Smart Antenna using MTM-MEMS Georgina Rosas a, Roberto Murphy a, Wilfrido Moreno b a Department of Electronics, National Institute of Astrophysics, Optics and Electronics, 72840, Puebla, MEXICO b Department of Electrical Engineering, University of South Florida, 33620 Tampa, Florida girosas@inaoep.mx, rmurphy@ieee.org, moreno@eng.usf.edu Abstract This article presents the design of a novel and compact coplanar antenna using Metamaterials (MTM) and Micro Electro Mechanical Systems (MEMS). The antenna is based on coplanar waveguide (CPW) technology; therefore, the signal and ground are on the same plane, presenting lower dielectric losses and high signal integrity. The designed antenna can be tuned in the frequency range from 5.3 to 5.8 GHz by MEMS capacitors, and it is useful for wireless communications applications, especially beam steering systems. Finally, the design, 3D full-wave simulations and MEMS simulations are presented. Index Terms Metamaterial (MTM), Transmission Line (TL), Coplanar (CPW), Composite Right/Left Handed (CRLH), Micro-Electro-Mechanical Systems (MEMS). I. INTRODUCTION Nowadays, with the advent of RF technology, electronic products demand more functions and higher performance, reduced dimensions and higher speeds, and higher output at a lower cost. The state-of-the-art in this technology requires the fusion of emerging technologies such as Metamaterials (MTM) and Micro Electro Mechanical Systems (MEMS) [1]-[4]. Together, they can revolutionize electronics by providing very small and reliable smart circuits at a minimal cost. Metamaterials are new artificial materials, that present unique electromagnetic properties, which are controllable and are not present in any known natural environment. Research in this field opens up new ways for innovation in communications, based on original designs that exploit singular properties such as simultaneously negative permittivity (ε) and permeability (µ), with antiparallel group velocity (v g ) and phase velocity (v p ), and negative refractive index (n) [5]-[6]. The proposed smart antenna consists of a CRLH-MTM structure with MEMS capacitors and four inductors connected to ground, as displayed in Figure 1. In this paper, a particular design, determining all quantities of interest is presented. Fig. 1. Proposed antenna using MTM and MEMS II. MTM-MEMS ANTENNA The parameters (L R, C R, L L, C L ) of a composite right/left handed Metamaterial (CRLH-MTM) structure are determined using the equations developed by [3] and following the methodology described in [7]. Using these, the values listed in Figure 2 are obtained: Fig. 2. Circuit Equivalent of a basic cell CRLH TL-MTM with L R = 1.437 nh, C L = 0.524 pf, C R =5747 pf, L L = 1.31nH.
A. MEMS Capacitor The MEMS capacitor consists of two parallel plates (two electrodes, one mobile-positive and one fixednegative), whose capacitance can be varied using the electrostatic principle. The MEMS capacitor can be fabricated using the surface micromachining technique, and it can be integrated in a CMOS chip. The process under consideration here is composed of four materials and five levels of masks on a silicon wafer (100 orientation, ρ>4000 Ω.cm) acting as the mechanical support. Titanium (Ti) and Gold (Au) are used as structural materials, with one suspended level for mechanical structures, and SU8 is used as the sacrificial material. Silicon dioxide SiO 2 and BenzoCyclobutene BCB are used as dielectrics. The capacitance between parallel plates is determined by: ε C 0 ε = rwl (1) d where ε 0, is the free space permittivity, ε r is the relative permittivity, W is the design width (W=596μm), L the length (L=596μm) and d is the gap between electrodes (d=1 5μm). Using these values in Equation (1), the capacitance varies from 0.69 to 1.57 pf, while the gap between plates can vary up to 4 microns. The structure of the variable capacitor is considered MEMS s techniques such as: dimples, holes, and some others that support functional stability, operation and a proper release of the structure [8-10]. In a real MEMS capacitor structure, the situation is more complicated than a simple lumped design. However, it happens to be a mechanic and elastic element, therefore it has instability points determined by the pull-in voltage; in electrostatic MEMS this is called the pull-in instability. The pull-in voltage equation can be defined as: [10]: Fig. 3. Displacement of mobile electrode and capacitance as a function of applied voltage. To avoid collapse, dimples were defined for the second level. Furthermore, these give the structure a more robust mechanical stability. The dimples were designed using the criteria for SUMMIT V [12] and 3D full-wave electromagnetic field simulations for radio frequency (RF), to obtain optimal results. Figure 4 ilustrates a schematic of the MEMS capacitor with all integrated elements (dimples and holes). Figure 4 shows stress simulations along the y axis. The different shades in Figure 4 indicate the distribution of mechanical stresses along the length of the positive (mobile) electrode. The MEMS capacitor was simulated under electrostatic actuation, as is shown in Figure 4(c). The electrostatic analysis was performed sweeping the applied voltage from 0 to 24V, obtaining a variation of 0 to 3 microns approximately. The effective stiffness (K eff ) can be obtained from for mechanic-electrostatic analysis. This has been extract from Coventorware simulations, and also, was calculated in accordance to [9] and [10]. V pi 8K eff g 0 = (2) 27ε WL 0 3 where K eff is the effective stiffness and g 0 is the initial gap, and all the other variables defined previously. Figure 3 shows the displacement of the movable electrode and the associated capacitance variation as a function of applied voltage, as given by (2). Furthermore, a detailed electric-mechanical analysis has been performed using Coventorware [11].
