Loretta Shirey

Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors.

Efforts to create reproducible surface-enhanced Raman scattering (SERS)-based chemical and biological sensors has been hindered by difficulties in fabricating large-area SERS-active substrates with a uniform, reproducible SERS response that still provides sufficient enhancement for easy detection. Here we report on periodic arrays of Au-capped, vertically aligned silicon nanopillars that are embedded in a Au plane upon a Si substrate. We illustrate that these arrays are ideal for use as SERS sensor templates, in that they provide large, uniform and reproducible average enhancement factors up to ∼1.2 × 10(8) over the structure surface area. We discuss the impact of the overall geometry of the structures upon the SERS response at 532, 633, and 785 nm incident laser wavelengths. Calculations of the electromagnetic field distributions and intensities within such structures were performed and both the wavelength dependence of the predicted SERS response and the field distribution within the nanopillar structure are discussed and support the experimental results we report.

Low-Loss, Extreme Subdiffraction Photon Confinement via Silicon Carbide Localized Surface Phonon Polariton Resonators

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. Using fabricated 6H-silicon carbide nanopillar antenna arrays, we report on the observation of subdiffraction, localized SPhP resonances. They exhibit a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7-24 cm(-1)), with quality factors of 40-135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λ(res)3/V(eff))1/3 = 50-200. Under certain conditions, the modes are Raman-active, enabling their study in the visible spectral range. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications.

Mie resonance-enhanced light absorption in periodic silicon nanopillar arrays.

Mie-resonances in vertical, small aspect-ratio and subwavelength silicon nanopillars are investigated using visible bright-field µ-reflection measurements and Raman scattering. Pillar-to-pillar interactions were examined by comparing randomly to periodically arranged arrays with systematic variations in nanopillar diameter and array pitch. First- and second-order Mie resonances are observed in reflectance spectra as pronounced dips with minimum reflectances of several percent, suggesting an alternative approach to fabricating a perfect absorber. The resonant wavelengths shift approximately linearly with nanopillar diameter, which enables a simple empirical description of the resonance condition. In addition, resonances are also significantly affected by array density, with an overall oscillating blue shift as the pitch is reduced. Finite-element method and finite-difference time-domain simulations agree closely with experimental results and provide valuable insight into the nature of the dielectric resonance modes, including a surprisingly small influence of the substrate on resonance wavelength. To probe local fields within the Si nanopillars, µ-Raman scattering measurements were also conducted that confirm enhanced optical fields in the pillars when excited on-resonance.

Imaging layers for 50 kV electron beam lithography: Selective displacement of noncovalently bound amine ligands from a siloxane host film

We report the development of an imaging layer technology for 50 kV electron-beam lithography based upon the displacement of noncovalently bound amine ligands from a siloxane host film. The patterned films were used as templates for the selective deposition of an electroless nickel film resulting in a positive tone imaging mechanism. The deposited nickel was sufficiently robust to function as an etch mask for pattern transfer by reactive ion etching. Metallized and etched patterns with linewidths to approximately 40 nm are demonstrated using an exposure dose of 500 μC/cm2.

The Role of Propagating and Localized Surface Plasmons for SERS Enhancement in Periodic Nanostructures

Periodic arrays of plasmonic nanopillars have been shown to provide large, uniform surface-enhanced Raman scattering (SERS) enhancements. We show that these enhancements are the result of the combined impact of localized and propagating surface plasmon modes within the plasmonic architecture. Here, arrays of periodically arranged silicon nanopillars of varying sizes and interpillar gaps were fabricated to enable the exploration of the SERS response from two different structures; one featuring only localized surface plasmon (LSP) modes and the other featuring LSP and propagating (PSP) modes. It is shown that the LSP modes determine the optimal architecture, and thereby determine the optimum diameter for the structures at a given incident. However, the increase in the SERS enhancement factor for a system in which LSP and PSP cooperatively interact was measured to be over an order of magnitude higher and the peak in the diameter dependence was significantly broadened, thus, such structures not only provide larger enhancement factors but are also more forgiving of lithographic variations.

