This material is based upon work supported by a start-up fund at the University of Minnesota, Twin Cities. Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program. A portion of this work was also carried out in the Minnesota Nano Center which receives partial support from the NSF through the NNCI program.

Caution: Several of the chemicals used in these syntheses are toxic, highly flammable, and may cause irritation and severe organ damage when touched or inhaled. Please wear appropriate personal protective equipment (PPE) when handling.
1. Preparation of the First Layer of Gold (Au) Nanopillar Arrays (Figure 2a-c and Figure 2e-g)
<ol>
	<li>Preparation of Copper (Cu) Seed layers for Au electroplating (Figure 2a, b and Figure 2e, f)
	<ol>
		<li>Use a 4&#34; high resistivity silicon (Si) wafer (resi...

<ol>
	<li>Xu, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flexible+visible-infrared+metamaterials+and+their+applications+in+highly+sensitive+chemical+and+biological+sensing.">Flexible visible-infrared metamaterials and their applications in highly sensitive chemical and biological sensing.</a> Nano Lett. 11 (8), 3232-3238 (2011).</li><li>Singh, R., Cao, W., Al-Naib, I., Cong, L., Withayachumnankul, W., Zhang, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrasensitive+terahertz+sensing+with+high-Q+Fano+resonances+in+metasurfaces.">Ultrasensitive terahertz sensing with high-Q Fano resonances in metasurfaces.</a> Appl. Phys. Lett. 105 (17), 171101 (2014).</li><li>Torun, H., Top, F. 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Mod. Optic. 56 (4), 554-557 (2009).</li><li>Melik, R., Unal, E., Perkgoz, N. K., Puttlitz, C., Demir, H. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flexible+metamaterials+for+wireless+strain+sensing.">Flexible metamaterials for wireless strain sensing.</a> Appl. Phys. Lett. 95 (18), 181105 (2009).</li><li>Naqui, J., Dur&#225;n-Sindreu, M., Mart&#237;n, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alignment+and+position+sensors+based+on+split+ring+resonators.">Alignment and position sensors based on split ring resonators.</a> Sensors. 12 (9), 11790-11797 (2012).</li><li>Chiam, S., Singh, R., Gu, J., Han, J., Zhang, W., Bettiol, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Increased+frequency+shifts+in+high+aspect+ratio+terahertz+split+ring+resonators.">Increased frequency shifts in high aspect ratio terahertz split ring resonators.</a> Appl. Phys. Lett. 94 (6), 064102 (2009).</li><li>Gil, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Varactor-loaded+split+ring+resonators+for+tunable+notch+filters+at+microwave+frequencies.">Varactor-loaded split ring resonators for tunable notch filters at microwave frequencies.</a> Electron. 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This fabrication technique has significant advantages for creating nano-scale structures over existing methods such as E-beam lithography and self-assembly. First, nano-scale structures can be realized over a large area (an entire wafer) using a photomask that features nanopillar arrays, which is not practical with an E-beam lithography process. Second, the fabrication process uses a traditional wafer scale micro fabrication process, which is much faster, simpler, and cheaper compared to E-beam lithography. Third, the at...

