The key of the via fabrication is the ball-bonding on a metal membrane on the bottom of the via cavity subsequently to the via hole formation. A second electrical flame off (EFO) cuts the wire on a spot above the upper substrate surface (Fig 2 a). The remaining hollow space of the cavity is subsequently filled with a dielectric, which acts both as a mechanical support for the via core and an insulator (Fig 2 b). The large diameter of the dielectric results in a very low capacitive coupling of the metal core towards the substrate. A grinding and polishing step of the frontside removes the remaining gold of the bond wire and he dielectric from the surface of the substrate. A final metal deposition on the frontside contacts the gold core of the via. The structuring of both metal layers forms the signal lines leading to the TSV (Fig. 2 c).

Proof-of-concepts with an overall aspect ratio of 1.5 and an aspect ratio of 12 of the metal core have been already successfully implemented. Smaller TSV diameters and higher aspect ratios are feasible but ultimately limited by the wire bonding process. The use of bond capillaries with smaller tip-diameters and/or thinner substrates would enable via diameters down to 100 μm and thus allows the use of this concept for scenarios with a TSV-pitch in that scale. Superior electrical characteristics in terms of low resistivity and low capacitive substrate coupling of the presented concept enable the cost-effective utilization of TSVs for sensitive applications, such as the heterogeneous integration of RF and capacitive MEMS sensors/devices and IC technology. The presented concept exclusively utilizes well-established standard processes and is therefore suitable for massproduction.

Contact:
Dipl.-Ing. Andreas Fischer, andreas.fischer@ee.kth.se