Available online at www.sciencedirect.com Procedia Engineering 29 (2012) 1292 1297 2012 International Workshop on Information and Electronics Engineering (IWIEE) Low Actuation Wideband RF MEMS Shunt Capacitive Switch Yasser Mafinejad a*, Abbas Z. Kouzani a*, Khalil Mafinezhad b, Akif Kaynak a a School of Engineering, Deakin University, Waurn Ponds, Victoria 3216, Australia b Department of Electrical Engineering, Ferdowsi University, Mashhad, Iran Abstract A wide bandwidth low actuation voltage RF MEMS capacitive shunt switch is designed and simulated. The proposed switch is designed for a low actuation voltage of 12 volts applicable to wireless systems. High frequency characteristics of the RF MEMS switches can be specified by coupling capacitors in up-state position C u. This capacitor is in trade-off with actuation voltage. In the proposed switch, the capacitor is compensated by incorporating two short high impedance transmission lines as a T matching circuit. Simulating the switch demonstrates great improvement in RF characteristics of the switch particularly in reflection coefficient in up-state position. 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Harbin University of Science and Technology Open access under CC BY-NC-ND license. Keywords: High impedance short transmission line; low actuation voltage; wideband; RF MEMS; shunt switch. 1. Introduction Micro electro mechanical systems (MEMS) have enabled a new generation of electronic devices, particularly switches. The switches can be made very small, and designed to deliver good bandwidth and linearity. MEMS switches can be employed in radio frequency RF circuits, and their performances could be made better than those of other standard switches such as FET, and PIN diodes. This is due to their good linearity, low noise, low power consumption, high electrical isolation, ultra wide frequency band. * * Corresponding author. Tel.: +86-0311-8799-4277. E-mail address: ymafinej@deakin.edu.au 1877-7058 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.01.129 Open access under CC BY-NC-ND license.
Yasser Mafinejad et al. / Procedia Engineering 29 (2012) 1292 1297 1293 RF MEMS switches are however faced with such issues as reliability and electrostatic discharge (ESD) stresses [1, 2], RF power handling [3, 4], and packaging. A key challenge is to develop high performance MEMS switches by lowering the actuation voltage for use in wireless communication applications. Different methods have been employed to tackle this issue including lowering the constant of the spring [5, 6], incorporating piezoelectric actuation [7-10], using tantalum nitride for transmission line and tantalum peroxide as dielectric layer [11], or other designs [12, 13]. Lowering the height of the gap of the coupling capacitor shunt switch deteriorates the RF performance of the switch in up-state position due the increase of up-state capacitor. In addition, it has negative effects on reliability and life cycle. In this work, this capacitor is eliminated by using a distributed inductance of two short high impedance transmission lines (HITLs). Thus, a wide bandwidth low actuation voltage RF MEMS capacitive shunt switch is proposed, designed, and simulated. It is expected that using the short HITLs will lead to lowering the cost of fabrication of the low actuation voltage MEMS switches. By optimizing the value of the parameters of the short HITLs, the desired switch can be achieved. 2. Proposed MEMS Switch The proposed switch has 6 layers (see Fig. 1(a)): a Si substrate (300 μm), an oxide layer (0.15 μm), CPW (1.2 μm = 2 times of the skin depth), a SiN layer (0.15 μm), and anchor and membrane layers (2 μm and 1 μm). The switch has two positions up-state and down-state. While the switch is in its up-state position, the signal propagates from the input to the output. If the actuation voltage is applied, switch is in its down-state position, and the signal is reflected because of the short circuit. The RF characteristics of a capacitive shunt switch depend on the capacitor of the switch in its up-state position. For lowering the actuation voltage, the gap-height should be decreased which leads to an increase in the up-state capacitor. This deteriorates the RF characteristics of the switch. The