Jooncheol Kim

Nanolaminated Permalloy Core for High-Flux, High-Frequency Ultracompact Power Conversion

Metallic magnetic materials have desirable magnetic properties, including high permeability, and high saturation flux density, when compared with their ferrite counterparts. However, eddy-current losses preclude their use in many switching converter applications, due to the challenge of simultaneously achieving sufficiently thin laminations such that eddy currents are suppressed (e.g., 500 nm-1 μm for megahertz frequencies), while simultaneously achieving overall core thicknesses such that substantial power can be handled. A CMOS-compatible fabrication process based on robot-assisted sequential electrodeposition followed by selective chemical etching has been developed for the realization of a core of substantial overall thickness (tens to hundreds of micrometers) comprised of multiple, stacked permalloy (Ni80Fe20) nanolaminations. Tests of toroidal inductors with nanolaminated cores showed negligible eddy-current loss relative to total core loss even at a peak flux density of 0.5 T in the megahertz frequency range. To illustrate the use of these cores, a buck power converter topology is implemented with switching frequencies of 1-2 MHz. Power conversion efficiency greater than 85% with peak operating flux density of 0.3-0.5 T in the core and converter output power level exceeding 5 W was achieved.

Microfabrication of toroidal inductors integrated with nanolaminated ferromagnetic metallic cores

We report microfabricated toroidal inductors with nanolaminated ferromagnetic metallic cores for chip-scale, high-power switching converters. The fabrication process of the toroidal inductor is based on individual manufacturing of partial windings (i.e. bottom and vertical conductors) and nanolaminated magnetic core, and integrating them by means of a drop-in approach. The nanolaminated ferromagnetic metallic cores presented in this paper consist of many multilayers of electrodeposited CoNiFe films, each layer with sub-micron thickness, with a total core thickness exceeding tens of microns. The beneficial magnetic properties (i.e. high saturation flux density and low coercivity) of CoNiFe alloys are well suited for chip-scale inductors as they achieve both large energy storage capacity as well as minimized volumetric core losses at high operating frequencies due to their nanolaminated structure. A drop-in integration approach, introduced to combine the microfabricated toroidal inductor windings with the magnetic cores, allows ease of integration. An advantage of this hybrid approach over monolithic fabrication in this application is the potential use of a wide variety of core materials, both microfabricated and bulk-fabricated, and which may or may not ultimately be CMOS-compatible. Exploiting this drop-in approach, 30-turn- and 50-turn-toroidal inductors integrated with nanolaminated CoNiFe cores, having 10 mm outer diameter and 1 mm thickness, have been successfully developed. Both types of inductors exhibit inductances higher than 1 µH at frequencies up to tens of MHz, showing ten times the inductance of an air core device with the same nominal geometry. The peak quality factor of the 30-turn-toroidal inductor reaches 18 at 1 MHz.

Monolithically-fabricated laminated inductors with electrodeposited silver windings

This paper presents batch microfabrication and experimental characterization of solenoid inductors with electrodeposited silver windings and laminated core. To enable high-frequency operation, the metallic magnetic core consists of multiple air-insulated, electroplated micron-thick permalloy (Ni80Fe20) laminations, with total core thicknesses up to tens of microns. The core is achieved by sequential electrodeposition of permalloy and copper, followed by selective removal of the copper, thereby releasing the core laminations. Electroplated silver is selected as winding material for its low resistivity and to withstand the copper etching required during core release. Release of the core laminations is the final inductor fabrication step, reducing potential process-induced damage to the core. Two inductors sharing one laminated core in a transformer geometry are fabricated, and exhibit inductances of approximately 100 nH at 5 MHz, resulting in an inductance density of 29 nH/mm2. This process demonstrates processing compatibility of using silver windings with highly-laminated, fully-integrated magnetics.

A MEMS lamination technology based on sequential multilayer electrodeposition

A MEMS lamination technology based on sequential multilayer electrodeposition is presented. The process comprises three main steps: (1) automated sequential electrodeposition of permalloy (Ni80Fe20) structural and copper sacrificial layers to form multilayer structures of significant total thickness; (2) fabrication of polymeric anchor structures through the thickness of the multilayer structures and (3) selective removal of copper. The resulting structure is a set of air-insulated permalloy laminations, the separation of which is sustained by insulating polymeric anchor structures. Individual laminations have precisely controllable thicknesses ranging from 500 nm to 5 µm, and each lamination layer is electrically isolated from adjacent layers by narrow air gaps of similar scale. In addition to air, interlamination insulators based on polymers are investigated. Interlamination air gaps with very high aspect ratio (>1:100) can be filled with polyvinylalcohol and polydimethylsiloxane. The laminated structures are characterized using scanning electron microscopy and atomic force microscopy to directly examine properties such as the roughness and the thickness uniformity of the layers. In addition, the quality of the electrical insulation between the laminations is evaluated by quantifying the eddy current within the sample as a function of frequency. Fabricated laminations are comprised of uniform, smooth (surface roughness <100 nm) layers with effective electrical insulation for all layer thicknesses and insulator approaches studied. Such highly laminated structures have potential uses ranging from energy conversion to applications where composite materials with highly anisotropic mechanical or thermal properties are required.

Nanopatterned Surfaces Based on Template-Assisted Multilayer Electrodeposition

Selective, template-assisted growth of electrodeposited, layered materials leads to the top-down designable realization of nanopatterned surfaces with a large surface area (>1 cm(2)) comprised of multi-dimensional, multiscale (10 nm-1 μm) features, without the need of standard nanolithography. This process opens a manufacturable route to functional nanodevices that rely on anisotropic, nanoscale surface structures with controlled dimensions.

