Jungkwun Kim

A Technology Overview of the PowerChip Development Program

The PowerChip research program is developing technologies to radically improve the size, integration, and performance of power electronics operating at up to grid-scale voltages (e.g., up to 200V) and low-to-moderate power levels (e.g., up to 50W) and demonstrating the technologies in a high-efficiency light-emitting diode driver, as an example application. This paper presents an overview of the program and of the progress toward meeting the program goals. Key program aspects and progress in advanced nitride power devices and device reliability, integrated high-frequency magnetics and magnetic materials, and high-frequency converter architectures are summarized.

Microfabrication of air core power inductors with metal-encapsulated polymer vias

This paper reports three-dimensional (3-D) microfabricated toroidal inductors intended for power electronics applications. A key fabrication advance is the exploitation of thick metal encapsulation of polymer pillars to form a vertical via interconnections. The radial conductors of the toroidal inductor are formed by conventional plating-through-mold techniques, while the vertical windings (up to 650 µm in height) are formed by polymer cores with metal plated on their external surfaces. This encapsulated polymer approach not only significantly reduces the required plating time but also exploits the relative ease of fabricating high-aspect-ratio SU-8 pillars. To form the top radial conductors, non-photopatternable SU-8 is introduced as a thick sacrificial layer. Two toroidal inductor geometries were fabricated and tested. The first inductor had an inner diameter of 2 mm, an outer diameter of 6 mm, 25 turns and a vertical via height of 650 µm. The second inductor had an inner diameter of 4 mm, an outer diameter of 8 mm, 50 turns and a vertical via height of 650 µm. Both inductor geometries were successfully fabricated and characterized in the frequency range of 0.1−100 MHz. Characterization results of the 25- and 50-turn inductors showed an average inductance of 76 and 200 nH, a low frequency (0.1 MHz) resistance of 0.2 and 1 Ω and a quality factor of 35 and 24 at 100 MHz, respectively. Finite-element simulations of the inductors were performed and agreed with the measured results to within 8%. The turn-to-turn breakdown voltage was measured to be in excess of 800 V and currents as high as 0.5 A could be successfully carried by the inductor windings.

Silicon-embedded 3D toroidal air-core inductor with through-wafer interconnect for on-chip integration

This paper presents a CMOS-compatible process for fabrication of 3D structures embedded in the volume of a silicon wafer, and capable of interconnection to circuitry on the wafer surface. The key challenge of embedding structures in the silicon substrate is processing inside deep silicon trenches. This difficulty is overcome by means of several key techniques: multilevel wafer etching; cavity shaping; fine proximity lithography at the bottom of trenches; and laminated dry-film lithography on complex 3D structures. As a technology demonstration, a topologically complex 3D toroidal inductor is fabricated in a deep silicon trench, and is coupled to the wafer surface with high-power, electroplated through-wafer interconnect. Inductors fabricated in these trenches achieved an overall inductance of 60 nH, dc resistance of 399 MΩ, and quality factor of 17.5 at 70 MHz.

Computer-controlled dynamic mode multidirectional UV lithography for 3D microfabrication

Computer-controlled dynamic mode multidirectional ultraviolet (UV) lithography has been demonstrated using a collimated UV light source, a substrate-holding stage equipped with two stepper motors (one for tilting and the other for rotation), a controller with programming software and a laptop computer. The tilting and rotational angles of the stage in motion are accurately controlled during UV exposure as programmed by the user to produce complex three-dimensional (3D) microstructures. Process parameters include the initial and final tilting and rotational angles of the stage, and the relative angular velocities of the two motors in addition to the normal fabrication process parameters of UV lithography such as optical dose, baking time, and developing time and condition. Symmetric patterns can be generated by a simple synchronous mode dynamic operation, where both the angular velocities of the tilting motion and the rotating motion are set equal or harmonically related. More complex and non-symmetric patterns can be obtained using a piecewise synchronous mode, where the relationship between the angular velocities of the two motors is described not with a single coefficient but with a set of coefficients. 3D structures fabricated from the synchronous mode operation include the four-leaf clover horn and the cardiac horn while the ones from the piecewise synchronous mode are a vertical triangular slab, a screwed wind vane and arbitrary shape horns. Ray trace simulation has been performed using a mathematical tool in a spherical coordinate system and the simulated 3D patterns show good agreement with the fabricated ones.

Size- and composition-dependent radio frequency magnetic permeability of iron oxide nanocrystals.

