Highly Ordered Porous Inorganic Structures via Block Copolymer Lithography: An Application of the Versatile and Selective Infiltration of the “Inverse” P2VP-b-PS System

Abstract

A facile and versatile strategy was developed to produce highly ordered porous metal oxide structures via block copolymer (BCP) lithography. Phase separation of poly(2vinylpyridine)-b-polystyrene (P2VP-b-PS) was induced by solvent vapor annealing in a nonselective solvent environment to fabricate cylindrical arrays. In this work, we thoroughly analyzed the effects of the film thickness, solvent annealing time, and temperature on the ordering of a P2VP-majority system for the first time, resulting in "inverse" structures. Reflectometry, atomic force microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy were used to characterize the formation of the highly ordered BCP morphology and the subsequently produced metal oxide film. At 40 min solvent annealing time, hexagonally close packed structures were produced with cylinder diameters ∼40 nm. Subsequently, the BCP films were infiltrated with different metal cations. Metal ions (Cr, Fe, Ni, and Ga) selectively infiltrated the P2VP domain, while the PS did not retain any detectable amount of metal precursor. This gave rise to a metal oxide porous structure after a UV/ozone (UVO) treatment. The results showed that the metal oxide structures demonstrated high fidelity compared to the BCP template and cylindrical domains presented a similar size to the previous PS structure. Moreover, XPS analyses revealed the complete elimination of the BCP template and confirmed the presence of the metal oxides. These metal oxides were used as hard masks for pattern transfer via dry etching as a further application. Silicon nanopores were fabricated mimicking the BCP template and demonstrated a pore depth of ∼50 nm. Ultimately, this strategy can be applied to create different inorganic nanostructures for a diverse range of applications, for example, solar cells, diodes, and integrated circuits. Furthermore, by optimizing the etching parameters, deeper structures can be obtained via ICP/RIE processes, leading to many potential applications.

High Aspect Ratio Nanoscale Pores through BCP-Based Metal Oxide Masks and Advanced Dry Etching

“…Through the contact of the BCP thin film with chloroform vapors, i.e., a nonselective solvent, unfavorable interactions between the P2VP (poly 2-vinylpyridine) and the PS blocks can be reduced, which leads to the vertical alignment along the surface plane of the domains. 32 Therefore, based on the two molecular weights studied in this work, different morphologies were obtained. Figure 1C presents topographical atomic force microscopy (AFM) images of the self-assembled polymeric thin films with the PS cylinders of varying dimensions, i.e., diameters of about 32 and 343 nm, being embedded within the P2VP matrix.…”

“…Phase separated P2VP- b -PS films were prepared according to previous studies. , Silicon wafers were cut into 4 cm 2 squares and subjected to ultrasonic cleaning in acetone for 20 min, followed by drying with a stream of N 2 . Solutions of BCP1 (1.5 wt %) and BCP2 (2.0 wt %) were prepared in a 1:4 mixture of toluene and tetrahydrofuran.…”

Morphology Engineering of the Asymmetric PS-b-P4VP Block Copolymer: From Porous to Nanodot Oxide Structures

“…It is also important to point out that solvent inclusion dramatically changes phase diagrams. Because of this, great care is needed to ensure that the swelling can be controlled enough to allow the various morphology changes to be properly controlled avoiding more random and poorly ordered films. ,,− Chloroform was chosen as the solvent because it can swell both blocks but has preferential affinity for the minority P4VP block. Experimental evidence has also shown chloroform’s preference for interacting with the P4VP block .…”

“…However, to achieve the porous pattern, they used a microphase separated PEO- b -PS BCP thin film, where PS, not PEO, is the cylinder forming phase and PEO is the majority block. In a study carried out by Esmeraldo Paiva et al, they also demonstrated the fabrication of vertically aligned pores on the substrate using P2VP- b -PS BCP, where again PS, not P2VP is the cylinder forming phase and P2VP is the majority block. However, forming continuous oxide films with vertical pores is less well demonstrated despite their potential use in, for example, sensor and diagnostic tests because of their high surface area and size selectivity.…”

Nanofabrication of hybrid nanomaterials: Macroscopically aligned nanoparticles pattern via directed self‐assembly of block copolymers

“…Moreover, block copolymer aided nanostructuring can be combined with traditional techniques such as lithography, resulting in block copolymer lithography. Paiva et al 94 following similar strategy were able to obtain highly ordered porous metal oxide structure packed as 40 nm diameter cylinders, which can be used diodes, and integrated circuits. Additionally, it provides the possibility of selective infiltration with metal cations offering high fidelity metal oxide structures.…”