Fast and Versatile Multiscale Patterning by Combining Template

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Fast and Versatile Multiscale Patterning by Combining Template-Stripping with Nanotransfer Printing Raphael F. Tiefenauer,† Klas Tybrandt,†,‡ Morteza Aramesh,† and János Vörös*,† †

Laboratory of Biosensors and Bioelectronics, ETH Zürich, 8092 Zürich, Switzerland Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, 601 74 Norrköping, Sweden

‡

S Supporting Information *

ABSTRACT: Metal nanostructures are widely used in plasmonic and electronic applications due to their inherent properties. Often, the fabrication of such nanostructures is limited to small areas, as the processing is costly, lowthroughput, and comprises harsh fabrication conditions. Here, we introduce a template-stripping based nanotransfer printing method to overcome these limitations. This versatile technique enables the transfer of arbitrary thin film metal structures onto a variety of substrates, including glass, Kapton, silicon, and PDMS. Structures can range from tens of nanometers to hundreds of micrometers over a wafer scale area. The process is organic solvent-free, multilayer compatible, and only takes minutes to perform. The stability of the transferred gold structures on glass exceeds by far those fabricated by e-beam evaporation. Therefore, an adhesion layer is no longer needed, enabling a faster and cheaper fabrication as well as the production of superior nanostructures. Structures can be transferred onto curved substrates, and the technique is compatible with roll-to-roll fabrication; thus, the process is suitable for flexible and stretchable electronics. KEYWORDS: template-stripping, plasmonics, nanowire array, nanotransfer printing, nanofabrication, multiscale

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fabrication techniques are preferable for large scale production. Standard photolithography lacks resolution for nanostructures, but modifications thereof can reach the nano regime. Phase mask lithography and interference lithography enable the creation of nanostructures down to ∼50 nm. Extreme UV Interference lithography enables sub 20 nm structures using synchrotron light.25,26 All those techniques are based on the interference of light; thus, the patterns are mostly limited to lines, grids, and circles. The multiplication of nanostructures using reusable templates created by serial structuring methods can bridge the gap between serial and parallel processing capabilities. Nanoimprint lithography is a well-known example. A template is imprinted into a photoresist layer followed by dry etching the photoresist residues in the grooves and further processing, e.g., metal evaporation and lift-off.8,27−29 Nanotransfer printing is another promising technique which also enables patterning on curved substrates. A pattern is transferred from a structured

anofabrication has become essential for the development of plasmonic devices,1,2 metamaterials, 3,4 sensors,5,6 label-free optical biosensing,7−9 surface enhanced spectroscopy,10−12 plasmonic photovoltaics,13,14 transistors,15,16 generators,17 and nonvolatile nanomemories.18 Structures in the nanometer regime exhibit other physical properties compared to bulk material.19 Within the sensing field, different functionalities and higher sensitivities can be achieved by shrinking device features down to the nanoscale. Until now, costly fabrication has been a bottleneck for the widespread application and market success of such nanostructure based devices. Better fabrication techniques are needed, which should be simple, fast, inexpensive, high-throughput, reproducible, and should allow for arbitrary multiscale patterns of good stability. Today, several methods exist for patterning metal nanostructures on surfaces. Standard and more recent techniques such as e-beam lithography, focused ion beam, and thermal scanning probe lithography can produce features down to sub-10 nm.20−24 However, these serial writing techniques cannot be used for mass production of large area nanostructures due to long fabrication times and high costs. Therefore, parallel © XXXX American Chemical Society

