Microstructure changes in nanoparticulate gold films under different thermal atmospheres and the effects on bondability

Sintering in a furnace under different thermal atmospheres of air, nitrogen (N2) or N2 bubbled through formic acid (FA/N2) was carried out for a spin-coated gold nanoparticle (NP) ink. Temperatures of 200, 250 and 290 °C were applied for each atmosphere. The size of the NPs was measured by transmission electron microscopy and the decomposition temperatures of the solvent and the organic capping molecules of the ink were determined by thermo-gravimetric analysis. The changes in the microstructure of Au NP films after sintering were studied using the field emission scanning electron microscopy, X-ray diffractometry, atomic force microscopy and focused ion beam analysis. Organic residues remaining on the film were detected by Fourier transform infrared spectroscopy and sheet resistance was measured using a four point probe for the calculation of resistivity. Wire bonding tests on the Au film were performed for bondability. The Au NP films sintered under air showed neck growth as temperature increased, while the films sintered under N2 showed grain growth, except for the film sintered at 290 °C. Coalescence and grain growth as well as porosity were observed in the film sintered under FA/N2. The infrared absorption peaks of stretch and deformation were found as organic residues, with C–O stretch peak only being detected in the film sintered under FA/N2. All of the samples represented a preferred Au (1 1 1) orientation. The film sintered under N2 showed good quality compared with those sintered under air or FA/N2 and the resistivity was about twice the bulk value. Wire bonding tests were successful in all the films sintered under air, N2 or FA/N2 atmospheres.