Gold TSVs (Through Silicon Vias) for High-Frequency III-V Semiconductor Applications

The PARADIGM (Photonic Advanced Research And Development for Integrated Generic Manufacturing) European Union Project focuses on design and fabrication of complex photonic integrated circuits (PICs) at low cost. A part of the project was to design a generic package for diverse PICs, and a co-planar waveguide with through silicon vias (TSV) was a requirement therefor. The through silicon vias should improve the high frequency performance of the waveguide due to better distribution of the ground level over the whole device. As conduction material inside the TSVs copper is a well-known and commonly used material. However copper being a well-known source of degradation in III-V semiconductors a copper free metallization system had to be developed. Gold is widely used in III-V devices to form contacts and should therefore be used as conduction material for the TSVs and the contacts on top of the device. A metallisation system based on gold was developed successfully. The TSVs had a diameter of 200 μm and were 400 μm deep (Aspect Ratio: 2). The co-planar waveguide and the TSVs were fabricated on 100mm wafers at Fraunhofer IZM using wafer-level-thin film technology. A reference sample without any TSV was fabricated as well. The diced samples with and without TSVs were electrically tested at CIP and evaluated with respect to their high frequency performance (0 - 40 GHz). Crosstalk and bandwidth (Bandwidths > 40 GHz were observed for a 10 mm long track) showed significantly better performance when TSVs were utilised in the device.