Recent advancements in two-photon polymerization (2PP) allowed for the precise and versatile fabrication of three-dimensional nano- and microscale structures, making it ideal for use in microelectronics, optoelectronics, photonics, and biomedical engineering. This research demonstrates the effective fabrication of multimaterial microstructures by an in situ material exchange technique using a commercial printhead system. Therefore, the effect of pre-existing microstructures on the laminar exchange flow was investigated, revealing areas of zero-flow and residual material. Residual material can be minimized with extended times of flow. In addition, the study shows a novel application idea for lateral multimaterial microlens printing. On large surfaces, we achieved very good lateral lens alignment as well as in situ material exchange. Lateral multimaterial printing provides substantial benefits over typical vertical lenses in optics and photonics integration. The technique included printing a core lenslet and peripheral lenslets utilizing custom materials for the in situ exchange. Overall, our results demonstrate the adaptability of multimaterial printing via in situ material exchange inside a commercial 2PP system, which broadens design options and improves functioning across multiple technological domains.

The fabrication of plasmonic 3D microstructures is still a challenge. In this study, the fabrication of gold‐nanoparticle (Au‐NP)‐filled acrylate‐based micron‐scale cuboids forming plasmonic 3D particles is investigated. UV curable acrylate is mixed with HAuCl 4 and spin‐coated onto a fused silica wafer. UV light and heat treatment of the layer result in polymerization of the acrylate and formation of Au‐NPs with a plasmonic resonance ranging from 535 to 550 nm. The plasmonic resonance wavelength decreases slightly with increasing UV dose. The resonance wavelength can be shifted by a subsequent annealing process. The decreasing resonance wavelength with increasing annealing temperature is discussed as a result of the increasing Au‐NPs. The size and the distribution of the resulting gold particles in the acrylate matrix is investigated by transmission electron microscopy. The Au‐NPs‐filled acrylate layer is laser structured using a UV‐femtosecond laser, which allows the fabrication of adjustable micro‐cuboids with edge lengths down to 15 μm. The laser‐cut micro‐cuboids are transferred into isopropanol and the plasmonic properties of the structures in the liquid are successfully demonstrated. The presented concept allows easy and large‐scale fabrication of plasmonic microstructures with independently adjustable plasmonic properties and microstructural size and shape.

Abstract Hot electron transistors (HETs) represent an exciting new device for integration into semiconductor technology, holding the promise of high‐frequency electronics beyond the limits of SiGe bipolar hetero transistors. With the exploration of 2D materials such as graphene and new device architectures, hot electron transistors have the potential to revolutionize the landscape of modern electronics. This study highlights a novel hot electron transistor structure with a record output current density of 800 A cm−2 and a high current gain α, fabricated using a scalable fabrication approach. The hot electron transistor structure comprises 2D hexagonal boron nitride and graphene layers wet transferred to a germanium substrate. The combination of these materials results in exceptional performance, particularly in terms of the highly saturated output current density. The scalable fabrication scheme used to produce the hot electron transistor opens up opportunities for large‐scale manufacturing. This breakthrough in hot electron transistor technology holds promise for advanced electronic applications, offering high current capabilities in a practical and manufacturable device.

Direct writing laser lithography has become increasingly attractive due to its mask-free nature, offering significant design flexibility and minimizing additional costs associated with new exposure masks. Among the various direct laser writing techniques, two-photon absorption direct laser writing stands out for its ability to fabricate very small features through nonlinear optical effects. Operating this technique in immersion, where the air gap between the lens and photoresist or probe is liquid-filled, has been shown to enable reduced feature sizes [D. P. Sanders, Chem. Rev. 110, 321 (2010).]. Previous studies have typically employed an immersion oil for this purpose [Jakkinapalli et al., J. Micromech. Microeng. 30, 125014 (2020).]. However, our experiments revealed that prolonged immersion times can lead to oil interacting with the photoresist, resulting in the formation of new compounds that are challenging to remove. Consequently, the altered photoresist becomes unresponsive to both exposure and development. In light of these findings, we conducted tests using alternative immersion liquids and propose a combination of different immersion liquids and air direct writing as a means to achieve optimal writing results.

