Digitally controllable variable high-Q MEMS capacitor for RF applications

Abstract

This paper describes the novel design of an electrostatic digitally controllable variable MEMS capacitor constructed using Cronos MUMPS technology and flipchip technology processing. The capacitor consists of an array of individual plates of equal area, which are connected to the bonding pads by springs of varying width. This creates a cascading snap-down effect when actuated by electrostatic forces. The capacitor has a measured Qfactor of 140 at 750MHz, and a tuning ratio of 4 1 .other digitally controlled capacitors have a very large tuning range [3], the eight individual MEMS capacitors used in that design increase the step size in the capacitance vs. voltage variation. In our new design the repeatability of capacitance vs. voltage is greatly improved as well as the reliability of the capacitor. In addition, electrostatic actuation results in higher switching speed and greater deflection stability than the thermally actuated variable capacitor [2].

MEMS variable-capacitor phase shifters part I: Loaded-line phase shifter

“…Q values of 104 and 145 at about 745 MHz have been reported [16]. The tuning ratio of the capacitor is 4:1 [6].…”

“…A digitally-controllable variable capacitor has been reported in [6]. This capacitor is mounted and measured in a TEM-mode coax-rectangular resonator [16].…”

“…The phase shifter design reported here makes use of a unique design for MEMS variable capacitors. Digital-type MEMS variable capacitors were developed previously at the University of Colorado at Boulder [6] for operation at around 1 GHz. Since then, this capacitor design has been modified for phase shifter applications at 26.5 GHz.…”

A high-tuning-range MEMS variable capacitor using carrier beams

“…However, the latter suffers from low self-resonance. In [4] and [5], an array of parallel-plate capacitors with different beam sizes that demonstrates a tuning range of 370% has been proposed. A twomovable-plates MEMS variable capacitor with a nitride layer between the two plates has been proposed in [1].…”

Vertically aligned carbon based varactors

“…The advancement in micromachining technologies in the last twenty years has introduced the new class of microelectromechanical (MEMS) varactors. 35,36 However, the development of MEMS varactors has not progressed at the pace of other MEMS devices, which can be explained by the abundance of discrete high quality factor silicon and GaAs varactors up to 30 GHz. 37 But there is still a pressing need for electromechanical varactors including the novel NEMS varactors: they have the potential of very high quality factors, they can be designed to withstand large radio frequency (rf) voltage swings, they exhibit a symmetric capacitance-voltage (C-V) response, and they can be inexpensively produced on top of high-resistivity silicon substrates.…”