impedance of 50Ω. The line spacing is 50μm and the signal line width is 78μm. Figure 6 shows the simulation results for the S 11 dispersion parameter. The simulated return losses at the point m 1 = 5.8GHz is 16.10dB with a 4% bandwidth (defined by S 11 < 10dB). This full wave simulation considers the design aspects for the MEMS capacitors, such as dimples and holes. This simulation shows greater losses than the previous simulation shown in Figure 8. Figure 6b shows the radiation pattern for the antenna resonating at 5.8GHz. (c) Fig. 4. Coventorware simulations Sketch of MEMS capacitor with dimples and holes, MEMS capacitor stress simulations along the y axis, and (c) Simulations along the z axis of a parallel-plate capacitor with an applied voltage of 24V. III. SIMULATION RESULTS In this paper, the smart antenna was simulated in HFSS v10 from Ansoft Corporation using it as a 3D full-wave electromagnetic field solver, using a high resistivity silicon substrate and a gold conducting layer with a thickness of 3 µm to reduce losses. The layout for the antenna only (without an integrated DC bias line) using an MTM-MEMS CRLH-TL basic cell of dimensions 1.397mm X 2.022mm, is shown in Fig. 5. It has a 1/37.02 λ 0 x 1/25.5 λ 0 footprint and is among the smallest in the literature, where λ 0 is the free space wavelength. Figure 5 shows the details of the MEMS capacitor as a tuning element. The dimple size is 15 x 15 μm for the mobile electrode and is of 25 x 25 μm for the support structure of the fixed electrode. The dimple base is isolated. The hole size is 15 x 15 μm. Each MEMS capacitor has ten holes. The antenna, consisting of two double MEMS capacitors and four inductors connected to ground, presents the following characteristics: The capacitor uses an area of 596μm x 596μm, and presents a nominal value of 1.048pF. The spiral inductor dimension are 200μm X 200μm, with a strip width of 20μm and a line spacing of 10μm, and provides an inductance value of 1.31nH. This structure was designed for a central frequency of 5.8GHz. The coplanar line (CPW) is designed for a characteristic Fig. 5. Layout of the smart antena of a cell MTM and Details distribuition of dimples and holes. The dispersion curve of the MTM-MEMS cell antenna is plotted in Figure 7 for a transition frequency of 5.8GHz, with Beta = 0. It shows the typical characteristics of a structured metamaterial; the negative sign of the slope demonstrates the existence of the negative phase velocity. The dispersion curve is obtained from a zero-resonator structure using the unwrapped phase of S 21. The CPW RF choke is designed for a central frequency of 5.8GHz and has a length of 2.58mm, and it is loaded with a capacitance of 0.845pF, as shown in Figure 9. It
is implemented on chip in the same fabrication processes that the smart antenna. The CPW transmision line is of the meander type, with a length of 90 electric degrees, which makes it possible to achieve high impedance ( > 1,500Ω at 5.3 to 5.8GHz) bias line which does not impact on the microwave performance of the device, as shown in Figure 9. The coplanar bias-t line is designed to have a characteristic impedance of 50Ω. The line spacing is 50μm, and the signal line width is 13μm. Fig. 7. Dispersion Diagram. It shows a frequency transition at 5.8GHz and a beta = 0. Fig.8. Simulated return loss of the smart antena with the following resonance frequencies m 1 = 5.3GHz, m 2 = 5.8GHz. Fig. 6. Simulated return loss of the antena with the following resonance frequencies m 1 = 5.8 GHz, and Radiation pattern for the antenna resonating at 5.8 GHz. Figure 8 shows the simulation results for the S 11 dispersion parameter, considering the change in capacitance. The simulated return losses at points m 1 = 5.3GHz and m 2 = 5.8GHz are 13.747 db and 20.15dB, respectively. This implies that the response of the antenna is very good at this range. The behavior can be improved overall by increasing the number of cells.
