4. Conclusion

This article shows the importance of adhesion energy in the direct bonding process between two silica or silicon surfaces. For these two surfaces to bond spontaneously, attractive forces must exist between them. These forces can be van der Waals, hydrogen bonding or covalent, but at low density. In all cases, however, these adhesion forces remain weak, and direct bonding requires very smooth surfaces on an atomic scale.

Surface roughness therefore plays a decisive role in molecular bonding: it determines the density of short-range interactions between surface asperities. The preparation of bonding surfaces involves careful polishing and cleaning, to eliminate contamination and achieve hydrophilic surfaces. The various surface polishing steps, including roughing, smoothing and polishing, are all designed in turn to reduce roughness and ensure the flatness required...