Si Interposers with 15-μm-thick Spiral Inductors and SrTiO3 Thin Film Capacitors for Novel 3D Stacked Buck Converters

We have developed a Si interposer with multilayer 15-μm-thick Cu wiring and SrTiO3 (STO) thin film capacitors to help create novel 3D stacked buck converters, which consist of a CMOS LSI that includes active components and a Si interposer with an LC output filter. A spiral inductor made using 15-μm-thick Cu wiring was stacked over an STO thin film capacitor in the Si interposer without any deterioration in capacitance and leakage current. A minimum line/space of 20 μm/20 μm and a minimum via-hole diameter of 30 μm were achieved to create 15-μm-thick Cu wiring patterns in the Si interposer. A newly developed chemically-amplified positive-tone resin was used as the interlayer dielectrics in the Si interposers to avoid under-exposure at the bottom of thick photo-sensitive resin and to obtain fine-pitch wiring patterns. The power efficiency of buck converters depends on the parasitic resistance of the embedded inductors. As a result, the 15-μm-thick inductor enhances the efficiency of an output current of 100 mA by 12% as compared to a 5-μm-thick inductor. The developed Si interposer with 15-μm-thick wiring layers will contribute to creating a 3D stacked buck converter for low-power and high-performance 3D integrated systems.