Investigations on Low Activation RF MEMS Shunt Switches

Abstract

Over the last decade, Radio Frequency (RF) Micro-Electro Mechanical System (MEMS) switches have replaced the conventional solid-state switches. The RF MEMS switches are found useful in many practical applications including wireless communications devices, reconfigurable antennas, software-defined radios, etc. In fact, the performance of such switches has improved over the years in terms of switching speed, isolation, power dissipation, requirement of activation voltage and reliability. There are many ways to classify the RF M`EMS switches. Based on the type of actuation, there are electrostatic, electromagnetic, piezoelectric and thermal types of switches. Out of these options, the electrostatic actuation is more preferred because of its advantages like low power consumption, fast switching and compatibility with electronic circuits. However, MEMS switches with electrostatic actuation require relatively high actuation voltage to activate the movable parts. This constraint makes the use of such switches limited for many practical applications, where low activation voltage is an essential requirement. This thesis focusses on investigations on the electrostatically actuated Micro-Electro Mechanical System (MEMS) capacitive shunt switches with emphasis on low activation voltage. The work is focused onthe designs of different RF MEMS shunt switches with low activation voltage. For all the proposed designs, the simulated results using standard design tools are obtained and verified with analytical results and are found to be in close agreement. The first proposed RF MEMS switch is composed of Pi(Π)-type piezoelectric cantilever, a coplanar wave transmission line (CPW) and contact electrodes. For the said design,the simulation and analytical results have been obtained. The performance of the Pi(Π)-type RF MEMS switch under different locations of bridges is discussed. a 400 μm separation between two bridges and various bridge widths (25-75 μm), the switch provides better frequency over the range of 51-75.5 GHz. At 250 μm separation between the bridges with the same bridge width, the Pi(Π)-type RF MEMS switch operates on the frequency range of about 31.3-56.5 GHz. The activation voltage is found to be about 24 V for a 3μm initial bridge air gap of the RF MEMS switch. The RF MEMS switch takes almost the same time to settle at up-state condition even if the time in down-state position varies. Another RF MEMS switch has serpentinetype bridge structure. This type of bridge structure offers more flexibility to the bridge and results in low spring constant and less activation voltage requirement. The performance of the serpentine RF MEMS switch with various designs of bridge arms is discussed for the same overlap area between the signal line and the bridge. The electrostatic results are discussed from flat to serpentine bridge arms. The activation voltage as low as 10.75 V with 1 μm thickness and 200 μm long bridge is obtained for the serpentine bridge RF MEMS switch. The bandwidth of the switch isachieved in the range of 21.3-50 GHz with a 30 μm signal line width and 90 μm bridge width. The third important design is a Crag-lagshaped RF MEMS switch with low spring constant and low activation voltage of the order of 4.7 V. Among all the designs of RF MEMS switch presented in the thesis, the Crag-lag bridge structure is more flexible with higher mechanical strength. In this design, there are two actuation electrodes and a separate signal line. Both electrodes and a signal line are arranged such that the whole arrangement isolates the DC static charge and the input operating signal. The switch operates on 4.7 V of activation voltage for a 3 μm bridge height, 5 μm × 5 μm vias, 95 μm × 205 μm actuation electrodes. This quantum of operating voltage can be compatible with CMOS operating voltage for on-chip fabrication. The return-loss in the up-state condition and the isolation in the down-state condition are found better than -20 dB over a wide frequency band of 14-41 GHz. The same switch yields insertion loss in up-state and return-loss in down-state condition lower than 0.5 dB over the same frequency band. Apart from switch designs, a phase shifter with a flat bridge type of RF MEMS switch is also designed and simulated. Nine flat bridges are connected in parallel on top of the signal line. Each bridge provides some phase shift depending upon the capacitance between the bridge and the signal line. The phase shifter provides phase shift of 45°, 60°, 90°, and 180° at actuation voltage of 6.5 V, 7 V, 10 V and 10.7 V, respectively, for 40 GHz of operating frequency. Based on the results of the designed RF MEMS switches and phase shifter, it is believed that all the proposed RF MEMS switches can operate in X to Ka bands. Moreover, it is possible to achieve excellent isolation and insertion loss properties using proposed RF MEMS switches. The proposed RF MEMS switches are suitable for use in various high frequency applications.