Thermal Transport and Mechanical Stress Mapping of a Compression Bonded GaN/Diamond Interface for Vertical Power Devices

In the last 20 years Gallium Nitride (GaN) electronics have outstripped conventional silicon devices in both efficiency and breakdown voltages. This enhanced performance of GaN devices comes at the cost of increased operating temperatures, kindling a need to develop more robust thermal management schema for such technologies. One such design proposes taking advantage of passive heat diffusion by bonding the GaN device to a high thermally conductive substrate, such as diamond. This would allow the heat generated in the GaN to flow into the diamond, reducing operating temperatures. To test this design, GaN and diamond die were bonded under 2 kN of force via an Ar activated intermetallic bonding layer of Ti/Au. The total bonding area of GaN/diamond stacks was quantified through confocal scanning acoustic microscopy (C-SAM), revealing both bonded and unbonded areas. These areas were confirmed to be bonded/unbonded using a combination of Focused Ion Beam milling and Transmission Electron Microscopy, with the unbonded regions being identified as a delamination at the Ti/diamond layer. Raman microscopy revealed a uniform compressive stress of &lt;80 MPa in the well bonded areas in addition to, stress oscillations where the sample transitions from being bonded to unbonded, corresponding to a net stress differential of up to Δ100 MPa. The thermal properties of the GaN/diamond stack were measured through spatially resolved frequency-domain thermoreflectance (FDTR), with the bonded area boasting a thermal boundary conductance (TBC) of &gt; 100 MW/m²K. FDTR also revealed the presence of micron-scale unbonded regions that showed up as fully bonded under C-SAM. Furthermore, Co-Local Raman/FDTR mapping was demonstrated for the first time with the mapping of a micron-scale unbonded region, showing that stress differentials border the low TBC region. Overall, our work demonstrates a new method for thermal management in vertical type GaN devices that maintains low intrinsic stresses while boosting thermal boundary conductances. As the delaminations seen in these samples are on the order of a possible device (200 microns), more needs to be done to mitigate their formation during initial device fabrication and to spot them post fabrication with more sensitive techniques than C-SAM, such as FDTR.<br/><b>Acknowledgements</b>: Sandia National Laboratories is a multi-mission laboratory managed and operated by the National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract No. DE-NA0003525. SAND2023-11173A