PRINCIPLES OF THE DEVELOPMENT OF A SILICA DIELECTRIC FOR MICROELECTRONICS PACKAGING

Recognizing that speed, size, reliability, and cost are the principal driving forces for advanced electronic packages, this review article d escribes the much needed development of a new, phase transformation-fr ee, single-phase silica dielectric with a dielectric constant (k) of a bout 4, the lowest among the inorganic oxides, and a coefficient of th ermal expansion (CTE) of about 3 ppm/degrees C, similar to that of Si. This dielectric, consisting largely of SiO2, represents a gain in med ia speed by about 50% over alumina dielectric, combined with an improv ement in reliability of the package by a factor of about 1000. The fea ture size and system cost can also be drastically reduced by using thi s 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 o f a strategy that prevents the cristobalite growth by incorporating a crystal growth inhibitor in the binary mixture of glasses. Since kinet ics, not thermodynamics, are shown to be the key to success of this st rategy, the roles of rate-controlling parameters are deliberately emph asized. A working model is delineated to identify compositions that yi eld a cristobalite-free silica dielectric with values of CTE that matc h those of Si and GaAs. Critical issues of co-firing between metals an d this dielectric are addressed within the context of multilayer packa ging fabrication. Finally, a list of measured properties is presented that clearly shows new opportunities for this silica dielectric.