*4.2. Challenges of Bonding GaN and Diamond Wafers*

SAB and VdW bonding are the most promising methods for bonding diamond GaN and diamond substrates. Both methods have a few advantages in comparison to the fabrication of GaN-on-diamond wafers by direct diamond CVD. To begin with, the GaN wafers without the nucleation and strain relief layers can be bonded to SCD plates that feature an extremely high κ, whereas the κ of the films close to the GaN on the GaNon-diamond wafers is quite low. In addition, only an extremely thin Si layer (or no layer, in the case of VdW bonding) is required to bond the two materials, which allows the *TBR*GaN/diamond to be significantly reduced. Finally, the bonding is performed at a temperature significantly lower than the 700–800 ◦C required to deposit good quality PCD (RT in the case of SAB, 300 ◦C in the case of VdW bonding), which means there is no residual stress (or that it is minimal) at the diamond/GaN interface.

 But the bonding of the wafers also presents some drawbacks. The area of available SCD substrates is quite small (only a few mm<sup>2</sup> ), which seriously compromises the scalability of the process. Bonding GaN wafers with large area PCD substrates could in principle overcome this limitation; however, bonding with PCD substrates is not reproducible in the case of VdW bonding. In a recent work, the SAB of GaN and PCD with a Mo/Au interlayer has been reported and the mechanical strength of the interface was evaluated, but the thermal characterization is still lacking.

– Despite these limitations, the potential of the technique has been recognized by Fujitsu and Mitsubishi, and it is expected that commercial devices will be available in a near future. The SAB of GaN HEMTs and large area PCD substrates is also regarded as an area worth of investigation, since the successful bonding of GaN and PCD would allow the fabrication of large area GaN/diamond wafers with improved heat extraction.
