Interfacial adhesion and microstructure of thick film metallized aluminum nitride substrates

The adhesive strength and fractured microstructure of aluminum nitride (AlN) substrate metallized with resistor thick-films followed by firing at 850 oC have been investigated via a pull-out rupture test loaded in tension. The AlN substrate used includes the as-received form and an oxidized form that was thermally treated at 1450 oC prior to the metallization. The thick-film paste includes both the as-received form (which is an RuO2-based resistor) and a mixture of the paste with varying fractions of submicrometer alumina powder (5–20 wt.%) for comparison purposes. When the as-received AlN is used as the substrate, rupture strength decreases monotonically from 1.2 to ∼0.1 MPa as the alumina loading (in the film) increased from 0 to 20 wt.%. The strength, however, remains relatively constant (∼0.5 MPa) for the pre-oxidized AlN case, i.e. the adhesion is found insensitive to the change of alumina loading. This strength dependency has been compared with their fractured microstructure and the possible fracture mechanism that caused the failure is discussed.