Development of a wide-tuning range and high Q variable capacitor using metal-based surface micromachining process

This study aims to improve the tuning range and quality-factor (Q) of micro variable capacitors for wireless communication applications. A suspending 0.5 μm-thick gold thin-plate with two-gap structure in one-to-three ratio of spacing is designed for the maximization of tuning range. To enhance effectively the flexural rigidity of top metal-plate and improve further the tuning range of the varactor, a double-cross-type microstructure with two vertical fixed-fixed beam springs and four horizontal fixed-guided cantilever beams is introduced. Besides, a glass substrate (Corning 7740) was used to reduce substantially the power dissipation and improve the Q-factor of variable capacitor. The new glass-based double-cross-type micro variable capacitor has demonstrated many superior performances, including the wide-tuning range (2100%, at 1.0 MHz with 6.0 V), the moderate capacitance (0.56 pF, at 2.4 GHz and without DC bias), 6.5 V pull-in voltage, and the high Q-factor (40.6, at 2.4 GHz). These characteristics approximately match with the theoretical derivation or simulated results from Agilent-ADS, Ansoft-HFSS, and IntelliSuite software.