The nanopatterning of surfaces and thin films with pattern dimensions of less than 100 nm is challenging for laser processing in particular in the case of large-area, low-cost fabrication. Self-assembly processes, however, provide a mechanism of pattern generation in this dimensional range offering an alternative fabrication method. The current work focuses on high-temperature, short-time laser annealing of PS-b-PMMA block copolymer (PS-b-PMMA BCP: poly(styrene-block-methyl methacrylate)) films on fused silica samples to achieve self-assembly into vertical lamellas with periods of approximately 50 nm. BCP samples were irradiated with a focussed CO2-laser beam for studying the influence of the laser power P-L and the scanning speed vs on the lamellae formation in BCP films. The formation of lamellae is observed in the centre of the laser track at sufficient laser irradiation (PL: 2 to 15 W, vs: 1 to 250 mms(-1)). With increasing laser irradiation, first the quality of the lamellas improves to a certain point but thereafter a partial degradation of the BCP and dewetting of the BCP film occurs. The partial degradation of the PMMA micro-phase of the ordered BCP results in a local self-developing process reducing the processing steps for nanopattern formation. The experimental results on laser-induced local self-assembly with irradiation times below 0.1 s are discussed in relation to laser-induced temperature field simulations. Combining self-assembly capabilities of BCP with local heating by laser beams can provide a tool for direct writing of hierarchical nano-/microscale patterns that is useful for various applications mimicking bio-inspired structures.

Guiding of the phase separation of a block copolymer (BCP) by an electric field perpendicular to the substrate is investigated in order to obtain vertical structures that can provide a mask for subsequent etching. Because of practical aspects, the substrate is bare Si without any neutral brush and the process time is limited to 1 h. A polystyrene-block polymethylmethacrylate lamellar material is employed in the study. For a unique guiding of the lamellar phase, an ordering mechanism orthogonal to the electric field is introduced by the interaction with the stamp in a thermal nanoimprint process. The naturally low surface energy of the stamp shall induce the formation of lamellae along the sidewalls of linear cavities. In order to fully utilize these two ordering mechanisms, the stamp sidewalls and the electric field, the imprint process is conducted in such a way that no residual layer remains below the stamp structures and the whole BCP is accumulated inside the cavities which are just partly filled. The electrically-assisted imprint process is studied analytically, considering the capacitive effects due to the local electric field in the cavity and in particular in the BCP. In addition, a numerical simulation is performed for the actual experimental conditions to compute the electric vector field in the BCP. In this way, an extensive understanding of the situation is gained which is the basis for choosing optimal experimental conditions for electrically-assisted thermal nanoimprint. Furthermore, the ambiguity of the electric field in a thermal nanoimprint process with partly filled cavities is addressed. The field shall induce vertical phase separation but, due to instabilities, it also may induce capillary bridges that represent replication defects. An improvement of the vertical phase separation by applying an electric field as high as 25 V/μm could be identified under specific experimental conditions. However, the guiding effect within the cavities and thus the long-range order of the lamellae remained limited. This may be due to a field strength too low in the BCP; in the present configuration, higher field strengths are prohibited by an electrical breakthrough.

Flexible composite stamps are commonly used in low pressure and low temperature processes, e.g., UV-assisted nanoimprint, as they provide a good conformal contact between stamp and substrate. The composite stamps investigated here consist of two layers, a thin hard top layer to enable stable nanometre-scaled structures with high aspect ratio and a soft backplane to ensure conformal contact. Stamps with two different material combinations were investigated, OrmoStamp /PDMS (polydimethylsiloxane) and h-PUA/s-PUA (polyurethane acrylate). The stability of both composite stamps was tested under harsh imprint conditions in a process at elevated temperature and pressure. Temperature and pressure loading results in strain in the top layer and may lead to break when the tensile strength is exceeded. To vary the stress level in the top layer, two different stamp designs were investigated, one with a thin backplane and one with a thicker backplane. The experimental results clearly show that a thin stamp (low stress level) is more stable than a thick stamp. Moreover, composite stamps are also suitable for a thermal imprint process when temperature and pressure remain limited.

