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

Abstract

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.  The Fabrication process.  The finished device with the magnetic core on the top and bottom layers.