Microanalyses of chemically etched thin film alumina-ferrite interfaces

Thin films of aluminium oxide were deposited on ferrite (Ni _x Zn_1−x Fe₂O₄) substrates by r.f. sputtering. The sputtered alumina films were not easily etched by hot phosphoric acid unlike readily etchable films prepared by physical deposition techniques. Microanalytical characterization of unetched films, partially etched films and interfacial regions was conducted to identify the microscopic features responsible for reluctant film etchability. The post-etched films were categorized as easily, partially and un-etchable (EE, PE and U respectively) and were examined using optical microscopy, SEM, XRD, EDS, XPS, AES, and TEM/STEM. TEM examination of cross-sections of partially etchable films revealed a non-uniform crystalline phase at the film-substrate interface. Electron diffraction data identified the phase asη-alumina although AES and EDS results suggest that the interfacial phase also contained some iron. The occurrence and orientation of theη-alumina phase was shown to depend on the orientation of the grains of the ferrite substrate.