Densification kinetics of glass films constrained on rigid substrates
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The kinetics of constrained-film sintering were studied in a borosilicate glass (BSG) + silica system because of their applications in microelectronic packaging technologies. Samples with a silica content by 20% by volume were prepared from slurries of powder mixtures in a commercial polyvinyl butyral (PVB) binder solution. Constrained films about 0.2 mm thick were formed by doctor-blade casting the slurries on silicon wafers. Free-standing films about 0.6 mm thick were also produced by casting the slurries on a treated mylar sheet for easy lift-off. Sintering experiments were carried out in a hot stage at temperatures between 715 °C and 775 °C. Shrinkage profiles of the free and constrained (shrinkage in thickness only) films were determined in situ using a custom-designed optical system. The densification rates measured in the constrained films were slower than those in the free films. However, the substrate constraint had no effect on the activation energy of densification which was found equal to 385 ± 10 kJ/mol, the same for both free and constrained films. A relation between the constrained-film and free-film densification profiles was derived using the viscous analogy for the constitutive equations of a porous sintering body.
I. INTRODUCTION Sintering of porous films on rigid substrates is an important process in the manufacture of microelectronic packages. For example, the manufacturing process of hybrid packages involves constructing porous metal and dielectric films on ceramic substrates and then sintering together at high temperatures.1 The metal/dielectric structure forms the electrical interconnecting network of the package. During the sintering process, the porous materials are constrained from shrinking in the substrate plane and allowed to shrink only in thickness. Constrained-film sintering can lead to undesirable defect formation, such as delamination or cracking of the film and distortion of the substrate.2"7 Currently, packaging engineers are limited to trial and error solutions to minimize these defects, e.g., adjusting material compositions and firing temperature-time profiles. A systematic problem-solving approach requires a basic understanding of the defect-formation mechanisms during constrainedfilm sintering. Studies of constrained-film sintering have been hampered by the lack of a suitable technique for realtime measurement of constrained densification kinetics. Methods currently available for measuring densification kinetics are dilatometry and thermomechanical analysis (TMA), which are not designed for constrained films. Only Garino and Bowen7 carried out an experimental study on constrained-film sintering. They measured densification kinetics of constrained and free-standing glass and ceramic films. An optical technique was deJ. Mater. Res., Vol. 10, No. 5, May 1995
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veloped to measure thickness shrinkage profiles in the constrained films by monitoring a reflected laser beam on a distant wall. The shrinkage profiles of free films
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