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

Abstract

The strong demand for 3-D 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. Adding a magnetic core is one of the solutions due to its enhancement of inductance density but it will also add complexity to the fabrication process. However, previous research shows that the performance enhancement brought by the magnetic core could 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, a polyimide layer is then coated and planarized on the Cu surface as insulation, and the upper layer magnetic core is directly sputtered on the Cu trace layer. For the lower magnetic core layer, a polyimide layer on the carrier wafer is prepared as the substrate and it is also patterned by the liftoff process. Then, the finished layers are aligned and bonded with a flip-chip bonder with high precision and efficiency. Before the whole fabrication process, the device patterns are designed and optimized by the finite-element analysis software. Finally, the finished devices are measured with the radio-frequency probe station. The result shows that the inductance density and the quality factor of the magnetic core inductor could be 3.61 nH/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 6.22, respectively, with the device size being only 0.95 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Compared with the air core inductor (ACI) 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> . These parameters are measured and extracted at 100 MHz and this is the potentially used frequency for the power delivery applications such as the in-chip integrated voltage regulators.