## 3 Results
Bonding energies obtained after annealing at 300 \({}^{\circ}\)C for 2 h with different chemical species are plotted in Figure 10 for various pHs. From these experiments, we deduce that increasing the amount of deposited hydroxide ions enhances silicon dioxide's direct bonding energies. A notable exception is ammonia (4.5 \(\times\) 10\({}^{-4}\) g/cm\({}^{3}\)), yielding the same bonding energy as neutral or acidic molecules. This may be attributed to its high vapor pressure; this molecule likely evaporates from wafer surfaces prior to bonding, thus no longer being present at the bonding interface in our experiment conditions. However, if a solution were found to retain ammonia at the bonding interface, an enhancement in bonding energy should be measurable. Noteworthy, the pH value of Figure 10 is only the pH value of the solution poured on the surface, not the pH of the bonding interface water itself. In order to investigate the link between adherence and deposited salts or molecules, we first tried to measure the number of species trapped at the interface. Direct measurements after wafer debonding and prior to annealing seem the most effective way of quantifying surface concentrations of molecules. For NaOH, total reflection X-ray fluorescence (TXRF) can be used to detect sodium atoms (refer to Table 3). Indeed, the Na\({}^{+}\) ionic concentrations vary depending on the concentration of the solution spread. As far as organic molecules are concerned, a wafer outgassing system (WOS) coupled with an automated thermal desorber-gas chromatography-mass spectrometer (ATD-GC-MS) can be used to quantify DEAE concentrations on wafer surfaces (just after wafer debonding). DEAE surface concentrations are comparable to Na  
Figure 10: Anhydroous direct bonding energy of SiO\({}_{2}\) bonding as a function of pH of the dispensed solution, for a 300 \({}^{\circ}\)C annealing during 2 h. The cross represents the sample breakage. Figure 9: Influence of NaOH concentration in DIW on the anhydrous direct bonding energy for various annealing temperatures. Crosses represent 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)/m\({}^{2}\)). concentrations (refer to Tables 3 and 4), confirming the effective embedding of this specific molecule at the bonding interface. Of course, all these measurements could have been performed directly after the solution pouring. Especially for organic molecules, their vapor pressure is imposed to perform the measurement as soon as possible after the deposition, which is not easy to put in place. The bonding is then used here as a convenient path to protect the surface waiting for the measurement to be ready. Clearly, the molecule concentration on the silicon surface is not easily deduced from its bulk spreading solution concentration. Indeed, from the last value, theoretical NaOH and DEAE surface concentrations can be calculated for a 1 cm\({}^{2}\) surface, assuming 0.8 monolayers of adsorbed solution [44]. This would yield concentrations four orders of magnitude lower than actually measured surface concentrations, indicating a substantial concentration effect during drying, with water molecules evaporating more rapidly than added molecules (except for ions). However, evaporation kinetic differences are not high enough to explain such surface concentrations of molecules. High ion concentrations at the surface suggest the presence of electrostatic interactions (double layer effect) at the charged silica surface [45]. Calculations of surface concentration have been performed using Poisson Boltzmann equations. They are, however, beyond the scope of this article. The pH value of the deposited solution is the key parameter in explaining the increase in bonding energy in this study.