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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

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.

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 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.

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.

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.

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.

Thickness and uniformity of the residual layer are major global quality criteria for nanoimprint lithography. When the total area of a stamp used in a mechanical replication process like hot embossing is composed of sections with different pattern density, the specific local stamp layout strongly affects residual layer uniformity. Taking the example of a positive stamp with sections of well-defined pattern density the effect of initial layer thickness on uniformity of the residual layer was investigated on the centimetre scale. Long-range uniformity is shown to be a result of local polymer flow in particular between sections of different pattern density. We found the effect to be independent of pattern size. It turned out, that initial layer thickness has to be chosen with regard to the polymer volume locally required for filling of the stamp cavities. Whereas overfilling should be avoided, underfilling may be beneficial for specific applications.

The characteristics and benefits of two types of cross-linking prepolymers with low glass transition temperature (Tg) for nanoimprinting are reported. They are soluble in organic solvents and their solutions can be processed like those of common thermoplastics. The imprinted patterns receive high thermal and mechanical stability through cross-linking polymerization. The course of the polymerization was investigated to determine the appropriate conditions for the imprint process. In thermally cross-linked polymers mr-I 9000, the cross-linking occurs during imprinting. Process time and temperature depend on the polymerization rate. Volume shrinkage during the polymerization does not adversely affect imprinting. Photochemically crosslinked polymers mr-L 6000 make possible imprint temperatures below 100 °C and short imprint times. The Tg of the prepolymer determines the imprint temperature. The cross-linking reaction and structural stabilization is performed after imprinting. 50-nm trenches and sub-50-nm dots confirm the successful application of the polymers.

A preliminary study of the suitability of vacuum chucks for fixing of stamp and sample in a hot embossing lithography process was performed. Four inch test chucks were designed, featuring circular grooves and isolated holes of 0.7 to 2 mm size. The effect of these grooves was tested by imprint into PMMA with an unpatterned stamp. The processed samples showed a polymer thickness variation correlated with the groove locations, which could be measured with a profilometer. The thickness variations found were of different type when polymers of different molecular weight were used. Under realistic embossing conditions, values of up to 70 nm were measured. From the experiments, an allowable groove width of

In order to reduce costs for nanometer-structured templates for hot embossing, potential ways to produce purely thermoset replica to be used as stamps are examined. Thermal expansion of thermosets under realistic process conditions is measured and its influence on the hot embossing of thermoplastic materials is studied. Thermal expansion coefficients of thermosets under different process conditions were found to vary between 3×10−5 and 2×10−4/ °C. Results are presented concerning the imprintability of both curable polymers on thermoset substrates and conventional resist materials on silicon.

Diamond-like carbon films were prepared by use of a hollow cathode discharge plasma source (HCD), operating at 13.56 MHz. At a distance of 6 cm from the plasma source, a water-cooled substrate holder was positioned, which was biased at 13.56 MHz as well. A plasma-enhanced chemical vapor deposition (PECVD) process was used. Helium as a carrier gas and methane as a source of carbon were used for diamond-like carbon (DLC) film deposition. DLC films were characterized with a micro Raman spectroscopy and spectroscopic ellipsometry. In the wavelength range of 300–900 nm the measured spectra were analyzed by a regression-fitting algorithm, according to the nominal sample structure in which the optical properties of DLC were described by a Cauchy dispersion model. Optimum film quality was obtained in a bias voltage range, between −(350–420) V, which produces the films with the highest index of refraction, close to 2.15; a wavelength of 550 nm; lowest optical band gap close to 1.3 eV; and a Vickers hardness of 30 GPa. It was found that, even in the stationary deposition, mode film thickness variations across a 5-inch wafer did not exceed ±3.5%. A maximum deposition rate of 70–80 nm/min at room temperature was achieved.

Some crucial limiting process steps of ITO/(n)a-Si:H/(p)c-Si solar cell manufacturing were investigated. The influence of amorphous silicon deposition temperature on the efficiency of heterojunction (HJ) solar cells as well as ITO deposition on the properties of ITO/a-Si:H/c-Si interfaces were studied. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and SIMS measurements were employed for the investigations. It was observed that the decrease in a-Si:H deposition temperature from 230 down to 100°C leads to a reduction in solar cell efficiencies to only 1%. It was found that the ITO deposition process itself can significantly modify the ITO/a-Si:H/c-Si interfaces due to the interaction of oxygen, and In and Sn atoms with silicon at the initial stage. It was concluded that this modification has a more significant impact on the quality of the HJ solar cells than on that of the a-Si:H layer.

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 reprodu...

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.

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. Furthe...

Thermal nanoimprint and sputtering, two low-cost techniques, are combined to define metallic patterns by means of lift-off. The typically positive sidewall angle of imprinted structures in combination with sputtering for layer deposition leads to a continuous metal layer which has to be ruptured at a definite position, at the best along the respective pattern base, so that lift-off can work. This issue is investigated taking polystyrene (PS) as the imprint polymer and toluene as the lift-off solvent. The impact of the geometry factors and the impact of soaking and the power of ultrasonic agitation are addressed both experimentally and analytically to derive the conditions for the successful lift-off of 250nm wide lines over large areas.

