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.

<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>

We do not have anything to disclose.

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 Au (middle)9, and solid Si with porous Au (right)20. The diffusion of the reactants was blocked in the first case, leading to nonlocalized etching and partial porosification of the imprinted Si, which correlates with the same issue in the conventional MACE process22,23. However, when the diffusion was enabled through porous networks (either embedded in Si or Au), high pattern transfer fidelity was observed, which leads to the conclusion that Mac-Imprint is a mass transport dependent process. Also, the imprinted Si surface was roughened after imprinting with porous Au (Figure 9a, right).
It was proposed that surface roughening originates from the porosity of the porous Au used. In order to test the hypothesis, a series of porous Au layers with various controlled pore volume fractions (PVF) was created according to sections 1.4 and 1.5 of the protocol and subsequently implemented for Mac-Imprint (Figure 9b)20. A direct relation between the stamp&#39;s PVF and imprinted Si surface roughness was observed, supporting the hypothesis. Additionally, after Mac-Imprint with low PVF stamps, Si was porosified, which was explained by hindered ES diffusion through undeveloped porous Au structure, resulting in delocalization of the etching front20. Thus, a developed and interconnected porous structure is critical for high pattern transfer fidelity during Mac-Imprint. Moreover, imprinted Si porosification was observed at medium PVF when a porous Au layer already had an interconnected porous network. This can be attributed to the high ratio between Au and Si surface areas and subsequent injection of the excessive holes into Si, which also leads to the etching front delocalization and, as a result, porous Si formation20. This process can be controlled through careful adjustment of the HF and H2O2 ratios in the ES.
Implementation of the porous Au stamps along with ES composition variations allows the manufacture of various 3D hierarchical patterns via Mac-Imprint that were previously published in the works of Azeredo et al.9 and Sharstniou et al.20 (Figure 9c).
Further investigations of porous Au/Si interface chemistry, in particular PVF-dependent etch rate and localization, along with imprinting system improvement, will help to make the Mac-Imprint process suitable for industrial scale applications in the future.

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.remotexs.ntu.edu.sg/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.

<table><tbody><tr><td>Acetone, &gt;99.5%, ACS reagent</td><td>Sigma-Aldrich</td><td>67-64-1</td><td>CAUTION, chemical</td></tr><tr><td>Ammonium fluoride, &gt;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&quot; x 0.125&quot;, 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&quot; x 0.125&quot;, 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 Technology</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, &gt;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&quot; x 0.125&quot;, 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 elastomer 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 generation during Mac-Imprint were two major criteria to analyze experimental success. The Mac-Imprint was considered successful if the Mac-Imprint stamp pattern was accurately transferred onto the Si and no porous Si is generated during the Mac-Imprint. The results of a suboptimal experiment (i.e., lack of pattern transfer fidelity along with porous Si generation during Mac-Imprint) are presented in Figure 9a (left).
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/61040/61040fig9.jpg" /> 
Figure 9: Representative results: (a) Mac-Imprint of solid Si and porous Si with solid Au film (left and middle, respectively) and solid Si with porous Au film (right). (b) Top-down SEM images of porous Au films with different pore volume fraction (top) and corresponding imprinted Si morphology (bottom). (c) SEM images of various patterns produced by Mac-Imprint. This figure is reprinted with permission9,20. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61040/61040fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 10" class="xfigimg" src="/files/ftp_upload/61040/61040fig10.jpg" /> 
Figure 10: Representative results of solid Si Mac-Imprint with porous Au stamp: (a) AFM scans of porous Au stamp (left) and imprinted solid Si (right) and (b) overlaid cross-sectional profiles of porous Au stamp (blue) and imprinted solid Si (red). This figure is reprinted with permission20. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61040/61040fig10large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 1" src="/files/ftp_upload/61040/61040suppfig1.jpg" /> 
Supplemental Figure 1: Photograph of spin coater control display. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61040/61040suppfig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 2" src="/files/ftp_upload/61040/61040suppfig2.jpg" /> 
Supplemental Figure 2: Magnetron sputter control software screenshots. (a) Evacuation of magnetron sputter chamber. (b) Sputtering control parameters. (c) Ventilation of magnetron sputter chamber. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61040/61040suppfig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 3" src="/files/ftp_upload/61040/61040suppfig3.jpg" /> 
Supplemental Figure 3: Potentiostat control software screenshot. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61040/61040suppfig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 4" src="/files/ftp_upload/61040/61040suppfig4.jpg" /> 
Supplemental Figure 4: Linear motorized stage and load cell control software screenshots. (a) Before Mac-Imprint and (b) during Mac-Imprint. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61040/61040suppfig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 5" src="/files/ftp_upload/61040/61040suppfig5.jpg" /> 
Supplemental Figure 5: Photograph of Mac-Imprint stamp to PTFE rod attachment process. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61040/61040suppfig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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 ...

