Kuhmann, J.F.J.F.KuhmannPech, D.D.Pech2022-03-032022-03-031996https://publica.fraunhofer.de/handle/publica/18949710.1109/68.544712With an in situ observation during flip-chip bonding under vacuum conditions, we demonstrate in this letter that accurate passive alignment can only be achieved when the oxide on the solder is removed. For this purpose, we used molecular hydrogen (H2) under vacuum conditions, in order to prevent the negative side effects when using fluxes or fluxlike chemicals. With this new, fluxless technology, we achieved at moderate bonding temperatures [e.g., 250 degrees C for SnPb(60/40)] and heating durations (<2 min) self-aligned FC-bonds well within 2 mu m residual offset. Using evaporated SnPb(60/40) and AuSn(80/20) solder bumps, FC-bonding experiments in vacuum (10-4 Pa) showed spontaneous alignment within 5 s but the accuracy of alignment for both solders was nevertheless always insufficient to meet the required tolerances for single-mode fiber-to-chip coupling.enflip-chip devicesintegrated circuit metallisationintegrated optoelectronicsmeasurement errorsoptical fabricationoptical fibre couplersin situ observationself-alignmentfc-bondingvacuumh2flip-chip bondingaccurate passive alignmentsolder oxide removalmolecular hydrogennegative side effectsfluxlike chemicalsfluxless technologymoderate bonding temperaturessnpb(60/40)heating durationsausn(80/20) solder bumpsself-aligned fc-bondsspontaneous alignmentalignment accuracychip optical fiber coupling250 c2 min5 ssnpbausn621535journal article