Name: metal-assisted electrochemical nanoimprinting of porous and solid silicon wafers
Uploaded: 2022-02-08T12:30:00
Description: Watch this Scientific Journal Video about Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers at JoVE.com

We acknowledge Dr. Keng Hsu (University of Louisville) for insights regarding this work; University of Illinois&#39;s Frederick Seitz Laboratory and, in memoriam, staff member Scott Maclaren; Arizona State University&#39;s LeRoy Eyring Center for Solid State Science; and the Science Foundation Arizona under the Bis grove Scholars Award.

CAUTION: Use appropriate safety practices and personal protective equipment (e.g., lab coat, gloves, safety glasses, closed-toe shoes). This procedure utilizes HF acid (48% wt) which is an extremely hazardous chemical and requires additional personal protective equipment (i.e., a face shield, natural rubber apron, and second pair of nitrile gloves that covers the hand, wrists, and forearms).
1. Stamp preparation for Mac-imprint
<ol>
	<li>PDMS mold fabrication
	<ol>
		<li>Pre...

<ol>
	<li>Ning, H., 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=Transfer-Printing+of+Tunable+Porous+Silicon+Microcavities+with+Embedded+Emitters.">Transfer-Printing of Tunable Porous Silicon Microcavities with Embedded Emitters.</a> ACS Photonics. 1 (11), 1144-1150 (2014).</li><li>Hirschman, K. D., Tsybeskov, L., Duttagupta, S. P., Fauchet, P. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silicon-based+light+emitting+devices+integrated+into+microelectronic+circuits.">Silicon-based light emitting devices integrated into microelectronic circuits.</a> Nature. 384, 338-341 (1996).</li><li>Cho, J., 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=Nanoscale+Origami+for+3D+Optics.">Nanoscale Origami for 3D Optics.</a> Small. 7 (14), 1943-1948 (2011).</li><li>Azeredo, B. P., 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=Silicon+nanowires+with+controlled+sidewall+profile+and+roughness+fabricated+by+thin-film+dewetting+and+metal-assisted+chemical+etching.">Silicon nanowires with controlled sidewall profile and roughness fabricated by thin-film dewetting and metal-assisted chemical etching.</a> Nanotechnology. 24 (22), 225305-225312 (2013).</li><li>Lin, C., Tsai, M., Wei, W., Lai, K., He, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Packaging+Glass+with+a+Hierarchically+Nanostructured+Surface:+a+universal+method+to+achieve+selfcleaning+omnidirectional+solar+cells.">Packaging Glass with a Hierarchically Nanostructured Surface: a universal method to achieve selfcleaning omnidirectional solar cells.</a> ACS Nano. 10 (1), 549-555 (2016).</li><li>Park, K. C., 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=Nanotextured+Silica+Surfaces+with+Robust+Superhydrophobicity+and+Omnidirectional+Broadband+Supertransmissivity.">Nanotextured Silica Surfaces with Robust Superhydrophobicity and Omnidirectional Broadband Supertransmissivity.</a> ACS Nano. 6 (5), 3789-3799 (2012).</li><li>Kim, J., Joy, D. C., Lee, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlling+resist+thickness+and+etch+depth+for+fabrication+of+3D+structures+in+electron-beam+grayscale+lithography.">Controlling resist thickness and etch depth for fabrication of 3D structures in electron-beam grayscale lithography.</a> Microelectronics Engineering. 84 (12), 2859-2864 (2007).</li><li>Deng, S., Zhang, Y., Jiang, S., Lu, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+three-dimensional+silicon+structure+with+smooth+curved+surfaces.">Fabrication of three-dimensional silicon structure with smooth curved surfaces.</a> Journal of Micro/Nanolithography, MEMS, and MOEMS. 15 (3), 0345031-0345036 (2016).</li><li>Azeredo, B. P., Lin, Y., Avagyan, A., Sivaguru, M., Hsu, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+Imprinting+of+Porous+Silicon+via+Metal-Assisted+Chemical+Etching.">Direct Imprinting of Porous Silicon via Metal-Assisted Chemical Etching.</a> Advanced Functional Materials. 26 (17), 2929-2939 (2016).</li><li>Azeredo, B., Hsu, K., Ferreira, P. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+Electrochemical+Imprinting+of+Sinusoidal+Linear+Gratings+into+Silicon.">Direct Electrochemical Imprinting of Sinusoidal Linear Gratings into Silicon.</a> The American Society of Mechanical Engineers - International Manufacturing Science and Engineering Conference. , 1-6 (2016).</li><li>Li, H., Niu, J., Wang, G., Wang, E., Xie, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+Production+of+Silicon+Nanostructures+with+Electrochemical+Nanoimprinting.">Direct Production of Silicon Nanostructures with Electrochemical Nanoimprinting.