Resistive Coupling Efficiency Criterion for Evaluating Substrate Shielding Structures of Transformers

This letter presents a new way to evaluate the transformer substrate effect by computing its resistive coupling efficiency coefficient eta_Kr. To assess eta_Kr, a set of transformers with and without shielding structures was fabricated using standard 0.18-mum CMOS process. The resistive coupling factor eta_Kr and efficiency eta_Kr were extracted. It was found that substrate and shielding effects are better quantified with eta_Kr than with G_max and S parameters. The larger the relative shielding dimensions, the higher the eta_Kr is. With eta_Kr as a figure of merit, we see clearly that a large 3-D patterned floating shield improves transformer performance more than a 2-D one. It was also found that shielding efficiency is correlated with shielding structure dimensions and that the performance of shielded transformers is less sensitive to temperature variations.