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The quality and the stability of devices prepared from polycrystalline layers of organic–inorganic perovskites highly depend on the grain sizes prevailing. Tuning of the grain size is either done during layer preparation or in a post-processing step. Our investigation refers to thermal imprint as the post-processing step to induce grain growth in perovskite layers, offering the additional benefit of providing a flat surface for multi-layer devices. The material studied is MAPbBr3; we investigate grain growth at a pressure of 100 bar and temperatures of up to 150 °C, a temperature range where the pressurized stamp is beneficial to avoid thermal degradation. Grain coarsening develops in a self-similar way, featuring a log-normal grain size distribution; categories like ‘normal’ or ‘secondary’ growth are less applicable as the layers feature a preferential orientation already before imprint-induced grain growth. The experiments are simulated with a capillary-based growth law; the respective parameters are determined experimentally, with an activation energy of Q ≈ 0.3 eV. It turns out that with imprint as well the main parameter relevant to grain growth is temperature; to induce grain growth in MAPbBr3 within a reasonable processing time a temperature of 120 °C and beyond is advised. An analysis of the mechanical situation during imprint indicates a dominance of thermal stress. The minimization of elastic energy and surface energy together favours the development of grains with (100)-orientation in MaPbBr3 layers. Furthermore, the experiments indicate that the purity of the materials used for layer preparation is a major factor to achieve large grains; however, a diligent and always similar preparation of the layer is equally important as it defines the pureness of the resulting perovskite layer, intimately connected with its capability to grow. The results are not only of interest to assess the potential of a layer with respect to grain growth when specific temperatures and times are chosen; they also help to rate the long-term stability of a layer under temperature loading, e.g. during the operation of a device.

The manufacturing of devices from methylammonium-based perovskites asks for reliable and scalable processing. As solvent engineering is not the option of choice to obtain homogeneous layers on large areas, our idea is to ‘upgrade’ a non-perfect pristine layer by recrystallization in a thermal imprint step (called ‘planar hot pressing’) and thus to reduce the demands on the layer formation itself. Recently, imprint has proven both its capability to improve the crystal size of perovskite layers and its usability for large area manufacturing. We start with methylammonium lead bromide layers obtained from a conventional solution-based process. Acetate is used as a competitive lead source; even under perfect conditions the resulting perovskite layer then will contain side-products due to layer formation besides the desired perovskite. Based on the physical properties of the materials involved we discuss the impact of the temperature on the status of the layer both during soft-bake and during thermal imprint. By using a special imprint technique called ‘hot loading’ we are able to visualize the upgrade of the layer with time, namely a growth of the grains and an accumulation of the side-products at the grain boundaries. By means of a subsequent vacuum exposition we reveal the presence of non-perovskite components with a simple inspection of the morphology of the layer; all experiments are supported by X-ray and electron diffraction measurements. Besides degradation, we discuss recrystallization and propose post-crystallization to explain the experimental results. This physical approach towards perovskite layers with large grains by post-processing is a key step towards large-area preparation of high-quality layers for device manufacturing.

When nanoimprint serves as a lithography process, it is most attractive for the ability to overcome the typical residual layer remaining without the need for etching. Then, ‘partial cavity filling’ is an efficient strategy to provide a negligible residual layer. However, this strategy requires an adequate choice of the initial layer thickness to work without defects. To promote the application of this strategy we provide a ‘guiding chart’ for initial layer choice. Due to volume conservation of the imprint polymer this guiding chart has to consider the geometric parameters of the stamp, where the polymer fills the cavities only up to a certain height, building a meniscus at its top. Furthermore, defects that may develop during the imprint due to some instability of the polymer within the cavity have to be avoided, with nanoimprint, the main instabilities are caused by van der Waals forces, temperature gradients, and electrostatic fields. Moreover, practical aspects such as a minimum polymer height required for a subsequent etching of the substrate come into play. With periodic stamp structures the guiding chart provided will indicate a window for defect-free processing considering all these limitations. As some of the relevant factors are system-specific, the user has to construct his own guiding chart in praxis, tailor-made to his particular imprint situation. To facilitate this task, all theoretical results required are presented in a graphical form, so that the quantities required can simply be read from these graphs. By means of examples, the implications of the guiding chart with respect to the choice of the initial layer are discussed with typical imprint scenarios, nanoimprint at room temperature, at elevated temperature, and under electrostatic forces. With periodic structures, the guiding chart represents a powerful and straightforward tool to avoid defects in praxis, without in-depth knowledge of the underlying physics.

