Cu-AlN direct bonds were made by the gas-metal eutectic method. Two kinds of AlN substrates from different suppliers were used for the research. The AlN surfaces were preoxidized in O2 at three different temperatures. An optional “anneal” in Ar was applied following the oxidation procedure. Two processes were used for the bonding.
The bond strengths and the failure modes were found to depend on the substrate used, the pretreatment of the AlN and bonding conditions. There was an optimum thickness for the Al2O3 layer on the AlN surface. High temperature annealing in Ar was demonstrated to effectively promote bond strength. The highest peel strength measured was 50 N/cm.
A thermal expansion mismatch between Al2O3 and AlN was considered to be the main factor which dominated the ultimate bond strength. This mismatch resulted in a residual stress in the Al2O3 layer. A measurement by XRD indicated a 300 MPa tensile stress existed in the surface Al2O3 after AlN oxidation. This made Al2O3/AlN the weakest interface and limited the bond strength rather than the eutectic bond.