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7 June 2011 Residual mechanical stress decrease in GaAs-based laser diodes via a bi-material investigation
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Bonding-induced mechanical stress in GaAs-based laser diodes is studied by numerical and experimental techniques. This stress, induced by the soldering processes, appears when cooling down the assembly because of Coefficient of Thermal Expansion (CTE) mismatch, and dimensional disparities. Detailed mechanisms taking place are not fully understood. Residual stress is also known to influence device reliability. Composite submounts studied are composed of a CuW heat spreader on an AlN bottom plate in standard and optimized designs to lower mechanical stress levels. CuW shows a high thermal conductivity and a matched CTE with GaAs. Plain AlN submounts are studied as a reference. The numerical technique is a Finite Element Method calculation to compute the stress tensor induced in GaAs-based laser diodes during the soldering process on submounts with 80-20 AuSn eutectic solder pads. Starting with 31 MPa on the plain AlN submount, the standard composite submount gives 23.5 MPa while the optimized version is as low as 12 MPa. The experimental technique consists of Degree of Polarization (DoP) measurements of the photoluminescence emitted by a planarized diode bonded to a submount. From the DoP, relative stress variations induced by the submount are estimated. Starting with DoP referenced at 100% on plain AlN submounts, the standard composite submount gives 46% DoP reduction while the optimized version is expected to exhibit a reduction larger than 65%. Composite submounts with reduced mechanical stress and preserved thermal properties were studied experimentally and theoretically. An optimized design allows reducing the mechanical stress by a factor 2.5 at least.
© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Julien LeClecH, Daniel T. Cassidy, François Laruelle, Mauro Bettiati, and Jean-Pierre Landesman "Residual mechanical stress decrease in GaAs-based laser diodes via a bi-material investigation", Proc. SPIE 8080, Diode-Pumped High Energy and High Power Lasers; ELI: Ultrarelativistic Laser-Matter Interactions and Petawatt Photonics; and HiPER: the European Pathway to Laser Energy, 808006 (7 June 2011);

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