Perovskites have high potential for future electronic devices, in particular, in the field of opto-electronics. However, the electronic and optic properties of these materials highly depend on the morphology and thus on the preparation; in particular, highly crystalline layers with large crystals and without pinholes are required. Here, nanoimprint is used to improve the morphology of such layers in a thermal imprint step. Two types of material are investigated, MAPbI3 and MAPbBr3, with MA being methylammonium, CH3NH3+. The perovskite layers are prepared from solution, and the crystal size of the domains is substantially increased by imprinting them at temperatures of 100–150 °C. Although imprint is performed under atmospheric conditions which, in general, enhances the degradation, the stamp that covers the layer under elevated temperature is able to protect the perovskite largely from decomposition. Comparing imprinting experiments with pure annealing at a similar temperature and time proves this. Furthermore, imprint is capable of patterning the surface of the perovskite layers; lines and spaces of 150 nm width were reproducibly obtained under imprint at 150 °C. Moreover, a through-layer patterning is possible by using the partial cavity filling approach. Although not yet optimized, this simple way to define isolated perovskite patterns within a layer simply by thermal nanoimprint is of impact for the preparation of devices, as patterning of perovskite layers by conventional techniques is limited.

The directed self-assembly (DSA) of block copolymers (BCP) has attracted high interest for the definition of nanostructures in an almost self-forming way when adequate boundary conditions are given. At present, grapho- and chemo-epitaxy are the workhorses but they require precisely patterned substrates to serve as the guiding pattern. Nanoimprint may replace this laborious pre-patterning of each substrate by employing an adequate stamp that can be used multiple times, inducing the guided DSA from the top of the film. Here, the DSA of BCPs is revisited in view of the specific nanoimprint situation. As a consequence, the BCP layer is imprinted in a partial cavity-filling mode, using a stamp of sufficient height provided with a conventional anti-sticking layer; substrate pre-treatment is minimized or rather avoided. Even with a highly preferential substrate it is possible to obtain vertical lamellae that are largely oriented in parallel to the stamp edges in PS-b-PMMA (polystyrene-block-polymethyl methacrylate) already after 3min of imprint. The vertical lamellae are at least 70nm high, freestanding on the substrate. Though optimization is required the results indicate the high potential of nanoimprint to simplify the DSA of BCPs for technical applications, also beyond Si technology.

Replica stamps fabricated of cross-linked materials are commonly used for nanoimprint lithography. Their mechanical properties depend on the preparation conditions and on the configuration used for curing as well. Typically, curing of a thin layer goes hand in hand with a shrinkage in the vertical direction. In a curing situation where this shrinkage is hindered, the cross-link density is reduced, resulting in a lower modulus. With respect to this issue, three cross-linkable materials are investigated, sylgard-polydimethylsiloxane, ultraviolet-polydimethylsiloxane, and OrmoStamp. The modulus is determined from the deflection of two-layer samples under temperature loading. Curing is performed under varying processing conditions; moreover, the impact of hindered shrinkage is addressed by experiments holding the layer thickness fixed during curing, a situation typical with automated stamp replication. The hindered shrinkage results in a modulus reduced by about 20%. The results obtained are vital for reproducible and comparable stamp preparation, beyond a single laboratory. A thorough and complete specification of replica stamp preparation does not only require information on the conventional processing parameters; it requires information on the curing configuration as well.

The preparation of transparent complex 3D structures over large areas at low cost paves the way for numerous optical applications. We prepared nanostructures on the surface of microstructures in a negative tone photoresist by a double imprint process combined with VUV-induced surface hardening. Successful preparation of the aforementioned structures asks for a compromise between the surface hardening of the nanostructures and the replication fidelity of the microstructures. Our results show that a VUV treatment time of at least 3s is required so that the cross-linked layer does not break-up. Adequate process parameters as well as their limiting values were identified by using high aspect ratio microstructures. The complex 3D structures were successfully replicated in UV-PDMS to provide long-term stable templates. To ensure effective separation despite of the undercuts a sacrificial mould technique is applied.

To improve the optical performance of Fresnel lenses, a technique for preparing them with antireflective structures of the moth-eye type is developed. Masters featuring such hierarchical structures are prepared in SU-8, a negative tone photoresist, by two consecutive thermal imprint steps. The moth-eye structures imprinted first are vacuum ultraviolet-treated at 172 nm to provide a surface-near the cross-linked layer that remains stable during the second imprint of the 100 μm sized Fresnel structures. A successful combination of both structure types is possible at an imprint temperature as low as 45 °C. This can be understood on the basis of the typical exposure and the crosslinking behavior of a chemically amplified negative tone resist like SU-8. The masters prepared in this way will be subjected to extrusion coating, the process of choice for future large area preparation of such structures in a single step.

