The role of capillary forces with nanoimprint is addressed. As the respective capillaries are the closed cavities of the stamp used for replication, the Laplace pressure together with the pressure of the gas phase inside the cavities dictate the equilibrium pressure in the polymer. Whether the cavities can be successfully filled, depends on the external pressure available from the imprint system, setting the pressure in the polymer, and on the sorption of any gaseous phase. Gas sorption is provided by the imprint polymer itself and may differ with different imprint materials and different imprint situations (imprint temperature, residual layer thickness). With soft stamps or composite stamps also the stamp itself may contribute to the overall gas sorption. Situations typical of different imprint techniques (thermal nanoimprint, UV-assisted nanoimprint, capillary force lithography) as well as of moulding during stamp replication are discussed in view of capillary effects. The discussion is illustrated by simple analytical estimations.

With nanoimprint, instabilities of the polymeric layer between the substrate and the stamp are observed when incomplete filling of the stamp cavities prevails, resulting in capillary bridges and meandering structures. Experiments show that instabilities may also affect the initial situation of a thermal nanoimprint process. Already at the beginning of the actual imprint step, the polymeric surface may be highly non-uniform, with undulations of the film thickness, polymeric bridges and de-wetted regions; thus, the starting point for imprint may differ substantially from the general expectation of a uniform spin-coated layer. A detailed instability analysis considering the combined effect of van der Waals forces, electrostatic forces and temperature gradients under realistic imprint conditions clearly identifies temperature gradients to be the cause for high initial layer non-uniformities. We found that during heat-up (before imprint) temperature differences of 10 °C may exist between the top and bottom hotplate of a thermal imprint system. These temperature differences result in the non-uniformities of the polymeric layer observed, when stamp and sample are heated under low-pressure contact, where some small but locally arbitrary gaps exist between polymer and stamp. This result asks for a reconsideration of the thermal nanoimprint procedure. It also confirms the high impact of temperature control for uniform, low-defect imprint results; this is critical not only during imprint, but also during heat-up.

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.

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 2 x 2 cm(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 60 mN per trench of 1 cm length were measured for lines with 11.5 mu m 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. (C) 2012 Elsevier B.V. All rights reserved.

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.

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.