Received: November 22, 2017 Accepted: February 20, 2018

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DOI: 10.1021/acsnano.7b08290 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano PDMS replica stamp onto a target substrate.30 The elastomer characteristics set limits in replica stamp resolution, adhesion, pattern fidelity due to merging of the surface topographic stamp patterns, and stamp reusability.31,32 Alternatively, a pattern is transferred to a flat PDMS stamp, which then can be further transferred to the target substrate. However, transferring the pattern onto PDMS requires a multistep process including harsh chemicals and only straight wires are possible due to a required angled evaporation.33 A further development of this process does not require a PDMS stamp anymore; however, the process limitations still apply.34 Template-stripping overcomes these limitations, can produce 3D structures, and also allows for stretchable target substrates.35−39 A metal film is deposited on a structured template and the top layer is subsequently peeled off by a target with an adhesive backing layer. This adhesive epoxy layer remains part of the resulting sample and the process only works on planar surfaces. The simple, large-scale, inexpensive fabrication of arbitrary nanostructures without additional processing steps or remaining residues remains a challenge. Here, we have developed a sub-50 nm replication technique to overcome these current limitations by combining the strengths of template-stripping and nanotransfer printing. The technique leverages a serially fabricated template by multiplying the pattern. We evaporate metal onto a template and strip the arbitrary pattern with an adhesive, transparent, supported poly(vinyl alcohol) (PVA) sheet. This flexible sheet can then be transferred to diverse target substrates including nonplanar, flexible, or stretchable surfaces. Thereafter, the PVA is simply dissolved in water, leaving an ultrastable pattern on the surface without the need for an adhesion layer. The template can be cleaned and reused over 50 times. The complete process only takes minutes and does not require additional photoresist processing, exposure, etching, or lift-off. Furthermore, alignment is possible and no residues are left, which allows for the transfer of multiple layers and enables integration into standard fabrication processes. The multiscale nature and the highthroughput potential makes this method attractive for large scale production.

Figure 1. The process flow of template-stripping based nanotransfer printing (TS-nTP). (a) Gold is evaporated onto a silicon template by e-beam evaporation. (b) The topmost layer is stripped by an adhesive supported PVA sheet (PVA*) and (c) is then transferred onto the target surface by lamination. (d) Subsequently, the support is peeled off and the PVA is simply dissolved in water. The template can be reused after etching away the remaining gold.

temperature transfer is also possible if the substrate does not endure this temperature, e.g., for SU-8 target substrates. The remaining PVA is dissolved in water in the last step (Figure 1d). The supporting layer is separated beforehand to ensure easy access of the water to the PVA. This process can occur at room temperature; however, using boiling water decreases the process time to 10 min due to the highly increased dissolution rate of PVA in water above 80 °C. The use of an appropriate adhesion layer is of paramount importance for a successful multiplication of nanostructures. Thin spin-coated PVA sheets provide a flat surface, which enable the use of templates with a wide range of aspect ratios between grooves and ridges. The increased adhesion at elevated temperatures and the high solubility in water are further key factors. PVA sheets alone, however, are delicate to handle due to their deformable nature. Thus, we spin-coat PVA onto a polyethylene naphthalate (PEN) foil as a supporting substrate. The adhesion of PVA to PEN is good enough for processing, but weak enough for an easy separation needed in the PVA dissolution step. This bendable, but not stretchable, supporting structure ensures easy handling, process stability as wells as the possibility of storing the metal patterns at an intermediate stage after template-stripping for a very long time. We have successfully transferred patterns more than a month after the template-stripping step. The metal coated template defines the pattern geometry. A template consists of grooves and ridges which are both covered by metal by e-beam evaporation. The template material is typically silicon, but can be diverse (e.g., SU-8) as long as the adhesion of the metal layer to the template is weaker than the PVA-metal adhesion. If necessary, the templates can also be treated with a nonsticky perfluoro-silane to lower the adhesion of the metal to the template. The template can be reused after the stripping by etching the remaining metal in the grooves (Figure 1c). In the case of gold, a standard etchant is sufficient and does not alter the template40 nor the silane coating. The templates are immersed in ethanol before the etchant is applied to overcome the wetting issue due to the hydrophobic nature of the nanostructures and the silane. This cleaning step enables the multiplication of the nanostructures manifold. We have performed more than 50

