Electroless copper/nickel films deposited on AIN substrates

Abstract

In this work, we studied the application of electroless copper/nickel (Cu/Ni) films deposited on aluminum nitride (AlN) substrates to high-frequency power GaAs device packaging. Experimental results showed metal films deposited on polished AlN surfaces possess a flatter surface, a finer grain structure, and a lower resistivity than those on unpolished surfaces. On unpolished AlN substrates, rough-surface-induced voids appear in the film interface during grain clustering, therefore deteriorating the electrical conductivity of the deposited layers. Pull-off tests revealed that the Cu/Ni films strongly adhered on both types of AlN substrates and that the adhesion strength exceeded 761 kg/cm(2). The Ni film remained a mixture of amorphous and microcrystalline structures, and the Cu film was polycrystalline. The resistivity of the Cu/Ni film was decreased by annealing process, which in turn decreased the number of crystal defects in the films. Subsequent Pb-Sn solder bumping experiments indicated the amorphous Ni(P) film was a good diffusion barrier layer since Sn could not diffuse through it.