Direct fabrication of pure metallic nanostructures is one of the main aims of focused electron beam-induced deposition (FEBID). It was recently achieved for gold deposits by the co-injection of a water precursor and the gold precursor Au(tfac)Me₂. In this work results are reported, using the same approach, on a different gold precursor, Au(acac)Me₂, as well as the frequently used platinum precursor MeCpPtMe₃. As a water precursor MgSO₄·7H₂O was used. The purification during deposition led to a decrease of the carbon-to-gold ratio (in atom %) from 2.8 to 0.5 and a decrease of the carbon-to-platinum ratio (in atom %) from 6–7 to 0.2. The purification was done in a regular scanning electron microscope using commercially available components and chemicals, which paves the way for a broader application of direct etching-assisted FEBID to obtain pure metallic structures.

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In electron optics, calculation of the electric field plays a major role in all computations and simulations. Accurate field calculation methods such as the finite element method (FEM), boundary element method and finite difference method, have been used for years. However, such methods are computationally very expensive and make the computer simulation challenging or even infeasible when trying to apply automated design of electrostatic lens systems with many free parameters. Hence, for years, electron optics scientists have been searching for a fast and accurate method of field calculation to tackle the aforementioned problem in the design and optimization of electrostatic electron lens systems. This paper presents a novel method for fast electric field calculation in electrostatic electron lens systems with reasonably high accuracy to enable the electron-optical designers to design and optimize an electrostatic lens system with many free parameters in a reasonably short time. The essence of the method is to express the off-axis potential in an axially symmetrical coordinate system in terms of derivatives of the axial potential up to the fourth order, and equate this to the potential of the electrode at that axial position. Doing this for a limited number of axial positions, we get a set of equations that can be solved to obtain the axial potential, necessary for calculating the lens properties. We name this method the fourth-order electrode method because we take the axial derivatives up to the fourth order. To solve the equations, a quintic spline approximation of the axial potential is calculated by solving three sets of linear equations simultaneously. The sets of equations are extracted from the Laplace equation and the fundamental equations that describe a quintic spline. The accuracy and speed of this method is compared with other field calculation methods, such as the FEM and second order electrode method (SOEM). The new field calculation method is implemented in design/optimization of electrostatic lens systems by using a genetic algorithm based optimization program for electrostatic lens systems developed by the authors. The effectiveness of this new field calculation method in optimizing optical parameters of electrostatic lens systems is compared with FEM and SOEM and the results are presented. It should be noted that the formulation is derived for general axis symmetrical electrostatic electron lens systems, however the examples shown in this paper are with cylindrical electrodes due to the simplicity of the implementation in the software.

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Structures fabricated using focused electron beam-induced deposition (FEBID) have sloped sidewalls because of the very nature of the deposition process. For applications this is highly undesirable, especially when neighboring structures are interconnected. A new technique combining FEBID and focused electron beam-induced etching (FEBIE) has been developed to fabricate structures with vertical sidewalls. The sidewalls of carbon FEBID structures have been modified by etching with water and it is shown, using transmission electron microscopy imaging, that the sidewall angle can be tuned from outward to inward by controlling the etch position on the sidewall. A surprising under-etching due to the emission of secondary electrons from the deposit was observed, which was not indicated by a simple model based on etching. An analytical model was developed to include continued etching once the deposit has been removed at the exposed pixel. At this stage the secondary electrons from the substrate then cause the adsorbed water molecules to become effective in etching the deposit from below, resulting in under-etched structures. The evolution of the sidewall angle during etching has also been experimentally observed in a scanning electron microscope by continuously monitoring the secondary electron detector signal.

