Low-temperature wafer bonding: a study of void formation and influence on bonding strength

The void formation has been systematically observed for low-temperature (120°C and 400°C) Si-Si and SiO₂-SiO₂ wafer bonding techniques in function of the annealing time (from 70 to 595 h), pressure (low vacuum and atmospheric) and surface pretreatments. Mixed solution (H₂SO₄ and H₂O₂) standard cleaning, warm nitric acid and O₂-plasma-assisted surface pretreatments have been considered and compared. The void formation is clarified according to the void distribution and the measurement of surface energy. Long annealing time periods are considered in order to reach the saturation of the interface chemical reactions. Our experiments demonstrate that the origin of voids appearing in low temperature O₂-plasma-enhanced wafer bonding is related to the great quantity of chemical reaction products. It has been shown that optimized O₂-plasma pretreatment time can lead to void-free, uniform and high surface energy (over 2.0 J/m²) wafer bonding. In the case of SiO₂-SiO₂ wafer bonding, our experimental results show that below a certain critical silicon dioxide thickness the reaction products cannot be absorbed totally and then voids occur. Presenting a higher surface energy than warm nitric acid O₂-plasma is an extremely promising surface pretreatment solution for the increasing demand of low-temperature wafer bonding techniques.