A Systematic Approach to Modeling Impedances and Current Distribution in Planar Magnetics

Planar magnetic components using printed-circuit-board windings are attractive due to their high repeatability, good thermal performance and usefulness for realizing intricate winding patterns. To enable higher system integration at high switching frequency, more sophisticated methods that can rapidly and accurately model planar magnetics are needed. This paper develops a lumped circuit model that captures the impact of skin and proximity effects on current distribution and electromagnetic fields in planar magnetics. This enables accurate predictions of impedances, losses, stored reactive energy and current sharing among parallel windings. This lumped model is also a circuit domain representation of electromagnetic interactions. It can be used to simulate circuits incorporating planar magnetics, to visualize the electromagnetic fields, and to extract parameters for magnetic models by simulations. The modeling results match with previous theories and finite-element-modeling results. A group of planar magnetic devices, including transformers and inductors with various winding patterns, are prototyped and measured to validate the proposed approach.Texas Instruments IncorporatedMassachusetts Institute of Technology. Center for Integrated Circuits and System

3-winding flyback transformer model extraction using time domain system identification

For low frequency applications, transformer model extraction has been widely investigated using both time and frequency domain data. However, the studies for high frequency transformers have been carried out in the frequency domain only. The reason is due to the complications in acquiring time domain data for high frequency transformer model estimation. This paper presents a methodology to deal with the numerical difficulties associated with time domain data collection, and to obtain a frequency-dependent model of a 3-winding flyback transformer using time domain system identification techniques. The obtained transformer model is experimentally verified

Magnetic Domains and Domain Wall Oscillations in Planar and 3D Curved Membranes

This dissertation presents a substantial contribution to a new field of material science, the investigation of the magnetic properties of 3D curved surfaces, achieved by using a self-assembled geometrical transformation of an initially planar membrane. Essential magnetic properties of thin films can be modified by the process of transforming them from a 2D planar film to a 3D curved surface. By investigating and controlling the reasons that influence the properties, it is possible to improve the functionality of existing devices in addition to laying the foundation for the future development of microelectronic devices based on curved magnetic structures. To accomplish this, it is necessary both to fabricate high-quality 3D curved objects and to establish reliable characterization methods based on commonly available technology. The primary objective of this dissertation is to develop techniques for characterizing the static and dynamic magnetic properties of self-assembled rolled 3D geometries. The second objective is to examine the origin of shape-, size- and strain/curvature-induced effects. The developed approach based on anisotropic magnetoresistance (AMR) measurement can quantitatively define the rolling-induced static magnetic changes, namely the induced magnetoelastic anisotropy, thus eliminating the need for microscopic imaging to characterize the structures. The interpretation of the AMR signal obtained on curved stripes is enabled by simultaneous visualization of the domain patterns and micromagnetic simulations. The developed approach is used to examine the effect of sign and magnitude of curvature on the induced anisotropies by altering the rolling direction and diameter of the 'Swiss-roll'. Furthermore, a time-averaged imaging technique based on conventional microscopies (magnetic force microscopy and Kerr microscopy) offers a novel strategy for investigating nanoscale periodic domain wall oscillations and hence dynamic magnetic characteristics of flat and curved structures. This method exploits the benefit of a position-dependent dwell time of periodically oscillating DWs and can determine the trajectory and amplitude of DW oscillation with sub-100 nm resolution. The uniqueness of this technique resides in the ease of the imaging procedure, unlike other DW dynamics imaging methods. The combined understanding of rolling-induced anisotropy and imaging DW oscillation is utilized to examine the dependence of DW dynamics on external stimuli and the structure's physical properties, such as lateral size, film thickness, and curvature-induced anisotropy. The presented methods and fundamental studies help to comprehend the rapidly expanding field of 3-dimensional nanomagnetism and advance high-performance magneto-electronic devices based on self-assembly rolling

