Investigations on Low Activation RF MEMS shunt switches

Abstract

Over the last decade, Radio Frequency (RF) Micro-Electro Mechanical System newline(MEMS) switches have replaced the conventional solid-state switches. The RF MEMS newlineswitches are found useful in many practical applications including wireless newlinecommunications devices, reconfigurable antennas, software-defined radios, etc. In newlinefact, the performance of such switches has improved over the years in terms of newlineswitching speed, isolation, power dissipation, requirement of activation voltage and newlinereliability. newlineThere are many ways to classify the RF M`EMS switches. Based on the type of newlineactuation, there are electrostatic, electromagnetic, piezoelectric and thermal types of newlineswitches. Out of these options, the electrostatic actuation is more preferred because of newlineits advantages like low power consumption, fast switching and compatibility with newlineelectronic circuits. However, MEMS switches with electrostatic actuation require newlinerelatively high actuation voltage to activate the movable parts. This constraint makes newlinethe use of such switches limited for many practical applications, where low activation newlinevoltage is an essential requirement. newlineThis thesis focusses on investigations on the electrostatically actuated Micro-Electro newlineMechanical System (MEMS) capacitive shunt switches with emphasis on low newlineactivation voltage. The work is focused onthe designs of different RF MEMS shunt newlineswitches with low activation voltage. For all the proposed designs, the simulated newlineresults using standard design tools are obtained and verified with analytical results and newlineare found to be in close agreement. newlineThe first proposed RF MEMS switch is composed of Pi(and#928;)-type piezoelectric newlinecantilever, a coplanar wave transmission line (CPW) and contact electrodes. For the newlinesaid design,the simulation and analytical results have been obtained. The performance newlineof the Pi(and#928;)-type RF MEMS switch under different locations of bridges is discussed. newline___________________________________________________________________________ newline___________________________________________________________________________ newlineviii newlineFor a 400 and#956;m separation between two bridges and various bridge widths (25-75 and#956;m), newlinethe switch provides better frequency over the range of 51-75.5 GHz. At 250 and#956;m newlineseparation between the bridges with the same bridge width, the Pi(and#928;)-type RF MEMS newlineswitch operates on the frequency range of about 31.3-56.5 GHz. The activation voltage newlineis found to be about 24 V for a 3and#956;m initial bridge air gap of the RF MEMS switch. newlineThe RF MEMS switch takes almost the same time to settle at up-state condition even newlineif the time in down-state position varies. newlineAnother RF MEMS switch has serpentinetype bridge structure. This type of bridge newlinestructure offers more flexibility to the bridge and results in low spring constant and newlineless activation voltage requirement. The performance of the serpentine RF MEMS newlineswitch with various designs of bridge arms is discussed for the same overlap area newlinebetween the signal line and the bridge. The electrostatic results are discussed from flat newlineto serpentine bridge arms. The activation voltage as low as 10.75 V with 1 and#956;m newlinethickness and 200 and#956;m long bridge is obtained for the serpentine bridge RF MEMS newlineswitch. The bandwidth of the switch isachieved in the range of 21.3-50 GHz with a 30 newlineand#956;m signal line width and 90 and#956;m bridge width. newlineThe third important design is a Crag-lagshaped RF MEMS switch with low spring newlineconstant and low activation voltage of the order of 4.7 V. Among all the designs of RF newlineMEMS switch presented in the thesis, the Crag-lag bridge structure is more flexible newlinewith higher mechanical strength. In this design, there are two actuation electrodes and newlinea separate signal line. Both electrodes and a signal line are arranged such that the newlinewhole arrangement isolates the DC static charge and the input operating signal. The newlineswitch operates on 4.7 V of activation voltage for a 3 and#956;m bridge height, 5 and#956;m × 5 and#956;m newlinevias, 95 and#956;m × 205 and#956;m actuation electrodes. This quantum of operating voltage can be newlinecompatible with CMOS operating voltage for on-chip fabrication. The return-loss in newlinethe up-state condition and the isolation in the down-state condition are found better newlinethan -20 dB over a wide frequency band of 14-41 GHz. The same switch yields newlineinsertion loss in up-state and return-loss in down newline