Adhesion‐Engineering‐Enabled “Sketch and Peel” Lithography for Aluminum Plasmonic Nanogaps

Abstract

in the near field and amplify both linear and nonlinear phenomena at the singlemolecule and single-particle level. [5][6][7][8][9] Choosing an appropriate plasmonic material is significant for different plasmonic applications, usually considering the cost, performance, and process compatibility. Among these materials, silver (Ag) is favored due to its low loss, which supports a strong plasmon resonance in the visible and near-infrared regions. [10] However, the oxidization of Ag in air greatly changes its optical properties, which severely limits the practical use of metallic Ag in plasmonics. In contrast, gold (Au) is a better choice due to its excellent chemical stability, but it suffers from a high intrinsic absorption of photon energy higher than 2.38 eV. As a result, making full use of Au for plasmon-enhanced spectroscopic applications, such as surface-enhanced Raman spectroscopy (SERS) and nonlinear optics in the entire visible region, is difficult.Compared to Au and Ag, aluminum (Al) is a more promising material for plasmonics, with the merits of oxidation stability superior to that of Ag and the extended optical response ranging from the ultraviolet to visible regime compared to that of Au. [11,12] Nevertheless, the optical loss of Al degrades the nanofocusing effect, which weakens the localization of light field in the nearfield of Al plasmonic nanostructures. For these reasons, tiny plasmonic nanogaps play a significant role in achieving large electric-field enhancement factor for practical applications of Al plasmonics.From the perspective of fabrication process, the plasmonic nanogaps in metallic structures are intrinsically a type of multiscale configuration, usually ranging from several nanometers to several hundreds of nanometers. To obtain plasmonic nanogaps, electron-beam lithography (EBL) is a key tool due to its advantages of high resolution and accuracy in patterning. [13,14] Traditionally, the whole area of final patterns separated by nanogaps should be fully exposed. However, this patterning strategy has limited resolution and accuracy in gap fabrication due to the large proximity effect caused by electron scattering. Furthermore, this kind of challenge becomes more apparent when fabricating nanogaps in plasmonic nanostructures, which commonly involves metal deposition and lift-off processes.Aluminum is one of the most significant plasmonic materials for its advantage of low cost, natural abundance, as well as the ultraviolet optical response. However, it is still very challengeable for the fabrication of aluminum plasmonic nanogaps, which greatly limits the applications of aluminum plasmonics considering the essential role of nanogaps for electric field enhancement. Here, the reliable patterning of aluminum plasmonic nanogaps employing a modified "Sketch and Peel" lithography strategy is demonstrated. By introducing a self-assembled monolayer to engineer the surface energy of the substrate, the adhesiveness of the aluminum film outside outline template is significantly decreased to implemen...

Plasmonic metal nanostructures with extremely small features: new effects, fabrication and applications

“…101 Furthermore, by introducing a self-assembled monolayer to engineer the surface energy of the substrate, the adhesiveness of the Al film outside outline template is significantly decreased to implement the selective peeling process, various plasmonics Al nanostructures with sub-20 nm feature sizes were successfully achieved by SPL approach, suggesting the tremendous potential and fabricating ability of the technology. [102][103][104] To optical properties and applications of artificial plasmonic metal atoms, the hybridization model for the plasmon response of complex nanostructures provides a simple effective guideline. 105 As shown in Fig.…”

Scalable Fabrication of Metallic Nanogaps at the Sub‐10 nm Level

“…[143] A number of variations to the SPL procedure have been reported. [144][145][146][147][148] For instance, instead of using a negative resist, it is possible to directly create nano-trenches in a deposited metal film by FIB milling before selectively removing the metal outside of the trenches by peeling, see Figure 14. [142] In the case of a silicon substrate, it has been shown that slight over-milling of the unwanted metal causes a thin layer of silicon atoms to be sputtered onto the side walls of the retained metal, which serves to protect the retained metal from damage during the subsequent peeling step.…”

“…SPL has also been extended to strongly adhesive metals such as aluminum by depositing a fluorine‐based self‐assembled monolayer on top of the substrate to modify its surface energy. [ 145 ] All in all, SPL is a versatile and reliable method for nanogap patterning that provides a good balance between speed and throughput as it requires only the outline of the patterns to be defined in the photoresist. However, it is best suited to weakly adhesive metals such as gold and silver or substrates treated with adhesion‐reducing surface modifiers, which may potentially compromise the mechanical stability of the resultant nanostructures.…”

Theoretical Design of a Bionic Spatial 3D-Arrayed Multifocal Metalens

“…However, the limitations of those existing fabrication techniques make it difficult to fabricate three-dimensional (3D) metamaterials [ 9 , 10 ]. Therefore, the two-dimensional (2D) metamaterials, metasurfaces, have been rapidly developed and applied owing to their relatively easy fabrication methods and technologies [ 11 , 12 , 13 , 14 , 15 , 16 ]. Moreover, metasurfaces can control the amplitude [ 17 , 18 , 19 , 20 ], phase [ 21 , 22 , 23 , 24 ], and polarization [ 25 , 26 , 27 , 28 ] of electromagnetic waves, leading to a fact that various applications based on their unique properties have been developed, such as metalens focusing [ 29 , 30 , 31 ], holographic imaging [ 32 ], vortex light modulation [ 33 ], and abnormal deflectors [ 34 , 35 ].…”