Terahertz (THz) metamaterials (MMs) have been developed for biomedical sensors and frequency-agile devices1,2,3,4,5,6,7,8,9,10,11. In order to improve the sensitivity and frequency selectivity of the THz MM sensors, a nanopillar-based split ring resonator (SRR) has been designed using displacement current generated inside gold (Au) nanopillar arrays to excite THz resonances with ultra-high quality factors (Q-factors) (~450) (Figure 1)12. Even though nanopillar-based SRRs show high Q-factors and promising sensing abilities, fabrication of such nanostructures with high aspect ratios (more than 40) and nano-scale gaps (sub-10 nm) over a large area remains challenging13.
The most commonly used technique to fabricate nano-scale structures is electron-beam lithography (EBL)14,15,16,17. However, the resolution of EBL is still limited due to the beam spot size, electron scattering, properties of the resist, and the development process18,19. In addition, it is not practical to fabricate nanostructures using EBL over a large area due to a slow process time and large process costs20. Another strategy to achieve nanostructures is to use a self-assembly technique21,22. By self-assembling metal nanocubes (NCs) in a solution and utilizing the electrostatic interaction and the association of polymer ligands between NCs, a well-organized one-dimensional NC array with nano-scale gaps can be achieved23. The nano-gap size depends on the polymer ligands between the NCs and can be controlled by applying different polymer materials with different molecular weights24,25,26. Self-assembly is a powerful technique for achieving scalable and cost-efficient nanostructures23. However, the fabrication process is more complicated compared to conventional micro and nano fabrication processes, and the control of nano-gap sizes is not precise enough for electronic device applications. In order to successfully fabricate nanopillar-based SRRs, a novel fabrication method should be invented to achieve the following goals: i) the fabrication process is easy to apply and is compatible with conventional micro and nano fabrication processes; ii) fabrication over a large area is applicable; iii) nano-gap sizes can be easily and precisely controlled with a 0.1 nm resolution and can be scaled down to 10 nm or less.
A novel fabrication method is demonstrated using the combination of an electroplating process and an atomic layer deposition (ALD) process to fabricate nanopillar-based SRRs. Since electroplating is a self-filling process with low cost, it is easy to fabricate structures over a large area. ALD is a chemical vapor deposition (CVD) process that can be precisely controlled by the reaction cycle during the process. The resolution of ALD thin film can be 0.1 nm, and the thin film is uniformly coated with a&#160;high quality, which is suitable to create nano-scale gaps27,28. Nanopillar-based SRR array with 10 nm gaps or less can be successfully fabricated over an area of 6 mm &#215; 6 mm. Both simulated and measured THz transmission spectra show resonant behaviors with ultra-high Q-factors and large frequency shifts, which proves the feasibility of the nanopillar-based SRRs mediated by displacement current. The detailed fabrication process is described below in the protocol section, and the video protocol can help practitioners to understand the fabrication process and avoid common mistakes associated with the fabrication of nanopillar-based SRRs.

<table border="0" cellpadding="0" cellspacing="0" width="1240">
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	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Silicon Wafer</td>
			<td style="width:295px;">Siltronic AG</td>
			<td style="width:199px;">N/A</td>
			<td style="width:437px;">100mm diameter, N-type, one-side polish, resitivity: 560-840 &#937;&#8226;cm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Chromium</td>
			<td style="width:295px;">Kurt J. Lesker Company</td>
			<td style="width:199px;">EVMCR35J</td>
			<td>99.95% pure</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Copper</td>
			<td style="width:295px;">Kurt J. Lesker Company</td>
			<td style="width:199px;">EVMCU40QXQJ</td>
			<td>99.99% pure</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">E-Beam Evaporator System</td>
			<td style="width:295px;">Rocky Mountain Vacuum Tech.</td>
			<td style="width:199px;">N/A</td>
			<td>RME-2000</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">S1813 Positive Photoresist</td>
			<td style="width:295px;">Microposit</td>
			<td style="width:199px;">10018348</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Spinner</td>
			<td style="width:295px;">Best Tools</td>
			<td style="width:199px;">S0114031123</td>
			<td>SMART COATER 100</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Mask Aligner</td>
			<td style="width:295px;">Midas</td>
			<td style="width:199px;">MDA-400LJ</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Digital Hot Plate</td>
			<td style="width:295px;">Thermo Scientific</td>
			<td style="width:199px;">HP131725</td>
			<td>Super-Nuvoa series, maximum temperature: 370 degree C</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">MF319 Developer</td>
			<td style="width:295px;">Microposit</td>
			<td style="width:199px;">10018042</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Acetone</td>
			<td style="width:295px;">Fisher Chemical</td>
			<td style="width:199px;">A18P-4</td>
			<td style="width:437px;">N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Isopropyl Alcohol</td>
			<td style="width:295px;">Fisher Chemical</td>
			<td style="width:199px;">A416-4</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Gold 25 ES RTU</td>
			<td style="width:295px;">Technic Inc.</td>
			<td style="width:199px;">391427</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Source Meter</td>
			<td style="width:295px;">Keithley</td>
			<td style="width:199px;">N/A</td>
			<td>2612 System SourceMeter</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Microscope</td>
			<td style="width:295px;">Omax</td>
			<td style="width:199px;">NJF-120A</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Profilometer</td>
			<td style="width:295px;">Tencor Instruments</td>
			<td style="width:199px;">N/A</td>
			<td>Alpha-Step 200</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">APS Copper Etchant 100</td>
			<td style="width:295px;">Transfene Company, Inc.</td>
			<td style="width:199px;">N/A</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">CE-5 M Chromium Mask Etchant</td>
			<td style="width:295px;">Transfene Company, Inc.</td>
			<td style="width:199px;">N/A</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Atomic Layer Deposition System</td>
			<td style="width:295px;">Cambridge Nano Tech inc.</td>
			<td style="width:199px;">N/A</td>
			<td>Savannah series</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Ion Mill Etching System</td>
			<td style="width:295px;">Intlvac Thin Film</td>
			<td style="width:199px;">N/A</td>
			<td>Nanoquest series</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Ultrasonic Cleaner</td>
			<td style="width:295px;">Crest Ultrasonics</td>
			<td style="width:199px;">N/A</td>
			<td>Powersonic series</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Hydrofluoric Acid</td>
			<td style="width:295px;">Sigma-Aldrich</td>
			<td style="width:199px;">244279</td>
			<td>Diluted to 5%</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:311px;">Field Emission Gun Scanning Electron Microscope</td>
			<td style="width:295px;">Jeol Ltd.</td>
			<td style="width:199px;">N/A</td>
			<td>JEOL 6700 series</td>
		</tr>
	</tbody>
</table>