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

Nanyang Technological University

Research

JoVE Journal

Engineering

4.0K Views.  Arizona State University. 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.

Video: Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers

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 ...

Soft Lithographic Functionalization and Patterning Oxide-free Silicon and Germanium

The development of hybrid electronic devices relies in large part on the integration of (bio)organic materials and inorganic semiconductors through a stable interface that permits efficient electron transport and protects underlying substrates from oxidative degradation. Group IV semiconductors can be effectively protected with highly-ordered self-assembled monolayers (SAMs) composed of simple alkyl chains that act as impervious barriers to both organic and aqueous solutions. Simple alkyl SAMs, however, are inert and not amenable to traditional patterning techniques. The motivation for immobilizing organic molecular systems on semiconductors is to impart new functionality to the surface that can provide optical, electronic, and mechanical function, as well as chemical and biological activity. 
Microcontact printing (&mu;CP) is a soft-lithographic technique for patterning SAMs on myriad surfaces.1-9 Despite its simplicity and versatility, the approach has been largely limited to noble metal surfaces and has not been well developed for pattern transfer to technologically important substrates such as oxide-free silicon and germanium. Furthermore, because this technique relies on the ink diffusion to transfer pattern from the elastomer to substrate, the resolution of such traditional printing is essentially limited to near 1 &mu;m.10-16
In contrast to traditional printing, inkless &mu;CP patterning relies on a specific reaction between a surface-immobilized substrate and a stamp-bound catalyst. Because the technique does not rely on diffusive SAM formation, it significantly expands the diversity of patternable surfaces. In addition, the inkless technique obviates the feature size limitations imposed by molecular diffusion, facilitating replication of very small (&lt;200 nm) features.17-23 However, up till now, inkless &mu;CP has been mainly used for patterning relatively disordered molecular systems, which do not protect underlying surfaces from degradation.
Here, we report a simple, reliable high-throughput method for patterning passivated silicon and germanium with reactive organic monolayers and demonstrate selective functionalization of the patterned substrates with both small molecules and proteins. The technique utilizes a preformed NHS-reactive bilayered system on oxide-free silicon and germanium. The NHS moiety is hydrolyzed in a pattern-specific manner with a sulfonic acid-modified acrylate stamp to produce chemically distinct patterns of NHS-activated and free carboxylic acids. A significant limitation to the resolution of many &mu;CP techniques is the use of PDMS material which lacks the mechanical rigidity necessary for high fidelity transfer. To alleviate this limitation we utilized a polyurethane acrylate polymer, a relatively rigid material that can be easily functionalized with different organic moieties. Our patterning approach completely protects both silicon and germanium from chemical oxidation, provides precise control over the shape and size of the patterned features, and gives ready access to chemically discriminated patterns that can be further functionalized with both organic and biological molecules. The approach is general and applicable to other technologically-relevant surfaces.

Here we describe a simple method for patterning oxide-free silicon and germanium with reactive organic monolayers and demonstrate functionalization of the patterned substrates with small molecules and proteins. The approach completely protects surfaces from chemical oxidation, provides precise control over feature morphology, and provides ready access to chemically discriminated patterns.

The development of hybrid electronic devices relies in large part on the integration of (bio)organic materials and inorganic semiconductors through a ...

Bioengineering

Micro-masonry for 3D Additive Micromanufacturing

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to a different substrate by utilizing elastomeric stamps. Transfer printing enables the integration of heterogeneous materials to fabricate unexampled structures or functional systems that are found in recent advanced devices such as flexible and stretchable solar cells and LED arrays. While transfer printing exhibits unique features in material assembly capability, the use of adhesive layers or the surface modification such as deposition of self-assembled monolayer (SAM) on substrates for enhancing printing processes hinders its wide adaptation in microassembly of microelectromechanical system (MEMS) structures and devices. To overcome this shortcoming, we developed an advanced mode of transfer printing which deterministically assembles individual microscale objects solely through controlling surface contact area without any surface alteration. The absence of an adhesive layer or other modification and the subsequent material bonding processes ensure not only mechanical bonding, but also thermal and electrical connection between assembled materials, which further opens various applications in adaptation in building unusual MEMS devices.

