Field-emission Scanning Probe Lithography Research Articles

Investigations on tip-based large area nanofabrication and nanometrology using a planar nanopositioning machine (NFM-100)

This paper explores large area application of tip-based nanofabrication by field emission scanning probe lithography and showcases the simultaneous possibility of atomic force microscopy on macroscopic scales. This is made possible by the combination of tip-based technology and a planar nanopositioning and nanomeasuring machine. Using long range atomic force microscopy measurement of regular grating structures, the performance of the machine is thoroughly characterized over the full 100 mm range of motion of the positioning machine, which was confirmed by repeated measurements. After initially focussing on achieving the minimum line width of < 40 nm in microscopic areas, a grating with a pitch of 1 μm is additionally fabricated over a total length of 10 mm, whereby the dimensions and deviations are also considered.

Formation of Anti-Etching Nanopatterns in Field-Emission Scanning Probe Lithography on Calixarene Films

Formation of Anti-Etching Nanopatterns in Field-Emission Scanning Probe Lithography on Calixarene Films

Mask-less nano-structuring of hydrogen terminated diamond using localized field emission scanning probe lithography (FE-SPL)

Sensing and manipulating a particle's spin promise the development of more energy efficient and ultra-fast devices. Diamond is a promising candidate for spintronics and quantum systems, where nano-structuring of diamond surfaces on nanometer length scales is required. Here, direct-write modification of a diamond surface at sub-50 nm resolution is demonstrated. This is achieved with an atomic force microscope tip used as a spatially localized field emission source or so-called field-emission scanning probe lithography on a sample in ambient conditions, which are rendered conductive only through the surface conductivity of hydrogen-terminated diamond. Mask-less direct structuring of the diamond surface is observed, creating features of depth in the range of 4–8 nm and linewidths of 70–150 nm, as well as the desorption of hydrogen from the surface achieving minimum linewidths of 40 nm. Both the linewidth and depth of etching appear to depend on the energy of emitted electrons and the resulting electron exposure dose. These findings demonstrate the possibility of controllably nano-structuring diamond surfaces with features over micrometer length scales and with sub-50 nm resolution, while held under ambient conditions.

Advanced Scanning Probe Nanolithography Using GaN Nanowires.

A fundamental understanding and advancement of nanopatterning and nanometrology are essential in the future development of nanotechnology, atomic scale manipulation, and quantum technology industries. Scanning probe-based patterning/imaging techniques have been attractive for many research groups to conduct their research in nanoscale device fabrication and nanotechnology mainly due to its cost-effective process; however, the current tip materials in these techniques suffer from poor durability, limited resolution, and relatively high fabrication costs. Here, we report on employing GaN nanowires as a robust semiconductor material in scanning probe lithography (SPL) and microscopy (SPM) with a relatively low-cost fabrication process and the capability to provide sub-10 nm lithography and atomic scale (<1 nm) patterning resolution in field-emission scanning probe lithography (FE-SPL) and scanning tunneling microscopy (STM), respectively. We demonstrate that GaN NWs are great candidates for advanced SPL and imaging that can provide atomic resolution imaging and sub-10 nm nanopatterning on different materials in both vacuum and ambient operations.

Silk as a biodegradable resist for field-emission scanning probe lithography

The patterning of silk allows for manufacturing various structures with advanced functionalities for optical and tissue engineering and drug delivery applications. Here, we propose a high-resolution nanoscale patterning method based on field-emission scanning probe lithography (FE-SPL) that crosslinks the biomaterial silk on conductive indium tin oxide (ITO) promoting the use of a biodegradable material as resist and water as a developer. During the lithographic process, Fowler-Nordheim electron emission from a sharp tip was used to manipulate the structure of silk fibroin from random coil to beta sheet and the emission formed nanoscale latent patterns with a critical dimension (CD) of ∼50 nm. To demonstrate the versatility of the method, we patterned standard and complex shapes. This method is particularly attractive due to its ease of operation without relying on a vacuum or a special gaseous environment and without any need for complex electronics or optics. Therefore, this study paves a practical and cost-effective way toward patterning biopolymers at ultra-high level resolution.

Field emission scanning probe lithography with GaN nanowires on active cantilevers

Field emission scanning probe lithography (FE-SPL) is based on the exposure of a resist covered substrate with low energy electrons emitted from an ultra-sharp tip placed in close vicinity to a sample. GaN nanowires (NWs) present high mechanical stability, suitable geometry for FE-SPL, and controllable electrical properties achieved by adjusting dopant concentration. Here, the authors will present long time exposure results performed using GaN NWs tips, mounted on active scanning probes, working as field electron emitters. Using GaN NW tips, features down to the sub-10 nm were achieved in the FE-SPL process. A systematic study of the field emission current stability, exposure reproducibility, and results on exemplary high-resolution exposure and nanostructure imaging done with the same GaN tips will also be presented.