its on-going support in the fabrication process of the device. REFERENCES Fig. 9. Layout of DC bias-t line for the antenna and S 11 and Z in simulation results of a CPW bias-t line. IV. CONCLUSION This paper has shown the design of a Smart MTM antenna, using MEMS capacitors and considering the most important factors in order to achieve mechanical stability in the antenna performance using dimples and holes. Additionally, the MEMS capacitor is a radiant element, as well as a MTM structure. In the fabrication process, the metal has been considered thicker in order to avoid conductor losses by the skin effect at low frequencies. This work has shown that MTM-MEMS technologies allow to simultaneously add variability, small size (much less than λ/4), broader bandwidth, low losses and design flexibility. These simulations encourage the fabrication of the structure, which will be undertaken in the near future. ACKNOWLEDGEMENT The authors wish to acknowledge CONACyT, Mexico, for the partial support of this work through Grant 83774- Y. Georgina Rosas also thanks CONACyT for the scholarship to undertake doctoral studies, Number 102735 and for the support in carrying out this research. Special recognition to the Nanomaterials & Nanomanufacturing Research Center (NNRC) at USF for [1] C. Caloz and T. Itoh. Application of the transmission line theory of left-handed (LH) materials to the realization of a microstrip LH transmission line, in Proc. IEEE-AP-S USNC/URSI National Radio Science Meeting, vol. 2, San Antonio, TX, pp. 412 415, June 2002. [2] C. Caloz and T. Itoh. Novel microwave devices and structures based on the transmission line approach of meta-materials, in IEEE-MTT Int l Symp., vol. 1, Philadelphia, PA, pp. 195 198, June 2003. [3] Christophe Caloz and Tatsuo Itoh, Electromagnetic Metamaterials: Transmission Line Theory and Microwave Applications, Copyright 2006, John Wiley & Sons, Inc. [4] Wei Tong, Zhirun Hu, Hong Chua, Philip Curtis, et al Left-Handed Metamaterial Coplanar Waveguide Components and Circuits in GaAs MMIC Technology, IEEE Transactions on Microwave Theory and Techniques, Vol 55, No.8, August 2007. [5] V.G. Veselago. The electrodynamics of substances with simultaneously negative values of ε and μ, Soviet Physics Uspekhi, vol. 10, no. 4, pp. 509 514, Jan., Feb. 1968. [6] R. A. Shelby, D. R. Smith, and S. Schultz. Experimental verification of a negative index of refraction, Science, vol. 292, pp. 77 79, April 2001. [7] G. Rosas, R. Murphy and A. Corona, Metamaterial MEMS Reconfigurable Transmission Line, XV Workshop Iberchip, Buenos Aires-Argentina, March 2009. [8] A Dec. K. Suyama, Micromachined Electro- Mechanically Tunable capacitors, IEEE Transactions IEEE Transactions on Microwave Theory and Techniques, Vol 46, No.12, December 1998. [9] S. Pamidighantam, R. Puers, et al, Pull-in Analysis of Electrostatically Actuated Beam Structures with Fixed- Fixed and Fixed-Free end Conditions, J. of Micromechanics and Mircoengineering, 12, pp. 458-464, Jun( 2002). [10] David A. Czaplewski, Christopher W. Dyck, A Soft Landing Waveform for Actuation of a Single-Pole Single- Throw Ohmic RF MEMS Switch, Journal Microelectromechanical Systems, Vol.15, No.6, December 2006. [11] Coventorware, http://www.coventor.com/coventorware.html [12] Sandia National Laboratories, http://www.mems.sandia.gov/