Lensless electron reflection microscopy using a coaxial point-source structure

A lensless image of the surface of a crystal is obtained by the reflection on this surface of a low-energy electron beam originated from a point source integrated in a coaxial structure. The point source is a sharp field emission tip and a free propagation of reflected electrons results from the shielding of the tip voltage provided by the coaxial structure. Images are obtained for an incidence angle between 3 and 45 degrees and for nA incident currents with a kinetic energy down to 40 V. On silicon surfaces a magnification up to a few thousands and a spatial resolution of 100 nm are demonstrated.     

A model of secondary electron imaging in the helium ion scanning microscope

A combination of the ‘semi-empirical’ model for secondary electron production and the TRIM routines which describe ion stopping power, scattering, and transport, has been used to construct a Monte Carlo simulation (IONiSE) that can quantitatively interpret the generation of secondary electrons (SE) from materials by fast helium ions. This approach requires that the parameters of the semi-empirical model be determined by fitting to experimental yield data but has the merit that, unlike more fundamental models, it can be applied with equal ease to both pure elements and complex compounds. The application of the model to predict the topographic yield variation of helium generated SE as a function of energy and material, and to investigate the ratio between SE generated by incident and backscattered ions, is demonstrated.     

Electron irradiation-induced destruction of carbon nanotubes in electron microscopes

Observations of carbon nanotubes under exposure to electron beam irradiation in standard transmission electron microscope (TEM) and scanning electron microscope (SEM) systems show that such treatment in some cases can cause severe damage of the nanotube structure, even at electron energies far below the approximate 100 keV threshold for knock-on damage displacing carbon atoms in the graphene structure. We find that the damage we observe in one TEM can be avoided by use of a cold finger. This and the morphology of the damage imply that water vapour, which is present as a background gas in many vacuum chambers, can damage the nanotube structure through electron beam-induced chemical reactions. Though, the dependence on the background gas makes these observations specific for the presently used systems, the results demonstrate the importance of careful assessment of the level of subtle structural damage that the individual electron microscope system can do to nanostructures during standard use.     

An electron microscope for the aberration-corrected era

Improved resolution made possible by aberration correction has greatly increased the demands on the performance of all parts of high-end electron microscopes. In order to meet these demands, we have designed and built an entirely new scanning transmission electron microscope (STEM). The microscope includes a flexible illumination system that allows the properties of its probe to be changed on-the-fly, a third-generation aberration corrector which corrects all geometric aberrations up to fifth order, an ultra-responsive yet stable five-axis sample stage, and a flexible configuration of optimized detectors. The microscope features many innovations, such as a modular column assembled from building blocks that can be stacked in almost any order, in situ storage and cleaning facilities for up to five samples, computer-controlled loading of samples into the column, and self-diagnosing electronics. The microscope construction is described, and examples of its capabilities are shown.     

Electron field emission properties of Co quantum dots in SiO2 matrix synthesised by ion implantation

In this work, Co ions were implanted into thermally oxidised SiO2 layers on silicon substrates. The implantation energy was 50 keV and the doses were 1, 3, 5 and 7 x 10(16) Co+/cm2. The field emission (FE) properties of these layers were studied and correlated with results from atomic force microscopy and transmission electron microscopy measurements. Other than that for the lowest dose sample, crystallised Co nanoclusters, with sizes ranging from 1.8 to 5.7 nm, are observed in these Co-implanted layers. The higher dose samples exhibit excellent FE properties and give an emission current of 1 nA at electric fields as low as 5 V/microm, for a dose of 5 x 10(16) Co+/cm2, compared with 120 V/microm for the lowest dose samples. We attribute the excellent FE properties of these layers to the formation of Co nanoclusters, with the electrical inhomogeneity giving rise to local field enhancement. Finally, repeatable staircase-like current-field (I-F) characteristics are observed in FE measurements of these higher dose samples as compared to conventional Fowler-Nordheim-type I-F characteristics in the lower dose sample. We believe this data may be a result of Coulomb blockade effects arising from the isolated low-capacitance metal quantum dots formed by controlled ion implantation.     