This paper introduces a 3D additive micromanufacturing strategy (termed &#8216;micro-masonry&#8217;) for the flexible fabrication of microelectromechanical system (MEMS) structures and devices. This approach involves transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Capillary Force Lithography for Cardiac Tissue Engineering

Cardiovascular disease remains the leading cause of death worldwide1. Cardiac tissue engineering holds much promise to deliver groundbreaking medical discoveries with the aims of developing functional tissues for cardiac regeneration as well as in vitro screening assays. However, the ability to create high-fidelity models of heart tissue has proven difficult. The heart&#8217;s extracellular matrix (ECM) is a complex structure consisting of both biochemical and biomechanical signals ranging from the micro- to the nanometer scale2. Local mechanical loading conditions and cell-ECM interactions have recently been recognized as vital components in cardiac tissue engineering3-5.
A large portion of the cardiac ECM is composed of aligned collagen fibers with nano-scale diameters that significantly influences tissue architecture and electromechanical coupling2. Unfortunately, few methods have been able to mimic the organization of ECM fibers down to the nanometer scale. Recent advancements in nanofabrication techniques, however, have enabled the design and fabrication of scalable scaffolds that mimic the in vivo structural and substrate stiffness cues of the ECM in the heart6-9.
Here we present the development of two reproducible, cost-effective, and scalable nanopatterning processes for the functional alignment of cardiac cells using the biocompatible polymer poly(lactide-co-glycolide) (PLGA)8 and a polyurethane (PU) based polymer. These anisotropically nanofabricated substrata (ANFS) mimic the underlying ECM of well-organized, aligned tissues and can be used to investigate the role of nanotopography on cell morphology and function10-14.
Using a nanopatterned (NP) silicon master as a template, a polyurethane acrylate (PUA) mold is fabricated. This PUA mold is then used to pattern the PU or PLGA hydrogel via UV-assisted or solvent-mediated capillary force lithography (CFL), respectively15,16. Briefly, PU or PLGA pre-polymer is drop dispensed onto a glass coverslip and the PUA mold is placed on top. For UV-assisted CFL, the PU is then exposed to UV radiation (&#955; = 250-400 nm) for curing. For solvent-mediated CFL, the PLGA is embossed using heat (120 &#176;C) and pressure (100 kPa). After curing, the PUA mold is peeled off, leaving behind an ANFS for cell culture. Primary cells, such as neonatal rat ventricular myocytes, as well as human pluripotent stem cell-derived cardiomyocytes, can be maintained on the ANFS2.

In this protocol, we demonstrate the fabrication of biomimetic cardiac cell culture substrata made from two distinct polymeric materials using capillary force lithography. The described methods provide a scalable, cost-effective technique to engineer the structure and function of macroscopic cardiac tissues for in vitro and in vivo applications.

Cardiovascular disease remains the leading cause of death worldwide1. Cardiac tissue engineering holds much promise to deliver groundbreaking medical ...

Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

Uniformly sized nanoparticles can remove fluctuations in contact hole dimensions patterned in poly(methyl methacrylate) (PMMA) photoresist films by electron beam (E-beam) lithography. The process involves electrostatic funneling to center and deposit nanoparticles in contact holes, followed by photoresist reflow and plasma- and wet-etching steps.

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

Utilization of Microscale Silicon Cantilevers to Assess Cellular Contractile Function In Vitro

The development of more predictive and biologically relevant in vitro assays is predicated on the advancement of versatile cell culture systems which facilitate the functional assessment of the seeded cells. To that end, microscale cantilever technology offers a platform with which to measure the contractile functionality of a range of cell types, including skeletal, cardiac, and smooth muscle cells, through assessment of contraction induced substrate bending. Application of multiplexed cantilever arrays provides the means to develop moderate to high-throughput protocols for assessing drug efficacy and toxicity, disease phenotype and progression, as well as neuromuscular and other cell-cell interactions. This manuscript provides the details for fabricating reliable cantilever arrays for this purpose, and the methods required to successfully culture cells on these surfaces. Further description is provided on the steps necessary to perform functional analysis of contractile cell types maintained on such arrays using a novel laser and photo-detector system. The representative data provided highlights the precision and reproducible nature of the analysis of contractile function possible using this system, as well as the wide range of studies to which such technology can be applied. Successful widespread adoption of this system could provide investigators with the means to perform rapid, low cost functional studies in vitro, leading to more accurate predictions of tissue performance, disease development and response to novel therapeutic treatment.

