Bonding Stress and Reliability of High-Power GaAs-Based Lasers

Abstract

This thesis documents a study of bonding stress and the reliability of GaAs-based lasers for high-power applications. GaAs-based lasers were bonded to oxygen-free high- conductivity (OFHC) copper heat sinks using a eutectic PbSn solder or a silver-filled conductive epoxy, and life tested. Epoxy-bonded devices were observed to have a larger failure rate on life test than solder-bonded devices. Bonding stress, as measured by the degree of polarization (DOP) of photoluminescence, was found to be the largest in epoxy-bonded devices. As well, the type of heat sink and bonding adhesive affected the stress in the laser material, with bonding stress increasing when there was a larger mismatch of coefficients of thermal expansion between the laser material, adhesive and heat sink. The reliability of the lasers was affected by the amount of force applied to unbonded laser chips. As the applied force increased on a chip centred on a groove, the rate of degradation in the output power increased. A limit in stress tolerance was observed in the lasers, which meant that larger amounts of stress would lead to increased rates of degradation in the output power. As well, the performance of lasers selected from a batch showing poor reliability degraded at an accelerated rate after several hours of operation under applied strain.