Claudia Lenk

Pattern-generation and pattern-transfer for single-digit nano devices

Single-electron devices operating at room temperature require sub-5 nm quantum dots having tunnel junctions of comparable dimensions. Further development in nanoelectronics depends on the capability to generate mesoscopic structures and interfacing these with complementary metal–oxide–semiconductor devices in a single system. The authors employ a combination of two novel methods of fabricating room temperature silicon single-electron transistors (SETs), Fowler–Nordheim scanning probe lithography (F-N SPL) with active cantilevers and cryogenic reactive ion etching followed by pattern-dependent oxidation. The F-N SPL employs a low energy electron exposure of 5–10 nm thick high-resolution molecular resist (Calixarene) resulting in single nanodigit lithographic performance [Rangelow et al., Proc. SPIE 7637, 76370V (2010)]. The followed step of pattern transfer into silicon becomes very challenging because of the extremely low resist thickness, which limits the etching depth. The authors developed a computer simulation code to simulate the reactive ion etching at cryogenic temperatures (−120 °C). In this article, the authors present the alliance of all these technologies used for the manufacturing of SETs capable to operate at room temperatures.

2D Simulation of Fowler-Nordheim Electron Emission in Scanning Probe Lithography

2D Simulation of Fowler-Nordheim Electron Emission in Scanning Probe Lithography For the manufacturing of quantum computers it will be necessary to routinely fabricate devices with critical dimensions down to the single-digit nanometer range. Since the high-costs and belated development of extreme UV lithography, we are focused on scanning probe lithography (SPL) utilizing Fowler-Nordheim emitted electrons for patterning of molecular resist materials. Our method is similar to the electron beam lithography with special electron emitters, i.e. our nanotips, differing in the much lower energy of the emitted electrons and the possibility to work at ambient conditions. Based on the thermo-mechanically actuated, piezoresistive cantilever technology our group has developed a first prototype of a scanning probe lithography platform able to image, inspect, align and pattern features down to single nanometer regime. Here, we present theoretical investigations of the electron emission and the surface exposure with the emitted electrons. Our simulation model and the used assumptions are described. The resulting electric field and electron density distributions are analyzed to gain deeper insights into relevance of the lithographic exposure parameters.

Six-axis AFM in SEM with self-sensing and self-transduced cantilever for high speed analysis and nanolithography

Merging two state-of-the-art surface research techniques, in particular, atomic force microscopy (AFM) and scanning electron microscopy (SEM), within a single system is providing novel capabilities like direct visual feedback and life-monitoring of tip-induced nanoscale interactions. In addition, the combination of AFM and SEM accelerates nanoscale characterization and metrology development. Here, the concept and first results of a novel AFM-integration into a high resolution scanning electron microscope and focused ion beam system for nanoscale characterization is presented. In this context, a six-axis AFM system using self-sensing thermomechanically transduced active cantilever was developed and integrated. The design of the developed AFM-integration is described and its performance is demonstrated. Results from combined examinations applying fast AFM-methods and SEM-image fusion, AFM-SEM combined metrology verification, and three dimensional-visualization are shown. Simultaneous operation of SEM and AF...

Large area fast-AFM scanning with active “Quattro” cantilever arrays

In this work, the fabrication and operation of an active parallel cantilever device integrating four self-sensing and self-actuating probes in an array is presented. The so called “Quattro” cantilever system is controlled by a multichannel field programmable gate array (FPGA) controller. The integrated cantilever devices are fabricated on the basis of a silicon-on-insulator wafer using surface micromachining and gas chopping plasma-etching processes [I. W. Rangelow, J. Vac. Sci. Technol., A 21, 1550 (2003)]. The unique design of the active cantilever probes provides both patterning and readout capabilities [Kaestner et al., J. Micro-Nanolithogr. MEMS 14, 031202 (2015)]. The thermomechanical actuation allows the individually operation of each cantilever in static and dynamic modes. This enables a simultaneous atomic force microscopy operation of all cantilevers in an array, while the piezoresistive read-out of the cantilever bending routinely ensures atomic resolution at a high imaging speed. The scanning ...

