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Multi-Length Scale Patterning of Functional Layers by Rollto-Roll Ultraviolet-Light Assisted Nanoimprint Lithography Markus Leitgeb, Dieter Nees, Stephan Ruttloff, Ursula Palfinger, Johannes Götz, Robert Liska, Maria R. Belegratis, and Barbara Stadlober ACS Nano, Just Accepted Manuscript • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 29, 2016

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Multi-Length Scale Patterning of Functional Layers by Roll-to-Roll Ultraviolet-Light Assisted Nanoimprint Lithography Markus Leitgeb1,2‡, Dieter Nees1‡, Stephan Ruttloff1, Ursula Palfinger1, Johannes Götz1, Robert Liska2, Maria R. Belegratis1, and Barbara Stadlober1‡* 1

JOANNEUM RESEARCH Forschungsgesellschaft mbH, Institute for Surface Technologies and

Photonics, Franz-Pichlerstraße, 8160 Weiz, Austria 2

Vienna University of Technology, Institute of Applied Synthetic Chemistry, Getreidemarkt 9,

1060 Wien, Austria *[email protected]

Nanoimprinting, UV-Nanoimprint Lithography, high-throughput, biomimicry, UV-curable resist, metal pattern

1 Environment ACS Paragon Plus

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ABSTRACT Top-down fabrication of nanostructures with high-throughput is still a challenge. We demonstrate the fast (> 10 m/min) and continuous fabrication of multi-length scale structures by Roll-to-Roll UV-Nanoimprint Lithography on a 250 mm wide web. The large-area nanopatterning is enabled by a multi-component UV-curable resist system (JRcure) with viscous, mechanical and surface properties that are tunable over a wide range to either allow for usage as polymer stamp material or as imprint resist. The adjustable elasticity and surface chemistry of the resist system enable multi-step self-replication of structured resist layers. Decisive for defectfree UV-nanoimprinting in Roll-to-Roll is the minimization of the surface energies of stamp and resist and the step-wise reduction of the stiffness from one layer to the next is essential for optimizing the reproduction fidelity especially for nanoscale features. Accordingly, we demonstrate the continuous replication of 3D-nanostructures and the high-throughput fabrication of multi-length scale resist structures resulting in flexible PET film rolls with super-hydrophobic properties. Moreover, a water-soluble UV-imprint resist (JRlift) is introduced that enables residue-free nanoimprinting in Roll-to-Roll. Thereby we could demonstrate high throughput fabrication of metallic patterns with only 200 nm line width.

As a design principle - very successfully exploited by nature - multi-length scale structures (which we understand as microstructures with superimposed nanoscale features) meanwhile are abundantly found in technical applications.1-8 Their realization requires nanostructuring techniques9-12 that are usually hard to implement in those real-life applications, where large areas, low costs, mechanical flexibility and ease of processing are required like in adhesive,13 drag reducing,14 anti-reflective15 or special wetting16 surfaces of plastic films, paper or textiles.


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Roll-to-roll nanoimprint lithography (R2R-NIL) is the only high-resolution patterning technique that can account for cost-effective production of nanostructures over square-meter large and flexible substrate areas.17,18 R2R-NIL has been established since 1998 as thermal imprinting by Tan et al.,19 but not before 2006 as R2R-UV nanoimprint lithography (R2R-UV-NIL) by Ahn et al. 20 Contrary to thermal imprinting, where a thermoplastic material is embossed at temperatures above its glass transition and hardened by subsequent cooling, in UVNIL the pattern that is transferred by a stamp in the imprint resist is fixed simultaneously by UVcuring. While the R2R-fabrication of multi-length scale structures with 3D form factor has not yet been shown, the realization of sub-100 nm features by roller-based UV-NIL was demonstrated by several groups15,21-26 whereby in all these reports the manufacturing was limited by the speed (≤ 5 m/min) and/or the width of the web (≤ 100 mm). The core idea of R2R-NIL is to replace the flat and rigid stamps by roller stamps. Thereby the imprinting is done continuously on a line resulting in more homogeneous and conformal pressure application, improved resist displacement, seamless pattern transfer and high throughput. On the one hand roller stamps can be topographically patterned rolling cylinders with either metallic19,20,27 or silicone28-30 surface and on the other hand bendable nanostructured stamps that are wrapped around a metal roller. Such flexible stamps (called shims) are promptly attached and detached and therefore enable a high degree of flexibility in manufacturing by decreasing changeover times. Meanwhile shims are common in many forms ranging from electroformed nickel shims15,31-33 to patterned polymer layers backed by a film substrate.21,34,35 In UV-NIL the resist to be patterned is typically an acrylate- or epoxy-based material.36-38 In a R2R process the requirements for such an imprint resist are manifold ranging from good substrate adhesion over high curing rate and low surface energy to low enough viscosity for fast


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spreading and filling of stamp cavities.39,40 Depending on whether being used as a robust flexible imprinting tool or as a soft nanopatterned film the elastic properties of the resist have to be adjusted. Inspiring in this context is the work of Choi et al. and Suh et al.36,41,42 illustrating the ability of PUA-based resists (Poly Urethane Acrylate) being usable either as molds or as patterned surface layers. Quite recently, they reported the usage of a very elastic PUA resist in slow roller-based UV-nanoimprinting of mushroom-like micro-pillars on a 100 mm web for dry adhesive films.43 Many applications involve production steps after the NIL process using the patterned resist layer as a mask. Exemplary here is the fabrication of submicron-spaced electrodes in organic thin film transistors (OTFT).44,45 Unfortunately, in most cases there remains a residual resist layer, stemming from an incomplete displacement of the resist by the raised stamp features. To allow further processing the residual layer has to be removed by anisotropic oxygen plasma etching, a process that is hard to implement in R2R manufacturing. Therefore residue-free nanoimprinting is an important target here. According to the squeeze flow model46 and stamp geometry considerations47 one condition for minimizing the residual layer thickness is that the viscosity of the resist should be as low as possible and that the amount of polymer resist on the substrate should be somewhat smaller than the void volume of the stamp. However, even with these conditions met, it theoretically takes infinite time to achieve zero residual layer thickness.46 Decisive for residue-free nanoimprinting is the interfacial chemistry of the substrate, resist and mold stack which should enable a spontaneous dewetting of the resist from the substrate induced by the mold. Residue-free imprinting of a low viscous UV-curable resist layer was demonstrated first by Auner et al.48 for hard stamps and by Choi et al.49 for soft stamps. Rolland et al.50 presented a “scum-free” molding process by squeezing different lipophilic biomaterial precursor


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solutions between a non-wetting substrate and a fluorinated elastomeric mold. This PRINT process was claimed to work in Roll-to-Roll for the fabrication of monodisperse shape-specific nanoparticles51, but was not applied for the nanopatterning of UV-curable resist layers useable as etch or lift-off masks. So far, in R2R-UV-NIL typically medium viscous (>100 mPa⋅s) resists were used, where the several tens of micron thick resist layers naturally prevent achieving a small residual layer thickness.20 In this context we demonstrate fast (> 10 m/min), continuous, multi-length scale (100 nm - 25 µm) patterning by R2R-UV-NIL on a 250 mm wide web. The large-area nanopatterning is enabled by a multi-component UV-curable resist system (JRcure) with viscous, mechanical and surface properties that are tunable over a wide range to either allow for usage as polymer stamp material or as imprint resist. The adjustable elasticity and surface chemistry of the resist system support high speed R2R-UV-NIL and the fabrication of high-pattern fidelity 3D-nanostructures with even undercut features, enabling e.g. super-hydrophobic properties as well as multi-step self-replication. Moreover, a water-soluble UV-imprint resist (JRlift) is introduced that allows for residue-free nanoimprinting even at high throughput. Thereby we could demonstrate the R2R fabrication of high definition metallic patterns - via a lift-off process - without the need for prior residual layer etching.


