Low‑Cost Metal‑Salt Nanocomposites Yield High‑Aspect‑Ratio, Super‑Hydrophobic, Low‑Reflectivity Silicon Nanostructures

Background

Surface nanostructures are increasingly demanded for their ability to tailor bulk material properties—enhancing wetting, thermal/electrical conductivity, self‑cleaning, anti‑icing, anti‑reflectivity, and gas‑barrier performance. Such “smart” coatings typically feature periodic pillars, cones, or porous textures, while recent studies highlight the versatility of random arrangements governed by statistical design.

Anti‑reflective coatings improve light absorption in solar cells, LEDs, camera lenses, and windows, while super‑hydrophobic surfaces, inspired by the lotus leaf, enable self‑cleaning and anti‑icing. Remarkably, both functionalities can coexist on a single nanoscale surface, as seen in the moth’s eye, which combines light‑trapping with liquid repellence.

Top‑down lithographic methods (optical, electron‑beam, nanoimprint) achieve high precision but at significant cost. Bottom‑up self‑assembly offers lower expense but traditionally yields either random or strictly periodic patterns, with limited feature sizes. Techniques such as nanosphere lithography, block‑copolymer self‑assembly, and self‑masking reactive‑ion etching have produced sub‑100 nm features, yet they often require specialized equipment or stringent processing conditions.

Our prior work demonstrated a spin‑coating and reactive‑ion‑etch approach to generate surface nanostructures. This study expands the metal‑salt repertoire to ANN and CNN, optimizing the process for sub‑20 nm resolution over large areas, and showcases superior anti‑reflective and super‑hydrophobic performance.

Methods/Experimental

We evaluated several metal salts as hard masks. Nickel salts were excluded due to magnetic properties incompatible with many cleanroom etchers. Aluminum and chromium nitrates—common hard‑mask metals—were selected. Their salts (Al(NO₃)₃·9H₂O and Cr(NO₃)₃·9H₂O) have low melting points (66 °C and 60 °C), promoting phase separation at modest temperatures. Both salts dissolve in dimethylformamide (DMF), the solvent chosen for its compatibility with poly(methyl methacrylate) (PMMA) and ability to produce uniform composite films.

PMMA (Mw = 996 kDa) was dissolved at 5 wt % in DMF. Separately, ANN or CNN were dissolved at 1–10 wt % in DMF. The two solutions were mixed 1:1 by volume, yielding a clear composite with 0.5–5 wt % metal salt and 5 wt % PMMA. The mixture was spin‑coated onto a pre‑cleaned silicon wafer (100 nm PMMA layer followed by 300 nm composite layer). Thermal annealing at 120 °C for 1 h induced phase separation, forming metal‑salt nano‑islands. Oxygen plasma (non‑switching RIE) etched the polymer matrix, leaving islands on silicon. Subsequent SF₆/C₄F₈ RIE etched the substrate, using the islands as a hard mask to form pillars or cones.

Fabrication flow: 1) Spin‑coat polymer–salt composite. 2) Thermal annealing to separate phases. 3) Oxygen plasma removes polymer, leaving salt islands. 4) Fluorine‑based RIE etches silicon, using islands as mask.

SEM of residual PMMA after oxygen plasma: (a) pure PMMA—etched cleanly; (b) PMMA + metal salt—polymer remains, confirming salt islands act as a micro‑mask.

Results and Discussions

Annealing Temperature Effects

Using a 1:10 ANN:PMMA ratio, films were annealed from 40 °C to 200 °C (1 h). Post‑etch SEM shows pillar formation across all temperatures; optimal uniformity occurs at 120 °C, where pillar diameters and spacings are most consistent.

SEM of silicon nanostructures annealed at various temperatures (a–i).

Effect of Metal Salt:Polymer Ratio

Higher salt loading increases pillar density and reduces diameter. For ANN:PMMA, ratios 1:10, 2:10, 3:10, and 5:10 produced pillars from sparse, >200 nm structures to dense, ~100 nm conical features—ideal for anti‑reflective applications. CNN:PMMA yields comparable dimensions due to similar chemistry.

ANN:PMMA pillar evolution with salt ratio (a = 1:10 to d = 5:10).

CNN:PMMA pillar evolution with salt ratio (a = 1:10 to d = 5:10).

Reflectivity measurements (PerkinElmer Lambda 35) reveal a dramatic drop: bare silicon reflects ~35 % in the visible, while 5:10 ANN:PMMA yields 2 % reflectance. The trend correlates with increased pillar density and tapered morphology.

Reflectivity spectra for ANN:PMMA (a–b) and CNN:PMMA (c). Structured wafers (d) show stark contrast to bare silicon.

Super‑hydrophobicity was confirmed via water contact angle on samples coated with trichloro(1H,1H,2H,2H‑perfluorooctyl) silane (FOTS). While flat silicon reaches 110°, the 3:10 ANN:PMMA nanostructures achieve 165.7°, matching the highest reported values.

Water contact angles for ANN and CNN nanostructures (a–b). Super‑hydrophobicity (>160°) achieved across all ratios.

Conclusions

Phase separation of metal‑salt–polymer nanocomposites provides a low‑cost, scalable route to high‑aspect‑ratio, sub‑50 nm silicon nanostructures. Using ANN or CNN salts with a 5:10 salt:PMMA ratio yields 2 % reflectivity and 165.7° water contact angles—suitable for photovoltaic, optical, and anti‑icing applications. Further enhancements are attainable by tuning salt concentration or etch chemistry. This methodology demonstrates a practical, economical alternative to conventional hard‑mask patterning techniques.