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

A fully back-end process for high-aspect ratio (HAR) Through Silicon Vias (TSVs) for 3D smart sensor systems where the sensors and the electronics are fabricated on the opposite sides of the wafer, is developed. Atomic Layer Deposition (ALD) 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 hrs for 300 μm deep and 20 μm wide TSVs) than with Cu bottom-up electroplating (>20hrs). The process is CMOS compatible and can be performed after the last metallization 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 190 mΩ is measured for 300 μm deep TSVs with an aspect ratio of 15.