Thermal management in devices like solar cells, light-emitting diodes, and lasers based on hybrid halide perovskite thin films is expected to be of paramount importance for optimal performance and reliability. As of yet, experimental data of thermal properties of non-iodine-based hybrid halide perovskites is very scarce. Here the thermal conductivity of methylammonium lead halide perovskite (CH3NH3PbX3 X= I, Br, and Cl) single crystals and thin films is analyzed by scanning near-field thermal microscopy. The thermal conductivity of CH3NH3PbX3 single crystals with X= I, Br, and Cl is found to be 0.34 ± 0.12, 0.44 ± 0.08, and 0.50 ± 0.05 W/(mK) at room temperature, respectively. Strikingly, similar thermal conductivities are determined for the corresponding thin-film samples. The thermal conductivity of MAPbI3 in the cubic phase (T > 55 °C) increases to (1.1 ± 0.1) W/(mK). In addition, the temperature dependence of the thermal conductivities and of thermal expansion coefficients of MAPbI3 around the phase t...

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. Display Omitted Thermal nano imprint with lamellar di block copolymers2 step imprint processFree standing vertical lamellar structuresImpact of a polystyrene brush

Cesium lead halide perovskites are of interest for light-emitting diodes and lasers. So far, thin-films of CsPbX3 have typically afforded very low photoluminescence quantum yields (PL-QY < 20%) and amplified spontaneous emission (ASE) only at cryogenic temperatures, as defect related nonradiative recombination dominated at room temperature (RT). There is a current belief that, for efficient light emission from lead halide perovskites at RT, the charge carriers/excitons need to be confined on the nanometer scale, like in CsPbX3 nanoparticles (NPs). Here, thin films of cesium lead bromide, which show a high PL-QY of 68% and low-threshold ASE at RT, are presented. As-deposited layers are recrystallized by thermal imprint, which results in continuous films (100% coverage of the substrate), composed of large crystals with micrometer lateral extension. Using these layers, the first cesium lead bromide thin-film distributed feedback and vertical cavity surface emitting lasers with ultralow threshold at RT that do not rely on the use of NPs are demonstrated. It is foreseen that these results will have a broader impact beyond perovskite lasers and will advise a revision of the paradigm that efficient light emission from CsPbX3 perovskites can only be achieved with NPs.

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. Display Omitted Calculation of the position of the neutral plane of a composite stamp when bendedPosition of the neutral plane accounts for the maximum bending stresses occurring.Correlation of maximum bending stresses with tensile strength.Neutral plane management reduces the risk of failure with flexible composite stamps substantially.

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. Display Omitted Complex 3D structures without vacuum processingShort wavelength treatment (172nm) in combination with nanoimprintCost-effective generation of complex 3D structures

Our study refers to the highly stretchable elastomer PDMS (polydimethylsiloxane), a material used with a wide range of applications. Its basic mechanical properties can be tuned, e.g., by varying the curing conditions; moreover, its surface properties can be tuned by modification techniques. We modified our PDMS by irradiating the samples with an excimer lamp at 172 nm. Such a treatment hardens the elastomer at the surface, and it becomes silicalike; the sample changes to a “quasi” two-layer system with a graded interface. When such samples are stretched, surface cracks occur beyond a critical strain. The increase of crack length with increasing strain is evaluated by means of video screenshots. The impact of the curing conditions is addressed by analyzing samples prepared at different cross-linking temperatures, resulting in differing bulk properties but similar surface properties. Crack length and crack velocity are evaluated with each sample based on single randomly chosen cracks. The results are discussed on the basis of theoretical concepts for channeling cracks in multilayer systems with polymeric substrates. Typically, with applications, random cracks should develop at high strain only and, if present, should propagate slowly along the surface but not into the depth of the sample. Our investigation shows that the mechanical material properties of the substrate are vital with respect to such stable cracking, rather than the surface properties. In particular, the curing conditions chosen for the substrate are essential to reduce cracking, a fact less regarded with applications so far.

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/mu m 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. Published by the AVS.

