Abstract
MEMS varactors are one of the important passive MEMS devices. Their applica- tions include use in VCOs, tunable impedance matching networks, tunable ¯lters, phase shifters, and true time delay lines. The shunt capacitive structure has been employed in most of the conventional MEMS varactor designs because of its simplicity. However, the capacitance ratio of this conventional shunt capacitive MEMS varactor is limited to 1.5 because of the MEMS Pull- In e®ect, which happens when the de°ection between the MEMS top and bottom metal plates increase beyond 1/3 of the airgap between the two metal plates. At that time, the top metal plate will quickly snap down. This e®ect is the major limitation in MEMS varactor designs and can cause nonlinearity and mechanically instability. In order to eliminate this Pull-In e®ect, the author employed the so-called MEMS extended tuning range structure. This structure utilizes a variable height top metal beam with separate actuation parts. The airgap between the center part of the top beam and the bottom plate has been designed to be less than 1/3 of the airgap between the top beam and the bottom actuation pads. When DC bias is applied to the actuation parts, the entire top beam will move down together. Consequently, before the Pull-In e®ect happens at the actuation parts, the center part has already traveled through its entire tuning range, which means that the capacitive ratio of this kind of MEMS varactor can go to in¯nity. A fabrication process employing a GaAs substrate has been designed based on surface micro- machining technology. The maximum capacitance ratio of the designed MEMS extended tuning range varactor is 5.39 with a Cmax value of 167 fF. Based on this MEMS varactor design, a Ka-band MEMS varactor based distributed true time delay line has been designed. This dis- tributed true time delay line includes a high impedance CPW transmission line with 70­ un- loaded impedance at 28 GHz and eight MEMS extended tuning range varactors based on the varactor design periodically loaded on the CPW line. The testing results show that a 56± phase delay variation has been achieved at 28 GHz. The measured insertion loss at 28 GHz is ¡1:07 dB at the up-state and ¡2:36 dB at the down-state. The measured return losses, S11 and S22, are both below ¡15 dB at 28 GHz and below ¡10 dB over the entire tested frequency range of 5 GHz to 40 GHz.
1. INTRODUCTION
MEMS varactors are one of the important passive MEMS devices. They have considerable ad- vantages compared with other semiconductor devices, including low loss, very high Q at mm-wave frequencies, high power handling capability, low power consumption, and high IIP3. The RF MEMS varactor can be employed in a phase shifter or true time delay line design to replace the GaAs Schottky varactor diode for low-loss, broadband, and high frequency applications in mod- ern communication, automotive and defense applications. It can also be used in low loss tunable circuits including matching networks, tunable ¯lters, and low noise oscillators.
2. RF MEMS EXTENDED TUNING RANGE VARACTOR
Conventional RF-MEMS varactors usually employ a shunt parallel plate capacitor whose capac- itance is determined by the spacing between a ¯xed bottom plate and a movable suspended top plate. Electrostatic actuation occurs when an electrostatic force is created by applying a DC voltage between the capacitor plates, thereby displacing the movable plate toward the ¯xed plate. How- ever, this shunt capacitance MEMS varactor structure su®ers from the so-called Pull-In e®ect [1]. It happens when the displacement between the two metal plates exceeds 1/3 of the entire air- gap. At that moment, the electrostatic attraction force loses balance with the mechanical restoring force and that causes the two metal plates to quickly snap into contact. The Pull-In e®ect is the major limitation in MEMS varactor designs.


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