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A Novel Three-State RF MEMS Switch for. Ultrabroadband (DC-40GHz) Applications. Yan-Qing Zhu, Lei Han, Li-Feng Wang, Jie-Ying Tang, and Qing-An Huang ...
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IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 8, AUGUST 2013

A Novel Three-State RF MEMS Switch for Ultrabroadband (DC-40GHz) Applications Yan-Qing Zhu, Lei Han, Li-Feng Wang, Jie-Ying Tang, and Qing-An Huang, Senior Member, IEEE Abstract— Design, fabrication, and measurement results of a lateral dc-contact RF microelectromechanical systems switch for ultrabroadband applications are presented in this letter. The switch is driven by a bidirectional cascaded electrothermal actuator, which can generate larger displacements and contact forces at two directions than traditional electrothermal actuators. Because of this bidirectional actuator, the proposed switch can not only realize the off-state to on-state shifting, but also provide an additional deep off-state. The proposed switch is fabricated by MetalMUMPs process, and measurement results show that the insertion loss is less than −0.5 dB and the initial isolation is better than −22.5 dB at 0–40 GHz range. At the deep off-state, the isolation better than −30 dB can be achieved at the whole frequency range 0–40 GHz. The measurement results agree well with the theory and design. Index Terms— Broadband, lateral contact, microelectromechanical systems (MEMS) switch, thermal actuation, three-state.

I. I NTRODUCTION

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OMPARED with conventional semiconductor switches, radio frequency microelectromechanical systems (RF MEMS) switches have a better high-frequency characteristics, low power consumption, and high linearity [1], which makes them to become as one of the most attractive devices for RF and microwave applications. The conventional RF MEMS switches are fabricated by the surfaces sacrificial process [2]. Most of them make use of thin metallic bridges. Susceptibility to plastic deformation and low power handling may be the problems of such thin metallic bridges [3]. Laterally actuated RF MEMS switches fabricated by several different micromachining processes are reported [4]–[6]. These switches have high power handling capability and excellent RF performance at low frequency bands, but their microwave performances are limited at high frequency bands because of high capacitance coupling between the signal lines and the contact plate. Therefore, we reported an electrostatically driven three-state switch [6]. This switch can not only realize the off-state to on-state shifting, but also provide an additional deep off-state. At the deep off-state, the switch has a better isolation at both high and low frequency

Manuscript received March 19, 2013; revised June 15, 2013; accepted June 17, 2013. Date of publication July 9, 2013; date of current version July 22, 2013. The work was supported by the National Natural Science Foundation of China under Grant 61106114. The review of this letter was arranged by Prof. C. P. Yue. The authors are with the Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing 210096, China (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2013.2269993

Fig. 1. SEM photograph of the proposed MEMS switch. The size of the switch is 1.1 mm × 1.6 mm. The structure parameters of the actuator are as follows: L 1 is 400 μm, L 2 is 410 μm, L 3 is 360 μm, W1 is 10 μm, W2 is 60 μm, Wg is 10 μm, α is 83.5°, and β is 81.1°. The thickness of the structure layer is 20 μm.

bands. The main drawbacks of this switch are its complex RF structure and high actuation voltage, which is 78 V. Besides, the insertion loss of this switch deteriorates with frequency fast, which limits its application to high frequency bands. To overcome the bottleneck for both RF performance and the area issue, we propose a new type three-state switch here, which is actuated by a novel thermal actuator that can produce a bidirectional motion. The test results show that this electrothermally driven switch has much lower insertion loss and higher isolation than the reported three-state MEMS switch, which are better than 0.5 and 30 dB at the frequency range of 0–40 GHz, respectively. This switch also has very simple RF structure and low actuation voltages of 0.3–0.55 V, which make that suitable for integration in the form of more complex RF switching networks for ultrabroadband applications. II. D ESIGN AND FABRICATION A SEM photograph of the designed three-state RF MEMS switch is shown in Fig. 1. The switch is based on a coplanar waveguide transmission line and a parallel configuration with two resistive gold–gold contacts. A silicon nitride structure is placed between the actuator and the mobile part of the contact for electrical and thermal isolation purposes (Fig. 1). The initial state of the switch is at the off-state, due to the fact that the contact plate does not contact the signal or the ground line when no dc voltage is applied. When an actuation voltage is applied at Pad C , and both Pad B and Pad A are grounded, the outer hot arm has a higher current density

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ZHU et al.: NOVEL THREE-STATE RF MEMS SWITCH

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Fig. 2. States of the thermal actuator when voltage is applied (a) pushed forward, (b) pulled back. Fig. 4.

