Fundamental principles and development of proximity-field nanopatterning toward advanced 3D nanofabrication

Abstract

Three-dimensional (3D) nanoarchitectures have offered unprecedented material performances in diverse applications like energy storages, catalysts, electronic, mechanical, and photonic devices. These outstanding performances are attributed to unusual material properties at the nanoscale, enormous surface areas, a geometrical uniqueness, and comparable feature sizes with optical wavelengths. For the practical use of the unusual nanoscale properties, there have been developments for macroscale fabrications of the 3D nanoarchitectures with process areas over centimeter scales. Among the many fabrication methods for 3D structures at the nanoscale, proximity-field nanopatterning (PnP) is one of the promising techniques that generates 3D optical holographic images and transforms them into material structures through a lithographic process. Using conformal and transparent phase masks as a key factor, the PnP process has advantages in terms of stability, uniformity, and reproducibility for 3D nanostructures with periods from 300 nm to several micrometers. Other merits of realizing precise 3D features with sub-100 nm and rapid processes are attributed to the interference of coherent light diffracted by phase masks. In this review, to report the overall progress of PnP from 2003, we present a comprehensive understanding of PnP, including its brief history, the fundamental principles, symmetry control of 3D nanoarchitectures, material issues for the phase masks, and the process area expansion to the wafer-scale for the target applications. Finally, technical challenges and prospects are discussed for further development and practical applications of the PnP technique.

Rationally Designed TiO2 Nanostructures of Continuous Pore Network for Fast‐Responding and Highly Sensitive Acetone Sensor

“…The fabrication details for 3D TiO 2 have been described in previous reports by the group. [27,37,38,[47][48][49]52,53] Briefly, for depositing a photoresist (PR) film by a spin-casting method, the edges of the electrodes were first covered with the commercial adhesives, thereby minimizing the undesired surface contamination during the PR coating. After an adhesion-promoting thin (≈2 µm) PR (SU-8 2, Microchem) layer was fabricated in the form of an open-window pattern at the electrode, [27] a relatively thick (≈10 µm) PR (SU-8 10, Microchem) layer that defines the 3D nanostructure was additionally coated over the pre-structured pattern by dropping a 1 g of PR and spin casting with 3000 rpm for 30 s. The thick PR layer was carefully soft baked in 2 steps: 65 °C for 30 min and 95 °C for 30 min.…”

“…Continuous nanonetworks were generated from 3D nanopatterning of epoxy‐based photoresist, successive atomic layer deposition (ALD) of TiO 2, and thermal removal of the 3D polymeric template resulted in highly porous TiO 2 thin‐shell nanostructure, namely 3D TiO 2 − . [ 34,38 ] The TiO 2 thin‐shell thickness was precisely controlled from 30 to 100 nm through the number of ALD cycles to optimize the porosity of 3D TiO 2 in terms of gas molecule accessibility and the TiO 2 interneck thickness in terms of electrical conduction. In addition, finite element method (FEM) simulation with a time‐dependent gas diffusion model was conducted to verify the effectiveness of our 3D TiO 2 for extremely high gas molecule accessibility under low (311 particle models) and high (1306 particle models) gas concentrations.…”

“…. [34,38] The TiO 2 thin-shell thickness was precisely controlled from 30 to 100 nm through the number of ALD cycles to optimize the porosity of 3D TiO 2 in terms of gas molecule accessibility and the TiO 2 interneck thickness in terms of electrical conduction. In addition, finite element method (FEM) simulation with a time-dependent gas diffusion model was conducted to verify the effectiveness of our 3D TiO 2 for extremely high gas molecule accessibility under low (311 particle models) and high (1306 particle models) gas concentrations.…”

Fabrication of Non‐Uniform Nanolattices with Spatially Varying Geometry and Material Composition

“…To allow more layers, the yield for each layer is important and can be improved by using lithographically patterned phase masks. [14,15,18] One future research topic could be improving the yield rate of the lithography process to achieve nanolattices with less defects and better periodicity. The demonstrated non-uniform nanolattice with varying porosity can also be applied as photonic reflector and non-uniform filter, which are areas of on-going research.…”

“…[8][9][10] Their periodic modulation can also be used to tailor wave behavior in photonic/phononic crystals. [11][12][13][14][15] Various parallel fabrication techniques have been developed for 3D nanostructures with homogeneous properties, such as holographic lithography, [16,17] phase-shift lithography, [14,15,[18][19][20][21][22][23] and nanospheres extend the advances of 3D nanostructures, more advanced techniques for creating and controlling non-uniform nanolattices are needed.…”