I. Introduction
On-chip integrated inductors are critical passive components in radio frequency (RF) integrated circuits. The wire-wound inductors consume large areas, even for an inductance () of a few nanohenry, which also suffer from high energy loss due to various parasitic effects, resulting in low-quality factors (). Progress was made in designing small stacked structures including high-thickness spiral inductors [1], stacked inductors and transformers [2], on-chip spiral inductors based on Si substrate [3]–[5]. Soft ferromagnetic alloys and granular films may be integrated into inductors for improvement. They induce large eddy currents and lead to low and self-resonance frequency due to low resistivity [6], [7].