Mounting of high power laser diodes on boron nitride heat sinks using an optimizedAu/Sn metallurgy

High power diode lasers become more and more important to industrial and medical applications. In contrast to low power applications, long cavity lasers or laser bars are used in this field and mounting quality influences considerably laser performance and life time. In this paper we focus on the solder metallurgy and stress-induced laser behavior after mounting. The laser chips have been bonded fluxless epi-side down on translucent cubic boron nitride (T-cBN) using Au/Sn solder. The laser behavior has been tested with different chip metallizations preserving the eutectic solder composition or forming the An rich zeta-phase during reflow. The resulting additional stress in the lasing region has been independently indicated by polarization measurements of the emitted light. A metallization scheme has been developed which forms the highly melting zeta-phase during soldering within a wide process window. This procedure yields better results then using eutectic Au/Sn which has a higher hardness than the zeta-phase. Laser diodes up to a cavity length of 2000 mum and an aperture of 200 mum have successfully been mounted on T-cBN. State of the art laser data, excellent thermal stability, high Nield and reliability have been obtained.