Characterization of the AIN-W Interface in a Cofired Multilayer AIN Substrate

The AIN–W Interfaces in a cofired multilayer AIN substrate were observed using an optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Optical and SEM observations showed an intricate intricatelocking AIN‐W grain structure at the interface. After the W pad was removed from the substrate with a NaOH etchant, the surface morphology of the W metal at the interface side was found to be very rough, with a small‐grain microstructure compared with that at the free surface side. Electron microprobe analyses using SEM revealed that there was no diffusion of either W or Al at the interface at the order of a few micrometer's resolution. Bright‐field images, dark‐field images and selected area electron diffraction (SAD) patterns using TEM indicated there was no secondary phase between AIN and W. However, scanning transmission electron microscopy using an energy dispersive X‐ray detector revealed that there was a 200‐nm thick W diffusion layer from the interface into the AIN ceramics. It was concluded that the high adhesion strength between the W conductor and the AIN substrate (>20 MPa) was not due to any secondary phase but to mechanical interlocking of AIN and W during cofiring.