fabrication of nanopillar-based split ring resonators for displacement current mediated resonances in terahertz metamaterials

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR is not affected by environmental characteristics such as the temperature and pressure surrounding the resonator. Electromagnetic radiation in THz frequencies is biocompatible, which is a critical condition especially for the application of the biomolecular sensing. However, the quality factor (Q-factor) and frequency responses of traditional thin-film based split ring resonator (SRR) MMs are very low, which limits their sensitivities and selectivity as sensors. In this work, novel nanopillar-based SRR MMs, utilizing displacement current, are designed to enhance the Q-factor up to 450, which is around 45 times higher than that of traditional thin-film-based MMs. In addition to the enhanced Q-factor, the nanopillar-based MMs induce a larger frequency shifts (17 times compared to the shift obtained by the traditional thin-film based MMs). Because of the significantly enhanced Q-factors and frequency shifts as well as the property of biocompatible radiation, the THz nanopillar-based SRR are ideal MMs for the development of biomolecular sensors with high sensitivity and selectivity without inducing damage or distortion to biomaterials. A novel fabrication process has been demonstrated to build the nanopillar-based SRRs for displacement current mediated THz MMs. A two-step gold (Au) electroplating process and an atomic layer deposition (ALD) process are used to create sub-10 nm scale gaps between Au nanopillars. Since the ALD process is a conformal coating process, a uniform aluminum oxide (Al2O3) layer with nanometer-scale thickness can be achieved. By sequentially electroplating another Au thin film to fill the spaces between Al2O3 and Au, a close-packed Au-Al2O3-Au structure with nano-scale Al2O3 gaps can be fabricated. The size of the nano-gaps can be well defined by precisely controlling the deposition cycles of the ALD process, which has an accuracy of 0.1 nm.

Fabrication schemes show each step (Figure 2a-x). Optical images (Figure 2y-ac) and scanning electron microscope (SEM) images (Figure 2ad-ag) were collected for the nanopillar-based SRRs at different fabrication steps. Animations (Figure 2a-c) illustrate the first layer of electroplated Au nanopillars and the second layer of electroplated Au films as well as the nano-gaps created between them. Figure 2d s...

Watch this Scientific Journal Video about Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials at JoVE.com

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

A protocol for the design and fabrication of a novel nanopillar-based split ring resonator (SRR) is presented.

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR ...

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.

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7.7K vistas.  University of Minnesota. 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 cyc...