Driven by the steadily increasing scale of integration in microelectronics much effort has been made in recent years to establish the technique of nanoimprint lithography (NIL) as a promising approach to time and cost-effective fabrication of nanometer scale patterns. NIL enables high throughput for mass fabrication in a simple way. Experiments have revealed that nanoimprint of small and periodic patterns in the nm range is much easier obtained than for μm scale patterns due to polymer transport phenomena. Thus a combination of NIL with optical lithography is considered to be advantageous for the definition of small patterns adjacent to large patterns. The multifunctionality of the material within one layer offers new routes in device design, in particular as multistep profiles are obtainable. In this contribution a combination of NIL and UV lithography is reported using polymers which are both imprintable and photosensitive. Based on an epoxy type negative tone chemically amplified resist system patterns were easily obtained using both techniques. In the first step a pattern relief was produced through imprinting. In the second step this relief was UV exposed in a contact printer and then developed. In this way three-step profiles could be generated within one single polymer layer.

Using different imprint systems and process conditions, the impact of the visco-elastic properties of polymers on the fidelity of pattern transfer in hot embossing lithography has been studied. The flow behaviour of the polymer, which is a highly viscous liquid at the investigated temperatures, has been shown to be the limiting factor in the replication of micrometer sized features, but does not adversely affect nanometer structuring. This is explained using hydrodynamic considerations. The effects during filling of the cavities formed between stamp and substrate are reported, and explained in accordance with recent publications, where two different types of filling mechanisms have been discussed. Large recessed stamp areas turn out to be much more critical for the process, as they may act as mask defects. They will not restrict the applicability of hot embossing lithography in practice when appropriate imprint conditions are chosen, which have to be enabled by the equipment.

We intend to apply guided self-assembly in thermal nanoimprint (T-NIL) for hierarchical pattern definition. To exploit the knowledge gained in a close-by field, capillary force lithography (CFL), comparable experiments are conducted and classified with respect to the surface profile obtained in the cavity, a key issue for controlled dewetting. Comparison of the results in literature for CFL with those obtained for T-NIL indicates unexpected differences between the two process types. Whereas CLF reports a transition from a W-shaped to a U-shaped meniscus with increasing polymer layer thickness, the opposite is the case for T-NIL. Similar differences exist in the theoretical description of the transition. With T-NIL it follows simple geometrical rules, nearly independent of temperature, whereas with CFL a temperature-dependent fitting of viscosity is applicable. We assign these differences to the different initial conditions. As a consequence, a simple transfer of CFL-rules to the control of guided self-assembly in T-NIL is not feasible.

A T-NIL/UVL hybrid process offers the potential to combine the advantages of thermal nanoimprint (T-NIL) - to define patterns in the nanometre range in parallel over large areas, with the advantages of photolithography in the ultraviolet range (UVL) - to define large patterns easily. It is attractive for the preparation of e.g. sensors, where typically nm-scaled electrodes are connected to macroscopic contacts. We address the critical issues of such a process that until now have hampered its application in praxis. Major aspects result from the required compatibility of both processes, asking for resists that are imprintable at temperatures well below any deterioration of the photoactive components. Furthermore, the lithography step has to be performed over a pre-patterned surface, which may require overexposure; the pre-patterned surface is also crucial during an eventually required post-exposure bake, where pattern loss may result from unintended viscous flow during crosslinking. Examples with SU-8 as a chemically amplified negative resist demonstrate that the hybrid process works, if some processing parameters are well tuned.

Preparation of a micro-zipper for component assembly requires an efficient anti-sticking layer between the two counterparts during preparation by moulding and during operation as well. In order to deposit a fluorinated silane on the PDMS-type materials used for zipper preparation, a UV-ozone pre-treatment at 172nm was investigated in detail. It was found that already at 2min of irradiation the surface features a sufficient amount of OH-groups for silane binding. Longer irradiation times were found to result in a surface layer of increasing brittleness that would prevent successful zipper operation.

Aromatic polymers based on methacrylates (linear polymers) and multifunctional allylesters (crosslinked polymers) have been prepared. They feature high dry etch resistance and good imprintability in a hot embossing process. The crosslinked polymers and their prepolymers were investigated in detail. Their characteristic molecular weight distribution and thermo-mechanical behaviour were studied with respect to the crosslinking process and compared with those of linear polymers. The thermal stability of imprinted patterns has been investigated for both, linear and crosslinked aromatic polymers. The latter ones show excellent stability up to test temperatures of 200 ^oC directly after the imprint process.

As a simple, low-cost alternative to rheological characterisation of an imprint polymer a novel approach is put forward, the derivation of imprint-relevant characteristic material parameters, here the viscosity, directly from imprint experiments. Imprints are performed at different temperatures with a fingerprint stamp, designed to allow replication of micron-sized linear structures without lateral constraint and independent from their neighbourhood. Under such conditions the squeezed flow theory applies and the viscosity can be determined from the imprint depth. For a typical material in thermal imprint, 25kg/mol PMMA, a broad agreement with rheological viscosity data could be obtained. Though the result requires critical assessment, the simplicity of the approach is appealing. Such procedure is promising for an easy access to material data for imprint process optimisation.