Ultralow-Capacitance Through-Silicon Vias With Annular Air-Gap Insulation Layers

Low capacitance is critical to the electric performance of through-silicon vias (TSVs). This paper reports the development and electrical characterization of ultralow-capacitance TSVs which use air gaps to replace the conventional silicon dioxide as the insulation layers. The air-gap TSVs are successfully fabricated by developing a sacrificial technology which uses void-free filling and selective etching of an annular benzocyclobutene polymer cladding that surrounds copper plugs. The capacitance and the leakage current are tested to characterize the electrical performance. The lowest effective dielectric constant of the air enables the capacitance of the air-gap TSVs to be as low as 24 fF, and the capacitance density is more than one order of magnitude lower than that of conventional SiO₂ liner TSVs. The leakage current to the substrate is 3 ×10^-13 A at 40 V, and no leakage current degradation occurs after a 40-cycle thermal shock test. The preliminary results demonstrate the new air-gap structure and the efficacy of air gaps in reducing TSV capacitance.