Ultra-high aspect ratio Si nanowires fabricated with plasma etching: plasma processing, mechanical stability analysis against adhesion and capillary forces and oleophobicity

Abstract

Room-temperature deep Si etching using time-multiplexed deep reactive ion etching (DRIE) processes is investigated to fabricate ultra-high aspect ratio Si nanowires (SiNWs) perpendicular to the silicon substrate. Nanopatterning is achieved using either top-down techniques (e.g. electron beam lithography) or colloidal polystyrene (PS) sphere self-assembly. The latter is a faster and more economical method if imperfections in diameter and position can be tolerated. We demonstrate wire radii from below 100 nm to several micrometers, and aspect ratios (ARs) above 100:1 with etching rates above 1 μm min(-1) using classical mass flow controllers with pulsing rise times of seconds. The mechanical stability of these nanowires is studied theoretically and experimentally against adhesion and capillary forces. It is shown that above ARs of the order of 50:1 for spacing 1 μm, SiNWs tend to bend due to adhesion forces between them. Such large adhesion forces are due to the high surface energy of silicon. Wetting the SiNWs with water and drying also gives rise to capillary forces. We find that capillary forces may be less important for SiNW collapse/bending compared to adhesion forces of dry SiNWs, contrary to what is observed for polymeric nanowires/nanopillars which have a much lower surface energy compared to silicon. Finally we show that SiNW arrays have oleophobic and superoleophobic properties, i.e. they exhibit excellent anti-wetting properties for a wide range of liquids and oils due to the re-entrant profile produced by the DRIE process and the well-designed spacing.

Hierarchical micro and nano structured, hydrophilic, superhydrophobic and superoleophobic surfaces incorporated in microfluidics, microarrays and lab on chip microsystems

“…Overhang structures where produced in Silicon by Susarrey et al [74] to study drop evaporation, and by Zeniou et al to study Si nanowire mechanical stability and hydrophobicity (see Fig. 4(vi)) [75]. Colloidal micro-nanosphere lithography has early enough been also proposed as an ''alternative'' cost-effective lithographic method [76].…”

“…4(vi) as discussed above). Controlling the first pulsing step allows control of the Si nanowire diameter under the sphere, and results in oleophobicity [75].…”

Review of nanostructured devices for thermoelectric applications

“…For its importance and utility also in other fields, such as integrated circuits and the fabrication of micro-electromechanical systems (MEMS), this technique has been improved and developed in the course of the years, with respect to both etching/polimerizating chemical agents and process temperature and step time [106]. Silicon pillars smaller than 100 nm with aspect ratios greater than 50:1 [109] or even 100:1 [110–111] have been fabricated, and thermoelectric power generators based on vertical DRIE fabricated nanowires, few micrometers long, have been tested [112–113]. …”

Subtractive Structural Modification ofMorphoButterfly Wings

“…Many examples of nanostructure fabrication have been reported using plasma-etching-based subtractive modifi cation processes. [25][26][27][28] In this work, we used oxygen plasma to systematically remove the surface chitin layer of butterfl y wings. As chitin reacted with the excited oxygen species, the thickness of the lamella layers gradually reduced (Figure 2 d), which resulted in both the shift of the refl ectance peak position and the reduction of refl ectance.…”