Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers

Aliaksandr Sharstniou; Stanislau Niauzorau; Ashlesha Junghare; Bruno  P. Azeredo

doi:10.3791/61040

JoVE Journal > Engineering

Ingegneria

Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers

Published: February 08, 2022

doi:

10.3791/61040

Aliaksandr Sharstniou¹, Stanislau Niauzorau¹, Ashlesha Junghare¹, Bruno P. Azeredo¹

¹The Polytechnic School,Arizona State University

Summary

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.

Abstract

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).

Introduction

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'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.

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). Please click here to view a larger version of this figure.

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.

Protocol

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

PDMS mold fabrication
1. Prepare the RCA-1 solution by mixing deionized pure (DI) water and ammonium hydroxide in the glass beaker in a 5:1 ratio (volume). Place the beaker with the mixture onto a stirring hotplate (see Table of Materials) and heat the mixture up to 70 °C. Measure the temperature of the mixture with a calibrated thermocouple and add 1 part of the hydrogen peroxide to the preheated mixture to obtain the RCA-1 solution. Wait until the RCA-1 solution starts to bubble vigorously (Figure 2).
2. Keep the RCA-1 solution at 70 °C.
3. Soak the Si master mold into the RCA-1 solution for 15 min.
4. Take the Si master mold out of the RCA-1 solution and thoroughly rinse with DI water.
5. Make the Si master mold hydrophobic. Put the Si master mold into a plastic Petri dish and place it inside a desiccator (see Table of Materials). Using a plastic pipette, add a few droplets of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOCS) onto a plastic weighing boat and place it inside the desiccator next to the plastic Petri dish with the Si master mold.
  NOTE: Place spacers underneath the Si master mold to elevate it from the bottom of the Petri dish. This will allow PFOCS to uniformly cover Si master mold and prevent PDMS sticking.
6. Close the desiccator lid. Connect the output of the desiccator to the vacuum pump (see Table of Materials) through a PVC tube. Start the vacuum pump. Set the pressure level to 30 kPa using the vacuum pump valve.
7. Open the desiccator valve and apply vacuum for 30 min.
8. While vacuum is applied to the desiccator, mix the base and curing agent provided in the silicone elastomer kit (PDMS) (see Table of Materials) in a 10:1 ratio (mass). Slowly stir the mixture with a glass spatula for 10-15 min.
9. Turn off the vacuum pump. Open the desiccator and remove the plastic weighing boat with PFOCS.
  NOTE: Remove spacers from underneath Si master mold.
10. Carefully pour PDMS over the Si master mold to completely cover it with the 2-3 mm layer of PDMS (Figure 3a).
11. Repeat step 1.1.6.
12. Degas the PDMS. Open the desiccator valve and apply vacuum for 20 min or until bubbles disappear.
13. Turn off the vacuum pump. Open the desiccator. Take out the plastic Petri dish with the PDMS-covered Si master mold and place it onto a hotplate (see Table of Materials) preheated to 80 °C (Figure 3b).
14. Cure the PDMS with Si master mold on the hotplate at 80 °C for 120 min (Figure 3b).
15. Remove the plastic Petri dish with cured PDMS from the hotplate. Using a scalpel, trim the edges of the cured PDMS inside the plastic Petri dish. Carefully take the cured PDMS with Si master mold out of the plastic Petri dish using tweezers.
16. Carefully remove all the PDMS that leaked underneath the Si master mold using a scalpel. Peel off the cured PDMS from the Si master mold using tweezers. Peel it off slowly, parallel to the direction of the Si master stamp pattern.
17. Crop the 2 x 2 cm PDMS mold with the pattern in the center using a scalpel. Store the PDMS mold in the plastic Petri dish with the pattern facing up.

Figure 2: RCA-1 cleaning process. (a) Solution heating and (b) Si cleaning. Please click here to view a larger version of this figure.

Figure 3: PDMS mold fabrication process. (a) Schematic representation of the process. (b) Photographs of the process steps. Please click here to view a larger version of this figure.

