## 3 Results  
As shown in Figure 4a, the bonding waves for the different samples are smooth, with a nearly constant velocity of approximately 20 mm/s, which is typical of standard  
Figure 3: Experimental setup for anhydrous DCB bonding energy characterization [33]. Figure 2: Principle of the DCB under prescribed displacement for direct wafer bonding energy measurement. \(\delta\) is the blade thickness, t the wafer thickness, and a the crack length, which is the sum of the blade level Xb, the IR debonded length measurement Xm, and the calculated post IR debonded length Xa [33]. hydrophilic bonding. This suggests that the adhesion energy is not affected by the different surface treatments. No bonding defects are detected, even when using scanning acoustic microscopy (see Figure 4b). Figure 5 shows the evolution of anhydrous direct bonding energies as a function of annealing temperature for various acidic and basic molecules. The concentrations were chosen to remain below the threshold for inducing particle contamination after spin drying. Although ammonia could have been used at a higher concentration, a similar concentration to the other molecules was chosen for comparative purposes. Regardless of the annealing temperature, the bonding energies increase in the presence of basic molecules. Expanding on this finding, the influence of the pH of the spin-dried solution is examined using strong base NaOH solutions at concentrations ranging from \(10^{-8}\) g/cm\({}^{3}\) to \(10^{-5}\) g/cm\({}^{3}\). A NaOH solution of \(10^{-5}\) g/cm\({}^{3}\) represents the maximum concentration enabling spontaneous bonding. Indeed, with a NaOH solution of \(10^{-4}\) g/cm\({}^{3}\), bonding does not occur due to excessive NaOH \"particles\" (Figure 6), detected using diffusion light scattering with a size threshold of 1000 nm. Figure 4: **(a) Infrared characterization of the bonding wave, taken midway through its propagation. The white spot in the middle of the first half of the image corresponds to the infrared camera’s image and is not a bonding defect. (b) Typical scanning acoustic microscopy image of the bonding studied. No bonding defects are detected. The pixel size is 10 μm. **  
Figure 5: Influence of various acidic and basic molecules on anhydrous direct bonding energy as a function of the annealing temperature. The cross represents the sample breakage, meaning that the bonding energy is higher than Si bulk material fracture energy (We have chosen arbitrarily to put the cross at 6.5 J/m\({}^{2}\)). These \"particles\" are probably sodium hydroxide crystals. Indeed, atomic force microscopy (AFM) did not reveal any changes in roughness (see Figure 7), and after DiW rinsing (Figure 8), fewer and smaller \"particles\" were otherwise detected on the wafer surface. Figure 9 shows that, at concentrations below \(10^{-5}\) g/cm\({}^{3}\), higher NaOH concentrations result in higher bonding energies whatever the annealing temperature. High pH thus yields high direct bonding energies, given the calculated spreading pH solution for each NaOH mass concentration (refer to Table 2). The influence of cations will be discussed later. \begin{table}
\begin{tabular}{c c} \hline \hline
**NaOH Solution Concentration** & **Calculated pH** \\ \hline \(10^{-5}\) g/cm\({}^{3}\) & 10.4 \\ \(10^{-6}\) g/cm\({}^{3}\) & 9.4 \\ \(10^{-7}\) g/cm\({}^{3}\) & 8.4 \\ \(10^{-8}\) g/cm\({}^{3}\) & 7.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Calculated pH of NaOH solutions that are spread on wafers prior to bonding. Figure 8: Diffusion light scattering images @90 nm of Si wafers after NaOH \(10^{-4}\) g/cm\({}^{3}\) solution dispense and DiW rinse. Figure 6: Diffusion light scattering images @1000 nm of Si wafers after water or NaOH solution dispense at two different concentrations. Figure 7: AFM (atomic force microscopy) images (scan size = 1 \(\upmu\)m) of SiO\({}_{2}\) surfaces after spreading of: (**a**) deionized water, (**b**) NaOH \(10^{-5}\) g/cm\({}^{3}\) solution, and (**c**) NaOH \(10^{-4}\) g/cm\({}^{3}\) solution.