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To design electron lens systems, applying a fully automated optimization routine has not yet been feasible, especially for the case where the optimization has many free variables of the lens system, such as all parameters that define the geometry of the lens electrodes and the voltage of each electrode. Hence, the study of the implementation of different optimization procedures has not yet been possible either. In one of our previous studies, we have proposed to use the so-called Second Order Electrode Method (SOEM) which performs the electrostatic field calculations in a very short time by the approximations of the field near the optical axis. There, using SOEM in field calculation, a Genetic Algorithm (GA) was successfully implemented to optimize the electron lens systems. One of the questions that has not been studied and answered in the literature yet, is whether the GA is the most suitable option among different optimization techniques for the design/optimization of electron lens systems. In this paper, by implementing the SOEM technique as the field calculation method, different optimization procedures are implemented and their performances are compared. For this study, a typical six electrode lens system is employed. The implemented optimization techniques include calculus-based local optimization (‘Fmin’) and metaheuristic methods such as GA, Particle Swarm Optimization (PSO), and Simulated Annealing (SA). The results demonstrate that the population-based global optimization techniques like GA and PSO significantly outperform single-based local optimization methods such as ‘Fmin’ and SA. Additionally, PSO shows slightly better performance than GA, although it cannot be concluded that PSO will always outperform GA for every electron lens design problem. Furthermore, in the comparison between the two single-based optimization techniques, the metaheuristic approach (SA) outperforms the calculus-based one (‘Fmin’). Hence, we recommend implementing metaheuristic, global, population-based optimization techniques like GA and PSO for the optimization electron lens systems.@en

In this work we demonstrate that ultra-thin (5 and 15 nm) MgO transmission dynodes with sufficient high transmission electron yield (TEY) can be constructed. These transmission dynodes act as electron amplification stages in a novel vacuum electron multiplier: the Timed Photon Counter. The ultra-thin membranes with a diameter of 30 μm are arranged in a square 64-by-64-array. The TEY was determined with a scanning electron microscope using primary electrons with primary energies of 0.75-5 keV. The method allows a TEY map of the surface to be made while simultaneously imaging the surface. The TEY of individual membranes can be extracted from the TEY map. An averaged maximum TEY of 4.6±0.2 was achieved by using 1.35 keV primary electrons on a TiN/MgO bi-layer membrane with a layer thickness of 2 and 5 nm, respectively. The TiN/MgO membrane with a layer thickness of 2 and 15 nm, respectively, has a maximum TEY of 3.3±0.1 (2.35 keV). Furthermore, the effect of the electric field strength on transmission (secondary) electron emission was investigated by placing the emission surface of a transmission dynode in close proximity to a planar collector. By increasing the electric potential between the transmission dynode and the collector, from -50 V to -100 V, the averaged maximum TEY improved from 4.6±0.2 to 5.0±0.3 at a primary energy of 1.35 keV with an upper limit of 5.5 on one of the membranes.

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The design of electrostatic electron lenses involves the choice of many geometrical parameters for the lens electrodes as well as the choice of voltages applied to the electrodes. The purpose of the design is to focus the electrons at a specific point and to minimize the aberrations of the lens. In a previous study, genetic algorithm optimization was introduced to aid the designer. For speeding up the electrostatic field calculations, new methods for analytical approximations of the field near the optical axis were introduced. In this paper, the influence of the main tuning parameters of the Genetic Algorithms is analyzed. The analysis is performed on a typical electrostatic lens systems including 6 electrodes. Different combinations of population sizes and number of generations are taken and the quality of the optimized lens system is compared. It is seen that within a constant computational effort (time or total number of system evaluations), a lower population size with a larger number of generations can achieve better results compared to having larger population size and fewer generations. The combination of Crossover Heuristic with Mutation Gaussian showed significantly better results compared to all other combinations of Mutations and Crossovers. Crossover Fraction is also evaluated to find the most suited values of this parameter.@en

Super-resolution fluorescence microscopy can be achieved by image reconstruction after spatially patterned illumination or sequential photo-switching and read-out. Reconstruction algorithms and microscope performance are typically tested using simulated image data, due to a lack of strategies to pattern complex fluorescent patterns with nanoscale dimension control. Here, we report direct electron-beam patterning of fluorescence nanopatterns as calibration standards for super-resolution fluorescence. Patterned regions are identified with both electron microscopy and fluorescence labelling of choice, allowing precise correlation of predefined pattern dimensions, a posteriori obtained electron images, and reconstructed super-resolution images.

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The effect of doping in Si3N4 membranes on the secondary electron yield is investigated using Monte Carlo simulations of the electron-matter interactions. The effect of the concentration and the distribution of the doping in silicon rich silicon nitride membranes is studied by using the energy loss function as obtained from ab initio density functional theory calculations in the electron scattering models of the Monte Carlo simulations. An increasing doping concentration leads to a decreasing maximum secondary electron yield. The distribution of the doped silicon atoms can be optimised in order to minimize the decrease in yield.

