Surface roughness at the nanometer scale limits the optical performance of reflective components for X-ray and extreme ultraviolet beam shaping. While sub-nanometer roughness can be achieved by polishing planar substrates, it remains challenging for con-tinuous three-dimensional topographies fabricated by grayscale direct-write lithography in polymer resists. In this work, mm-long linear grayscale slopes are introduced as a cali-bration platform to distinguish between form, waviness, and roughness contributions. Process optimization reduces artefacts such as gray-value discretization and stitching, while replication into PMMA combined with the TASTE process enables a reduction of intrinsic roughness below 2 nm. Laser scanning confocal and atomic force microscopy are used as complementary techniques to assess surface quality across spatial scales. The re-sults provide insight into the origin of roughness in novolak-based resists and its evolu-tion through the fabrication chain, highlighting material limitations and paths toward smooth polymer optics.

Nanoimprint lithography (NIL) is a high-resolution parallel patterning method based on molding. It has proven resolution down to the nanometer range and can be scaled up for large areas and high throughput. Its main characteristic is that the surface pattern of a mold is imprinted on a material that is displaced locally by using the difference in hardness of the mold and the moldable material, thus replicating its surface topography. This can be achieved by shaping a thermoplastic film by heating and cooling (T-NIL) or a photosensitive resin followed by a curing process for hardening (UV-NIL). In lithography, the local thickness contrast of the thin molded film can be used as a masking layer to transfer the pattern onto the underlying substrate. Therefore, NIL will be an alternative in fields in which electron-beam lithography and photolithography do not provide sufficient resolution at reasonable throughput. Direct imprint enables applications where a modified functional surface is needed without pattern transfer. NIL is currently used for high-volume manufacturing in different applications, like patterned sapphire substrates, wire grid polarizers, photonic devices, lightguides for AR/VR devices, metalenses, and biosensors for DNA analysis, and is being tested for semiconductor integrated circuit chips.

Initial studies indicate that structured polymer surfaces can support the attachment and biofilm formation of bacteria and thereby provide enhanced positive effects of beneficial bacteria, for instance in biocontrol in aquacultures. In this study, we demonstrate a test platform to further explore the surface topography for bacterial attachment and biofilm growth. It is based on a cyclic olefin copolymer (COC) materials platform, and nanoimprint technology was used for the replication of microstructures. The use of nanoimprint technology ensures precise micropattern transfer, enabling easy prototyping. Further, the process parameters of the mold preparation and nanoimprinting are discussed, with the purpose of optimizing the polymer pattern profile. This study has the potential to identify promising surfaces for biofilm growth of beneficial bacteria.

The combination of imprinting and extrusion overprinting of composite glass materials offers a pathway for the fabrication of fully integrated functional microfluidic devices. Utilizing novel low-temperature phosphate glasses, originally developed for 3D extrusion printing, imprinting now has been achieved with soft stamps. The current hybrid process demonstrates superior resolution compared to previously reported extrusion techniques, presenting possibilities for applications such as open microfluidic channels that can subsequently be sealed by extrusion printing. The water-based microparticle glass-ink enables patterning at room temperature and its solidification occurring through water evaporation within the glass particle scaffold. Following the demolding, sintering is conducted at a temperature of 485 °C, resulting in dense glass elements with porosity levels below 1 %. For straightforward microfluidic channel structures featuring channels ranging from 100-200 µm in width, the shape is fully preserved during demolding and sequential annealing. The shrinkage observed is almost isotropic. The channels were successfully tested in an open microfluidic setup and initial tests were performed with the closed channels.

Straight, almost vertical sidewalls can be achieved in thick negative-tone resist using focused laser beam-based direct write lithography (DWL). This is possible despite beam divergence and Gaussian intensity distribution that cause non-homogeneous illumination over the resist thickness and non-sharp edges between exposed and nonexposed areas. Three write modes with different focal lengths, numerical apertures and resolutions were used in our DWL 66+ pattern generator from Heidelberg Instruments. By variation of dose, vertical adjustment of location of focus, and multipass exposure, up to 88 degrees sidewalls were achieved in 60 & mu;m high and 100 & mu;m wide ridges. The results show the potential of DWL to achieve resist structures with larger height up to 200 & mu;m, higher resolution down to 5 & mu;m, aspect ratio of over 8 and defined sidewall shape.