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RESULTS AND DISCUSSION Pilot line for Roll-to-Roll UV-nanoimprinting In our R2R-UV-NIL machine (Figure 1a) the liquid UV-curable imprint resist is coated on a 250 mm wide PET web by reverse gravure printing and then transported to the imprinting unit at a certain velocity (Figure 1b). The imprint resist is based on an appropriate, thus soft and lowviscous, formulation of JRcure or JRlift. The shim is mounted on a steel roller and is either made of Ni or a patterned polymer layer coated on a PET substrate. The polymer shim is based on a somewhat harder formulation of the JRcure resist resulting in a stiff, but flexible layer after UVcuring (Figure 1c). Its fabrication is described in detail in the Methods section. The pressure that is needed to transfer the topographic shim pattern into the resist is applied by a soft-rubber coated counter roller pressing the substrate against the shim and furthermore levelling the waviness of the substrate. In consequence of the small contact area between resist and shim in the nip, the R2R-UV-NIL unit enables to maintain high pressure uniformity across the web width. The UV-nanoimprinting process takes about a half turn of the imprint roller with the imprint pressure being maintained by the web tension, the while the curing is done by means of a high power Hg-UV lamp (2.2 W/cm2) radiating through the back of the transparent PET substrate. A detachment roller (not shown in Figure 1) alleviates the separation of stamp and resist in a “peeling” way which is decisive when large areas have to be patterned defect-free at a certain throughput (≥ 5 m/min).52


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Figure 1. a) Photograph of the R2R-UV NIL machine supporting a web width of 250 mm and a web speed up to 30 m/min. b) Scheme of the R2R-UV-NIL unit. By reverse gravure printing the liquid imprint resist (e.g. JRcure) is coated on a PET web, which is fed at a certain speed into the UV-NIL unit. The imprint shim is wrapped around the imprint roller and attached either magnetically or by bonding. In addition to the web tension a rubber-coated counter roller is needed to apply a well-defined imprint pressure. The UV-curing is done through the reverse side of the transparent PET film, optionally under inert atmosphere, resulting in a cured and patterned resist layer on the PET substrate. c) Photographs of Ni and polymer shims (629 mm × 250 mm). The left shim is made from electroplated Nickel (Ni shim), the right is fabricated as an inverse copy of the Ni shim by R2R-UV-NIL in a high modulus composition of JRcure on a thick (> 100 µm) PET film substrate. The right photograph illustrates the mechanical flexibility of the polymer shim.


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Criteria for an imprint resist system used for polymer shims and imprinted resist layers According to a thermodynamic approach the maximum inaccuracy ε in the reproduction of any stamp or shim feature that is the result of a nanoimprinting process follows the relation:53

~ ∙

(1)

In (1) E is the Young’s modulus of the stamp representing a measure of its stiffness, is the solid-resist interfacial energy and R0 is the radius and height of the structures on the master stamp (in Ref. 53 a cylindrical shape of the structures with an aspect ratio of 1:1 was assumed). The polymeric stamp material has to fulfill several requirements: (i) For an imprinting process with high resolution and good structure fidelity (implying ε < 0.1) these are a high microscopic stiffness E and a low interfacial energy , both of which increase the replication accuracy for features with small R0. Nevertheless, for roller-based nanoimprinting the material should not become brittle after curing and maintain its macroscopic mechanical flexibility; therefore E should not be too high. (ii) For fast nanoimprinting it is important to facilitate the demolding step, which is achieved by reducing the force required to separate stamp and cured resist. The separation force is the lower, the lower is the interfacial energy between stamp and cured resist with = + − 2 ∙ . For a minimal the surface energies of stamp and cured resist are required to converge ( ~ ). (iii) Defect-free nanoimprinting requires defect-free demolding and is enabled by an as low as possible stamp-resist adhesion energy ( = 2 ∙ )54. Thus both and should be minimized and nearly equal. Furthermore, full C=C double bond conversion on the acrylate-based stamp’s surface is mandatory for enabling a


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defect-free pattern transfer by preventing chemical bonding with the (also acrylate-based) imprint resist during curing. In contrast to the polymer shim the elasticity of the cured nanoimprinted resist layer should be as high as possible. 53 (i) A clean separation between stamp and nanoimprinted layer (defectfree demolding) is enabled by a resist with a low modulus (after curing) and a low surface energy. This would even allow for the transfer of 3D patterns with undercut features. (ii) Moreover, the pattern fidelity is enhanced by minimized shrinkage after solidification.

Composition of two resist systems: JRcure and JRlift Along these criteria we constructed the UV-crosslinking resist system JRcure that is based on five tightly linked components at variable mass fractions. Figure 2 provides an overview on the four principal components of JRcure and in Table S 1 of the Supporting Information the used materials are listed. The JRcure resist system consists of urethane acrylate oligomers (UAO) for adjusting the stamp’s stiffness and reducing the shrinkage – various materials with different functionality and molar masses were investigated here (Figure 2a). Acrylate monomers are added as reactive thinners (T) for decreasing the viscosity and tuning the network (crosslinking density) – here a trimethylol propane triacrylate compound with three reactive acryloyl groups and a variable number n of ethoxy spacers groups (TMP(EO)3nTA) was tested as well as hexanedioldiacrylate (HDDA) with two acryloyl groups (Figure 2b). In addition, mono- or tri-functional thiol monomers serve as reaction accelerators for improving the conversion and tuning the elasticity as well as preventing oxygen inhibition (Figure 2c). Finally, various anti-adhesion additives were tested, i.e. releasing agents (RA), most of them being either perfluoropolyetherbased (PFPE) or perfluoroalkyl-based (PFA) acrylates that become part of the polymer network


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and are needed to lower the resist’s surface energy (Figure 2d). Of course, a photoinitiator (PI) is necessary to start the radical polymerization reaction, where here, mainly Norrish type I materials such as α−hydroxyl ketones and phosphine oxides were tested.

Figure 2. Materials of the JRcure resist system. The abbreviations are displayed next to the structural formula, the reactive groups are highlighted by a red box: a) Aliphatic low molecular urethane acrylate oligomer (UAO) with a functionality of four (E8210 with ca. 600g/mol), b) Reactive thinners (T): Trimethylolpropane ethoxylated triacrylate (TMP(EO)3nTA), hexanedioldiacrylate (HDDA), c) Thiol monomers: trimethylolpropane tris(3-mercaptopropionate) (Trithiol), 1-dodecanthiol (Dodecanthiol) d) Releasing agents: 1H,1H,2H,2H-perfluoro-n-decylacrylate (AC812), perfluoropolyether-urethane dimethacrylate (MD700).


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A JRcure composition that is optimized as an imprint resist with high elasticity is called JRresist, whereas a JRcure composition optimized for a shim with high surface layer stiffness after patterning and curing is called JRstamp. Furthermore, a second imprint resist material class was developed – JRlift, a UV-curable, but still lift-off capable resist. This JRlift resist (not shown in Figure 2, components are listed in Table S 1) consists of a non-crosslinking monoacrylate material with ultralow viscosity (acryloyl morpholine), thiol monomers for curing under atmosphere, a surfactant additive for adjusting the surface energy and a photoinitiator. JRlift represents a water-soluble resist composition, where the resist is used as a patterned sacrificial layer and can be lifted off after nanoimprinting.

