CONSTRAINED-FILM SINTERING OF A GOLD CIRCUIT PASTE

We studied the constrained-film sintering of a gold circuit paste used in microelectronic packaging applications. Optical techniques were de veloped to determine the shrinkage profiles of constrained and free fi lms and stresses generated during sintering in the constrained films. Constrained films approximately 60 mu m thick were made by multiple sc reen-printing of the gold paste on rigid alumina substrates, while the free films were obtained by peeling off portions of the gold films fr om the substrate after binder burnout. Constrained films for stress me asurement were made by multiple screen-printing on an oxidized 25 mu m thick silicon substrate. Sintering runs were done in a hot stage at t emperatures between 650 degrees C and 900 degrees 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, determ ined by wafer curvature measurement, rose rapidly to a maximum level o f 510 kPa during the initial stage of sintering and then gradually dec reased. The reduction in the sintering potential due to the hydrostati c stress is not large enough to completely account for the retarded de nsification in constrained films. SEM micrographs of the film microstr uctures after sintering showed no significant difference in grain grow th kinetics between the constrained and free films. However, the activ ation 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 retar ded densification kinetics in the constrained gold films is due to (i) the reduction in the sintering potential by the hydrostatic stress an d (ii) a change in the dominant sintering mechanism from grain-boundar y diffusion in the free films to lattice diffusion in the constrained films.