Juan L. Sepulveda, Lisa Valenzuela, and Stewart W. Wilson
In addition to traditional heat-sinking in packaging of microelectronic dies, more-demanding applications are emerging for copper/tungsten (Cu/W) 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 Cu/W. 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 Cu/W 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 Cu/W heat-sink bases. Applications include medical, scientific, and fiberoptic-based communication networks, among others.
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.
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.
Standard Cu/W technology
Metal-matrix composites having copper as the continuous matrix phase have been used extensively during the last two decades. The use of a refractory metal such as tungsten (or molybdenum) as reinforcement is favored. Copper/tungsten composites traditionally have been produced using infiltration technology by which parts are produced by copper infiltration of a tungsten preform. The parts are then mechanically ground to final shape.
Processes have been developed recently that are based on powder metallurgy technology that produces net or nearly net-shape parts. A mixture of refractory metal powder and copper-containing powder is milled and agglomerated using spray drying to produce flowable power.
The powder is compacted into green (unfired) parts using conventional uniaxial mechanical or hydraulic dry presses and fired in a hydrogen environment at temperatures that can reach 1450°C. Parts can be machined at this point if special features or surface finish is required. The parts are nickel (Ni) plated and gold (Au) plated for most optoelectronics applications. A die bonded to a ceramic base may be brazed onto the copper composite heat sink. Other users may attach the semiconductor die directly to the Cu/W mount.
The main advantage of the powder-metallurgy process is the ability to produce parts that are suitable for use with only slight additional processing immediately after firing. Minimal postfiring machining results in considerable cost savings, and most of the precision machining is concentrated in the area where the laser diode is mounted.
Another advantage of the processed Cu/W is its very fine grain and homogeneous microstructure that allows for the machining of perfectly square, sharp, and defect-free knife edges where edge emitters can be mounted. In contrast, the Cu/W microstructure manufactured by traditional infiltration technology is characterized by copper pools and very large tungsten grains, which results in machined parts that exhibit a larger number of grain pullouts and pits.
Functionally graded Cu/W composites
A functionally graded Cu/W metal-matrix-composite mount is made of a minimum of two metal compositions and has at least two discrete portions in the x-y plane: a high-thermal-conductivity, high-coefficient-of-thermal-expansion functional insert and a surrounding body with lower thermal conductivity and a lower coefficient of thermal expansion.
The functional insert (or functional core) is intimately bonded to the surrounding body. This body constrains the thermal expansion of the insert in the x and y direction during temperature excursions in such a manner that the resulting thermal expansion for the functional core is equal to that of the surrounding body. The expansion along the z axis is insignificant for all practical purposes in the final end use.
Functionally graded materials for laser-diode mounts have been manufactured using standard Cu/W grade formulations for the surrounding body of the mount and higher-copper-content formulations for the functional core. Typically, a 50/50 Cu/W formulation has been used for the functionally graded material core, and a 15/85 Cu/W formulation has been used for the surrounding body. This results in an effective thermal conductivity of 320 W/mK in the core, with an effective coefficient of thermal expansion of 7.11 ppm/°C. The semiconductor laser die is mounted on top of the functional core as in the case of a centrally mounted device (see figure).
Copper/tungsten FGM substrates are used in high-power laser-diode-manufacturing applications in which it is mandatory to keep the die cool and stress-free during temperature excursions. Using an FGM design and commonly available copper and tungsten makes it possible to build a laser mount where the thermal conductivity is optimized at the same time that the thermal expansion is reduced to a level that matches the semiconductor device it houses. The FGM approach makes best use of the high thermal-conductivity properties of the copper and the low thermal-expansion coefficient of the tungsten. This is perhaps the only approach that allows for control of the thermal dissipation ability and expansion properties independent of each other. Although the physical properties of the material are not changed, the effective performance of the FGM composite as a whole provides the properties desired for optoelectronics packaging.
Mounting laser dies
Optoelectronics applications that benefit from conventional Cu/W heat sinks or advanced FGM configuration are laser-diode mounts and submounts for pulsed and continuous-wave semiconductor laser applications, light-emitting diodes, and detectors. Semiconductor laser dies traditionally have been mounted on one of the mount edges or on the center of the mount, but more-complex assemblies containing metallized multilayer ceramics bonded onto a Cu/W base have also been built. In these cases, the die could be mounted directly over the Cu/W base or on top of a pedestal built as an integral part of the base. They allow for the incorporation of other high-frequency electrical components that are electrically insulated from the Cu/W base.
JUAN L. SEPULVEDA is director of technical marketing and LISA VALENZUELA is a marketing communications specialist in the Powder Metal Products Electronics Products Division, Brush Wellman Inc., 6100 S. Tucson Blvd., Tucson, AZ 85706; e-mail: [email protected]; [email protected]. STEWART W. WILSON is manager of the Optoelectronics Packaging, Telecom Products Division, Opto Power Corp., 3321 East Global Loop, Tucson, AZ 85706; e-mail: [email protected].