</a> ACS Applied Electronic Materials. 1 (7), 1070-1075 (2019).</li><li>Kim, K., Ki, B., Choi, K., Lee, S., Oh, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resist-Free+Direct+Stamp+Imprinting+of+GaAs+via+Metal-Assisted+Chemical+Etching.">Resist-Free Direct Stamp Imprinting of GaAs via Metal-Assisted Chemical Etching.</a> ACS Applied Materials &amp; Interfaces. 11 (14), 13574-13580 (2019).</li><li>Zhang, J., 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=Contact+electrification+induced+interfacial+reactions+and+direct+electrochemical+nanoimprint+lithography+in+n-type+gallium+arsenate+wafer.">Contact electrification induced interfacial reactions and direct electrochemical nanoimprint lithography in n-type gallium arsenate wafer.</a> Chemical Science. 8, 2407-2412 (2017).</li><li>Zhan, D., 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=Electrochemical+micro/nano-machining:+principles+and+practices.">Electrochemical micro/nano-machining: principles and practices.</a> Chemical Society Reviews. 46 (5), 1526-1544 (2017).</li><li>Li, X., Bohn, P. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metal-assisted+chemical+etching+in+HF+/+H2O2+produces+porous+silicon.">Metal-assisted chemical etching in HF / H2O2 produces porous silicon.</a> Applied Physics Letters. 77 (16), 2572-2574 (2000).</li><li>Chartier, C., Bastide, S., Levy-Clement, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metal-assisted+chemical+etching+of+silicon+in+HF+-+H2O2.">Metal-assisted chemical etching of silicon in HF - H2O2.</a> Electrochimica Acta. 53, 5509-5516 (2008).</li><li>Chattopadhyay, S., Li, X., Bohn, P. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In-plane+control+of+morphology+and+tunable+photoluminescence+in+porous+silicon+produced+by+metal-assisted+electroless+chemical+etching.">In-plane control of morphology and tunable photoluminescence in porous silicon produced by metal-assisted electroless chemical etching.</a> Journal of Applied Physics. 91 (9), 6134-6140 (2002).</li><li>Torralba, E., 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=3D+patterning+of+silicon+by+contact+etching+with+anodically+biased+nanoporous+gold+electrodes.">3D patterning of silicon by contact etching with anodically biased nanoporous gold electrodes.</a> Electrochemistry Communications. 76, 79-82 (2017).</li><li>Bastide, S., 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=3D+Patterning+of+Si+by+Contact+Etching+With+Nanoporous+Metals.">3D Patterning of Si by Contact Etching With Nanoporous Metals.</a> Frontiers in Chemistry. 7, 1-13 (2019).</li><li>Sharstniou, A., Niauzorau, S., Ferreira, P. M., Azeredo, B. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+nanoimprinting+of+silicon.">Electrochemical nanoimprinting of silicon.</a> Proceedings of the National Academy of Sciences. 116 (21), 10264-10269 (2019).</li><li>Niauzorau, S., Ferreira, P., Azeredo, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+of+Porous+Noble+Metal+Films+with+Tunable+Porosity+by+Timed+Dealloying.">Synthesis of Porous Noble Metal Films with Tunable Porosity by Timed Dealloying.</a> The American Society of Mechanical Engineers - International Manufacturing Science and Engineering Conference. , 1-4 (2018).</li><li>Geyer, N., 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=Model+for+the+Mass+Transport+During+Metal-Assisted+Chemical+Etching+with+Contiguous+Metal+Films+As+Catalysts.">Model for the Mass Transport During Metal-Assisted Chemical Etching with Contiguous Metal Films As Catalysts.</a> The Journal of Physical Chemistry C. 116 (24), 13446-13451 (2012).</li><li>Li, L., Liu, Y., Zhao, X., Lin, Z., Wong, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Uniform+Vertical+Trench+Etching+on+Silicon+with+High+Aspect+Ratio+by+Metal-Assisted+Chemical+Etching+Using+Nanoporous+Catalysts.">Uniform Vertical Trench Etching on Silicon with High Aspect Ratio by Metal-Assisted Chemical Etching Using Nanoporous Catalysts.</a> ACS Applied Materials and Interfaces. 6 (1), 575-584 (2014).</li></ol>

Mac-Imprint stamps and prepatterned Si chips (p-type, [100] orientation, 1-10 Ohm&#8729;cm) were prepared according to sections 1 and 2 of the protocol, respectively. The Mac-Imprint of prepatterned Si chip with stamps containing 3D hierarchical patterns was performed according to section 3 of the protocol (Figure 9). As shown in Figure 9a, different configurations of Mac-Imprint were applied: solid Si with solid Au (left), porous Si with solid ...

