Water stress corrosion at wafer bonding interface during bond strength evaluation

Abstract

Wafer-to-wafer bonding is becoming a critical unit process step with the increase in the demand for 3D integration. The widely used bonding method is plasma activated bonding, where it is essential to ensure sufficient bond strength to withstand the stresses generated during subsequent processes and achieve multi-layer stacking or layer transfer. Although the double cantilever beam method is widely used to evaluate wafer bond strength, significant variation occurs due to water stress corrosion at the bonding interface. These factors have not been adequately addressed, and precise measurement conditions have yet to be established. In this study, a semi-auto tool was introduced to conduct measurements under an inert atmosphere, whereby the influence of water stress corrosion was eliminated. Furthermore, the study elucidated the effects of the crystal orientation and blade insertion conditions. The impacts of interfacial voids and the annealing conditions on bond strength were revealed. Observation of the progress of delamination during measurements under an inert atmosphere enabled the effective estimation of the moisture content that remained inside the wafers, which then contributed to the selection of appropriate annealing conditions.