Brice Jamieson

Review of On-Chip Inductor Structures With Magnetic Films

A comparison of on-chip inductors with magnetic materials from previous studies is presented and examined. Results from on-chip inductors with magnetic material integrated into a 90 nm CMOS processes are presented. The inductors use copper metallization and amorphous Co-Zr-Ta magnetic material. Inductance densities of up to 1700 nH/mm2 were obtained thanks to inductance increases of up to 31 times, significantly greater than previously published on-chip inductors. With such improvements, the effects of eddy currents, skin effect, and proximity effect become clearly visible at higher frequencies. Co-Zr-Ta was chosen for its good combination of high permeability, good stability at high temperature (> 250degC), high saturation magnetization, low magnetostriction, high resistivity, minimal hysteretic loss, and compatibility with silicon technology. The Co-Zr-Ta alloy can operate at frequencies up to 9.8 GHz, but trade-offs exist between frequency, inductance, and quality factor. Our inductors with thick copper and thicker magnetic films have dc resistances as low as 0.04 Omega, and quality factors of up to 8 at frequencies as low as 40 MHz.

Improved electronic and magnetic properties of reduced graphene oxide films

We demonstrate semi-metallic transport in graphene oxide layers treated with an organic acid through a nearly linear current-vs.-voltage relationship and the weak temperature dependence of resistance from high temperatures down to 20 K. Additionally an energy gap was observed below 17 K due to the formation of local barriers by residual oxygen groups and disorder in reduced graphene oxide (RGO) sheets. At higher temperatures resistance shows a negative T2 temperature dependence. Temperature dependent magnetization measurements showed a phase transition from diamagnetic to ferromagnetic at around 10 K, in agreement with the electronic transport properties of the RGO films.

High efficiency Si integrated micro-transformers using stacked copper windings for power conversion applications

This paper details the design, fabrication, and characterization of silicon integrated micro-transformers. Two types of race-track shaped micro-transformers, single copper winding or single layer metal (SLM) and double copper winding or double layer metal (DLM) were designed and fabricated using standard CMOS processing. The DLM devices have higher inductance density than SLM devices realized within similar footprint area. The design study showed that the efficiency of micro-transformers increased from 37% for SLM designs to over 75% for DLM transformers at 20 MHz.

Shape-independent permeability model for uniaxially-anisotropic ferromagnetic thin films

A permeability model based on the three-dimensional shape demagnetization effect is developed to estimate the permeability of any uniaxially-anisotropic ferromagnetic thin-film for integrated micromagnetic structures. The model is validated by comparison to measured ferromagnetic thin films (NiFe and CoPRe) with different anisotropies and saturation magnetizations. The dependence of permeability on film-thickness is modeled as a function of the demagnetization effect and verified against fabricated samples of varying thicknesses.

Invited) Integrated Microinductors on Semiconductor Substrates for Power Supply on Chip

Enterprise Ireland (commercialisation fund technology development project CFTD/2008/331); Higher Education Authority (“INSPIRE” Higher Education Authority via PRTLI4)

Effect of magnetizing field on the martensitic transformations in a melt spun NiMnGa alloy

The investigation addresses the effect of magnetizing field on the magnetic properties of melt spun Ni52.84Mn19.6Ga27.56 (at%) alloy ribbons. Magnetization behaviour at different fields was observed using a superconducting quantum interference device magnetometer for heating and cooling cycles. The plots showed distinct changes in magnetization around the characteristic temperatures at austenitic start and finish (AS, AF), martensitic start and finish (MS, MF). With increasing field AS, MF were unaffected. In the range of martensitic start and its finish temperature, the zero field cooled and field cooled measurements indicated magnetization drops indicating antiferromagnetic interactions, which is characteristic of the martensitic phase formation. It was shown from x-ray diffraction analysis that the low martensitic fraction in the majority austenite phase induced the splitting in the L21 austenitic ordering. This was further corroborated by the evidence of a few martensitic plates around grain boundaries at room temperature which is close to martensitic start temperature.