RESULTS AND DISCUSSION Nanostructures via Template-Stripping Based Nanotransfer Printing. The procedure for the template-stripping based nanotransfer printing (TS-nTP) is illustrated in Figure 1. It is a four-step process consisting of a metal deposition step, a template-stripping and a transfer step, and a step to dissolve the carrier substrate. The top layer of a metal-deposited template is stripped with an adhesive supported PVA layer (Figure 1a,b). This flexible supported PVA sheet is softly laminated onto the template ensuring that no air is being trapped. No vacuum chamber is needed. A pressure-less lamination ensures that deposited metal is stripped only from the top of the structured template despite the flexibility of the adhesive sheet (Figure 1b). This process occurs above the glass transition temperature of PVA at 120 °C, which increases its adhesion strength. After cooling down to room temperature, the two sheets are separated without applying any shearing forces. The resulting metal pattern on the adhesive sheet is transferred onto a substrate (Figure 1c). The sheet is laminated under pressure, followed by applying pressure from above with a soft stamp for 30 s. The process temperature is ideally 120 °C, which makes the PVA sheet stick to the substrate as well. This increases the stability of the process. However, a lower B


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ACS Nano cycles without any visible degradation of the template or remaining residues (see Figure 2a).

Figure 3. Comparison of structures (a) before and (b) after stability testing (gold on glass). The structures were tested by peeling with different types of tapes and immersion in an ultrasonic bath for more than 2 h. No changes except intentional manual scratches are visible. The gold regions appear darker in these transmission microscope images due to their limited transparency.

The optical properties of the transferred nanostructures are most important for potential plasmonic and biosensing applications. Figure 4 shows the transmission spectrum of 10 nm thick samples with a pitch of 300 nm and a line width of 60, 90, and 115 nm, respectively. Figure 2. SEM images of a template and the transferred nanostructures. The widths of the nanowires are 65 nm with a pitch of 300 nm. (a) Intact silicon template after 50 transfer cycles. (b) The same template after template-stripping of a 10 nm gold layer before cleaning. The gold layer was fully stripped from the ridges but not from the grooves. The typical gold islands from the 10 nm deposition are clearly visible in the grooves. (c) Transferred 10 nm thick gold pattern on a glass substrate. Gold islanding leads to visible cracks. (d) Transferred 20 nm thick gold pattern on a glass substrate. Gold islanding has vanished and conductive nanowires are obtained.

Structural and Functional Characterization of Transferred Structures. Figure 2 shows SEM images of transferred gold on glass and the corresponding template at different stages of the process. The nanowire arrays show a direct replication of the mold. Figure 2c,d shows nanowires with a thickness of 10 and 20 nm, respectively. While transferring 20 nm Au results in intact nanowires, the 10 nm pattern shows line breaks and incomplete wires. This is due to the known islanding behavior of thin gold films indicating a limit due to the material itself.41 Such nanowire arrays can be produced at a large area (see Figure S1). The used template is shown in Figure 2a and the remaining gold after stripping is shown in Figure 2b. Gold is only stripped from the ridges despite using a template with a sidewall angle of 60 degrees. This indicates, that the exact shape of the grooves is not important as long as the edges of the ridges are reasonably sharp. Blunt edges lead to a lower resolution since metal from the side walls could also be picked up. The transferred gold structures onto glass have a superior stability compared to evaporated gold on glass, which is wellknown to have a weak adhesion.41 In our method, no adhesion layer is required to improve the adhesion between glass and gold, which enables faster and cheaper processing. We tested the stability by means of tape peeling tests, ultrasound, and temperature shocks (Figure 3a,b). This observed strong adhesion and stability is surprising and is most likely due to a van der Waals force dominated direct contact42,43 of the gold to the substrate caused by the conformal soft−hard contacting method of the ultraflat gold pattern to the substrate. However, further investigations are needed to understand the exact underlying mechanism.

Figure 4. Spectrophotometry of transferred ordered nanowire arrays on glass. The widths of the nanowires are 65, 90, and 115 nm. The height and pitch are 10 and 300 nm, respectively.