Three-dimensional micro- and nanoscale patterning and texturization of semiconductors enables numerous applications in various areas, such as optoelectronics1,2, photonics3, antireflective surfaces4, super hydrophobic, and self-cleaning surfaces5,6 among others. Prototyping and mass-producing 3D and hierarchical patterns has been successfully accomplished for polymeric films by soft lithography and nanoimprinting lithography with sub-20 nm resolution. However, transferring such 3D polymeric patterns into Si requires the etching selectivity of a mask pattern during reactive ion etching and thus limits the aspect ratio, and induces shape distortions and surface roughness due to scalloping effects7,8.
A new method called Mac-Imprint has been achieved for parallel and direct patterning of porous9 and solid Si wafers10,11 as well as solid GaAs wafers12,13,14. Mac-Imprint is a contact-based wet etching technique that requires contact between substrate and a noble metal-coated stamp possessing 3D features in the presence of an etching solution (ES) composed of HF and an oxidant (e.g., H2O2 in the case of Si Mac-Imprint). During the etching, two reactions occur simultaneously15,16: a cathodic reaction (i.e., the H2O2 reduction at the noble metal, during which positive charge carriers [holes] are generated and subsequently injected into Si17) and an anodic reaction (i.e., Si dissolution, during which the holes are consumed). After sufficient time in contact, the stamp&#39;s 3D features are etched into the Si wafer. Mac-Imprint has numerous advantages over conventional lithographical methods, such as high throughput, compatibility with roll-to-plate and roll-to-roll platforms, amorphous, mono- and polycrystalline Si and III-V semiconductors. Mac-Imprint stamps can be reused multiple times. Additionally, the method can deliver a sub-20 nm etching resolution that is compatible with contemporary direct writing methods.
The key to attaining high-fidelity imprinting is the diffusion pathway to the etching front (i.e., contact interface between catalyst and substrate). The work of Azeredo et al.9 first demonstrated that ES diffusion is enabled through a porous Si network. Torralba et al.18, reported that in order to realize solid Si Mac-Imprint the ES diffusion is enabled through a porous catalyst. Bastide et al.19 and Sharstniou et al.20 further investigated the catalyst porosity influence on ES diffusion. Thus, the concept of Mac-Imprint has been tested in three configurations with distinct diffusion pathways.
In the first configuration, the catalyst and substrate are solid, providing no initial diffusion pathway. The lack of reactant diffusion leads to a secondary reaction during imprinting that forms a layer of porous Si on the substrate around the edge of the catalyst-Si interface. The reactants are subsequently depleted, and the reaction stops, resulting in no discernable pattern transfer fidelity between the stamp and substrate. In the second and third configurations, the diffusion pathways are enabled through porous networks introduced either in the substrate (i.e., porous Si) or in the catalyst (i.e., porous gold) and high pattern transfer accuracy is attained. Thus, the mass transport through porous materials plays a critical role in enabling the diffusion of reactants and reaction products to and away from the contact interface9,18,19,20. A schematic of all three configurations is shown in Figure 1.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61040/61040fig1.jpg" /> 
Figure 1: Schematics of Mac-Imprint configurations. This figure highlights the role of porous materials in enabling the diffusion of reacting species through the substrate (i.e., case II: porous Si) or in the stamp (i.e., case III: catalyst thin film made of porous gold). <a href="https://www.jove.com/files/ftp_upload/61040/61040fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In this paper, the Mac-Imprint process is thoroughly discussed, including stamp preparation and substrate pretreatment along with Mac-Imprint itself. The substrate pretreatment section within the protocol includes Si wafer cleaning and Si wafer patterning with dry etching and substrate anodization (optional). Further, a stamp preparation section is subdivided into several procedures: 1) PDMS replica molding of Si master mold; 2) UV nanoimprinting of a photoresist layer in order to transfer the PDMS pattern; and 3) catalytic layer deposition via magnetron sputtering followed by dealloying (optional). Finally, in the Mac-Imprint section the Mac-Imprint setup along with the Mac-Imprint results (i.e., Si surface 3D hierarchical patterning) is presented.

