Constrained-film sintering of a gold circuit paste

Abstract

We studied the constrained-film sintering of a gold circuit paste used in microelectronic packaging applications. Optical techniques were developed to determine the shrinkage profiles of constrained and free films and stresses generated during sintering in the constrained films. Constrained films approximately 60 μm thick were made by multiple screen-printing of the gold paste on rigid alumina substrates, while the free films were obtained by peeling off portions of the gold films from the substrate after binder burnout. Constrained films for stress measurement were made by multiple screen-printing on an oxidized 25 μm thick silicon substrate. Sintering runs were done in a hot stage at temperatures between 650 °C and 900 °C. The densification rates were much lower in the constrained films than those in the free films. The in-plane tensile stresses in the constrained films, determined by wafer curvature measurement, rose rapidly to a maximum level of 510 kPa during the initial stage of sintering and then gradually decreased. The reduction in the sintering potential due to the hydrostatic stress is not large enough to completely account for the retarded densification in constrained films. SEM micrographs of the film microstructures after sintering showed no-significant difference in grain growth kinetics between the constrained and free films. However, the activation energy for densification was found to be very different between the two types of films, 90.1 ± 4.3 kJ/mole for the free film and 188.8 ± 6.7 kJ/mole for the constrained film. We suggest that the retarded densification kinetics in the constrained gold films is due to (i) the reduction in the sintering potential by the hydrostatic stress and (ii) a change in the dominant sintering mechanism from grain-boundary diffusion in the free films to lattice diffusion in the constrained films.