Neuromodulation treatments are based on the functional interface between the brain and man-made electrodes. Chronic inflammation around the implant, however, occurs as a result of mechanical mismatching between soft brain tissue and the stiff metallic electrode. After water uptake, cellulose films should prevent encapsulation of the electrode. In this work, we reinforced cellulose films with silk fibre networks, improving tear strength by a factor of four to seven and maintaining flexibility, characterised by an elastic modulus between 100 and 200 MPa. Whereas sputtered gold on flat films exhibited restricted adhesion, micro-patterning guaranteed reasonable adhesion of nanometre-thin gold to cellulose substrates. Micro-patterned cellulose films coated in 80 nm-thin gold retained conductivity for strains as large as 30%, while sheet resistance increased by a factor of about 30. Fabrication of the biocompatible electrodes is efficient and compatible with large-scale production.

For the characterization of the mechanical deformation of materials at microscopic length scales, image processing of a high-quality surface pattern was used. We imprinted speckle patterns onto a thin polymer film attached to the surface of flat and curved metal substrates using flexible molds and soft-thermal nanoimprint lithography. High optical contrast was achieved by mixing black dye into the film generating high absorption in the elevated structures, and by adding titania nanoparticles as fillers to the recessed areas to induce diffuse scattering. For accessing resolution suitable to detect deformation at an individual grain level, the structure sizes were scaled down from 20 μm to 2 μm. For both structure sizes imaging was tested using a digital image correlation setup, that enables 3D imaging of samples with angles of up to 10° of inclination.

Laser-based pattern generators are versatile tools for transferring design layouts into resist structures. The non-linear response and the proximity effects caused by the interaction of the laser beam with the low-contrast photoresist are particularly critical for the creation of shapes with sloped, stepped and continuous topographies. Cross-sections were taken to compare topological differences of the measurement results with the graytone design. For simple shapes, an iterative adjustment of the global dose distribution is sufficient, while for more complex shapes the local correction of dose values using mathematical models is preferred. For the one-step exposure of the corrected map, it requires the careful assessment of a contrast curve and knowledge of various parameters. The coefficient of determination R2 as a unitless single figure of merit proved useful in the quantitative comparison of the two methods, and demonstrated the capability of current model-based approaches for shape optimization towards the target design within one cycle.

Neurodegenerative diseases can be treated using a functional interface between the physically soft tissue such as brain and the man-made electrodes. The orders of magnitude harder neural probes cause local injuries, due to periodic micromovements owing to breathing and pulsatile blood flow leading to encapsulation and related collapsing signals. An alternative to the currently used neural implant films including polyimide, poly(p-xylylene), SU-8 - epoxy-based negative photoresist, liquid crystal polymer, and benzocyclobutene is the natural polymer cellulose with an elastic modulus between 100 and 200 MPa. This article elucidates the measurement of the mechanical properties of bare as well as mono- and double-layer silk-reinforced cellulose in phosphate-buffered saline using a universal testing machine. In addition, the article contains electron microscopy data of these micro-structured, gold-coated films subsequent to peel-off tests to access the impact of micro-structures on gold adhesion on cellulose. These imaging data were completed by electron micrographs of mechanically loaded gold-coated cellulose films to demonstrate the impact of micro-structures on crack formation. Finally, the phosphate-buffered saline-induced swelling of the micro-structure was visualized by electron micrographs obtained before and after two-month storage in air and phosphate-buffered saline, respectively.

Roll-to-roll extrusion coated films were bonded onto polymer parts by back injection molding (BIM). The polypropylene (PP) coated polyethylene terephthalate (PET) films were pre-patterned with microstructured V-shaped grooves with 3.2 µm and 53 µm width, and other geometries. Bonding on PET and poly(methyl methacrylate) (PMMA) parts was facilitated by either higher tool or melt temperatures but was particularly enhanced by applying a mild oxygen plasma to the backside of the PET film prior to injection of the polymer melt. Silver wires from conductive nanoparticle ink were embedded into the PP coating during the BIM process by controlled collapse of the V-grooves. Thus, the feasibility of combining standard carrier film materials for printed flexible electronics and packaging into a non-flat polymer part was demonstrated, which could be a helpful step towards the fabrication of polymer parts with surface functionality.