Droplet-induced self-folding processes enable the easy and cost-effective fabrication of millimeter to submillimeter 3D structures from planar templates. These templates were fabricated by laser cutting of polymer foils that offer a high flexibility in design. The interaction of water droplets with template surfaces induces a surface tension force that causes the deformation of the laser-cut templates needed to form the 3D structures. In this study, laser patterning of 25 mu m thick polyimide (PI) foils by UV ultrashort pulse laser ablation was used to systematically investigate the effect of hinge geometry on the bending and self-folding process of cubes. The deposition of water droplets on the laser-structured samples leads to forces that move the side faces of the cube template causing a defined deformation of the hinges of the PI template and resulting in a bending angle between hinged template regions. The bending angle was determined as a function of hinge geometry and water droplet volume. The bending angle is increased with increasing droplet volume below a certain maximum but decreased with increasing hinge thickness and width. Finally, 2D laser cut templates with optimized v-shaped hinge structures was sucessful formed into a cube by laser droplet induced self folding.

In recent years, more and more research was conducted to explore smaller and smaller systems that become similar to an actual micro/nano-robot. The major roadblock regarding their real world implementation is the highly restricted available volume. In this paper, we introduce folding - hence an origami technology using Al2O3 as building material, which is compatible with current Si technology. High quality 50 nm thin Al2O3 film is grown by atomic layer desposition, patterned, thinned and then released from the Si subtrate using SF6 plasma etching. The realized free standing Al2O3 structure would fold itself at predefined regions due to the stress in the as deposited films. We believe such technology could offer a new possibility to tackle the problem of efficiently using the volume. Al2O3 could act as both the structural origami material and the functional gate dielectric material for electronics. This approach enables the feasibility of patterning devices and circuits on every Al2O3 facet of a 3D object, while the inside volume of this object is still available for 3D bulk device components. We demonstrate the optimized etching process as well as an emperical improvement of the folding hinges and of the overall structural stability.

In the past two decades, direct laser writing (DLW) technologies have seen tremendous growth. However, strategies that enhance the printing resolution and the development of printing material with assorted functionalities are still sparser than expected. Herein, a cost-effective method to tackle this bottleneck is presented. Semiconductor quantum dots (QDs) are selected to carry out this task, most importantly via surface chemistry modification to enable their copolymerization with themonomers, resulting in transparent composites. The evaluations indicate that the QDs show great colloidal stability and their photoluminescent properties are well-preserved. This allows further exploration of the printing characteristics of such composite material. It is shown that in the presence of the QDs, the material provides a much lower polymerization threshold with faster linewidth growth, indicating that the QDs form a synergetic relationship with the monomer and the photoinitiator, widening the dynamic range of the material and thus increasing the writing efficiency for broader fields of applications. Lowering the polymerization threshold reduces the minimum achievable feature size by ≈32%, which is well-matched with STED-based (i.e., stimulated-emission depletion microscopy) methods in writing 3D structures. The study further elucidates the mechanism of the synergetic behavior, further guiding the future development of functional materials for DLW-related printing technologies.

Plasmonic sensing in the infrared region employs the direct interaction of the vibrational fingerprints of molecules with the plasmonic resonances, creating surface-enhanced sensing platforms that are superior to traditional spectroscopy. However, the standard noble metals used for plasmonic resonances suffer from high radiative losses as well as fabrication challenges, such as tuning the spectral resonance positions into mid- to far-infrared regions, and the compatibility issue with the existing complementary metal-oxide-semiconductor (CMOS) manufacturing platform. Here, we demonstrate the occurrence of mid-infrared localized surface plasmon resonances (LSPR) in thin Si films hyperdoped with the known deep-level impurity tellurium. We show that the mid-infrared LSPR can be further enhanced and spectrally extended to the far-infrared range by fabricating two-dimensional arrays of micrometer-sized antennas in a Te-hyperdoped Si chip. Since Te-hyperdoped Si can also work as an infrared photodetector, we believe that our results will unlock the route toward the direct integration of plasmonic sensors with the on-chip CMOS platform, greatly advancing the possibility of mass manufacturing of high-performance plasmonic sensing systems.

