This paper demonstrates the resonant power supply noise reduction effects of STO thin film decoupling capacitors, which are embedded in interposers. The on-interposer STO capacitor consists of SrTiO₂ whose dielectric constant is about 20 and is sandwitched by Cu films in an interposer. The on-interposer STO capacitors are directly connected to the LSI PADs so that they provide large decoupling capacitance without package leadframe/bonding wire inductance, resulting in the reduction of the resonant power supply noise. The measured power supply waveforms show significant reduction of the power supply noise, and the Shmoo plots also show the contribution of the STO capacitors to the robust operations of LSIs.