Photoresist channel‐constrained deposition of electroless metallization on ligating self‐assembled films

Patterned, selective electroless deposition has been achieved using exposed and developed photoresists, produced with UV and e‐beam exposure sources, to create channels for constrained metal growth on ligating self‐assembled film surfaces. This process is attractive for the production of high‐resolution, etching‐resistant features for semiconductor integrated circuit (IC) fabrication, as well as for the fabrication of patterned metal lines for IC‐level and (PWB)‐level interconnects. Etched metal features with linewidths of 150 nm have been demonstrated.

Nanolithography in polymethylmethacrylate: An atomic force microscope study

This work addresses the formation and development of very dense nanolithographic resolution patterns in polymethylmethacrylate (PMMA) of two molecular weights (MWs), 950 and 50 K. The patterns were lithographically defined by a JEOL JBX-5DII e-beam lithography system operated at 50 kV with a Gaussian probe standard deviation of 8–10 nm. The lithographic patterns consisted of pads of widths from 1 to 20 μm with a 50 nm gap of exposure in the center and single pass line gratings of periods 40, 60, 80, and 100 nm. Atomic force microscope images were acquired. Latent images of the 50 nm gap were observed over a much larger range in dose and pad size, than observed in developed samples. The morphologies of the two unpatterned resists were comparable exhibiting a granular type structure with average particle diameters of 52–53 nm and root mean square surface roughness of 0.2–0.3 nm. In developed patterns, the morphologies and resolution of the two MW resists were very different. The 950 K resist exhibited bette...

Nanolithography by displacement of catalytic metal clusters using an atomic force microscope tip

The use of catalytically active nanoclusters as a novel material for atomic force microscope (AFM) nanolithography is demonstrated. Films were prepared from colloidal Au nanoparticles and giant Pd clusters. Lithographic patterns were generated using the contact area of the AFM tip to physically displace nanoclusters, forming two-dimensional patterns on silicon oxide and functionalized silicon surfaces. Linewidth was found to depend on the force applied to the nanoparticles and the number of tip passes used to generate the pattern. Conditions were optimized to clear scanned areas using minimum applied force. Patterned films were used as templates for the selective deposition of electroless metal, which served as a robust plasma etch mask for pattern transfer into the underlying substrate to a depth of 200 nm. Minimum linewidths of approximately 35 nm were achieved in etched samples.

Electron-beam nanolithography, acid diffusion, and chemical kinetics in SAL-601

The work presents a unique investigation of the role of postexposure bake (PEB) on resist insolubility, PEB reaction kinetics, and the high resolution behavior of Microposit™ SAL-601. Patterns of 20–100 μm rectangles and single pass isolated lines were written on a JEOL JBX-5DII e-beam lithography system operated at 50 kV. The effects of PEB temperatures of 90–110 °C for periods of 1–11 min on resist insolubility and linewidth were examined. The samples were developed in MF-322 or acetone. The range of patterns allowed measurement of both the resist exposure curves and the line spread for each PEB condition. Insolubility in MF-322 can result from protection reactions and/or crosslinking reactions. Acetone insolubility, a characteristic of crosslinked resist, was observed at PEB temperatures of 100 °C and above. At 90 °C, acetone insolubility (crosslinking) was observed only after a PEB of 11 min. From the resist exposure curves for a sequence of PEB times, reaction orders for the PEB processes that led to...

Thin silicon nitride films for reduction of linewidth and proximity effects in e‐beam lithography

A thin silicon nitride layer (50–300 nm), deposited on a semiconductor substrate, prior to resist deposition, greatly enhances the resist resolution during electron beam lithography. The resolution enhancement was manifested by smaller feature sizes, for a given dose and full resolution of individual array elements at higher doses than on a bare semiconductor substrate. The effect has been observed in 50–100 nm of both SAL‐601 and PMMA spun onto silicon nitride coated Si and GaAs substrates. The samples were patterned with a 50 keV, 15 nm diam probe generated by a JEOL JBX‐5DII system. Improved resolution was found on two types of silicon nitride film grown by chemical vapor deposition: one deposited at 800 °C on Si and the other deposited at 200 °C on GaAs. Linewidth reductions in SAL‐601 of 40% at low doses and an order of magnitude at high doses were observed on silicon nitride coatings of 50–300 nm thickness. In PMMA, the resolution enhancement was less than in SAL‐601, with only a 15% linewidth reduc...