Fabrication and characterization of well-aligned plasmonic nanopillars with ultrasmall separations

We show the fabrication of well-aligned gold and silver nanopillars with various array parameters via interference lithography followed by ion beam milling and compare the etching rates of these two metallic materials. Silver is suitable for fabricating ultrafine arrays with ultrasmall separations due to high milling rates. The optical properties of the fabricated nanopillars are specifically characterized from both normal incidence and oblique incident angles. Tunable surface plasmon resonances are achieved with varying structural parameters. Strong coupling effects are enabled when the separation between adjacent nanopillars is dramatically reduced, leading to useful applications in sensing and waveguiding.

Due to important advances in nanofabrication techniques, plasmonic nanostructures and related devices are presently gaining tremendous technological significance in nanophotonics and optics. Nanostructures could provide intriguing possibilities for resolving those challenges and improving device performance. Well-aligned nanopillars with perpendicular orientations to the substrate are becoming the main building blocks for new optical devices with promising potential applications [41]. Here we explore, experimentally and theoretically, the optical properties of periodic nanopillars perpendicularly aligned on the supporting substrate. Combination of interference lithography (IL) and ion beam milling (IBM) techniques enables scalable fabrication of such nanopillars with excellent dimensional control and high uniformity. Detailed experimental results show that silver (Ag) has a much higher etching rate than gold (Au) under the same milling conditions, making Ag a perfect candidate for the construction of plasmonic ultrasmall features. In addition, nanopillar arrays with ultrasmall inter-pillar separations are fabricated and optically characterized.

Methods
Quartz substrates were first cleaned with acetone in an ultrasonic bath followed by isopropyl alcohol (IPA) and deionized water washing and finally blow-dried with a nitrogen gun. Subsequently, Au or Ag films with different thicknesses were deposited on quartz substrates with 4-nm titanium as the adhesion layer by electron beam evaporation (Auto 306, Edwards, Crawley, UK) at a base pressure of about 3 × 10 −7 mbar. In order to minimize the deposition-introduced roughness, low evaporation rates were applied (less than 0.03 nm/s). Afterwards, positive resist (S1805, Dow, Midland, MI, USA) was used to define nanopillar arrays on the metal (Au or Ag) layer supported by a quartz substrate (refractive index = 1.46) with a laser holography system using a 325-nm helium-cadmium laser, serving as the IBM mask after development.
During the IBM process (Microetch 1201, Veeco Instruments, Plainview, NY, USA), argon was ionized and accelerated in an electric field to a high energy level. Argon ions struck the target materials while the sample plate rotated, ensuring homogeneous removal of waste material and straight sidewalls in all features with nearly zero undercutting. The work plate was cooled and tilted 10°to the normal of the incident beam to ensure even uniformity of the ion bombardment. At last, resist residue was removed by Microposit Remover 1165 (Rohm and Haas, Philadelphia, PA, USA) and cleaned up with IPA and deionized water. Detailed milling parameters are summarized in Table 1. The measured milling rate for Au and Ag is 23 and 61 nm/min, respectively.
Compared with other fabrication methods, IL has idiographic advantages. For instance, IL allows for processing a complete substrate with one single exposure or several times of full-area exposures to define complex patterns. More importantly, IL can offer the possibility to construct homogeneous micro-or nanometer-structured surfaces  on areas with wafer scale that is either impossible or extremely time consuming with other patterning techniques. In addition, one can precisely control the geometry of the arrays in a wide range by changing the processing parameters such as the incident angle and exposure time. As shown in Figure 1, nanopillars with varying profiles are achieved by accurately controlling the milling conditions. One can clearly observe cone-shaped particles in Figure 1a, which were achieved by oblique milling. In Figure 1b, normal round-shaped nanopillars are shown. Rough fringes are caused by redeposition which is almost inevitable in all ion-involved milling processes. Further, Figure 1c demonstrates nanopillars with ultrasmall separations. Note that the round shapes are replaced by quadrate outlines since the individual nanopillars are approaching each other. Smallest features of approximately 10 nm are realized. Figure 1d shows the cross sections of pagoda nanopillars with high aspect ratios (100-nm average diameter and 270-nm height). The optical properties of the fabricated nanopillars under normal incidence were measured using a commercial system (UV-VIS-NIR microspectrophotometer QDI 2010™, CRAIC Technologies, Inc., San Dimas, CA, USA).
A × 36 objective lens with the numerical aperture of 0.5 was employed with a 75-W xenon lamp which provided a broadband spectrum. Using a beam splitter, the partial power of the incident light beam was focused onto the sample surface through the objective lens. The spectrum acquisition for all measurements was performed with a sampling aperture size of 7.1 × 7.1 μm 2 . Transmission and reflection were measured with respect to the light through a bare quartz substrate and an aluminum mirror, respectively. To characterize the optical properties from oblique angles, an ellipsometry setup (Uvisel, Horiba Jobin Yvon, Kyoto, Japan) was employed with a broadband light source. Figure 2a demonstrates the scanning electron microscopy (SEM) image of the top view of the fabricated Ag nanopillars with 400-nm periodicity. As can be seen, the fringe of the nanopillars presents a brighter color than the other areas due to different contrast which is caused by materials redeposition during milling. Figure 2b is the optical image of nanopillars supported by a quartz substrate with the size of 1.5 × 1.5 cm 2 . The corners show defects caused by fabrication imperfections since the pattern area is limited during holography and uneven distribution of resist during spin coating. The extinction spectra for nanopillar arrays with varying periodicities are plotted in Figure 2c. One can clearly observe tunable LSPRs and redshift of resonance peaks with increasing periodicities. Besides, relatively large full width at half maximum can be seen for resonance peaks after 900 nm. Figure 3a shows the atomic force microscopy (AFM) image of the Au nanopillar array with 450-nm periodicity. As can be seen, nanopillars with uniform shapes are achieved. The measured reflectance spectra of nanopillar arrays with different incident angles (40°to 70°in 10°i ncrements) as a function of wavelength are plotted in Figure 3b. Tunable plasmon resonance with varying incident angles can be observed. Figure 3c shows the electric near-field distribution of a single nanopillar at 30°to the incidence normal at the wavelength of 430 nm calculated by using CST microwave studio. During simulations, one unit cell was considered which consisted of a vertically oriented cylindrical Au nanopillar. Periodic boundary conditions were assigned to the lateral walls and Floquet ports were imposed on top and bottom of the unit cell to mimic an infinite periodic array with a periodicity of p = 450 nm. The nanopillar has a radius of r = 100 nm and a height of h = 200 nm. A fifth-order Drude-Lorentz model was employed to fit the measured permittivity of Au [42]. It is observed that at the wavelength corresponding to the peak of specular reflection for each angle of incidence case, the electric field exhibits curl-like patterns, concentrating near the vertical surface of the nanopillar.