Material selection for JRcure in terms of surface energy properties According to relation (1), the better the structure reproduction fidelity and thus the achievable critical dimension (CD), the lower the interfacial energy has to be. Furthermore, the force needed to separate the polymeric stamp from the cured imprint resist is reduced when they have equal surface energies ( ~ ) and the work of adhesion between them is low. Both requirements can be fulfilled by a minimized surface energy of the polymeric stamp that is close to that one of the cured imprint resist . For decreasing the surface energy, several UV-curing releasing agents were tested as additives in the JRcure matrix (see material list in Table S 1). The idea was to cover a wide range of surface energies by using additives with different hydrophobic groups like (per)fluoroalkyl, polydimethylsiloxane or perfluoropolyether groups. Concentration series of these additives were prepared in the matrix solution with the composition UAO:T:PI = 48.5:48.5:3 and the surface energy of the cured resists was determined as described in the Methods section. It should be


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mentioned that when adding efficient RAs the composition of the matrix itself has virtually no influence on . During the UV-curing (5 mW/cm2 for 1 min) different phases may be adjacent

to the resist’s surface and therefore may lead to different surface energy values

. The

adjacent phases are either cleaned native nickel oxide (NiOx) or NiOx surfaces treated with a self-assembled monolayer of fluorocarbon-based phosphonic acid (NiOx + SAM), if curing is done against a nickel shim surface, or Ar, if curing is done in inert atmosphere.55 The hydrophobic SAM decreases the surface energy from ~ 56 mN/m for NiOx to 13 mN/m for NiOx + SAM.

In Figure 3a

values (disperse and polar parts) of several JRcure resists, differing only

in the type of RA mixed at a mass fraction of 1% in the matrix (UAO:T:PI = 48.5:48.5:3), are compared for curing against Ar ( ) or NiOx ( ). It is clear that all additives induce lowering of the resist’s surface energy as compared to the one with the pure matrix, when curing is done in Argon atmosphere. Furthermore, some additives (MD40, MD700, PFPE-A1 and AC812) are more effective than others in lowering the surface energy. In contrast, none of the additives could decrease substantially, when NiOx was the adjacent phase during curing. In Table 1 the values of all investigated resist compositions are listed for the different curing situations (Ar, NiOx, NiOx+SAM) for an additive mass fraction of 1% in the standard matrix composition (UAO:T:PI = 48.5:48.5:3). The additives were classified according to their hydrophobic groups. Curing against the SAM-treated NiOx shim was done only for those additives that were more effective in decreasing the surface energy under Ar curing. In addition, from concentration series of the releasing agents, the turnover point , indicating at which mass fraction of RA the resulting surface energy of the cured resist becomes virtually constant, can be extracted.


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Figure 3. (a) Polar and disperse parts of the surface energy of JRcure resists for the tested releasing agents at a mass fraction of 1%. The curing was done against Ar atmosphere (left column) or against a cleaned native NiOx shim surface (right column) (b) Surface energy of the cured resist depending on the concentration of the additive perfluoropolyether acrylate (PFPE-A1) for different adjacent phases during UV-curing (Ar, NiOx and NiOx + SAM). (c) Determination of the concentration turnover point of the additive PFPE-A1 for different UVcuring situations.

!"

~ 0.25% for curing against SAM-treated NiOx. No is observed

for curing against NiOx, whereas = 0.016% for curing against Ar. Schematic depiction of the processes involved at the interface for curing of an additive containing resist against (d) a NiOx surface with ~ 56 mN/m (top) and (e) a NiOx surface covered with a fluorocarbonphosphonic acid SAM (see structural formula) with ~ 13 mN/m. Both the additive (e.g. MD700) and the SAM are schematically illustrated by polar heads (red for additive and blue for SAM) and non-polar curved tails.


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Table 1. Surface energy ( ) and calculated turnover point ( ) of JRcure resists for different adjacent phases during UV-curing as a function of the type of releasing agent (RA). is determined at 1% mass fraction of RA.

γrNiOx+SAM

[mN/m]

TPNiOx+SAM [%]

no4

50

0.25

14

12

no

48

1.30

24

0.22

12

no

51

no

40

2

0.04

12

no

52

0.40

18

PFA

2

0.56

22

no

49

-5

-

PFA

1

0.30

21

no

46

-

-

2

0.04

23

no

50

-

-

RA

Type

F1

TPAr [%]

PFPE-A1

PFPE2 + PFA3

1

AC 812

PFA

MD 40

γrAr [mN/m]

TPNiOx [%]

0.02

12

1

0.22

PFPE

2

MD 700

PFPE

PF 3320 PF 3510

BYK3500 PDMS6 Matrix

47

γrNiOx

55

[mN/m]

52

Three important conclusions may be drawn from a comparison of Figure 3a with Table 1. First, for curing in Ar the additives MD40, MD700, PFPE-A1 and AC812 all deliver resist compositions with very low surface energies < 14 mN/m. (Note that the surface energy value of PTFE is about 20 mN/m). However, the turnover point of the resist with the PFPE-A1 additive is the lowest, meaning that least PFPE-A1 is needed to minimize .

1

Functionality, number of reactive acryloyl groups per molecule

2

perfluoropolyether

3

perfluoroalkyl

4

no” means that no TP was observed

5

„-„ means that this condition was not investigated

6

polydimethylsiloxan


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Second, for curing against the NiOx surface, none of the agents were able to substantially decrease the resist’s surface energy , only a slight decrease of 5-15% compared to the pure JRcure matrix (with = 55 mN/m) is observed (Figure 3a). Obviously, it is energetically not very favorable for the non-polar tails of the surface active additives to adsorb at the phase border to the polar NiOx surface (with a high surface energy of = 56 mN/m). Consequently, no turnover behavior was observed in any of the JRcure formulations for curing against NiOx. Third, if the resist is cured against a SAM-treated NiOx, the surface energy

!"

could be

decreased for the additives PFPE-A1, AC812 and MD700, but most effectively for PFPE-A1, with

!"

approaching . No substantial decrease was observed for a resist containing

MD40 molecules. The interplay of these arguments motivated us to proceed mainly with PFPE-A1 as the releasing agent and in some cases with AC812. The dependence of the resist’s surface energy on the mass fraction of PFPE-A1, compared to the three different UV-curing situations, is detailed in Figure 3b. Curing against Ar results in a rapid decrease of the resist’s surface energy with increasing PFPE-A1 concentration, whereas no substantial decrease is observed for curing against NiOx. In contrast, the SAM-coverage of the nickel oxide surface induces a similar decrease of the resist’s surface energy (

!"

~ 14 mN/m) as curing against Ar. Figure 3c

illustrates the graphical determination of the turnover point following the method of Bresler and Hagen for extraction of the critical micelle concentration of surfactants in water.56 At the concentration the liquid resist’s surface is fully covered with the molecules of the additive. The as-extracted for the PFPE-A1 containing JRcure resist is

!"

= 0.25 % and

= 0.016 % for curing against SAM-treated NiOx and Ar gas, respectively. Over the whole


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investigated PFPE-A1 concentration range, no turnover behavior is observed for curing against NiOx. In Figure 3d,e a model for the processes that may occur at the interface during UVnanoimprinting is depicted. When the resist is in contact with Ar gas the amphiphilic molecules of the releasing agent (e.g. MD700, as drawn in Figure 3d) form a monolayer at the liquidgaseous interface, with the hydrophobic tails pointing out into the gas phase57,58 (Figure 3d left). When a high surface energy NiOx stamp is brought into contact with the liquid resist, the molecules will have no energetic motivation to align at the interface with their non-polar tails pointing towards NiOx, the monolayer of RA molecules will disappear and the cured resist will show a similarly high surface energy as the NiOx stamp (Figure 3d middle, right). Finally, when the stamp is detached from the cured resist the substantial adhesion energy between the two high energy surfaces might result in sticking of the resist to the stamp. Contrary, if a SAM-treated NiOx stamp with its low surface energy is pressed into the liquid resist it is energetically more favorable for the RA molecules to remain at the interface with outpointing tails (Figure 3e left). Thus, after curing, a structured polymer resist layer is obtained covered with a fluorinated monolayer resulting in a surface with an energy almost as low as the one of the SAM-treated NiOx stamp (Figure 3e middle). This facilitates a sticking-free detachment of stamp and cured imprint resist (Figure 3e right). This process enables the design of resist surfaces with tunable and self-reproducing interface energies, meaning that the surface energy of the additive molecules containing cured resist is determined by the surface energy of the adjacent phase. The lower the surface energy of the adjacent phase is, the higher is the driving force for amphiphilic molecules in the liquid resist for adsorbing at the solid-liquid interface and the lower becomes the surface energy of the cured


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resist. Thus the surface energy of the master stamp material can be “copied” to the next generations being it polymer stamps or imprinted and cured resist layers. Accordingly, in a R2RUV-NIL process, as it is sketched in Figure 1b, it is decisive for high quality nanoimprinting to cure against a shim with an as low as possible surface energy.

