Design and modeling of high-Q variable width and spacing, planar and 3-D stacked spiral inductors

Layout optimized planar spiral inductors using variable width (W) and spacing (S) across their turns are known to exhibit higher quality factors (Q). In this paper, we explore the performance improvement of 3-dimensional, series-stacked, parallel-stacked, single ended, and symmetric inductor configurations with variable W&S across their turns. Parameterized cells for the aforementioned complex variable W&S inductor structures are developed in cadence using SKILL scripts for automatic generation of optimized inductor layouts with improved quality factors. The analyzed standard (constant W&S) and layout optimized (variable W&S) inductors are fabricated in a 0.18 μm silicon on insulator process. Measurement results show more than 20% improvement in quality factor for the 3-D stacked variable W&S inductor topologies. Furthermore, an accurate, scalable, and broadband compact inductor model was developed and is shown to capture the improved Q characteristics across the analyzed inductor topologies. A complementary LC-tank voltage controlled oscillator with layout optimized inductors, operating over 2.4-2.5 GHz frequency range was simulated using these developed compact models and is shown to exhibit an improved phase noise characteristics with better figure of merit.