Photoresist UV nanoimprinting
1. Cleave a 2.5 x 2.5 cm Si chip out of the Si wafer using a scriber.
2. Repeat steps 1.1.1-1.1.4 to clean the Si chip.
3. Bring SU-8 2015 photoresist out of the refrigerator and let it stay at room temperature (RT) for 10-15 min prior to spin coating.
4. Open the spin coater lid (see Table of Materials). Place the Si chip inside the spin coater onto the vacuum chuck (Figure 4a).
5. Connect the output of the spin coater to the vacuum pump through the PVC tube. Start the vacuum pump. Set the pressure level to 30 kPa using the vacuum pump valve.
6. Select a spin coating procedure with the following parameters: spread at 500 rpm for 10 s with acceleration 100 rpm/s, spin at 2,000 rpm for 30 s with acceleration 300 rpm/s.
  NOTE: Step 1.2.6 will produce a 20 µm thick SU-8 2015 layer.
7. Apply vacuum to the vacuum chuck by pressing "VAC ON" on the spin coater display. Refer to Supplemental File (Figure S1).
8. Pour 1.5 mL of SU-8 2015 photoresist onto the center of the Si chip.
9. Close the spin coater lid. Start spin coating by pressing "START". Refer to Supplemental File (Figure S1).
10. Open the spin coater lid. Turn off the vacuum by pressing "VAC OFF". Refer to Supplemental File (Figure S1). Take out the Si chip with the spin-coated layer of SU-8 2015 photoresist using tweezers (Figure 4a).
11. Carefully place the PDMS mold on the photoresist-coated Si chip with the pattern facing down. Manually flatten the PDMS mold. Put a UV transparent glass plate on the backside of the PDMS resulting in 15 g/cm² weight applied to the PDMS mold (Figure 4b).
12. Perform constant UV exposure for 2 h using a 6 W UV bulb (see Table of Materials) placed 10 cm away from the Si wafer surface.
13. Peel off the PDMS mold from the Si chip using tweezers. Peel off slowly in the direction parallel to the direction of the cured SU-8 2015 pattern.

Figure 4: Photoresist UV nanoimprinting process. (a) Photographs of photoresist spin coating. (b) Schematics and photographs of UV nanoimprinting. Please click here to view a larger version of this figure.

Gold catalyst thin film deposition by magnetron sputtering
1. Attach the Si chips with a patterned SU-8 2015 photoresist layer onto a 4 inch Si wafer using double-sided polyimide tape.
2. Open the chamber of the magnetron sputter (see Table of Materials). Place the 4 inch Si wafer with attached Si chips onto a rotational plate. Close the plate solid shutter by pressing the "Solid" button in the control software. Refer to Supplemental File (Figure S2b).
  NOTE: The "Solid" button will turn green when the shutter is closed.
3. Place Cr and Au targets (see Table of Materials) onto the magnetron guns connected to the DC power supply. Place an Ag target (see Table of Materials) onto the magnetron gun connected to the RF power supply. Set the distance between targets and rotational plate to 8.5 inches.
4. Close the chamber of the magnetron sputter and start evacuating the chamber by pressing "Pump Down" and "Turbo Enable" in the control software. Leave it overnight. Refer to Supplemental File (Figure S2a).
5. Turn on the DC and RF power supplies. Open the Cr gun shutter by pressing "Gun 1 Open" in the control software. Set the DC power supply to 100 W in the control software. Refer to Supplemental File (Figure S2b).
6. Set the "Thickness Controlled Process" to 200 Å. Enable the rotation of the rotational plate by pressing the "Cont" and "Rotation" buttons in the control software. Refer to Supplemental File (Figure S2b).
7. Set the deposition pressure to 3 mTorr. Refer to Supplemental File (Figure S2b).
8. Set the Ar flow rate to 50 sccm in the control software. Enable the DC power supply by pressing "DC supply" in the control software. Change the Ar flow rate to 5 sccm. Refer to Supplemental File (Figure S2b).
9. Start the crystal thickness monitor and tare the thickness by pressing the "START" and "ZERO THICKNESS" buttons respectively in the control software. Refer to Supplemental File (Figure S2b).
10. Start the thickness-controlled process by pressing "Thickness Controlled Process". Open the plate solid shutter by pressing "Solid". Tare the thickness monitor one more time by pressing "ZERO THICKNESS". Refer to Supplemental File (Figure S2b).
11. After the sputtering ends, close the plate solid shutter by pressing "Solid". Stop the thickness monitor by pressing "STOP". Refer to Supplemental File (Figure S2b).
12. Open the Au gun shutter by pressing "Gun 2 Open". Set the DC power supply to 35 W. Refer to Supplemental File (Figure S2b).
13. Set the "Thickness Controlled Process" to 800 Å. Enable the rotation of the rotational plate by pressing the "Cont" and "Rotation" buttons. Refer to Supplemental File (Figure S2b).
14. Repeat steps 1.3.7-1.3.11.
15. Vent the magnetron sputter chamber by pressing "Press to Vent" in the control software. Refer to Supplemental File (Figure S2c). The resulting structure is a solid Au Mac-Imprint stamp (Figure 5).
  NOTE: Perform steps 1.4 and 1.5 only if stamps with porous catalytic films are required.

