Subwavelength features have been successfully demonstrated in near-field lithography. In this study, the point spread function (PSF) of a near-field beam spot from a plasmonic ridge nanoaperture is discussed with regard to the complex decaying characteristic of a non-propagating wave and the asymmetry of the field distribution for pattern design. We relaxed the shape complexity of the field distribution with pixel-based optical proximity correction (OPC) for simplifying the pattern image distortion. To enhance the pattern fidelity for a variety of arbitrary patterns, field-sectioning structures are formulated via convolutions with a time-modulation function and a transient PSF along the near-field dominant direction. The sharpness of corners and edges, and line shortening can be improved by modifying the original target pattern shape using the proposed approach by considering both the pattern geometry and directionality of the field decay for OPC in near-field lithography.

Quantitative analysis of line edge roughness (LER) is very important for understanding the root causes of LER and thereby improving the pattern quality in near-field lithography (NFL), because LER has become the main limiter of critical dimension (CD) control as the feature size of nanostructures is scaled down. To address this challenge, the photoresist point-spread function of NFL with a contact plasmonic ridge nanoaperture can be employed to account for the physical and chemical effects involved in the LER-generation mechanism. Our theoretical and experimental results show that the sources of LER in NFL mainly come from the aerial image, material chemistry, and process. Importantly, the complicated decay characteristics of surface plasmon waves are demonstrated to be the main optical contributor. Because the evanescent mode of surface plasmon polaritons (SPPs) and quasi-spherical waves (QSWs) decay in the lateral direction, they can induce a small image log-slope and low photoresist contrast, leading to a large LER. We introduce an analytical model and demonstrate the relationship between LER and CD to estimate the pattern quality in NFL. We expect that these results can provide alternative approaches to further improve pattern uniformity and resolution, which can lead to advanced nanopatterning results in NFL.

We introduce a plasmonic resonance ridge aperture capable of sensing changes in refractive index and absorption with nanoscale resolution. Using this aperture, we devised a plasmonic near-field scanning nanoscope (PNSN) to record images of heterogeneous nanostructures. Compared to a conventional near-field scanning optical microscope (NSOM) that measures light scattered by the sample, the PNSN directly measures the change in a beam reflected from the aperture to detect buried objects. Using the PNSN we recorded images of nanoscale rectangular groove arrays on a SiO(2) substrate with patterns typical of a dynamic random access memory circuit. By comparing the experimental and calculated image of the nanostructure, we estimate the resolution of PNSN to be ~20 nm, which is ~50% smaller than the near-field spot generated by the aperture. Also, we theoretically analyzed the feasibility of the PNSN detecting an object underneath a metal film.

We successfully demonstrate the practical application of plasmonic lithography to fabricate nanoimprint masters. Using the properties of a non-propagating near-field, we achieve high-speed multi-scale patterning with different exposure time during the scanning. We modulate the width of the line patterns using a pulse light source with different duty cycles during the scanning of the probe. For practical application in plasmonic lithography, we apply a deep reactive ion etching process to transfer an arbitrary fluidic channel into a silicon substrate and fabricate a high-aspect-ratio imprint master. Subsequently, we carry out the imprint process to replicate the fluidic channel with an aspect ratio of 7.2. For pattern width below 100 nm, we adopt a three-layer structure of photoresist, hard layer, and polymer to record the near field and form a hard mask and transfer mask. Using the multilayer structure, we fabricate high-resolution nanoimprint masters in a silicon substrate with an aspect ratio greater than 1.

We report an extension of plasmonic lithography to nanoscale 2.5-dimensional (2.5D) surface patterning. To obtain the impulse response of a plasmonic lithography system, we described the field distribution of a point dipole source generated by a metallic ridge aperture with a theoretical model using the concepts of quasi-spherical waves and surface plasmon–polaritons. We performed deconvolution to construct an exposure map of a target shape for patterning. For practical applications, we fabricated several nanoscale and microscale structures, such as a cone, microlens array, nanoneedle, and a multiscale structure using the plasmonic lithography system. We verified the possibility of applying plasmonic lithography to multiscale structuring from a few tens of nanometres to a few micrometres in the lateral dimension. We obtained a root-mean-square error of 4.7 nm between the target shape and the patterned shape, and a surface roughness of 11.5 nm.