Lumped-circuit model for the proposed switch.

Fig. 3. Simulated displacements and temperature of the actuator versus applied voltage. The temperature of the actuator is measured at a specific point on the beam where the temperature is the highest at that time.

Fig. 5. Effect of the contact resistance Rd on the isolation of deep off-state.

whereas the middle cold arm and the inner hot arm have a lower current density. Because of its lower resistance, the majority of electrical current travels through the cold arm to the anchors. Correspondingly, the inner hot arm exhibits a very low current density that results in low Joule heating [7]. Thus, U-beam actuators both bend toward the inside, as shown in Fig. 2(a). Both of them give a push force to the cascaded bent beam. Therefore, the bent beam moves upward. This kind of deformation causes the contact plate to contact with the signal line and farther away from the ground lines so that the switch turns to on-state. When an actuation voltage is applied at Pad A , and ground at Pad B and Pad C , the actuator deforms oppositely, as shown in Fig. 2(b). This kind of deformation causes the contact plate to become farther away from the signal line and contact the ground lines. This situation is defined here as the deep off-state because the isolation of this state will be larger than the initial off-state. Finite element method by ANSYS software is used to analyze the thermal actuator of the device. The simulation displacements and temperature of the actuator versus applied voltage is shown in Fig. 3. The simulated actuation spring constant and release spring constant are both 103 N/m. The contact gap between the contact plate and the signal lines is designed to be 10 um (the real gap depends on the thickness of the sputtered gold on the sidewall), whereas the gap between the contact plate and the ground line is 8 um. For push forward state (on-state), when actuation voltages are 0.3–0.39 V, the simulated total contact and release force is 0.3–1.1 and 0.6 mN, respectively. For pull back state (deep off-state), when actuation voltages are 0.45–0.51 V, the simulated total contact and release force is 0.1–0.3 and 0.8 mN, respectively.

The structure-based lumped-circuit models for the MEMS series switch are shown in Fig. 4. The contacts are modeled as resistors or capacitors depending on the state of the switch. The lateral contact structure has two contact areas that are between the input and output ports. The coupling capacitance contains two series of the Cc in parallel with C g . The Cc is the off-state capacitance because of the distance of the overlapping part between the contact plate and each broken signal line. The C g is the signal-line coupling capacitance because of the gap between each broken signal line. The Rc is half of the contact resistance when the switch is at on-state. The Rd and Cd are the resistor and capacitor between the ground line and the movable structure, respectively. On-state and off-state are equivalent to switch between Rc and Cc . At the deep offstate, Cc becomes smaller than its initial state because of the larger distance between the signal line and the movable contact plate. In addition, Cd is switched to Rd resulting in the resistor connection between the movable element and the ground line. These two reasons make larger isolation possible. The isolation of the deep off-state of the switch deteriorates with the increase in the contact resistance Rd , as shown in Fig. 5. The switch is manufactured using MetalMUMPs technology [8]. The SEM photograph of the proposed MEMS switch is shown in Fig. 1. In this process, gold is chosen as the contact metal because of its low resistivity, good stability, and efficiency in RF signal propagation. The main problem with Au is its propensity for high adhesion, which may lead to failure if restoring forces are not large enough to break a contact [9]. Because of the large restoring force and small contact areas of our switch, no adherence problems should be experienced.

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Fig. 6. On-state S-parameter measured results and the fitted results by the lumped-circuit model. The fitted results: Rc = 0.2–0.4 .

off-state can achieve maximum improvement of −9.5 dB. The validity of the model is demonstrated by the fact that its RF performances are almost the same as that of the measured data, as shown in Figs. 6 and 7. The actuation/releasing time of the fabricated switch are also measured. The actuation time is 28.5 ms for the transition from the off- to the on-state, and the releasing time is 1.9 ms for the transition from the on- to the off-state. The actuation time is much larger than the releasing time because of the electrothermal actuator generates movement through an expansion of its materials caused by the Joule effect, which has a long actuation time. Once the voltage is removed, the actuator, however, begins to cool down quickly, and the switch turns to off-state. The high spring constant of the nickel beams is also another important factor that helps the switch turn off. Although the switching time of the proposed switch is longer than its electrostatic counterparts, it could find several applications where the switching time requirement is less stringent such as redundancy networks, multiband receiver band selection networks, or automated test equipment. IV. C ONCLUSION

Fig. 7. Off-state and deep-off-state S-parameter measured results and the fitted results by the lumped-circuit model. Vact is the actuation voltage of the switch at the deep off-state. The fitted results: Cc = 3.6 fF, C g = 1.6 fF, and Cd = 1.2 fF. When Vact is 0.55 V and Rd = 1–10 .

