Ultrasoft magnetic materials processing and characterisation

Abstract

The objective of this research was to develop, characterise and test post-processed magnetic materials used for in-package or on-chip devices such as micro-inductors. The electrochemical post-processing has been shown to improve the magnetic properties of the NiFe alloy. The aim of this research was further to test this process on alloys such as CoNiFe as well as integrating these ‘ultrasoft’ magnetic materials into on-chip micro-inductors for device functionality testing. A material with a hysteresis of Hc = <10 A/m is referred to as an “ultrasoft” material.

Magnetic devices such as inductors and transformers are essential devices in power supplies. Power supplies have been decreased in size and increased in speed and efficiency over the past few decades with the creation of on-chip power supply circuitries. One key barrier in the miniaturisation of these circuitries in their power storage and transfer devices, inductors, and transformers, which currently occupy ~30% of the circuit’s volume.

Decreasing the volume of these devices requires the improvement of their efficiency and functionality, which relates to the magnetic materials used in these devices. Ferrites have been used extensively over the past decades due to their low cost and low loss performance. Unfortunately, ferrite’s low magnetic flux density and resistivity make them less suitable for miniature devices or devices which work at high frequencies. Current alternative high flux materials are limited by high cost or high-power loss and work at very low frequencies. Therefore, there is a need for more efficient magnetic material for improved power supplies.

The newly created post-processed magnetic materials and devices were tested for magnetic and electrical properties such as inductance, inductance density, DC resistance, quality factor, and high-frequency response of the devices. They were compared with devices supporting cores made of the as-deposited version of the materials.

Oxi-NiFe, the post-processed version of NiFe, saw a noticeable increase in both its electrical and magnetic properties as a material, as well as showing improved properties as a core in the inductor devices, in comparison to NiFe and the NiFe devices. CoNiFe also showed improvement in both electrical and magnetic properties, as well as device improvements when used as a core, in comparison to the standard NiFe. Oxi-CoNiFe saw diminished properties when compared to CoNiFe.