Halide perovskites are currently of interest for a variety of optoelectronic applications. While, typical wet-chemical preparation techniques afford relatively rough polycrystalline layers, we have recently demonstrated that thermal imprint is a powerful post-deposition processing tool that affords extremely smooth perovskite thin-films with crystals that extend over tens of microns laterally.[1,2] A comparative study of optical, morphological and thermal properties (e.g. thermal conductivity) reveals some striking similarity of pressed MAPbX3 thin-films and their single crystalline analogues.[3] More recently, we successfully used thermal imprint also for entirely inorganic halide perovskite materials, such as CsPbBr3. While as-deposited CsPbBr3 layers are typically discontinuous and rough with a large number of pinholes, thermal imprint at relatively low temperature and pressure (150°C, 100 bar) will be shown to turn them into dense, smooth and pinhole-free thin films, which show substantially enhanced luminescence quantum yield and in contrast to pristine CsPbBr3 layers even enable room-temperature amplified spontaneous emission (ASE). Perovskite thin films patterned by thermal nanoimprint with photonic resonator structures will be shown to afford hybrid and entirely inorganic distributed feedback lasers, with ultra-low lasing thresholds.[2,4] [1] A. Mayer et al. J. Vac. Sci. & Techol. B 2017, 35, 06G803. [2] N. Pourdavoud et al. Adv. Mater. Technol. 2018, 3, 1700253. [3] R. Heiderhoff et al. J. Phys. Chem. C 2017, 121, 28306. [4] N. Pourdavoud et al. Adv. Mater. 2017, 29, 1605003.

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.

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.

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.

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 recently re-discovered class of organometal-halide perovskites hold great promise for solar cells, LEDs and lasers.[1] Today, their potential has not been fully unlocked partially because of the lack of suitable nano-patterning techniques, which are mandatory to create resonator structures, waveguides etc. with a maximum level of precision directly into perovskite layers. Their chemical and thermal instability prevents the use of established wet-chemical patterning techniques.[2] In contrast to conventional inorganic semiconductors, crystal binding in these perovskites includes significant contributions of van der Waals interactions among the halide atoms and Hydrogen bonding.[3] The formation enthalpy per unit cell is only about 0.1eV in MAPbI3.[4] Here, we take advantage of the “soft-matter properties” of organo-metal halide perovskites and demonstrate that photonic nano-structures can be prepared by direct thermal nano-imprint lithography in MAPbI3 and MAPbBr3 at relatively low temperatures (

The transfer of systems from nature to technical schemes is still a challenge as the fabrication of such complex systems in a cost efficient way while keeping the intended technical functionality requires appropriate materials and fabrication techniques. Here the multi-scale hierarchy of the gecko foot was transferred into membrane-actuated gecko-inspired adhesion elements (AE) to study the adhesion to surfaces. Therefore, a three level hierarchical polydimethylsiloxane (PDMS) structure comprising a membrane for actuating and the micron-patterned AE were mould from a microtechnical fabricated master into PDMS by thermal and UV curing. The micron-patterned AE consist of dot or line micron patterns on top of cuboid posts with 15 µm height that representing the second hierarchical level. For master fabrication lithographic masking, wet and dry etching as well as SU8 3D lithography was applied. The adhesion of such AE on smooth, rough and chemically modified glass surfaces was measured by determining the shear force for several different AE designs and PDMS materials. A slip of the micron sized PDMS AEs at the adhesion test sample surface was measured. This slip of the PDMS elements being in contact to the surface can enhance the accepted shear force and the measured anisotropy of the shear force for linear AEs. The results found can contribute to the optimization of PDMS AE for technical surfaces with chemically and topographically inhomogeneous properties.

Graphical abstractDisplay Omitted Combination of nanoimprint lithography of negative tone resist with a UV-flood exposure.Pattern definition in the remaining residual layer.Use of interference and diffraction effects along the imprinted edges to define structures in 50nm range. Residual layer lithography makes use of pattern frequency doubling with nanoimprinted negative tone photoresists that are subjected to an UV-flood exposure, with a residual layer of a thickness of about a quarter-wavelength. A height of the imprinted step of about 260nm provides ideal conditions for destructive interference along the imprinted edges. Under such conditions the centre of this residual layer is well exposed, whereas diffraction and interference effects result in under-exposure along the edges of the imprinted patterns. After flood exposure and post exposure bake the residual layer in the underexposed regions can simply be removed by development which results in trenches along the imprinted edges. Patterns 60-70nm in width could be achieved by an imprint with stamp geometries in the range of the exposure wavelength. The transfer into Cr metal structures by sputtering and lift-off provides a suitable etch mask.

