductor and the signal is transmitted by the line. By applying excitation voltage on two lower and upper electrodes, an electrostatic force is created between the electrodes which cause the upper electrode (bridge) to be drawn downwards and be absorbed by the lower electrode which causes short circuit. In terms of connection type, switches are divided as metal-metal connection and metal-insulator-metal (capacitive). Since we use capacitive switch in this paper, thus we must place a dielectric layer of SiO 2 or si 3 N 4 on the lower electrode to create a capacity when the switch is on. The loss in parallel switches is due to impedance mismatch which may be minimized by accurate calculations. Electrostatic excitation The basis of this excitation is that by applying the voltage between panels and creating opposite charges in both panels, based on the gravity available between panels of an electric field and finally, an electrostatic force is created which causes the panels to be absorbed by each other. By applying the voltage between two panels, we create an electric force which causes the moving panel to be drawn downward and causes short circuit in the

3 transmission path. The voltage in which, the distance between two panels is equal to 0.66 of total distance, is called pull-down voltage or V S. this voltage is one of the most important parameters in switch parameters.! "!#$ )* + %&!'( In which K is the effective spring constant, h up is the initial height of beam when the switch is off, 0 is the Permeability of vacuum, E is theyoung's modulus, A is the area of two electrodes which are placed in front of each other, W is the width of electrode, L is the length of the electrode and t is the thickness of the electrodes. CPW transmission line Same page waveguide is a one-way transmission line with 3 conductors. Same page waveguide includes two earth conductor and one central conductor. The schematic this type of transmission line is shown in Figure 2. This Figure has been designed in 3Dbuilder environment of Intellisute software. Figure 2. Cpw transmission line schematic One of the most important characteristics of transmission line is characteristic impedance. In this paper, our purpose is to design switch dimensions in order to have characteristic impedance of 50 Ohms. For this purpose, by using TX-Line 2003 software, the switch dimensions are measured. Figure 3. TX-Line 2003 software schematic

4 Ohms. As it may be observed with input dimensions, the characteristic impedance of cpw transmission line is 52 Equivalent circuit of cpw transmission line the equivalent circuit of this switch and relations related to impedance and its approximation around resonant frequency is mentioned as following(g.m. Rebeiz, 2003). Figure 4. Equivalent circuit of cpw transmission line -. /, )*+ 0-1, % )5* Here, the switch model is considered to be independent of transmission line. When the switch is in up mode, by using the parallel capacitive formula, we can achieve a relevant approximation for it. When switch is in the down mode, since the distance between two electrodes is equal to dielectric layer, so the Capacitance amount of this case is measured easily. % 67 9: $ 2 % 8 );* The Impedance of the capacitor will reduce with increase of frequency, thus isolation in serial switch will reduce with frequency increase and in parallel switches, and it would increase with frequency increase. Manufacturing process of RF MEMS Switch For simulating manufacturing process of RF MEMS capacitive switch, we use Intellisuite software. By using 3Dbuilder, we design the switch with several different layers. The manufacturing processesare shown below (Stephen A.campbell, 2001).

5 Level0: Si Level1. Sio 2 Level2. Cpw Level3. Sio 2 Level4. Bridge post Figure 5. Fabrication switch The bridge layer is designed as 3 following formats and we call them form a, b, c. thus, since we have considered 3 forms for the bridge layer, 3 types of switch is made.

6 Form a Form b Form c Figure 6. Designed forms of bridge After designing switches, we must perform thermo-electromechanical analysis, which is given in the next chapter. RESULTS OF SWITCH ANALYSIS We perform thermo-electromechanical analysis for each switch in Intellisuite software and in each case, we measure the pull-down voltage. This voltage is one of the most important parameters in RF MEMS switch.most people who work on RF MEMS switches are looking for a way to reduce this voltage. In this paper, we are trying to reduce this voltage, by designing 3 different forms for the bridge layer. Schematics of designed switches are given in Figure7.