Figure 5: Catalytic stamp preparation process. (a) Schematics of the thin film deposition. (b) Photographs of the magnetron sputtering system. (c) Photograph of dealloying process with representative porous gold SEM images. Please click here to view a larger version of this figure.

Silver/Gold catalyst thin film deposition by magnetron sputtering
1. Repeat steps 1.3.1-1.3.14. In step 1.3.13 set the thickness-controlled process to 500 Å instead of 800 Å.
2. Open the Au and Ag guns shutter by pressing "Gun 3 Open". Set the DC and RF power supplies to 58 W and 150 W respectively. Refer to Supplemental File (Figure S2b).
  NOTE: Step 1.4.2 will provide an Ag/Au alloy with composition 60/40 (volume)
3. Set the "Timed Process" to 16.5 min in the control software. Enable the rotation of the rotational plate by pressing the "Cont" and "Rotation" buttons. Refer to Supplemental File (Figure S2b).
  NOTE: Steps 1.4.3-1.4.8 of the protocol will produce a 250 nm thick Ag/Au alloy layer.
4. Set the air flow rate to 50 sccm. Enable the DC and RF power supplies by pressing "DC Supply" and "RF Supply" respectively. Change the air flow rate to 5 sccm. Refer to Supplemental File (Figure S2b).
5. Start the crystal thickness monitor and tare the thickness by pressing "START" and "ZERO THICKNESS" respectively. Refer to Supplemental File (Figure S2b).
6. Start the time-controlled process by pressing "Timed Process". Open the plate solid shutter by pressing "Solid". Tare the thickness monitor one more time by pressing "ZERO THICKNESS". Refer to Supplemental File (Figure S2b).
7. After the sputtering ends, close the plate solid shutter by pressing "Solid". Stop the thickness monitor by pressing "STOP". Refer to Supplemental File (Figure S2b).
8. Repeat step 1.3.15.
  NOTE: The resulting structure is an Ag/Au alloy sputtered Mac-Imprint stamp.
Silver/Gold catalyst thin film dealloying
1. Mix DI water and nitric acid in the glass beaker in 1:1 ratio (volume). Let it cool down to 30 °C.
2. Place the beaker with the mixture onto a stirring hotplate and submerge the perforated polytetrafluoroethylene (PTFE) sample holder into the mixture. Heat the mixture up to 65 °C with constant stirring at 100 rpm. Constantly measure the temperature of the mixture with a calibrated thermocouple.
3. Place the Si chips with the patterned SU-8 2015 layer sputtered with Ag/Au alloy into the mixture and dealloy for 2-20 min²¹.
4. After dealloying, quench samples in RT DI water for 1 min.
5. Take the Si chips out of the DI water and thoroughly rinse with DI water.

