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 $\zeta^{\prime}$ 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 $\zeta^{\prime}$ 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.