Integration of CVD Diamond Heat Spreaders with High-Power Amplifier MMICs for Enhanced Thermal and RF Performance

CVD diamond can enable significant performance enhancements in a range of industrial applications due to its superlative material properties. However, certain diamond properties such as its chemical inertness, low thermal expansion coefficient, high stiffness and high yield stress, while desirable in some applications, can present challenges of integration in others. In this paper we present the successful die-attach integration of metallised CVD diamond heat spreaders with high power RF amplifier monolithic microwave integrated circuits (MMICs), resulting in a substantial improvement in RF performance.

Heat spreaders with room temperature thermal conductivity between 1000 and 2200 Wm^-1K^-1 were metallised with Ti/Pt/Au for facilitating adherence to the diamond and bonding to the semiconductor die. A nano-silver sinter paste was dispensed on each metalized diamond heat spreader using a time and pressure-controlled dispenser and gold metalized MMIC dies wet mounted to the diamond using a die-bonder. Sintering was then carried out by a pressureless sintering process under nitrogen atmosphere, using a temperature profile for drying and then sintering the silver nanoparticles. This process was repeated to attach the die-diamond stack to a CuMo cooling block.

The RF device tested was a 400 W GaN-on-SiC S-band (~3 GHz) amplifier MMIC, and the performance impact of the diamond heat spreaders was evaluated by comparing to identical amplifiers mounted directly to a CuMo block. In all cases it was found that the diamond heat spreaders led to a significant decrease in MMIC temperature and a decrease in thermal resistance of the MMIC package by up to 30%. Thermal simulations show that the amplifiers with diamond heat spreaders can be operated at their maximum junction temperature of 250°C at 10× to 100× longer pulse widths than the device without a diamond heat spreader.

These results successfully demonstrate an attachment method applicable to the integration of CVD diamond to a range of electronic and optoelectronic materials and devices.