Densification and Crystallization Kinetics of Cordierite Glass-Ceramic Coatings on Rigid Substrates
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glass undergoes crystallization. Glass-ceramics are useful materials for making ceramic packaging substrates because of their tailorable properties and sinterability at low temperatures. Many glassceramic compositions[ 12-14] have been developed to have: low dielectric constants for fast signal transmission; thermal coefficients of expansion (TCEs) matched to those of semiconductors for better reliability; and densification profiles suitable for cofirability with high-conductivity metals, such as copper, silver, or gold. 483 Mat. Res. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society
EXPERIMENT The glass powder used in this work is a crystallizable magnesium-aluminosilicate glass from Sem-Com Co., Inc. (P.O. Box 8428, Toledo, OH 43623). The glass is composed of 51% Si0 2, 24% MgO, 21% A120 3 which is off from stoichiometric cordierite (2MgO.2AI20 3 .5SiO 2) which is the major phase crystallizing out of the glass[13,14]. The glass also contains small amounts of sintering aids: P20 5, to promote the formation of a-cordierite, a low-expansion form of cordierite; and B20 3 which raises the crystallization temperature thus allowing densification to occur prior to crystallization. The raw powder had an average particle size of 10 ptm. It has a density of 2.66 g/cm 3 and a thermal expansion coefficient (TCE) of 4.6 ppm in the amorphous state. The density of cordierite is about 2.2 and its TCE is 4.1 ppm. Silicon and aluminum nitride were chosen as the substrates to minimize TCE compatibility problems. A two-stage milling procedure was used to prepare the glass for slip-casting onto the substrates to form uniform coatings[15,161. Polyvinyl butyral (Monsanto Butvar B-79) was used as the binder for the mixture. Constrained film samples were prepared by casting the slurry on 0.5 mm (20-mil) thick silicon wafer and 0.64 mm (25 mil) aluminum nitride using a doctor blade. The dried coatings were between 150 to 200 j.tm thick. Freestanding tape was produced by casting part of the slurry on a coated mylar sheet for easy peel-off. The tapes were air dried followed by a 3 hour drying process at 115'C. This procedure ensured a strong adhesion between the tape and the silicon substrate in the green state enabling it to withstand the subsequent sample preparation without debonding. The coated substrates were cut into 5 mm by 6 mm rectangles. Part of the tape was stripped from each rectangular substrate leaving a bare surface about 2 mm wide. The relative green density of the coatings was 0.60 with an estimated error of 2%. Constrained sintering experiments were carried out in a furnace with a cavity approximately 1 cm across and change in film thickness was monitored optically with a He-Ne laser beam.[10,11] A sample is placed on top of the thermocouple platform and two alumina spacers are laid out, one on the film and the other on the bare substrate. A highly polished rectangular silicon is then positioned across the spacers such that it forms an angle to the sample plane. The silicon is used to reflect the laser beam onto a
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