The laser cavity electric connection line with SnAg solder for laser flip chip bonding

Previously we reported an efficient butt-coupling method to couple the laser light to a photonic integrated circuit (PIC) waveguide by integrating a multi-finger edge coupler in the PIC waveguide input port. In this paper, we demonstrate how current can be provided to the flip-chip bonded laser diode by fabricating an electric connection line using under-bump metallization (UBM) and an economic SnAg solder. In addition, a polyimide passivation layer was developed to protect the electric connection line and provide solder confined structure. Due to the large step height of the laser cavity, controlling the polyimide thickness control inside the cavity via spin coating is a challenge. We show that this passivation thickness inside the cavity can be reduced via etching. The electric line connects to the redistribution line (RDL) through the side wall UBM. The measured resistance of the electric line and solder in series is 0.4 Ohm.