Super-wetting, wafer-sized silicon nanowire surfaces with hierarchical roughness and low defects

This paper reports the fabrication of wafer-sized silicon nanowire (SiNW) surfaces using a modified metal-assisted chemical etching method. The complete fabrication and coating process can be performed in less than three hours, is easily size-scalable, and produces surfaces with very low surface defects, complex, hierarchical surface roughness, and large nanowire height. These surfaces exhibit extreme wettabilities depending on surface coating: oxidized SiNW surfaces are superhydrophilic, while surfaces coated with a fluorinated hydrocarbon are superhydrophobic. The wetting and morphological properties of SiNW surfaces made with one and two etches of different duration are characterized vis-à-vis their effect on water drop mobility. Compared to a single etch process, a double etch followed by coating with a fluorinated hydrocarbon more efficiently produces SiNW surfaces with high contact angles on which microliter-sized water drops roll-off at approximately 0° tilt angle. Due to their very low friction, extreme wetting properties, ease of fabrication, low-cost, and large-sizes, these SiNW surfaces may be advantageous in microfluidic devices, bioanalysis systems sensitive to cross-contamination that require disposable substrates, and other applications.