Coaxial Through-Silicon-Vias Using Low-κ SiO2 Insulator

This paper reports the design, fabrication, and test of a novel coaxial through-silicon-via (TSV) using low-κ SiO2 insulator and fabricated on normal silicon substrate for high-frequency applications. The coaxial TSV consists of an annular Cu conductor deposited on the outer sidewall of a central Si post, an annular Cu shielding layer deposited on the inner sidewall of an annular trench that surrounds the Si post, and an annular low-κ SiO2 insulator in-between the two Cu layers. The low-κ SiO2 insulator, with the advantage of low cost and good high-frequency performance, is formed by vacuum-assisted spinfilling and thermal curing of liquid precursors. The Cu shielding layer prevents the propagation of electric fields into the lossy silicon substrate and blocks external interferences, and the low-κ SiO2 insulator reduces the dielectric loss and the parasitics of the TSVs. The measured S21 and S11 of TSVs are, respectively, -0.48 dB and -14.91 dB at 10 GHz, and are -1.48 dB and -11.73 dB at 40 GHz, indicating that the coaxial TSVs achieve excellent propagation properties at high-frequency that are impossible for normal TSVs.