RECONFIGURABLE ANTENNA STRUCTURES USING MEMS TECHNOLOGY 1
Kagan Topalli1, Emre Erdil2, Ozlem Aydin Civi1 Dept. of Electrical-Electronics Eng., Middle East Technical University, Ankara, Turkey 2 Dept. of Information Technology, Capital Markets Board of Turkey, Ankara, Turkey
[email protected],
[email protected]
1. INTRODUCTION The tunable characteristics of RF MEMS (Microelectromechanical Systems) components enable their integration with antennas providing numerous advantages such as reconfigurability in polarization, frequency, and radiation pattern [1]. Furthermore, the monolithic fabrication of these components with antennas can reduce parasitic effects, losses and costs. This paper presents two tunable frequency antennas integrated with RF MEMS capacitors to tune the operating frequencies. The antennas are designed using Ansoft HFSS, prototypes are fabricated and experimental results are compared with the simulations. Approximately 1 GHz tunability for the desired frequencies has been achieved. 2. DESIGN, SIMULATION AND MEASUREMENT OF RECONFIGURABLE ANTENNAS The first structure is a reconfigurable rectangular slot antenna. A rectangular slot antenna itself shows a dual frequency operation with a significant amount of cross-polar component in H-plane for the second resonant frequency. Insertion of the stub to the edge opposite to the 50 Ω CPW feeding line reduces the cross-polar component in H-plane to a level lower than -30 dB. The length of the stub is selected to be nearly quarter-guided wavelength in the upper band. The dimensions of the stub, i.e. the characteristic impedance of the stub, affect the resonant frequencies and the separation between the frequencies [2]. 6 MEMS cantilever type capacitors are placed onto the stub to change the characteristic impedance of the stub, and to dynamically load the antenna. The general view of the rectangular slot antenna loaded with MEMS capacitors is shown in Fig. 1 (a). The anchors of these cantilevers are attached to the stub. Two cantilevers supported by these anchors are suspended over the conductor carrying the RF signal. As can be seen from the cross-sectional view in Fig. 1 (b), these cantilever type capacitors resemble a “T-wing” structure which can be actuated electrostatically by applying DC voltage between the RF signal line and suspended cantilevers [3]. The capacitance takes place in the overlapping area of the cantilevers and the conductor carrying the RF signal. The cantilevers are designed at 2 µm height when they are not actuated and they are lowered down to 1.4 µm. By bending cantilevers from 2 µm to 1.4 µm, the loading capacitance increases resulting in a change in the characteristic impedance of the stub which provides shift in the resonant frequencies. The simulation results for 5-12 GHz band are given in Fig. 2 (a). The resonant frequencies of this structure when the cantilevers is at 2 µm occur at 8.48 GHz (10 dB BW: 4.2 %) and 10.53 GHz (10 dB BW: 10 %). As the height of the cantilevers moves down to 1.4 µm, the resonant frequencies shifts down to 7.3 GHz (10 dB BW: 1.6 %) and 10.2 GHz (10 dB BW: 11.7 %), respectively. The antenna radiates broadside for all of the four resonances and increasing the capacitance by lowering down the cantilevers does not cause any adverse effect on the radiation patterns.
MEMS cantilevers 2 µm
Silicon Nitride Base Metal 250 µm
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Fig. 1. (a) General view of the rectangular slot antenna loaded with 6 MEMS cantilever type capacitors. (b) Side view of the cantilevers.
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Fig. 2. (a) Simulation results for the reflection coefficient characteristics of the rectangular slot antenna for the cantilevers are at 2 µm and 1.4 µm height, (b) The measurement result compared to the simulations for various cantilever heights. The reflection coefficient characteristic of the rectangular slot antenna is measured using CPW RF probes. Fig.
2 (b) shows the measurement results of the structure compared with the simulation results for the cantilevers with different height. It can be seen that the measured antenna has a cantilever height of approximately 4 µm which is designed to be at 2 µm. The change in the cantilever height is also verified with SEM and surface profiler measurement. The cantilevers are bended up to 8 µm at the tip which is due to the stress gradient occurred in the structural layer gold plating.
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Fig. 3. SEM views of the fabricated slot antenna (a) general view of the loading section (b) close-up of the cantilevers suspended on the stub. Fig. 4 (a) shows the comparison between the measured reflection coefficient characteristics of the structure without MEMS capacitors and the structure with MEMS capacitors. This result shows the significant effect of the loading capacitors on the characteristic impedance of the stub and antenna characteristics. The MEMS cantilevers are electrostatically actuated using DC voltage applied between the signal and ground of CPW using bias tee. The pull-in voltage for the cantilevers is about 32 volts. Fig. 4 (b) gives the reflection coefficient characteristics for different actuation voltages up to the pull-in voltage. The characteristics shifts towards lower frequencies continuously with the increase in the DC applied voltage on the cantilevers. These results show that the lower and the higher resonant frequencies can be tuned from 9.87 GHz to 9.48 GHz and from 12 GHz to 11.12 GHz, respectively.
