An optimized face-down bonding process for laser diode packages

We present an optimized face-down die attachment for single-mode ridge-waveguide laser diodes (LDs) using Au80Sn20 solder. Based on this improved technique, a bonding process window has been determined through microstructure and mechanical investigation of the solder joint, and electro-optical characterization of LDs. Metallographical investigation shows that the solder joint comprises of a layer of δ phase near the LD/solder and solder/heatsink interfaces, ζ phase at the center of the solder joint, and a layer of (Au, Ni)Sn intermetallic compound (IMC) at the solder/heatsink interface. The δ phase shifted to the interfaces, while ζ phase Au/Sn compound is remaining at the center of the joint, is postulated by the lower surface tension in δ phase. Mechanical shear testing demonstrates good bonding integrity, which meets the requirements specified by MIL-STD-883C. Fracture surface characterization shows brittle fracture occurring within the LD, at the GaAs/SiN interface. This optimized bonding process can achieve optical improvement of 2.5-3× compared to unbonded LDs due to good thermal management.