The new trend towards minimally invasive millimeter-sized and free-floating distributed implants promises to enable emerging applications, such as chronic neural recording with minimal damage to the surrounding tissue. However, wireless power transmission (WPT) to these medical devices is quite challenging. The magnetic field produced by external transmitter (Tx) coils at the position of small implants can be considered homogeneous to separate the optimization of Tx and receiver (Rx) coils for efficient WPT. This paper focuses on the optimization of the solenoid-type Rx coils, which are suitable for this application. We have developed an analytical model of solenoid coils that includes the impact of tissue and coating around the coils, verified through simulations and measurements. Using the proposed model, under a given size restriction and a specific load, we find the optimal operating frequency and coil geometry to maximize a figure of merit (FoM) for the Rx that includes the loaded quality factor and its internal efficiency as well as a factor related to the coupling coefficient. For a millimeter-sized coil, the optimal operating frequency for the Rx and the number of turns are found to be 500 MHz and six, respectively, if the coil is closely wound using AWG36 copper wires. If the pitch is also optimized, then 700 MHz and four turns provide the best FoM for the solenoid Rx.

Analytical Modeling and Optimization of Small Solenoid Coils for Millimeter-Sized Biomedical Implants

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.

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MEMS inductor fabrication and emerging applications in power electronics and neurotechnologies.

Construction of cobalt phthalocyanine sensitized SnIn4S8/g-C3N4 composites with enhanced photocatalytic degradation and hydrogen production performance

Intel's microprocessors are powered by high-frequency fully-integrated switching regulators implemented on die. Current products use non-magnetic inductors designed in the package substrate. This article investigates the use of magnetic thin film inductors fabricated on silicon wafers as an alternative that improves conversion efficiency while achieving higher inductance per area density. This article builds on the earlier work done on thin film inductors at Intel, and explores the feasibility of using them in a high volume product. A number of different design variations are studied to look at tradeoffs between efficiency, saturation characteristics, transient response, and voltage ripple. This is accomplished through a combination of passive measurements using a vector network analyzer (VNA) and active measurements on a fully functioning integrated voltage regulator system.

Study of Thin-Film Magnetic Inductors Applied to Integrated Voltage Regulators

The emergence of smartphones and other smart systems is driving new trends in electronics scaling that goes beyond transistors or active devices, to include all the system components such as packaging substrates, passive components, thermal structures, power sources, and the system interconnections. Current system components are at milliscale, creating a $10^{3}$ to $10^{6}$ scaling gap with the packaging interfaces at microscale, and transistors at nanodimensions. With current microstructured materials, component miniaturization also degrades performance metrics such as efficiency, tolerance or precision, thermal and frequency stability. Nanostructured materials and processes can potentially miniaturize these system components, while simultaneously enhancing the performance. These nanostructured components are assembled close to the active devices, resulting in ultraminiaturized and ultrathin systems with 3-D integration of passives with actives. This paper shows the impact of nanostructured materials toward enhancing the performance and miniaturization of power and radio-frequency (RF) passive components in emerging smart systems. Opportunities for nanostructured materials in improving the power density and efficiency of capacitors and inductors in power-supply modules are reviewed in the first part of the paper. The impact of nanostructured magnetic, dielectric and magneto-dielectric films on emerging RF subsystems is illustrated in the last part of the paper.

System Scaling With Nanostructured Power and RF Components

Seeking light sources from Si-based materials with an emission wavelength meeting the requirements of optical telecommunication is a challenge nowadays. It was found that the subband emission centered near 1200 nm can be achieved in phosphorus-doped Si quantum dots/SiO2 multilayers. In this work, we propose the phosphorus/boron co-doping in Si quantum dots/SiO2 multilayers to enhance the subband light emission. By increasing the B co-doping ratio, the emission intensity is first increased and then decreased, while the strongest integrated emission intensity is almost two orders of magnitude stronger than that of P solely-doped sample. The enhanced subband light emission in co-doped samples can be attributed to the passivation of surface dangling bonds by B dopants. At high B co-doping ratios, the samples transfer to p-type and the subband light emission from phosphorus-related deep level is suppressed but the emission centered around 1400 nm is appeared.

Enhanced subband light emission from Si quantum dots/SiO2 multilayers via phosphorus and boron co-doping.