A graphene-based three-terminal barristor device was proposed to overcome the low on/off ratios and insufficient current saturation of conventional graphene field-effect transistors. In this study, we fabricated and analyzed a novel graphene-based transistor, which resembles the structure of the barristor but uses a different operating condition. This new device, termed graphene adjustable-barriers transistor (GABT), utilizes a semiconductor-based gate rather than a metal-insulator gate structure to modulate the device currents. The key feature of the device is the two graphene-semiconductor Schottky barriers with different heights that are controlled simultaneously by the gate voltage. Due to the asymmetry of the barriers, the drain current exceeds the gate current by several orders of magnitude. Thus, the GABT can be considered an amplifier with an alterable current gain. In this work, a silicon-graphene-germanium GABT with an ultra-high current gain (ID/IG up to 8 × 106) was fabricated, and the device functionality was demonstrated. Additionally, a capacitance model is applied to predict the theoretical device performance resulting in an on-off ratio above 106, a swing of 87 mV/dec, and a drive current of about 1 × 106 A/cm2.

Multi-photon laser-lithography has become a versatile technology for writing complex 3D structures in high-resolution. Most efforts are focusing on single material printing. To realize multi-material printing, two main steps have to be carried out consecutively: developing the previous material and introducing the next material to the print side. So far, this is done ex-situ, and alignment errors are unavoidable. In this work, a fast in situ material exchange is demonstrated. The designed setup can effectively replace the print material in the optical focus, enabling the writing of detailed 3D structures while conserving efficiency and precision.

3D self-folding microarchitectures have been studied enormously since the past decade, because of the potential of utilizing the third dimension to reach a new level of device integration. However, incorporating various functionalities is a great challenge, due to the limited folding force and choice of materials. In particular, self-folding microarchitectures with advanced optical properties have yet to be demonstrated. Here, a unique folding technique is developed, namely vacuum microforming, successfully demonstrating the self-folding of microcubes that can be completed within 30 ms, a few orders of magnitudes faster as compared to various established strategies reported so far. Simultaneously, a metal-insulator-metal (MIM) plasmonic nanostructure is fabricated, invoking strong gap plasmon to obtain a wide and robust angle-independent optical behavior and high environmental sensitivity that is close to the theoretical limit. It is successfully proven that such superb plasmonic properties are well preserved in 3D architectures throughout the folding process. The nanofabrication method together with the self-folding strategy not only provide the fastest folding process so far, compatible for high-volume fabrication, but also create new opportunities in integrating various functionalities, more specifically, optical properties for untethered optical sensing and identification.

This work describes the fabrication of anisotropically etched, faceted pyramidal structures in amorphous layers of silicon dioxide or glass. Anisotropic and crystal-oriented etching of silicon is well known. Anisotropic etching behavior in completely amorphous layers of silicon dioxide in combination with purely isotropic hydrofluoric acid as etchant is an unexpected phenomenon. The work presents practical exploitations of this new process for self-perfecting pyramidal structures. It can be used for textured silica or glass surfaces. The reason for the observed anisotropy, leading to enhanced lateral etch rates, is the presence of thin metal layers. The lateral etch rate under the metal significantly exceeds the vertical etch rate of the non-metallized area by a factor of about 6-43 for liquid and 59 for vapor-based processes. The ratio between lateral and vertical etch rate, thus the sidewall inclination, can be controlled by etchant concentration and selected metal. The described process allows for direct fabrication of shallow angle pyramids, which for example can enhance the coupling efficiency of light emitting diodes or solar cells, can be exploited for producing dedicated silicon dioxide atomic force microscopy tips with a radius in the 50 nm range, or can potentially be used for surface plasmonics.

