MEMS variable capacitor with electrically floating plate

Abstract

In this work, a MEMS variable capacitor with a new actuation principle was proposed and demonstrated. Recently, MEMS variable capacitors have been actively researched for achieving higher $\mathcal{Q}$ -factor, tuning ratio and self-resonant frequency than those of the semiconductor on-chip varactors. Based upon a simple two-plate capacitance equation, known actuation principles to date can be classified into three categories: changing the dielectric constant (ε) in between two plates; tuning the gap (d) between two plates; and varying the overlap area (A) between two comb structures. Among them, the tunable-dielectric method showed higher $\mathcal{Q}$ than the others so far thanks to reducing series resistance by the elimination of mechanical spring in the RF signal path. The propose a new MEMS variable capacitor that resembles the tunable-gap one at first glance, but provides much better $\mathcal{Q}$ by eliminating the inevitable spring resistance in the RF signal path using a new actuation method of moving electrically floating plate. In the proposed variable capacitor, the DC bias and RF signal between two fixed bottom plates are applied in the proposed method. The E-field is confined between the top electrically-floating plate and two bottom plates, and then the E-field generates a net downward force because the system naturally tends to move toward increasing the capacitance of the system. In this way, better $\mathcal{Q}$ can be achieved by eliminating the mechanical spring in the RF signal path. Based on this concept, fundamental characteristics of proposed variable capacitor was measured and compared. First, the Crab-leg spring design shows the maximum tuning ratio of 41%, $\mathcal{Q}$ of 27.1 at 5GHz when the tuning bias was 0V (0.3 pF), and $\mathcal{Q}$ of 26.7 at 5GHz when the tuning bias was 5V (0.37 pF). Regardless of the applied biases, Q was almost the same at around 5GHz. Second, the T-type spring design shows maximum tuning ratio of 14...