Metrological stages such as the nano-positioning and nano-measurement machine (NPMM) can position single-digit nanometer accurately on centimeter working volumes. However, their measurement system requires a feedback to the arbitrary shaped specimen by another probe. The differential confocal microscopy (DCM) offers the possibility to have a sensitivity down to that single-digit nanometers but suffers from noise and aberration. Recently the principle of the LockIn filtering could be successfully adapted in DCM and therefore achieved a high SNR. Contrary to the there employed acoustically driven tunable GRIN lens (TAG lens) at the objective, we demonstrate a microelectromechanical system (MEMS), an AFM cantilever, as an ultrafast oscillating pinhole in front of the detector. Its first resonance at 96kHz makes it very competitive regarding acquisition speed, but the low oscillation amplitude lowers contrast. By principle inheriting the possibility to compensate a change in reflectivity, we present another advancement for the evaluation of the resulting differential signal to make it robust against sample induced systematic depth errors, e.g. a tilt-angle. This could be advantageous for DCM with static beam-paths, as well. Potentially, the highest improvement can be achieved in conjunction with the NPMM’s highly accurate measurement interferometers, because the residual error for the depth of a specimen under the influence of varying aberration is kept below 20nm.
KEYWORDS: Interferometers, Nanofabrication, Mirrors, Actuators, Near field optics, Control systems, Manufacturing, Nanotechnology, Physics, Atomic force microscopy
Although the field of optical lithography is highly investigated and numerous improvements are made, structure sizes smaller than 20 nm can only be achieved by considerable effort when using conventional technology. To cover the upcoming tasks in future lithography, enormous exertion is put into the development of alternative fabrication technologies in particular for micro- and nanotechnologies that are capable of measuring and patterning at the atomic scale in growing operating areas of several hundred square millimetres. Many new technologies resulted in this process, and are promising to overcome the current limitations1, 2, but most of them are demonstrated in small areas of several square micrometers only, using state-of-the-art piezo stages or the like. At the Technische Universitat Ilmenau, the NanoFabrication Machine 100 (NFM-100) was developed, which serves as an important experimental platform for basic research in the field of scale-spanning AFM tip-based and laser-based nanomeasuring and nanofabrication for simultaneous subnanometre measuring and structuring on surfaces up to Ø100 mm. This machine can be equipped with several probing systems like AFM, laser focus probes and 3D-micro probes as well as tools for different nanofabrication technologies like tip-based technologies, optical technologies and mechanical two-dimensional technologies in a large working range with subnanometre reproducibility and uncertainty. In this paper, the specifics and advantages of the NFM-100 will be described as well as nanofabrication technologies that are currently worked on e.g. advanced scanning proximal probe lithography based on Fowler-Nordheim-electron-field emission, direct laser writing and UV-nanoimprint lithography.
To keep up with Moore’s law in future, the critical dimensions of device features must further decrease in size. Thus, the nano-electronics and nano-optics manufacturing is based on the ongoing development of the lithography and encompasses also some unconventional methods. In this context, we use the Nanopositioning and Nanomeasuring Machine (NPMM) to generate features in resist layers by means of Direct Laser Writing (DLW),1 Field Emission Scanning Probe Lithography (FE-SPL)2 and Soft UV-Nanoimprint Lithography (Soft UV-NIL)3 with highest accuracy. The NPMM was collaboratively developed by TU Ilmenau and SIOS Meßtechnik GmbH.4 The tool provides a large positioning volume of 25 mm × 25 mm × 5 mm with a positioning resolution of 0.1 nm and a repeatability of less than 0.3 nm over the full range. Previously a single electron transistor (SET) working at room temperature generated by FE-SPL has been demonstrated.5 However, the throughput is limited because of the serial writing scheme making Tennant’s law (At ∼ R5 ) valid.6 Here, At is the areal throughput and R the lithographic resolution. Thus, patterning of the whole NPMM positioning area by FE-SPL is very time consuming. In order to address this problem, different strategies and/or combinations are conceivable. In this work a so-called Mix-and-Match lithography is conducted. A fast generation of structures in the sub-micron range is possible by means of DLW. By this, features such as electrical wires, contact patches for bonding or labels are generated in resist. Subsequently, we use FE-SPL in order to define the actual nano-scaled features for quantum or single electron devices. In combination, DLW and FE-SPL are maskless lithography strategies, hence, offering completely novel opportunities for rapid nanoscale prototyping of largescale resist patterns. An explanation of this technique is given in a previous publication.7 Furthermore, after reactive ion etching, the sample can be used as template for Soft UV-NIL, thus resulting in a high-throughput process chain for future quantum and/or single electron devices.