Combining microstructures of different dimensions benefits from hybrid manufacturing strategies that use nanoimprint for generating regular large area structures and laser lithography for larger grayscale topography. While the individual processes are straightforward, due to the thermoplastic property of the positive resist used for grayscale lithography, diffraction on surface gratings and degradation of photoactive substances require a careful choice of the order of process steps, and balance of process, temperatures, and dimensions.

Long-term interfacing with neural tissue is key for the diagnosis and therapy of neurological disorder. Coatings with dedicated micro- and nanostructures have been proposed, such as gold nanowires, platinum nanostructured by electrochemical roughening, columnar and porous titanium nitride, carbon nanotubes, and conductive polymers. The performance of these coatings, however, is limited because of the mechanical mismatch between implant and neural tissue. Herein, we show that micro- and nanostructured, soft and conductive elastomer films can be obtained by depositing gold on nanometer-thin thiol-functionalized polydimethylsiloxane (PDMS) films. Additionally, microstructured polyether ether ketone (PEEK) films enable directional ordering in topology. The formation of soft and conductive PDMS films with oriented wrinkles on the macroscopic scale was controlled by the ratio between the metal/elastomter thicknesses and the depth of thermally imprinted trenches. Four-point probe measurements revealed that the electrical conductivity is one order of magnitude higher than that of recently presented hydrogel formulations. Nano-indentations proved that the submicrometer-thin conductive elastomer exhibit an average elastic modulus well below 10 MPa. This material system can be made tens of micrometers thin, and, therefore, has the potential to address several challenges of current implantable neural interfaces for the central nervous system, e.g. fabrication of softer and more flexible micrometer-thin spinal cord arrays.

In this work, dynamic mechanical properties of amorphous silicon and scale effects were investigated by the means of nanoindentation. An amorphous silicon sample was prepared by plasma-enhanced chemical vapor deposition (PECVD). Next, two sets of the samples were investigated: as-deposited and annealed in 500 °C for 1 hour. A three-sided pyramidal diamond Berkovich’s indenter was used for the nanoindentation tests. In order to determine the strain rate sensitivity (SRS), indentations with different loading rates were performed: 0.1, 1, 10, 100 mN/min. Size effects were studied by application of maximum indentation loads in the range from 1 up to 5 mN (penetrating up to approximately one-third of the amorphous layer). The value of hardness was determined by the Oliver–Pharr method. An increase of hardness with decrease of the indentation depth was observed for both samples. Furthermore, the significant dependence of hardness on the strain rate has been reported. Finally, for the annealed samples at low strain rates a characteristic “elbow” during unloading was observed on the force-indentation depth curves. It could be attributed to the transformation of ( β -Sn)-Si to the PI (pressure-induced) a-Si end phase.

The integration of lamps and exposure systems for the curing of resists during or after imprint is an integral part of ultraviolet–assisted nanoimprint lithography. Devices that require several lithography steps and precise overlay will need an imprinting process capable of addressing registration issues. Imprint tools based on drop-on-demand ultraviolet nanoimprinting are broadly divided into wafer steppers and whole substrate tools. Imprint lithography relies on the parallel orientation of the imprint template and the substrate. Inaccurate orientation may yield a residual layer that is nonuniform across the imprint field. The imprint tools include precision self-leveling flexure systems to passively align the imprint mask and substrate to be parallel during the imprint process. Imprint is also widely used for the surface patterning of thermoplastic materials and glass, such as for counterfeit tags, microfluidics, micro-optics, and biomimetics.

In back injection molding, a polymer film (most commonly a decorative label) is inserted into the mold and fused with the polymer product by injection of the polymer melt from behind. By placing a bendable film into a mold cavity and by injection of polymer melt, the film conforms to the outlines of the cavity, thus enabling the decoration of elements with non-planar surfaces. This technique allows for pre-patterning of films by a planar process, and then convert these into the surface skin of a non-planar molded part. In this research we demonstrate the integration of silver wires onto the surface of a curved polymer part. As an example, we used poly (methyl methacrylate) films of different thicknesses that were pre-structured with micrometer-sized V-grooves, filled with silver nanoparticle ink and placed into the cavity of a commercial injection molding tool. The effect of the back injection molding process on unfilled and filled V-grooves was evaluated for different process parameters. In most cases, the silver wires remained undamaged and their electrical conductivities remained essentially similar to those in planar films. The resulting surface-integrated silver wires were characterized by scanning electron microscopy and electrical resistivity measurements.

