Hi Jung Kim

A megahertz switching DC/DC converter using FeBN thin film inductor

A dual spiral sandwiched thin film inductor with a dimension of 5 mm /spl times/ 5 mm was fabricated by a LIGA-like micromachined process. FeBN magnetic films were used as a core material. The inductance and quality factor value of the inductor were measured approximately 1 /spl mu/H and 4 up to 5 MHz, respectively. Using the FeBN film inductor, a hybrid dc/dc converter (15 mm /spl times/ 12 mm /spl times/ 1.5 mm) was designed and fabricated. The circuit topology of the converter was a zero voltage switching clamp voltage (ZVS-CV) buck converter. An input of 3.6 V was bucked to 2.7 V at a switching frequency of 1.8 MHz. The maximum power was 1.5 W. The measured efficiency of the buck converter reached a maximum value of 80% and kept stable up to 300 mA of load currents at 1.8 MHz.

A microfabricated transformer for high-frequency power and signal conversion

A new microtransformer (1:1) for high frequency power or signal conversion has been designed, fabricated and characterized in this work. An innovative laterally laminated-core structure has been adopted and implemented by using new micromachining techniques. Using the fabricated microtransformer, a voltage gain of -4 dB has been achieved over the frequency range of 7-11 MHz. The coupling coefficient between the primary and secondary coils is higher than 0.9 up to 5 MHz. The transformer realized in this research functionally works as a basic transformer for power or signal conversion.

Development of FeCo-based thin films for gigahertz applications

For improving high-frequency characteristics and soft magnetic properties of (Fe/sub 7/Co/sub 3/)B-based thin films, the effects of Ni addition and an oblique deposition method on the microstructures have been examined. The Ni addition to (Fe/sub 7/Co/sub 3/)B films was found to enhance B segregation on grain boundaries, which resulted in the formation of a well-defined columnar structure and the increase of film resistivity. Utilizing the columnar structure, large in-plane anisotropy was induced by an oblique deposition method which enabled the control of the inclination angle of the columns. As a result, the ferromagnetic resonance of (Fe/sub 7/Co/sub 3/)/sub 71/Ni/sub 7/B/sub 22/ thin films reached 6.2GHz.

Micromachined Semi-encapsulated Spiral Inductors For Microelectromechanical Systems (MEMS) Applications

Semi-encapsulated spiral inductors for microelectromechanical systems (MEMS) applications have been designed, fabricated, and characterized using micromachining and electroplating techniques. The inductors are fabricated on a Pyrex glass wafer using a simple three mask process and have an area of 4 mm by 4 mm. Devices consist of electroplated copper conductor lines with width 50 /spl mu/m, thickness 25 /spl mu/m, and spacing 30 /spl mu/m; they are semi-encapsulated with an electroplated Ni-Fe (81%/19%) magnetic core. At the low frequency of 10 kHz, the inductance of the devices was measured as 1.5 /spl mu/H, while they exhibited a low resistance of only 3 ohms. Their low resistance and reasonably high inductance makes them ideal in MEMS applications as either sensors or actuators.

Room temperature electron-mediated ferromagnetism in a diluted magnetic semiconductor: (Ga, Mn)N

We present the electron-induced ferromagnetic ordering and magnetotransport of epitaxial (Ga1-xMnx)N films with low Mn concentration (x=0.0006–0.005) grown by plasma-enhanced molecular beam epitaxy (PEMBE). The (Ga,Mn)N films were found to exhibit n-type conductivity, ferromagnetic ordering with Curie temperature in the range 550–700 K, in-plane magnetic anisotropy, and negative magnetoresistance (MR) in the range of 4–300 K. Our results are indicative of ferromagnetic semiconducting (Ga,Mn)N films due to the coupling of electron spins and localized Mn moments.

2D and 3D simulation of toroidal type thin film inductors

There have been remarkable advances in semiconductor industry but the miniaturization of magnetic devices is progressed very slowly. So many studies are carried out overcoming the gap. To develop small size devices, proper design and property simulation of the devices are requested to spare the efforts before fabrication processes. However, there is a few of simulation studies have been reported. In this work, we fabricated thin film toroidal inductors using permalloy as a core material and measured the properties to estimate 2D and 3D simulation in the magnetic devices. The 3D simulation shows more agreeable results than the 2D simulation. On the other hand, when we changed core materials, the inductor using Fe-Hf-C-N nanocrystalline alloys as core material shows the better properties than using permalloy and Fe-Zr-C-N alloys.

Microstructure and magnetic properties of oxidized Fe-Pt films with high rate order-disorder transformation

Structural and magnetic properties of air-annealed Fe/sub 1-x/Pt/sub x/ films with a high rate order-disorder transformation were analyzed by transmission electron microscopy, Auger electron microscopy, Rutherford backscattering spectroscopy and vibrating sample magnetometer. The air-annealed films consisted of three layers, that is, crystalline Fe oxide layer, Fe-Pt upper layer with small grains, and Fe-Pt bottom layer with large grains from the film surface. Those layers were nonmagnetic, magnetically hard, and with magnetically hard and soft phase, respectively. The Fe content in Fe-Pt metallic layer decreased and then increased with increasing film depth from the oxide/metal interface.

Magnetic Force Microscopy (MFM) Study of Remagnetization Effects in Patterned Ferromagnetic Nanodots

Periodic magnetic nanodot arrays were successfully produced on glass substrates by interference laser lithography and electron beam lithography methods. Magnetic force microscopy (MFM) observation was carried out on fabricated nanodot arrays. MFM tip induced magnetization effects were clearly observed in ferromagnetic elliptical nanodots varying in material and aspect ratio. Fe-Cr dots with a high aspect ratio show reversible switching of the single domain magnetization state. At the same time, Co nanomagnets with a low aspect ratio exhibit tip induced transitions between the single domain and the vortex state of magnetization. The simple nanolithography is potentially an efficient method for fabrication of patterned magnetic arrays.

Surface topology and magnetic properties of FePt alloyed thin films

Dependency of Ar pressure on the surface topology and magnetic properties of as-deposited and air-annealed 120 nm thick FePt thin films were investigated. The higher the Ar pressure was, the rougher the surface of as-deposited films became. After annealing in the air, the surface became much rougher which was due to formation of Fe oxide on the surface. The oxidation was much accelerated in the films with rougher surface. This accelerated oxidation resulted in a high order-disorder transformation rate and higher coercivity.

Soft magnetic properties of Permalloy multilayered films with ceramic intermediate layers

Multilayer thin films of Permalloy with SiO/sub 2/, Si/sub 3/N/sub 4/, Al/sub 2/O/sub 3/, or AlN as a ceramic intermediate layer were prepared by rf magnetron sputtering. The effects of intermediate layer materials on the soft magnetic properties, internal stress, and microstructure of multilayer thin films with various Permalloy thickness were investigated. Effective permeability /spl mu//sub eff/, coercive force H/sub c/, and internal stress /spl sigma/ of multilayers with a fixed intermediate layer (20 /spl Aring/), and a thickness of the permalloy layer, varying from 200 to 5000 /spl Aring/ (number of magnetic layers from 50 down to 2) were measured. Multilayered structures with ceramic intermediate layer thickness of 20 /spl Aring/ and Permalloy layer thickness of less than 500 /spl Aring/ had the lowest coercive force and the highest effective permeability.