Modified Face-Down Bonding of Ridge-Waveguide Lasers Using Hard Solder

A modified face-down bonding technique of ridge-waveguide laser diodes (LDs) using 80Au20Sn solder has been performed. For ease of manufacturability, a bonding window with good bonding integrity and improved optical performance was determined. Metallographical investigation showed that the solder joint comprised of a layer of delta phase compound near the solder/heatsink interface, a layer of (Au,Ni)Sn intermetallic compound (IMC) at the solder/heatsink interface, and zeta' phase Au/Sn compound at the center of the solder joint. The delta phase shifted to the interfaces after reflow was postulated by its lower surface tension than zeta' phase Au/Sn compound. Good bonding integrity was observed with LD residues still adhering onto the bond pad after die shear testing. Scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analyses of the fracture surface showed that the fracture occurred within the LD, at the GaAs/SiN interface. LDs bonded with this modified bonding process achieved an optical improvement of 2.5-3X compared to the unbonded LDs due to its good thermal management. These bonded LDs further exhibited good long-term reliability with no significant degradation in optical performance and no significant microstructure evolution in the solder joint after 500 thermal cycling test.