Important aspects of investigating optical excitations in semiconductors using a scanning transmission electron microscope

Since semiconductor structures are becoming smaller and smaller, the examination methods must also take this development into account. Optical methods have long reached their limits here, but small dimensions are also a challenge for electron beam techniques, especially when it comes to determining optical properties. In this paper, electron microscopic methods of investigating optical properties are discussed. Special attention is given to the physical limits and how to deal with them. We will cover electron energy loss spectrometry as well as cathodoluminescence spectrometry. We pay special attention to inelastic delocalisation, radiation damage, the Čerenkov effect, interference effects of optical excitations and higher diffraction orders on a grating analyser for the cathodoluminescence signal.     

Comparison of different models for the generation of electron backscattering patterns in the scanning electron microscope

An electron backscattering pattern (EBSP) is formed on a fluorescent (or other) screen from the faster scattered electrons when a single-crystal region of a solid sample is illuminated by a finely focused electron beam (EB). The EBSP is very similar in appearance to the electron channeling pattern (ECP) that is obtained in the scanning electron microscope (SEM) by rocking the beam about a point on the surface of a single crystal. It has been suggested that the mechanisms that give rise to EBSP and ECP are related by reciprocity. If this is indeed the case, then the models that are used to explain them should be the same except for the direction in which the electrons travel through the specimen. The two-event “diffraction model” for EBSP (diffuse scattering followed by diffraction) fails this condition, leading to the conclusion that the “channeling in and channeling out” model for EBSP is more likely to be correct. This has been described rigorously by Reimer (1979, 1985). It is named after the title used by Joy (1994). An attempt is made here to describe this model in a simple way.     

Diffraction patterns of artificial two-dimensional crystals synthesized in situ in an environmental scanning transmission electron microscope

In this study, we demonstrated the use of electron‐beam‐induced deposition for synthesis of artificial two‐dimensional crystals with an in situ scanning transmission electron microscope. The structures were deposited from W(CO)₆ in an environmental scanning transmission electron microscope on a 30‐nm‐thick Si₃N₄ substrate. We present clear electron beam diffraction patterns taken from those structures. The distance between the diffraction peaks corresponded to the dot spacing in the self‐made surface crystal. We propose using these arrays of dots as anchor points for making artificial crystals for diffraction analysis of weakly scattering or beam‐sensitive molecules such as proteins.     

Integration of a high‐NA light microscope in a scanning electron microscope

We present an integrated light‐electron microscope in which an inverted high‐NA objective lens is positioned inside a scanning electron microscope (SEM). The SEM objective lens and the light objective lens have a common axis and focal plane, allowing high‐resolution optical microscopy and scanning electron microscopy on the same area of a sample simultaneously. Components for light illumination and detection can be mounted outside the vacuum, enabling flexibility in the construction of the light microscope. The light objective lens can be positioned underneath the SEM objective lens during operation for sub‐10 μm alignment of the fields of view of the light and electron microscopes. We demonstrate in situ epifluorescence microscopy in the SEM with a numerical aperture of 1.4 using vacuum‐compatible immersion oil. For a 40‐nm‐diameter fluorescent polymer nanoparticle, an intensity profile with a FWHM of 380 nm is measured whereas the SEM performance is uncompromised. The integrated instrument may offer new possibilities for correlative light and electron microscopy in the life sciences as well as in physics and chemistry.     

Reduction of charging effects using vector scanning in the scanning electron microscope.

We describe a vector scanning system to reduce charging effects during scanning electron microscope (SEM) imaging. The vector scan technique exploits the intrinsic charge decay mechanism of the specimen to improve imaging conditions. We compare SEM images obtained by conventional raster scanning versus vector scanning to demonstrate that vector scanning successfully reduces specimen-charging artifacts.     

Resolution in low voltage scanning electron microscopy

As the energy of an electron beam is reduced, the range falls and the secondary electron yield rises. A low voltage scanning electron microscope can therefore, in principle, examine without damage or charging samples such as insulators, dielectrics or beam sensitive materials. This paper investigates the way in which the choice of beam energy affects the spatial resolution of a secondary electron image. It is shown that for samples which are thin compared to the electron range, the edge resolution and contrast in the image improve with increasing beam energy. In samples that are thicker than the electron range, the resolution can be optimized at either high or low energies, but low energy operation will produce images of higher contrast. At an energy of 2 keV or less beam interaction limited resolutions of the order of 3 nm should be possible.     

A novel method for acquiring large-scale automated scanning electron microscope data

Recent software and hardware advances in the field of electron backscatter diffraction have led to an increase in the rate of data acquisition. Combining automated stage movements with conventional beam control have allowed researchers to collect data from significantly larger areas of samples than was previously possible. This paper describes a LabVIEW? and AutoIT^© code which allows for increased flexibility compared to commercially available software. The source code for this software has been made available in the online version of this paper.     

