Structures and methods of making a dielectric region in a bulk silicon (Si) substrate of a mixed-signal integrated circuit (IC) provide a high-Q passive resonator. Deep trenches within the bulk Si substrate in directions are expanded by wet etching to form contiguous cavities, which are filled by Si oxide to form a dielectric region. The dielectric region enhances the quality (Q) of an overlying passive resonator, formed in metallization layers of the mixed-signal IC.