Nanostructured Soft Magnetic Multilayers with Tunable Properties for On-Chip Micro-Magnetic Devices.

Abstract

Miniaturization of the RF passive devices and DC-DC converters is key to achieving lighter, faster and more efficient mobile devices, and high-conversion ratio DC micro-grids, as the 5th generation (5G) wireless network and Internet of Things (IoT) paradigms emerge. In order to realize this objective, however, the biggest challenge remains shrinking the size of the chip-integrated magnetic components (e.g., micro-inductors, micro-transformers). Due to their flux amplification properties and high operating frequencies, integrated thin film magnetic cores with high permeability based on amorphous and polycrystalline magnetic alloys promise further device miniaturization, lower energy loss and thus lower power operation [1], [2]. Yet, integrating these magnetic films on the silicon complementary metal oxide semiconductors (Si-CMOS) platform is technologically very challenging, since for a significant inductance enhancement, several-micrometer-thick films with ultra-low losses need to be deposited. Moreover, leveraging this gain requires complex tailoring of the device architecture and magnetic thin film properties, since maximizing simultaneously the inductance, frequency bandwidth and peak quality factor is very difficult [3]. In this work, we present an economical method of manufacturing magnetic thin films, which allows combining soft magnetic materials with complementary properties, e.g., high saturation magnetization, low coercivity, high specific resistivity and low magnetostriction. Soft magnetic multilayered thin films based on the Ni78.5Fe21.5, Co91.5Ta4.5Zr4, Fe52Co28B20, Fe65Co35 alloy materials were deposited on 8” bare Si and Si/200nm-thermal-SiO2 wafers in an industrial, high-throughput Evatec LLS EVO II magnetron sputtering system [4]. The sputtered multilayers consisted of stacks of alternating 80nm-thick ferromagnetic layers and 4nm-thick Al2O3 dielectric interlayers. Since the substrate cage rotates continuously, such that the substrates face different targets (e.g., NiFe, FeCoB, CoTaZr) alternatively (Fig. 1a), each ferromagnetic sublayer in the multilayer stack can exhibit a nano-layered structure with very sharp interfaces as revealed by X-ray reflectometry (XRR) and transmission electron microscopy (TEM) (Fig. 1b,c). We adjusted the thickness of these individual nanolayers by changing the cage rotation speed and the power of each cathode, which is an excellent mode to engineer new, composite ferromagnetic materials with tunable properties. The ferromagnetic layers were deposited by DC sputtering at a pressure of $1.7 \times 10 ^{-3}$ mbar using Ni-21.5%Fe, Fe-28%Co-20%B (at.%) and Co-4.5%Ta-4%Zr long life (~250 kWh) targets, whereas the dielectric Al2 O3 interlayers were deposited by RF sputtering from monoblock Al2 O3 targets at a pressure of $5 \times 10 ^{-3}$ mbar. We introduced the in-plane magnetic anisotropy in these multilayered thin films during sputtering by a linear magnetic field parallel to the wafer plane, which is designed such that the magnetic field of the magnetron located behind the opposite target is not perturbed. In-plane hysteresis loops (along the EA and HA directions) measured by means of magneto-opto Kerr effect (MOKE) and B-H looper revealed that the coercivity $( H_{c})$, anisotropy field $( H_{k})$ and magnetostriction of these thin films can be tuned with the thickness of the individual magnetic nanolayers (Fig. 2). The behavior of the coercive field for these nanolaminated films was explained by the random-anisotropy model based on grain size considerations (Fig. 2a). The nanolaminated structure exhibited a uniform magnetization throughout its structure with good microwave properties, as revealed by broadband (100 MHz - 10 GHz) RF spectra. The classical Landau-Lifschitz-Gilbert (LLG) model could describe well the experimental behavior of the magnetization dynamics of the multilayered structure (Fig. 2c,d). Since the nanolaminated multilayered structures exhibited no degradation upon post-annealing up to a temperature of $300 ^{\circ}\mathrm {C}$, these materials are very promising for on-chip micro-magnetic devices.