Due to the novel nature of the diamond-based devices under development, there is currently no standard package identified for use with this semiconductor. The main focus of the GreenDiamond WP7 task is the analysis of advanced materials suitable for developing the required packages. WP7 aimed also to design and develop high quality/cost-effective solutions for the study and optimization of high temperature (300ºC) and high blocking voltage (15kV) packages that are adapted to diamond-based semiconductor devices.
To achieve this objective, a fixed package topology based on the SOT-227 industry standard was chosen. The reasons are that the SOT-227 structure allows the packaging of high power single die devices and that it shows the same constitutive elements as multi-chip high-power modules. On the lower part of the proposed package structure, there is a metallic baseplate providing mechanical support, thermal conduction and heat spreading capability towards the final heat extraction system (heatsink, cold-plate, etc.). Pure Cu has been selected as the reference baseplate material, but more advanced options with best matched CTE values have been assessed such as CuMo alloy and Cu-diamond MMC (metal matrix composite). On top of the baseplate, a ceramic substrate provides thermal conduction and electrical isolation between the active device and the external circuit frame. Here, AlN and Si3N4 DCB (Direct Copper Bonded) substrates have been considered, showing good thermal properties and relatively low CTE values. On top of the Cu pads and tracks of the ceramic substrate, the active device is fixed with a die-attach layer. In this case, AuGe solder alloy (~360ºC melting point) and sintered Ag particles have been considered. Particular attention was paid to the Ag sintering solution, analyzing the effect of the processing conditions (pressure, temperature, time) on the final die-attach layer properties. Finally, the semiconductor die and its topside interconnections (wire-bonding) are protected with an encapsulant. This element shows the main limitations in terms of maximum working temperature and three silicone-based references have been analyzed for their use in SOT-227 packages in the range of the 250ºC operating temperature.