An evaluation process of polymeric adhesive wafer bonding for vertical system integration

Bonding of wafers using dielectric polymer thin films as bonding adhesives is one of key approaches to monolithic vertical system integration. As the first step for the integration, properties of the polymer desired for baseline adhesive material are studied, followed by the evaluations on bonding results obtained from wafer pairs bonded between a silicon and a glass wafers. Four sequential evaluation procedures are performed after bonding; (1) optical inspection is made to measure the fraction of bonded area, (2) a razor blade test is taken to separate the bonded wafers, (3) a four-point bending method is used to measure bonding strength quantitatively, (4) FTIR is applied to analyze changes in chemical structure. Bonding integrity is evaluated how the intentionally created debond areas of the bonded wafers are changed after the subsequent thinning processes. To date, benzocyclobutene (BCB), Flare™, and methylsilsesquioxane (MSSQ) and Parylene-N have been considered as bonding adhesives. Among these polymer materials, BCB and Flare™ are selected as initial adhesive materials after a screening process through optical inspection. Wafer pairs bonded using Flare™ have higher bond strength than those using BCB. However, the bond strength of wafer pairs bonded using BCB is still higher than that at the interface between Cu and porous low-k inter-level dielectrics (ILD) (a back end interconnect structure), indicating almost 100% of bonded area reproducibly. In the light of such evaluation results, BCB has been eventually selected as a baseline adhesive material for the wafer bonding prcess for vertical system integration.