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.