Self-aligned, fluxless flip-chip bonding technology for photonic devices

The self-aligned flip-chip (FC) bonding technique is a very attractive means for the assembly of photonic devices containing multiple optical as well as electrical waveguide interconnects. In this article we propose a fluxless FC-bonding technology which nevertheless ensures efficient solder oxide reduction by applying molecular hydrogen (H₂) under vacuum conditions. Bonding experiments were carried out in a newly developed FC-bonder of which some interesting details are reported. Reproducable bonding accuracies below the required tolerances for fiber to tapered waveguide coupling of ⩽3 μm have been achieved. Using eutectic tin-lead (SnPb 60/40) solder and platinum (Pt) as a wettable pad metallization these bonding results have been obtained at moderate temperatures (250°C) and heating durations (2 min). An investigation on the oxidation kinetics of molten SnPb 60/40 solder confirms the ability of H₂ to reduce solder oxides when thermodynamic boundary conditions are met. The leach resistance of the Pt thin-film metallization (300 nm) has been prooved by SIMS depth profiles for the required bonding temperatures and durations