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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

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

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.

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 (

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.

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

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.

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.

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]

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.

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.

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

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...

This paper presents a study concerning the macroscale deformation of soft stamps for nanoimprint lithography. Composite stamps consisting of two or three layers were prepared with varying process parameters, and the bow of the stamps was investigated with an optical profilometer. Deflection measurements were compared with results from a mechanical model in order to prove its validity. The composite stamps were prepared from polydimethylsiloxane (S-PDMS) and OrmoStamp® layers on different substrate materials (silicon or AF-32-glass). The influence of selected preparation parameters on the deformation was investigated, such as layer thickness, curing, and hardbake temperatures, as well as UV-exposure intensities. It is discussed how an appropriate choice of preparation parameters can reduce the deformation, too. By decreasing the curing intensity of a thin OrmoStamp layer, coated on top of a silicon wafer, from 200 mW/cm2 to 2 mW/cm2, the deformation was reduced from 6 μm to 0. It is shown that the mechanic...

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.

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.

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.

We have studied the suitability of poly(methylmethacrylate) (PMMA) of different molecular weight for large area pattern transfer by embossing structures above the glass transition temperature (Tg) of the specific polymer. The molecular weight covers a range of one order of magnitude (Mw∼5.0×104–8.1×105). This range was chosen in order to obtain information regarding the flow properties that we expect to depend strongly on the molecular weight at a specific temperature. Large area pattern transfer were tested by applying a stamp with both densely packed and isolated features. The feature size ranged from 100 μm down to 450 nm. At a processing temperature of 90°C above Tg we found clear indications that flow is sufficient to transfer large, isolated features even into the polymer with the highest Mw. Problems of incomplete material transport can be related to local inhomogeneities of the imprint due to a lack of parallelism between the stamp and the sample. At 50°C above Tg incomplete flow effects were observed over the whole area and for all molecular weights. This was observed only with large, isolated structures whereas small, periodic features showed a well defined transfer.

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.

Nanoimprint is addressed as a novel technique to define nanometer-scaled patterns, in view of largely regular patterns as typical for photonic devices. The main techniques, thermal nanoimprint and ultraviolet nanoimprint, are characterized with respect to their system parameters as well as their processing parameters. Based on simple analytical equations the most important issues for these nanoimprint techniques are discussed and brought into a quantified form. A major impact is laid on the pattern size dependence and the need for anti-sticking precautions. The former, though not of basic impact for the highly regular devices in photonics, shows up when positive or negative type stamps are used. The latter is an ultimate must for successful separation of the imprinted sample from the stamp. It is aimed to assist the reader in developing its own critical view of nanoimprint, in the most positive sense possible. There is no doubt that photonic devices are one of the pattern types most suitable for successful definition by nanoimprint when the guidelines developed here are followed.

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 ...

The main nanoimprint lithography techniques are highlighted in this paper. In addition to details concerning the materials used and the basic processing techniques, some of the major recent developments are summarized. Critical issues are addressed and some ideas for improvement are given. The paper aims to give an introduction to the techniques and a survey of the actual status without going into the details of the specific techniques.

The idea of subdividing the conventional hot embossing process into subsequent process steps is introduced as a modular hot embossing concept. Based on the EVG 520HE the process is separated into modules so that typical temperature cycle times can be circumvented because the imprint is carried out as an isothermal embossing process where the applied pressure is released at a temperature above Tg. Before detachment of stamp and imprinted substrate cooling is applied in a separate module. The full benefit of such a modular concept is achieved when every module is equipped with its own embossing tool and therefore with its own template. We have investigated a new way of preparing working-stamps, which are cost-efficient namely the polymeric replications of Si templates. We have chosen mr-L 6000, an UV curable imprint resist as a working-stamp material. It was possible to obtain a complete cross-link without PEB, which is beneficial with respect to pattern fidelity conservation. Furthermore XPS measurements have shown that a silanization of the cured resist is possible to serve as an anti-adhesion layer. IR alignment as a part of the modular concept and its issues have been discussed on the basis of the EVG450 bond aligner by using Si templates and Si substrates. It is shown that an embossing tool enabling alignment may affect pattern transfer.

Nanoimprint was performed in very thin layers of polystyrene (PS) in order to define a mask with minimum CD loss for a subsequent etch process at minimum etching time for opening of the mask windows after imprinting. The initial polymer layer thickness was chosen as to fill the stamp cavities with nearly no surplus of polymer material. The residual layers after imprint were in the range of 50 nm and could be cleared at 50% overetch within 90 s in an oxygen RIE step. As there was not enough polymer material available for a complete filling of the cavities when a residual layer remains, filling defects occurred. High imprint temperature and thus low viscosity led to formation of deep defects in the imprint and thus the mask to be formed by embossing. Lift off experiments revealed that within the defective regions the remaining polymer layer thickness was smaller than the imprinted residual layer. In order to avoid such mask defects the imprint temperature had to be reduced.