Material selection for JRcure in terms of mechanical properties and conversion The mechanical characteristics of the UV-cured JRcure film are determined by the properties of the urethane acrylate oligomers, the reactive thinners, the thiols and the photoinitiator. Generally speaking, the nature and structure of the polymer backbone and the number of reactive acryloyl groups per molecule, parameterized by the functionality F, are the crucial factors. Long aliphatic or polyether chains are typically very flexible and may lower the Young’s Modulus, while for short chains or inflexible (e.g. cyclic) building blocks and moieties the opposite is the case. Furthermore, a higher functionality leads to a denser network of the polymer chains.42,59 In the following sections the influence of each JRcure component on the mechanical properties and the conversion is investigated and discussed. The simplest formulations only contain a thinner, a photoinitiator and an additive; the addition of UAOs and/or thiols allows for a further fine-tuning of certain properties. a.) Reactive thinners are monomers with varying functionality; they are much smaller molecules than the urethane acrylate oligomers. These compounds are used to lower the liquid resist’s viscosity to values well below 1 Pa⋅s, a necessity that enables the coating of JRcure films in the R2R machine by reverse gravure printing at high web speed. Another advantage of lowviscosity resists is the complete and fast filling of any stamp cavities. The used thinners are based either on HDDA with F = 2 or on TMP(EO)3nTA with F = 3.


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Figure 4. (a) Reduced modulus E* (right axis) and degree of double bond conversion DBC (left axis) for a JRcure formulation with base composition 96.5% TMP(EO)3nTA, 3% KL200 and 0.5 % PFPE-A1 plotted in dependence of the number of ethoxy groups (EO) per side chain n; Curing: 5 min at 5 mW/cm2. (b) Dynamic viscosity η for concentration series of different UAOs (E8210, G4425), reactive thinner (TMP(EO)9TA) and trithiol in a liquid matrix of TMP(EO)3TA, 5% KL200 and 0.5% PFPE-A1, Curing: 5 min at 5 mW/cm2. (c) DBC and reaction kinetics for a concentration series of E8210 in TMP(EO)3TA, 0.5% PFPE-A1 and 3% KL200; Curing at 5 mW/cm2.The curves are fitted according to the equation DBC (t) = DBCmax⋅(1-exp(t/τ)0.3). The inset illustrates the dependence of DBC20 on η. (d) Dependence of E* on the mass fraction of either the oligomers E8210 and G4425 or of the monomer TMP(EO)9TA or of Trithiol in the matrix TMP(EO)3TA, 5% KL200 and 0.5% PFPE-A1. The TMP(EO)3TA fraction was decreased according to the fraction increase of the other components; Curing: 5 min at 2.2 W/cm2. (e) IR spectra of JRcure samples irradiated with different doses. Composition of the samples: 35% E8210 in TMP(EO)3TA, 5% KL200, 0.5% PFPE-A1. (f) DBC and reaction kinetics for a Trithiol concentration series in the matrix TMP(EO)3TA, 0.5% PFPE-A1 and 5% KL200; curing with 5 mW/cm2. The curves are fitted according to the equation DBC = DBCmax⋅(1-exp(t/τ)β with β being either 0.3 (0%, 10%) or 1 (20%, 30%).


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TMP(EO)3nTA contains three reactive acryloyl double bonds and has variable ethoxy spacers, described by the parameter n (number of ethoxy groups per side chain with n = 0, 1, 2, 3). The aim was to select the best out of the four TMP(EO)3nTA components with regard to their influence on the mechanical properties of the stamp material and the degree of conversion of the cured resists. The mechanical properties of the resist after curing, parameterized by the reduced modulus E*, were determined by nanoindentation experiments (see Methods section), whereas the degree of double bond conversion DBC (correlating to a certain extent with the degree of crosslinking) was extracted from ATR-IR spectra (Attenuated Total Reflection-IR, see Methods section). In Figure 4a, both E* and DBC are compared for JRcure compositions based on 96.5% TMP(EO)3nTA, 3 % PI and 0.5 % RA, whereby the thinner has either none, one, two or three ethoxy groups per side chain. The diagram shows that DBC increases linearly with n in the range 0 ≤ n ≤ 2, but for the thinner with three ethoxy groups per side chain (TMP(EO)9TA) the conversion rate levels off and reaches a maximum value of DBC = 0.97. E* shows exactly the opposite behavior; it decreases linearly with n. This can be understood as follows: An increasing number of ethoxy groups per molecule enhances the mobility of the acryloyl groups and hence increases DBC. On the other hand, more ethoxy groups decrease the stiffness of the cured resist because of the flexibility of the polyethoxy chains and the increase of the mesh size. Accordingly, the cured resist’s modulus E* decreases with n (approximately linear). With respect to maximizing both the stiffness (proportional to E*) and the DBC there is a trade-off, parameterized by the number of ethoxy groups. For the preparation of polymer stamps, we therefore selected TMP(EO)3TA as reactive thinner in the JRstamp matrix. For formulations with this thinner we measured intermediate, but reasonable values for both, DBC and the modulus,


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namely E* ~ 1.4 GPa and DBC = 0.95 for 5 min curing at a power of 5 mW/cm2. In JRresist formulations, where a higher elasticity (smaller E* of the cured resist) is advantageous for defect-free demolding, TMP(EO)9TA might be the better choice as the thinner. b.) For the urethane acrylate oligomers (UAO) various materials (Table S 1) were investigated in view of the influence of their functionalities and molar masses on DBC, E* and the dynamic viscosity η. The results of these investigations are compiled in Table S 2 and are discussed in detail in Chapter 2 of the Supporting Information. In short summary, higher functional UAOs induce higher stiffness in the cured resists stemming from a denser polymer network, which might be advantageous for stamps. But simultaneously these higher functional UAOs hardly reach conversion levels of more than 60% (DBC < 0.6). Obviously, there are opposite dependencies of DBC and E* on the functionality of the UAOs. Good compromise candidates are the aliphatic urethane acrylate oligomers E8210 and G4425 with F = 4. As far as the liquid resist’s viscosity is concerned, the UAOs play a decisive role. For different liquid JRcure formulations the dependence of η on the respective mass fractions of (i) two different UAOs with the same functionality (E8210, G4425), of (ii) the monomer TMP(EO)9TA (at the expense of TMP(EO)3TA) and of (iii) Trithiol is plotted in Figure 4b. It is evident that η is solely determined by the mass fraction of the oligomers; an observation that is equally valid for UAOs with much higher molar masses (see Table S 2). In particular, η increases exponentially with the oligomer concentration, whereas it remains constant at a value around 60-70 mPa⋅s upon addition of the TMP(EO)9TA monomer with the long side chains or upon addition of Trithiol. At a mass fraction of 65% the resist with the E8210 molecules has a lower viscosity (η ~ 0.6 Pa⋅s) than the resist with the G4425 molecules (η ∼1 Pa⋅s), which is due to the lower molar mass of the former. E8210 has one of the smallest molar masses (600 g/mol) that