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		<tr>
			<td>Acetone, &#62;99.5%, ACS reagent</td>
			<td>Sigma-Aldrich</td>
			<td>67-64-1</td>
			<td>CAUTION, chemical</td>
		</tr>
		<tr>
			<td>Ammonium fluoride, &#62;98%, ACS grade</td>
			<td>Sigma-Aldrich</td>
			<td>12125-01-8</td>
			<td>CAUTION, hazardous</td>
		</tr>
		<tr>
			<td>Ammonium hydroxide solution, 28-30%, ACS reagent</td>
			<td>Sigma-Aldrich</td>
			<td>1336-21-6</td>
			<td>CAUTION, hazardous</td>
		</tr>
		<tr>
			<td>AZ 400K developer</td>
			<td>Microchemicals</td>
			<td>AZ 400K</td>
			<td>CAUTION, chemical</td>
		</tr>
		<tr>
			<td>BenchMark 800 Etch</td>
			<td>Axic</td>
			<td>BenchMark 800</td>
			<td>Reactive ion etching</td>
		</tr>
		<tr>
			<td>Chromium target, 2&#34; x 0.125&#34;, 99.95% purity</td>
			<td>ACI alloys</td>
			<td>ADM0913</td>
			<td>Magnetron sputter chromium target</td>
		</tr>
		<tr>
			<td>CTF 12</td>
			<td>Carbolite Gero</td>
			<td>C12075-700-208SN</td>
			<td>Tube furnace</td>
		</tr>
		<tr>
			<td>Desiccator</td>
			<td>Fisher scientific Chemglass life sciences</td>
			<td>CG122611</td>
			<td>Desiccator</td>
		</tr>
		<tr>
			<td>F6T5/BLB</td>
			<td>Eiko</td>
			<td>F6T5/BLB 6W</td>
			<td>UV bulb</td>
		</tr>
		<tr>
			<td>Gold target, 2&#34; x 0.125&#34;, 99.99% purity</td>
			<td>ACI alloys</td>
			<td>N/A</td>
			<td>Magnetron sputter gold target</td>
		</tr>
		<tr>
			<td>Hotplate KW-4AH</td>
			<td>Chemat tecnologie</td>
			<td>KW-4AH</td>
			<td>Leveled hotplate with uniform temperature profile</td>
		</tr>
		<tr>
			<td>Hydrofluoric acid, 48%, ACS reagent</td>
			<td>Sigma-Aldrich</td>
			<td>7664-39-3</td>
			<td>CAUTION, extremly hazardous</td>
		</tr>
		<tr>
			<td>Hydrogen peroxide, 30%, ACS reagent</td>
			<td>Fisher Chemical</td>
			<td>7722-84-1</td>
			<td>CAUTION, hazardous</td>
		</tr>
		<tr>
			<td>Isopropyl alcohol, &#62;99.5%, ACS reagent</td>
			<td>LabChem</td>
			<td>67-63-0</td>
			<td>CAUTION, chemical</td>
		</tr>
		<tr>
			<td>MLP-50</td>
			<td>Transducer Techniques</td>
			<td>MLP-50</td>
			<td>Load cell</td>
		</tr>
		<tr>
			<td>Nitric acid, 70%, ACS grade</td>
			<td>SAFC</td>
			<td>7697-37-2</td>
			<td>CAUTION, hazardous</td>
		</tr>
		<tr>
			<td>NSC-3000</td>
			<td>Nano-master</td>
			<td>NSC-3000</td>
			<td>Magnetron sputter</td>
		</tr>
		<tr>
			<td>Potassium hydroxide, 45%, Certified</td>
			<td>Fisher Chemical</td>
			<td>1310-58-3</td>
			<td>CAUTION, chemical</td>
		</tr>
		<tr>
			<td>Rocker 800 vacuum pump, 110V/60Hz</td>
			<td>Rocker</td>
			<td>1240043</td>
			<td>Oil-free vacuum pump</td>
		</tr>
		<tr>
			<td>Silicon master mold</td>
			<td>NILT</td>
			<td>SMLA_V1</td>
			<td>Silicon chip with pattern</td>
		</tr>
		<tr>
			<td>Silicon wafers, prime grade</td>
			<td>University wafer</td>
			<td>783</td>
			<td>Si wafer</td>
		</tr>
		<tr>
			<td>Silver target, 2&#34; x 0.125&#34;, 99.99% purity</td>
			<td>ACI alloys</td>
			<td>HER2318</td>
			<td>Magnetron sputter silver target</td>
		</tr>
		<tr>
			<td>SP-300</td>
			<td>BioLogic</td>
			<td>SP-300</td>
			<td>Potentiostat</td>
		</tr>
		<tr>
			<td>SPIN 150i</td>
			<td>Spincoating</td>
			<td>SPIN 150i</td>
			<td>Spin coater</td>
		</tr>
		<tr>
			<td>SPR 200-7.0 positive photoresist</td>
			<td>Microchem</td>
			<td>SPR 220-7.0</td>
			<td>CAUTION, chemical</td>
		</tr>
		<tr>
			<td>Stirring hotplate</td>
			<td>Thermo scientific Cimarec+</td>
			<td>SP88857100</td>
			<td>General purpose hotplate</td>
		</tr>
		<tr>
			<td>SU-8 2015 negative photoresist</td>
			<td>Microchem</td>
			<td>SU-8 2015</td>
			<td>CAUTION, chemical</td>
		</tr>
		<tr>
			<td>SYLGARD 184 Silicone elastomere kit</td>
			<td>DOW</td>
			<td>4019862</td>
			<td>CAUTION, chemical</td>
		</tr>
		<tr>
			<td>T-LSR150B</td>
			<td>Zaber Technologies</td>
			<td>T-LSR150B-KT04U</td>
			<td>Motorized linear stage</td>
		</tr>
		<tr>
			<td>Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOCS), 97%</td>
			<td>Sigma-Aldrich</td>
			<td>78560-45-9</td>
			<td>CAUTION, hazardous</td>
		</tr>
	</tbody>
</table>

