RF MEMS Extended Tuning Range Varactor and Varactor Based True Time Delay Line Design

MEMS varactors are one of the important passive MEMS devices. Their applications include use in VCOs, tunable impedance matching networks, tunable filters, 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 PullIn effect, which happens when the deflection 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 effect is the major limitation in MEMS varactor designs and can cause nonlinearity and mechanically instability. In order to eliminate this Pull-In effect, 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 effect 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 infinity. A fabrication process employing a GaAs substrate has been designed based on surface micromachining 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 distributed true time delay line includes a high impedance CPW transmission line with 70 Ω unloaded 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 28GHz and below −10 dB over the entire tested frequency range of 5GHz to 40 GHz.