Atomistic simulation analysis of plasma surface activation in wafer-to-wafer oxide fusion bonding

Using a variety of modeling techniques in different scales, we gained insights of surface chemical reactions and built the possible bonding mechanism during the surface activation and hydration processes by combining with the experimental results. Experimentally, we measured the bonding strength of the thermal silicon dioxide (TOX) with surface activation process, N-2 and O-2 plasma under the same plasma conditions. The results showed that the bonding strength with the N-2 treatment is higher than that with the O-2 treatment. Using the quantum chemistry calculation, we built the mechanism explaining how O/N-radicals modify the surface of SiO2 to increase the bond density. O atoms binding with H, N and C in the SiO2 films to create OH, CH2O, CO, HNO and nitic oxides (NO) groups. NO-groups are hydrolyzed to form additional surface OH during the deionized water rinsing step, increasing the density of Si-O-Si formation and total bonding energy. Atomistic molecular dynamic simulation also confirmed how nitrogen atoms introduce silanol and siloxane group to the surface during the surface activation treatment and the hydration reaction. We also studied the population effects of N atom on bonding formation, indicating more N atoms exist will lead to better bonding. We extended our work to explain why bare-Silicon (Si) film with same surface activation method diminishes the bonding strength unlike SiO2 wafer. During a deionized water rinse, the hydrophobicity of bare-Si created a condensed water on modified bare-Si surface regardless of a surface activation. This generated condensed water would cause the slip of wafer during a bonding process and lead a weak bonding strength. We believe this study would be beneficial to develop the process to enhance a bonding energy of wafer-to-wafer bonding.