Controlled deposition of anti-sticking layers based on the use of evaporation cell.Tuning of process time and uniformity by multiple-cell arrangement.Uniformity of surface energy better than ?4% over 100mm diameter.Low surface energy (?10-15mN/m) obtained after 40min at 120?C. We developed a novel process for the deposition of anti-sticking layers from the gas phase based on the use of evaporation cells. The cells are prepared from Teflon and consist of small silane containers with an orifice. Temperature is used to provide a silane-containing gas phase in the containers; the temperature is well below the boiling point of the silane. The number of cells can be varied to improve the uniformity with respect to surface energy, to scale-up the process to larger diameters and to reduce the processing time required. The impact of cell number, processing temperature and processing time is investigated with respect to uniformity and with respect to the minimum times and temperatures required. The concept works well and shows potential for the control of surface energy beyond anti-sticking purposes.

Display Omitted 5µm high polymeric bridges induced by a temperature difference.Silica particles as spacers between stamp and substrate.Rough estimation of mechanical properties of silica particles. Instability-induced pattern formation is easily realized during thermal nanoimprint when a temperature difference is provided between stamp and substrate. The use of this effect for pattern definition requires spacers between stamp and substrate to install a gap of definite height. We test the use of silica particles to serve as spacers with variable height for such an approach. Then, there is no need to prepare stamps with a specific design especially for this purpose, as any stamp available can be used. It turns out that the silica particles are not ideally suited to serve as reliable spacers - they are too soft, when Si stamps are used, but too hard when replica stamps made from OrmoStamp are used. The experiments allow to roughly estimate the mechanical properties of the particles. Nonetheless, it is demonstrated that a formation of isolated polymeric bridges of 5µm in height is feasible.

Graphical abstractDisplay Omitted A semi-analytical approach in a matrix form to handle the intensity distribution within photoresist layers.Thermal nanoimprint with well-defined residual layer (dR≤30nm) and without physical self-assembly defects.Residual layer removal by development. This paper presents a semi-analytical approach for the calculation of the mean relative intensity within photoresist layers of a given thickness on silicon - the mean intensity is the one obtained after a post exposure bake to remove standing wave effects. The approach is based on the handling of analytically determined intensity values in a matrix form. Transmission, reflection and absorption in an air/resist/substrate configuration are considered to calculate the intensity for a single wavelength or a multiple wavelength exposure. Swing curves of the mean relative intensity as a function of the total resist thickness indicate a novel application in the context of nanoimprint: a residual layer-free imprint can be obtained with residual layers of up to about 30nm, as such thin layers always remain underexposed. Thus, when a negative tone photoresist is imprinted and is flood exposed after the imprint, any thin residual layer will be removable in a simple development step, thus avoiding any breakthrough etch. This is of particular interest for a further use of the imprinted structures with lift-off.

The copy of structures in the same tone as the original asks at least for a double replication. Each replication generation will suffer from shrinkage of the replication material used, due to curing or thermal contraction. The impact of shrinkage is addressed in a basic study by simulation and experiment. The main replication materials investigated are OrmoStamp and SU-8. Presently, the preparation of anisotropic adhesion elements with hierarchical structures is investigated. The structures are micrometer-sized isolated pads with nanometer-sized self-aligned ripples (laser-induced periodic surface structures) on top. The initial structures are available as isolated photoresist patterns on Si. The double replication of this polymeric master raises questions with respect to its chemical and mechanical stability. The authors report an optimized replication process with an OrmoStamp intermediate template and a final replication in an elastomer. The anisotropic character of the so prepared hierarchical elastomeric adhesion elements is demonstrated by measurement.The copy of structures in the same tone as the original asks at least for a double replication. Each replication generation will suffer from shrinkage of the replication material used, due to curing or thermal contraction. The impact of shrinkage is addressed in a basic study by simulation and experiment. The main replication materials investigated are OrmoStamp and SU-8. Presently, the preparation of anisotropic adhesion elements with hierarchical structures is investigated. The structures are micrometer-sized isolated pads with nanometer-sized self-aligned ripples (laser-induced periodic surface structures) on top. The initial structures are available as isolated photoresist patterns on Si. The double replication of this polymeric master raises questions with respect to its chemical and mechanical stability. The authors report an optimized replication process with an OrmoStamp intermediate template and a final replication in an elastomer. The anisotropic character of the so prepared hierarchical elastom...

