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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.

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.

In a thermal imprint process, contact angles may evolve in two situations, between the stamp and the polymer, when a cavity remains unfilled, or, in cases where the polymer locally dewets the substrate, between the substrate and the polymer. For two polymers of different polarity, such contact angles are determined experimentally and compared to values calculated from surface energy data. In doing so, the specific temperature dependence of surface energies of a thermoplastic polymer as well as the typical course of an imprint process, where cooling times often exceed the imprint times, are considered. The differences between experimental and theoretical values are critically discussed. The results indicate that the respective adhesive layers govern the surface status of the substrate and the stamp as well.

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.

Based on the well-known fact that thermoplastic polymers feature a decrease of viscosity at increased frequency we propose a novel tool for frequency assisted thermal nanoimprint. The system is equipped with a stepper motor to drive into contact and to apply a static loading. In addition a piezo-unit is available that allows the superposition of the static load with a dynamic excitation. Detailed analysis of the frequency response of the overall system makes obvious that the frequency range available is limited not only by the frequency cut-off of the piezo amplifier, but also by its power or rather by the limited output current available. As a consequence the maximum frequency at full displacement is only 10 Hz. Nonetheless this should be enough to reduce the viscosity of typical imprint polymers at a low imprint temperature. The measurement system is sensitive enough to detect the small changes induced by the polymeric layer in the imprint stack, when the temperature is raised to typical imprint temperatures. Decay times for the residual force during imprint with a conventional imprint stack could be obtained from a relaxation experiment, where the piezos are used as step-displacement sources. The data are in excellent agreement with values calculated from dynamic rheological characterization experiments.

During hybrid lithography of thermal nanoimprint with optical lithography, where both steps are performed within one single resist layer, changes of the lithographic performance of the material used during the first step, the thermal imprint, are essential for the overall lithography result. In order to characterize such changes two positive tone photoresists, AZ-1505 and ARP- 3510, were characterized by measurement of dose curves (relative resist layer thickness remaining after exposure and development) after a specific temperature treatment simulating the thermal imprint step. Obviously the two materials are affected in a somewhat different way. After temperature treatment at 120°C, ARP-3510 still features a higher sensitivity than AZ-1505, but the latter shows a higher contrast.

In order to utilise both techniques in the best and easiest way, thermal nanoimprint (T-NIL) and optical/UV lithography (UVL) were combined in a hybrid process to pattern the same resist layer. Typically, T-NIL as the first step defines the small, submicron patterns, UVL as the second step the larger patterns, well above 1 μm. The hybrid process is performed with a conventional Si stamp for imprint and a conventional photomask for UVL and does not require any special infrastructure. Conventional, commercially available positive and negative tone resists were used, SU-8 and AR-P 3510. In the case of SU-8, a chemically amplified negative resist with low glass temperature, the imprint is uncritical and does not result in any optical degradation. However, the post exposure bake required to finalise the crosslink reaction has to be adapted and performed as a temperature ramp. In the case of AR-P 3510 the imprint step is critical, as the photoactive component is degenerated at suitable imprint temperatures. We found that at temperatures up to 130°C this degradation can be compensated by an increase of the exposure dose. Though more critical in processing, the positive tone resist features a better definition of the transition region between imprinted patterns and UV patterns.

For the preparation of interdigitated sensor devices with nanometre sized electrodes a low-cost route is followed. The central technique used for electrode definition is nanoimprint. To imprint the larger contact areas as easy as the electrodes, the contacts are broken down into a grid. In order to end up with a highly uniform residual layer the concept of 'partial cavity filling' is utilised, resulting in an almost negligible layer thickness. The metallic electrodes are defined by sputtering and lift-off directly after imprint, where a previous etching of the residual layer is not required. The results show that the concept works. With this strategy, preparation of an interdigitated sensor requires nothing but spin-coating, nanoimprinting and sputtering/lift-off.

When minimal residual layers are aimed at in the field of thermal imprint lithography (T-NIL) 'partial cavity filling' offers a promising method to succeed. There, a filling of any of the cavities a stamp provides is avoided by a reduction of the initial layer thickness. Due to this unusual procedure, some challenges evolve during the imprint of complex types of structures. We report about effects that go along with partial cavity filling like physical self assembly and recovery. Furthermore we will give some suggestions how they may be reduced or even can be avoided.

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.

