This review contains a comparative study of reported fabrication techniques of gallium based liquid metal alloys embedded in elastomers such as polydimethylsiloxane or other rubbers as well as the primary challenges associated with their use. The eutectic gallium–indium binary alloy (EGaIn) and gallium–indium–tin ternary alloy (galinstan) are the most common non-toxic liquid metals in use today. Due to their deformability, non-toxicity and superior electrical conductivity, these alloys have become very popular among researchers for flexible and reconfigurable electronics applications. All the available manufacturing techniques have been grouped into four major classes. Among them, casting by needle injection is the most widely used technique as it is capable of producing features as small as 150 nm width by high-pressure infiltration. One particular fabrication challenge with gallium based liquid metals is that an oxide skin is rapidly formed on the entire exposed surface. This oxide skin increases wettability on many surfaces, which is excellent for keeping patterned metal in position, but is a drawback in applications like reconfigurable circuits, where the position of liquid metal needs to be altered and controlled accurately. The major challenges involved in many applications of liquid metal alloys have also been discussed thoroughly in this article.

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Fabrication methods and applications of microstructured gallium based liquid metal alloys

A tunable active inductor is presented where the novel topology enables both the inductance and series resistance to be varied. With a discrete MMIC realization of this active inductor, Q-factors in excess of 15 000 have been measured over a wide range of inductance values. Applications for these active inductors include high-performance narrow-band filters, voltage controlled oscillators, and analog phase shifters. Analytical equations for the novel active inductor and a 3-resonator filter are given. The measured performance for a monolithic 2 GHz filter achieves a mean insertion loss of 0.9 dB, passband ripple of ±0.7 dB, with a 3 dB bandwidth of 70 MHz, and an excellent out-of-band rejection which exceeds 50 dB up to 18 GHz

Monolithic narrow-band filter using ultrahigh-Q tunable active inductors

Inductors are primary elements in many radio frequency circuitries and devices. This review gives a comprehensive survey on the developments and performances of fixed and variable RF MEMS inductors. MEMS inductors are the core of this review due to their high-yield performances and the wide choices of possible tuning techniques. First, the factors that constrain high-performance micro coils and the conventional solutions to overcome them are highlighted. Next, this paper systematically reviews varieties of MEMS inductors starting with the fixed inductors and according to inductors' topologies and performance enhancement techniques. In addition, the variable types of inductors are subsequently discussed, mainly from the point of view of their tuning techniques. Many quantitative comparisons are given in terms of the values of quality factor, inductance, self-resonance frequency, and tuning range. This will provide readers with an overall evaluation of different studies and assist them in choosing the inductors' topologies and layouts that best suit their applications. This review also sheds light on different and most common RF MEMS inductors applications, including RF communication devices and wireless sensors and actuators applications. Finally, this paper summarizes the best fabrication and tuning approaches inferred from the reviewed works and discusses some design guidelines to achieve better inductor performance.

RF MEMS Inductors and Their Applications--A Review

This paper reports a micropatterned variable inductor that utilizes ferrofluid as the movable magnetic core. Ferrofluid is displaced over the planar inductor using the magnetic field provided by another “actuation” coil aligned to the inductor, varying permeability distribution on the inductor to control its inductance. Superimposing a bias field on the varying magnetic field uniquely enables repelling of the fluid from the inductor, as well as switching between repelling and attracting motions simply by changing the direction of the driving current to the actuation coil. Microfabricated devices are tested to reveal that the repelling-mode operation provides 6.1 × larger inductive changes than those available with the attracting mode. The continuous modulation of inductance is demonstrated with a tuning range of 16% using input currents up to 1.2 A. Frequency tuning using a resonant tank formed with the device is also demonstrated to show a 76-ppm/mA frequency sensitivity to the input current.