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Large-area transmission dynodes were fabricated by depositing an ultra-thin continuous film on a silicon wafer with a 3-dimensional pattern. After removing the silicon, a corrugated membrane with enhanced mechanical properties was formed. Mechanical metamaterials, such as this corrugated membrane, are engineered to improve its strength and robustness, which allows it to span a larger surface in comparison to flat membranes while the film thickness remains constant. The ultra-thin film consists of three layers (Al₂O₃/TiN/Al₂O₃) and is deposited by atomic layer deposition (ALD). The encapsulated TiN layer provides in-plane conductivity, which is needed to sustain secondary electron emission. Two types of corrugated membranes were fabricated: a hexagonal honeycomb and an octagonal pattern. The latter was designed to match the square pitch of a CMOS pixel chip. The transmission secondary electron yield was determined with a collector-based method using a scanning electron microscope. The highest transmission electron yield was measured on a membrane with an octagonal pattern. A yield of 2.15 was achieved for 3.15 keV incident electrons for an Al₂O₃/TiN/Al₂O₃ tri-layer film with layer thicknesses of 10/5/15 nm. The variation in yield across the surface of the corrugated membrane was determined by constructing a yield map. The active surface for transmission secondary electron emission is near 100%, i.e. a primary electron generates transmission secondary electrons regardless of the point of impact on the corrugated membrane.

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The (secondary) electron emission from multilayered Al2O3/TiN membranes has been investigated with a hemispherical collector system in a scanning electron microscope for electrons with energies between 0.3 and 10 keV. These ultra-thin membranes are designed to function as transmission dynodes in novel vacuum electron multipliers. Two different types, a bi-layer and a tri-layer, have been manufactured by means of atomic-layer deposition (ALD) of aluminum oxide and sputtering of titanium nitride. The reflection and transmission electron yield (σR, σT) have been measured for both types of membranes. In comparison, the tri-layer membranes outperformed the bi-layer membranes in terms of transmission electron yield for films with the same effective thickness. The highest transmission electron yield was measured on an Al2O3/TiN/Al2O3 film with layer thicknesses of 5/2.5/5 nm, which had a maximum transmission electron yield {σ T max(E0) of 3.1 (1.55 keV). Furthermore, the bi-layer membranes have been investigated more in-depth by performing an additional measurement using a positive sample bias to separate the transmitted fraction ηT and the transmission secondary electron yield δT. The transmitted fraction was used to determine the transmission parameter p, which characterizes the interaction of primary electrons (PEs) in thin films. The transmission secondary electron yield was used to compare the energy transfer of PEs in films with different thicknesses.

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Super-resolution structured illumination microscopy (SIM) has become a widely used method for biological imaging. Standard reconstruction algorithms, however, are prone to generate noise-specific artifacts that limit their applicability for lower signal-to-noise data. Here we present a physically realistic noise model that explains the structured noise artifact, which we then use to motivate new complementary reconstruction approaches. True-Wiener-filtered SIM optimizes contrast given the available signal-to-noise ratio, and flat-noise SIM fully overcomes the structured noise artifact while maintaining resolving power. Both methods eliminate ad hoc user-adjustable reconstruction parameters in favor of physical parameters, enhancing objectivity. The new reconstructions point to a trade-off between contrast and a natural noise appearance. This trade-off can be partly overcome by further notch filtering but at the expense of a decrease in signal-to-noise ratio. The benefits of the proposed approaches are demonstrated on focal adhesion and tubulin samples in two and three dimensions, and on nanofabricated fluorescent test patterns.

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High resolution dense lines patterned by focused electron beam-induced deposition (FEBID) have been demonstrated to be promising for lithography. One of the challenges is the presence of interconnecting material, which is often carbonaceous, between the lines as a result of the Gaussian line profile. We demonstrate the use of focused electron beam-induced etching (FEBIE) as a scanning electron microscope (SEM)-based direct-write technique for the removal of this interconnecting material, which can be implemented without removing the sample from the SEM for post processing. Secondary electron (SE) imaging has been used to monitor the FEBIE process, and atomic force microscopy (AFM) measurements confirm the fabrication of well separated FEBID lines. We further demonstrate the application of this technique for removing interconnecting material in high resolution dense lines using backscattered electron (BSE) imaging to monitor the process.

