Development of Ordered, Porous (Sub-25 nm Dimensions) Surface Membrane Structures Using a Block Copolymer Approach

Abstract

In an effort to develop block copolymer lithography to create high aspect vertical pore arrangements in a substrate surface we have used a microphase separated poly(ethylene oxide) -b- polystyrene (PEO-b-PS) block copolymer (BCP) thin film where (and most unusually) PS not PEO is the cylinder forming phase and PEO is the majority block. Compared to previous work, we can amplify etch contrast by inclusion of hard mask material into the matrix block allowing the cylinder polymer to be removed and the exposed substrate subject to deep etching thereby generating uniform, arranged, sub-25 nm cylindrical nanopore arrays. Briefly, selective metal ion inclusion into the PEO matrix and subsequent processing (etch/modification) was applied for creating iron oxide nanohole arrays. The oxide nanoholes (22 nm diameter) were cylindrical, uniform diameter and mimics the original BCP nanopatterns. The oxide nanohole network is demonstrated as a resistant mask to fabricate ultra dense, well ordered, good sidewall profile silicon nanopore arrays on substrate surface through the pattern transfer approach. The Si nanopores have uniform diameter and smooth sidewalls throughout their depth. The depth of the porous structure can be controlled via the etch process.

Figure 1

AFM images (2 × 2 μm) of hexagonal ordered PEO-b-PS thin film following solvent annealing in Chloroform at different temperature of (a) 40 °C, (b) 50 °C, (c) 60 °C and (d) 70 °C for 30 min. (e,f) SEM images of the film at 60 °C.

Figure 2

(A) Solvent annealing of the spin coated film in Chloroform produces hexagonally arranged PS cylinders perpendicular to the substrate in PEO matrix, (B) Modification of PEO matrix creates nanoporous template where PS cylinders exposed to the film-air interface, (C) Iron oxide precursor solution spin coated onto the template, (D) Iron oxide nanohole arrays were prepared by UV/ozone treatment, (E) Si nanopores with iron oxide at top were fabricated by consecutive silica and silicon ICP etch processes, (F) Ordered Si nanopore arrays were formed after removal of oxide mask.

Figure 3

(a) AFM (2 × 2 μm) (b) SEM and (c and d) cross-sectional TEM images of hexagonal ordered nanoporous network after chemical etching/modification of PEO matrix by anhydrous ethanol. XPS C1s spectra of the thin films (e) before and (f) after ethanol treatment.

Figure 4

(a) AFM (2 × 2 μm) (b,c) SEM images of hexagonal ordered nanoporous network prepared by spin coating the metal ion precursor solution followed by UV/ozone treatment. (d) XPS Fe 2p spectra of the as-prepared iron oxide nanopores.

Figure 5

(a,b) Cross-sectional TEM images hexagonal ordered nanoporous iron oxide network. (c) represents corresponding elemental mapping for Fe O, Si.

Figure 6

SEM images hexagonal ordered nanoporous Si patterns formed by pattern transfer for different etch time (a) 1 min, (b) 2 min, (c) 3 min, and (d) 4 min.

Figure 7

(a,b) Cross-sectional TEM images hexagonal ordered nanoporous Si pattern for the 2 min etch time. (c) EDAX and corresponding elemental mapping for Fe O, Si (d) HRTEM image of Si nanopore.