SiO2-SiO2 die-to-wafer direct bonding interface weakening

Nowadays silicon (Si) based die-to-wafer (D2W) direct bonding technique is of great interest for enabling advanced 3D-integrated device applications. The small size of bonded dies makes an important difference from standard wafer-to-wafer direct bonding in terms of bonding mechanism or bonding strength evolution, because it allows rapid lateral diffusion of the chemical species related to direct bonding at the interface. In the case of hydrophilic SiO 2 -SiO 2 direct bonding, water plays an essential role for bonding area evolution. Control of sample storage condition is therefore quite important because the humidity of storage environment may affect water diffusion at the bonding interface. However, there is a lack of in-depth study about the reliability of the SiO 2 -SiO 2 D2W direct bonding due to the difficulty of evaluating the strength of the bonding interface. In this paper, using a previously developed modified four-point flexion test, we have systematically investigated the effect of sample storage before or after post-bonding annealing on SiO 2 -SiO 2 D2W direct bonding energies. Weakening of the SiO 2 -SiO 2 D2W direct bonding interfaces was observed during sample storage in humid ambiences after post-bonding annealing. This unexpected weakening effect could be explained by the water stress corrosion which occurs around asperity contacts of the bonded SiO 2 surfaces due to water molecule diffusion at the bonding interface after post-bonding annealing.