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Seven gold(I) N-heterocyclic carbene (NHC) complexes were synthesized, characterized, and identified as suitable precursors for focused electron beam-induced deposition (FEBID). Several variations on the core Au(NHC)X moiety were introduced, that is, variations of the NHC ring (imidazole or triazole), of the alkyl N-substituents (Me, Et, or iPr), and of the ancillary ligand X (Cl, Br, I, or CF₃). The seven complexes were tested as FEBID precursors in an on-substrate custom setup. The effect of the substitutions on deposit composition and growth rate indicates that the most suitable organic ligand for the gold precursor is triazole-based, with the best deposit composition of 15 atom % gold, while the most suitable anionic ligand is the trifluoromethyl group, leading to a growth rate of 1 × 10⁻² nm³/e⁻.

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The design of an electrostatic electron optical system with five electrodes and two objective functions is optimized using multiobjective genetic algorithms (MOGAs) optimization. The two objective functions considered are minimum probe size of the primary electron beam in a fixed image plane and maximum secondary electron detection efficiency at an in-lens detector plane. The time-consuming step is the calculation of the system potential. There are two methods to do this. The first is using COMSOL (finite element method) and the second is using the second-order electrode method (SOEM). The former makes the optimization process very slow but accurate, and the latter makes it fast but less accurate. A fully automated optimization strategy is presented, where a SOEM-based MOGA provides input systems for a COMSOL-based MOGA. This boosts the optimization process and reduces the optimization times by at least ∼10 times, from several days to a few hours. A typical optimized system has a probe size of 11.9 nm and a secondary electron detection efficiency of 80%. This new method can be implemented in electrostatic lens design with one or more objective functions and multiple free variables as a very efficient, fully automated optimization technique.

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Micro- and nano-patterns are gaining increasing attraction in several fields ranging from nanoelectronics to bioengineering. The mechanical properties of the nanostructures (nanopillars, nanotubes, nanowires, etc.) are highly relevant for many applications but challenging to determine. Existing mechanical characterization methods require mounting the testing setup inside a scanning electron microscope (SEM) and additional sample modification. Here, we propose two atomic force microscopy (AFM) methods, based on contact mode imaging (CMI) and force spectroscopy imaging (FSI), to determine the mechanical characteristics of individual micro- and nanopillars as fabricated, without using SEM. We present the working principles of both methods and two case studies on nanopillars fabricated by additive manufacturing methods: two-photon polymerization (2PP) and electron beam induced deposition (EBID). Various mechanical parameters were determined using CMI and FSI, respectively. For the 2PP nanopillars, we measured the stiffness (13.5 ± 3.2 N/m and 15.9 ± 2.6 N/m), the maximum lateral force (883.0 ± 89.5 nN and 889.6 ± 113.6 nN), the maximum deflection (64.2 ± 13.6 nm and 58.3 ± 14.24 nm), the failure stress (0.3 ± 0.03 GPa and 0.3 ± 0.02 GPa), and the adhesion force (56.6 ± 4.5 µN and 58.6 ± 5.2 µN). For the EBID nanopillars, we measured the failure stress (2.9 ± 0.2 GPa and 2.7 ± 0.4 GPa). The similar results obtained using both techniques confirmed the efficacy and consistency of the methods. The proposed methodologies have the potential of enabling otherwise impossible measurements particularly when the specimens need to be tested under wet conditions, such as patterns for mechanobiological studies.

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In electron optics, the design of electron lens systems is still a challenge. To optimize such systems, the objective function which should be calculated, depends on the electric potential distribution in the space created by the lenses. To obtain the electric potential, the existing methods are generally based on some mathematical techniques which need to mesh the space of the lens system and derive the electric potential at all mesh points. Hence, calculation of the objective function for such systems are computationally expensive. Therefore, applying a fully automatic optimization routine has not yet been feasible, especially for lens systems with many free variables. Hence, the study of objective-function landscape of such problems has not yet been performed. One of the questions of interest for optical designers, that has not been studied in the literature, is whether this problem can be solved by a local optimizer or is it necessary to apply a global optimizer. Recently we succeeded in implementing a method (based on a so-called SOEM (Second Order Electrode Method) technique) which calculates the electric potential in a fast and reasonably accurate way. In this paper, that method, is implemented to perform the study of local versus global optimization for electron lens design. The global optimization method here is performed by GA (Genetic Algorithm). The objective function is taken to be the probe size of the electron beams at the image plane. The results of our study show that the objective function of this problem has many local minima and the optimization of such problems cannot be handled by a local optimizer. GA is shown to perform well by overcoming these multiple-local minima to arrive at a global minima.