The positive tone resist AZ 1505 is characterized with respect to hybrid lithography that uses thermal nanoimprint lithography to define the nanometric part of a lithography pattern in a first step, followed by optical lithography to define the micron-scaled patterns in the same resist layer in a second step. The parameters investigated are glass temperature and sensitivity (dose curves), both after thermal loading in a typical imprint sequence. The glass transition of the multicomponent resist formulation is derived from stress measurements and the dose curves are evaluated from development rate monitoring via laser interferometry at 532 nm. The results show that both parameters are affected by thermal loading, but adequate choice of the processing parameters makes it possible to compensate for the thermal degradation of the photoresist as long as the imprint temperature chosen does not exceed 130 °C. Based on the characterization results, successful hybrid lithography and lift-off has been demonstrated ...

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.

In order to investigate the nonuniformity occurring below wider patterns during thermal nanoimprint lithography, the pressure situation is analyzed in detail. A balance of vertical forces shows that the gas pressure within the cavities is negligible, whereas the effective pressure acting on the stamp structures in contact to the polymer is increased compared to the externally applied pressure. This effective pressure has a high hydrostatic component, which may result in local stamp deformation. Imprint with a low molecular weight polymer provides evidence that the stamp structures become compressed under high pressure, resulting in a nonuniform residual layer beneath the imprinted patterns. In contrast, bending deformation of single stamp structures is found to be negligible. Pressure reduction is effective to reduce stamp compression, improving the uniformity of the residual layer. With typical imprint polymers of medium molecular weight, however, pressure reduction reduces the overall imprint depth.

Imprints into thin thermoplastic layers in thermal imprint often go along with phenomena such as physical self-assembly and recovery, which impair the resulting replication quality. To avoid recovery, in particular within larger structures, a polymer with extremely low Mw (polystyrene with a molecular weight of 1kg∕mol) is investigated. In contrast to expectations, the polymer develops a strong recovery. This occurs at such low viscosities that the imprint is also impaired by strong physical self-assembly effects. The imprint is further hampered by the fact that, due to the very low glass-transition temperature, a layer preparation without prebake is required. In addition the suitable temperature window for imprint is extremely small (T≈40°C) so that process control is hardly feasible. Polystyrene with such low Mw is not suitable for thermal imprint.

To improve control during the filling of the channels of a microfluidic device with the fluid under investigation, a new concept is presented: channels with a diamond-shaped cross section. These channels easily allow one to distinguish an unfilled channel from a filled channel by simple optical microscopy. The idea is based on the fact that incident light is totally reflected with unfilled channels, whereas it is mostly transmitted with filled channels. Preparation of such channels in thin SU-8 layers on a glass was performed via a double replication of a Si template with undercut trenches. Functionality tests with filling materials spanning the range of refractive indices of typical organic media provided an excellent contrast between filled and unfilled channels, thus demonstrating the capacity of this concept.

Three dimensional linear structures with definite undercuts of different sizes were replicated in the first step in an elastomeric material and, in the second replication step, in a negative tone resist. Undercut masters for the first step were prepared from Si by laser ablation and anisotropic wet etching. For the second step the elastomeric replica, featuring the inverse undercut, served as a master. The first replication was performed by molding or imprinting, where the prepolymer of the elastomer was cross-linked within the master in an oven or in a thermal press, respectively. The second replication of the undercut elastomeric structures into the negative tone resist was performed successfully by molding. Comparison of the Si masters with the second replication revealed shrinkage-induced differences. Undercuts of up to 60% of the overall pattern width were successfully replicated.

To address the importance of the parameter glass transition temperature Tg for the choice of an adequate process temperature in a thermal nanoimprint process, three polymers with different values of Tg were investigated with respect to their imprint behaviors (poly)vinyl-chloride, (poly)styrene, and (poly)methyl-methacrylate. In order to evaluate the imprint results obtained at different temperatures, the volume of the polymer squeezed into the cavities was observed, and the temperature margins for the appearance of typical pattern size dependent failure types were derived. It turned out that despite the comparable molecular weight of these materials, the imprint temperature has to be chosen at different levels above Tg. Thus, Tg is not sufficient to characterize imprint polymers, and, in addition to the molecular weight, the viscosity at a definite temperature should be known. Tg alone can only indicate a lower limit for the process temperature.

Thermal imprint into polymer layers that are thin compared to the pattern height allows for imprints with extremely low and uniform residual layers where lift-off works without previous dry etching. The authors report about possible restrictions of this method such as unintended self-assembling and recovery of polymer underneath the imprinted structures. Both can be attributed to structure size properties and temperature influence and hinder a subsequent successful lift-off. Taking the example of two different temperatures and two different initial layer thicknesses the authors investigate their impact on the resulting structure shape and potential defects. In addition, to qualitatively estimate the residual layer thickness the authors apply lift-off and discuss the results with focus on the usability of this approach as a lithography technique.