Ultra-Smooth Patterned Metals for Plasmonics and Bio-Sensing

PI's: Sang-Hyun Oh, David Norris,

Surface plasmons (SPs) are electromagnetic surface waves bound to a metal interface by coupling to the free electron plasma in metals. Due to their evanescent nature, SP waves are not limited by diffraction, and can provide confinement of light on scales much smaller than the free-space wavelength. The possibility of nanofocusing and manipulation of optical fields has generated intense interest in the rapidly developing field of plasmonics for applications ranging from solar cells to bio-sensing . Noble metals such as Ag, Au that are required for plasmonics, however, are not amenable to large-area patterning at the required nanometer-scale resolution, because (1) the as-deposited metal films are rough, and (2) no simple high-throughput method exists to pattern these metal films over a large area. Existing fabrication techniques such as focused ion beam (FIB) milling or e-beam lithography don't lend themselves to large-area patterning. To address this fundamental challenge of engineering plasmonic materials, Norris and Oh collaborated and developed a new fabrication method that simultaneously solves both problems by combining template stripping with precisely patterned silicon substrates to obtain ultra-smooth metallic films patterned with nanoscale grooves, bumps, pyramids, ridges, and holes.

The simple procedure illustrated in Fig. 1A can provide a powerful solution for plasmonics. We use inexpensive silicon wafers to benefit from well-developed microelectronics fabrication techniques. After the wafer is patterned, e.g. with lithography or FIB, it is coated with a thin metal film and a layer of epoxy. Since the adhesion of the metal layer to the silicon is poor, the epoxy-metal bilayer can then simply be peeled off the substrate to reveal a patterned structure with a surface roughness determined by the silicon wafer template. The silicon template can then be used again. For example, Fig. 1B shows a silicon substrate with circular concentric grooves defined by FIB. We then thermally evaporated 275 nm of silver on this substrate, added epoxy, and peeled off the bilayer. Figure 1C, 1D, and 1E indicate the quality of the silver "bull's eye" structure that is obtained. In particular, electron micrographs taken at glancing incidence (e.g., Fig. 1E) are extremely effective at exposing surface roughness. These images reveal the ultra-smooth patterned interfaces. Using a very simple and high-throughput approach, we've demonstrated a variety of high-performance plasmonic structures (Fig. 2).

Besides the capability to produce ultra-smooth patterned structures from a variety of metals, our method can also fabricate highly reproducible, sharp pyramidal tips with radii of curvature as small as 10 nm (inset to Fig. 2D). More importantly, with our template stripping approach, we can produce an entire array of such pyramids that are attached to a smooth metal film. This has clear advantages for plasmonics in terms of the well controlled placement, orientation, and integration of the structures.

Figure 1:(a) Template stripping process steps. (b) Silicon template. (c)-(e) Ultrasmooth patterned metallic surface.

Figure 2: (a)-(e) Various ultra-smooth patterned metallic surfaces. The pyramids were made with anisotropic wet etching with KOH of the silicon template.