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are achievable for a urethane acrylate oligomer with four acryloyl groups. As such, E8210 would be the best choice for trouble-free coating in the R2R machine. Furthermore, the E8210 oligomer has a high enough functionality to allow for a reasonably high degree of conversion and also a sufficiently large modulus can be expected. In Figure 4c DBC is plotted as a function of irradiation time for a JRcure formulation with varying E8210 oligomer concentration (0% - 65%). The curing was done at a power of 5 mW/cm2. For all concentrations DBC20 - the DBC value after 20 min of curing - lies above 0.7. Please note, that DBC is influenced by the mobility and flexibility of the oligomer molecules in the JRcure matrix. The higher the concentration of E8210 the lower is the degree of conversion. This is attributed to the higher viscosity for mixtures containing more E8210 thus resulting in a decreased mobility of the radicals and an increased tendency to trap them in the jellifying polymer network. Consequently, DBC20 decreases linearly with the η as is displayed in the inset of Figure 4c. In such complex multifunctional acrylate resist systems the kinetics of the polymerization and crosslinking reaction is described by a stretched exponential function (fitted lines in Figure 4) according to the equation DBC ~ DBCmax (1-exp(t/τ)β) .60-63 This function is characterized by the relaxation time τ, by DBCmax, which is the maximum achievable degree of conversion and the exponent β describing the self-limiting polymerization as a fractal-time stochastic process. More details on stretched exponentials are found in Chapter 3 of the Supporting Information (see Table S 3). Concerning the influence of the UAO concentration on the mechanical properties, nanoindentation experiments reveal a linear correlation between oligomer content and stiffness; the higher the mass fraction, the higher E* (filled circles and squares in Figure 4d). The weakening influence of the UAO concentration on DBC was circumvented by using a significantly higher irradiation power (2.2 W/cm2) as provided by the UV lamp in the R2R 21 Environment ACS Paragon Plus

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machine. It has to be emphasized that DBC increases significantly with the irradiation dose. With the high power lamp a practically 100% converted JRcure resist (DBC ~ 1) was achieved showing almost no dependence on the E8210 mass fraction (see Figure S 1 in Chapter 4 of the Supporting Information). The dramatic influence of the irradiation power is also reflected in the decrease of the C=C peak at 810 cm-1 with increasing dose and its complete disappearance for samples irradiated over 10 min at 2.2 W/cm2 in the R2R machine (black line in Figure 4e). This sample has no measurable C=C double bonds from acryloyl groups at the surface. A comparison of Figure 4a and Figure 4d reveals, that the increase of the irradiation power from 5 mW/cm2 to 2.2 W/cm2 results in a jump of the modulus of the oligomer-free JRcure composition from 1.4 GPa to 3.6 GPa. Finally, increasing the UAO content from 0% to 80% yields a linear increase of the modulus from 3.6 GPa to 5 GPa (Figure 4d). As expected, increasing the ratio of TMP(EO)9TA to TMP(EO)3TA leads to a reduction of the materials’ modulus after curing (stars in Figure 4d). The obtained inverse linear relationship between the mass fraction of TMP(EO)9TA and E* is consistent with the inverse linear correlation between chain length and stiffness (filled circles in Figure 4a). The more ethoxy groups per side chain, the longer are these flexible links resulting in a wider network mesh and thus a more flexible and less stiff material. A resist with 94.5% TMP(EO)9TA has a modulus of only E* = 0.3 GPa. More aspects of the influence of the irradiation power and type of thinner on the modulus are discussed in Chapter 4 of the Supporting Information. To summarize our findings on the UAOs, JRstamp formulations will always contain a substantial amount of urethane acrylate oligomers with low molar mass and an intermediate functionality (F ~ 4) resulting in a low viscous and thus solution-processible resist that can be


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almost 100% converted and has a reasonably large modulus (> 4 GPa). For JRresist formulations the addition of oligomers is not beneficial in many cases. c.) As an alternative to pure acrylate-based systems also mixed thiol-acrylate UV-resists were investigated. In thiol-acrylate polymerization the thiol-ene step growth hetero-polymerization competes with the chain growth homo-polymerization of the acrylates and therefore delays the gel point.64 As a result, a higher mobility of the reactive species in the system and accordingly shorter reaction times and higher final conversion degrees are achieved. Furthermore, the stress built up in an acrylate network is reduced due to the delayed gelation. This leads to a more uniform network which manifests in a narrower glass transition region being shifted to lower temperatures. The latter also is expected to lower the Young’s modulus at room temperature. 64,65 In addition, the low shrinkage upon solidification of thiol-acrylate systems will guarantee a high patterning fidelity. Finally, thiol-acrylate systems also alleviate the problem of oxygen inhibition of free-radical acrylate polymerization. Trithiol (trimethylolpropane tris(3-mercaptopropionate) is chosen because of its similarity to TMP(EO)3TA both in structure and molar mass (see Table S 1). So the structure-related influence of the thiol on viscosity and diffusion mechanisms could be ruled out. Figure 4f shows the dynamic behavior of the conversion DBC(t) for a concentration series of trithiol in a matrix of TMP(EO)3TA, 0.5% RA, and 5% PI. As expected, the addition of thiol significantly increases DBC, for higher trithiol concentrations (≥ 20%) a completely converted system is achieved already after very short curing times. For JRcure compositions with more than 20% trithiol the reaction kinetics are described rather by a simple than a stretched exponential thus supporting the model of decreased volume shrinkage-strain in thiol-acrylate polymerization (see Table S 4 in Chapter 3 of the Supplementary Information). However, as is obvious from Figure 4d, the


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addition of trithiol at the expense of TMP(EO)3TA up to a mass fraction of 20% lowers the elastic modulus nearly equally as does TMP(EO)9TA. This is attributed to the narrowing of the glass transition region. At a higher trithiol concentration (> 20%) the modulus decreases rapidly to only E* = 200 MPa. A similar rapid drop of E* is observed for a trithiol concentration series in TMPTA (Figure S 2 in Chapter 5 of the Supporting Information). In thiol-acrylate photopolymerization a (linear) reduction of the glass transition temperature TG with increasing thiol to acrylate ratio was reported accompanied by a fast decrease of the modulus (up to two orders of magnitude).64 Hence, we believe that the decrease and sudden drop of E* with the addition of trithiol to the TMP(EO)3nTA matrix is caused by the reduction of TG. For the reasons described above, only a smaller amount of thiol (≤ 10%) is added to JRstamp formulations in order to get sufficiently hard polymer stamps, whereas higher thiol fractions can be advantageous for soft JRresist formulations. Here, in some cases (such as the R2R-fabrication of high aspect ratio structures or high resolution patterns with undercut details) fast curing, high elasticity and finally, a high degree of conversion are essential.

d.) The influence of the photoinitiator material on the reaction kinetics is described in Chapter 6 of the Supplementary Information (Figure S 3). KL200 is chosen as the initiator in most JRcure mixture (at concentration of 3-5%), because it is liquid and therefore easily soluble in the liquid resist.

