Coaxial Through-Silicon Via Micro-Transformers: Design, Fabrication, and Power Conversion Demonstration

In this paper, new through-silicon via (TSV) micro-transformers are designed, fabricated, and demonstrated for high-frequency LLC dc-dc converter applications. The micro-transformers are implemented by electrodepositing thin-film magnetic material onto via sidewalls and forming two coaxial Cu layers inside vias as primary and secondary windings. The primary and secondary windings of multiple vias can be connected in series or in parallel through on-chip metallization to achieve different turn ratios. As a proof of concept, basic 1:1 TSV micro-transformer units were first fabricated, achieving a high inductance density of 124 nH/mm², an L/R_DC ratio of 815 nH/Ω, a coupling factor of 0.92, and a peak Q of 14.9 at 6.5 MHz. By connecting the primary and secondary windings in series or in parallel, different transformer turn ratios and inductance values can be realized. In our demonstration, a 1:1 transformer, formed by connecting five basic 1:1 transformer unit in series, has been successfully used in an LLC dc-dc converter with a switching frequency of 10 MHz and an output power of up to 500 mW.