Fully back-end TSV process by Cu electro-less plating for 3D smart sensor systems

A fully back-end process for high-aspect ratio through-silicon vias (TSVs) for 3D smart sensor systems is developed. Atomic layer deposition of TiN provides a highly conformal barrier as well as a seed layer for metal plating. Cu electro-less plating on the chemically activated TiN surfaces is applied to uniformly fill the TSVs in a significantly shorter time (2 h for 300 mu m deep and 20 mu m wide TSVs) than with Cu bottom-up electroplating (>20 h). The process is CMOS compatible and can be performed after the last metalization step, making it a fully back-end process (VIA-last approach). Wafers containing metal interconnections on both sides are in fact used as demonstrator. Four-terminal 3D Kelvin structures are fabricated and characterized. An average resistance value of 650 m Omega is measured for 300 mu m deep TSVs with an aspect ratio of 15. The crosstalk between adjacent TSVs is also measured by means of S-parameters characterization on dedicated RF test structures. The closest TSVs (75 mu m) show a reciprocal crosstalk of less than -20 dB at 30 GHz.