Principles of the development of a silica dielectric for microelectronics packaging
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Principles of the development of a silica dielectric for microelectronics packaging Tapan K. Gupta Westinghouse Electric Corporation, Electronic Systems Group, Baltimore, Maryland 21203
Jau-Ho Jean National Tsing Hua University, Hsinchu, Taiwan, Republic of China (Received 1 March 1995; accepted 15 September 1995)
Recognizing that speed, size, reliability, and cost are the principal driving forces for advanced electronic packages, this review article describes the much needed development of a new, phase transformation-free, single-phase silica dielectric with a dielectric constant (k) of about 4, the lowest among the inorganic oxides, and a coefficient of thermal expansion (CTE) of about 3 ppmy±C, similar to that of Si. This dielectric, consisting largely of SiO2 , represents a gain in media speed by about 50% over alumina dielectric, combined with an improvement in reliability of the package by a factor of about 1000. The feature size and system cost can also be drastically reduced by using this dielectric. It is made from a mixture of binary borosilicate glasses that normally exhibit an undesirable characteristic of precipitating cristobalite during sintering that severely weakens the structure. The most important aspect of this article is the design and development of a strategy that prevents the cristobalite growth by incorporating a crystal growth inhibitor in the binary mixture of glasses. Since kinetics, not thermodynamics, are shown to be the key to success of this strategy, the roles of rate-controlling parameters are deliberately emphasized. A working model is delineated to identify compositions that yield a cristobalite-free silica dielectric with values of CTE that match those of Si and GaAs. Critical issues of co-firing between metals and this dielectric are addressed within the context of multilayer packaging fabrication. Finally, a list of measured properties is presented that clearly shows new opportunities for this silica dielectric.
I. INTRODUCTION
The driving forces for the advanced ceramic electronic package for the digital world are higher characteristic speed of the media, smaller feature size of the conductors, greater reliability against mechanical, thermal, and environmental perturbations, and continuing lower cost.1 These requirements translate to a complex materials/process system wherein the ceramic ensures the lowest available dielectric constant, the metal exhibits the highest achievable electrical conductivity, and the technology provides the greatest attainable wiring density. Furthermore, these properties must exhibit little or no dependence on temperature, frequency, voltage, current, and time. When these features are combined in a ceramic substrate that also possesses a coefficient of thermal expansion matched to that of Si, and exhibits a thermal conductivity, and a mechanical strength and an environmental stability that are at least equivalent J. Mater. Res., Vol. 11, No. 1, Jan 1996
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