## 4 Discussion
Plasma treatment modifies the surface structure over a depth of approximately three to five nanometers through ion bombardment [60]. This modification of the subsurface silica atomic order likely facilitates water penetration and enhances silica--water hydrolysis kinetics, even at room temperature. This effect is further amplified in the presence of increased pH. For instance, the addition of DEAE immediately after plasma treatment and just before bonding leads to an even more rapid increase in bonding energy at room temperature over time (see Figure 15). In the absence of plasma treatment,  
Figure 14: Silicon dioxide thickness measured by ellipsometry as a function of etching time in different solutions [59]. Figure 15: Comparison of plasma and DEAE effects on silicon dioxide direct bonding energy at room temperature aging the bonding in the clean room environment. merely increasing the pH at room temperature is insufficient to achieve a tenfold increase in adherence. This suggests that additional energy is required for water to penetrate the asperity volume when the asperities have not been exposed to plasma treatment. Thermal annealing, in the absence of plasma treatment, could supply this energy. The mechanism of asperity volume hydrolysis aligns most consistently with the observed results. At present, asperity hydrolysis--referred to as the second mechanism--offers the most straightforward and coherent physical explanation for these phenomena. In conclusion, based on the plasma treatment results, the second mechanism appears to be the correct one. An additional argument supporting the asperity hydrolysis phenomenon can be drawn from the work of [PERSON] et al. on the SAB bonding mechanism [61]. In their study, the authors demonstrated that surface dangling bonds alone are insufficient to achieve high bonding energy. They used hydrophobic silicon surfaces and soft SAB activation to highlight the role of the amorphous silicon layer in the SAB mechanism. The soft SAB activation was sufficient to remove all slane bonds from the silicon surface, leaving reactive dangling bonds without producing a thick amorphous layer. One of the hydrophobic surfaces studied featured atomic terraces obtained through silicon epitaxial layer deposition, resulting in extremely low roughness. [PERSON] et al. showed that only this exceptionally smooth surface could achieve high bonding energy, whereas a standard silicon surface with typical roughness (approximately 0.13 nm RMS) could not, even in the presence of reactive dangling bonds. If the roughness asperities of the silicon surface are not softened by the amorphization induced by SAB activation (which happened with a longer activation time), the dangling bonds remain too far apart to react effectively. Of course, for the extremely low roughness surface, the need for a soft amorphous layer is less pronounced, and a good bonding energy is obtained even after a soft SAB activation. With a surface having a standard roughness, the presence of a soft amorphous silicon layer is therefore essential, in addition to the dangling bonds. This layer facilitates the widening of asperities and enables dangling bonds to form covalent linkages across the bonding interface. Consequently, for the hydrophilic bonding of this study (which has similar roughness as Lomonaco's standard silicon surface), the enhancement of adherence is not primarily dependent on the catalysis of silanol condensation. Indeed, taking as a reasonable hypothesis that the surface density of dangling bonds and silanol one is roughly equal, even if silanol condensation were as energetically favorable as the recombination of dangling bonds, the atoms from both surfaces would still need to be in close proximity to react, as observed in the Lomonaco study. Surface asperities must be softened to allow silanol condensation, even if the condensation kinetics are very fast. Thus, silanol condensation kinetics do not appear to play the primary role.