Abstract
In order to reduce the size of high-frequency communication devices, the general technology trend is to integrate both the active and passive components in the same substrate. The active components are generally fabricated on low resistivity silicon substrate. However, silicon-based on-chip inductors usually suffer serious quality factor degradation at high frequency due to capacitive coupling in silicon substrates. Therefore, the goal of the study is to focus on how to improve the quality factor and inductance density of Si-based inductors. In order to reduce inductor’s substrate loss, we use thick low-k dielectric or air-bridge technology to isolate the silicon substrate as well as thick electroplating copper to reduce the series resistance of the inductor. Since the inductance and quality factor also depend on the dimensions of inductors, we have designed inductors with various dimensions to study the relationship between inductors’ properties and their physical dimensions. For conventional planar spiral inductors, the way to raise the inductance-to-area ratio is to increase the number of metal turns. However, the cost to be paid is the increased parasitic capacitance and the reduced quality factor of the spiral inductors. A double-level inductor fabricated by standard integrated circuits technology has been proposed to increase the inductance density. Nevertheless, it has low quality factor mainly due to the severe parasitic capacitance between conductor levels. Therefore, we fabricate double-level suspended inductors with large inter-level spacing, which can effectively reduce the parasitic capacitance and result in high quality factor. The other way to enhance inductance density is to integrate magnetic materials with inductors. Although inductors with magnetic materials have better inductance, they have a decreased quality factor due to the increasing series resistance. In addition to the inductor’s property enhancement, an appropriate device circuit model for predicting inductor’s properties is also important. One approach employed here is to utilize a data fitting technique to obtain the series inductance and resistance values of a spiral inductor. The other equivalent circuit components are calculated from the materials properties and physical dimensions of the inductor. Therefore, the designers would be able to determine quickly the inductor properties of the spiral inductors of interests from the results of the limited tested structures. The other inductor’s physical model is devised for simulating the properties of solenoid inductors. All the inductive, resistive, and capacitive equivalent circuit elements can be correlated to the physical parameters and material properties of the solenoid inductors.