| Wafer scale 3D integration is recognized as an emerging technology to increase the performance of ICs. When bonding with processed ICs, the bonding process must be compatible with IC back-end processing. The fraction of bonded area was examined by optical inspection and BCB was selected as the baseline glue after achieving reproducible void-free bonding. Bond strength at the glue interface of bonded wafers was quantified by four-point bending. Using four point bending, the following effects of BCB glue on the bonding integrity were evaluated; (1) employment of adhesion promoter, (2) BCB glue thickness and (3) material stack. When the adhesion promoter is used, bond strength increases at both BCB bonds of 2.6 μm and 0.4 μm. These results also demonstrate that BCB glue thickness affects the bond strength at the glue interface with thicker glue layers corresponding to higher bond strength. The decrease in bond strength observed for thin BCB is due to a decrease of plastic dissipation energy, Gplastic, which is proportional to BCB thickness. In both bonded wafer pairs that include a PECVD oxide deposited silicon wafer and a glass wafer, bond strengths are linearly proportional to BCB thickness. With these results, the relationship between Gplastic , and bond breaking energy, Gtip, and BCB thickness, t is observed to be Gplastic ≈ 0.3 · Gtip · t. The effects of thermal cycling on bond strength and residual stress at the interface between BCB and a PECVD oxide, and the thermal stability of BCB were evaluated by four point bending and wafer curvature measurements. Stress relaxation of the PECVD oxide layer during thermal cycling leads to a decrease in the deformation energy due to residual stress, G residual, and to an increase in bond strength. In thermal cycling performed at temperatures of 350 and 400°C, it is observed that the relaxation of residual stress occurs predominantly during the first thermal cycle. Conclusively, the BCB process for wafer-to-wafer bonding applications is stabilized after four cycles at a temperature of 400°C. Thermal cycling performed at a temperature 450°C leads to cohesive failure within the BCB layer with low bond strength (<0.5 J/m2). |
