Characteristics of indium bump for flip-chip bonding used in polymeric-waveguide-integrated optical interconnection systems

We have flip-chip-bonded vertical-cavity surface-emitting laser (VCSEL) arrays on polymeric-waveguide-integrated optical interconnection systems. Using indium solder bumps, thermal damage to the polymeric waveguide can be avoided. Fracture occurs between the indium solder bumps and the VCSEL chip pad during the die shear test. It is inferred that both the low bonding temperature and the oxide layer formed on the surface of the indium solder prevent the bump from interacting with the chip pad. To reveal the microstructures of the joints between the bump and the chip pad, several specimens are cut into cross sections and polished. Scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) spectroscopic system is used to examine the microstructures and analyze the element compositions. Also, the optoelectronic characteristics of VCSEL arrays that were flip-chip-bonded under different bonding conditions are compared by current-voltage (I-V) and light-current (L-I) inspection.