Fine-tunable plasma nano-machining for fabrication of 3D hollow nanostructures: SERS application

Abstract

Novel processing sequences for the fabrication of artificial nanostructures are in high demand for various applications. In this paper, we report on a fine-tunable nano-machining technique for the fabrication of 3D hollow nanostructures. This technique originates from redeposition effects occurring during Ar dry etching of nano-patterns. Different geometries of honeycomb, double ring, nanotube, cone and crescent arrays have been successfully fabricated from various metals such as Au, Ag, Pt and Ti. The geometrical parameters of the 3D hollow nanostructures can be straightforwardly controlled by tuning the discharge plasma pressure and power. The structure and morphology of nanostructures are probed using atomic force microscopy (AFM), scanning electron microscopy (SEM), optical emission spectroscopy (OES) and energy dispersive x-ray spectroscopy (EDS). Finally, a Ag nanotube array was assayed for application in surface enhanced Raman spectroscopy (SERS), resulting in an enhancement factor (EF) of 5.5 × 10, as an experimental validity proof consistent with the presented simulation framework. Furthermore, it was found that the theoretical EF value for the honeycomb array is in the order of 10, a hundred times greater than that found in nanotube array.

A wafer-scale fabrication method for three-dimensional plasmonic hollow nanopillars

“…Moreover, for atomically flat surfaces at normal incidence, redeposition depends on the ion's mean free path length in the process chamber. 75 This is an extreme example, but it suggests that IBE at normal incidence only enables redeposition whenever the initial deposited material already has some roughness. Control over the redeposition rate is critical for the Au-HNP plasmonic structures.…”

“…In this respect, Jeon et al 50 achieved similar 3D architectures with NIL, but in our case, we have pattern-ability by SHL and show the wafer-scale homogeneity. Lastly, Mehrvar et al 75 showed the complexity of using a capacitively coupled RIE for sidewall redeposition. Utilizing an IBE gives a more controllable platform for sidewall redeposition.…”

“…This is a different mechanism than those reported in other works where the etching was performed at normal incidence. 54,55,65,77 In this work, tuning the incidence angle can yield different etch rates on the substrate and sidewalls and thus yield varying redeposition rates. Moreover, for atomically flat surfaces at normal incidence, redeposition depends on the ion's mean free path length in the process chamber.…”

Large Dense Periodic Arrays of Vertically Aligned Sharp Silicon Nanocones

“…The partial pressure of sputtered silicon is highest at the center of the SiNW, which results in a lower etch rate. Studies conducted on the redeposition of metals under a polymer mask subdued to CM-ICP RIE show that indeed the redeposition process is pressure-dependent [37]. To lower the probability of redeposition, the substrates were etched in an IBE system at a much lower process chamber pressure than that during CM-ICP RIE, being 0.17 mTorr instead of 18 mTorr.…”

“…In other literature works, maskless etching and sharpening of structures were suggested based on this electric field effect [36]. However, a more likely explanation for the observed tip formation and redeposition effect is the pressure range in which the inductively coupled plasma RIE (ICP RIE) functions [37]. The impeding argon ions (Ar + ) sputter the silicon which in turn increases the pressure and decreases the mean free path length near the surface, leading to a higher lateral velocity component near the substrate that enables sputter redeposition [38,39].…”