A Compact Differential Inductor with Improved Self-Resonance Frequency

An original multi-layer structure is proposed for the realization of monolithic differential inductors. Compared with standard differential topologies of equal inductance, it can reduce the area occupation by more than 50% and increase the self-resonance frequency fSR up to 30%. However, because of the high resistivity of the lower metal layers, the quality factor Q decreases by 25−40% and makes this inductor mainly interesting for applications like chokes. The higher fSR is obtained through an optimized sequencing of wires. In fact, a second stacked differential inductor without wiring optimization gives no benefits in fSR, but only in area reduction, and shows the importance of a proper layout. The novel structure was fabricated in a silicon-germanium technology; measurements report fSR = 5.66 GHz for a 14-nH inductance. An analytical model for singleand multi-layer differential inductors is also introduced. The model is in good agreement with experimental results, with a 5% error in fSR and a 6% error in inductance.