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Fig. 4. (a) Comparison between the unloaded and loaded slot antenna structure. (b) The reflection coefficient characteristics for different actuation voltages. The second structure employs the idea of loading one of the radiating edges of the microstrip patch antenna with a stub on which RF MEMS bridge type capacitors are placed periodically. The general view of the microstrip patch antenna loaded with 5 bridge type MEMS capacitors on CPW stub is shown in Fig. 5 (a). As in the previous design the electrical parameters of the CPW stub can be tuned by changing the height of the RF MEMS capacitors loading the stub. Therefore, the CPW stub provides a variable load to the radiating edge it is connected, resulting in tunability in the resonant frequency. To load the microstrip patch antenna with a coplanar waveguide, the antenna is loaded by a microstrip stub and a tapered line to provide the appropriate transition from microstrip stub to the CPW. The capacitance takes place in the overlapping area between the capacitor and the signal line of the CPW. Simulation results obtained using Ansoft HFSS indicates that the resonant frequency can be tuned from 17.1 GHz to 15.95 GHz with the change of MEMS bridge height from 2 µm to 1.4 µm defined by the mechanical limitation of MEMS capacitors. Fig. 5 (b) shows the related reflection coefficient characteristics. It should be noted that the antenna radiates broadside at the corresponding resonant frequencies. 0.9 mm
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Fig. 5. (a) General view of the microstrip patch antenna loaded with RF MEMS bridge type capacitors. (b) reflection coefficient characteristics of the structure. 4. FABRICATION Both antenna structures are fabricated on a 500 µm-thick Pyrex 7740 glass substrate. The process flow is as follows: (a) 300/3000 Å-thick Cr/Au is evaporated as a seed layer for gold electroplating. (b) 2 µm of Au is deposited via electroplating and Cr/Au seed layer is etched. (c) 2000-4000 Å-thick SixNy is deposited as dielectric layer by PECVD process and patterned by RIE. (d) 100/2500 Å-thick Ti/Cu layer is sputtered and anchor regions are patterned by etching. (e) The regions for sacrificial layer deposition are defined and 2 µm of copper is deposited via copper
electroplating. (f) Structural metal regions are defined and 1 µm of Au is deposited via electroplating for structural metal. (h) Sacrificial layer is removed and the devices are released using critical point dryer. 5. CONCLUSION This paper presents two reconfigurable antennas using MEMS technology. The first structure is reconfigurable dual-frequency rectangular slot antenna integrated with MEMS cantilever type capacitors. The simulations results show that the structure has a dual frequency behavior where both of the resonant frequencies can be reconfigured dynamically. By the actuation of MEMS cantilevers, the lower resonant frequency shifts 1.2 GHz, whereas the higher resonant frequency has a shift of 330 MHz without any distortion on the radiation pattern. The device has been fabricated and a shift of 390 MHz for the lower resonant frequency and 880 MHz for the higher resonant frequency has been achieved. The second structure employs the idea of loading one of the radiating edges of the microstrip patch antenna with a stub on which RF MEMS bridge type capacitors are placed periodically. Simulation results obtained using Ansoft HFSS indicates that the resonant frequency can be tuned from 17.1 GHz to 15.95 GHz with the change of MEMS bridge height from 2 µm to 1.4 µm. This structure is currently in fabrication. Measurement results will be presented and compared with the simulation results at the symposium.
ACKNOWLEDGMENTS This research is supported by The Scientific and Technical Research Council of Turkey (TUBITAK-EEEAG102E036), Turkish State Planning Organization (DPT), and AMICOM (Advanced MEMS for RF and Millimeter Wave Communications) Network of Excellence under 6th Framework Program of European Union. REFERENCES [1] G. M. Rebeiz, “RF MEMS theory, design, and technology,” Hoboken, NJ: John Wiley & Sons, 2003. [2] Daniel Llorens, Pablo Otero, and Carlos Camacho-Peñalosa, “Dual-band, single CPW port, planar-slot antenna,” IEEE Trans. on Antennas and Propagation, Vol. 51, Jan. 2003, pp.137-139. [3] M. Unlu, K. Topalli, H. Sagkol, S. Demir, O. Aydin Civi, S. Koc, and T. Akin, "New MEMS Switch Structures for Antenna Applications," 2002 IEEE AP-S International Symposium and USNC/URSI National Radio Science Meeting, p.134, San Antonio, Texas, 16-21 June 2002.
