Implementation of Air-Gap Through-Silicon-Vias (TSVs) Using Sacrificial Technology

Using air-gaps to replace conventional silicon dioxide as the insulators of through-silicon-vias (TSVs) has the possibility to improve the electrical performance and some thermal reliability issues of TSVs. This paper reports the implementation of TSVs with air-gap insulators by developing a polymer sacrificial technology. The sacrificial technology and the key fabrication processes are investigated in detail, including spin-coating of poly propylene carbonate (PPC) on the sidewalls of blind vias, copper chemical-mechanical polishing, PPC grinding, and PPC pyrolysis to form air-gaps. To address the technical challenge in coating thin and conformal PPC sacrificial claddings in blind vias, a vacuum-assisted solvent refilling technique is developed. Air-gap TSVs are successfully fabricated and the electrical performances are characterized. The accumulation capacitance of the air-gap TSVs is 48 fF, and the leakage current is as low as 1.22 pA at bias voltage of 20 V. Finite element simulation shows that air-gaps are able to reduce thermal stresses. The preliminary results demonstrate the feasibility of the sacrificial technology and the good electrical performance of air-gap TSVs.