Development and implementation of a rotating nanoimprint lithography tool for orthogonal imprinting on edges of curved surfaces

Uniform molding and demolding of structures on highly curved surfaces through conformal contact is a crucial yet often-overlooked aspect of nanoimprint lithography (NIL). This study describes the development of a NIL tool and its integration into a nanopositioning and nanomeasuring machine to achieve high-precision orthogonal molding and demolding for soft ultraviolet-assisted NIL (soft UV-NIL). The process was implemented primarily on the edges of highly curved plano-convex substrates to demonstrate structure uniformity on the edges. High-resolution nanostructures of sub-200-nm lateral dimension and microstructures in the range of tens of microns were imprinted. However, the nanostructures on the edges of the large, curved substrates were difficult to characterize precisely. Therefore, microstructures were used to measure the structure fidelity and were characterized using profilometry, white light interferometry, and confocal laser scanning microscopy. Regardless of the restricted imaging capabilities at high inclinations for high-resolution nanostructures, the scanning electron microscope (SEM) imaging of the structures on top of the lens substrate and at an inclination of 45° was performed. The micro and nanostructures were successfully imprinted on the edges of the plano-convex lens at angles of 45°, 60°,and 90° from the center of rotation of the rotating NIL tool. The method enables precise imprinting at high inclinations, thereby presenting a different approach to soft UV-NIL on curved surfaces

Fundamental investigations in the design of five-axis nanopositioning machines for measurement and fabrication purposes

The majority of nanopositioning and nanomeasuring machines (NPMMs) are based on three independent linear movements in a Cartesian coordinate system. This in combination with the specific nature of sensors and tools limits the addressable part geometries. An enhancement of an NPMM is introduced by the implementation of rotational movements while keeping the precision in the nanometer range. For this purpose, a parameter-based dynamic evaluation system with quantifiable technological parameters has been set up and employed to identify and assess general solution concepts and adequate substructures. Evaluations taken show high potential for three linear movements of the object in combination with two angular movements of the tool. The influence of the additional rotation systems on the existing structure of NPMMs has been investigated further on. Test series on the repeatability of an NPMM enhanced by a chosen combination of a rotary stage and a goniometer setup are realized. As a result of these test series, the necessity of in situ position determination of the tool became very clear. The tool position is measured in situ in relation to a hemispherical reference mirror by three Fabry-Pérot interferometers. FEA optimization has been used to enhance the overall system structure with regard to reproducibility and long-term stability. Results have been experimentally investigated by use of a retroreflector as a tool and the various laser interferometers of the NPMM. The knowledge gained has been formed into general rules for the verification and optimization of design solutions for multiaxial nanopositioning machines

High optical contrast nanoimprinted speckle patterns for digital image correlation analysis

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

Tip- and laser-based 3D nanofabrication in extended macroscopic working areas

The field of optical lithography is subject to intense research and has gained enormous improvement. However, the effort necessary for creating structures at the size of 20 nm and below is considerable using conventional technologies. This effort and the resulting financial requirements can only be tackled by few global companies and thus a paradigm change for the semiconductor industry is conceivable: custom design and solutions for specific applications will dominate future development (Fritze in: Panning EM, Liddle JA (eds) Novel patterning technologies. International society for optics and photonics. SPIE, Bellingham, 2021. https://doi.org/10.1117/12.2593229). For this reason, new aspects arise for future lithography, which is why enormous effort has been directed to the development of alternative fabrication technologies. Yet, the technologies emerging from this process, which are promising for coping with the current resolution and accuracy challenges, are only demonstrated as a proof-of-concept on a lab scale of several square micrometers. Such scale is not adequate for the requirements of modern lithography; therefore, there is the need for new and alternative cross-scale solutions to further advance the possibilities of unconventional nanotechnologies. Similar challenges arise because of the technical progress in various other fields, realizing new and unique functionalities based on nanoscale effects, e.g., in nanophotonics, quantum computing, energy harvesting, and life sciences. Experimental platforms for basic research in the field of scale-spanning nanomeasuring and nanofabrication are necessary for these tasks, which are available at the Technische Universität Ilmenau in the form of nanopositioning and nanomeasuring (NPM) machines. With this equipment, the limits of technical structurability are explored for high-performance tip-based and laser-based processes for enabling real 3D nanofabrication with the highest precision in an adequate working range of several thousand cubic millimeters