metal-assisted electrochemical nanoimprinting of porous and solid silicon wafers

Metal-assisted electrochemical imprinting (Mac-Imprint) is a combination of metal-assisted chemical etching (MACE) and nanoimprint lithography that is capable of direct patterning 3D micro- and nanoscale features in monocrystalline group IV (e.g., Si) and III-V (e.g., GaAs) semiconductors without the need of sacrificial templates and lithographical steps. During this process, a reusable stamp coated with a noble metal catalyst is brought in contact with a Si wafer in the presence of a hydrofluoric acid (HF) and hydrogen peroxide (H2O2) mixture, which leads to the selective etching of Si at the metal-semiconductor contact interface. In this protocol, we discuss the stamp and substrate preparation methods applied in two Mac-Imprint configurations: (1) Porous Si Mac-Imprint with a solid catalyst; and (2) Solid Si Mac-Imprint with a porous catalyst. This process is high throughput and is capable of centimeter-scale parallel patterning with sub-20 nm resolution. It also provides low defect density and large area patterning in a single operation and bypasses the need for dry etching such as deep reactive ion etching (DRIE).

Scanning electron microscope (SEM) images, optical microscope scans (Figure 9), and atomic force microscopy (AFM) scans (Figure 10) were obtained in order to study the morphological properties of the Mac-Imprint stamps and imprinted Si surfaces. The cross-sectional profile of the imprinted solid Si was compared to that of the used porous Au stamp (Figure 10). Pattern transfer fidelity and porous Si g...

Watch this Scientific Journal Video about Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers at JoVE.com

Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers

A protocol for metal-assisted chemical imprinting of 3D microscale features with sub-20 nm shape accuracy into solid and porous silicon wafers is presented.

Metal-assisted electrochemical imprinting (Mac-Imprint) is a combination of metal-assisted chemical etching (MACE) and nanoimprint lithography that is ...

