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

The PARADIGM (Photonic Advanced ResearchAnd Development for Integrated Generic Manufacturing) European Union Project focuses on design and fabrication ofcomplex photonic integrated circuits (PICs) at low cost. A partof the project was to design a generic package for diverse PICs, and a co-planar waveguide with through silicon vias (TSV) wasa requirement therefor. The through silicon vias shouldimprove the high frequency performance of the waveguide dueto better distribution of the ground level over the wholedevice. As conduction material inside the TSVs copper is a well-known and commonly used material. However copperbeing a well-known source of degradation in III-Vsemiconductors a copper free metallization system had to bedeveloped. Gold is widely used in III-V devices to formcontacts and should therefore be used as conduction materialfor the TSVs and the contacts on top of the device. Ametallisation 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 werefabricated on 100mm wafers at Fraunhofer IZM using waferlevel-thin film technology. A reference sample without anyTSV was fabricated as well. The diced samples with andwithout TSVs were electrically tested at CIP and evaluatedwith respect to their high frequency performance (0 -- 40 GHz). Crosstalk and bandwidth (Bandwidths > 40 GHz wereobserved for a 10 mm long track) showed significantly betterperformance when TSVs were utilised in the device.