High Density Solenoidal Series Pair Symmetric Inductors and Transformers

Abstract

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 \(\mu \) m 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 GHz within \(250\times 250\) \(\mu \) m \(^2\) 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.