An Effective Approach of Reducing the Keep-Out-Zone Induced by Coaxial Through-Silicon-Via

Keep-out-zone (KOZ) is a conservative way to prevent any devices/cells from being impacted by the through-silicon via (TSV)-induced stress. In this paper, an effective approach was proposed of reducing the KOZ induced by coaxial TSV, by using the structure of coaxial-annular TSV, without decreasing the electrical performance of coaxial TSV. The analytical model was developed appropriate for the thermal stress induced by both coaxial and coaxial-annular TSVs, and was verified by the finite element method. The KOZs induced by coaxial and coaxial-annular TSVs were compared in detail, and the effects of Cu plasticity, TSV material, TSV size, and inner metal plating ratio of coaxial-annular TSV were also studied. The electrical characteristics of different TSVs were compared by employing ANSYS' HFSS, and a feasible fabrication process for coaxial-annular TSV was suggested. It could be concluded that: 1) a 1.6-μm (22.2%) drop of KOZ for coaxial-annular TSV could be reached as compared with that of coaxial TSV; 2) coaxialannular TSV was proved to offer the same superior signal integrity with coaxial TSV, improving S21 by about 93% at 5 GHz and 60% at 20 GHz compared with ordinary cylindrical and annular TSVs; and 3) the coaxial-annular TSV is realizable.