Semicrystalline Reg-P3HT (regio-regular poly-3-hexylthiophene) is a promising material for organic electronics. It features relatively high charge mobility and enables easy preparation because of its solubility. Due to its high optical and electrical anisotropy, the size, number and orientation of the ordered domains are important for applications. To control these properties without limitation from crystalline domains existing after spin coating, thermal nanoimprint is performed beyond the melting point. The state of the art of measurement to analyze the complex morphology is X-ray diffraction (XRD). We address an alternative measurement method to characterize the material by its optical properties, spectroscopic ellipsometry. It provides information on the degree of order from the typical fingerprint absorption spectrum. In addition, when the material is modeled as a uniaxial layer, an anisotropy factor can be derived. The results obtained from spectroscopic ellipsometry are in accordance with those from XRD. In particular, spectroscopic ellipsometry is able to distinguish between order along the backbone and order in π – π stacking direction, which is important with respect to conductivity.

To reduce the overall costs, replica stamps have become more and more relevant in the past years. These replicas often are composite stamps with at least two layers, a structured top layer and a backplane. In most cases, the composite stamps are rigid, which limits their use for rolling processes or low-pressure imprints. Flexible composite stamps avoid these limitations. However, due to the flexibility, preparation-induced bending may occur. A concept to avoid such a bending will be introduced and verified by the preparation of a flat and flexible composite stamp consisting of a structured OrmoStamp top layer and a polydimethylsiloxan (PDMS) backplane. To increase the adhesion between both layers, two measures will be adopted: one relies on an increase in the surface energy of the PDMS and the other one on an increase in the interface area. Finally, these flat and highly flexible composite stamps are tested in a critical imprint situation, where particles are dusted to the substrate before imprint. Imprint defects and stamp damage due to these particles will be examined.

With thermal processing of multilayer systems' prevention of wrinkling is challenging, but when wrinkling results in well-controlled patterns, this self-forming process is interesting for a lot of applications, in particular, when mechanically stable and transparent materials are involved. The authors use the widely used negative tone photoresist SU-8; the hard top layer is realized by vacuum ultraviolet-treatment. Applicability of the established wrinkling theories to this SU-8 system is verified. The control of wrinkling is provided by masked lithography and nanoimprint. In combination, these two techniques allow controlling the wrinkling area and, moreover, are capable of inducing anisotropy to the wrinkling pattern. An analytical calculation of the anisotropy is presented and compared to experimental results. The calculation is based on the bending stiffness of two orthogonal directions. Two types of imprinted line-structures are investigated, normal and inverted V-grooves. Wrinkling amplitudes and anisotropies found are in good accordance with theoretical results.

Flexible composite stamps are interesting for thermal nanoimprint as well as for UV-assisted nanoimprint as they provide conformal contact to a substrate without the need of applying high pressure. However, they are also prone to bending when handled during mounting, contact establishment and separation after the imprint step. As they consist of different materials bonded together, internal stresses during bending may lead to stamp failure, typically rupture of the top layer. In order to minimize the risk of failure, the stress within the top layer must not go beyond the material-inherent tensile strength, which, for materials typically used to prepare the top layer, often is not available and may depend on the layer thickness and the specific preparation of the composite stamp. Minimization of the maximum bending stress is therefore required to reduce the risk of failure with flexible composite stamps. Such minimization is obtained easily by controlling the position of the so-called ‘neutral plane’. Stresses are highly reduced when the neutral plane is located near the interface between top layer and backplane. We address this issue experimentally as well as theoretically. Diligent choice of the layer thicknesses in combination with the modulus of the materials involved is required to implement this concept. The results obtained confirm the suitability of ‘neutral plane management’ for minimizing the risk of failure with flexible composite stamps.

3D structures are required for a large range of applications, e.g. for light management purposes, which often require time-consuming and costly preparation techniques. Here, we report on a combination of simple low-cost techniques to provide complex 3D structures. The material used is SU-8, a negative tone photoresist. The techniques combined are optical lithography, nanoimprint as well as a vacuum ultraviolet exposure at 172nm. The latter allows depth-selective crosslinking, thus providing a surface skin to SU-8 due to a strong absorption at this wavelength. In combination with optical lithography closed micro-channels were realized. In combination with two subsequent imprint steps (capillary force lithography and thermal imprint) complex 3D structures combining micro- and nano-patterns were prepared over a large area without costly vacuum processing.

