Tungsten Copper Mounts
Copper Tungsten Mounts Keep Diode Lasers Cool
In addition to traditional heat sinking in packaging of microelectronic dies, more demanding applications are emerging for copper/tungsten (WCu) metal-matrix composites (MMCs) as mounts and submounts for semiconductor laser diodes. Currently, the majority of semiconductor laser diodes are mounted on a mount or submount made out of WCu. Improved thermal expansion match between the heat sink and the die, coupled with the current trend of increasing die size and power-dissipation requirements, has made WCu the material of choice for packaging laser diodes. This is particularly true for die larger than 1000 µm in any direction. Copper/tungsten provides the needed thermal dissipation and good thermal expansion match. Some laser diodes are mounted directly on oxygen free high purity copper, on a beryllia or aluminum nitride ceramic substrate, or even on a diamond substrate.
The majority of power semiconductor laser diodes manufactured for wavelengths in the 800 to 1550 nm range have benefited from the improved performance of the new WCu heat sink bases. Applications include medical, scientific, and fiberoptic-based communication networks, among others.
Copper Tungsten Mounts Changing Conventions
Conventional copper tungsten heat sink bases provide thermal conductivity between 170 and 220 W/mK and a reduced coefficient of thermal expansion that matches the semiconductor dies for diode manufacturing (5.6-9.0 ppm/°C). Laser dies are typically built on gallium arsenide (GaAs) substrates using processes such as molecular-beam epitaxy or metal-organic chemical-vapor deposition. The final chemical composition may include indium gallium arsenide (InGaAs), indium aluminum gallium arsenide (InAlGaAs), aluminum gallium arsenide (AlGaAs), indium gallium arsenide phosphide (InGaAsP), or indium gallium phosphide (InGaP). Recently, indium gallium nitride (InGaN) lasers have been manufactured on a sapphire substrate using a layer of epitaxially laterally overgrown GaN to match the lattice energy between the sapphire and the semiconductor.
Different tungsten copper mounts were modeled using the finite boundary value solution technique. Performance was compared using 160-W/mK WCu material as the baseline and high-purity copper (thermal conductivity = 398 W/mK) for top-end performance. Based on thermal-resistance reduction, an improvement of 19.1% was obtained for the 200-W/mK material. The functionally graded 320-W/mK material was about 47.54% better, with the top-end material providing a 56.88% improvement over the standard. Corresponding reduction in junction temperature is also shown.Technical developments using functionally graded materials (FGMs) push the performance envelope of copper/tungsten to thermal conductivity levels around 320 W/mK. This performance level is comparable to the thermal performance provided by copper. These thermal-management solutions are pursued using common, readily available materials such as copper and tungsten.
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