Ion-plated metal/ceramic interfaces

Elemental Cu and Ti films have been deposited onto magnesia-alumina-silica ceramic substrates with a plasma-aided physical vapour deposition (ion-plating) process. This paper presents results of an investigation on how the structure and chemistry of the metal/ceramic interface is modified as a function of the deposition process parameters (e.g. the applied bias voltage and current). Emphasis is placed on determining how the strength of the metal/ceramic interface is controlled by the interphase boundary structure and chemistry. Structural and chemical analyses of the film, interface and substrate regions have employed scanning Auger microprobe, secondary ion mass spectroscopy, SEM/STEM-energy dispersive X-ray and TEM/SEM imaging and microdiffraction techniques. A cross-section sample preparation procedure for direct TEM/SEM studies of metal/ ceramic interfaces is discussed. The strength of the Cu/ceramic interface was found to be strongly influenced by both applied substrate bias voltage and substrate roughness. While evaporated Cu films on substrates polished with standard metallographic techniques had essentially zero adhesion, films deposited with an applied substrate bias showed increasing adhesive strength with increasing bias (in excess of 70 MPa, the limit of the employed epoxy-bonded-stud adhesion test, with -3 KV applied bias). Microchemical analysis indicated that this enhanced adhesion is directly correlated with the development of a chemically graded interface region. The adhesive strength of the ion plated Cu films was also found to be improved with increasing substrate smoothness. The behaviour of Ti, because of its inherent reactivity, was found to be quite different from that of Cu. Although the generally superior adhesion of Ti was evident for films either evaporated or deposited with low bias voltage/current, this adhesion actually decreased for films deposited with high bias voltage/current. Interfacial TEM studies show that this effect results from the formation of a compound at the Ti/ceramic interface region. Results indicate that the thickness of this apparently brittle compound plays a critical role in film adhesion.