Microelectrical characterizations of junctions in solar cell devices by scanning Kelvin probe force microscopy

Scanning Kelvin probe force microscopy was applied to the microelectrical characterizations of junctions in solar cell devices. Surface Fermi-level pinning effects on the surface potential measurement were avoided by applying a bias voltage (V_b) to the device and taking the V_b-induced potential and electric field changes. Two characterizations are presented: the first is a direct measurement of Bi-induced junction shift in GaInNAs(Bi) cells; the second is a junction-uniformity measurement in a-Si:H devices. In the first characterization, using Bi as a surfactant during the molecular beam epitaxy growth of GaInNAs(Bi) makes the epitaxial layer smoother. However, the electrical potential measurement exhibits a clear Bi-induced junction shift to the back side of the absorber layer, which results in significant device degradation. In the second characterization, the potential measurement reveals highly non-uniform electric field distributions across the n–i–p junction of a-Si:H devices; the electric field concentrates much more at both n/i and i/p interfaces than in the middle of the i-layer. This non-uniform electric field is due possibly to high defect concentrations at the interfaces. The potential measurements further showed a significant improvement in the electric field uniformity by depositing buffer layers at the interfaces, and this indeed improved the device performance.     

Near-field reflection backscattering apertureless optical microscopy: application to spectroscopy experiments on opaque samples, comparison between lock-in and digital photon counting detection techniques

An apertureless scanning near-field optical microscope (ASNOM) in reflection backscattering configuration is designed to conduct spectroscopic experiments on opaque samples constituted of latex beads. The ASNOM proposed takes advantage of the depth-discrimination properties of confocal microscopes to efficiently extract the near-field optical signal. Given their importance in a spectroscopic experiment, we systematically compare the lock-in and synchronous photon counting detection methods. Some results of Rayleigh's scattering in the near field of the test samples are used to illustrate the possibilities of this technique for reflection backscattering spectroscopy.     

High-sensitivity detection of proteins using gel electrophoresis and atomic force microscopy

We have developed a method to detect specific proteins with a high sensitivity using a gel electrophoresis method and force measurement of atomic force microscopy (AFM). Biotinylated proteins were separated by electrophoresis and fixed with cross-linking chemicals on the gel, followed by direct force measurement between the biotinylated proteins on the gel and a streptavidin-modified tip of an AFM cantilever. We were able to achieve a high enough sensitivity to detect the picogram order of the biotinylated proteins by evaluating the frequency of the interaction force larger than 100 pN in the force profile, which corresponds to the rupture force of interaction between streptavidin and biotin.     

Primary considerations for image enhancement in high-pressure scanning electron microscopy

The mechanisms of electron beam scattering are examined to evaluate its effect on contrast and resolution in high-pressure scanning electron microscopy (SEM) techniques reported in the literature, such as moist-environment ambient-temperature SEM (MEATSEM) or environmental SEM (ESEM). The elastic and inelastic scattering cross-sections for nitrogen are calculated in the energy range 5–25 keV. The results for nitrogen are verified by measuring the ionization efficiency, and measurements are also made for water vapour. The effect of the scattered beam on the image contrast was assessed and checked experimentally for a step contrast function at 20 kV beam voltage. A considerable degree of beam scattering can be tolerated in high-pressure SEM operation without a significant degradation in resolution. The image formation and detection techniques in high-pressure SEM are considered in detail in the accompanying paper.     

Three-dimensional imaging in double aberration-corrected scanning confocal electron microscopy, part I: elastic scattering

A transmission electron microscope fitted with both pre-specimen and post-specimen spherical aberration correctors enables the possibility of aberration-corrected scanning confocal electron microscopy. Imaging modes available in this configuration can make use of either elastically or inelastically scattered electrons. In this paper we consider image contrast for elastically scattered electrons. It is shown that there is no linear phase contrast in the confocal condition, leading to very low contrast for a single atom. Multislice simulations of a thicker crystalline sample show that sample vertical location and thickness can be accurately determined. However, buried impurity layers do not give strong, nor readily interpretable contrast. The accompanying paper examines the detection of inelastically scattered electrons in the confocal geometry.     