7 Schematic form a Schematic form b Schematic form c Figure7. RF MEMS Switches schematic with different forms of bridge layer Form a Form b Form c Figure 8. Switches bridge layer bending by applying electrostatic excitation

8 Figure 9. Diagram of distance of z according to applied voltage for switch, form a) Figure10. Diagram of distance of z according to applied voltage for switch, form b) Figure 11. Diagram of distance of z according to applied voltage for switch, form c)

9 According to Figures, it is observed that pull-own voltage for form a, is lower than other two forms. The amount of this voltage for all types of forms is as follows. For calculating V P, we assume that the voltage in which the total distance reaches 0.66 of total distance, is the V P (Ali Mansoor Pasha et al. and s.k.thakur et al, 2009). Here,since the total distance is 1.5 micrometer, so the V p is obtained in distance 1.5*0.66 or 1 micro meter. That is why in the Figures, a horizontal line has been drawn in front of 1micro meter. Table 1. Voltage value for each form Bridge forms V p Form a 8.2 v Form b 9.2 v Form c 9 v CONCLUSION High frequency switches wildly have applied in defense aerospace and wireless communication, radar systems application. For the purpose at a suitable RF switch for any kind of application at first should be consider required function features such as band width excited voltage and switching velocity. In this paper, first we designed and calculated dimensions of same page waveguide for having characteristic impedance of 50 ohms in Tx-Line software. Then, we simulated the switch in Intellisuite software and performed thermo-electromechanical analysis. The results of analysis show that excitation voltage may be controlled by changing the shape of the bridge and reduced to desirable amount. According to above shapes, we conclude that form a has more suitable performance than two other forms, because it has less Vp. REFERENCES Bochobza-Degani O, Nemirovaski Y Modeling the pull-in parameters electrosatatic actuators with a novel lumped two degrees of freedom pull-in model, Sens. Actuators A (2002) Chaterjee S, Pohit G.2009.A large deflection model for the pull-in analysis of electrostatically actuatedmicrocantilever beams, J. Sound Vib. 322 (2009) Haslina J, Fong LN, Nurul A, Yunus Md '' Design and Simulation of High Performance RF MEMS Series Switch'', RSM2011 Proc., Kota Kinabalu, Malaysia, /11/$ IEEE Hyman D.et al '' GaAs-compatible surface-micromachined RF MEMS switch'', Electron Lett Katehi LPB, Harvey JF, Brown E MEMS and micromachined circuits for High- Frequency applications '' IEEE MTT,Vol. 50, No. 3, PP Mansoor Pasha A, Asghar Saqib M '' Design Optimization for Low Voltage DC Contact RF MEMS Shunt Switch '', IEEE, Mojahedi M, Moghimi Zand M, Ahmadian MT Static pull-in analysis of electrostatically actuated microbeams using homotopy perturbation method, Appl. Math. Model Nathanson HC, Newell WE, Wickstrom RA, Davis JR The resonant gate transistor, IEEE Trans. Electron Dev Rahaeifard M, Kahrobaiyan MH, Asghari M, Ahmadian MT Static pull-in analysis of microcantilevers based on the modified couple stress theory, jo u rn al hom epage: Sensors and Actuators A: Physical, SNA-7486; No. of Pages 5 Rebeiz GM RF MEMS Theory, Design, and Technology, John Wiley & Sons, Rong H, Huang QA, Nie M, Li W.2004 An analytical model for pull-in voltage of clamped clamped multilayer beams, Sens. Actuators A Stephen A, campbell '' the science and engineering of microelectronic fabrication '', oxford. new York, Taylor GI The coalescence of closely spaced drops when they are at different electric potential, Proc. R. Soc. A Thakur SK, SumithraDevi KA, Ranjitha I '' Performance of low loss RF MEMS Fixed Fixed capacitive switch characterization'', IEEE, Zhang Y, Zhao Y Numerical and analytical study on the pull-in instability of micro-structure under electrostatic loading, Sens. Actuators A