Material selection for a lift-off capable imprint resist (JRlift) The essential part of the JRlift formulation is acryloyl-morpholine (ACMO), a water soluble mono-acrylate compound that is curable by UV-initiated free radical polymerization and has a viscosity as low as 12 mPa⋅s. Contrary to the trimethylolpropane family with F = 3, ACMO has


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only one reactive group and thus prevents crosslinking. As a consequence of the absence of crosslinking the resulting polymer is much softer and soluble in many solvents, particularly in water. JRlift has a reduced hardness well below 100 MPa, thus being an attractive candidate for a low modulus imprint resist. The good solubility of the cured JRlift in water is explained with the morpholine group being part of the ACMO monomer and its high polarity. Therefore the JRlift formulation can be liftedoff in water after being imprinted and UV-cured. Non-ionic surfactants (such as Triton X-100) are beneficial to adjust the surface energy of JRlift. The addition of a thiol monomer with only one thiol group (dodecanethiol) guarantees the acceleration of the polymerization during the fast UV-curing step by delaying the gelation of the polymerizing system and furthermore suppresses the oxygen inhibition without inducing a crosslinking of the acrylate group. The surface energy properties of JRlift are of particular interest. Since the ACMO monomer has a high polarity and thus surface energy, JRlift has a large contact angle with non-polar surfaces like the SAM treated NiOx (see Figure S 4 in Chapter 7 of the Supporting Information). The low viscosity of the material, combined with its small interfacial energy to the substrate allows for a complete resist displacement by the stamp during imprinting thus resulting in a residual-layer free imprint pattern. 48 The interfacial energies play a dominant role in residue-free imprinting because of the interplay between the surface energies of the flexible stamp (shim), the substrate material (PET) and the resist (JRlift), which is described by the spreading parameter during nanoimprinting #$% #$% = &'/ ) − &'/* +,- − * +,-/ )


(2)

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Equation (2) is an adaption of de Gennes’66 expression for the spreading parameter. During the coating step the substrate should be wetted by the resist, which is described by a positive spreading parameter #./ #./- = &'/ − &'/* +,- − * +,-/ ≥ 0

(3)

Accordingly, the resist should have a lower surface energy than the substrate and the resistsubstrate interfacial energy should be minimal, corresponding to a small contact angle of JRlift on the PET substrate (2 < 10° according to Figure S 4). As determined by contact angle measurements the surface energy of the PET substrate is &'/ = 48 89/8, the surface energy of JRlift is * +,-/ = 44 89/8 and the interfacial energy between PET and JRlift is &'/ = 4 89/8, so #./- ≅ 0, which is consistent with a wetting of the PET substrate by JRlift. On the other hand a stamp-induced dewetting during imprinting is only observed if the spreading parameter #$% of the system PET/JRlift/shim is negative according to #$% = &'/ ) − &'/* +,- − * +,-/ ) < 0

(4)

In Table 2 the surface energy values of the different tested shim surfaces like NiOx, SAMtreated NiOx (NiOx+SAM) and AlOx are listed as well as the interfacial energies of these shim materials with the PET substrate and with the imprint resist JRlift. Obviously, the shim-induced dewetting of JRlift during nanoimprinting (equivalent to #$% < 0) is fulfilled for all shim materials. This is attributed to the interfacial energy * +,-/ ) being high enough. However, for a * +,-/ ) , that is too high, we expect sticking of the resist on the shim due to a high adhesion energy WA between shim and resist. According to Table 2 WA is minimal for the SAM-


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treated NiOx surface. Therefore, the optimum resist-stamp combination, allowing for residue-free imprinting without sticking, is the combination of JRlift with a NiOx+SAM shim. Table 2. Surface and interfacial energies for different combinations of shims and imprint resist JRlift as determined by contact angle measurements. Shim

Surface energy mN/m

γshim/air

Interfacial energy

γshim/PET

Interfacial energy

γJRlift/shim

Spreading Parameter S NIL

Adhesion Energy WA

NiOx

56

1

39

- 42

2⋅√44⋅√56 = 99

NiOx+SAM

15

9

11

-6

2⋅√44⋅√15 = 51

AlOx

42

2

22

-24

2⋅√44⋅√42 = 86

Apart from surface energy considerations, residual layer-free imprinting also needs a low viscosity resist to enable an easy displacement of the resist by the stamp. As was reported by Choi et al. 49 and Martin et al. 67 the resist dewetting starts as a hole-type dry patch and grows at a velocity V that is given by =

|!@AB |C

>

∙

;∝ ∙

(5)

Here the resist viscosity η, the shim modulus E, the film thickness h and the spreading parameter #$% are the relevant parameters. The lower the viscosity and the more negative #$% , the faster and easier is the dewetting. Since the viscosity of JRlift is in the range of 12 mPa⋅s and therefore much lower than that of most commercially available imprint resists, the dewetting process is considerably accelerated. A sufficiently high negative spreading coefficient combined with a low shim surface energy will promote residue-free imprinting in a fast roller-based process under avoidance of resist-to-shim sticking. Finally, also a small resist layer thickness and


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a perfect adjustment of h with the height of the elevated shim structures R0 are needed.47 Both dimensions should lie close together, since shim structures which are too deep w.r.t. the layer thickness could trap air bubbles, whereas stamp structures too flat will not offer enough free volume to resorb the displaced resist. It is also interesting to note that a very high stiffness of the shim (high modulus E) is expected to decrease the upper velocity limit for residue-free imprinting.

In summary, several resist compositions for usage either as polymer stamp material (JRstamp), as nanopatterned resist layer (JRresist) or as lift-off capable imprint resist (JRlift) were derived that are all adjusted to R2R-nanoimprinting: Cured JRstamp mixtures with 35-80% E8210 in TMP(EO)3TA (or HDDA) with 3-5% photoinitiator and 0.5-1% releasing agent showed moduli in the range 4 ≤ E* ≤ 6 GPa and full conversion (DBC ~1) as well as a sufficiently low viscosity (η < 1 Pa⋅s) in the uncured state, thus making them a good choice as a polymer stamp material. The addition of PFPE-A1 or AC812 as releasing agent lowers the surface energy and guarantees defect-free demolding of the cured imprinted resist film from SAM-treated NiOx surfaces. In contrast to the polymeric stamp the stiffness of the JRresist should be small to enhance clean separation and enable 3D patterning. For R2R imprinting and web speeds up to 15 m/min JRresist formulations with about 0-20% UAO in TMP(EO)9TA with 3-5% photoinitiator (KL200) and 0.5-1% releasing agent (PFPE-A1, AC812) were tested successfully as nanoimprinted resist layers. Low modulus, low viscosity as well as fast and complete conversion are provided also by oligomer-free thiol-acrylate resist systems with 10-20% Trithiol in a matrix of TMP(EO)6TA, 3% photoinitiator and 0.5% additive. Moreover, curing under ambient


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atmosphere is possible in a thiol-acrylate material due to the suppression of oxygen inhibition by the thiol groups. The JRlift formulation is based on a solution of 74.5-89.5% ACMO with 5-20% dodecanethiol, 5% photoinitiator and 0.5% additive and is especially attractive, since it constitutes a water-soluble, lift-off capable UV-curable imprint resist material with very low viscosity. Due to its potential for residue-free imprinting the JRlift makes a subsequent residual etch step obsolete which is a big advantage in R2R fabrication. Thus the JRlift allows for direct translation of high resolution imprint patterns in other functional layers (such as metals).

Micro- and nanopatterning of JRcure by R2R-UV-NIL Figure 5 illustrates the excellent self-reproducibility of the JRcure resist system by means of a gallery of Scanning Electron Microscopy (SEM) images. A JRcure formulation with high mass fraction of oligomer (JRstamp1) and a rather stiff monomer thinner is used for the batch fabrication of a polymer shim from a small scale silicon master with critical dimensions (CD) down to 200 nm and an aspect ratio of 5:1 (Figure 5a).. This pattern with small protruding (male) structures on the polymer shim is perfectly replicated in the somewhat softer JRstamp2 material with the more flexible TMP(EO)3TA thinner (Figure 5b). In Figure 5c line/space structures in JRstamp1 with CD = 200 nm and a height of 1000 nm are shown as they appear on a polymer shim that is later used for R2R-UV-NIL. A typical detail of ingoing (female) 200 nm structures replicated in JRstamp2 by R2R-UV-NIL at a web speed of 5 m/min is displayed Figure 5d. The large-area polymer shim was fabricated by a manual step and repeat process as described in the Methods section (see Figure S6 in the Supporting Information).


