MEMS inductor fabrication and emerging applications in power electronics and neurotechnologies

Abstract

MEMS inductors are used in a wide range of applications in micro- and nanotechnology, including RF MEMS, sensors, power electronics, and Bio-MEMS. Fabrication technologies set the boundary conditions for inductor design and their electrical and mechanical performance. This review provides a comprehensive overview of state-of-the-art MEMS technologies for inductor fabrication, presents recent advances in 3D additive fabrication technologies, and discusses the challenges and opportunities of MEMS inductors for two emerging applications, namely, integrated power electronics and neurotechnologies. Among the four top-down MEMS fabrication approaches, 3D surface micromachining and through-substrate-via (TSV) fabrication technology have been intensively studied to fabricate 3D inductors such as solenoid and toroid in-substrate TSV inductors. While 3D inductors are preferred for their high-quality factor, high power density, and low parasitic capacitance, in-substrate TSV inductors offer an additional unique advantage for 3D system integration and efficient thermal dissipation. These features make in-substrate TSV inductors promising to achieve the ultimate goal of monolithically integrated power converters. From another perspective, 3D bottom-up additive techniques such as ice lithography have great potential for fabricating inductors with geometries and specifications that are very challenging to achieve with established MEMS technologies. Finally, we discuss inspiring and emerging research opportunities for MEMS inductors.

Keywords: Engineering; Nanoscience and technology.

Fig. 1. Number of publications on MEMS inductor applications in the last 30 years.

Please see supplementary material for source data

Fig. 2. 3D illustrations of four categories of MEMS inductor.

Category (i): 2D on-substrate inductors such as spiral inductors (a) and racetrack inductors (b). Category (ii): 3D on-substrate solenoidal inductors with a magnetic bar core (c). Category (iii): 2D in-substrate spiral inductor (d). Category (iv): 3D in-substrate toroidal inductor (e). The directions of current flows are depicted by the yellow arrows

Fig. 3. Number of publications on MEMS inductor fabrication technologies over time.

Please see supplementary material for source data

Fig. 4. 2D micromachined spiral inductors.

Suspended windings and air gaps made by a removing a sacrificial layer (Copyright IEEE 2004) and b etching the underlying substrate (Copyright MDPI 2011). c 2D micromachined racetrack inductor with a magnetic thin-film core (Copyright IEEE 2013)

Fig. 5. 3D on-substrate inductors.

a 3D air-core solenoidal inductor with dome-shaped windings (Copyright IEEE 2018). b 3D magnetic-core solenoidal inductor with a profile of 150 µm (Copyright IEEE 2019). c 3D magnetic-core solenoidal inductor with a tall profile of 1 mm, fabricated by a high-aspect-ratio SU-8 process for vertical windings (Copyright IOP 2015)

Fig. 6. A TSV-based process for fabricating a 3D in-substrate toroidal inductor (Copyright Springer Nature 2018).

a Fabricated inductors, including air-core toroidal inductors, a toroidal transformer, a solenoid inductor, a spiral inductor, and an inductor with an arbitrary “DTU” core shape. b Four main steps for fabricating the toroidal inductor (Copyright IEEE 2018)

Fig. 7. Other processes.

SEM images of microinductors fabricated by a a self-rolled-up membrane (S-RUM) (Copyright Elsevier 2002) and b the surface micromachining and post release folding process (Copyright Springer Nature 2015). c Wire bonding (Copyright IOP 2010)

Fig. 8. Microcoils and nanocoils made by emerging 3D nanofabrication technologies.

SEM images of coil-like 3D microstructures: a Metallic 3D triple helix structure, printed with FluidFM (Copyright Wiley 2016). b 3D microcoil with a diameter of 200 μm, a height of 60 μm and five turns, printed using TPS (Copyright Leibert 2019). c 3D Moebius strip with a triangular cross section and individual wire dimensions of approximately 25 nm, fabricated by FEBID (Copyright AIP 2019)

Fig. 9. Ice lithography.

a Schematic diagram of the process of ice lithography pattering. Example of ice lithography patterning, where the lift-off process is used to melt the ice layer. b Patterning of the Ti/Au layer on an AFM tip. c An SEM image of a patterned Ti/Au wire on a microcantilever. d A TEM image of a three-bladed pattern of palladium (Pd) metal patterns on a free-standing Si₃N₂ membrane. e Patterned parallel lines on octane. f SEM images of a 3D pyramidal nanostructure, made of Ag layers, fabricated at room temperature using a water ice resist for patterning (Copyright ACS 2012, 2018)

Fig. 10. Overview of the timeline of notable developments of PwrSoC and PwrSiP using MEMS inductors.

Copyright IEEE 2000, 2011, 2011, 2013, 2015, 2016, 2019, 2018, 2019

Fig. 11. Characteristics of MEMS inductors that are used in power converters.

a Quality factor versus frequency. b Inductance density versus a figure of merit ($\mathrm{FOM}=\sqrt{Q_{\mathrm{DC}}\cdot Q_{\mathrm{AC}}}/V$)

Fig. 12. Magnetic brain stimulation techniques.

a Transcranial magnetic stimulation (TMS) (adapted from reference, Copyright Elsevier 2020): (i) The coil induces a strong magnetic field exciting or inhibiting cortical activity. (ii) A summary of different formulations of pulse patterns—different protocols can be implemented by pairing more pulses with various effects. (iii) Simulated area of activation by the TMS. b Silicon MEMS implant with a half-turn coil (adapted from reference, Copyright AAAS 2016): (i) The microfabricated microcoil implant. (ii) Schematic of the coil orientation over the apical dendrites of cortical pyramidal neurons: (top) perpendicular orientation, which resulted in very weak electric fields along the neuron and did not produce spiking, and (bottom) parallel orientation, which produced robust spiking. (iii) Stimulus waveforms composed of pulses that consisted of one full period of a 3-kHz sinusoid with an amplitude of 112 mV. (iv) Coils inserted into the whisker motor cortex (M1). Ten-hertz stimulation resulted in protraction of the whiskers (upward deflections) on the right side (top), whereas 100-Hz stimulation induced retraction (downward deflections) (bottom). (v) Mean amplitudes of the peak whisker movements for each stimulus condition

Fig. 13. Timeline of the discoveries and development in the field of micromagnetic neurostimulation.

Transcranial stimulation (Copyrights Elsevier 1985), minicoils, microinductor and MEMS inductor neurostimulation, Springer Nature 2012, AAAS 2016