Quantitative analysis of backscattered-electron contrast in scanning electron microscopy

Backscattered-electron scanning electron microscopy (BSE-SEM) imaging is a valuable technique for materials characterisation because it provides information about the homogeneity of the material in the analysed specimen and is therefore an important technique in modern electron microscopy. However, the information contained in BSE-SEM images is up to now rarely quantitatively evaluated. The main challenge of quantitative BSE-SEM imaging is to relate the measured BSE intensity to the backscattering coefficient η and the (average) atomic number Z to derive chemical information from the BSE-SEM image. We propose a quantitative BSE-SEM method, which is based on the comparison of Monte–Carlo (MC) simulated and measured BSE intensities acquired from wedge-shaped electron-transparent specimens with known thickness profile. The new method also includes measures to improve and validate the agreement of the MC simulations with experimental data. Two different challenging samples (ZnS/Zn(O_xS_1–x)/ZnO/Si-multilayer and PTB7/PC₇₁BM-multilayer systems) are quantitatively analysed, which demonstrates the validity of the proposed method and emphasises the importance of realistic MC simulations for quantitative BSE-SEM analysis. Moreover, MC simulations can be used to optimise the imaging parameters (electron energy, detection-angle range) in advance to avoid tedious experimental trial and error optimisation. Under optimised imaging conditions pre-determined by MC simulations, the BSE-SEM technique is capable of distinguishing materials with small composition differences.     

Software acceleration techniques for the simulation of scanning electron microscope images

A scanning electron microscope (SEM) simulator was developed based on the models used in the MONSEL software. This simulator extends earlier work by introducing an object-oriented framework and adding optimization methods based on precomputation of electron trajectories. Several optimizations enable speedup by factors of 5-100 on a single processor over unoptimized simulations without introducing additional approximations. The speedup for a particular surface depends on the self-similarity of the surface at the scale of the electron penetration depth. We further accelerate by parallelizing the calculations for a total speedup of about 100-2000 on 30 processors. The goal of this work was to create a system capable of simulating a quantitatively accurate SEM image of a relatively unconstrained surface. Results of this work include simulation software, optimization algorithms, performance measurements with various optimizations, and examples of simulated images.     

Objective comparison of scanning ion and scanning electron microscope images

Common and different aspects of scanning electron microscope (SEM) and scanning ion microscope (SIM) images are discussed from a viewpoint of interaction between ion or electron beams and specimens. The SIM images [mostly using 30 keV Ga focused ion beam (FIB)] are sensitive to the sample surface as well as to low-voltage SEM images. Reasons for the SIM images as follows: (1) no backscattered-electron excitation; (2) low yields of backscattered ions; and (3) short ion ranges of 20–40nm, being of the same order of escape depth of secondary electrons (SE) [=(3–5) times the SE mean free path]. Beam charging, channeling, contamination, and surface sputtering are also commented upon.     

Conditions for imaging emulsions in the environmental scanning electron microscope

Environmental scanning electron microscopy (ESEM) is a technique capable of imaging volatile and/or insulating samples in their natural state, without prior specimen preparation. It is thus a powerful potential tool for the study of the structure and dynamics of emulsions and other complex liquid systems, at a resolution greater than that obtainable by conventional optical microscopy. We present images of a variety of liquid systems containing micron-scale and smaller features. The morphology of these systems may be clearly discerned. The contrast observed between the liquid phases was consistent with the model proposed by Stokes et al. (1998). The limits of resolution were determined by sample motion and by beam damage effects; under optimum conditions, resolution of a few tens of nanometers was obtained. This compares favourably with conventional and confocal optical microscopy. In some samples, thin films (solid or liquid) could be observed at the liquid/gas interface. Some of these films were so thin that they did not completely obscure the underlying structure of the bulk sample.     

Nanotip electron gun for the scanning electron microscope

Experimental nanotips have shown significant improvement in the resolution performance of a cold field emission scanning electron microscope (SEM). Nanotip electron sources are very sharp electron emitter tips used as a replacement for the conventional tungsten field emission (FE) electron sources. Nanotips offer higher brightness and smaller electron source size. An electron microscope equipped with a nanotip electron gun can provide images with higher spatial resolution and with better signal-to-noise ratio. This could present a considerable advantage over the current SEM electron gun technology if the tips are sufficiently long-lasting and stable for practical use. In this study, an older field-emission critical dimension (CD) SEM was used as an experimental test platform. Substitution of tungsten nanotips for the regular cathodes required modification of the electron gun circuitry and preparation of nanotips that properly fit the electron gun assembly. In addition, this work contains the results of the modeling and theoretical calculation of the electron gun performance for regular and nanotips, the preparation of the SEM including the design and assembly of a measuring system for essential instrument parameters, design and modification of the electron gun control electronics, development of a procedure for tip exchange, and tests of regular emitter, sharp emitter and nanotips. Nanotip fabrication and characterization procedures were also developed. Using a "sharp" tip as an intermediate to the nanotip clearly demonstrated an improvement in the performance of the test SEM. This and the results of the theoretical assessment gave support for the installation of the nanotips as the next step and pointed to potentially even better performance. Images taken with experimental nanotips showed a minimum two-fold improvement in resolution performance than the specification of the test SEM. The stability of the nanotip electron gun was excellent; the tip stayed useful for high-resolution imaging for several hours during many days of tests. The tip lifetime was found to be several months in light use. This paper summarizes the current state of the work and points to future possibilities that will open when electron guns can be designed to take full advantage of the nanotip electron emitters.     

