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>
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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.

Research

JoVE Journal

Engineering

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

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Fabrication of VB2/Air Cells for Electrochemical Testing

Fabrication of VB2/Air Cells for Electrochemical Testing

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Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

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

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

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

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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 ...