A Novel Double-Sided Etching and Electroplating Fabrication Scheme for Coaxial Through-Silicon-Vias in 3-D Integration

Compared to conventional through-silicon-via (TSV) technology, coaxial TSVs can provide better radio frequency (RF) transmission performance in terms of reduced transmission loss and enhanced impedance matching in 2.5-D/3-D heterogeneous integration of RF microsystems. This article presents a novel fabrication scheme for coaxial TSVs comprised of Cu-pillar inner conductors, annular benzocyclobutene (BCB) insulators, and annular Cu outer conductors. Complete Cu conductors are achieved by the proposed double-sided etching and electroplating method, in which the outer and inner conductors are fabricated from the front and back sides of the wafer, respectively. Besides, a thick BCB insulator without voids is realized based on the vacuum-assisted spin-filling technique. Due to the good feasibility of the fabrication processes, the dimensions of the coaxial TSV can be flexibly designed to meet the requirements for impedance matching. Coaxial TSVs with a height of 85~\mu m, an inner conductor diameter of 45~\mu m, and an insulator thickness of 53~\mu m are successfully fabricated. Measurement results show that the TSVs exhibit a low leakage current between the inner and outer conductors of 1.28 pA at 20 V, and the return loss and insertion loss are better than -16 and -0.35 dB up to 40 GHz, respectively. Such compact and low-loss coaxial TSV structure together with its fabrication scheme facilitates the miniaturized, high-density, and high-performance 2.5-D/3-D heterogeneous integration of microsystems at RF and millimeter-wave (MMW) frequencies.