Surface-hardened polydimethylsiloxane (PDMS) with random cracks is studied by means of double replication. The PDMS samples are prepared under different curing conditions, resulting in a different Young's modulus of the bulk. To modify the surface, an excimer lamp at 172 nm is used. The samples are stretched uniaxially until the first cracks appear. As an evaluation under strain is hard or rather impossible, the idea is to replicate the cracked sample in a curable material and to characterize the cracks by inspection of the replica. To protect the sample from mechanical loading, this replication is done by molding in OrmoStamp on glass; these replicas are used for optical inspection to determine the crack spacing. As a characterization of the depth and width of the cracks is highly facilitated when cleaving is enabled, a second replication is performed into a thin layer of SU-8 on Si; these second replica are analyzed by secondary electron microscopy of cross sections. They provide a realistic picture of the crack shape. It is found that the curing conditions affect the crack shape; a U-shape occurs with a low bulk modulus, whereas a V-shape occurs with a high bulk modulus. The parameters width, depth, and spacing are largely unaffected by the curing conditions. This work provides a background to understand the behavior of random cracks, which is, e.g., useful to design a system with controlled cracks that remain stable. An important finding is that as soon as cracking occurs at a certain strain, already a number of cracks develop, yet without any further stretching. The cracks behave independently from neighboring cracks. Upon further stretching, new cracks develop, and the crack width and depth remain similar. Therefore, these random cracks are stable, and sample failure does not occur up to a strain of at least 40%.Surface-hardened polydimethylsiloxane (PDMS) with random cracks is studied by means of double replication. The PDMS samples are prepared under different curing conditions, resulting in a different Young's modulus of the bulk. To modify the surface, an excimer lamp at 172 nm is used. The samples are stretched uniaxially until the first cracks appear. As an evaluation under strain is hard or rather impossible, the idea is to replicate the cracked sample in a curable material and to characterize the cracks by inspection of the replica. To protect the sample from mechanical loading, this replication is done by molding in OrmoStamp on glass; these replicas are used for optical inspection to determine the crack spacing. As a characterization of the depth and width of the cracks is highly facilitated when cleaving is enabled, a second replication is performed into a thin layer of SU-8 on Si; these second replica are analyzed by secondary electron microscopy of cross sections. They provide a realistic picture of ...

When nanoimprint is not used for lithography purposes (NIL), but for the direct patterning of polymeric layers, high aspect ratio patterns may be of interest for a number of applications. The definition of such patterns in a nanoimprint process deals with two aspects, a successful filling of the high aspect ratio cavities of the stamp used, followed by a successful separation of the high aspect ratio structures defined in the polymeric layer on the substrate, from the stamp. These two aspects are addressed by shedding light to the impact of capillary effects during the filling of high aspect ratio cavities, and to the deformation processes involved in the separation of the stamp from the polymeric structures, where adhesional energies have to be overcome without cohesional failure. Both aspects are discussed in terms of the geometries involved, the stamp geometries as well as the polymeric layer thickness, and correlations with thermally-assisted (T-NIL) and UV-assisted (UV-NIL) processing are deduced. The aspects discussed are typical of a nanoimprint situation with thin polymeric layers on hard substrates.

With residual-free imprint, physical self-assembly of the polymer within partly-filled cavities has to be avoided to ensure the masking capability of nanoimprint when used as a lithography technique. In this regard we investigate the impact of the velocity of pressure application by use of a motor-driven imprint system. Slowing-down the velocity of pressure application maintains a smooth surface of the polymeric layer in the cavities, minimizing the risk of local contact between polymer and stamp with moderate filling levels of the cavities. At a high filling level of the cavities, the formation of self-assembly cannot be avoided, as van der Waals interactions between polymer surface and stamp become effective to drive the formation of instabilities. The experimental results obtained can be understood by stability analysis. Understanding of the processes leading to the formation of physical self-assembly is vital to exploit the full potential of nanoimprint for processing.