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Figure 5. SEM images of imprinted structures illustrating the self-reproducibility of the JRcure material system. (a) First imprint in JRstamp1 (50% E8210, 46.5% HDDA, 3% KL200, 0.5% PFPE-A1) yielding polymer shim A with 200 nm wide and 1000 nm high male structures (line height: line width = 5:1 = aspect ratio) and vertical sidewalls (90°). (b) Second imprint with polymer shim A in JRstamp2 (50% E8210, 46.5% TMP(EO)3TA, 3% KL200, 0.5% PFPE-A1) yielding female structures with CD = 200 nm and aspect ratio 5:1, the inset is a zoom to the center. (c) Male structures in JRstamp1 resist with CD = 200 nm and 1000 nm height on a polymer shim B used for R2R-UV-NIL. The sample is tilted by 30°. (d) Female structures with CD = 200 nm done by R2R-UV-NIL with polymer shim B at 5 m/min in a JRstamp2 formulation.


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The homogeneity and high pattern quality extending all over the width of the web prove a perfect tuning of shim and resist materials in roller-based nanoimprinting. Apart from the very low interfacial energy between JRstamp1 and JRstamp2 limited polymer shrinkage further facilitates the defect-free demolding and is therefore advantageous for R2R-UV nanoimprinting. Although the lateral structure dimensions are perfectly reproduced in R2R, AFM profiles unveiled ~ 5% height shrinkage upon UV-curing. The mechanisms behind polymer shrinkage will be investigated in some detail in an upcoming publication. Apart from the molding of high resolution patterns at high throughput we succeeded to fabricate more complex 3D-like structures with the JRcure resist system at lower speed (~1 m/min). In Figure 6a-d the replication of a natural diatom template (Actinocyclus ehrenbergii) in a soft formulation of JRresist is shown. The second imprint (Figure 6d) represents a perfect replication of the natural diatom structure (Figure 6a) without any loss in detail. As illustrated in the close-up (Figure 6c) of the first imprint even slightly undercut features and bumps with just a few hundreds of nanometers in dimension are reproduced. Finally, the replication of undercut features on a curved surface with dimensions down to 300 nm in a soft JRresist formulation is demonstrated for artificial diatom skeleton templates (Figure 6e, fabricated by 3D laser lithography).68 By means of SEM images the development of a round lens-shape structure with triangular grating is illustrated throughout the multistep nanoimprinting process (Figure 6e-h). It is remarkable how perfect the inversely curved 3D features were transferred in the JRstamp material (Figure 6f,g). This is only possible for a material with low surface energy after curing and enough elasticity to allow for a defect free separation of template and JRstamp layer. By means of such a JRstamp tool the 2. imprint of the 3D lens-shape structure in the JRresist layer looks basically identical to the template (Figure 6h).


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Figure 6. Multi-step nanoimprinting of complex 3D structures inspired by diatom skeleton geometries. In all examples the polymer shims (1. imprint) are based on a JRstamp formulation with 35% E8210, 59% TMP(EO)3TA, 5% KL200, 1% PFPE-A1, whereas the 2. imprints are done in the much softer JRresist formulation 76% TMP(EO)6TA, 20% Trithiol, 3% KL200, 1% PFPE-A1 using shims from the 1. imprint. a) Detail of a frustule from Actinocyclus ehrenbergii, a natural diatom, serving as an imprint template, b) 1. imprint of the natural diatom skeleton in JRstamp, and c) detail of the 1. imprint, d) 2. imprint of the diatom skeleton, e) Template with an artificial diatom skeleton with a curved surface fabricated by 3D laser lithography, f) 1. imprint of an artificial diatom skeleton imprinted in JRstamp, g) detail of the 1. imprint illustrating the replication of structures on a curved surface with dimensions down to 300 nm and h) 2. imprint in the soft JRresist formulation.


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The JRcure resist system also supports the R2R fabrication of high-aspect ratio structures with undercut features, as they are found on a silicon template patterned by e-beam lithography. The Si-spikes (Figure 7a) are transferred into a JRstamp layer on a PET film, which serves as a polymer shim for the roller-based imprinting of a soft and highly elastic oligomer-free JRresist formulation (Figure 7b). According to the SEM images (Figure 7c) the reproduction of the highaspect ratio (4:1) spikes with submicron-sized undercut features in JRresist by R2R-UV-NIL is nearly perfect.

Figure 7. Imprinting of complex 3D structures by means of R2R-UV-NIL at 1 m/min. The polymer shims (1. imprint, not shown here) are imprinted in a JRstamp formulation with 35% E8210, 59.5% TMP(EO)3TA, 5% KL200, 1.3% AC812, whereas the 2. imprints are done in the much softer JRresist formulation 93.7% TMP(EO)9TA, 5% KL200, 1.3% AC812. a) Si-template including a regular spike pattern, each spike has an aspect ratio of 4:1, b) 2. imprint in the soft JRresist layer by means of the polymer shim, c) detail of two R2R-imprinted spikes illustrating the perfect replication of undercut features with aspect ratio 4:1.

The potential of the JRlift resist for residual-layer free imprinting enables us to fabricate highresolution metal patterns in a high-throughput process by supplementing the R2R-UV-NIL step with a metallization step and a subsequent resist lift-off. The metallization, consisting of a 3 nm - thick chromium and 30 nm - thick aluminum layer, is done by thermal evaporation under


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high vacuum. A SEM-cross section of the R2R-imprinted JRlift resist edge on the PET substrate is shown in Figure 8a, b. The zoom in Figure 8b proves that in the imprinted region the resist is fully displaced by the protruding part of the SAM-treated NiOx shim. No trace of a residual JRlift layer is visible in the SEM image. Accordingly, it is possible to transfer the resist pattern inversely into a metal layer without any need of a prior residue etch step. For pattern transfer the JRlift is lifted-off in water, thus leaving the metal layer only in the imprinted regions. This metallization process (see Methods Section for more details) based on a R2R-UV-imprint at 10 m/min, was used to fabricate high-resolution metal structures on PET substrates with line/space and grid design and with CD = 400 nm (Figure 8c,d) or 200 nm (Figure 8e,f). We note that the high-resolution metallization process also works, when not only the UV-NIL step, but also the metallization and the lift-off are done in R2R. High-throughput fabrication of metal patterns with sub-µm dimensions on large area, flexible substrates provides an avenue to manifold applications ranging from metal-grid based transparent conductive substrates and electrode layers, over printed electronic devices with finely spaced electrodes to films with plasmonic structures applicable in versatile optoelectronic and biosensor components.


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Figure 8. High-resolution metal patterns based on residual-layer free R2R-UV-NIL of JRlift (89.5% ACMO, 5% dodecanethiol, 5% KL200 and 0.5 % Triton X-100), the web speed was 10 m/min. A SAM-treated NiOx shim was used as the stamp. a) SEM cross section of the imprinted JRlift layer and b) zoom of the imprinted resist edge, illustrating that no trace of resist is observed in the imprinted region, c) Microscope image of aluminum lines with 400 nm line width after lifting-off the R2R-imprinted JRlift layer, d) SEM image of an aluminum grid pattern with CD = 400 nm, e) Microscope image of aluminum lines with 200 nm line width after liftingoff the R2R-imprinted JRlift, f) SEM image of an aluminum grid pattern with CD = 200 nm. 35 Environment ACS Paragon Plus

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Finally, the application of flexible film surfaces comprised of multi-length scale structures for large-area biomimicry is shown by means of a plastic film containing a shark-skin inspired riblet surface that was fabricated by R2R-UV-NIL at high throughput. Such riblet surfaces are known to reduce the viscous drag of fast moving objects both in air and fluids. 14, 69 Overall, the riblets shown in Figure 9a have a triangular cross section with micrometer dimension, but each rim is very sharp with a radius of only 170 nm (Figure 9b and inset). These multi-length scale riblets are replicated at 15 m/min in a hydrophobic JRstamp formulation in order to get a drag-reducing superhydrophobic surface. Indeed, placing a water droplet onto this surface reveals its water repelling nature showing a water contact angle well above 160° (Figure 9c) with a roll-off angle below 2°. Moreover, in a microscopic side view the droplet is sitting on the riblets’ rims without penetrating through as is expected for the Cassie regime (Figure 9d). In Chapter 8 of the Supplementary Information a video is included that shows water droplets dancing upon a R2Rfabricated riblet resist surface on a 150 mm wide film. Such riblets can be patterned in a very fast and cost-effective way over 100s of square meters resulting in a large-area film surface which combines two functionalities – drag-reduction and water repellence.