High-density and high-frequency inductors (>5 MHz) enable miniaturization of power modules, and also integration of voltage regulator modules into the processor packages for higher efficiency and switching frequencies Several advances in magnetic materials and substrate process integration are needed to meet the performance, size, and cost requirements This paper highlights two substrate-embedded inductor approaches using magnetic paste and pre-fabricated magnetic thick laminate sheets An innovative cavity-filling process using paste printing and hot pressing is developed to integrate magnetic paste with high magnetic filler loading into the core of standard organic laminate substrates The hot-pressed metal/polymer composite magnetic cores showed a permeability of ~55 at 10 MHz The other approach utilizes pre-fabricated thick magnetic sheets with a permeability of ~93 at 10 MHz Standard panel-scale processes are adapted for high-volume manufacturing at low cost The electrical performance of the inductors from both approaches showed good correlation between the simulated and measured data Compared with air-core inductors with the same structure, the advanced thin-film inductors with magnetic sheet showed nine times improvement in inductance Both these approaches can be scaled to higher frequencies with further innovation in magnetic composites

Substrate Embedded Thin-Film Inductors With Magnetic Cores for Integrated Voltage Regulators

This paper investigates the trade-offs in inductance density and quality factor of inductors with and without magnetic-cores through modeling, design, fabrication and model validation through characterization. Two type of inductors, one for power-supply and the other for EMI filters, are investigated. Parametric analysis was performed to study the enhancement in inductance density without compromising the current handling and efficiency. For EMI filter inductors, the inductance and quality factor (Q) were improved by 13X and 4X respectively with and without magnetic cores. For power inductors, the inductance and quality factor were improved by 5X and 1.6X respectively. The fabricated magnetic core inductors achieved 5X reduction in thickness, compared to the stage-of-the art air-core inductors which use multilayered and thick solenoid.

Magnetic Materials and Design Trade-Offs for High Inductance Density, High-Q and Low-Cost Power and EMI Filter Inductors

Integrated power conversion and delivery are the key to achieve high-efficiency and high-performance data processing. Power inductors with high power density and low dc resistance are the key enabler to realize miniaturized voltage regulators (VRs) with high efficiency. With the integrated voltage regulators (IVRs), interconnection length from the battery to the loads can be dramatically reduced, resulting in lower conduction loss and higher power efficiency. This article demonstrates substrates with embedded high-power-density solenoid inductors with an ultralow dc resistance for the IVRs. By incorporating advanced metal–polymer magnetic composites as the cores, the inductors show a high inductance density of 7 nH/mm2 with an ultralow dc resistance of 10 $\text{m}\Omega $ . With a low thickness of $ , the power inductors can be embedded into substrates and positioned close to the loads to realize the IVRs. A substrate-compatible panel-level process was developed to embed the magnetic-core inductors with high throughput and low cost. Compared with the air-core inductors, the magnetic core achieved four times improvement in inductance for the same dc resistance, indicating substantial footprint reduction and improvement in efficiency.

Substrate-Embedded Low-Resistance Solenoid Inductors for Integrated Voltage Regulators

This paper describes advances in integrated ultra-thin wireless power module components for Internet of Things (IoT) and wireless sensor applications. A typical wireless module integrates both an inductive link for wireless power transfer, and supercapacitor as a storage element. The performance of the inductive link is enhanced with innovative high-permeability and low-loss magnetic films for improved inductive coupling between the receiver and the transmitter. Up to 4X enhancement in power transfer efficiency is demonstrated with the innovative magnetic films. Energy storage is achieved through a thinfilm micro fabricated supercapacitor layer with interdigitated graphene-based planar electrodes and solid-state electrolytes for easier integration. The component properties demonstrated through this work are projected to achieve high power levels with ultra-thin form-factors.

Teng Sun

Papers

Enhanced subband light emission from Si quantum dots/SiO2 multilayers via phosphorus and boron co-doping.

Substrate Embedded Thin-Film Inductors With Magnetic Cores for Integrated Voltage Regulators

Magnetic Materials and Design Trade-Offs for High Inductance Density, High-Q and Low-Cost Power and EMI Filter Inductors

Substrate-Embedded Low-Resistance Solenoid Inductors for Integrated Voltage Regulators

Ultra-Thin Wireless Power Module with Integration of Wireless Inductive Link and Supercapacitors