We investigate the size- and composition-dependent ac magnetic permeability of superparamagnetic iron oxide nanocrystals for radio frequency (RF) applications. The nanocrystals are obtained through high-temperature decomposition synthesis, and their stoichiometry is determined by Mössbauer spectroscopy. Two sets of oxides are studied: (a) as-synthesized magnetite-rich and (b) aged maghemite nanocrystals. All nanocrystalline samples are confirmed to be in the superparamagnetic state at room temperature by SQUID magnetometry. Through the one-turn inductor method, the ac magnetic properties of the nanocrystalline oxides are characterized. In magnetite-rich iron oxide nanocrystals, size-dependent magnetic permeability is not observed, while maghemite iron oxide nanocrystals show clear size dependence. The inductance, resistance, and quality factor of hand-wound inductors with a superparamagnetic composite core are measured. The superparamagnetic nanocrystals are successfully embedded into hand-wound inductors to function as inductor cores.

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.

Automated dynamic mode multidirectional UV lithography for complex 3-D microstructures

Automated multidirectional UV exposure has been demonstrated using a collimated UV source and a movable stage equipped with two computer controlled motors and a microcontroller for complex three dimensional (3-D) microstructures. The stage is tilted and rotated using independently controlled motors during UV exposure as programmed by the user. Various dynamic operations with the synchronized angular velocities of tilting and rotation have been performed using an SU-8 substrate and a reverse-side exposure scheme. Fabricated structures using the dynamic mode include a vertical reverse triangular slab, a quadruple reverse triangular slab, a cardiac horn, screwed wind vane shapes with double blades and quadruple blades. Ray tracing simulation using mathematical equations has been performed to analytically confirm the shapes of resultant structures.

Lithographically defined integrable air-lifted bow-tie antennas

A vertically placed monopole type bow-tie antenna has been fabricated using dynamic mode multidirectional ultraviolet (UV) lithography and polymer metallization on a non-transparent printing wiring board. A photopatternable liquid state polyurethane is utilized to define a bow-tie antenna backbone with a height of 4.5mm, a top portion width of 3mm, a bottom portion width of 800µm, and a flare angle of 30°, which allows a resonant radiation frequency of 12 GHz. The printing wiring board with a thickness of 508 µm has a drilled hole with a diameter of 800 µm through which backside tilting exposure has been performed to define the vertical bow-tie slab. Conductor backed coplanar waveguide (CB-CPW) has been used for feeding the air-lifted bow-tie antenna. The measurement results show a resonant radiation frequency of 12.34GHz and a 10dB-bandwidth of 7%. It shows an omnidirectional far-field radiation pattern based on simulation. Different shapes of air-lifted backbone for 12GHz resonant radiation has been fabricated and the performance has been simulated using a high frequency structure simulator tool.

A compact 5GHz WLAN notched bluetooth/UWB antenna

Among short-range wireless communication systems, Ultra-wideband (UWB) technology, with a frequency allocation of 3.1–10.6 GHz, has gained a lot of popularity from researchers and the wireless industry alike due to the high data transfer rate (110–200 Mb/s) and low power consumption. However, since UWB covers the frequency spectrum of Wireless Local Area Network (WLAN) operating at 5.2 GHz (5150–5350 MHz) and 5.8 GHz (5725–5825MHz), a frequency notched function in the UWB antenna design is greatly desirable to avoid interferences from WLAN. Since 2002, various antenna structures [1, 2] and methods to notch a specific frequency band [1, 3, 4] have been introduced. Moreover, a planar integrated antenna working on both Bluetooth and UWB has been recently introduced for systems operating in those two communication systems [5, 6].

Silicon-embedded toroidal inductors with magnetic cores: Design methodology and experimental validation

An approach to the ultimate integration and miniaturization of MEMS-based 3-D magnetic components involves embedding the volume of the magnetic structures within the volume of the silicon wafer itself, exploiting microfabricated windings to create current paths, and utilizing embedded magnetic cores within the limited footprint of these components to boost the magnetic performance. However, this embedding approach imposes volumetric and microfabrication constraints that require an unusual magnetic component optimization methodology compared to wire-wound inductors and PCB inductors. These constraints dictate embedded toroidal inductors with non-overlapping windings and thin magnetic cores, and impose additional limitations on inductor design parameters such as pattern resolution, the number of winding turns and winding thickness; these constraints complicate the trade-offs to be made in designing core-integrated inductors. A design methodology encompassing these constraints is therefore needed. For a targeted inductance value within a given footprint, our design methodology addresses an inductor with a maximized quality factor based on the trade-offs between copper loss and core loss. To illustrate this methodology, silicon-embedded inductors with iron powder cores are designed and fabricated; a quality factor of 24 is achieved at 30 MHz.