A Tunable Ferrofluid-based Polydimethylsiloxane (PDMS) Microchannel Inductor for Ultra High Frequency Applications

Abstract

In this work, a tunable ferrofluid-based polydimethylsiloxane (PDMS) microchannel inductor with high quality factor and high tuning range is proposed. For this project, PDMS is used to create a microchannel with a width and height of 0.53 mm and 0.2 mm respectively. The microchannel is then used to cover the whole design of a solenoid inductor. A solenoid inductor is designed using wire bonding technique where lines of copper and bond wires are used to form a solenoid winding on top of silicon substrate. A light hydrocarbon based ferrofluid EMG 901 660 mT with high permeability of 5.4 is used. The ferrofluid-based liquid is injected into the channel to enhance the performance of a quality factor. A 3D full-wave electromagnetic fields tool, ANSYS HFSS is used in this work to simulate the solenoid inductor. The results obtained in this work gives a quality factor of more than 10 at a frequency range of 300 MHz to 3.3 GHz (Ultra High Frequency range). The highest quality factor is 37 which occurs at a frequency of 1.5 GHz, provides a high tuning range of 112%. Keyword:Ferrofluid INTRODUCTIONRecently, researches on a tunable MEMS inductors are widely found in literatures. A tunable MEMS inductor provides a broad range of functions especially in RF and microwaves device applications such as radio frequency (RF) power amplifiers, low noise amplifier (LNA) and voltage-controlled oscillator (VCO) [1]. Commonly, tuning configuration of the inductors can be divided into discretes and continuous variable inductors. Both approaches have different functionality, for example, a discrete tunable inductor is often developed using microrelays or microswitches which are used for controlling the length of the inductor coils [2], [3]. For continuous tunable inductor, a method of displacing the magnetic cores of the solenoid inductor is done in order to have a variation of inductance values [3]. Both methods are seen to have a better robustness and design flexibility compare to a tunable capacitor. For that reason, current researches are focusing on tunable inductor for Ultra High Frequency (UHF) applications.However, the design of tunable inductor(s) is hard to be realized especially at giga-hertz (GHz) range [4]. The difficulty is caused by a relatively smaller size of inductor is required at such frequency range. Plus, with the requirement of high quality factor and good tuning range makes it harder for the design to be realized especially on silicon substrate. On top of that, minituarization of inductor is more difficult due to an ohmic losses and eddy-current losses in a metal traces. Other than that, skin effect also plays its part at high frequency. Most of the current are having a tendency to flow at the outer surface of the conductor. This effect