Estimation of mass thickness response of embedded aggregated silica nanospheres from high angle annular dark‐field scanning transmission electron micrographs

In this study, we investigate the functional behaviour of the intensity in high‐angle annular dark field scanning transmission electron micrograph images. The model material is a silica particle (20 nm) gel at 5 wt%. By assuming that the intensity response is monotonically increasing with increasing mass thickness of silica, an estimate of the functional form is calculated using a maximum likelihood approach. We conclude that a linear functional form of the intensity provides a fair estimate but that a power function is significantly better for estimating the amount of silica in the z‐direction. The work adds to the development of quantifying material properties from electron micrographs, especially in the field of tomography methods and three‐dimensional quantitative structural characterization from a scanning transmission electron micrograph. It also provides means for direct three‐dimensional quantitative structural characterization from a scanning transmission electron micrograph.     

Transmission EBSD from 10 nm domains in a scanning electron microscope

The spatial resolution of electron diffraction within the scanning electron microscope (SEM) has progressed from channelling methods capable of measuring crystallographic characteristics from 10 μm regions to electron backscatter diffraction (EBSD) methods capable of measuring 120 nm particles. Here, we report a new form of low‐energy transmission Kikuchi diffraction, performed in the SEM. Transmission‐EBSD (t‐EBSD) makes use of an EBSD detector and software to capture and analyse the angular intensity variation in large‐angle forward scattering of electrons in transmission, without postspecimen coils. We collected t‐EBSD patterns from Fe–Co nanoparticles of diameter 10 nm and from 40 nm‐thick Ni films with in‐plane grain size 15 nm. The patterns exhibited contrast similar to that seen in EBSD, but are formed in transmission. Monte Carlo scattering simulations showed that in addition to the order of magnitude improvement in spatial resolution from isolated particles, the energy width of the scattered electrons in t‐EBSD is nearly two orders of magnitude narrower than that of conventional EBSD. This new low‐energy transmission diffraction approach builds upon recent progress in achieving unprecedented levels of imaging resolution for materials characterization in the SEM by adding high‐spatial‐resolution analytical capabilities.     

Contrast in high-resolution scanning electron microscope images

Current scanning electron microscopes, equipped with field emission guns and high-performance immersion lenses, can achieve spatial resolutions of the order of 1 nm in both secondary and backscattered imaging modes over a wide range of operating energies. The generation and interpretation of images with nanometre-scale resolution relies on a detailed knowledge, and application, of electron-solid interactions. This paper develops the practical steps required to produce a high-resolution image, and discusses the principles which govern image interpretation. Attention is focused primarily on materials which are low in atomic number and density, such as biological tissue, but the results apply after appropriate scaling of the physical parameters to most other materials.     

Imaging of semiconducting polymer blend systems using environmental scanning electron microscopy and environmental scanning transmission electron microscopy

This study has investigated the potential of environmental electron microscopy techniques for studying the structure of polymer‐based electronic devices. Polymer blend systems composed of F8BT and PFB were examined. Excellent contrast, both topographical and compositional, can be achieved using both conventional environmental scanning electron microscopy (ESEM) and a transmission detector giving an environmental scanning transmission electron microscope (ESTEM) configuration. Controllable charging effects present in the ESEM were observed, giving rise to a novel voltage contrast. This shows the potential of such contrast to provide excellent images of phase structure and charge distributions.     

Characterization of an oil-degrading Microcoleus consortium by means of confocal scanning microscopy, scanning electron microscopy and transmission electron microscopy

A consortium of microorganisms with the capacity to degrade crude oil has been characterized by means of confocal laser scanning microscopy (CLSM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The analysis using CLSM shows that Microcoleus chthonoplastes is the dominant organism in the consortium. This cyanobacterium forms long filaments that group together in bundles inside a mucopolysaccharide sheath. Scanning electron microscopy and transmission electron microscopy have allowed us to demonstrate that this cyanobacterium forms a consortium primarily with three morphotypes of the heterotrophic microorganisms found in the Microcoleus chthonoplastes sheath. The optimal growth of Microcoleus consortium was obtained in presence of light and crude oil, and under anaerobic conditions. When grown in agar plate, only one type of colony (green and filamentous) was observed.     

Contrast in the transmission mode of a low-voltage scanning electron microscope

The contrast thicknesses (x_k) of thin carbon and platinum films have been measured in the transmission mode of a low-voltage scanning electron microscope for apertures of 40 and 100 mrad and electron energies (E) between 1 and 30 keV. The measured values overlap with those previously measured for E (≥ 17keV) in a transmission electron microscope. Differences in the decrease of x_k with decreasing E between carbon and platinum agree with Wentzel-Kramer-Brillouin calculations of the elastic cross-sections. Knowing the value of x_k allows the exponential decrease ∝ exp(—x/x_k) in transmission with increasing mass-thickness (x = ρt) of the specimen and the increasing gain of contrast for stained biological sections with decreasing electron energy to be calculated for brightfield and darkfield modes.