Adjustable, (super)hydrophobicity by e-beam deposition of nanostructured PTFE on textured silicon surfaces

Polytetrafluoroethylene (PTFE)-like films, produced by electron beam (e-beam) deposition, have shown higher hydrophobicity than those deposited by RF sputtering under similar deposition rates. It was found that this results from both surface chemical composition and nano-roughness. X-ray photoelectron spectroscopy measurements revealed that larger moieties of CF2 and CF3 groups were present to reduce surface energy in the e-beam deposited films. RF sputtering led to a higher degree of PTFE target fragmentation producing a different perfluorinated film on the Si substrate. Scanning electron microscopy and atomic force microscopy measurements revealed a much larger rms roughness on the film surfaces produced by e-beam (25.13 nm, at 20 mA) than those by RF sputtering (2.42 nm, at 100 W), and allowed a broad power spectrum density analysis with determination of the κ B wetting parameter. In addition, the e-beam deposited films presented a linear increase of contact angle with applied electron current in the range under study (5–20 mA). This allows easy water repellency adjustment, up to 159 ± 2°. For a superhydrophobic state with self-cleaning, a micro-pyramid structure was wet etched on the Si wafer, followed by PTFE deposition, and a very low contact angle (163 ± 2°) and hysteresis was attained (<3°). These first results indicate that e-beam PTFE deposition with adjustable hydrophobicity may become a useful technique for integrated production with present Si microelectronics technology and for Si solar cells.