Our protocol provides a new patterning method for silicon that enables for the creation of three-dimensional hierarchical microstructures that enable the design of metasurface-based microoptic elements and waveguide technologies. This protocol allows for the replication of 3D structures from polymeric and hard molds into monocrystalline and porous silicon wafers in a single step. And at the same time, providing sub 100 nanometer resolutions in all three directions.Porous silicon and silicon itself are great materials for fabricating optical biosensors and infrared optical devices. However, we expect this technique to expand to the group III-V semiconductors and even beyond. Stamp to substrate tip and tilt alignment is extremely important for a uniform Mac-imprint.Following this protocol, the alignment can be performed by mounting stamp to the holder while it is in contact with the substrate. To prepare a stamp for the Mac-imprint, firstly clean the silicon master mold using RCA-1 solution according to the steps described in the protocol and then place a clean silicon master mold into a plastic Petri dish inside of a desiccator. Use a plastic pipette to add a few drops of PFOCS to a plastic weigh boat and place the weigh boat next to the dish with the master mold.Turn on the vacuum pump, open the desiccator valve, and apply vacuum for 30 minutes. While the vacuum is being applied, use a glass spatula to mix the base and curing agent from a silicon elastomer kit at a 10-to-one ratio for 10 to 15 minutes. At the end of the desiccation, remove the weigh boat from the desiccator and spacers from underneath silicon master mold, then carefully cover the master mold with a two to three millimeter layer of the freshly prepared PDMS.To degas the PDMS after opening the desiccator valve, apply vacuum for an additional 20 minutes or until the bubbles disappear. At the end of the second desiccation period, transfer the dish onto an 80 degree Celsius hot plate. After two hours, use a scalpel to trim the edges of the cured PDMS inside the plastic Petri dish and use tweezers to carefully remove the PDMS mold from silicon master mold.For photoresist UV nanoimprinting, use a scriber to cleave a 2.5 by 2.5 centimeter silicon chip out of the silicon wafer, cleaning it using RCA-1 solution according to the steps described in the protocol and then place the clean chip onto the vacuum chuck in a spin coater. Set the spin coating parameters as indicated to apply a 20 micrometer thick layer of photoresist onto the chip and press VAC ON to apply vacuum to the system. Pour 1.5 milliliters of SU-8 2015 photoresist onto the center of the chip and close the spin coater lid.Then press Start to start the spin. At the end of the spin, press VAC OFF to turn off the vacuum and use tweezers to remove the photoresist-coated chip. Carefully place the PDMS mold onto the photoresist-coated silicon chip pattern side down and manually press the mold into the chip.Place a UV transparent glass plate on top of the PDMS to apply 15 grams per square centimeter of pressure to the mold and chip and expose the setup to six watts of UV light for two hours. At the end of the irradiation period, use tweezers to slowly remove the mold from the chip in the direction parallel to the direction of the cured photoresist pattern. To deposit a 250 nanometer thick gold/silver alloy layer onto the chip, firstly deposit 20 nanometer thick layer of chromium and 50 nanometer thick layer of gold as it is described in the text protocol.Next, click GUN 1 OPEN to open the gold and silver gun's shutter. Set the time to process to 16.5 minutes and set the DC set point to 58 and RF set point to 150. Click Rotation and set the argon flow rate to 50 standard cubic centimeters per minute.Click Argon, DC Supply and RF Supply. When the signal plateaus, set the argon control to five. Click Start in zero thickness to start the crystal thickness monitor and to tear the thickness respectively.Click Timed Process to start the time-controlled process and click PLATEN SHUTTER Solid to open the platen shutter. Click zero thickness again. When the sputtering ends, click Solid to close the platen solid shutter and press Press to Vent to vent the magnetron sputter chamber.To de-alloy the silver/gold alloy coated Mac-imprint stamp, first mix deionized water and nitric acid at a one-to-one ratio in a glass beaker and place the beaker onto a stirring hot plate. Submerge a perforated PTFE sample holder in the mixture and heat the solution up to 65 degrees Celsius with constant stirring at 100 revolutions per minute. When the solution has reached the target temperature, place the patterned gold/silver alloy-coated Mac-imprint stamp into the holder for two to 20 minutes.After de-alloying, quench the Mac-imprint stamp in room temperature deionized water for one minute. To perform stamp to substrate alignment, place stamp facing down on top of silicon chip in electrochemical cell, add a droplet of SU-8 on the back of the stamp. Bring PTFE rod in contact with SU-8 and cure it in place in dry conditions under UV light for two hours.The contact is about 86 millimeters from home position. To perform a Mac-imprinting operation, clean patterned silicon chip using RCA solution according to the steps described in the protocol and then place it into the center of an electrochemical cell and position the cell under a PTFE rod with the Mac-imprint stamp. Mix the etching solution of hydrofluoric acid and hydrogen peroxide at a 17-to-one ratio inside a PTFE beaker.After five minutes, use a plastic pipette to add the etching solution to the electrochemical cell. To position the Mac-imprint stamp in contact with the pattern chip, bring the stamp down by about 86 millimeters and then move the stamp down from the contact position by about 300 to 1, 000 micrometers to achieve the desired contact force. Maintain the Mac-imprint stamp in contact with the chip from one to 30 minutes before clicking Home to return the rod to the home position.Use a pipette to carefully aspirate the etching solution from the cell and rinse the imprinted silicon chip with isopropyl alcohol and deionized water. Then dry the chip with clean dry air. Scanning electron microscope images can be obtained to study the morphological properties of the gold-coated Mac-imprint stamps and the resulting imprinted silicon surfaces.In this representative analysis, the cross-sectional profile of the imprinted solid silicon obtained by atomic force scan was compared to that of the used porous gold stamp. Pattern transfer fidelity and porous silicon generation during Mac-imprint were two major criteria to analyze the experimental success. The Mac-imprint was considered successful if the stamp pattern was accurately transferred onto the silicon and no porous silicon was generated during the Mac-imprint.Here, the results of a suboptimal experiment in which a lack of pattern transfer fidelity and a porous silicon generation occurred during the Mac-imprint can be observed. It is important to remember that Mac-imprint of silicon involves hydrofluoric acid, which is life-threatening substance and following a safety protocol and wearing appropriate personal protective equipment are paramount. Once you utilize stamps coated in porous gold to facilitate mass transport.Following this procedure, the composition of the etching solution or the contact force could be varied to study the kinetics of the process or the stamp durability.

