IN-SITU OBSERVATION OF THE SELF-ALIGNMENT DURING FC-BONDING UNDER VACUUM WITH AND WITHOUT H-2

With an in situ observation during flip-chip bonding under vacuum cond itions, 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 (Hz) under vacuum conditions, in order to prevent the negative side effects when using fluxes or fluxli ke chemicals. With this new, fuxless technology, we achieved at modera te bonding temperatures [e.g., 250 degrees C for SnPb(60/40)] and heat ing durations (<2 min) self-aligned FC-bonds well within 2 pm residual offset, Using evaporated SnPb(60/40) and AuSn(80/20) solder bumps, FC -bonding experiments in vacuum (10(-4) Pa) showed spontanous alignment within 5 s but the accuracy of alignment for both solders was neverth eless always insufficient to meet the required tolerances for single-m ode fiber-to-chip coupling.