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. Display Omitted Combination of micro- and nanostructures without laborious writing techniquesLimitation of a surface-hardened SU-8 layer with high aspect ratio structuresSuccessful replication of complex 3D structures in UV-PDMS

Applications in various sectors of microtechnology such as MEMS, microfluidics, and lab on chip call for reliable low-cost joining approaches that are detachable and re-closable, providing a tight fit at the micron level. Dovetail-like structures are a promising approach to realize the female side of such connectors and moulding is a suitable technique to produce the counterpart that completes the joint. Force-displacement measurements were performed for 2x2cm^2 master-replica assemblies, prepared from Si and PDMS (polydimethylsiloxane) and featuring dovetail-like structures with different undercuts. For measurement a miniaturised force-displacement tester was set up. Maximum retention forces of up to about 60mN per trench of 1cm length were measured for lines with [email protected] wide undercuts. Finite element modelling using a linear elastic model was employed to simulate the local distribution of stress and strain in the PDMS under loading with an applied tensile force. The simulation results approximate the measurements well. The quantitative results obtained can be used for designing miniaturized connectors with undercuts.

Photonic nanostructures are created in organo-metal halide perovskites by thermal nanoimprint lithography at a temperature of 100 °C. The imprinted layers are significantly smoothened compared to the initially rough, polycrystalline layers and the impact of surface defects is substantially mitigated upon imprint. As a case study, 2D photonic crystals are shown to afford lasing with ultralow lasing thresholds at room temperature.

For a simple and low-cost preparation of e.g. sensors featuring large areas of nano-scaled line fields connected to micron-sized contacts we propose hybrid lithography of thermal nanoimprint and optical lithography in a reversed order. In the reversed order process the first step is lithography, followed by thermal nanoimprint and finally development. When the imprint is performed into a positive tone resist with a minimum residual layer remaining, any residual polymer is cleared during development, thus avoiding an additional dry etch step. This allows easy metal definition by sputtering and lift-off directly after development. Compared to a conventional sequence, the reversed order process allows higher imprint temperatures and reduces the exposure doses needed substantially, thus improving the quality of the imprint and of the lithography step as well. Line fields of several millimetres length could be successfully prepared. Based on the results obtained a novel approach for low-cost sensor preparation is proposed.

The replication of 3D structures with undercuts by nanoimprint technologies needs appropriate, rigid masters. The fabrication of 3D microstructured masters with inclined, undercut structures by photolithography and reactive ion beam etching into fused silica is demonstrated. The ion beam incidence angle defines the sidewall inclination of the undercuts and the shape of the structures. Specific characteristics of 3D structures in particular of the undercuts and of the bottoms are the result of the competing processes of polymer deposition and silica etching and the alteration of the average ion current density distribution due to etching depth increasing and mask degradation during the cyclic etching from opposite directions. Optimized structures can be achieved with a high number of short etching cycles.

Tailoring of surfaces or interfaces by patterning is a well-known approach used for the improvement of optical and electrical properties of thin film optical devices, and nanoimprint is a strong candidate to realise this at low cost over large areas. For such purposes we provide stamps with random texture. Contamination masking is used instead of lithography to further reduce the costs. Reactive ion etching in SF"6/O"2 at high oxygen content is effective to provide pyramidal structures of about 300-500nm in height. Loading affects the etch result and has to be considered when establishing a process suitable for large area processing. The geometry of the pyramids is characterised by SEM and AFM measurements, and also by thermal imprint to show the shape of the grooves in between the pyramids. Optical characterisation in a single-axis goniometer-type set-up documents a substantial broadening of the angular distribution of the intensity of the reflected light compared to a flat surface, well suited for the purpose envisaged.

Layers from fluorinated trichlorosilanes are in widespread use for the prevention of sticking in nanoimprint. It is generally assumed that these layers are monolayers, bonded to the substrate, and internally cross-linked. The authors have investigated a gas phase deposition process for such layers in detail, varying the amount of antisticking agent and the deposition time in a vacuum process. The quality of the layers was determined from the layer thickness and the contact angle was measured. In accordance with some recent literature, claiming that formation of dense cross-linked monolayers from fluorocarbon-trichlorosilanes is not possible, the results indicate that the deposited layers are not monolayers. Instead the authors suggest that they are even thinner disordered but cross-linked layers. Obviously monolayer formation is not required for sticking prevention.

In a thermal imprint process, contact angles may evolve in two situations, between the stamp and the polymer, when a cavity remains unfilled, or, in cases where the polymer locally dewets the substrate, between the substrate and the polymer. For two polymers of different polarity, such contact angles are determined experimentally and compared to values calculated from surface energy data. In doing so, the specific temperature dependence of surface energies of a thermoplastic polymer as well as the typical course of an imprint process, where cooling times often exceed the imprint times, are considered. The differences between experimental and theoretical values are critically discussed. The results indicate that the respective adhesive layers govern the surface status of the substrate and the stamp as well.

