Providing high quality diamond wafer metallization

While benefits in device thermal management by using diamond are apparent, implementation of diamond heat spreaders into laser and LED chips is not obvious because of mechanical stress concerns caused by the lower CTE of diamond.

Diamond is an electrical insulator. It is metallized-typically using titanium/platinum/gold (Ti/Pt/Au)-on top and bottom surfaces for solder attachment to the laser chip and for solder attachment to the heat sink on the opposite side. Prior to metallization, the top side of the heat spreader is polished smooth. This flat (less than 80 nm Ra or arithmetic average) top is used to solder-attach laser chips or LEDs, whose crystalline substrate materials-gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN)-are sensitive to tensile fracture. The smooth diamond surface minimizes chip fracture issues for the assembly. The rougher (less than 5 µm Ra) bottom of the heat spreader is used to thermally attach it to the heat sink where surface finish is not an issue for solder or epoxy attachment reliability.

Because the CTE of diamond (1.5 ppm/°C) is lower than the fragile crystalline chip substrates, it shrinks less than the chip upon solidification of the attachment solder. High-temperature attachment solders of AuSn and AuGe (280°C and 356°C eutectic) have a higher Young’s modulus and a larger CTE (13 to 16 ppm/°C), than the chip CTE (5.8 ppm/°C for GaAs). The stiffness of these hard solders also helps minimize transfer of tensile load to the laser chip from the differential thermal contraction of the chip and the heat-spreader assembly.

While the cost for CVD diamond is higher on a volume basis compared to other heat-spreader materials, its bill-of-material cost is low-typically 1% to 2% of the module bill-of-materials cost, excluding the cost of the laser chip-and its use enables increases of device optical power of up to 300% at the same device junction temperature. The use of diamond heat spreaders in multiple market applications to provide dramatic device power and reliability performance improvements will continue to reduce its portion of total device cost.

CSMH provides high-quality diamond wafers, which can be used as heat sinks, windows, etc. For lasers, it provides customized services such as diamond metallization to ensure increased power and device life.