High-performance 3D on-chip air-gap spiral inductor and its application to filter design

The air-gap three-dimensional symmetric inductor is developed by using an MMIC air bridge approach to realize a suspended inductor. The proposed inductor has a very high quality factor achieving a Q factor of about 30 for a 2.4 nH air-gap symmetric inductor. This paper also presents a miniature semi-lumped low-pass filter composed of a cascade of the proposed elevated low-loss lumped inductors and ultra-low impedance air gap thin film microstrip line (TFMS) sections. This results in a wide stopband up to a high frequency and, when combined with high quality factor suspended inductors, also gives a sharp roll-off at the cutoff frequency. The combined characteristics of these elements make for excellent low-pass filter performance. Finally, compact and harmonic suppression semi-lumped stubs based on a CPW/thin film microstrip combination using 3D on-chip air-core elevated symmetric inductors are proposed.