Materials Today
Volume 39, October 2020, Pages 89-97
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Research
Cascade domino lithography for extreme photon squeezing

https://doi.org/10.1016/j.mattod.2020.06.002Get rights and content

Abstract

Squeezing photons into deep sub-wavelength volumes and few-nanometer gaps has led to the investigation of interesting phenomena, including strong coupling, quantum plasmonics, nonlinearity enhancement, nonlocality, and molecular junctions. The common configuration of bowtie nanoantennas has been extensively studied owing to the great enhancement of the localized electromagnetic field. The enhancement rapidly increases as the tips become sharper and the gap becomes narrower. However, reliable fabrication of nanoantennas that are extremely sharp with sub-10 nm gaps is statistically prohibited due to the fundamental limitation of the proximity effect of electron beam lithography. Here, an intuitive “fall-to-rise” scheme is proposed and experimentally validated using a new concept of cascade domino lithography. In this report, we successfully establish a controllable lithography method of making extremely sharp bowtie nanoantennas with sub-1 nm radius of curvature reaching the size of a gold nanocluster as well as single-digit-nanometer gaps between such sharp tips. In addition, a proof-of-concept application of surface enhanced Raman spectroscopy is demonstrated along with rigorous full-wave electromagnetic simulations and numerical analysis. This control of falling nanostructures opens up an unexplored gateway towards conquering the limitations of experimentally exploring the realm of plasmonics down to the sub-nanometer regime.

Introduction

Squeezing photons into deep sub-wavelength volumes and few-nanometer gaps has demonstrated and enabled an abundance of unprecedented phenomena, including strong coupling [1], quantum plasmonics [2], [3], [4], [5], nonlinearity enhancement [6], enhanced light–matter interactions [7], [8], [9], [10], [11], [12], and molecular junctions [13]. Plasmonic nanostructures have been proven to achieve gigantic field enhancements in a small volume, a so called “hotspot”. The intensity and location of the hotspot depend on the shape, size, and dielectric environment of the nanoparticles. In general, antenna theory predicts that electric fields can be dramatically enhanced in the close vicinity of sharp metal tips [14] or in sub-nanometer scale gaps.

As the feature size and/or gap size of plasmonic nanoparticles shrink, light interactions with plasmonic structures (or nanoantennas) become more pronounced. In this context, a classic platform (namely square, circular, or triangular plasmonic dimers) has been developed to confine light into a very narrow space, upon which more complex geometric platforms are built. Once the distance between the dimers decreases down to the single-digit nanometer scale, light–matter interactions are maximized and enable intriguing applications such as quantum plasmonics [15], light generation from inelastic electron tunneling [16] and single nanoparticle trapping [17]. Particularly, enhanced electromagnetic fields squeezed into a deep sub-wavelength volume have been actively employed to substantially enhance light–molecule interactions, such as the absorption, emission, and scattering process of single molecules. Molecular Raman scattering signals can work as a fingerprint with a distinct spectrum associated with molecular vibrational modes and have been widely used for label-free molecular sensing but suffer from a low signal intensity. A plasmonic nanostructure can dramatically improve the signal intensity using the enhanced light–matter interaction, this technique is called surface enhanced Raman spectroscopy (SERS).

Bowtie nanoantennas consisting of two metallic triangles with opposing tips have been widely adopted for deep sub-wavelength field confinement and enhancement. These serve as a promising yet simple candidate for quantum plasmonics and ultra-sensitive molecular sensing such as SERS [18], [19]. In order to make sharper and narrower-gap bowties for stronger field enhancements, many different approaches have been adopted including but not limited to electron beam lithography (EBL) [20], [21], [22], colloidal lithography [23], nanostencil lithography [24], and focused ion beam milling (FIB) using gallium (Ga) or helium (He) ions [25], [26], [27]. EBL and FIB-made bowties with gap sizes of around 10–20 nm have been successfully demonstrated. Unfortunately, intrinsic hurdles such as the electron beam proximity effect in EBL, and the large ion beam sizes and undesired damage from ions in Ga-based FIB, make it a considerable challenge to fabricate a bowtie with a nanogap of less than 10 nm while simultaneously maintaining a sub-nanometer level sharpness. The He-based FIB process shows better performance [26], [27] i.e., sub-5 nm gap size, but this process requires very sensitive fabrication conditions including proximal effect, milling time and additional milling steps to accurately produce the required gap size, which means that it is difficult to fabricate the precise nanoantennas uniformly over large areas.

Here, an intuitive “fall-to-rise” scheme using cascade domino lithography (CDL) to achieve those two targets (sharp tip and few-nanometer gap) simultaneously is proposed and experimentally validated. In this report, we successfully establish a controllable lithography method of making extremely sharp bowtie nanoantennas of sub-1 nm radius of curvature (ROC) reaching the size of a gold nanocluster as well as a single-digit-nanometer gap between such sharp tips. We further develop this unique technique for single sharp tips to generate arrays of sharp bowties. SERS measurements are performed using our proposed bowties and compared with those fabricated by the conventional EBL and lift-off process. Both the theoretical and experimental results confirm the role of our advanced ROC and gap size in the gigantic field enhancement and confinement.

Section snippets

Mechanism and working principle of cascade domino lithography

During the EBL process, interactions between the primary and secondary electrons, resist, and substrate result in a developed pattern area wider than the scanned pattern area. This proximity effect reduces the chance to fabricate sharp edges and nanogaps (Fig. 1a and Fig. S1). However, statistically speaking, by fabricating hundreds or thousands of such bowties as shown in scanning electron microscope (SEM) image of Fig. 1a, a skilled EBL operator should stand some chance of producing one or

Conclusion

In conclusion, a new concept of CDL, inspired by the art of collapsing dominoes, is experimentally demonstrated. It overcomes the long-standing challenges of nanolithographic fabrication of bowtie nanoantennas with sub-nanometer sharpness and single-digit nanometer gaps, simultaneously empowering high-yield, consistent repeatability, and extreme field enhancement of focused light with functionalized applications in SERS (Table S2). Such fascinating features of bowties made by CDL in a reliable

Materials and methods

Device fabrication: The sharp tip fabrication started on a 500 μm thick silicon substrate. Using a standard EBL process (Elionix ELS-7800, 80 kV, 50 pA), the half bowtie patterns were defined on the MMA (Microchem, MMA (8.5) MAA EL-8)/PMMA (Microchem, 495 PMMA A2) bilayer positive tone resist having different solubility in the developer. This bilayer process produces a T-shaped profile of the resist after development, and such a profile is favorable for the lift-off process because it can

CRediT authorship contribution statement

Inki Kim: Investigation, Methodology, Visualization, Writing - original draft, Writing - review & editing. Jungho Mun: Software, Formal analysis, Writing - original draft, Writing - review & editing. Kwang Min Baek: Validation, Formal analysis, Resources. Minkyung Kim: Software, Resources. Chenglong Hao: Resources. Cheng-Wei Qiu: Resources, Data curation, Writing - review & editing. Yeon Sik Jung: Resources, Data curation. Junsuk Rho: Conceptualization, Investigation, Methodology, Writing -

Acknowledgements

This work was financially supported by the National Research Foundation (NRF) grants (NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2019R1A5A8080290, NRF-2018M3D1A1058998, NRF-2015R1A5A1037668) funded by the Ministry of Science and ICT (MSIT) of the Korean government. I.K. acknowledges the NRF Global Ph.D. fellowship (NRF-2016H1A2A1906519) funded by the Ministry of Education of the Korean government. The authors acknowledge Kyoung-Duck Park (UNIST, Korea) for the NSOM measurement in

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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