Magnetoelectric Sensor Systems and Applications

In the field of magnetic sensing, a wide variety of different magnetometer and gradiometer sensor types, as well as the corresponding read-out concepts, are available. Well-established sensor concepts such as Hall sensors and magnetoresistive sensors based on giant magnetoresistances (and many more) have been researched for decades. The development of these types of sensors has reached maturity in many aspects (e.g., performance metrics, reliability, and physical understanding), and these types of sensors are established in a large variety of industrial applications. Magnetic sensors based on the magnetoelectric effect are a relatively new type of magnetic sensor. The potential of magnetoelectric sensors has not yet been fully investigated. Especially in biomedical applications, magnetoelectric sensors show several advantages compared to other concepts for their ability, for example, to operate in magnetically unshielded environments and the absence of required cooling or heating systems. In recent years, research has focused on understanding the different aspects influencing the performance of magnetoelectric sensors. At Kiel University, Germany, the Collaborative Research Center 1261 “Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics”, funded by the German Research Foundation, has dedicated its work to establishing a fundamental understanding of magnetoelectric sensors and their performance parameters, pushing the performance of magnetoelectric sensors to the limits and establishing full magnetoelectric sensor systems in biological and clinical practice

Optical Properties of MacEtch-Fabricated Porous Silicon Nanowire Arrays

The increasing demand for complex devices that utilize unique, three-dimensional nanostructures has spurred the development of controllable and versatile semiconductor fabrication techniques. However, there exists a need to refine such methodologies to overcome existing processing constraints that compromise device performance and evolution. Conventional wet etching techniques (e.g., crystallographic KOH etching of Si) successfully generate textured Si structures with smooth sidewalls but lack the capabilities of controllably producing high aspectratio structures. Alternatively, dry etching techniques (e.g., reactive-ion etching), while highly controllable and capable of generating vertically aligned, high aspect-ratio structures for IC technologies, introduce considerable sidewall and lattice damage as a result of high-energy ion bombardment that may compromise device performance. Metal-assisted chemical etching (MacEtch) provides an alternative process that is capable of anisotropically generating high aspect-ratio micro and nanostructures using a room temperature, solution-based technique. This fabrication process employs an appropriate metal catalyst (e.g., Au, Ag, Pt, Pd) to induce etching in several semiconducting materials (e.g., Si, GaAs) submerged in a solution containing an oxidant and an etchant. The MacEtch process resembles a galvanic cell such that cathodic and anodic half reactions take place at the catalyst/solution interface and catalyst/substrate interface, respectively. At the cathode, the metal catalyzes the reduction of the oxidant resulting in the generation and accumulation of charge carriers (e.g., holes, h+) that are subsequently injected into the underlying substrate at the anode. This results in the formation of oxide species that are preferentially dissolved by the etchant. Thus, MacEtch provides a tunable, top-down, catalytic fabrication technique enabling greater process control and versatility for generating high aspect-ratio semiconductor structures. In this thesis, Au and Au/Pd catalyzed MacEtch is used to generate ultradeep Si micropillar structures, and porous SiNW (p-SiNW) arrays with enhanced optical properties. Using a combination of Au-MacEtch and a crystallographic KOH etch, Si micropillars with ~100 μm height were fabricated with up to 70 μm clearance between pillars to allow efficient fluid flow for optical detection of viral particles. Alternatively, porous SiNW arrays fabricated via AuPd- MacEtch demonstrated broadband absorption ≥ 90% from 200 – 900 nm and were shown to outperform RCWA-simulated SiNW arrays with similar morphologies. Additionally, photoluminescence (PL) spectra collected from as prepared p-SiNW showed significant enhancement in intensity centered near 650 nm as etch depth increased from 30 μm to 100 μm, attributed to an increase in the porous volume. Using atomic layer deposition (ALD) the p-SiNW were passivated using alumina (Al2O3) and hafnia (HfO2) thin films in addition to ITO thin films deposited via sputtering. PL intensity also increased after ALD passivation, attributed to a quenching effect on non-radiative SRH recombination sites on the NW surfaces, with a red shift in the peak wavelength as ALD film thickness increased from 10 nm to 50 nm, resulting from strain effects acting on the NW themselves. These results show promise in such micropillar and coated and uncoated p-SiNW structures towards applications in microfluidic devices, and indoor light-harvesting and outdoor solar-based technologies