On the design of high performance spiral inductors for communication system in a package (CSIP)

Abstract

This paper presents the fabrication and characterization of high performance spiral inductors with a magnetic core on glass substrates. The porous anodic aluminum oxide (AAO) membranes with average pore diameter of 70 nm and thickness of 18 µm were fabricated and electroplated with Ni to serve as a magnetic core for spiral inductors. The 3.5 turns spiral inductors were fabricated on Ni-AAO membranes by using standard photolithography and Cu electroplating processes. The fabricated spiral inductors has a dimension of 1370 µm x 1010 µm with 5µm thickness, 15 µm trace width, 15 µm spacing, 400 µm outer diameter, and 220 µm inner diameter. The RF characterization of spiral inductors was performed over a frequency range of 1 -20 GHz. The inductance of spiral inductors with a magnetic core was measured to be 10.12 nH and enhanced 21% at 5 GHz compared to an air core of the same coil size. The quality factor of 14.48 was obtained at 5 GHz. The AAO template with ferromagnetic nanowires promises a great potential in serving as a magnetic core for fabrication of high performance spiral inductors for CSIP. IntroductionThere is a strong demand for high quality and performance integrated RF components such as, transformers, filters, oscillators, regulators, and matching networks for communication system in a package (CSIP). One of the fundamental passive elements for implementation of those components into CSIP is inductors. Among various means in integrating inductors into CSIP, spiral inductors have attracted most attention owing to its compact design and small form factor. However, inductance and quality factor of the spiral inductors do not scale efficiently with the number of turns, leading to excessive area consumption and limited operating frequency [1]. Thus, it is essential to increase inductance and quality factor at high operating frequency and compact size of spiral inductors for realization of CSIP.To further miniaturize the size, lower the cost, improve the performance, and increase the operating frequency of spiral inductors for CSIP, integration of ferromagnetic materials into an air core inductor has attracted much research interest. Ferromagnetic materials act as the flux-amplifying components in spiral inductors and correspondingly, inductance (L) and quality factor (Q) enhancement and area occupation reduction can be achieved in these inductors [2]. Different combinations of ferrites and soft magnetic metal-alloy films have been reported for integration into spiral inductors attributed to their magnetic and electrical properties such as high saturation magnetization, controllable anisotropy, single domain state, and high resistivity [1,[3][4][5][6][7][8]. However, spiral inductors with ferromagnetic cores suffer from poor