Constrained-film sintering of cordierite glass-ceramic on silicon substrate
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Constrained-film sintering of cordierite glass-ceramic on silicon substrate Jesus Noel Calata, Allen Matthys, and Guo-Quan Lu Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received 11 July 1997; accepted 14 November 1997)
The densification behavior of cordierite glass-ceramic films constrained on a rigid silicon substrate was studied in the temperature range from 900 ±C to 1000 ±C. An optical setup was used to obtain the thickness versus time profiles and in-plane stresses of the constrained glass-ceramic during isothermal sintering. The thickness profiles showed a rapid shrinkage due to sintering followed by an expansion corresponding to crystallization of the glass-ceramic. Measurements of in-plane stresses in constrained-sintering films showed a rapidly rising tensile stress during densification followed by a slight drop during crystallization. In films sintered above 950 ±C, the tensile stress rose rapidly again near the end of crystallization, suggesting a further densification in a mostly crystallized film. Scanning electron microscope (SEM) micrographs of the film cross sections revealed the formation and growth of large pores along the interface between the glass-ceramic and silicon substrate that may have contributed to the observed film expansion. These pores are substantially larger than the initial pore size in the films, indicating that they were formed during sintering. We believe that poor wetting of the glass-ceramic on silicon may have contributed to the formation of the porous structure at the interface.
I. INTRODUCTION
In the fabrication of microelectronic packaging substrates, one often encounters the problem of constrainedfilm sintering. This could arise from mismatched sintering kinetics between different materials in a structure of cofired multilayered metal/ceramic substrates1 or in the case of ceramic and/or metal layers fired directly on a rigid substrate (e.g., a heat sink), for the construction of a hybrid package.2,3 Constrained sintering can lead to serious defects in fired parts, such as camber, low density, or high porosity that can shorten the lifetime of these parts or make them unusable.4,5 An understanding of the formation of defects due to constrained-film sintering is still relatively rudimentary because of limited experimental work brought about by experimental difficulties. In the absence of suitable experimental techniques, others have to resort to experiments involving indirect measurements as well as theoretical modeling to obtain insights into the process.6–9 Recently, we developed simple optical techniques to measure shrinkage profiles of free and constrained films and inplane stresses in constrained films. These techniques have been successfully used to study the densification behavior of glass and metal films on rigid substrates.10,11 In this study, we report our findings on the densification behavior of cordierite glass-ceramic constraine
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