Atomic force microscopy characterization of the chemical contrast of nanoscale patterns fabricated by electron beam lithography on polyethylene glycol oxide thin films

The present paper shows that atomic force microscopy (AFM) imaging of friction force and phase lag in ambient air can be used to characterize the chemical contrast induced by electron beam (EB) irradiation on polyethylene glycol oxide (PEO) surface. Time-of-flight secondary emission mass spectroscopy measurements showed that the EB irradiation generates chemical contrast on PEO surface by decreasing the ether bond density. The AFM measurements showed smaller phase lag and lower friction and adhesive forces on the EB irradiated PEO surface, as compared to the non-irradiated PEO surface. While the chemical contrast in friction force had a linear dependence on the EB irradiation dose, the dependence of the chemical contrast in the phase lag was strongly non-linear. As the friction and adhesive forces depended on the AFM probe hydrophilicity and air humidity, the contrast in friction and adhesive forces is ascribed to different capillary condensation of ambient water vapour at the AFM tip contact with the EB irradiated and non-irradiated PEO surfaces, respectively.     

Imaging of radiation-sensitive samples in transmission electron microscopes equipped with Zernike phase plates

We have optimized a bright-field transmission electron microscope for imaging of high-resolution radiation-sensitive materials by calculating the imaging dose n(0) needed to obtain a signal-to-noise ratio (SNR)=5. Installing a Zernike phase plate (ZP) decreases the dose needed to detect single atoms by as much as a factor of two at 300 kV. For imaging larger objects, such as Gaussian objects with full-width at half-maximum larger than 0.15 nm, ZP appears more efficient in reducing the imaging dose than correcting for spherical aberration. The imaging dose n(0) does not decrease with extending of chromatic resolution limit by reducing chromatic aberration, using high accelerating potential (U(0)=300 kV), because the image contrast increases slower than the reciprocal of detection radius. However, reducing chromatic aberration would allow accelerating potential to be reduced leading to imaging doses below 10 e(-)/A(2) for a single iodine atom when a CS-corrector and a ZP are used together. Our simulations indicate that, in addition to microscope hardware, optimization is heavily dependent on the nature of the specimen under investigation.     

Depth sectioning in scanning transmission electron microscopy based on core-loss spectroscopy

Recent and ongoing improvements in aberration correction have opened up the possibility of depth sectioning samples using the scanning transmission electron microscope in a fashion similar to the confocal scanning optical microscope. We explore questions of principle relating to image interpretability in the depth sectioning of samples using electron energy loss spectroscopy. We show that provided electron microscope probes are sufficiently fine and detector collection semi-angles are sufficiently large we can expect to locate dopant atoms inside a crystal. Furthermore, unlike high angle annular dark field imaging, electron energy loss spectroscopy can resolve dopants of smaller atomic mass than the supporting crystalline matrix.     

Three-dimensional imaging in double aberration-corrected scanning confocal electron microscopy, part II: inelastic scattering

The implementation of spherical aberration-corrected pre- and post-specimen lenses in the same instrument has facilitated the creation of sub-Angstrom electron probes and has made aberration-corrected scanning confocal electron microscopy (SCEM) possible. Further to the discussion of elastic SCEM imaging in our previous paper, we show that by performing a 3D raster scan through a crystalline sample using inelastic SCEM imaging it will be possible to determine the location of isolated impurity atoms embedded within a bulk matrix. In particular, the use of electron energy loss spectroscopy based on inner-shell ionization to uniquely identify these atoms is explored. Comparisons with scanning transmission electron microscopy (STEM) are made showing that SCEM will improve both the lateral and depth resolution relative to STEM. In particular, the expected poor resolution of STEM depth sectioning for extended objects is overcome in the SCEM geometry.     

Field enhancement factor and field emission from a hemi-ellipsoidal metallic needle

We present an exact solution for the electrostatic field between a metallic hemi-ellipsoidal needle on a plate (as a cathode) and a flat anode. The basic idea is to replace the cathode by a linearly charged thread in a uniform electric field and to use a set of “image” charges to reproduce the anode. We calculate the field enhancement factor on the needle surface and ponderomotive force acting on the needle. Using the Fowler–Nordheim theory we obtain an exact analytical formula for the total current.     

