Experimental analysis of irregularly shaped octagonal on-chip inductors for improving area-efficiency in CMOS RFICs for millimeter wave applications

This article deals with the analysis of irregularly shaped single turn octagonal spiral inductors for millimeter-wave and sub-THz CMOS IC designs. Simulations and experimental results, along with theoretical formulations, are used to characterize these irregular structures. This article proposes a novel approach for efficient use of silicon chip area by reshaping the on-chip inductors used in millimeter wave (mm-wave) applications without compromising the performance of the inductors. Especially in CMOS RFICs when a space constraint exists in either $X$- or $Y$-direction in their layout, such reshaping can be attempted. Moreover, two novel methods of reshaping the inductors are proposed and studied thoroughly. The study of these irregular shapes has interesting conclusions, which are validated through on-wafer measurements. Certain methods of reshaping result in inductors which do not have degradation in their quality factors ($Q$), while other approaches degrade the $Q$. Based on these insights, a design methodology is proposed for designers who need to reshape their inductors to irregular structures while not compromising on the quality factor. The measurement results agree with the simulations and prove that the proposed reshaping is practically possible.