Spectacular progress in developing advanced Si circuits with reduced size, along the track of Moore's law, has been relying on necessary developments in wet cleaning of nanopatterned Si wafers to provide contaminant free surfaces. The most efficient cleaning is achieved when complete wetting can be realized. In this work, ordered arrays of silicon nanopillars on a hitherto unexplored small scale have been used to study the wetting behavior on nanomodulated surfaces in a substantial range of surface treatments and geometrical parameters. With the use of optical reflectance measurements, the nanoscale water imbibition depths have been measured and the transition to the superhydrophobic Cassie-Baxter state has been accurately determined. For pillars of high aspect ratio (about 15), the transition occurs even when the surface is grafted with a hydrophilic functional group. We have found a striking consistent deviation between the contact angle measurements and the straightforward application of the classical wetting models. Molecular dynamics simulations show that these deviations can be attributed to the long overlooked atomic-scale surface perturbations that are introduced during the nanofabrication process. When the transition condition is approached, transient states of partial imbibition that characterize intermediate states between the Wenzel and Cassie-Baxter states are revealed in our experiments.

In semiconductor manufacturing, potential wetting issues with aqueous chemistries are becoming a concern as feature dimensions are continuously scaled down and novel materials with different wetting properties are introduced in new technology nodes. The wetting behavior of silicon nanopillars with different dimensions and surface modifications has been studied using static contact angle, decoration by etching, and attenuated total reflection infra-red spectroscopy (ATR-FTIR). The contact angle measurements showed a consistent deviation from the classic wetting models for patterned substrates with an hydrophilic surface termination. Under these conditions the decoration and ATR-FTIR studies gave evidence for partial wetting, with residual gas lasting for more than 30 min. It is proposed that this was resulting from the formation of long-lasting surface nanobubbles localized in-between or at the bottom of nanopillars. On the other hand the residual gas volume estimated by ATR-FTIR seemed too small to explain the contact angle deviations. It is proposed that the apparent extension of the superhydrophobic regime to lower contact angles resulted from modifications of the wettability of the surface of nanopillars caused by the manufacturing process. Both the formation of nanobubbles and the extension of the superhydrophobic regime may present challenges for aqueous cleaning in semiconductor manufacturing.