MEMS inductor fabrication and emerging applications in power electronics and neurotechnologies

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.

Fabrication of 3D air-core MEMS inductors for very-highfrequency power conversions

We report a fabrication technology for 3D air-core inductors for small footprint and very-high-frequency power conversions.Our process is scalable and highly generic for fabricating inductors with a wide range of geometries and core shapes.We demonstrate spiral,solenoid,and toroidal inductors,a toroidal transformer and inductor with advanced geometries that cannot be produced by wire winding technology.The inductors are embedded in a silicon substrate and consist of through-silicon vias and suspended windings.The inductors fabricated with 20 and 25 turns and 280-350μm heights on 4-16 mm2 footprints have an inductance from 34.2 to 44.6 nH and a quality factor from 10 to 13 at frequencies ranging from 30 to 72 MHz.The air-core inductors show threefold lower parasitic capacitance and up to a 140% higher-quality factor and a 230% higher-operation frequency than silicon-core inductors.A 33 MHz boost converter mounted with an air-core toroidal inductor achieves an efficiency of 68.2%,which is better than converters mounted with a Si-core inductor(64.1%).Our inductors show good thermal cycling stability,and they are mechanically stable after vibration and 2-m-drop tests.