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.
KEYWORDS: Scanning electron microscopy, Atomic force microscope, Electron microscopes, Metrology, Electron beams, Nanofabrication, Diamond, Scanning probe lithography, Microscopy, Overlay metrology
An integration of atomic force microscopy (AFM) and scanning electron microscopy (SEM) within a single system is opening new capabilities for correlative microscopy and tip-induced nanoscale interactions. Here, the performance of an AFM-integration into a high resolution scanning electron microscope and focused ion beam (FIB) system for nanoscale characterization and nanofabrication is presented. Combining the six-axis degree of freedom (DOF) of the AFM system with the DOF of the SEM stage system, the total number of independent degree of freedom of the configuration becomes eleven. The AFM system is using piezoresistive thermomechanically transduced cantilevers (active cantilevers). The AFM integrated into SEM is using active cantilevers that can characterize and generate nanostructures all in situ without the need to break vacuum or contaminate the sample. The developed AFM-integration is described and its performance is demonstrated. The benefit of the active cantilever prevents the use of heavy and complex optical cantilever detection technique and makes the AFM integration into a SEM very simple and convenient. Results from combined examinations applying fast AFM-methods and SEM-image fusion, AFM-SEM combined metrology verification, and tip-based nanofabrication are shown. Simultaneous operation of SEM and AFM provides a fast navigation combined with sub-nm topographic image acquisition. The combination of two or more different types of techniques like SEM, energy dispersive x-ray spectroscopy, and AFM is called correlative microscopy because analytical information from the same place of the sample can be obtained and correlated [1]. We introduced to the SEM/FIB tool correlative nanofabrication methods like field-emission scanning probe lithography, tip-based electron beam induced deposition, and nanomachining/nanoidentation.
Atomic Force Microscopes are capable to provide non-destructive high resolution, CD-metrology and precise defect analysis. However, a conventional AFM has not enough throughput for today’s large scale semiconductor manufacturing. The primary point remains the increase of the scanning area in case of large wafers, masks, displays or dies. Cantilever array-based AFMs are intended to increase the imaging throughput by parallelizing the work of many AFM probes that may be practiced by parallel AFM systems that are capable to operate autonomously. An active cantilever scheme makes it possible to sense electronically the deflection and individually to control the actuation of every cantilever in the array. Each cantilever in the array represents a self-sustaining AFM-hardware system for metrology and imaging. In that, the multiple parallel probes are forming many AFMs capable to work independently.
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.
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.
Cost-effective generation of single-digit nano-lithographic features could be the way by which novel nanoelectronic devices, as single electron transistors combined with sophisticated CMOS integrated circuits, can be obtained. The capabilities of Field-Emission Scanning Probe Lithography (FE-SPL) and reactive ion etching (RIE) at cryogenic temperature open up a route to overcome the fundamental size limitations in nanofabrication. FE-SPL employs Fowler-Nordheim electron emission from the tip of a scanning probe in ambient conditions. The energy of the emitted electrons (<100 eV) is close to the lithographically relevant chemical excitations of the resist, thus strongly reducing proximity effects. The use of active, i.e. self-sensing and self-actuated, cantilevers as probes for FE-SPL leads to several promising performance benefits. These include: (1) Closed-loop lithography including pre-imaging, overlay alignment, exposure, and post-imaging for feature inspection; (2) Sub-5-nm lithographic resolution with sub-nm line edge roughness; (3) High overlay alignment accuracy; (4) Relatively low costs of ownership, since no vacuum is needed, and ease-of-use. Thus, FE-SPL is a promising tool for rapid nanoscale prototyping and fabrication of high resolution nanoimprint lithography templates. To demonstrate its capabilities we applied FE-SPL and RIE to fabricate single electron transistors (SET) targeted to operate at room temperature. Electrical characterization of these SET confirmed that the smallest functional structures had a diameter of only 1.8 nanometers. Devices at single digit nano-dimensions contain only a few dopant atoms and thus, these might be used to store and process quantum information by employing the states of individual atoms.
Sub-10 nanometer lithography is opening a new area for beyond-CMOS devices. Regarding to single nano-digit manufacturing we have established a new maskless patterning scheme by using field-emission, current controlled Scanning Probe Lithography (cc-SPL) in order to create optical nanodevices in thin silicon-on-insulator (SOI) substrates. This work aims to manufacture split ring resonators into calixarene resist by using SPL, while plasma etching at cryogenic temperatures is applied for an efficient pattern transfer into the underlying Si layer. Such electromagnetic resonators take the form of a ring with a narrow gap, whose 2D array was the first left-handed material tailored to demonstrate the so-called left-hand behavior of the wave propagation. It is shown that the resonance frequency can be tuned with the feature size of the resonator, and the resonance frequency can be shifted further into near infrared or even visible light regions.
Next-generation electronic and optical devices demand high-resolution patterning techniques and high-throughput fabrication. Thereby Field-Emission Scanning Probe Lithography (FE-SPL) is a direct writing method that provides high resolution, excellent overlay alignment accuracy and high fidelity nanopatterns. As a demonstration of the patterning technology, single-electron transistors as well as split ring electromagnetic resonators are fabricated through a combination of FE-SPL and plasma etching at cryogenic temperatures.
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