Results and discussion
As mentioned above, Ag has a much higher etching rate than Au under the same milling parameters using ion beams. Therefore, Ag has a larger selectivity than Au with the same resist mask (fixed thickness) for milling. Figure 4a,b shows the top-view and oblique-view SEM images of Ag nanopillar arrays with ultrasmall gap sizes, respectively. The average measured smallest gap width is approximately 10 nm. Dome-shaped profiles can be observed from Figure 4b, which is mainly caused by materials redeposition during the milling process. Note that the gaps between neighboring nanopillars have been milled through to the surface of the substrate. Typical fabrication imperfections are highlighted with red circles.
The measured absorbance spectra for two Ag nanopillar arrays with different periodicities and ultrasmall inter- pillar separations are plotted in Figure 5. The LSPRs in nanopillars can be described as a series of longitudinal standing waves with an increasing number of harmonics at shorter wavelengths. In addition, the LSPRs are laterally confined and bounded between adjacent nanopillars. The spectra also show the effect of periodicity variation and reveal different regimes. Very little radiative coupling occurs when the diffraction edge is on the high-energy side of the main LSPR since the allowed diffracted orders have higher energy than the plasmon resonance. Most of the LSPRs confined within the nanopillar array exist as higher-order modes. Note that the standing waves within the nanopillars can be influenced by the coupling of transverse plasmon modes between nanopillars, leading to different resonances described for separate nanopillars. Additionally, Fano-type line shapes are observed which result from the interference between directly transmitted and scattered energy. Such nanopillars have great potential for sensing purposes due to significantly enhanced near-field intensity which can be clearly observed from the inset of Figure 5, possessing the key advantage of plasmonic-based sensors which may enable new opportunities for sensing geometries and strategies.

Conclusions
To conclude, we have demonstrated the fabrication of well-aligned plasmonic nanopillars by combining IL and IBM techniques. Using arrays with different geometric parameters, tunable plasmon resonances are simply achieved. It is found that Ag has a much higher milling rate than Au under the same experimental conditions, which makes Ag suitable for constructing fine nanostructures with ultrasmall features and high aspect ratios. The optical properties of the fabricated nanopillars are characterized both experimentally and theoretically. The approach developed in this work may trigger new applications in plasmon-assisted sensing and detecting.