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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.

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.

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.

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 ...

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...

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

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

The properties of the thermosetting polymer mr-I-9000, developed and used for nanoimprint lithography (NIL), were investigated by means of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The spectra of mr-I-9000 were compared with those of PMMA and show the presence of aromatic rings and a high amount of carbon–carbon and carbon–hydrogen bonds. As a consequence, the mask selectivity of mr-I-9000 in an oxygen and tetrafluoromethane reactive ion etching processes is higher than that for PMMA and slightly different for the cross-linked and non-cross-linked state.

Performance and functionality of devices prepared from organic semicrystalline polymers strongly depend on the size and the orientation of the ordered crystalline domains, as the conductivity is distinctly different in the different lattice directions. This investigation addresses the potential of thermal nanoimprint to control the size of the ordered domains as well as their orientation with respect to the substrate. Poly-3-hexylthiophene (P3HT) is chosen as one of the prominent semicrystalline polymers available at high quality. In order to control the ordering of the domains freely, the imprint is performed at a temperature beyond the melting point to eliminate the domains already existing after spin-coating and soft-bake. To avoid a degradation of P3HT at such a high temperature under oxygen contact—the imprint system used operates in air—a novel imprint procedure is employed, where a distinct prepressure is applied already during the heating of the imprint stack (stamp/substrate with P3HT layer). To ...

Flexible stamps are common for roll-to-roll processing but less common with planar processing, although they offer a number of benefits as, e.g., an improved conformal contact at reduced pressure. A simple way to realize such a flexible stamp is to use a two layer system with a structured top layer and a flexible backplane. The structured top layer is most easily obtained by molding, the backplane provides the flexibility envisaged. For use in a thermal nanoimprint process, a high thermal stability is required for both. This investigation addresses the preparation of flexible composite stamps with OrmoStamp as the structured top layer and polyimide as the flexible backplane. The process recommended for stamp preparation with OrmoStamp has to be modified to avoid bending after the hard bake that is required after ultraviolet-curing of the material to obtain a high stability of the top layer. Reduction of bending is advised, in particular, for large area stamps, where the hard bake step is in conflict with ...

Relief of residual stress in an imprinted polymer may affect the replication fidelity by leading to recovery. The level of stress induced in the polymer depends on the method of imprint. For example, a “soft” imprint with an elastomeric stamp uses capillary forces to fill the cavities whereas a “hard” imprint with a rigid stamp relies on external pressure. To study the effect of residual stress after imprint, both methods are applied with different imprint times to vary the level of residual stress, as the stress remaining relaxes with imprint time. To visualize the residual stress a temperature treatment is performed after imprint. This temperature treatment allows recovery within a convenient experimental time. A comparison of the shape of the imprinted structures before and after temperature treatment clearly shows that with a hard imprint at short imprint times a considerable amount of stress remains in the polymer, in particular when the residual layer is thin and the imprinted stamp structures are wide. With a soft imprint residual stress is not evident. Similar results are obtained with a thermoplastic material and a crosslinking material (SU-8); however, with the latter recovery remains limited due to a decrease of mobility during crosslinking. Residual stress is of major importance for the replication fidelity with short imprint processes.

The wrinkling of SU-8 may be useful for microelectromechanical systems and optical applications, but highly regular and periodic wrinkles are the most convenient for such applications. This paper reports wrinkles generated on SU-8 by exposure to 172 nm-wavelength ultraviolet under ozone. To identify the relevant processing parameters, the impact of the SU-8 layer thickness and the effects of the postexposure bake temperature and UV-ozone treatment time are investigated. With 5 μm-thick SU-8 layers, distinct wrinkles are obtained with an ultraviolet-ozone treatment of 3 min and at a postexposure bake temperature of at least 140 °C. To guide the wrinkling, topography is induced on the SU-8 surface via capillary force lithography. By using a stamp with equidistant cavities possessing different widths, the structure size dependence is investigated. Owing to local stress relaxation, single or double wrinkles form along the elevated lines depending upon the line width. Within the gaps between the lines, wrinkles oriented perpendicular to the lines develop induced by the nonrelaxed compressive stress. Wrinkling represents a simple alternative for covering the surface of SU-8 with largely self-forming regular structures that may act as transmissive optical elements.

Although the direction of flow during imprint for amorphous polymers is accessible via simulations, a direct observation in the experiment is difficult. The authors address an indirect method, the detection of imprint-induced ordering in the residual layer by optical inspection of the imprint result. The method relies on imprint materials featuring optical anisotropy when existing in an ordered state, ordering being induced by the imprint process proceeding under shearing flows. Organic polymers with π-electrons along the backbone were chosen to test the approach; similar materials with regular and random chemical chain structure were compared, where only the polymer with regular chemical structure exhibits optical anisotropy after imprint, due to its semicrystalline nature. A clear correlation of the anticipated flow direction in the residual layer with optical anisotropy in transmission was possible. Beyond visualization of the direction of flow, the experiments allowed to attribute the local ordering observed in the residual layer with poly 3-hexylthiophene-2.5-diyl to a flow-induced stretching of the polymeric network occurring at the beginning of the squeeze process.

