Thermal Management of Hotspots Using Diamond Heat Spreader on Si Microcooler for GaN Devices

A diamond heat spreader has been applied on the hybrid Si microcooler for the improvement of the hotspots cooling capability for GaN devices. The microwave chemical vapor deposition diamond heat spreader under tests is of thickness 400 μm and thermal conductivity as high as 1500 ~ 2000 W/mK, and is bonded through the thermal compression bonding process at chip level. Eight hotspots, each of size 450 × 300 μm², were fabricated on a Si thermal test chip to mimic the heating areas of eight GaN units. Heat dissipation capabilities were studied and compared through experimental tests and thermal/fluid simulations, and consistent results have been obtained. Using the diamond heat spreader, to dissipate 70-W heating power, the maximum chip temperature can be reduced by 40.4% and 27.3%, compared with the structure without a heat spreader and the one with a copper heat spreader, respectively. While maintaining the maximum hotspot temperature under 160°C, 10-kW/cm² hotspot heat flux can be dissipated. The thermal effects of the heat spreader thickness, the diamond thermal conductivity, and the bonding layer are investigated. Based on the simulation results, the higher power density of the GaN device can be dissipated, while maintaining the peak gate temperature under 200°C. The concentrated heat flux has been effectively reduced using a diamond heat spreader, and much better cooling capability of the Si microcooler has been achieved for high-power GaN devices.