Imprinting micro- and nanostructures on non-planar surfaces has gained prominence in various fields such as optoelectronics, photonics and biomedical implants. It has been implemented for applications such as optical sensor arrays and optical fibers. Nanoimprint lithography (NIL) is a low cost, high resolution nanofabrication process. In this work, soft UV-NIL process is used in which a flexible stamp is used which makes it ideal for imprinting on curved surfaces such as plano-convex lens. However, the substrate to stamp positioning for successful transfer of patterns is crucial and needs to be addressed. The Nanopositioning and Nanomeasuring machine (NPMM), developed in the Collaborative Research Center (of the German Research Foundation) of TU Ilmenau, provides a unique solution to the challenges of positioning and alignment. Therefore, a UV-LED assisted small scale NIL-setup was designed, developed and integrated into the NPMM and it was further realized for carrying out fabrication of micro- and nanostructures on silicon chips and planoconvex lenses. In addition to scanning electron microscopy (SEM) and atomic force microscopy (AFM) characterization, the structures were further characterized using a focus sensor. The utilized focus sensor is an optical sensor developed at the Institute for Process Measurement and Sensor Technology of TU Ilmenau. It was observed that the imprinted structures were of considerably good fidelity. Thus, a distinctive integrated imprinting process for flat and non-flat surfaces was developed and implemented.
In view of the increasing demands on precision optics, microelectronics and precision mechanics nanoscale structuring processes are of great interest. It is becoming more and more important to apply a large number of structures that are as small as possible to ever larger areas with high reliability and to increase the number of structures per area element (packing density). The straightness and uniformity of these structures, as well as the positioning accuracy during the fabrication of such narrow lines and points are at the center of the increase of the packing density. A further decisive role is played by the development of suitable sensors and tools for the production and measurement of these structures. The development and the combination of a new laser based probe for the measurement and a direct laser writing (DLW) tool for the creation of sub-micro structures forms the core of this topic. The new sensor is based on a confocal measuring principle. A fiber coupling is used to avoid thermal influences. At the same time, the fiber end itself serves as a confocal pinhole. For the process tool, comprehensive investigations of laser and resist parameters are necessary. The first results are shown. These two parts are investigated separately and combined at the end of the work. In order to achieve the necessary positioning accuracy, the tool is integrated into the Nanopositioning and Measurement Machine (NPMM).
The positioning range of ultra precise positioning systems is restricted to certain hundreds of micrometers in lateral direction due to the application of piezoelectrical stages. Using galvanometer scanner, the range is limited even to tens of micrometer or by restriction of accuracy. Direct laser writing processes can be done with a combination of both positioning techniques, but structures in the millimeter range can only be achieved by using stiching methods. The nanopositioning and nanomeasuring machine NMM-1, developed at TU Ilmenau together with SIOS Meβtechnik GmbH, provides a large positioning volume of 25 mm × 25 mm × 5 mm with a positioning resolution in the sub-nanometer range. A combination of this precise positioning machine with common laser writing tools like UV direct laser writing or two-photon polymerization enables to produce structures with metrological traceability. The laser writing techniques base on both one-photon absorption and two-photon processes in the used photosensitive materials. Therefore, a 405 nm fiber-coupled laser diode for the one-photon processes and a 805 nm femtosecond laser are guided through an objective onto the positioning stage of the NMM-1. In first experiments, structures with sub-10 μm linewidths could be achieved. Furthermore, a probe system consisting of a hologram laser unit with a wavelength of a 650 nm is applied to the setup to control the focus spot of the machining laser onto the sample’s surface. That leads to a further decrease in the linewidths of the structures.
Direct Laser Writing techniques like two-photon-polymerization or UV-lithography have become common tools for the micro- and nanofabrication of precise devices like photonic crystals. A decrease in the size of structures of special devices requires a significant better resolution of the laser beam system that can be determined by using different photoinitiators or a second depletion laser for STED-lithography.
However, besides the optical limits for the resolution of the laser system due to diffraction effects, the positioning systems for the laser beam or the sample stage lead to further imprecisenesses. To benefit from the high resolution techniques for the structuring process, the need for highly accurate positioning systems has dramatically grown during the last years. A combination of lithographic techniques with a nanopositioning and nanomeasuring machine NMM-1, developed at the TU Ilmenau, enables high precision structuring capability in an extended range. The large positioning volume of 25mm x 25mm x 5mm with a resolution in the sub-nanometer range is a good condition for ultra precision manufacturing with large area 3D-Laser-Lithography. Advantages and disadvantages as well as further developments of the NMM-1 system will be discussed related to current developments in the laser beam and nanopositioning system optimization. Part of the further development is an analysis of the implementability of additional ultra precise rotational systems in the NMM-1 for the unlimited addressability perpendicular to the surface of a hemisphere as key strategy for multiaxial nanopositioning and nanofabrication systems.
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