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.

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Keywords air gap, CPW, low-pass and band-pass filters, Symmetric inductor, TFMS
Conference Asia-Pacific Microwave Conference, APMC 2011
Hettak, K., Ross, T., Elgaid, K., Thayne, I.G., Wight, J. S, & Morin, G.A. (2011). High-performance 3D on-chip air-gap spiral inductor and its application to filter design. In Asia-Pacific Microwave Conference Proceedings, APMC (pp. 1070–1073).