2. Silicon substrate patterning and cleaning

Substrate preparation for solid Si imprinting with porous catalyst
1. Oxidize the 4 inch Si wafer at 1,150 °C for 24 h in an O₂ flow of 4 sccm.
2. Take SPR 220 7.0 photoresist out of the refrigerator and let it stay at RT for 10-15 min prior to spin coating.
3. Open the spin coater lid. Place the Si wafer inside the spin coater onto the vacuum chuck.
4. Connect the output of the spin coater to the vacuum pump through a PVC tube. Start the vacuum pump. Set the pressure level to 30 kPa using the vacuum pump valve.
5. Select a spin coating procedure with the following parameters: spread at 400 rpm for 30 s with acceleration 200 rpm/s, spin at 2,000 rpm for 80 s with acceleration 500 rpm/s.
  NOTE: Step 2.1.5 will produce a 9 µm thick SPR 220 7.0 layer.
6. Apply vacuum to the vacuum chuck by pressing "VAC ON" on the spin coater display.
7. Pour 5 mL of SPR 220 7.0 photoresist at the center of the 4 in Si wafer.
8. Close the spin coater lid. Start spin coating by pressing "START".
9. Open the spin coater lid. Turn off the vacuum by pressing "VAC OFF". Take out the 4 inch Si wafer with the spin-coated layer of SPR 220 7.0 photoresist using tweezers.
10. Place the Si wafer with the spin-coated layer of SPR 220 7.0 photoresist onto a hotplate preheated to 110 °C and prebake for 2 min. Let cool for 1 min.
11. Expose the photoresist layer through the mask with a square mesas pattern that has following parameters: width = 500 µm and spacing = 900 µm. Flood exposure for 10 s to achieve a 150 mJ/cm² dosage.
12. Develop the exposed photoresist layer in 4:1 (volume) of developer: DI water for 3 min. Rinse the sample with DI water and check the features in microscope.
13. Place the Si wafer with the developed SPR 220 7.0 photoresist onto a hotplate preheated to 120 °C and hard bake for 5 min. Let cool for 1 min.
14. Etch the oxide layer in reactive ion etching equipment for 20 min using the following parameters: pressure = 100 mT, O₂ flow = 3 sccm, CF₄ flow = 24 sccm, power = 250 W.
15. Remove the SPR 220 7.0 layer using acetone, then rinse with isopropyl alcohol (IPA) and DI water.
16. Perform etching in a 30% KOH bath (weight) at 80 °C for 100 min with constant stirring at 175 rpm to create mesas on the Si wafer.
17. Remove the oxide layer with buffered oxide etch solution.
18. Thoroughly flush with DI water.
  NOTE: Si wafer patterning mask layout and single patterned chip is shown in Figure 6.

Figure 6: Si wafer patterning mask layout (A) and single patterned chip (B). Please click here to view a larger version of this figure.

Substrate preparation for porous Si imprinting with solid catalyst
1. Repeat step 2.1.
2. Coat the back of the patterned 4 inch Si wafer with nickel and anneal at 320 °C in a rapid thermal annealing chamber in N₂ for 3 min.
3. Cleave 2.5 x 2.5 cm Si chips out of the patterned 4 inch Si wafer using a scriber.
4. Place the Si chip inside the bottom part of the electrochemical cell (EC). Place an O-ring on the top of the Si chip. Place the top part of the EC on and tighten the screws.
5. Set the galvanostatic regime in the potentiostat (see Table of Materials) control software. Refer to Supplemental File (Figure S3). Connect a working electrode to the Si chip and the counter electrode to the platinum electrode (Figure 7).
6. Carefully fill the EC with HF and insert a cylindrical platinum electrode from the top to 5 mm above the Si chip surface (Figure 7b).
7. Apply current density of 135 mA/cm² for 120 s by pressing the green Start button in the potentiostat software. Refer to Supplemental File (Figure S3).
8. Carefully suck the HF out of the EC with a plastic pipette.
9. Thoroughly flush with DI water.
  NOTE: The Si anodization process and Si chip with a porous Si layer are shown in Figure 7.

Figure 7: Photographs of substrate porosification procedure (Si anodization). (a) PC-controlled potentiostat connected to two-electrode electrochemical cell. (b) Electrochemical cell with platinum electrode. (c) Si chip with a porous Si layer. Please click here to view a larger version of this figure.

