We prepared a 635-μm-thick AlN ceramic plate and a 30-μm-thick Al foil. The average roughness (Ra) of the surface in AlN plate was ∼0.5 μm. Ra of the Al foil was 0.06 and 0.19 μm in its mirror and matte surfaces, respectively. The matte surface of Al foil was directly bonded to the AlN plate through a surface activation process using an Ar fast atom beam.

In a preparatory study we fabricated an AlN/Al foil/AlN junction by bonding AlN plates to the both sides of an Al foil. We observed the cross section of AlN/Al foil/AlN junction by using scanning electron microscopy. We found that the AlN plate and Al foil were bonded to each other without gaps at the both AlN/Al interfaces although Ra of AlN plates and Al foils was much larger than typical Ra of semiconductor substrates that were bonded using SAB (≤~1 nm).

We prepared an SiC epi substrate by growing an n⁺-SiC buffer layer and an n^--SiC epi layer on an n⁺-4H-SiC (0001) substrate. We formed an ohmic contact on the backside of the epi substrate by evaporating an Al/Ni/Au and annealing at 1000 °C. Then we made a 300-μm-ϕ Ni/Au Schottky contact on the top by evaporation and lift-off. An Al layer was evaporated on the top of ohmic contact so that an SiC Schottky diode die was fabricated. Then the Al surface of ohmic contact of SiC die was bonded to the mirror surface of Al foil on AlN plate using SAB. Note that no intermediate layers such as solder were inserted at bonding interfaces.

We measured the I-V characteristics between the Schottky contact of SiC die and the surface of Al foil at temperatures between room temperature and 300 °C. A reverse-bias current was ~10^-7 A/cm² at a reverse-bias voltage up to 6 V for temperatures lower than 250 °C. A larger reverse-bias current (~10^-6 A/cm²) was observed in measurement at 300 °C. In the forward characteristics, lower turn-on voltages and larger series resistances were observed for higher temperatures. It is notable that these features are typically observed for Schottky diodes. In addition, no change was observed in appearance of the SiC die/Al foil/AlN junction even after the measurement at 300 ℃. Although the bonding strength in Al foil/AlN interfaces is still unclarified, these results suggest that SAB technologies could be useful for realizing solder-free modules of power devices.

Acknowledgment—The Al foil employed in the work was supplied from Toyo Aluminium K.K.