This investigation follows the idea to characterise polymers for thermal imprint, in contrast to the classical rheological characterisation route, directly by means of an imprint process. For this purpose the polymers under investigation, a PS and a PMMA of comparable molecular weight (350kg/mol) are imprinted at different temperatures under conditions of partial cavity filling. The development of characteristic effects, namely physical self-assembly, limited imprint depth and recovery is investigated. Imprintability test results are correlated with parameters obtained from rheology, in particular with the viscosity, the minimum flow time constant and the elastic compliance. In addition, the temperature behaviour is discussed in terms of the WLF-parameters. It turns out, that PMMA requires lower molecular weights than PS for efficient thermal imprint. Differences in the recovery behaviour of both materials can be understood from the elastic compliance and time constants measured. An optimum viscosity for thermal imprint at around 10^6Pas is estimated.

Imprint of thin PDMS layers with undercut trenches features a behaviour different from the one known from binary structures. In order to compare the filling behaviour of such geometries a moulding process with preheated masters was performed. Due to the temperature accelerated curing of the reactive mixture this method enabled the identification of different filling states by 'freezing' of an incomplete filling situation. The different filling states could be discerned by means of the final location of the 'frozen' air bubbles and the filling length of the trenches. It turned out that, in contrast to the behaviour of binary trenches, the filling of undercut trenches proceeds primarily in lateral direction along the trench. This behaviour could be understood by pinning of the contact line between liquid and solid at outer edges.

Under mechanical load, the reaction of polymers with a low molecular weight features small time constants. To test their actual behaviour in a thermal imprint, two polystyrenes (PS 10kg/mol, PS 30kg/mol) were imprinted under partial cavity filling conditions. The imprints were inspected regarding recovery and physical self-assembly of the polymer by means of an optical microscope. The results show that such low molecular weight polymers obviously are not suited for thermal imprint. In contrast to polymers with higher molecular weight, physical self-assembly and recovery occur at an identical imprint temperature, so that processing conditions for defect-free imprint cannot be identified. In case of a polymer with higher molecular weight this is possible by adequate choice of the processing temperature.

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.

Thermal imprints into polymer layers, which are thin compared to the structure height of the stamp provide the advantage that minimum residual layers can be achieved. On the other hand, due to the specific properties of thermoplastic polymers, recovery, i.e. a release of stored imprint energy, may occur after separation of sample and stamp. The intensity of this recovery strongly depends on the imprint temperature. Unfortunately a reduction of recovery by means of increased imprint temperature often generates other problems like unwanted physical self-assembly of the polymer. Another parameter that affects the development of recovery is the structure size of the imprinted patterns. This contribution is about the structure size dependency of the intensity of polymer recovery. By means of lift-off imprints of different structure sizes and types are tested after the imprint at two different temperatures.

In order to develop an easy to handle, low-cost procedure to derive imprint-relevant characteristic polymer data from simple imprint experiments, a fingerprint stamp was designed and tested. It contains line and dot structures of different, well-graded sizes in the micron range, where the gaps in between are chosen as to assure independent intrusion of the single patterns into the polymer layer. By imprint of this stamp into a well-characterised polymer, PS 350k, a feasibility test was performed. A pattern size dependent change of the imprint characteristics was observed during the imprint under high-shear into thin layers. This is taken as a hint that polymer characterisation from such experiments is indeed possible. Such data can be adopted to optimise an imprint process for mixed pattern sizes ranging from the micron down to the nanometre scale.

The separation of masters with 3D undercut structures from thin spin-coated layers of PDMS (Sylgard 184) on Si was investigated. In order to identify typical failure types and to figure out potential limitations imposed by the material, masters with different undercut widths were prepared. Imprint was chosen as the replication technique, where only thin base layers remain on the substrate below the 3D undercut structures. Two typical failure types were observed during separation, neck failure and base layer failure. It was found that surface quality of the master is a major impact factor. Small defects in the surface of the PDMS induced by the master result in large area tear-off of the undercut replica during separation. Defect-free masters could be successfully imprinted and separated with base layer thicknesses down to about [email protected] for undercut widths of up to [email protected]

The evolution of the profile of an imprinted line under squeeze-flow dominated thermal nanoimprint is investigated experimentally. The results indicate, that in the very first moment the stamp intrusion into the polymer is fast due to shear thinning at the periphery of the feature, proceeding along with the build-up of internal stress in the feature centre. Relaxation of this stress proceeds according to the flow time constants. At processing times that are short compared to the time constants, stress relaxation will not be completed. Then the polymer recovers its initial shape in parts, and this recovery is responsible for the final profile shape. As time constants can be tuned by temperature, these investigations are helpful to define adequate low temperature imprint conditions when mixed pattern sizes are imprinted into spin-coated layers.