Performance and functionality of devices prepared from organic semicrystalline polymers strongly depend on the size and the orientation of the ordered crystalline domains, as the conductivity is distinctly different in the different lattice directions. This investigation addresses the potential of thermal nanoimprint to control the size of the ordered domains as well as their orientation with respect to the substrate. Poly-3-hexylthiophene (P3HT) is chosen as one of the prominent semicrystalline polymers available at high quality. In order to control the ordering of the domains freely, the imprint is performed at a temperature beyond the melting point to eliminate the domains already existing after spin-coating and soft-bake. To avoid a degradation of P3HT at such a high temperature under oxygen contact—the imprint system used operates in air—a novel imprint procedure is employed, where a distinct prepressure is applied already during the heating of the imprint stack (stamp/substrate with P3HT layer). To enable efficient recrystallization in the form “shaped” by the imprint mold, the novel procedure features, in addition, a slow cool-down from the processing temperature (245 °C) down to 150 °C. During recrystallization the orientation of the ordered domains is driven primarily by the interaction of the side-chains of the P3HT molecules with the antisticking layer of the mold, where the pressure still applied forces ordering, too. The potential of this approach is characterized by x-ray diffraction measurements. The results clearly show that the novel procedure investigated here is effective not only to avoid the degradation of P3HT at a high temperature but also to provide ordered domains that are more than two times larger than those existing before imprint, after spin-coating. The results obtained so far with grating patterns are encouraging and indicate new possibilities of controlling the conductivity of devices prepared from semicrystalline organic polymers.

Display Omitted 'Residual layer free imprint' by use of exposure on reflecting substrates.'Residual layer lithography' by use of interference effects to define patterns of about 50nm.'Hybrid lithography' to overcome pattern size limitations of T-NIL and UV-L.'3D multilevel patterns' by use of diffraction effects. The combination of nanoimprint with optical lithography in a conventional mask aligner, both low-cost techniques, provides a number of novel but simple options for pattern definition with photoresists beyond nanoimprint or optical lithography. The effects exploited are based on two issues related to the interfaces at the bottom (substrate/resist) and at the top (resist/air); the bottom interface leads to a specific exposure situation with a reflective substrate like silicon, resulting in standing waves and a swing of the mean intensity in the resist; the top interface represents a pre-patterned surface providing phase shifting and diffraction when exposed. Both facts can be used to realize specific patterning results. We will show 'residual layer free imprint', frequency doubling with 'residual layer lithography', 'hybrid lithography' to overcome the pattern size dependence of nanoimprint as well as 'multi-level structures' resulting from diffraction patterns along the walls of lithography-defined grooves or inside the resist when thick layers are used. Major parameters for all effects are the resist thickness as well as the geometries of the stamp, in particular the stamp height. Furthermore, hierarchical wrinkling patterns can be induced by adding an exposure in the deep ultraviolet range to the hybrid process. The physical background of the effects will be addressed and examples of the single techniques are shown, indicating the requirements. With the physical understanding at hand the engineer is able either to make use of the effects shown or to avoid them, simply by choosing the boundary conditions of the combination process accordingly. The successful combination of nanoimprint with optical lithography relies on the fact that the imprint process does not change the properties of the imprinted material, in this case the optical ones, so that these can be exploited in a further processing step, namely lithography.

Flexible stamps are common for roll-to-roll processing but less common with planar processing, although they offer a number of benefits as, e.g., an improved conformal contact at reduced pressure. A simple way to realize such a flexible stamp is to use a two layer system with a structured top layer and a flexible backplane. The structured top layer is most easily obtained by molding, the backplane provides the flexibility envisaged. For use in a thermal nanoimprint process, a high thermal stability is required for both. This investigation addresses the preparation of flexible composite stamps with OrmoStamp as the structured top layer and polyimide as the flexible backplane. The process recommended for stamp preparation with OrmoStamp has to be modified to avoid bending after the hard bake that is required after ultraviolet-curing of the material to obtain a high stability of the top layer. Reduction of bending is advised, in particular, for large area stamps, where the hard bake step is in conflict with preparation of a flat stamp and limits applicability for thermal nanoimprint. A small scale almost flat flexible composite stamp is prepared and employed for low-pressure nanoimprint; it provides full area imprint at a reduced pressure of 25 bar. The replicated structures, lines of 300 nm, are of high quality and image the stamp structures. Stamps prepared in this way allow a tuning of the flexibility simply by changing the thickness of the backplane.