III. M EASUREMENTS AND D ISCUSSION The RF response of the fabricated switch is measured under atmospheric conditions using the Agilent N5244A PNA-X Vector Network Analyzer with 150-um pitch coplanar probes. A short-open-load-through standard on-wafer calibration technique is employed. The switch on-state characterization with insertion loss better than 0.5 dB at the frequency range 0–40 GHz, whereas that of the return loss is from 39.1 to 18.0 dB is shown in Fig. 6. The lumped-circuit model is employed to fit the measured S-parameters, as shown in Fig. 6. The measured and modeled S-parameter for the fabricated MEMS switch are shown in Fig. 7. In the initial off-state mode, the measured isolation are from 61.4 to 30.3 dB at 0.5–15 GHz, and higher than 22.5 dB up to 40 GHz. At the deep off-state, when Vact is 0.5 V, the isolation of the switch can only be improved by 2–3 dB. Presumably, the isolation characteristic at the deep off-state can be improved simply with a good contact, which can be achieved with a higher actuation voltage. When Vact increases to 0.55 V, the isolation higher than 30 dB at the whole frequency bands can be achieved. Compared with the initial off-state, the isolation of deep

The objective of this research was to design a RF MEMS switch for ultrabroadband communication applications. Through actuating a novel bidirectional electrothermal actuator, the switch not only realized the off-state to on-state shifting, but also provided an extra deep off-state. The switch exhibited excellent RF performance to millimeter wave frequencies owing to its lateral contact mechanism. The insertion loss was less than −0.5 dB and the initial isolation was better than −22.5 dB at 0–40 GHz range. At the deep off-state, the isolation better than −30 dB can be achieved at the whole frequency bands. R EFERENCES [1] G. M. Rebeiz and J. B. Muldavin, “RF MEMS switches and switch circuits,” Microw. Mag., vol. 2, no. 4, pp. 59–71, 2001. [2] W. B. Zheng, Q. A. Huang, X. P. Liao, et al., “RF MEMS membrane switches on GaAs substrates for X-band applications,” J. Microelectromech. Syst., vol. 14, no. 3, pp. 464–471, 2005. [3] J. B. Rizk, E. Chaiban, and G. M. Rebeiz, “Steady state thermal analysis and high power reliability considerations of RF MEMS capacitance switches,” in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2002, pp. 239–242. [4] M. Daneshmand, S. Fouladi, R. R. Mansour, et al., “Thermally actuated latching RF MEMS switch and its characteristics,” IEEE Trans. Microw. Theory Tech., vol. 57, no. 12, pp. 3229–3238, Dec. 2009. [5] M. Tang, A. Q. Liu, and A. Agarwal, “A compact DC-20 GHz SPDT switch circuit using lateral RF MEMS switches,” in Proc. Asia-Pacific Microw. Conf., Dec. 2005, pp. 117–120. [6] L. F. Wang, L. Han, J. Y. Tang, et al., “Lateral contact three-state RF MEMS switch for ground wireless communication by actuating rhombic structures,” J. Microelectromech. Syst., vol. 22, pp. 10–12, Feb. 2013. [7] R. Venditti, J. S. H. Lee, Y. Sun, et al., “An in-plane, bi-directional electrothermal MEMS actuator,” J. Micromech. Microeng., vol. 16, no. 10, pp. 2067–2070, 2006. [8] A. Cowen, B. Dudley, E. Hill, et al., (2010). MetalMUMPs Design Handbook [Online]. Available: http://www.memscap.com/ products/mumps/metalmumps/reference-material [9] I. Schiele, J. Huber, B. Hillerich, et al., “Surface-micromachined electrostatic microrelay,” Sens. Actuators A, Phys., vol. 66, no. 1, pp. 345–354, 1998.