Influence of the grain boundary phase on the mechanical strength at aluminum nitride substrate surfaces

The influence of grain boundary phases on the mechanical strength at aluminum nitride (AlN) substrate surfaces, after a wet chemical process, was studied using AlN substrates doped with 2 wt% CaO or 4 wt% Y₂O₃. Mechanical strength was measured by pin pull-off, pin bending tests and Vickers hardness measurements. Test specimens were obtained by the processes of lapping, polishing, thin-film metallizing, photoengraving, plating, and brazing. The test results showed that the mechanical strength at the surface of Y₂ O₃-AlN substrates was higher than at CaO-AlN surfaces, where it was under the adhesion strength between the metallization layer and the AlN substrate. The mechanical strength at the surface of the CaO-AlN substrate was lost during patterning and plating. This was accompanied by an important weight loss in HNO₃ and K₂ O·n(B₂O₃) aqueous solution, due to dissolution of the grain boundary phase. The reason for the difference in the mechanical surface strength of AlN substrates after patterning and plating is that the CaO-AlN grain boundary phase dissolves in the chemical solution in the process more easily than the Y₂O₃-AlN gram boundary phase. The Y₂O ₃-AlN substrate showed unchanged pin pull-off and bending strength even after thermal cycling (1000 cycles) and 1000 h of a pressure cooker test. Although the CaO-AlN grain boundary phase seemed to cover all the grains, thermal conductivity was only slightly affected, because the grain boundary film was very thin