metal-assisted-electrochemical-nanoimprinting-porous-solid-silicon

Research

JoVE Journal

Engineering

Nanomoulding of Functional Materials, a Versatile Complementary Pattern Replication Method to Nanoimprinting

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding combined with layer transfer enables the replication of arbitrary surface patterns from a master structure onto the functional material. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes. In particular we demonstrate the fabrication of patterned transparent zinc oxide electrodes for light trapping applications in solar cells.

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes.

We describe a nanomoulding technique which  allows low-cost nanoscale patterning of functional materials, materials stacks  and full devices. Nanomoulding ...

Fabrication of VB2/Air Cells for Electrochemical Testing

A technique to investigate the properties and performance of new multi-electron metal/air battery systems is proposed and presented. A method for synthesizing nanoscopic VB2 is presented as well as step-by-step procedure for applying a zirconium oxide coating to the VB2 particles for stabilization upon discharge. The process for disassembling existing zinc/air cells is shown, in addition construction of the new working electrode to replace the conventional zinc/air cell anode with a the nanoscopic VB2 anode. Finally, discharge of the completed VB2/air battery is reported. We show that using the zinc/air cell as a test bed is useful to provide a consistent configuration to study the performance of the high-energy high capacity nanoscopic VB2 anode.

Fabrication of VB2/Air Cells for Electrochemical Testing

A protocol is presented to study multi-electron metal/air battery systems by using previous technology developed for the zinc/air cell. Electrochemical testing is then performed on fabricated batteries to evaluate performance.

A technique to investigate the properties and performance of new multi-electron metal/air battery systems is proposed and presented. A method for ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned and bonded silicon wafers. Unlike confinements in porous materials often used for these types of experiments3, bonded wafers provide predesigned uniform spaces for confinement. The geometry of each cell is well known, which removes a large source of ambiguity in the interpretation of data.
Exceptionally flat, 5 cm diameter, 375 &#181;m thick Si wafers with about 1 &#181;m variation over the entire wafer can be obtained commercially (from Semiconductor Processing Company, for example). Thermal oxide is grown on the wafers to define the confinement dimension in the z-direction. A pattern is then etched in the oxide using lithographic techniques so as to create a desired enclosure upon bonding. A hole is drilled in one of the wafers (the top) to allow for the introduction of the liquid to be measured. The wafers are cleaned2 in RCA solutions and then put in a microclean chamber where they are rinsed with deionized water4. The wafers are bonded at RT and then annealed at ~1,100 &#176;C. This forms a strong and permanent bond. This process can be used to make uniform enclosures for measuring thermal and hydrodynamic properties of confined liquids from the nanometer to the micrometer scale.

A method for permanently bonding two silicon wafers so as to realize a uniform enclosure is described. This includes wafer preparation, cleaning, RT bonding, and annealing processes. The resulting bonded wafers (cells) have uniformity of enclosure ~1%1,2. The resulting geometry allows for measurements of confined liquids and gasses.

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned ...