up-state position of the switch with two short HITLs could be modeled by the electrical circuit shown in Fig. 1(b). In this circuit, the switch is modeled by the C, L and R components. The C component is the more dominating component at high frequency in this model causing a mismatch at the input and output of the switch. We therefore match the circuit by using two additional short HITLs incorporating two inductors (L TL ) at the input and output of the switch. This structure forms a T matching circuit as shown in Fig. 1(b). A graphical illustration of the switch is presented in Fig. 1(c-d). The parameters of the switch are given in Table 1. (a) (b) (c) Fig. 1. (a) Physical structure of the switch. (b) Equivalent circuit of the proposed switch. (c) Graphical illustration of the switch. (d) Graphical illustration of the CPW (d)
1294 Yasser Mafinejad et al. / Procedia Engineering 29 (2012) 1292 1297 It should be noted that one other goal of the design parameters is to increase the life cycle of the switch. For this issue, we limit the gap-height to 2 μm. The shunt switch initially is in its up-state position and the switch is on. The state of the switch changes to the down-state and off-position when it is actuated. In the up-state position, the switch should be matched with input and output circuits. Because of the small capacitance between the membrane and transmission line in the up-state position, there is not an ideal match and a part of the RF signal is bypassed to ground by this capacitor. The reflection coefficient at the input is: (1) The values of S 11 equals or less than -10 db guarantee a good matching at the input in the up-state position. For Z 0 = 50 Ω at 30 GHz, the value of the up-state capacitor C u is 50.3 ff. C u of the low actuation voltage switches are between 20 ff to 100 ff [4, 6, 14] for the RF frequency of up to V band. For small values of C u, the area of the membrane decreases and the height of the gap increases. (2) The k factor in (2) represents the effect of fringing capacitance. When k=1, the fringing capacitance in the up-state position between membrane and ground is comparable to the ideal capacitance. k mainly depends on the gap-height. For small value of g (< 0.1 μm), the fringing capacitance is negligible and k=1. For g=2 μm and the proposed dimension of the membrane, k is calculated from the EM simulation as about 1.2. Using (2), the effective area of the switch (A) is 9472 μm 2. W is the width of the central line of the CPW whose values is 80 μm.the length of the membrane of the CPW in our design is L=118.4 μm. Table 1. Parameters of the proposed MEMS switch S 11 up state S 21 up state S 11 down state S 21 down state Frequency Actuation Voltage < -10-0.5 > -0.5 < < -10 Ka to V < 12 2.1. Calculation of the actuation voltage One of the more commonly used beams for lowering the effective spring constant is folded flexures [15]. For this structure, the spring constant and actuation voltage can be calculated as follows. (3) (4) where Young s modulus for gold E is 80 Gp, the thickness of the beam t is 1.5 μm, the gap between the transmission line and the membrane g is 2 μm, the membrane effective area parameters W 1 is 80 μm, W 2 is 118.4 μm, and l is 80 μm. The actuation voltage which is calculated for these values is less than 12 V.
Yasser Mafinejad et al. / Procedia Engineering 29 (2012) 1292 1297 1295 2.2. Estimation of Z h The characteristic impedance of a short HITL (Z h ) is a function of the dimension of the t-line, h t (depth) and ε r of the substrate. The critical value for Z h is 84 Ω. For Z h less than this value, the switch would not be matched. Through calculations, the optimized and proper value for Z h is 90 Ω. For Z h larger than this value, the t-line s width is increased excessively. The switch should be matched at the input and output (S 11 =0 db). By equating S 11 =0 db, the short HITL can be calculated as follows: (5) where Z 0 =50 Ω, Z h =90 Ω, C u =50.3 ff, F=30 GHz, λ=4.8 mm. The equation is used to calculate l (l =760 μm). Two values are found for l but due to the variation of the phase shift along the transmission line, the shorter l (760 μm) is selected because it makes the circuit less sensitive to frequency band. 3. Simulation Results The scattering