ResearchFields

A Low-Loss MEMS Tunable Capacitor with Movable Dielectric

RF tunable capacitors are key building blocks in many wireless communication applications, such as voltage controlled oscillators (VCO), tunable filters and phase shifters. The devices typically require a wide tuning range and high quality factor. Compared to semiconductor varactors, MEMS tunable capacitors have the potential for an extended tuning range, higher linearity and low loss. However, traditional MEMS capacitor designs clearly exhibit a Q factor versus actuation voltage trade-off: the Q of micromechanical tunable capacitor is limited by resistive losses in the suspension beams, which are often made narrow and long to make it soft for low voltage actuation. A method of moving electrically floating plate has been proposed to provide much better Q by eliminating the inevitable spring resistance in the RF signal path. However, the tuning ratio is relatively low (41%) because of the pull-in effect. In this paper, we introduce a novel MEMS tunable capacitor with floating plate, which has high tuning ratio of 367% and a Q of 56 at 1GHz.

Fig. 1 presents conceptual schematic view of the tunable capacitor design, identifying key components and RF signal path. The figure illustrates a movable electrically floating dielectric suspended beside the top and the bottom plates of a capacitor by mechanical springs. Unlike the traditional designs, in our prototype, the RF signal does not pass though springs. Thus, a better Q can be achieved by eliminating the high resistive loss in the spring. The gap between the dielectric and the plates can be adjusted by a DC voltage on drive combs, making the capacitor tunable in large range without pull-in effect.

Fig. 2 illustrates schematic views of the actual structure design with different test methods for comparison purpose. In Fig. 2(a), the RF signal flows through the floating dielectric from the top plates to the bottom plates; while in Fig. 2(b), the RF signal passes through spring from the top plates to the anchor, causing high loss in the spring.

The image of the whole device and a section of it taken under a Scanning Electron Microscope (SEM) are provided in Fig. 3. The tunable capacitor was measured using Agilent E4980A precision LCR meter. Fig. 4 shows the capacitance change vs. actuation voltage, indicating a 367% tuning ratio for floating dielectric case of Fig. 2 (a), and a 172% tuning ratio for the case of Fig. 2(b). The Q factor was calculated based on the measured capacitance and equivalent series resistance. The results show that the proposed capacitor with floating dielectric has a Q factor of 56 at 1GHz, while the capacitor with signal path in spring has a low Q factor of 0.35 at 1GHz, because of a high spring loss.