High Density Solenoidal Series Pair Symmetric Inductors and Transformers

A high density symmetric inductor using a novel combination of solenoidal wound series stacked spirals is proposed in this paper. Series stacking in the individual sections increases overall inductance density, while solenoidal winding pushes the quality factors (Q) to higher frequencies. The proposed inductor achieves more than 65% improvement in peak-Q value and 100% higher peak-Q frequency and self resonance frequency, while occupying 20% lesser area when compared with a standard symmetric inductor with crossovers. Implemented in a high resistivity 0.18 μm CMOS silicon-on-insulator process with dual-thick metal stack, the proposed inductor achieves 70-nH inductance and a Q of 11.3 operating at 1.6 Gilz within 250 × 250 μm² area. This translates to a record figure-of-merit of 12.2, which is highest in air core symmetric inductor literature. Further, the proposed inductor configuration is extended to realize a planar transformer with very high turns ratio of 9.25 using only two metals.