The morphology of highly regular, semi-crystalline P3HT (poly 3-hexylthiophene-2.5-diyl) after thermal nanoimprint is studied and compared to that of its amorphous counterpart. Differential scanning calorimetry measurements of both materials provide the glass transition temperature (below room temperature) as well as the melting temperature (T m ≈ 235 °C) and the crystallization temperature (T c ≈ 200 °C) of the semi-crystalline polymer. Imprint experiments are performed at temperatures below and above the melting temperature of the crystallites. The samples imprinted with line structures in the range of 135–500 nm are investigated by scanning electron microscopy and transmission measurements. In agreement with T m, the investigations indicate that the crystallites do not melt when semi-crystalline P3HT is imprinted below T m. After imprint above T m, no more indication of the crystallites is found. Furthermore, physical self-assembly only occurred after imprint beyond T m with semi-crystalline P3HT, indicating a fully amorphous state. The morphology observed—the roughness of the surface within partly filled cavities when crystallites are present—correlates well with the results from optical measurements.

The preparation of nanostructures by guiding the phase separation of a block copolymer (BCP) is an attractive technique to overcome the limitations of conventional lithography. Well-established methods for guiding are to provide a topographical pattern (grapho-epitaxy) or a chemical pattern (chemo-epitaxy) on the substrate before BCP application. This paper reports on an alternative technique, the guiding of the BCP without pre-patterning the substrate, by providing the guiding pattern from the top, by nanoimprint. This approach offers the benefit of a multiple use of the same stamp. In the work reported here, we use an elastomeric stamp and examine capillary force lithography (CFL) with respect to guiding. The processing temperature is chosen high in order to provide fast phase separation. For the control of the surface energy of the substrate, a silane deposition from the gas phase is utilized. The ordering behavior of a lamellar polystyrene–polymethylmethacrylate BCP with CFL is studied. In particular, the issues specific with phase separation in an imprint situation are addressed, the locally differing layer thickness and the fact that two surface energies contribute to the ordering process, the one of the substrate and the one of the stamp. Though further optimization is required to make use of CFL for efficient guiding, the first results reported here indicate the potential of this technique for stamp-induced guiding of BCPs. As it allows a multiple use of the stamp, such guiding is interesting and may replace the time- and cost-consuming pre-patterning of each substrate.

Low-pressure imprint is interesting to avoid stamp deformation, stamp failure as well as polymer recovery. When large-area stamps are prepared with a stepping procedure, low pressure is required to optimize the stitching. However, with low-pressure imprint, conformal contact between stamp and substrate is critical. Admittedly, the imprint pressure required for conformal contact depends on the stamp material and its thickness. To get an idea to which extent the imprint pressure can be reduced with a flexible stamp, we compared different stamp materials and stamp architectures, single-layer stamps and two-layer stamps. The two-layer stamps are replica stamps, where the structures were replicated in a thin layer of OrmoStamp, fixed by a backplane. On the background of plate theory, we deduce the pressure reduction compared to a Si stamp by calculating the respective pressure ratio, independent from geometries. In addition, temperature-induced issues are addressed which are of relevance for a thermal imprint process. These issues are related to the mismatch between the thermal expansion coefficients of the stamp and the substrate, and in case of a two-layer stamp, to the mismatch between the backplane material and the top layer. The latter results in temperature-induced stamp bending. On the basis of simple analytical calculations, the potential of single-layer stamps and two-layer stamps with respect to thermal imprint at reduced pressure is discussed and guidelines are provided to assess the imprint situation when replica stamps are used for imprint. The results demonstrate the attractiveness of two-layer stamps for reduced pressure nanoimprint, even in a temperature-based process.

Nanostructured, functional materials attract increasing attention for various applications. Therefore, the fabrication of nanometer structures in polymers for masking purposes and their transfer into functional materials is highly requested. One promising approach is the transfer of vertical lamellas fabricated by self-assembling processes from 104 k poly(styrene)-block-methyl methacrylate (PS-b-PMMA) block copolymer (DiBCP) into fused silica by dry etching. The similar to 50 nm high DiBCP lamellas were fabricated by a standard procedure comprising spin-on, thermal annealing, and wet developing in acetic acid. The perpendicular orientation of the lamellar DiBCP that is required for dry etching pattern transfer was verified and inclination angles of maximal 8 degrees were found. These DiBCP lamellas that have a period of 48 +/- 1 nm were transferred by a reactive ion beam etching process (RIBE) with CF3H into fused silica wafers. The etched fused silica lamellas feature a size of 21 +/- 4 nm, a lamella period of 48 +/- 2 nm, and a height of 81 +/- 3 nm for 4 min etching time. For 6 min etching time we specified a lamella feature size of 18 +/- 4 nm, a lamella period of 47 +/- 3 nm, and a height of 114 +/- 4.5 nm. (C) 2015 Elsevier B.V. All rights reserved.