A study of stacked and miniature 3-D inductor performance for radio frequency integrated circuit design

The performance of stacked and miniature three-dimensional spiral inductors is analyzed and compared to standard planar coils. For this purpose, nine of these new structures have been fabricated in a 0.35-μm four-metal SiGe process. According to the measurement results, some of the proposed stacked inductors occupy only 48% of the area of a conventional planar inductor with the same inductance value and work frequency. The area reduction is even more significant with the miniature 3-D structures, which occupy only 22% in some cases, and translate the inductor self-resonance frequency to higher values than the conventional stacked inductors. In spite of this area reduction, these new structures employ metal levels close to the substrate, which significantly degrades the quality factor. So the standard planar coils continue to be the best choice if the designer requires high-quality inductors. However, stacked and 3-D miniature structures could be a better solution if the area saving is the circuit major priority.