| A major reliability issue in microelectronic packaging is adhesion of the many dissimilar materials employed for their electrical, mechanical, and cost-saving characteristics. Even a small area of weak adhesion between adjacent layers can quickly grow to a breach in the hermetic seal protecting a chip from its harsh operating environment. Once moisture enters the package, electrical contacts can be shorted, severely limiting the operational lifetime of the device. Silane coupling agents show promise as a method to augment adhesion of packaging interfaces, particularly those between polymers and inorganic materials. This study employs x-ray photoelectron spectroscopy (XPS), contact angle measurements, and fracture mechanics-based testing methods to clarify the important factors in selection and use of silanes as adhesion promoters. Fifteen silanes of varying functionalities and chain lengths were studied under a variety of experimental conditions to determine their effect can adhesion of BCB polymer to silicon oxide.; It was found that the hydrolysis and condensation reactions required for silane deposition could be controlled by timing the addition of spin-coat solution components to the mixture. By varying the hydrolysis time, it was shown that higher surface coverages and comparable adhesion energies could be obtained without the use of a propylamine catalyst commonly used in solution preparation. When values for a single silane are compared, adhesion energy was found to exhibit a weak dependence on both hydrolysis time and silane solution concentration. Comparison of the adhesion energies for silanes of varying functionalities reveals that the potential for covalent bonding between silane and BCB is not the only important criterion for silane selection. Only after adhesion energies were normalized for differences in surface coverage was a result obtained that could have been predicted based on chemical reactivity. Finally, adhesion energy results for silanes of varying alkyl chain length were compared for existing models of polymer chain entanglement. An increase in adhesion energy with increasing chain length was observed for CH3-terminated silanes, while the opposite trend was observed for vinylfunctional silanes. When quantified, neither trend is consistent with the predictions of entanglement-based models. |