Planar Variable Inductor Controlled by Ferrofluid Actuation

This special issue presents devices and the results of a tunable microwave microsystem associating RF circuits and microfluidic components. A channel is buried inside the substrate of a microstrip waveguide. This channel is located beneath a resonant stub. With this configuration a microfluidic passive tunable filter can be fabricated. Dielectric fluids are used to disrupt the electric field in a microstrip structure and thus modify the effective permittivity of the substrate. In this work, a notch filter is realized with an open-ended quarter-wavelength stub placed on top of a hollow SU-8 structure. This structure offers two advantages: channels can easily be fabricated and a reduction of SU8 losses. The filter shows a good performance; the initial cut-off frequency of 25 GHz shifts more than 20% when deionized water is used in fluidic channels. And the shape of RF function is kept throughout the range.

https://iopscience.iop.org/article/10.1088/0960-1317/22/7/074005/pdf

Development of a microsystem based on a microfluidic network to tune and reconfigure RF circuits

We propose a new type of on-chip micro-variable inductor fabricated by using microelectronic technology and lamination process. The proposed inductor is a dual circular coil and has an inductance of few nH. The fundamental idea is to place a liquid droplet between the metal turns of a coil in order to modify the capacitive/resistive coupling between metal tracks and hence to change the stored magnetic energy. In this study salt water has been used to fill partly or totally the channels constituting these three-dimensional coils. The numerical modeling allows us to obtain inductance values according to the liquid position, which can rise from zero to 100% of the total length of channels. The SU-8 resin was used to realize the microfluidic channels and Au as metallic tracks. The measured electrical characteristics show that these devices can be used up to 1.6 GHz frequency. The relationship between the inductance and the liquid position indicates that the tuning range of the inductance is approximately 107% (from 5.4 to 2.6 nH at a frequency of 1.6 GHz). Without liquid, the peak quality factor is 12, and the self-resonance frequency is 4 GHz; when the channels are completely full, these parameters become respectively 6 and 4.3 GHz.

https://iopscience.iop.org/article/10.1088/0960-1317/21/2/025018/pdf

Fabrication and evaluation of an on-chip liquid micro-variable inductor

A novel variable radio-frequency (RF) microelectromechanical system (MEMS) inductor using micro-pump actuator is presented, which inductance is of- nH for GHz applicationsThe variable inductor has micro-pump fluid conductor as mercury and a spiral inductor fabricated on a Si CMOS chip Micropump activates the fluid displacement by using passive microvalves The fluid moves between inter-spires distance and shortening the path length of the current through the structure; leading to reduction of the stored magnetic energy, and hence the inductance To reduce the substrate loss and thus increase the quality factor of the inductor, the silicon underneath spiral inductor is etched away Electrical performance of variable inductor is analyzed using a 3D electromagnetic simulation tool (Ansoft) At 83 GHz, the simulated inductor is continuously varied from 222 nH to 088 nH, ie, the variable range is above 100% The proposed variable inductor is perspective key component for the multi-band RF circuits such as electrically controllable matching circuits and wide tuning range voltage controlled oscillator (VCO)

Wide-range RF MEMS variable inductor using micro pump actuator

This letter presents the fabrication and the modeling of a continuously tuned microfluidic capacitor. On one hand its electrodes are realized by classical electrodeposition techniques and on the other hand, the lamination of SU-8 films allows superposing some microfluidic channels. According to the experimental results, the capacitor value increases continuously following the deionized (DI) water penetration in microchannels. The capacitance variations are comprised between Cmin = 0.52 pF and Cmax = 18.5 pF, allowing a wide tuning range that reaches 3460% at 500 MHz. The quality factor decreases from Qmax = 69 when the capacitor is empty to Qmin = 5.3 when it is fully filled with DI water. We have also investigated the theoretical aspects of our device by modeling the electric field and the current distributions inside the channels: when they are entirely filled with the DI water, the electric field is cancelled.

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Fabrication and modeling of a capacitor microfluidically tuned by water

The effect of a dielectric liquid on the tunability of capacitor operating in RF domains is evaluated. The RF measurement shows a high variation of the resonant frequency accompanied with a low insertion loss. Moreover, the fluid positions between electrodes modify the capacitance value up to Tr = 6660% at 600 MHz. The quality factor decreases in response of water filling from Q max = 51.9 when it is empty to Q min = 1.49 when it is fully filled. According to the finite-element method analysis, the change of the dielectric permittivity influences the capacitor performances. Essentially, the tuning range of the capacitance and the quality factor could reach, respectively: Tr = 7660% and Q min = 35.

https://hal.laas.fr/hal-01415341/document

M.A. Kallala

Papers

Fabrication and evaluation of an on-chip liquid micro-variable inductor

Wide-range RF MEMS variable inductor using micro pump actuator

Fabrication and modeling of a capacitor microfluidically tuned by water

Tunable MEMS capacitor: influence of fluids

Liquid RF MEMS variable inductor