While researchers of ever more advanced NGL systems are still struggling to demonstrate the feasibility to manufacture features well below 100 nm at an affordable cost and a reasonable throughput, nanoimprint technologies are emerging as a possible answer to these challenges. 100 nm patterns are imprinted with a fully patterned 4 inch diameter stamp in a low-temperature embossing process. In low temperature imprinting processes with polymers having very low glass transition temperatures heating and cooling cycles are minimized. This enables to increase the throughput of a hot embossing process, which is important for potential industrial applications.

Sidewall lithography was used as a low cost fabrication process for high aspect ratio stamps for hot embossing. The stamps, featuring up to 1 pm pattern height and an aspect ratio of about 5:1 were tested in a typical hot embossing process at 50 bar. Mechanical stability of the crystalline stamp structures was achieved after optimization of the respective dry etch process. At imprint temperatures of 140°C PMMA was imprinted over areas of four inch diameter. After residual layer removal in the trenches Al electrodes of 500 nm height and 200 nm width could be defined by evaporation and lift off.

One of the key questions concerning the concept of a system for hot embossing lithography is whether or not it should provide for imprinting under vacuum. We have performed experiments comparing the embossing in vacuum and in atmospheric pressure in a semi-automated imprint system. The stamps used were fully patterned, 10 cm diameter with pattern sizes ranging from 400 nm to 100 μm. It turned out that vacuum enhances the large area uniformity of the imprint by avoiding an air cushion remaining between stamp and sample during automated contact after a non-contact assembly and alignment step. Lower molecular weight polymers turned out to be more sensitive to uniformity deviation than higher molecular weight materials. Detailed analysis showed that defects typically found for relatively high processing temperatures, caused by overheated compressed air, remaining solvent in the polymer layer or even beginning polymer decomposition could be reduced substantially under vacuum embossing conditions, where the excess volume of the polymer is evacuated and free to accommodate gaseous constituents. The best result with complete cavity filling and negligible defects was obtained for imprint of a 99 kg/mol polymer at 200°C and 50 bar under vacuum. Residual layers measured across the diameter of the sample were 44.5 nm ± 9.8 nm. The non-uniformity of the residual layer is a result of the locally different pattern sizes and pattern densities of the stamp, typical for all mechanical patterning processes.

In order to reduce the cost for stamps featuring nanometer structures in a hot embossing lithography (HEL) process the production and performance of working stamps made of thermoset polymer are of interest. Fabrication of stamps made of thermosetting material no silicon substrates by hot embossing with 2 X 2 cm 2 templates and their replication by HEL has already been demonstrate. In this paper the enlargement of this principle to 4 inch wafer- scale is presented. Two procedures to obtain working stamps by hot embossing are compared, one solely based on hot embossing, the other enhanced by additional UV-exposure. The produced working stamps are tested for performance under standard embossing conditions are the topic of anti-sticking layers, a key issue in all large area imprint applications is addressed. Two methods of tailoring adhesion properties of thermosets are proposed, plasma-depositing a fluorinated film and coating with a self-assembled monolayer of fluoroalkyltrichlorosilane, only the former of which was employed successfully. The achieved fidelity of pattern replication with working stamps and imprints thereof is assessed by cross-sectional SEM investigation, showing only the UV-enhanced method to be well suited for the task of obtaining low-cost replications of silicon stamps.

Stamp fabrication for nanoimprinting can be significantly simplified, when specialized crosslinking polymers are applied to pattern definition. The polymer patterns can be used as stamps themselves. Two possibilities are reported: (1) An e-beam sensitive resist was developed, which enables the fabrication of polymer-on-silicon stamps. Patterns with a feature size of 70 nm could be created. (2) Full plastic stamps were obtained by a casting-moulding technique, which enable pattern transfer from any conventional mould. The quality of the two stamp variants were proved by imprinting experiments.

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.

We summarize flow and filling effects detected during hot embossing lithography (HEL), which reflect the visco-elastic properties of the polymers as well as the specific processing conditions and stamp pattern configurations. In particular a delayed elastic recovery is reported, which is found in larger elevated stamp regions. The consequences of these effects for mask definition by HEL are discussed, and strategies for wafer-scale imprint are de-duced. Successful imprint of 4' wafer is demonstrated under adequate processing conditions.

Replication of the e-beam master into thermosetting polymers was investigated for low cost working stamp fabrication for nanoimprint lithography (NIL). Negative as well as positive replicas from a master were fabricated in a thermal replication step from duroplastic material. Without any further treatment these working stamps were able to imprint thermoplastic polymers. To demonstrate the procedure we used the custom polymers mr-I-8130 and mr-I-9030 as well as PMMA for comparison. All replications were done in a NIL process. The replication quality is excellent when a suitable imprint procedure is used.

Within the last few years a number of new techniques have emerged for definition of nm-scale patterns. They may be summarised under concepts like bottom-up or top-down, addressing the basic direction of pattern assembly. Bottom-up techniques based on the self assembly offer a very high innovation potential. At the other hand, top-down techniques are somewhat nearer to state of the art technology. These latter may be optimally suited for bridging the gap between conventional techniques and the high end self assembly.

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.