Localized surface plasmon resonances (LSPRs) have been widely explored in various research fields because of their excellent ability to condense light into a nanometer scale volume. However, it suffers quite often from the broadening of the LSPR linewidths, resulting in low quality factors. Among the causes of the broadening, fabrication inaccuracies are crucial yet challenging to evaluate. In this paper, we designed a type of metal-insulator-metal structure as an example via the colloidal self-assembly approach. We then demonstrated a facile approach to identify the origin of the discrepancies in between spectra obtained from experiments and simulations. Through a series of simulations in accordance with the experimental results, we could confirm that the predominant influencing factors are the presence of defects, as well as feature size variations, though they impact the spectral response in different ways. For similar plasmonic systems, our results enabled a more cost-effective optimization process in lieu of rather intensive and iterative experimentations, which will pave the way to automated fabrication and optimization, as well as integrated design. Furthermore, our results also indicated that the typical defect ratio that is introduced via the colloidal self-assembly approach has only limited impact on the resulting plasmonic resonances, proving that for similar plasmonic structure designs, colloidal self-assembly methods can provide a reliable and efficient alternative in the field of nanofabrication of plasmonic systems.

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.

Providing freeform full 3D fabrication on a large scale is the current technology trend (Figure 1) [1]. 3D fabrication is frequently discussed along with additive fabrication and next generation industrialization, where on-demand fabrication, or printing, of desired work pieces without expensive tooling from any imaginable material can be done on-the-spot and everywhere in the world. This bold vision needs to be proven first; however, 3D printing of essential parts has already replaced traditional fabrication. For example, the aviation industry is working hard on integrating 3D printing of metals and there are already 3D printed components in recent generations of airplanes [6]. Finally, we should not forget about the established sector of at home polymer 3D printing and the professional prototyping as well as functional device fabrication all being based on polymers. When it comes to the smallest pieces that drive the evolution of our tech-society, namely nanoelectronic systems and microsystems, the visions for true 3D technologies at the micro and even the nanoscale are also bold. Quoting a pioneer in nanoimprint lithography (Stephen Chou, Princeton) [7]: “[...] the commercialization of nanotechnology critically depends upon our ability to manufacture, if you want to use nanostructures commercially – you need to have the nanofabrication abilities”. It is as simple as that, and it is obviously true for 3D patterning. A very good candidate for high-resolution 3D printing and fabrication, even at the industrial scale, is femtosecond laser processing which makes use of non-linear optical effects to finally beat the diffraction limit by multi-photon lithography and stimulated emission depletion. Jonušauskas et al. and Stender et al. broadly discuss the technology in two review articles in this special issue. In the lab, of course, there are plenty of ideas and curiosities that might be realized with this technology being available already since the end of the last century. Applications may reach from photonic and mechanical metamaterials [8] all the way to photonic chip bonding [9] and smart mesoscopic objects and particles – just name it. On the industrial and applied scale, refractive and diffractive micro-optics play an important role. This has become very clear at the NIL Industrial Day 2019 in Aachen [http://www.nil-industrialday.org/]. Such optics are not new, however, new applications such as structured light applications and high-performing, light-weight optics for virtual and augmented reality wearables bring new challenges for traditional lithography. Now, techniques such as multilevel and grayscale patterning will become even more interesting. Especially using direct writing methods such as grayscale laser, electron or ion beam lithography allows for fabricating optical elements with challenging specifications for the field of view, illumination homogeneity and photon efficiency. Some of the major potential techniques in the field are covered in this special issue, including an excellent review on electron beam grayscale lithography by Unno et al. All these techniques have in common that they seem very laborious on the first look and might for that reason still live an abandoned life in niche applications waiting for the killer-application. It might be knocking on our doors just this moment. In addition, pattern transfer and high-volume replication is required to bring most of the valuable, original polymer or resist patterns into more durable and end-user friendly materials. This special issue wants to motivate more industrial researchers to use the exceptional potential we have today with the integration of such 3D techniques into design and into fabrication flows. 3D lithography is just another lithography method in our toolbox. Let us make use of it. Another key technique is required to help in accessing the high-volume fabrication and multiplication of original patterns realized with the mentioned 3D techniques. Here, we come back to the initial quote borrowed from nanoimprint lithography – a technique which also originally peaked in attention at the end of the last century as next generation lithography techniques for semiconductor *Corresponding authors: Dr. Robert Kirchner, Technische Universität Dresden, Faculty of Electrical and Computer Engineering, Institute of Semiconductors and Microsystems, 01062 Dresden, Germany, e-mail: robert.kirchner@tu-dresden.de; and Dr. Jun Taniguchi, Tokyo University of Science, Department of Applied Electronics, Faculty of Industrial Science and Technology, Tokyo, Japan, e-mail: junt@te.noda.tus.ac.jp