Hybrid lithography is applied to the negative tone resist ma-N 405, successfully combining thermal nanoimprint lithography (to define submicron patterns) with conventional photolithography (for micrometer-pattern definition). When employing such a hybrid technology scheme, one often finds it difficult to achieve a good pattern fidelity in the overlapping areas due to uneven exposure relating to the formation of standing waves during exposure. To avoid such standing waves, one has to infer an antireflective layer. Alternatively, one can make a short heat treatment. Such a heat treatment levels the uneven exposure but may lead to a reflow of the preimprinted pattern. In order to minimize reflow during this postexposure bake, flood exposure is integrated into the processing sequence whereby the dose required must be optimized. The results show that with an adequate choice of dose for flood exposure and masked exposure, the reflow can be avoided, and the standing waves are reduced to an acceptable state. Particularly, these conditions make it possible to remove residual layers of up to 30 nm simply by development. The mixed patterns defined by hybrid lithography in this way can be successfully transferred into a metal pattern.

Self-assembly defects limit the suitability of partial cavity filling in thermal nanoimprint for residual layer minimization. As they represent thermodynamically stable structures already the formation of such defects has to be avoided—because once developed they cannot be removed. One impact parameter for the formation of self-assembly defects is the quality of the stamp surface, in particular its roughness. The authors compared stamps with flat surfaces obtained by potassium hydroxide etching of (110)-Si with stamps prepared by dry-etching, featuring vertical grooves or horizontal scallops (typical of deep-etching in a Bosch process). Under comparable conditions, the stamps with the vertical grooves feature the highest amount of self-assembly defects. In contrast, horizontal scallops avoid defect formation. Thus, the surface quality of the stamp sidewalls is important for a high quality imprint.

Hybrid lithography combining thermal nanoimprint with optical lithography is an effective means to prepare structures with differing geometries (nanometer, micrometer) over large areas, e.g., for sensor applications. For the realization of this technique, the suitability of photoresists for this process has to be investigated. In case of negative tone resists, the cross-linking induced or provided by exposure may be amplified in the imprint step due to heating times in the minute range. As cross-linking may impede the imprint, a subtle adjustment of the exposure dose with respect to the imprint temperature envisaged is required for successful processing. With an imprint temperature of 100 °C, good results for hybrid lithography could be obtained with two commercially available negative tone resists: ma-N 405 and ARN 4410. The latter could successfully be used for a subsequent liftoff. The best results were obtained when the resists were exposed before the imprint step.

Dynamic agitation during imprint is investigated as a means to reduce the imprint temperature of thermoplastic materials. The imprint system used consists of a stepping motor and a piezo unit as the main components. The motor controls the working point of the system, and a sinusoidal motion of the piezo unit (3 piezos at 120° with respect to each other) superimposes an additional dynamic excitation. The system was characterized with respect to its mechanical behavior by exploiting analogies between mechanical and electrical quantities. The results show that the system can be operated at a frequency of up to about 60 Hz and that the piezos act as a dynamic motion source in the system. First imprint experiments with polystyrene at 100 °C indicate the potential of this frequency-assisted imprint technique.

Two low-cost techniques suitable for large area processing are combined to define submicrometer metal structures, thermal nanoimprint lithography, and sputtering. With very low residual layers after thermal nanoimprint, as obtained under partial cavity filling conditions, it is possible to skip the etching step for residual layer removal before sputtering. As imprinted structures typically feature a positive sidewall angle, the authors investigate the morphology of sputtered layers on sloped sidewalls. Independent from inclination, the sputtered layers exhibit a columnar structure with only a slight taper, where the crystallites extend perpendicular to the substrate through the whole layer thickness. This structure allows penetration of the solvent, swelling of the polymer, and lifting of the top layer as long as the adhesion to the substrate within the contact area is sufficiently high, thus enabling successful lift-off.

Thermal nanoimprint (T-NIL) is uniquely suitable for combinational lithography because it proceeds without cross-linking and does not rely on changes in properties of the imprint material. Most typical are combinations with other lithography techniques. Combinations with optical lithography may help to circumvent the pattern size dependence of nanoimprint and help to remove the residual layer by a simple development step. This work focuses on a hybrid processing combining T-NIL with optical lithography by using a single resist layer. Critical issues are the imprintability of typical photoresists and the fact that lithography has to be performed over a prepatterned topography. A well-known positive tone photoresist, AZ 1500, is compared with a previously investigated negative tone resist, SU-8. Gel permeation chromatography reveals typical differences of these photoresists compared to typical imprint polymers. The results demonstrate that molecular mobility is a more important indicator for viscosity of photoresists than molecular weight. Additionally, a simple construction method was found to allow prediction of features typically encountered during hybrid patterning.