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To allow researchers to fabricate micro- and nano-devices on a small scale, without having to use complex cleanroom facilities, a single tool is proposed in which a variety of typical cleanroom techniques and processes is combined. This ‘cleanroom’ in SEM tool, based on a scanning electron microscope (SEM), integrates several add-on tools, such as a miniature plasma source for sputtering and cleaning purposes, a miniature thermal evaporator for metal deposition, and facilities to enable in-situ selective atomic layer deposition. The cleanroom techniques and processes selected for integration in the ‘cleanroom’ in SEM tool are discussed, and the design and fabrication of the add-on tools are presented. Finally the proofs of principle of the plasma source, evaporator and in-situ selective ALD process are experimentally demonstrated.

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Line-edge roughness (LER) is often measured from top-down critical dimension scanning electron microscope (CD-SEM) images. The true three-dimensional roughness profile of the sidewall is typically ignored in such analyses. We study the response of a CD-SEM to sidewall roughness (SWR) by simulation. We generate random rough lines and spaces, where the SWR is modelled by a known power spectral density. We then obtain corresponding CD-SEM images using a Monte Carlo electron scattering simulator. We find the measured LER from these images, and compare it to the known input roughness. We find that, for isolated lines, the SEM measures the outermost extrusion of the rough sidewall. The result is that the measured LER is up to a factor 2 less than the true on-wafer roughness. The effect can be accurately modelled by making a top-down projection of the rough edge. Our model for isolated lines works fairly well for a dense grating of lines and spaces, as long as the trench width exceeds the line height.

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One of the methods to create sub-10 nm resolution metal-composed 3D nanopillars is electron beam-induced deposition (EBID). Surface nanotopographies (e.g., nanopillars) could play an important role in the design and fabrication of implantable medical devices by preventing the infections that are caused by the bacterial colonization of the implant surface. The mechanical properties of such nanoscale structures can influence their bactericidal efficiency. In addition, these properties are key factors in determining the fate of stem cells. In this study, we quantified the relevant mechanical properties of EBID nanopillars interacting with Staphylococcus aureus (S. aureus) using atomic force microscopy (AFM). We first determined the elastic modulus (17.7 GPa) and the fracture stress (3.0 ± 0.3 GPa) of the nanopillars using the quantitative imaging (QI) mode and contact mode (CM) of AFM. The displacement of the nanopillars interacting with the bacteria cells was measured by scanning electron microscopy (50.3 ± 9.0 nm). Finite element method based simulations were then applied to obtain the force-displacement curve of the nanopillars (considering the specified dimensions and the measured value of the elastic modulus) based on which an interaction force of 88.7 ± 36.1 nN was determined. The maximum von Mises stress of the nanopillars subjected to these forces was also determined (3.2 ± 0.3 GPa). These values were close to the maximum (i.e., fracture) stress of the pillars as measured by AFM, indicating that the nanopillars were close to their breaking point while interacting with S. aureus. These findings reveal unique quantitative data regarding the mechanical properties of nanopillars interacting with bacterial cells and highlight the possibilities of enhancing the bactericidal activity of the investigated EBID nanopillars by adjusting both their geometry and mechanical properties.@en

We measure the transmission secondary electron yield of nanometer-thick Al2O3/TiN/Al2O3 films using a prototype version of a Timed Photon Counter (TiPC). We discuss the method to measure the yield extensively. The yield is then measured as a function of landing energy between 1.2 and 1.8 keV and found to be in the range of 0.1 (1.2 keV) to 0.9 (1.8 keV). These results are in agreement to data obtained by a different, independent method. We therefore conclude that the prototype TiPC is able to characterise the thin films in terms of transmission secondary electron yield. Additionally, observed features which are unrelated to the yield determination are interpreted.@en