Soft nanoimprint lithography on curved surfaces

Diese Arbeit befasst sich mit den Herausforderungen des Prägens auf gekrümmten Oberflächen. Die Prägung auf gekrümmten Oberflächen hat aufgrund der steigenden Nachfrage nach Anwendungen in der Optik, bei biomedizinischen Implantaten und Sensoren eine hohe Relevanz gewonnen. Gegenwärtig ist es aufgrund der begrenzten Tiefenschärfe des Projektionssystems eine Herausforderung, Strukturen auf gekrümmten Oberflächen mit Hilfe der konventionellen Photolithographie zu strukturieren. Daher wird zunehmend die Nanoimprint-Lithographie (NIL) eingesetzt. NIL ermöglicht die gleichmäßige Abformung von Strukturen auf stark gekrümmten Oberflächen. Sie ist im Vergleich zu herkömmlichen Methoden nicht durch optische Effekte wie Streuung, Beugung und Interferenz in einem Substrat begrenzt. In dieser Arbeit wird ein weicher UV-NIL-Prozess entwickelt, optimiert und für die Strukturierung auf nicht ebenen Oberflächen implementiert. Es handelt sich um einen mechanischen Prozess, der auf der Übertragung von Strukturen von einem flexiblen und transparenten weichen Polydimethylsiloxan (PDMS) Stempel auf das Substrat in Gegenwart von UV-Licht basiert. Die Entformung ist ein wichtiger Aspekt für einen erfolgreichen NIL-Prozess. Die induzierte Scherspannung während der Trennung des Stempels vom Substrat kann zu strukturellen Verzerrungen führen, insbesondere an den Rändern der gekrümmten Oberfläche, wo die Neigung hoch ist. Das weiche NIL wurde angewandt, da es im Vergleich zur konventionellen Lithographie nicht von komplexen Maschinen, strengen Umgebungsbedingungen und optischen Einschränkungen abhängig ist. Eine hochpräzise Nanopositionierungs- und Nanomessmaschine (NPMM) wurde eingesetzt, um eine kontrollierte Positionierung des Substrats zum Stempel zu ermöglichen. Ein kompakter und einstellbarer weicher UV-NIL-Prozess wurde entworfen, montiert und in die NPM-Maschine integriert. Die erfolgreiche Umsetzung des grundlegenden Prozesses im NPMM schuf die Grundlage für die Herstellung eines NIL-Werkzeugs, das mit einer Drehvorrichtung kombiniert und in das NPMM integriert wurde. Die Kombination ermöglicht fünf Bewegungsfreiheitsgrade für das orthogonale Formen und Entformen an den Kanten gekrümmter Substrate. Das rotierende NIL-Werkzeug spricht die allgemein übersehene Herausforderung der Strukturierung und Charakterisierung an den Kanten gekrümmter Substrate an. Unabhängig von den eingeschränkten Messmöglichkeiten bei hohen Neigungen für hochauflösende Nanostrukturen war es möglich, die Strukturen sowohl auf dem Linsensubstrat als auch bei einer Neigung von 45° im REM abzubilden. Es hat sich gezeigt, dass es an der Kante eines gekrümmten Substrats mit einem Krümmungsradius von ca. 25 mm bei einer Oberflächennormalen, die um 45° geneigt ist, ein Imprinting durchführen kann. Die Methode ermöglicht präzise Prägefähigkeiten bei hohen Neigungen und stellt damit einen neuartigen Ansatz von weichem NIL auf gekrümmten Oberflächen dar.This thesis addresses the challenges of imprinting on curved surfaces. The imprinting on curved surfaces has gained high relevance due to the increasing demand for application in optics, biomedical implants and sensors. Currently, it is challenging to pattern structures on curved surfaces using conventional photolithography due to the limited depth of focus of the projection system. Herein, rises the use of Nanoimprint Lithography (NIL). NIL allows the uniform molding of structures on highly curved surfaces. It is not limited by optical effects such as scattering, diffraction and interference in a substrate as compared to conventional methods. In this thesis, soft UV-NIL process is developed, optimized and implemented for patterning on non-flat surfaces. It is a mechanical process based on the transfer of structures from a flexible and transparent soft polydimethylsiloxane (PDMS) mold to the substrate in the presence of UV light. Demolding is an important aspect for a successful NIL process. The induced shear stress during the separation of the stamp (mold) from the substrate can lead to structural distortions, specifically on the edges of the curved surface, where the inclination is high. Soft NIL was applied as it is not dependent on complex machines, strict environmental conditions and optical limitations as compared to conventional lithography. A high-precision Nano-positioning and Nano-measuring Machine (NPMM) was used to provide controlled positioning of substrate to the stamp. A compact and adjustable soft UV-NIL process was designed, assembled and integrated into the NPM machine. The successful implementation of the fundamental process in the NPMM created a foundation for making a NIL tool to be combined with a rotary device and integrated into the NPMM. The combination enables five degrees of freedom of motion for orthogonal molding and de-molding on edges of curved substrates. The rotating NIL tool addresses the generally overlooked challenge of patterning and characterization on the edges of curved substrates. Regardless of restricted measurement capabilities at high inclinations for high nanostructures, it was possible to perform SEM images on top of the lens substrate as well as at an inclination of 45°. It has displayed imprinting capabilities on the edge of curved substrate with radius of curvature of approximately 25 mm at a surface normal titled to 45°. The method enables precise imprinting capabilities at high inclinations, thereby presenting a novel approach of soft NIL on curved surfaces