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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.

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.

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]

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.

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.

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.

Micron-sized dot-like arrowhead structures were replicated by moulding with an elastomer. In order to address the impact of prepolymer viscosity on stamp filling of the structures and the impact of elasticity on stamp separation of these structures three different PDMS materials were investigated. The moulding results obtained for different geometries of the arrowhead structures were evaluated with respect to typical defects. The results could be well understood in view of the material properties derived from simple characterisation measurements. Sylgard 184 provided the best replication results.