New method for characterizing paper coating structures using argon ion beam milling and field emission scanning electron microscopy

We have developed a new method for characterizing microstructures of paper coating using argon ion beam milling technique and field emission scanning electron microscopy. The combination of these two techniques produces extremely high-quality images with very few artefacts, which are particularly suited for quantitative analyses of coating structures. A new evaluation method has been developed by using marker-controlled watershed segmentation technique of the secondary electron images. The high-quality secondary electron images with well-defined pores makes it possible to use this semi-automatic segmentation method. One advantage of using secondary electron images instead of backscattered electron images is being able to avoid possible overestimation of the porosity because of the signal depth. A comparison was made between the new method and the conventional method using greyscale histogram thresholding of backscattered electron images. The results showed that the conventional method overestimated the pore area by 20% and detected around 5% more pores than the new method. As examples of the application of the new method, we have investigated the distributions of coating binders, and the relationship between local coating porosity and base sheet structures. The technique revealed, for the first time with direct evidence, the long-suspected coating non-uniformity, i.e. binder migration, and the correlation between coating porosity versus base sheet mass density, in a straightforward way.     

Parameters and mechanisms governing image contrast in scanning electron microscopy of single-walled carbon nanotubes

Image formation of single-walled carbon nanotubes (SWNTs) in the scanning electron microscope (SEM) is peculiarly sensitive to primary electron landing energy, imaging history, sample/substrate geometry, electrical conductivity, sample contamination, and substrate charging. This sensitivity is probably due to the extremely small interaction volume of the SWNTs' monolayered, nanoscale structures with the electron beam. Traditional electron beam/bulk specimen interaction models appear unable to explain the contrast behavior when directly applied to SWNTs. We present one systematic case study of SWNT SEM imaging with special attention to the above parameters and propose some physical explanations for the effect of each. We also demonstrate that it is possible to employ voltage biasing to counteract this extrinsic behavior, gain better control of the image contrast, and facilitate the interpretation of SWNT images in the SEM.     

Topographic and material contrast in low-voltage scanning electron microscopy

Two scintillation backscattered electron (BSE) detectors with a high voltage applied to scintillators were built and tested in a field emission scanning electron microscope (SEM) at low primary beam energies. One detector collects BSE emitted at low take-off angles, the second at high takeoff angles. The low take-off detector gives good topographic tilt contrast, stronger than in the case of the secondary electron (SE) detection and less sensitive to the presence of contamination layers on the surface. The high take-off detector is less sensitive to the topography and can be used for detection of material contrast, but the contrast becomes equivocal at the beam energy of 1 keV or lower.     

Advances in TEM orientation microscopy by combination of dark-field conical scanning and improved image matching

A new approach to automatic TEM‐based orientation microscopy is presented, which is based on a combination of the techniques of dark-field conical scanning and improved image matching, and a diffraction pattern simulation method. For indexing, a full experimental diffraction pattern is compared to all possible pre-calculated diffraction patterns for the given structure by image matching. In order to speed up this relatively calculation-intensive algorithm, polar transformation and, most important, circular projection that increase the speed of pattern indexing by a factor of about 50 are proposed. A microstructure of submicron scale and crystallographic orientations in nanocrystalline materials are measured successfully. It is proposed that the taken approach of dark-field conical scanning and improved image matching may be, in principle, better suited for TEM-based orientation microscopy than serial orientation mapping.     

Freeze-fracturing for conventional and field emission low-temperature scanning electron microscopy: the scanning cryo unit SCU 020

A dedicated cryopreparation system, the SCU 020 (Balzers), is introduced and described in detail for use in low-temperature scanning electron microscopy (LTSEM). The basic unit consists of two parts: (i) a high-vacuum preparation chamber equipped with a cold-stage, motor-driven fracturing microtome, planar magnetron (PM) sputter source, quartz-crystal thin-film monitor, Meissner cold trap, and turbo molecular pump stand; and (ii) a second part (separated from the first by a sliding, high-vacuum valve) residing in the SEM chamber. This is equipped with an anti-contamination cold trap, a fully movable goniometer cold stage (having motor drives for x, y, and rotation) and replaces the SEM's original stage (Raith). The SCU 020 is entirely self contained allowing independence from, and synchroneity with, the SEM of choice. LTSEM micrographs of specimen (that are fully frozen hydrated or partially freeze-dried) surfaces or fracture faces, without or with various metal coatings, can be examined over a broad temperature range (-150 to +50°C). This is made possible by the combined application of the two, independently controlled, cold stages and the on-line, high-vacuum, specimen cryo transfer between them. In-situ etching is simple and straightforward. Intramembranous particles and membrane fracture steps, typically imaged in transmission electron microscopy (TEM), are resolved by PM sputtering with platinum at low specimen temperature and high-resolution LTSEM in a field emission microscope.