Orthogonally polarized light is coupled to the gold nanowire arrays and localized surface plasmons are excited, which modulate the optical transmission spectrum resulting in a specific peak. The spectrum shows excellent peak properties with a spectral full width at half-maximum (fwhm) of 83, 91, and 127 nm for the different line widths. The redshift and the increase in intensity with increasing line width is in accordance to Schider et al.44 Plasmonic biosensing is commonly based on the analysis of the peak shift.45 Thus, a sharp peak with high intensity to the background is advantageous. The sharp peaks indicate that the coherence is preserved despite the bending of the structures during the transfer process. The conductivity of structures is a further indicator of their functionality and is crucial for several types of integrated chips as well as for sensing applications.46 The conductivity of the nanowire arrays was measured with a 4 point-measuring setup. The 20 nm thick nanowire arrays showed excellent conductivity, with a typical resistance of 20 Ω for the 1333 parallel 500 μm long nanowires with a 20 × 60 nm2 cross section. This corresponds to a bulk resistivity of 7 × 10−8 Ωm, which is about 3 times higher than the bulk resistivity of gold C


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Figure 5. Reflection light microscopy pictures of test structures on different substrates (brighter areas are gold). (a) Original silicon template and (b) a gold covered template. The structures from this template were transferred onto (c) glass, (d) a silicon wafer, (e) Kapton foil, and (f) PDMS.

Figure 6. Structures can be transferred onto curved substrates. (a,b) Test structures on a glass vial. (c) Roll-to-roll fabrication with a custommade setup.

Figure 7. (a) SEM image of two stacked 10 nm thick gold structures. Gold islanding behavior of a double sheet part top left. Top right shows a nanowire array on a sheet, bottom left a sheet on a nanowire array, while bottom right is a nanowire grid resulting from transferring nanowires on a nanowire array. The top structure conforms to the lower one and forms an electrical connection. (b) Integration of nanowire grids in a sensing chip enabling electrical 4-point measurements.

(2.44 × 10−8 Ωm). The discrepancy can be explained by the size effect, which predicts an increased resistivity47−49 when one of the wire dimensions approaches the electron mean free path.50,51 While the 10 nm thick arrays were not conductive due to gold islanding, they could be made conductive by creating a grid out of two layers of the same arrays rotated by 90° (see Figure 7a). This way, the gaps are bridged by alternative routing. With such conductive nanowire arrays, plasmonic and electronic dual sensors similar to already published systems could be achieved.52−54

The process is independent of the shapes due to the intrinsic properties of the template-stripping. Even multiscale structures can be fabricated in one step. Deeper trenches enable thick metal layers, but also multiple patterning cycles without the need of cleaning the template. Large gaps between the structures can also be transferred because the hard supporting structure prevents the sagging of the soft PVA during pick-up. Aspect ratios of more than 1:100 (depth:width of the groove) have been tested successfully. Additionally, such multiscale templates can be replicated over a large area. Therefore, this D


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ACS Nano method can bridge the gap between nanofabrication to microfabrication in one step as an all-in-one technique. One notable advantage of the technique is that it allows for transferring onto almost arbitrary surfaces due to the enhanced adhesion of the structures and the flexibility of the supported PVA sheets. Next to glass, we successfully targeted silicon wafers, Kapton foils (Kapton E, DuPont, USA), and PDMS (Sylgard 184, Dow, USA) (see Figure 5a−f and Figure S2). Silicon compatibility opens up opportunities to integrate the transfer technique into standard manufacturing processes. The ability of transferring onto Kapton and PDMS is of particular interest for flexible and stretchable electronics. The flexibility further allows to conformably transfer onto round surfaces. Figures 6a,b show transferred gold patterns on a glass vial. A common limitation of nanofabrication is the scalability. The easy handling of the supported PVA sheets allows for the fabrication of arbitrary areas. We successfully tested 4-in. wafer fabrication. Furthermore, the flexibility of the supported PVA membrane allows for roll-to-roll compatibility of the transfer printing (Figure 6c). Cylindrical templates and a continuous metal evaporation would further ensure roll-to-roll compatibilities of the template-stripping step. Thus, the process is scalable beyond batch processing toward industry scale. The transfer process is a stand-alone technique, leaving no residues. Multiple layers can be fabricated on top of each other while the adhesion remains (see Figure 7a). In fact, the new layer fully conforms on the previous one and forms an electrical connection. The different layers can be aligned since the supported PVA sheets are transparent. Furthermore, the process is fully embeddable in conventional cleanroom processing due to the stability of the structures. Figure 7b shows a chip with leads for four-point contacting of 9 nanowire grids. The fabrication process included spin-coating photoresist, exposure, etching, e-beam evaporation, plasma cleaning, and lift-off.