Mix-and-match lithography and cryogenic etching for NIL template fabrication

Field Emission Scanning Probe Lithography (FE-SPL) is an enabling technology for prototyping of sub-10 nm high-performance electronic devices. However, due to the serial writing scheme this technology is rather slow. The purpose of this work is the demonstration of a mix-and-match process in combination with cryogenic etching in order to fabricate templates for Nanoimprint Lithography (NIL). We describe (i) the fabrication of a micron-sized electrode layout on a 1.5 mm × 1.5 mm area by means of fast Direct Laser Writing (DLW), (ii) the subsequent stitching of nano-sized features on the same resist layer using high-resolution FE-SPL and (iii) the pattern transfer into silicon. IC manufacturing requires both, large throughputs with critical feature size smaller than 5 nm. Therefore, the motivation of our project is to develop a lithographic process for nano-electronic devices with critical feature size smaller than 5 nm, while the rest of the device sizes are 100 nm or larger. Thus, a possible SET template was designed for future high-throughput NIL.

Line edge roughness metrology software

A line edge roughness analysis software is developed based on the Canny edge detection algorithm with a double threshold, where threshold values are obtained by Otsu’s method. The performance of the software is demonstrated on features with a 200-nm nominal pitch generated by current-controlled, field-emission scanning probe lithography. Two lithographic modes are applied: (a) direct self-development positive mode and (b) image reversal mode. Atomic force imaging is used to analyze the line edge roughness. This is followed by a benchmarking study, where findings are compared to those provided by metroler software (Fractilia, LLC). This work is the first report on both line edge roughness involving imaging using the same exposure setup and latent image line edge roughness—made possible thanks to the resolving power of imaging through noncontact AFM. The authors are presenting a comparison of patterning through image reversal of the calixarene molecular glass resist from negative-tone to positive-tone as well as direct-write. In image reversal, a close match was observed between the proposed analysis and metroler software for line edge roughness and linewidth.

Correlative Microscopy and Nanofabrication with AFM Integrated with SEM

Abstract

Nanoscale lift-off process using field emission scanning probe lithography

Nanoelectronics manufacturing requires an ongoing development of lithography and also encompasses some “unconventional” methods. In this context, the authors use field emission scanning probe lithography (FE-SPL) to generate nanoscaled electronic devices. For the generation of future novel quantum devices, such as single-electron transistors or plasmonic resonators, patterning of features in the sub-10 nm regime as well as a defined metallization is necessary. In terms of metallization, the authors take advantage of the well-known lift-off process for creating narrow gap junctions. Narrow gap electrodes have found wide approval in the formation of narrow gap junctions and can be employed for the investigation of the electrical properties of molecules. In the lift-off process presented here, two sacrificial layers (50 nm polymethylglutarimide and 10 nm calixarene) have been deposited and patterned by FE-SPL. Subsequently, the sample was treated with tetraethyl-ammonium hydroxide in order to ensure an undercut. Afterward, a layer of 10 nm thick Cr has been deposited on top and finally the sacrificial films have been removed, leaving behind only the chromium film deposited directly on the substrate. In this work, the authors will present the utilization of novel active cantilevers with diamond coated silicon tips for FE-SPL purposes in order to generate chromium metal features by lift-off for the generation of future quantum devices. In this context, they will present the integration of an ultrananocrystalline diamond (UNCD) layer deposited on the tip of an active silicon cantilever. Electron emission and FE-SPL capabilities of UNCD coated silicon tips are evaluated. The authors demonstrate a reliable fabrication scheme of sub-15 nm coplanar narrow gap metal electrodes.

High-throughput process chain for single electron transistor devices based on field-emission scanning probe lithography and Smart Nanoimprint lithography technology

Next generation electronic devices like single electron transistors (SETs) operating at room temperature (RT) demand for high-resolution patterning techniques and simultaneously cost-effective, high-throughput manufacturing. Thereby, field-emission scanning probe lithography (FE-SPL) is a direct writing method providing high-resolution and high-quality nanopatterns. SET devices prepared by FE-SPL and plasma etching at cryogenic substrate temperature were shown to operate at RT [C. Lenk et al., Microelectron. Eng. 192, 77 (2018); Z. Durrani, M. Jones, F. Abualnaja, C. Wang, I. W. Rangelow, M. Kaestner, S. Lenk, C. Lenk, and A. Andreev, J. Appl. Phys. 124, 144502 (2018); I. W. Rangelow et al., J. Vac. Sci. Technol. B 34, 06K202 (2016)]. Nevertheless, FE-SPL lacks in writing speed and large area manufacturing capability required for industrial device manufacturing. This can be overcome by combining FE-SPL with nanoimprint lithography (NIL), which enables the replication of high-resolution features on large areas and provides high throughput. In this work, the authors will review a high-throughput process chain for RT-SET fabrication based on reproducing FE-SPL prepared masters by NIL and etching.

