The strong demand for 3D packaging technology has accelerated the miniaturization of passive devices, and inductors are facing challenges because the inductance value will be sacrificed when the size is shrinking. The magnetic core is one of the solutions due to its enhancement of inductance density but it will also add complexity to the fabrication process. Previous research [1-3] shows that the performance enhancement brought by the integration of magnetic will be shadowed by the process complexity. A novel method to fabricate the magnetic core inductors on flexible substrates is proposed in this work. The inductor has a stacking structure of three layers: the Cu trace layer, and the two layers of magnetic core (under and above the Cu). The Cu trace is patterned by the wet etch process on the flexible PCB, and the upper layer magnetic core is directly sputtered on the Cu trace layer with the polyimide layer being planarized on the Cu surface as insulation. A polyimide layer on the carrier wafer is prepared as the substrate for the lower magnetic core layer and the pattern is also created by the liftoff process. Then, the two finished layers are aligned and bonded with a flip-chip bonder with high precision and efficiency. The whole process is shown in Fig.1. Before the whole fabrication process, the device patterns are designed and optimized by the finite element analysis software to extract the parameters for comparison. And finally, the finished devices (shown in Fig.2) are measured with the radiofrequency probe station. The result shows that the inductance density and the quality factor of the magnetic core inductor could be 3.61 nH/mm2 and 6.22, respectively, with the device size being only 0.95 mm2. Compare with the air core inductor with the same Cu trace pattern, the magnetic core inductor elevates the inductance by 10% and still keeps the same low profile, of which the thickness of the whole device is within 100 um. These parameters are measured and extracted at 100 MHz and this is the widely used frequency for the power delivery applications such as the in-chip integrated voltage regulators.

![](https://assets.underline.io/uploads/markdown_image/1/image/9d73279a565e56c01aa0293d4f0c5845.jpg)

The Fabrication process.

![](https://assets.underline.io/uploads/markdown_image/1/image/0fc585fb76a9a86add06825af359c037.jpg)

The finished device with the magnetic core on the top and bottom layers.

A Low-cost Novel Method to Fabricate Integrated Magnetic Core Inductor Embedded in Organic Substrate

Magnetic core has been widely used in integrated inductor as it can provide a high inductance and reduce the device size due to its high permeability [1]. At the same time, the relatively high loss tangent and saturation current will diminish the performance enhancement in the high frequency and high current applications, respectively [2]. The dilemma has resulted in the investigation of magnetic core materials, among which CoZrTaB demonstrated a low Hk, high resistivity and low loss tangent below 500 MHz, which is compatible with most of the integrated voltage regulators’ working frequency [3]. Here we fabricated CoZrTaB thin films with sputtering on different substrates and characterized them comprehensively. The CoZrTaB thin films have been fabricated in single layer and multi-layer structures (Fig. 1(a)), of which the VSM results, shown in Fig. 2, suggest that the Hk change of the multi-layer structure can be neglected while it can suppress the eddy current at high working frequency. Compared with the continuously sputtered 400nm CoZrTaB thin film, the multi-layer thin film shows a more desirable hysteresis loop with a single domain. The surface roughness of the Si and two kinds polyimide was extracted with the AFM, followed by the VSM characterization to verify the performance degradation. In addition, the material characterization such as XRD and RBS have been conducted to acquire the information of phase and element ratio of the CoZrTaB thin film, as shown in Fig.1 (b)(c). A comparison is made with the previous reported results from Gardner [4] and Koh [5] to see the successful implantation of the CoZrTaB thin film on different substrates.References: [1] Xu, Wei, et al. "Performance enhancement of on-chip inductors with permalloy magnetic rings." IEEE Electron Device Letters 32.1 (2010): 69-71. [2] Bechir, M. H., et al. "Magnetic inductor model which take into account permeability and loss tangent of the magnetic material." 2015 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP). IEEE, 2015. [3] Burton, Edward A., et al. "FIVR—Fully integrated voltage regulators on 4th generation Intel® Core™ SoCs." 2014 IEEE Applied Power Electronics Conference and Exposition-APEC 2014. IEEE, 2014. [4] Gardner, Donald S., et al. "Integrated on-chip inductors with magnetic films." 2006 International Electron Devices Meeting. IEEE, 2006. [5] Koh, Kisik, et al. "High frequency microwave on-chip inductors using increased ferromagnetic resonance frequency of magnetic films." 2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS). IEEE, 2015.

The Improvement of the CoZrTaB Thin Films on Different Substrates for Flexible Device Applications

**Session chair:** Guijun Li, Hong Kong University of Science and Technology



IOD - Power Devices, Induction

technical paper

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A Low-cost Novel Method to Fabricate Integrated Magnetic Core Inductor Embedded in Organic Substrate

The Improvement of the CoZrTaB Thin Films on Different Substrates for Flexible Device Applications

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