Highly Laminated Soft Magnetic Electroplated CoNiFe Thick Films

The fabrication and characterization of highly laminated (~40 layers), thick (~40 μm) films of magnetically soft cobalt-nickel-iron are presented. Thick film fabrication is based on automated sequential electrodeposition of alternating CoNiFe and copper layers, followed by selective copper removal. The film, comprised tens of 1 μm thick laminations, exhibits saturation flux density of 1.8 T and coercivity of approximately 1.3 Oe. High-frequency film characterization took place in a 36-turn test inductor, which demonstrated constant inductance of 1.6 μH up to 10 MHz, indicating suppressed eddy-current loss. Quality factor exceeding 40 at 1 MHz, surpassing the performance of similarly fabricated Permalloy (Ni80Fe20) films.

Electrodeposited Nanolaminated CoNiFe Cores for Ultracompact DC–DC Power Conversion

Laminated metallic alloy cores (i.e., alternating layers of thin film metallic alloy and insulating material) of appropriate lamination thickness enable suppression of eddy current losses at high frequencies. Magnetic cores comprised of many such laminations yield substantial overall magnetic volume, thereby enabling high-power operation. Previously, we reported nanolaminated permalloy (Ni80Fe20) cores based on a sequential electrodeposition technique, demonstrating negligible eddy current losses at peak flux densities up to 0.5 T and operating at megahertz frequencies. This paper demonstrates improved performance of nanolaminated cores comprising tens to hundreds of layers of 300-500-nm-thick CoNiFe films that exhibit superior magnetic properties (e.g., higher saturation flux density and lower coercivity) than permalloy. Nanolaminated CoNiFe cores can be operated up to a peak flux density of 0.9 T, demonstrating improved power handling capacity and exhibiting 30% reduced volumetric core loss, attributed to lowered hysteresis losses compared to the nanolaminated permalloy core of the same geometry. Operating these cores in a buck dc-dc power converter at a switching frequency of 1 MHz, the nanolaminated CoNiFe cores achieved a conversion efficiency exceeding 90% at output power levels up to 7 W, compared to an achieved permalloy core conversion efficiency below 86% at 6 W.

Hypodermic-needle-like hollow polymer microneedle array using UV lithography into micromolds

This paper presents a polymer hollow microneedle array for transdermal drug delivery that is fabricated using UV photolithography and a single-step micromolding technique. This fabrication process patterns a 6×6 array of 1mm tall high-aspect-ratio hollow microneedles with sharp beveled tips and 150µm diameter side-opened lumens. The geometry of the beveled tip and the position of the lumen are defined simultaneously by a two-dimensional lithography mask pattern and the topography of the micromold. This three-dimensional geometry improves insertion performance and potentially the drug delivery efficiency without additional fabrication processes. These hypodermic-needle-like microneedles have been successfully constructed, packaged, and tested for fluidic functionality and skin penetrability.

Surface tension-driven assembly of metallic nanosheets at the liquid-air interface: Application to highly laminated magnetic cores

This paper presents a fabrication technique to develop highly laminated structures comprising stacked thin films, in which the structures are based on surface tension-driven assembly at the liquid-air interface. When multiple metallic films are removed from a liquid solution, there is a surface tension-driven coalescence and self-alignment of the wetted films, resulting in thick metallic microstuctures comprised of many layers of metallic nanosheets after evaporation of the liquid. If the liquid contains a dissolved material, each sheet can further be coated with the material prior to assembly. Based on this technique, we developed laminated structures comprising hundreds of nanoscale layers of alternating metallic film and non-conducting polymer. Electroplated Co44Ni37Fe19 (Cobalt-Nickel-Iron alloy) and a commercial Novec 1700 solution (3M, Minnesota) were utilized for the metallic film and the liquid solution, respectively. However, the assembly process inherently allows a variety of materials to be exploited. Theoretical analysis and experimental results were compared, demonstrating a critical gap between the metallic films, below which capillary force is sufficient for driving self-assembly of the films. As an exemplary application of this technique, highly laminated magnetic cores comprising 600 layers of 500 nm thick CoNiFe that are insulated by 100 nm thick polymer were prepared. A 15-turn toroidal inductor with the fabricated magnetic core exhibited a constant inductance of 2.5 μH up to 30 MHz with a quality factor over 70 at 1 MHz.

Anisotropic nanolaminated CoNiFe cores integrated into microinductors for high-frequency dc–dc power conversion

This paper presents a rectangular, anisotropic nanolaminated CoNiFe core that possesses a magnetically hard axis in the long geometric axis direction. Previously, we have developed nanolaminated cores comprising tens to hundreds of layers of 300–1000 nm thick metallic alloys (i.e. Ni80Fe20 or Co44Ni37Fe19) based on sequential electrodeposition, demonstrating suppressed eddy-current losses at MHz frequencies. In this work, magnetic anisotropy was induced to the nanolaminated CoNiFe cores by applying an external magnetic field (50–100 mT) during CoNiFe film electrodeposition. The fabricated cores comprised tens to hundreds of layers of 500–1000 nm thick CoNiFe laminations that have the hard-axis magnetic property. Packaged in a 22-turn solenoid test inductor, the anisotropic core showed 10% increased effective permeability and 25% reduced core power losses at MHz operation frequency, compared to an isotropic core of the identical geometry. Operating the anisotropic nanolaminated CoNiFe core in a step-down dc–dc converter (15 V input to 5 V output) demonstrated 81% converter efficiency at a switching frequency of 1.1 MHz and output power of 6.5 W. A solenoid microinductor with microfabricated windings integrated with the anisotropic nanolaminated CoNiFe core was fabricated, demonstrating a constant inductance of 600 nH up to 10 MHz and peak quality factor exceeding 20 at 4 MHz. The performance of the microinductor with the anisotropic nanolaminated CoNiFe core is compared with other previously reported microinductors.