Template-Stripping. The template was covered with metal in the desired pattern thickness by e-beam evaporation. The template was laminated by the supported PVA sheet without applying pressure with a silicone pasting roller on a 120 °C hot plate. After cooling down, the PVA sheet and the template were separated. Pattern Transfer. The supported PVA sheet with the metal pattern was laminated using a silicone pasting roller onto the target substrate on a 120 °C hot plate with pressure followed by applying pressure from above with a soft stamp, for 30 s. Dissolving PVA. The PEN was peeled off, leaving the pattern and PVA sheet on the target substrate. Thereafter, the PVA was dissolved in a boiling water bath for 2 times 5 min with a water rinsing step in between and after the baths, followed by drying the structures with a nitrogen gun. Template Cleaning. The remaining gold on the template was removed with an iodide based standard gold etchant (Gold etchant standard, Sigma-Aldrich, Switzerland) for typically 10 s after a quick ethanol rinse. Optical Measurements. The optical measurements were conducted with a spectrometer SpectraPro 2150 (Princeton Instruments, US) and halogen lamp illumination in transmission mode. Stability Tests. Structures were peeled 5 times each with office tape, lab tape, Kapton tape, and copper tape followed by a 2 h immersion in an ultrasonic bath in 2% SDS at 50 °C. The sample also underwent 2 times 5 min immersion in boiling water with a cold water rinse directly onto the structures followed by drying with a nitrogen gun at 5 bar pressure.

CONCLUSIONS In summary, we demonstrated a low-cost practical nanofabrication technique with scaling potential. The templatestripping based nanotransfer printing bridges the gap between serial and parallel nanofabrication, enabling the fabrication of arbitrary multiscale structures onto diverse planar and nonplanar substrates. These optically and electrically active structures exhibit outstanding adhesion and are fully integrable into standard fabrication techniques. Multiple nanostructure layers can be created in a short time. The transfer technique is scalable to high-throughput production batch processing and has roll-to-roll compatibility because of the multiplication of the templates and the flexibility of the supported PVA sheets. This simple, fast and inexpensive technique has the potential to widen the application area of nanostructures beyond research.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08290. Figures S1 and S2 (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

János Vörös: 0000-0001-6054-6230 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Thanks to S. Wheeler for setup constructions and to M. Lanz and A. Rossi for their support in the cleanroom. The nanowire templates were obtained from H. Solak (PSI) and the test template design from the Embedded MEMS lab course (ETH). This work was supported by ETH Zurich, the Swiss National Science Foundation as part of the NCCR Molecular Systems Engineering, and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO Mat LiU No 2009 00971).

MATERIALS AND METHODS

REFERENCES

Templates. The nanowire templates were fabricated based on extreme UV interference lithography.26 The test structures were fabricated with standard photolithography. Fabricated templates were plasma cleaned followed by an incubation with Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (Sigma-Aldrich, Switzerland) for 1 h by chemical vapor deposition. The supported PVA sheets were prepared by spin-coating a 15% PVA solution (Mowiol 18−88, Sigma-Aldrich, Switzerland) in water onto a 125 μm thick PEN foil (Teonex Q 125 μm, Synflex, Germany), at 500 rpm for 30 s. Subsequently, the sheets were cured on a hot plate at 60 °C for 10 min.

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