We report on the fabrication of very high-resolution discrete four-resist-level grayscale patterns in poly(methyl methacrylate) with just 6-nm step height and down to 32-nm step width using dose-modulated, grayscale electron beam lithography and a low-contrast resist-developer system. This direct pattern writing is important for replication in high-volume manufacturing of diffractive optics. An innovative concept of unexposed auxiliary spacers helped to enhance the discrete character of the multi-level patterns. For pattern step widths between 100 and 32 nm, a transformation toward blazed gratings with increasingly continuous-slope character was obtained. All high-resolution patterns were prepared in a single exposure and development process from an initially about 30-nm thin film. The pattern roughness due to a relatively large polymer molecular weight was reduced using selective thermal annealing with only minimally affecting the global pattern shape by reflow. The results will enable further approaches toward single-digit vertical and prospectively single-digit lateral resolution grayscale patterns.

State-of-the-art, polymeric, refractive micro-optics simultaneously require an ultrasmooth three-dimensional (3-D) surface and a precise geometry for excellent optical performance with minimal stray light. In earlier work, we have established a surface finishing process for thermoplastic polymer master structures that is only effective on the surface and does not affect the designed optical geometry, thus enabling polishing without touching. Therewith, the high curvature corners of a 50-mu m-tall optical diffuser device were maintained while the surface roughness was reduced to about 10-nm root mean square. For this, 3-D master structures were first fabricated by direct write laser-lithography with two-photon polymerization. The master structures were replicated into poly(methyl methacrylate) through a poly(dimethyl siloxane) intermediate replication stamp. Finally, all structures were surface-polished by selective high-energy photon exposure and thermal postprocessing. In this work, we focus on the comparison of the surface smoothening using either postprocessing or dedicated direct writing strategies. For this comparison, strategies for modifying the exposed voxel size and the writing discretization being the primary source of roughness were tested by sweeping the laser exposure dose for two different resist materials and objectives. In conclusion, the postprocessing smoothening resulted in a lower roughness compared to a direct writing strategy-even when 50-nm vertical discretization steps were used-and still enabled 10 times shorter writing times. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)

We characterize the impact of high-energy, 172 nm vacuum ultraviolet photons on the molecular weight and the glass transition temperature of poly(methyl methacrylate). We found that the molecular weight is reduced strongly on the surface of the exposed samples with a continuous transition towards the unexposed bulk material being located below the modified region. The glass transition temperature was found to be significantly lowered in the exposed region to well below 50 degrees C compared to that of the 122 degrees C of the bulk region. We could use this material contrast to selectively reflow the top surface of the exposed samples only. This allowed us to create ultra-smooth micro-optical structures by post-processing without influencing the overall geometry that is required for the optical functionality.