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.
Wet chemical etching is a key process in fabricating silicon (Si) nanostructures. Currently, wet etching of Si is proposed to occur through the reaction of surface Si atoms with etchant molecules, forming etch intermediates that dissolve directly into the bulk etchant solution. Here, using in situ transmission electron microscopy (TEM), we follow the nanoscale wet etch dynamics of amorphous Si (a-Si) nanopillars in real-time and show that intermediates generated during alkaline wet etching first aggregate as nanoclusters on the Si surface and then detach from the surface before dissolving in the etchant solution. Molecular dynamics simulations reveal that the molecules of etch intermediates remain weakly bound to the hydroxylated Si surface during the etching and aggregate into nanoclusters via surface diffusion instead of directly diffusing into the etchant solution. We confirmed this model experimentally by suppressing the formation of nanoclusters of etch intermediates on the Si surfaces by shielding the hydroxylated Si sites with large ions. These results suggest that the interaction of etch intermediates with etching surfaces controls the solubility of reaction intermediates and is an important parameter in fabricating densely packed clean 3D nanostructures for future generation microelectronics.
An approach for wet-chemical atomic layer etching (WALE) of semiconductors is described. The surface chemistry of InAs was investigated for HCl/H 2 O 2 solutions suitable for controlled etching in the low etch rate range (<0.1-10 nm min −1 ). Kinetic studies were performed using inductively coupled plasma -mass spectrometry (ICP-MS). As for GaAs and InGaAs, the importance of the Cl − ion for the etching kinetics is demonstrated and a chemical reaction scheme is presented to help understand the surface chemistry. A detailed study of an alternative two-step etching process was performed. A quantitative ICP-MS analysis of the oxide formed in O 3 /H 2 O solution and the dissolution in HCl was performed suggesting that the removal of oxidized In products is the slow step in the dissolution reaction. The reoxidation of oxide-free InAs (100) surfaces in air is discussed. The etch rate range and the surface morphology control after etching show that the investigated wet-chemical approach for atomic-layer etching is a valid candidate for advanced CMOS processing.
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