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic electrolyte, and an O2-breathing cathode. The aprotic electrolyte is a solution of lithium salt with aprotic solvent; and porous carbon is commonly used as the cathode substrate. To improve the performance, an electrocatalyst is deposited onto the porous carbon substrate by certain deposition methods, such as atomic layer deposition (ALD) and wet-chemistry reaction. The as-prepared cathode materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray absorption near edge structure (XANES). A Swagelok-type cell, sealed in a glass chamber filled with pure O2, is used for the electrochemical test on a battery test system. The cells are tested under either capacity-controlled mode or voltage controlled mode. The reaction products are investigated by electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and Raman spectroscopy to study the possible pathway of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). This protocol demonstrates a systematic and efficient arrangement of routine tests of the aprotic Li-O2 battery, including the electrochemical test and characterization of battery materials.

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

A protocol for the electrochemical testing of an aprotic Li-O2 battery with the preparation of electrodes and electrolytes and an introduction of the frequently used methods of characterization is presented here.

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic ...

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Electrochemical Etching and Characterization of Sharp Field Emission Points for Electron Impact Ionization

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. The results of studies in which the etching current and the molarity of the NaOH solution used in the etching process were varied are presented. The investigation of the geometry of the tips, by imaging them with a scanning electron microscope, and by operating them in field emission mode is also described. The field emission tips produced are intended to be used as an electron beam source for ion production via electron impact ionization of background gas or vapor in Penning trap mass spectrometry applications.

A method for electrochemically etching field emission tips is presented. Etching parameters are characterized and the operation of the tips in field emission mode is investigated.

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. ...

Fabrication of White Light-emitting Electrochemical Cells with Stable Emission from Exciplexes

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active layer which consists of blue-fluorescent poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and &#960;-conjugated triphenylamine molecules. This white light emission originates from exciplexes formed between PFD and amines in electronically excited states. A device containing PFD, 4,4&#39;,4&#39;&#39;-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), Poly(ethylene oxide) and K2CF3SO3 showed white light emission with Commission internationale de l&#39;&#233;clairage (CIE) coordinates of (0.33, 0.43) and a Color Rendering Index (CRI) of Ra = 73 at an applied voltage of 3.5 V. Constant voltage measurements showed that the CIE coordinates of (0.27, 0.37), Ra of 67, and the emission color observed immediately after application of a voltage of 5 V were nearly unchanged and stable after 300 sec.

The authors present a method for fabricating stable white-light-emitting electrochemical cells utilizing emission from exciplexes formed between a blue-emitting fluorene polymer and aromatic amines.

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active ...

Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser (laser-SARPES), and demonstrate a power of this technique for studying solid state physics. Laser-SARPES achieves two great capabilities. Firstly, by examining orbital selection rule of linearly polarized lasers, orbital selective excitation can be carried out in SAPRES experiment. Secondly, the technique can show full information of a variation of the spin quantum axis as a function of the light polarization. To demonstrate the power of the collaboration of these capabilities in laser-SARPES, we apply this technique for the investigations of spin-orbit coupled surface states of Bi2Se3. This technique affords to decompose spin and orbital components from the spin-orbit coupled wavefunctions. Moreover, as a representative advantage of using the direct spin detection collaborated with the polarization-variable laser, the technique unambiguously visualizes the light polarization dependence of the spin quantum axis in three-dimension. Laser-SARPES dramatically increases a capability of photoemission technique.

Here, we combine polarization-variable 7-eV laser with spin- and angle-resolved photoemission technique to visualize the spin-orbital coupling effect in solid states.

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser ...

A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This FOSP relies on a silicon Fabry-Perot interferometer (FPI) attached to the fiber end, referred to as Si-FOSP in this work. The Si-FOSP generates an interferogram determined by the optical path length (OPL) of the silicon cavity. Measurand alters the OPL and thus shifts the interferogram. Due to the unique optical and thermal properties of the silicon material, this Si-FOSP exhibits an advantageous performance in terms of sensitivity and speed. Furthermore, the mature silicon fabrication industry endows the Si-FOSP with excellent reproducibility and low cost toward practical applications. Depending on the specific applications, either a low-finesse or high-finesse version will be utilized, and two different data demodulation methods will be adopted accordingly. Detailed protocols for fabricating both versions of the Si-FOSP will be provided. Three representative applications and their according results will be shown. The first one is a prototype underwater thermometer for profiling the ocean thermoclines, the second one is a flow meter to measure flow speed in the ocean, and the last one is a bolometer used for monitoring exhaust radiation from magnetically confined high-temperature plasma.

This work reports an innovative silicon-tipped fiber-optic sensing platform (Si-FOSP) for high-resolution and fast-response measurement of a variety of physical parameters, such as temperature, flow, and radiation. Applications of this Si-FOSP span from oceanographic research, mechanical industry, to fusion energy research.

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This ...