Utilization of Microscale Silicon Cantilevers to Assess Cellular Contractile Function In Vitro

This protocol describes the use of microscale silicon cantilevers as pliable culture surfaces for measuring the contractility of muscle cells in vitro. Cellular contraction causes cantilever bending, which can be measured, recorded, and converted into readouts of force, providing a non-invasive and scalable system for measuring contractile function in vitro.

The development of more predictive and biologically relevant in vitro assays is predicated on the advancement of versatile cell culture systems which ...

Synthesis, Functionalization, and Characterization of Fusogenic Porous Silicon Nanoparticles for Oligonucleotide Delivery

With the advent of gene therapy, the development of an effective in vivo nucleotide-payload delivery system has become of parallel import. Fusogenic porous silicon nanoparticles (F-pSiNPs) have recently demonstrated high in vivo gene silencing efficacy due to its high oligonucleotide loading capacity and unique cellular uptake pathway that avoids endocytosis. The synthesis of F-pSiNPs is a multi-step process that includes: (1) loading and sealing of oligonucleotide payloads in the silicon pores; (2) simultaneous coating and sizing of fusogenic lipids around the porous silicon cores; and (3) conjugation of targeting peptides and washing to remove excess oligonucleotide, silicon debris, and peptide. The particle&#8217;s size uniformity is characterized by dynamic light scattering, and its core-shell structure may be verified by transmission electron microscopy. The fusogenic uptake is validated by loading a lipophilic dye, 1,1&#39;-dioctadecyl-3,3,3&#39;,3&#39;-tetramethylindocarbocyanine perchlorate (DiI), into the fusogenic lipid bilayer and treating it to cells in vitro to observe for plasma membrane staining versus endocytic localizations. The targeting and in vivo gene silencing efficacies were previously quantified in a mouse model of Staphylococcus aureus pneumonia, in which the targeting peptide is expected to help the F-pSiNPs to home to the site of infection. Beyond its application in S. aureus infection, the F-pSiNP system may be used to deliver any oligonucleotide for gene therapy of a wide range of diseases, including viral infections, cancer, and autoimmune diseases.

We demonstrate the synthesis of fusogenic porous silicon nanoparticles for effective in vitro and in vivo oligonucleotide delivery. Porous silicon nanoparticles are loaded with siRNA to form the core, which is coated by fusogenic lipids through extrusion to form the shell. Targeting moiety functionalization and particle characterization are included.

With the advent of gene therapy, the development of an effective in vivo nucleotide-payload delivery system has become of parallel import. Fusogenic ...

A Versatile Method of Patterning Proteins and Cells

Substrate and cell patterning techniques are widely used in cell biology to study cell-to-cell and cell-to-substrate interactions. Conventional patterning techniques work well only with simple shapes, small areas and selected bio-materials. This article describes a method to distribute cell suspensions as well as substrate solutions into complex, long, closed (dead-end) polydimethylsiloxane (PDMS) microchannels using negative pressure. This method enables researchers to pattern multiple substrates including fibronectin, collagen, antibodies (Sal-1), poly-D-lysine (PDL), and laminin. Patterning of substrates allows one to indirectly pattern a variety of cells. We have tested C2C12 myoblasts, the PC12 neuronal cell line, embryonic rat cortical neurons, and amphibian retinal neurons. In addition, we demonstrate that this technique can directly pattern fibroblasts in microfluidic channels via brief application of a low vacuum on cell suspensions. The low vacuum does not significantly decrease cell viability as shown by cell viability assays. Modifications are discussed for application of the method to different cell and substrate types. This technique allows researchers to pattern cells and proteins in specific patterns without the need for exotic materials or equipment and can be done in any laboratory with a vacuum.

This report describes a simple, easy to perform technique, using low pressure vacuum, to fill microfluidic channels with cells and substrates for biological research.

Substrate and cell patterning techniques are widely used in cell biology to study cell-to-cell and cell-to-substrate interactions. Conventional patterning ...

Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers

Podsumowanie

Przeglądaj więcej filmów

Rozdziały w tym wideo

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

Soft Lithographic Functionalization and Patterning Oxide-free Silicon and Germanium

Micro-masonry for 3D Additive Micromanufacturing

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

Capillary Force Lithography for Cardiac Tissue Engineering

Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Utilization of Microscale Silicon Cantilevers to Assess Cellular Contractile Function In Vitro

Synthesis, Functionalization, and Characterization of Fusogenic Porous Silicon Nanoparticles for Oligonucleotide Delivery

A Versatile Method of Patterning Proteins and Cells