Review Article: Active scanning probes: A versatile toolkit for fast imaging and emerging nanofabrication

With the recent advances in the field of nanotechnology, measurement and manipulation requirements at the nanoscale have become more stringent than ever before. In atomic force microscopy, high-speed performance alone is not sufficient without considerations of other aspects of the measurement task, such as the feature aspect ratio, required range, or acceptable probe-sample interaction forces. In this paper, the authors discuss these requirements and the research directions that provide the highest potential in meeting them. The authors elaborate on the efforts toward the downsizing of self-sensed and self-actuated probes as well as on upscaling by active cantilever arrays. The authors present the fabrication process of active probes along with the tip customizations carried out targeting specific application fields. As promising application in scope of nanofabrication, field emission scanning probe lithography is introduced. The authors further discuss their control and design approach. Here, microactua...

Initiation of atrial fibrillation by interaction of pacemakers with geometrical constraints

Atrial fibrillation (AF) is the most common arrhythmia of the heart in industrialized countries. Its generation and the transitory behavior of paroxysmal AF are still not well understood. In this work we examine the interaction of two activation sources via an isthmus as possible cause for the initiation of fibrillation episodes. For this study, the electrophysiological model of Bueno-Orovio, Cherry and Fenton is adapted to atrial electrophysiology, both for physiological and electrophysiologically remodeled conditions due to AF. We show that the interaction of the pacemakers, combined with the geometrical constraints of the isthmus, can produce fibrillatory-type irregularities, which we quantify by the loss of spatial phase coherence in the transmembrane voltage. Transitions to irregular behavior occur when the frequencies of the pacemakers exceed certain thresholds, suggesting that AF episodes are initiated by frequency changes of the activating sources (sinus node, ectopic focus).

Identification of response classes from heavy metal‐tolerant soil microbial communities by highly resolved concentration‐dependent screenings in a microfluidic system

Summary One of the most important constituents in soil is the microflora, mainly containing bacteria and fungi with high metabolic versatility and very complex intra- and interspecific interactions. Co-occurrence of several micro-organism species in soil regulates growth or suppression of single species, either by mutual tolerance or by induction of defence mechanisms, which may result in the release of secondary metabolites for growth suppression of coexisting species. Accumulations of heavy metals in soils can further affect the growth of soil microbial communities; this however is strongly dependent on the capability of micro-organisms to tolerate heavy metals. Until now, there is no fast and reliable method available to study the growth of microbial communities in highly resolved concentration spaces with environmentally relevant toxic substances such as heavy metals and to identify the tolerance thresholds of micro-organism communities of selected soils. Here, we present a new methodological approach for the assessment of the growth–response behaviour of soil microbial communities in response to increasing heavy metal concentration (copper) using the droplet-based micro-segmented flow technique. Therefore, micro-organism-containing soil slurries from contact with metal artefacts from archaeological excavations and from the surface of early copper-mining areas were studied by separate cultivation in segments in the sub-μL range and growth, and fluorescence was characterized after cultivation by combined micro-flow-through photometry and fluorimetry. Highly resolved dose–response data provided copper tolerance thresholds of the soil communities of the different soils. Concentration-dependent growth patterns of the micro-organisms in the segments could be observed and allowed to distinguish response groups with characteristic distribution of photometric and fluorimetric measurement values. It is assumed that these response groups are caused by a sample characteristic growth of metal-tolerant microbial communities with characteristic critical metal concentrations for growth inhibition. The clear transitions between the groups in small concentration intervals are probably due to sharp transitions between growth and no growth of dominant micro-organism species at the critical metal concentration. The investigations demonstrate the potential of droplet-based microfluidic techniques for ultra-miniaturized ecological studies and its suitability for the assessment of tolerance thresholds of soil microbial communities from heavy metal-contaminated areas.

Fabrication of optical nanodevices through field-emission scanning probe lithography and cryogenic etching

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.

Field-emission scanning probe lithography with self-actuating and self-sensing cantilevers for devices with single digit nanometer dimensions

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.

Single nano-digit and closed-loop scanning probe lithography for manufacturing of electronic and optical nanodevices

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.