Flip chip mounting of laser diodes with Au/Sn solder bumps: bumping, self-alignment and laser behavior

A homogeneously melting eutectic Au-Sn solder bump has been developed and used for selfaligning fluxless laser diode flip chip bonding. The eutectic solder is deposited by stacking alternating e-beam evaporated Au and Sn layers. A special `under-bump metallization´ made of a WSi_0.4N_0.2 barrier layer metallurgically separates the Au-Sn solder from the chip metallization. Therefore the bumps can be exposed to multiple reflow and soldering cycles. This results in a wide parameter window for the flip chip bonding process especially if a WSi_0.4N_0.2 barrier layer is also included in the substrate metallization. In contrast to the barrier containing substrates electroplated Au footprint pads cause a rapid growth of the gold-rich ζ-phase and spontaneous local freezing during bonding. The parameter window is thus strongly narrowed in this case. An optimized bumping scheme was integrated into the processing of 1.55 μm (InGaAsP/lnP) laser diodes having both p- and n-contacts on the top side. The laser data have not been influenced by the flip chip bonding with the developed Au-Sn bumps