Abstract
We present an innovative, economical method for manufacturing soft magnetic materials that may pave the way for integrated thin film magnetic cores with dramatically improved properties. Soft magnetic multilayered thin films based on the Fe-28%Co20%B (at.%) and Co-4.5%Ta4%Zr (at.%) amorphous alloys are deposited on 8” bare Si and Si/200nm-thermal-SiO2 wafers in an industrial, high-throughput Evatec LLS EVO II magnetron sputtering system. The multilayers consist of stacks of alternating 80-nm-thick ferromagnetic layers and 4-nm-thick Al2O3 dielectric interlayers. Since in our dynamic sputter system the substrate cage rotates continuously, such that the substrates face different targets alternatively, each ferromagnetic sublayer in the multilayer consists of a fine structure comprising alternating CoTaZr and FeCoB nanolayers with very sharp interfaces. We adjust the thickness of these individual nanolayers between 0.5 and 1.5 nm by changing the cage rotation speed and the power of each gun, which is an excellent mode to engineer new, composite ferromagnetic materials. Using X-ray reflectometry (XRR) we reveal that the interfaces between the FeCoB and CoTaZr nanolayers are perfectly smooth with roughness of 0.2-0.3 nm. Kerr magnetometry and B-H looper measurements for the as-deposited samples show that the coercivity of these thin films is very low, 0.2-0.3 Oe, and gradually scales up with the thickness of FeCoB nanolayers, i.e. with the increase of the overall Fe content from 0 % (e.g. CoTaZr-based multilayers) to 52 % (e.g. FeCoB-based multilayers). We explain this trend in the random anisotropy model, based on considerations of grain size growth, as revealed by glancing angle X-ray diffraction (GAXRD), but also because of the increase of magnetostriction with the increase of Fe content as shown by B-H looper measurements performed on strained wafers. The unexpected enhancement of the in-plane anisotropy field from 18.3 Oe and 25.8 Oe for the conventional CoTaZr- and FeCoB-based multilayers, respectively, up to ∼48 Oe for the nanostructured multilayers with FeCoB/CoTaZr nano-bilayers is explained based on interface anisotropy contribution. These novel soft magnetic multilayers, with enhanced in-plane anisotropy, allow operation at higher frequencies, as revealed by broadband (between 100 MHz and 10 GHz) RF measurements that exhibit a classical Landau-Lifschitz-Gilbert (LLG) behavior.
Highlights
The imperative for lighter, faster and “smarter” mobile devices, combined with the expected exponential growth in wireless data transfer toward 2020 and beyond, due to the rise of the Internet of Things (IoT) paradigm shift and 5G networks, puts device manufacturers under enormous pressure
Intel states that the on-chip inductors in their DC-DC converters used for power management in multi-core processors occupy approximately a quarter of the total available chip area, making them rather costly components,[6] illustrating the urgent need to shrink these components for the generation mobile devices
Multilayers I and II were modeled by FeCoB/Al2O3 and CoTaZr/Al2O3 bilayer stacks with four periods, whereas the multilayers III and IV were modeled by X/Al2O3 bilayer stacks with four periods, where the ferromagnetic layers X consist of FeCoB/CoTaZr bilayer stacks with 40 and 39 periods, respectively
Summary
The imperative for lighter, faster and “smarter” mobile devices, combined with the expected exponential growth in wireless data transfer toward 2020 and beyond, due to the rise of the Internet of Things (IoT) paradigm shift and 5G networks, puts device manufacturers under enormous pressure. Intel states that the on-chip inductors in their DC-DC converters used for power management in multi-core processors occupy approximately a quarter of the total available chip area, making them rather costly components,[6] illustrating the urgent need to shrink these components for the generation mobile devices Due to their flux amplification properties and high operating frequencies, integrated thin film magnetic cores with high permeability promise further device miniaturization, lower energy loss and lower power operation. ∼1.5-2 T) and low hysteresis loss, soft magnetic layers based on amorphous alloys, e.g. FeCoB and CoTaZr, currently receive great attention for their potential for GHz frequencies applications.[10–12] While these amorphous materials have ρ in the range 100-130 μΩ·cm, larger than that of polycrystalline metals (e.g. for Permalloy ρ ∼ 20 μΩ·cm13), it is not large enough for high frequency operation due to the eddy current loss. While this work is limited to the case of FeCoB and CoTaZr soft magnetic alloy materials, our method could be applied to more than two materials, and other material combinations, including magnetic/nonmagnetic ones
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