Florian Herrault

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.

An electromagnetic energy scavenger from direct airflow

This paper presents two types of electromagnetic power generators exploiting direct conversion of airflow into mechanical vibration: 1) a windbelt-based vibratory linear micro-generator targeting strong airflows, and 2) a Helmholtz resonator based generator capable of scavenging energy from weaker airflows, i.e. environmental airflows. Both devices consist of two tightly coupled parts: a mechanical resonator, which produces high-frequency mechanical oscillation from quasi-constant airflow; and a permanent magnet/coil system, which generates electrical power from the resonators motion. The proposed micro-generators obviate the typically required matching of the resonant frequencies of the scavenger and the ambient energy sources it taps. This enables a device that is simpler, smaller, and higher-frequency than previously reported resonant power generator. The windbelt-based micro-generator demonstrated a peak-to-peak output voltage of 81mV at 0.53kHz, from an input pressure of 50kPa. The Helmholtz-resonator based micro-generator achieved a peak-to-peak output voltage of 4mV at 1.4kHz, from an input pressure of 0.2kPa, which is equivalent to 5m/s (10mph) of wind velocity

A Microfabricated Wireless RF Pressure Sensor Made Completely of Biodegradable Materials

A wireless RF MEMS pressure sensor made entirely of biodegradable materials is presented. Such biodegradable sensors may be appropriate for short-term, acute medical implantation applications as they potentially eliminate the need for implant extraction when sensing is no longer required. The biodegradable sensors described here require structural materials for pressure sensing, dielectric materials for insulation, and conducting materials for formation of electrical elements and wireless links. Zinc/iron bilayers were used as the sensor conductor material, and known biodegradable polymers poly-L-lactide and polycaprolactone were used as dielectric and structural materials. Zinc, which otherwise degrades very slowly on its own under biological conditions, is galvanically activated when electrically connected to iron in saline, greatly increasing the total degradation rate of the conductors. To avoid contact of the biodegradable materials with the strong chemicals or solvents that are typically used in conventional MEMS fabrication, embossing, multilayer folding, and lamination were combined with traditional techniques during fabrication. The fabricated sensor was wirelessly tested in both air and 0.9% saline and demonstrated a linear frequency response with external applied pressure. A sensitivity of 39 kHz/kPa was measured in the 0-20 kPa pressure range in air and initially in saline. After immersion in saline for 20 h, the sensor stabilized, remaining stable and functional for 86 h with a sensitivity of -54±4 kHz/kPa.

Design optimization of an 8 W, microscale, axial-flux, permanent-magnet generator

This paper presents the design optimization and characterization of a microscale, permanent-magnet (PM) generator, capable of supplying 8 W of dc power to a resistive load at a rotational speed of 305 000 rpm. The generator is a three-phase, axial-flux, PM machine, consisting of a stator with Cu surface windings and a multi-pole SmCo PM rotor. Optimization of the machine geometries has enabled a 30% improvement in power density (for the same rotational speed) over a previously reported machine. Furthermore, these design improvements, in combination with higher rotational speeds, have enabled a >7x improvement in total output power and a net power density of 59 W cm -3 .

Ultraminiaturized High-Speed Permanent-Magnet Generators for Milliwatt-Level Power Generation

This paper presents the design, fabrication, and characterization of millimeter-scale rotary electromagnetic generators. The axial-flux synchronous machines consist of a three-phase microfabricated surface-wound copper coil and a multipole permanent-magnet (PM) rotor measuring 2 mm in diameter. Several machines with various geometries and numbers of magnetic poles and turns per pole are designed and compared. Moreover, the use of different PM materials is investigated. Multipole magnetic rotors are modeled using finite element analysis to analyze magnetic field distributions. In operation, the rotor is spun above the microfabricated stator coils using an off-the-shelf air-driven turbine. As a result of design choices, the generators present different levels of operating frequency and electrical output power. The four-pole six-turn/pole NdFeB generator exhibits up to 6.6 mWrms of ac electrical power across a resistive load at a rotational speed of 392 000 r/min. This milliwatt-scale power generation indicates the feasibility of such ultrasmall machines for low-power applications. [2008-0078].

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.

Fabrication and Performance of Silicon-Embedded Permanent-Magnet Microgenerators

This paper focuses on the design, fabrication, and characterization of silicon-packaged permanent-magnet (PM) microgenerators. The use of silicon packaging favors fine control on shape and dimensions in batch fabrication and provides a path toward high rotational speeds (1Mr/min), a requirement for ultimate compactness of microgenerators. The successful silicon packaging of these microgenerators consisted of three essential elements: (1) a winding scheme allowing both nonplanar fabrication and through-wafer interconnects; (2) laminations built into the silicon for enhanced electrical performance; and (3) a balancing scheme for the heavy PM rotor to ensure its maximum performance. The devices were fabricated using bonded silicon wafers, integrated magnetics, and an electroplated metal. The mechanical strength of the 12-mm-diameter silicon-packaged PM rotors was evaluated at high rotational speeds using an external spindle drive. Speeds up to 200000 r/min were achieved prior to a mechanical rotor failure. The generators were electrically characterized, and an output power in excess of 1 W across a resistive load of 0.32 ¿ was measured at a maximum speed. A 225% power increase was also experimentally determined due to the addition of a laminated stator back iron.

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.

Cooling performance of micromachined self-oscillating reed actuators in heat transfer channels with integrated diagnostics

This paper presents heat transfer (HT) performance of small-scale MEMS-enhanced self-powered oscillating actuators for applications in highly-compact high-power air-cooled heat exchangers. Commercial air-cooled heat sinks are typically much larger than the systems they must cool due to large air-side thermal resistance. Our work is applying MEMS technologies to reduce this thermal resistance via the integration of small-scale oscillating actuators into small heat exchangers. Improved HT performance either yields smaller heat sinks or higher heat fluxes. These mm-scale actuators were built using MEMS manufacturing technologies such as laser micromachining, lamination, and/or metal patterning and etching. Conceptually, oscillating reeds inserted into the channels of an air-cooled heat sink induce small-scale motions in low-Reynolds number flows, which increases HT efficacy. Using MEMS-enhanced reed actuators, we experimentally demonstrated local HT enhancement up to 250% in microfabricated channels monitored by integrated temperature sensors.