Linear polystyrene of different weight averaged molecular weight (M"w=58kg/mol and 353kg/mol) and a narrow molecular weight distribution was imprinted to point out differences in the flow behaviour caused by the polymer's molecular weight in a typical hot embossing/thermal nanoimprint process. Residual layer thickness and uniformity as well as defects were taken as a measure to qualify the imprint process. Since polymer viscosity is addressed as the main material parameter, a full thermo-mechanical characterisation of the two polymers via parallel plate dynamic viscosimetry was performed prior to imprint. The main process parameter responsible for viscosity, the temperature, was varied from about 30 to 150K above the glass transition. In addition the amount of polymer supplied for the process was varied, i.e. the initial spin-coated polymer layer thickness. Situations representing 'underfilling', 'filling' and 'overfilling' of the stamp cavities in specific regions were investigated. Squeezed flow simulations were performed taking into account the increase of pattern size and the filling of cavities during the ongoing imprint, which have substantial impact on the residual layers obtained. The results show that shear thinning is an issue in a high pressure hot embossing system. It governs the initial phase of the imprint featuring a rapid decrease of the polymer height. From theoretical considerations, a model could be derived for the imprint procedure under shear thinning in a constant pressure system which was able to interpret the observed phenomena. Despite the macroscopic nature of the thermo-mechanical analysis, the standard viscosimetric data for the polymers appeared to be sufficient to understand the experimental results.

Imprint processes can be described by the equations of squeezed flow. In case they account, in a modified form, for the filling of the stamp cavities during the imprint process they predict that the imprint activity virtually stops after complete filling of these cavities and a thick residual layer remains. Without cavity filling no interference of the imprint process occurs and very thin residual layers are obtained. This concept of partial cavity filling is tested during the imprint of fields of periodic 200nm lines, and the imprint temperature was reduced to prevent the formation of self assembly induced defects. Very low residual layers and good aspect ratios were obtained even at temperatures of 130^oC for commercial polystyrene with a glass temperature of 105^oC.

UV curing of resists after imprint is of interest for mix and match applications or when the thermo-mechanical stability of the resist has to be increased. In those cases the imprint into a curable resist like the mr-L 6000XP is followed by a UV exposure. This exposure is different from an exposure during UV lithography, as the pre-patterned resist results in a lateral intensity modulation, depending on the pattern geometries (width, distance, height, residual thickness) and depending on potential polarisation directions as well, which may be induced by reflecting elements within the optical path. We have simulated the exposure of line patterns of different geometries under polarisation of the incident light under directions parallel and perpendicular to the imprinted lines. Corresponding UV curing experiments after imprint suggest that the stabilisation should be feasible at adequate increase of the exposure dose independent from the polarisation direction.

The imprint behaviour of commercial polystyrene, featuring a wide molecular weight distribution, is compared to polymers used as standards with a narrow molecular weight distribution. All polymers were characterised prior to imprint. The differences in their characteristics are discussed, with respect to standard thermo-mechanical behaviour and with respect to nanoimprint in a hot embossing process. Shear rate effects and recovery are discussed. Recovery results in a local non-uniformity of the residual layer within the imprinted features. As long as the reaction time constants of the polymer remain low compared to the embossing time to avoid recovery, the use of shear rate effects during nanoimprint of thermoplastic polymers is favourable for throughput improvement as it allows processing at reduced times and reduced temperatures.

The mechanical state of polymer flows induced by wide area thermal nanoimprint was investigated using instrumented indentation testing. Polystyrene films of narrow distribution molecular weight both slightly above and well above the chain entanglement weight spacing were imprinted with a range of feature shapes under nominally equivalent flow conditions. Following imprint, pristine, unflowed, and flowed regions of the polymer were tested for indentation modulus and hardness using sharp pyramidal and blunt spherical indenters. No difference in mechanical state for these various regions was observed. The implications of these results for the imprint process itself as well as the mechanical testing of imprinted patterns are discussed.