3. Mac-Imprinting setup

Stamp to PTFE rod fixation
1. Place the reference Si chip inside the bottom part of the EC. Place the Mac-Imprint stamp on top of the reference Si chip with the pattern facing down.
2. Attach the PTFE rod to the load cell (see Table of Materials) through a double-sided threaded screw. Connect the structure to the software-controlled motorized linear stage (see Table of Materials) through a metal bracket.
3. Add a small droplet of SU-8 2015 photoresist on the back of the Mac-Imprint stamp.
4. Bring the PTFE rod in contact with a SU-8 droplet by setting the "Move Relative" command 173,500 steps from the home position and pressing the "Write" button in the stage control software. Refer to Supplemental File (Figure S4a).
5. Cure the SU-8 2015 photoresist droplet with a 6 W UV bulb for 2 hours. Refer to Supplemental File (Figure S5).
6. Bring the PTFE rod with the attached Mac-Imprint stamp into the home position by setting the "Home" command and pressing "Write" in the stage control software. Refer to Supplemental File (Figure S4a).
7. Assemble the EC.
Mac-Imprinting operation
1. Clean the patterned Si chip according to steps 1.1.1-1.1.4.
2. Place the patterned Si chip at the center of an EC. Position the EC under the PTFE rod with the Mac-Imprint stamp (Figure 8).
3. Mix the ES of HF and H₂O₂ in the 17:1 ratio (volume) inside a PTFE beaker. Let the ES stay for 5 min before etching.
  NOTE: The suggested ratio leads to solution parameter ρ = 98%¹⁶. The ratio can be changed in order to suppress or promote the etching rate.
4. Carefully pour the ES into the EC using a plastic pipette.
5. Bring the PTFE rod with the attached Mac-Imprint stamp in contact with the patterned Si chip by setting the "Move Relative" command 173,500 steps from the home position and pressing the "Write" button. Refer to Supplemental File (Figure S4a).
6. Next, set 600-2,000 steps and press "Write" to obtain loads in the range of 4-10 lbf. Measure load values through a software-controlled load cell. Refer to Supplemental File (Figure S4b).
7. Hold in contact during Mac-Imprint (Figure 8c). The Mac-Imprint time varies from 1-30 min.
8. Move the PTFE rod with the attached Mac-Imprint stamp into the home position by pressing "Home". Refer to Supplemental File (Figure S4a). Carefully aspirate the ES out of the EC with a plastic pipette.
9. Rinse the imprinted Si chip using IPA and DI water.
10. Dry the imprinted Si chip with clean, dry air.

Figure 8: Photographs of Mac-Imprint setup (A), stamp before (B) and after (C) contact with Si chip. Please click here to view a larger version of this figure.

Representative Results

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).

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. Please click here to view a larger version of this figure.

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. Please click here to view a larger version of this figure.

Supplemental Figure 1: Photograph of spin coater control display. Please click here to view a larger version of this figure.

Supplemental Figure 2: Magnetron sputter control software screenshots. (a) Evacuation of magnetron sputter chamber. (b) Sputtering control parameters. (c) Ventilation of magnetron sputter chamber. Please click here to view a larger version of this figure.

Supplemental Figure 3: Potentiostat control software screenshot. Please click here to view a larger version of this figure.

Supplemental Figure 4: Linear motorized stage and load cell control software screenshots. (a) Before Mac-Imprint and (b) during Mac-Imprint. Please click here to view a larger version of this figure.

Supplemental Figure 5: Photograph of Mac-Imprint stamp to PTFE rod attachment process. Please click here to view a larger version of this figure.

Discussion

Mac-Imprint stamps and prepatterned Si chips (p-type, [100] orientation, 1-10 Ohm∙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'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.

Divulgazioni

The authors have nothing to disclose.

Acknowledgements

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

Materials

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

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Sharstniou, A., Niauzorau, S., Junghare, A., Azeredo, B. P. Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers. J. Vis. Exp. (180), e61040, doi:10.3791/61040 (2022).