Effect of Oxygen Plasma Treatment and Aging on the Spontaneous Formation of Surface Nanobubbles on Silicon.

The formation of gaseous, spherical cap-shaped domains (so-called ″surface nanobubbles″) at the solid-liquid interface is a topic of fundamental interest due to the possible effects of nanobubbles on surface cleaning, wetting, and nanoscale patterning. This work investigates the spontaneous formation of surface nanobubbles on oxygen plasma-treated Si by PeakForce quantitative nanomechanics (PFQNM) imaging, X-ray photoelectron spectroscopy, and water contact angle measurements. Large quantities of surface nanobubbles with sub-10 nm height and sub-100 nm base width are observed on oxygen plasma-treated Si surfaces that have been ″aged″ (stored in a plastic or glass container) in ambient conditions. Several days of aging time are required for surface nanobubbles to form on oxygen plasma-treated Si, at which point the bubbles are remarkably consistent in their properties across samples aged 5-12 days. The presence of surface nanobubbles on plasma-treated aged Si surfaces runs contrary to prior reports of infrequent nanobubble formation on Si/SiOx. Surface characterization supports a theory of atmospheric hydrocarbon adsorption increasing Si hydrophobicity to a sufficient extent that nanobubble nucleation can occur. The results are of importance in Si-based fabrication processes employing oxygen plasma treatment with subsequent DI water immersion.