parameters of the switch with and without the short HITLs are calculated by using EM3DS, and presented in Fig. 2. As can be seen, the scattering parameters are improved in the up-state position for the switch with the short HITLs, and are desirable for the Ka and V band. Fig. 2(a) and 2(b) illustrate S 11 and S 21 for the switch with and without the short HITLs. Fig 2(a) shows that S 11 is less than - 10 db for the frequency of up to 60 GHz. Furthermore, there is desirable matching at 30GHz for S 11 less than -45 db, which is desired for this switch. Fig 2(b) illustrates that the insertion loss, S 21, for the switch with transmission line in the up-state position is less than -0.9 db from DC up to 65 GHz. Fig. 2(c) and 2(d) show that the added short HITLs do not have significant effects on the RF parameter in the downstate position. As can be seen from the figures, the S 11 and S 21 of the down-state position for the switch are desirable for the frequency of greater than 25 GHz. Therefore, the switch provides desirable RF characteristics (S 11 and S 21 ) for the Ka and V frequency band. 4. Conclusion A RF MEMS capacitive coupling shunt switch was designed and simulated. Low actuation voltage can be achieved, partially by a proper structure design of a cantilever for minimum spring constant, decreasing the gap, and increasing the area of the membrane. Decreasing the gap and increasing the area enlarge the up-state capacitor Cu. By integrating a specific short HITL as inductance at the input and output of the switch, Cu can be eliminated at the desired frequency. Simulation of this design by EM3DS shows that the scattering parameters are better than the estimated values for the very large frequency band from 25 GHz up to 50 Ghz. For our specific switch, the actuation voltage was found to be 12 V. Acknowledgements The authors wish to acknowledge for the support Professor Marco Farina (Università Politecnica delle Marche) and MEM Research for the EM3DS software.
1296 Yasser Mafinejad et al. / Procedia Engineering 29 (2012) 1292 1297 (a) (b) (c) (d) Fig. 2. (a) S 11 for up-state position, with and without short HITL. (b) S 21 for up-state position, with and without short HITL. (c) S 11 for down-state position, with and without short HITL. (d) S 21 for down-state position, with and without short HITL References [1] Malmqvist, R., et al. Design, packaging and reliability aspects of RF MEMS circuits fabricated using a GaAs MMIC foundry process technology. in Microwave Conference (EuMC), 2010 European. 2010. [2] Ruan, J., et al. ESD stress in RF-MEMS capacitive switches: The influence of dielectric material deposition method. in Reliability Physics Symposium, 2009 IEEE International. 2009. [3] Balachandran, S., et al. High power nanocrystalline diamond RF MEMS- A combined look at mechanical and microwave properties. in Microwaves, Communications, Antennas and Electronic Systems, 2008. COMCAS 2008. IEEE International Conference on. 2008. [4] Seong-Dae, L., et al., A novel pull-up type RF MEMS switch with low actuation voltage. Microwave and Wireless Components Letters, IEEE, 2005. 15(12): p. 856-858. [5] Ching-Liang Dai, H.-J.P., Mao-Chen Liu, Chyan-Chyi Wu, Lung-Jieh Yang, Design and Fabrication of RF MEMS Switch by the CMOS Process. Tamkang Journal of Science and Engineering, 2005. 8(3): p. 197-202. [6] Calaza, C., et al., Electromechanical characterization of low actuation voltage RF MEMS capacitive switches based on DC CV measurements. Microelectronic engineering, 2007. 84(5-8): p. 1358-1362. [7] Lee, H.C., J.Y. Park, and J.U. Bu, Piezoelectrically actuated RF MEMS DC contact switches with low voltage operation. Microwave and Wireless Components Letters, IEEE, 2005. 15(4): p. 202-204. [8] Mahameed, R., et al., Dual-beam actuation of piezoelectric AlN RF MEMS switches monolithically integrated with AlN contour-mode resonators. Journal of Micromechanics and Microengineering, 2008. 18: p. 105011. [9] Goldsmith, C., et al. High-cycle life testing of RF MEMS switches. 2007: IEEE. [10] Park, J.H., et al., A fully wafer-level packaged RF MEMS switch with low actuation voltage using a piezoelectric actuator. Journal of Micromechanics and Microengineering, 2006. 16: p. 2281.
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