Field-emission scanning probe lithography tool for 150 mm wafer

The development of next nodes of nano-electronic devices requires mask-less techniques for fast prototyping and analysis of ultimately down-scaled devices or for fabrication of templates for nanoimprint based high-volume manufacturing. Moreover, the atomic force microscopy (AFM) of large surfaces with acceptable speed becomes an issue with the introduction of large-sized wafers. The authors have designed an AFM system which is capable of field-emission scanning probe lithography on 150 mm wafers providing superior stitching accuracy better than 3 nm. The system is also providing noncontact, high-resolution 3D imaging employing active probes (i.e., piezoresistive self-sensing and thermo-mechanically self-actuated probes) and capable to operate with an array of four cantilevers. A high-precision X-Y-θ stage with 10 nm positioning accuracy and with 360° rotation capability enables the highest placement precision and cost effective large scanning field imaging.

Atomic force microscope integrated with a scanning electron microscope for correlative nanofabrication and microscopy

More than 40 years after its invention, the atomic force microscopy (AFM) can be integrated with scanning electron microscope (SEM) instruments as an increasingly capable and productive characterization tool with sub-nanometer spatial resolution. The authors have designed and developed an AFM instrument capable to be integrated into any SEM or in a combination of SEM with a focused ion-beam (FIB) tool. 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. For the first time, they introduced to the SEM/FIB tool correlative nanofabrication methods like field-emission scanning probe lithography, tip-based electron beam induced deposition, and nanomachining. The combination of all these methods provides a completely new nanotechnology instrument, which should be seen as a tool for correlative nanofabrication and microscopy. Thus, it provides for the first time the capabilities of a stand-alone instrument with the capabilities of nondestructive three-dimensional tip-based metrology and nanofabrication into the combined SEM/FIB tool. In this article, the authors describe all these methods in detail and present a brief example of the results obtained. They demonstrate that the self-sensing, self-actuating cantilevers (called active cantilevers) equipped with Diamond tip are a versatile toolkit for fast imaging and emerging nanofabrication. 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.

Experimental study of field emission from ultrasharp silicon, diamond, GaN, and tungsten tips in close proximity to the counter electrode

The patterning process in field-emission scanning probe lithography (FE-SPL), a high-resolution and cost-effective method for nanofabrication, is based on the field emission of electrons from ultrasharp tips in close proximity to a sample (distances below 100 nm). Thereby, the emitted electrons expose directly an ultrathin resist film. The field enhancement at the tip apex is crucial for the field emission current, which follows the Fowler–Nordheim theory. Despite the success of FE-SPL in nanofabrication, systematic experimental studies of the field-emission process, including the determination of the tip radius and tip-to-sample distance during the measurement, for these small tip-to-sample distances and different tip materials are lacking. To resolve this issue, experimental measurements of the field-emission current for tip–sample proximity distances below 100 nm were performed. For this purpose, the developed AFM in SEM system was modified,1,2 which enables one to monitor the tip–sample distance with a high accuracy using SEM while simultaneously recording the field-emission current. The authors present experimental results of the dependence of the field-emission current on the tip shape, tip material, applied voltage, and tip–sample distance. Therefore, the emission characteristics of silicon, diamond, GaN, and tungsten tips are shown. The knowledge about the field-emission process for small tip-to-sample distances will help to understand and improve the current FE-SPL, regarding also the choice of tip material. Furthermore, these measurements enable the detailed comparison with current FE models beyond state-of-the-art since all necessary parameters (voltage, current, tip diameter, and tip-to-sample distance) could be measured and controlled during the FE experiment due to the unique experimental system.

Sharp GaN nanowires used as field emitter on active cantilevers for scanning probe lithography

Field emission scanning probe lithography (FE-SPL), which offers sub-10 nm resolution under ambient conditions, strongly relies on the quality and shape of the applied scanning probe tip. The technological development of AFM systems is also strongly connected to the performance of the cantilever. Thereby, the probes evolved from a simple passive deflection element to a complex MEMS through integration of functional groups, such as piezoresistive detection sensors and bimaterial based actuators. Here, the authors show actual trends and developments of miniaturization efforts for fabrication of active cantilevers with mounted sharp GaN and diamond nanowires. Both materials exhibit a higher mechanical stability than the typically applied silicon, which is supposed to increase tip lifetime and reproducibility of the lithography process. Furthermore, diamond offers a high chemical stability, whereas electrical and optical properties of GaN are tunable. They present preliminary results on the applicability of GaN nanowires and diamond tips, mounted on active scanning probes, as field emitter for FE-SPL. Therefore, field emission measurements, stability of field emission current, and exemplary high resolution features generated with the tested tips are shown.

