Oxygen Plasma and Humidity Dependent Surface Analysis of Silicon, Silicon Dioxide and Glass for Direct Wafer Bonding

Surface and interface characteristics of substrates are critical for reliable wafer bonding. Understanding the elemental and compositional states of surfaces after various processing conditions is necessary when bonding dissimilar materials. Therefore, we investigated the elemental and compositional states of silicon (Si), silicon dioxide (SiO2) and glass surfaces exposed to oxygen reactive ion etching (O2 RIE) plasma followed by storage in controlled humidity and/or ambient atmospheric conditions to understand the chemical mechanisms in the direct wafer bonding. High-resolution X-ray Photoelectron Spectroscopy (XPS) spectra of O2 RIE treated Si, SiO2 and glass showed the presence of Si(-O)2 resulting in highly reactive surfaces. A considerable shift in the binding energies of Si(-O)2, Si(-O)4 and Si(-OH)x were observed only in Si due to plasma oxidation of the surface. The humidity and ambient storage of plasma activated Si and SiO2 increased Si(-OH)x due to enhanced sorption of hydroxyls. The amounts of Si(-O)2 and Si(-OH)x of Si varied in different humidity storage conditions which are attributed to crystal-orientation dependent surface morphology and oxidation. The O2 RIE plasma induced high surface reactivity and humidity induced Si(-OH)x can play an important role in the hydrophilic wafer bonding with low temperature heating.