Measuring Surface Energy and Reactivity of SiO2 Using the Van Oss Theory and Three Liquid Contact Angle Analysis

Surface energies $\gamma^T$ can characterize reactivity for Wet NanoBonding$^{\mathrm{TM}}$ of Si(100) and SiO$_2$, a $200^{\circ}$C process where surfaces cross-bond. The Van Oss theory models $\gamma^T$ via 3 interaction energies, $\gamma^{LW}$ for Lifshitz-Van der Waals (LW) interactions, $\gamma^-$ for electron acceptors and $\gamma^+$ for donors, with $\gamma^T=\gamma^{LW}+2\sqrt{\gamma^+\gamma^-}$. To calculate $\gamma^{LW}$, $\gamma^+$, and $\gamma^-$, contact angles for 3 different liquids are measured in a Class 100 hood. For precision, 4-8 droplets are used instead of 1. Three SiO$_2$/Si(100) structures are analyzed: amorphous thermal a-SiO$_2$, HF-etched thermal a-SiO$_2$, and ordered 2 nm-thick c-SiO$_2$ produced by the Herbots-Atluri (H-A) process. In thermal a-SiO$_2$ surfaces, $\gamma^T=45\pm 2\frac{mJ}{m^2}$, while in more defective HF-etched a-SiO$_2$, $\gamma^T=57.5+/-2 \frac{mJ}{m^2}$. Because HF-etching yields a $\gamma^T$ closer to $\gamma^T$ of H$_2$O ($72\pm 0.4 \frac{mJ}{m^2}$), HF-etching makes the surface more hydrophilic. In contrast, in hydrophobic, ordered 2nm-thick H-A c-SiO$_2$, $\gamma^T=37.3\pm 2 \frac{mJ}{m^2}$. In ordered c-SiO$_2$, $\gamma^{LW}=.98\gamma^T$. However, for etched a-SiO$_2$, $\gamma^{LW}=.65\gamma ^T$ and $\gamma^-=.48\gamma^T$.