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Figure 9. (a) SEM image of multi-length scale riblet structures in a hydrophobic JRstamp formulation (44% E8210, 50% HDDA, 5% KL200, 1% PFPE-A1) fabricated by R2R-UV-NIL at 15 m/min on a 250 mm plastic web, (b) Close-up of the rim of the riblets, the inset represents a transmission electron microscopy (TEM) image of the rim cross section (prepared by Focused Ion Beam), the small circle has a radius of 170 nm, (c) Water droplet on the riblet surface illustrating the film surface’s superhydrophobic nature, (d) Microscopic side view of a water droplet sitting on the rims of the riblets.

CONCLUSION In summary, this work illustrates the power of R2R-UV-NIL as a versatile tool for high-speed continuous fabrication of micro- and nanopatterns with high aspect ratios, high resolution and 3D-like form factor on flexible substrates. Decisive for a successful roller-based imprinting


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technology was the design and the realization of a tunable and flexible resist system, the JRcure system. By varying the respective mass fractions of oligomers, reactive thinners and thiols and adapting the curing conditions, the elastic modulus could be varied over two orders of magnitude (between 50 MPa and 5 GPa). The addition of UV-curable antiadhesive additives influences the surface energy of the UV-cured imprinted resist and enabled to copy the surface energy from shim to the first JRcure layer and from one JRcure layer to the next. Accordingly, it is possible to self-reproduce flexible JRcure polymer shims (JRstamp) and to change the JRcure surface energy from very hydrophilic (60 mN/m) to very hydrophobic (12 mN/m). Finally, the introduction of a low-viscous, oligomer-free imprint resist (JRlift) that allows for residue-free imprinting and water-based lift-off enables the R2R-fabrication of fine metal patterns with critical dimensions down to 200 nm on large flexible substrates. Large-area superhydrophobic film surfaces exploiting imprinted multi-length scale structures demonstrate the application potential of high-throughput R2R-UV-NIL in biomimicry.


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METHODS/EXPERIMENTAL Releasing agents The concentration series of several releasing agents were prepared with a defined matrix solution (48.5 % UAO (E8210), 48.5% thinner (HDDA), 3% photoinitiator (KL200)) by dilution series. The investigated releasing agents and their suppliers are listed in Table 1 and Table S 1. A defined amount of releasing agent was placed in a bottom flask using analytical scales and filled up with matrix solution to obtain a certain mass fraction. The dilutions were prepared by thinning with matrix solution. The particular mixtures containing releasing agent were stirred, with a certain amount afterwards transferred to a bottom flask, and then finally a defined mass of matrix solution was added to obtain a certain mass fraction. Some droplets of the corresponding solution were placed on a smooth piece of cleaned nickel, or SAM-coated nickel or aluminum and then covered with a polyethylenetherephtalate (PET)-film. Afterwards the resist was cured for one minute at 5 mW/cm2 with a Hg-vapor-lamp (Waldmann UV 236 A from Waldmann Medizintechnik, λ > 300 nm). Finally the cured resist was peeled off together with the PET substrate. In a second experiment droplets of the solutions were placed on a glass substrate and distributed with a bar-coater leading to a liquid film thickness of about 10 µm. These samples were cured under the same irradiation parameters, but under argon atmosphere to avoid oxygen inhibition.

Surface treatment of nickel shims Nickel shims were covered with SAM forming molecules to obtain a hydrophobic surface. At first the pieces were treated with oxygen plasma for 10 minutes at a power of 200 Watt and a


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plasma pressure of 0.02 mbar in oxygen plasma chamber (Prep-2). The phosphonic acid derivates (1H,1H,2H,2H-perfluoro-n-decylphosphonic acid and 1H,1H,2H,2H-perfluoro-noctylphosphonic acid) used in this work were dissolved in isopropanol to get solutions with a molar concentration of 2 mmol/L. Then the plasma treated nickel pieces were put into this solution for one hour, afterwards rinsed with isopropanol and dried in an oven for 10 minutes at 150°C. After this treatment the surface energy of SAM-covered NiOx pieces decreased from about 56 mN/m to 13 mN/m.

Contact angle measurements Concentration series of the investigated releasing agents were prepared with an acrylate based matrix solution as described above. The obtained liquid resists were cured in argon atmosphere and the surface energies of the obtained polymeric films were measured by means of the contact angle method. The contact angles of water and diiodomethane droplets on the cured films were measured with the Drop Shape Analyzer DSA 100 from KRUESS at the surface of the cured resists. With the measured contact angles (three measurements with each liquid were done per sample) the dispersive and polar part of the cured resist’s surface energy were calculated according to the OWRK model 54 based on the known values for the surface energy of water and diiodomethane. 70

Alternatively, the model of Wu, which takes the harmonic instead of geometric mean between

disperse and polar components, can be used for the calculation of interfacial energies in the low surface energy regime 54.


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Dynamic viscosity The dynamic viscosity of several resist formulations was measured using a MC 200 rheometer from Paar Physika with a cone-plate system. The cone’s diameter was 50 mm, with a 5°angle with respect to the plate. The measured temperature was 25°C and the shear rate observed ranged from 10 sec-1 to 1000 sec-1.

ATR-IR spectroscopy The degree of double bond conversion DBC was determined with IR spectroscopy. Infrared spectra were recorded with a Tensor 27 Attenuated Total Reflection (ATR)-IR spectrometer from Bruker Optics using a diamond crystal as ATR element. The DBC was determined by using equation (i), where A(0)810 and A(0)1720 represent the areas of the peaks in the spectrum at 810 cm-1 and 1720 cm-1 before curing. DBCGt) =

IGJ)KL ⁄IGJ)LMC NIGO)KL ⁄IGO)LMC

IGJ)KL ⁄IGJ)LMC

(S1)

A(t)810 and A(t)1720 represent the peak areas at the same wavenumbers after the curing time t. The peak at 810 cm-1 is related to stretching of C=C bonds and decreases with curing time, and the absorption peak at 1730 cm-1 results from stretching-oscillations of the C=O groups. 71 The C=O groups do not take part in the polymerization reaction, thus the resulting peak from these groups does not change with increasing curing time and its area serves as a reference. The samples for the IR-spectroscopy were all prepared in the same manner with slightly different spin coating parameters resulting in comparable thickness values between 13 - 16.5 µm. A PET-film (Melinex from Dupont) was fixed to microscope cover glasses (15 ×15 mm2) by means of a double-faced adhesive tape. On the surface of the PET-film, a liquid resist film (250


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µl) was spun. To get a value for the thickness of the cured films the same procedure was done without putting PET-film onto the cover glass slides. After curing for one minute (5 mW/cm2, Ar atmosphere) the obtained thickness was measured with a surface profiler (Dektak 150 from Veeco).

Nanoindentation for determination of reduced modulus To obtain comparable data all samples were produced the same way: Some droplets of the liquid resist were placed on a glass slide. A PET-film was not used, because the flexibility of this material led to unreproducible results in nanoindentation experiments. A liquid thickness of 10 µm was achieved by using a bar coater (Model 358 from Erichsen). Afterwards the samples were cured for 5 minutes at 5 mW/cm2 with the Waldmann UV-source under argon atmosphere. Another set of samples was pre-cured for 1 minute at 5mW/cm2 and then mounted on the imprint cylinder of the R2R-UV-NIL-module in front of the high power UV-lamp (IST). Now curing was performed at 2.2 W/cm2 for 0.5-10 minutes under nitrogen flow (

Multilength Scale Patterning of Functional Layers ... - ACS Publications

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