Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials

This protocol uses gold electroplating and atomic layer deposition methods to create novel nanopillar-based split ring resonator meta materials with nanoscale gaps. Each fabricated split ring resonator consists of thousands of gold nanopillars and is driven by displacement current across the nanogaps between them which excites terahertz resonances. The magnitude of displacement current is proportional to the size of the nanogaps.

The size of the nanogaps can be tuned by changing the cycles of the atomic layer deposition process which results in changes to the resonant frequencies and amplitude. An enhanced quality factor of approximately 450 can be achieved, which is 40 times more than the quality factor of 11 that can be achieved by thin-film split ring resonators. A frequency shift 17 times larger than that of thin film-based resonators can also be observed.

Both simulated and measured transmission spectra demonstrate that the nanopillar-based meta materials are ideal candidates for high sensitive sensors and frequency-agile terahertz devices. Analytical results from scanning electron microcopy show the fabricated nanopillar-based split ring resonator, two adjacent gold nanopillars, as well as the 10 nanometer aluminum oxide gaps between them. Nanopillar-based split resonators contain gold nanopillars and 10 nanometer aluminum oxide gap.

This structure excite inductive and capacitive resonance via displacement current. They provide more energies to it, enhance the energies towards these to high quality factor, and large frequency shift. Because of the high Q value and the large frequency shift, the nanopillar-based split resonator are suitable for various application, including biomedical and chemical sensors, and frequency tuning devices.

The main advantage of this technique over existing methods like electron beam lithography and self assembly is that this method combines a sequential electricoplating process and atomic layer deposition to realize nanoscale structures over a large area, with only conventional microfabrication process. The fabrication process is much faster and simpler compared to electron beam lithography and self assembly. High aspect ratio nano structures can be easily created with precisely controlled feature sizes.

First, deposit and adhesion promoter layer and a seed layer onto a flat substrate. Then, pattern the nanopillars using photolithography and electroplating. Uniformly coat a nanoscale dielectric layer on the entire structure, using atomic layer deposition.

A second adhesion promoter layer and seed layer are then deposited, followed by an electroplating deposition. Finally, create a C-shape split-ring resonator by partially etching the nanopillars and dielectric nanogaps, using photolithography and ion milling. Then, remove the photo resist using acetone in an ultrasonication bath.

Begin the substrate preparation with a four-inch high resistivity silicone wafer. Clean the silicone wafer with acetone, isopropyl alcohol, and deionized water. Then, blow dry with nitrogen gas.

Use an electron beam evaporator to deposit a five nanometer chromium adhesion promoter layer and a 10 nanometer copper seed layer. Cut the wafer into two centimeter by 2.5 centimeter pieces. Then, spin coat a two micrometer layer of S1813 Photoresist, at 2, 000 RPM, for 60 seconds.

Bake the substrate at 115 degrees celsius for 60 seconds. Now, expose the Photoresist to the nanopillar mask using a mask aligner, with approximately 15 milliwatts per centimeter squared of ultraviolet light, for 22 seconds. The nanopillar mask features five millimeter by five millimeter nanopillar arrays.

Develop the Photoresist in MF-319 developer with agitation, for 90 seconds. Rinse the sample with deionized water, and then blow dry it with nitrogen gas. Remove the top section of the Photoresist on the substrate using acetone to expose the copper seed layer for electrode connection.

Then, dip the sample in gold electroplating solution and electroplate an 800 nanometer gold layer for approximately eight minutes. Remove the Photoresist using acetone. Rinse with deionized water, and blow dry with nitrogen gas.

Remove the chromium and copper layer on the surface of the sample by dipping the sample in chromium etchant for 10 seconds, followed by copper etchant for 10 seconds. Now, uniformly coat the sample with a 10 nanometer aluminum oxide layer using atomic layer deposition. Use an electron beam evaporator to deposit a second five nanometer chromium adhesion promoter layer, as well as a second 10 nanometer copper seed layer, on top of the 10 nanometer aluminum oxide layer.

Dip the sample in gold electroplating solution for 16 minutes, to electroplate a 400 nanometer gold film on the substrate. Spin coat a two micrometer layer of S1813 Photoresist at 2, 000 RPM for 60 seconds. Bake the substrate at 115 degrees celsius for 60 seconds.

Expose the Photoresist to the C-shape split-ring resonator mask using a mask aligner with approximately 15 milliwatts per centimeter squared of UV light, for 22 seconds. Develop in MF-319 developer for 90 seconds. Etch the structures outside the Photoresist pattern using an ion mill system for approximately 30 minutes.

Remove the Photoresist using acetone in an ultrasonication bath, then rinse the sample with acetone, isopropyl alcohol, and deionized water, then blow dry with nitrogen gas. Alternatively, if air nanogaps are preferred, dip the sample in 5%hydrogen fluoride solution for five minutes, to remove the aluminum oxide. Inspect the nanopillar-based split ring resonators under an optical microscope.

Optical imaging shows two five millimeter by five millimeter nanopillar-based split ring resonator arrays on a silicone substrate. The zoomed-in, optical microscope image shows the C-shape nanopillar-based split ring resonator arrays. Gold nanopillar and aluminum oxide nanogaps can be observed in the zoomed-in image of a single split ring resonator.

Analytical results from scanning electron microscopy also showed the fabricated nanopillar-based split ring resonator, two adjacent gold nanopillars, as well as aluminum oxide gaps between them. Both five nanometer aluminum oxide gaps and 10 nanometer aluminum oxide gaps can be easily fabricated by controlling the cycles of the aluminum oxide atomic layer deposition process. These results show that this fabrication process can produce thousands of nanopillar-based split ring resonators on a large-scale substrate.

The process allows for precise control over the size of the nanogaps, and can easily produce high aspect ratio nanostructures, compared to electron beam lithography and self assembly. The nanopillar-based split ring resonators contain thousands of gold nanopillars and nanoscale gaps. The resonant frequencies and amplitudes of the nanopillar-based split ring resonators can be easily tuned by changing the size of the nanogaps, which makes them suitable for various applications, including biomedical sensors, and frequency-agile devices.

After watching this video, you should have a good understanding of how to make gold nanopillar-based split ring resonators using a two-step gold electroplating and anatomic layer deposition process to create aluminum oxide nanoscale gaps between gold nanopillars. It is important to understand the working principle of the process and choose the right materials and thicknesses for each material used in the process.