Bonding-Induced Strain Effects in InP DFB Components Soldered p-Side-Up on AlN Substrates

Bonding-induced strain is shown to have significant impact on the performance of distributed feedback (DFB) lasers mounted p-side up on AlN carriers using AuSn solder. Degree of polarization (DOP) of photoluminescence was used to estimate top-side longitudinal strain profiles in InP chips soldered to AlN carriers. Asymmetric strain profiles were revealed, the orientation of which are shown to be dependent on bonding tool coplanarity. Solder profiles measured on the same chips by scanning electron microscopy (SEM) were found to be nonuniform. Finite-element method (FEM) simulations were used to confirm that the asymmetric strain profiles resulted from solder nonuniformity caused by the bonding process. The FEM simulations were extended to analyze the effects of various bonding parameters on the top-side longitudinal strain profiles in InP chips, and suggestions are made for minimizing strain variations. The measured strains were included in a DFB laser model, and are shown to cause changes in slope efficiency and threshold current. These changes in slope efficiency and threshold current with bonding compare well with data collected from a large ensemble of DFB laser devices measured before and after the mounting process.