Room-temperature single dopant atom quantum dot transistors in silicon, formed by field-emission scanning probe lithography

Electrical operation of room-temperature (RT) single dopant atom quantum dot (QD) transistors, based on phosphorous atoms isolated within nanoscale SiO2 tunnel barriers, is presented. In contrast to single dopant transistors in silicon, where the QD potential well is shallow and device operation limited to cryogenic temperature, here, a deep (∼2 eV) potential well allows electron confinement at RT. Our transistors use ∼10 nm size scale Si/SiO2/Si point-contact tunnel junctions, defined by scanning probe lithography and geometric oxidation. “Coulomb diamond” charge stability plots are measured at 290 K, with QD addition energy ∼0.3 eV. Theoretical simulation gives a QD size of similar order to the phosphorous atom separation ∼2 nm. Extraction of energy states predicts an anharmonic QD potential, fitted using a Morse oscillator-like potential. The results extend single-atom transistor operation to RT, enable tunneling spectroscopy of impurity atoms in insulators, and allow the energy landscape for P atoms in SiO2 to be determined.

Field emission from diamond nanotips for scanning probe lithography

Single-digit nanometer lithography is a basic requirement for beyond CMOS devices. To address this lithography challenge, a variety of different lithographic methods were developed. Here, the authors present the possibility of field emission scanning probe lithography (FE-SPL) with a diamond tip in order to enhance the lifetime of the used tip. A superior mechanical hardness and a good electron emission stability even after a total of 48 h of lithographic patterning by FE-SPL were proven, and features with half pitches down to 15 nm have been fabricated.

Theoretical investigation of the enhancement factor for a single field emitter in close proximity to the counter electrode

Field emission of electrons enables the patterning process in field-emission scanning probe lithography. A nanotip mounted at an active cantilever is used as an electron field emitter. Since the field enhancement at the tip apex is crucial for the Fowler–Nordheim emission current, the electric field and its enhancement based on a finite-element method (FEM) calculation are studied. The influences of the tip radius, the tip opening angle, and the tip–sample distance are considered. The numerically obtained enhancement factor γp is compared to various models found in the literature. Only three of them agree with the FEM data well. For small ratios of tip–sample distance and tip radius, the data resemble a “hemisphere on a post” model, and for larger ratios, it is better modeled by a “hemiellipsoid on a plane” model despite its “hemisphere on a cone” shape.

Nanopatterning on calixarene thin films via low-energy field-emission scanning probe lithography

Field-emitted, low-energy electrons from the conducting tip of an atomic force microscope were adopted for nanolithography on calixarene ultrathin films coated on silicon wafers. A structural evolution from protrusion to depression down to a 30 nm spatial resolution was reproducibly obtained by tuning the sample voltage and exposure current in the lithography process. Close analyses of the profiles showed that the nanostructures formed by a single exposure with a high current are almost identical to those created by cumulative exposure with a lower current but an equal number of injected electrons. Surface potential imaging by Kelvin probe force microscopy found a negatively charged region surrounding the groove structures once the structures were formed. We conclude that the mechanism related to the formation of a temporary negative state and molecule decomposition, rather than thermal ablation, is responsible for the low-energy field-emission electron lithography on a calixarene molecular resist. We hope that our elucidation of the underlying mechanism is helpful for molecular resist design and further improving the reproducibility and throughput of nanolithography.

Nanofabrication by field-emission scanning probe lithography and cryogenic plasma etching

Building low-power and high-density circuits requires new devices, which can be based for example on single electron effects. Single electron transistors (SET), which can operate at room temperature (RT), are candidates with high potential for the post-CMOS era. However, their fabrication relies typically on a statistical fabrication of quantum dots or positioning of nanoparticles or molecules between predefined electrodes. These methods hamper a scaled-up fabrication of RT-SETs. Here, we present a route for reproducible fabrication of RT-SETs on the basis of field-emission scanning probe lithography (FE-SPL) and cryogenic reactive ion etching. Due to the unique capabilities of our FE-SPL tool, enabling pre- and post-inspection of features, highly reliable patterning and precise feature alignment are obtained. The fabricated devices exhibit single electron effects at RT. A combination of this method with nanoimprint lithography would enable a high throughput and reproducible way of RT-SET fabrication.