Monolithic integration of integrated circuits onto macroscopic polymer parts can be done by adding individual electronic elements after the polymer part has been molded or by adding a film with the entire circuit prepared in advance. In the first case, the patterning of non-flat surfaces requires a patterning method with tools that can follow the outline of a 3D topography, in the second case a flexible film that needs to be deformed. This approach has been used for thermoforming and is now also favored for printed electronics [1,2]. This way, fabrication methods for low-cost flexible electronics can be used to prepare the polymer films. We placed a 175 μm-thick polymeric film into the crowned mold of an injection-molding tool that constitutes a concave cylinder lens with a radius of 30 mm and a depth of 10 mm, with a convex cylinder at the opposite side. The film (PMMA: Evonik Plexiglas film 99524, Tg 113°C) was patterned by thermal nanoimprint (T-NIL) at 180°C and was pressed into this crowned surface by the viscous polymer (Evonik Plexiglas 7N, Tg 110°C) injected onto the backside of the film. The tool temperature (40°C) was low enough that the 260°C hot melt was cooled down to ~100°C when it reaches the film upon entering the mold cavity, thus it was able to deform the thermoplastic film without erasing its topography. The resulting back-injected cylinder-like element consists of the film that is covering the top of a 3 mm thick convex polymer body. Two cases were implemented: When the film is patterned towards the mold surface, the resulting cylinder has a surface topography constituted by the film. When it is patterned towards the injecting polymer, the surface structures are embedded. The latter is an interesting alternative if not only surface structures are used, but films that are already processed, e.g., covered with electrodes. We used both cases for the fabrication of components covered with a metal wire array (Fig. 1). Our process has, in comparison to inkjet printing used for printed electronics, resolutions down to sub-μm and relies on nanoparticle self-confinement in V-grooves [3]. Similarly, films with open microchannels were back-injection molded (Fig. 2). The shape of the 3.1 μm wide Vgrooves is slightly flattened by the process, thus enabling capillary filling in an open microfluidic approach [4]. In both cases, the wires are electrically conducting. Combining nanoimprint lithography and additive patterning techniques may be a viable strategy to create low-cost devices on large areas or even free-form components. Such strategies can be applied to various designs of electrical circuits that enable to build complex devices as planned within the SFA project FOXIP [5]. Applications would be LEDs and sensors embedded in polymer components.

This contribution summarizes advanced lithographic methods for polymeric 3D topographies based on the modification of the polymer molecular weight and applying thermal polymer reflow. Initial structures realized with grayscale electron beam and multi-photon lithography were reshaped due to thermal annealing close to the glass transition temperature following high-energy radiation. This allowed for new topographical functionalities such as aspheric and ultra-smooth freeform micro and nano-optics. The covered methods have in common that they exploit a specific contrast in molecular weight that enables post-processing and thus a transformation of the initial pattern by polymer reflow into a new surface topography or shape. Also, simulation methods are quickly summarized.

This is a topical review on advanced lithographic methods used to create 3D topographies. We start the discussion with the principal capabilities of grayscale electron beam lithography and multi-photon lithography for initial patterning. The innovative structures in this work become only possible by combining the initial patterns with methods to reshape them. By this, new functionalities such as aspheric, freeform and ultra-smooth surfaces can be provided. All the covered methods have in common that they exploit a specific material contrast that enables a post-processing and thus a transformation of the initial pattern by polymer reflow into a new surface topography or shape. To enhance the understanding of current reflow methods, we review the history of polymer reflow and discuss its simulation. In-depth examples cover optical and biomimetic applications. Furthermore, we provide new results and new insights into dynamic material changes during thermoplastic reflow. An outlook on emerging 3D MEMS fabrication using thermoplastic polymer reflow for actuation will illuminate the way towards smart, i.e, stimuli responsive 3D structures. This review is mainly meant for applied scientists and engineers.