Optimisation of the environmental scanning electron microscope for observation of drying of matt water-based lacquers

Environmental scanning electron microscopy (ESEM) modifies conventional SEM through the use of a partial gas pressure in the microscope specimen chamber. Like conventional SEM, it has the resolution to image structure on the submicron lengthscale, but can also tolerate hydrated specimens if water vapour is used in the specimen chamber. This ability to image aqueous specimens leaves ESEM uniquely placed to study in situ drying in polymer latexes. However, there are two key practical difficulties associated with in situ drying. First, the size of the latex particles: larger latex particles are typically around 500 nm in diameter. Although ESEM can resolve structure on this lengthscale without difficulty, the magnification required results in radiation damage of the specimen due to the electron beam. This means that a given region can be imaged only once during film formation, so the evolution of particular features cannot be followed. Second, the change from ambient temperature and pressure to the ESEM conditions of 7 degrees C and 7.5 torr (100 Pa) can subject the specimen to a very high evaporation rate, which can disrupt film formation. The inclusion of a drop of water in the specimen chamber is shown largely to alleviate this, enabling successful imaging of film formation in the lacquer. Instead of the polymer latex itself, this work concentrates on a matting lacquer with silica inclusions. The silica matting agent particles are 1-10 microm in size, allowing for a lower magnification to be used, massively reducing specimen damage. Furthermore, the contrast during drying is much enhanced in the presence of silica. The images reveal the silica as bright regions against a darker background of polymer and water. Film formation shows the transition from a uniform, featureless aqueous solution to a polymer film with silica particles present on the surface. The appearance of individual silica particles can be followed. The particles are generally revealed quite early, after a few minutes of drying time. As film formation progresses, these same particles appear larger and more distinct. Few new particles are revealed at longer film formation times.     

Reproducibility and accuracy of spatial measurements from digitally captured images in the environmental scanning electron microscope

Accurate spatial measurements in a scanning electron microscope (SEM) require calibration of the magnification as a function of working distance and microscope operating conditions. This work presents the results of the calibration of an environmental SEM for the accurate spatial measurement of dimensions and areas in experiments, both for the measurement of strain in steel specimens under applied loads and the measurement of dimensional changes in timber with changes in relative humidity.     

A new method for low-magnification in the environmental scanning electron microscope

A device has been developed and used successfully on two models of the environmental scanning electron microscope that allows low-magnification imaging of about 30x, significantly better than the original 200x low-magnification imaging limit. This was achieved by using an additional aperture to limit the pressure at a point where it will not block the electron beam, and a larger aperture plate for the combination final aperture/secondary electron signal collection surface that also does not block the electron beam significantly.     

A case of Leukonychia with scanning electron microscope observation

Leukonychia is a medical term for white discoloration appearing on nails. The pathophysiologic mechanisms that cause white discoloration are not entirely clear. We processed a case of leukonychia with scanning electron microscope observation and found many crispy, obviously dissociated "layers" in the lower part of the white nail plate. The dissociated "layers" were composed of thick, loose, coarse keratin bundles intertwined with each other. We believe the dissociated "layers" are related to the clinically noted white discoloration appearing on the nails.     

The role of induced contrast in images obtained using the environmental scanning electron microscope

Generation of contrast in images obtained using the environmental scanning electron microscope (ESEM) is explained by interpretation of images acquired using the gaseous secondary electron detector (GSED), ion current, and the Everhart-Thornley detector. We present a previously unreported contrast component in GSED and ion current images attributed to signal induction by changes in the concentration of positive ions in the ESEM chamber during image acquisition. Changes in positive ion concentration are caused by changes in electron emission from the sample during image acquisition and by a discrepancy between the drift velocities of negative and positive charge carriers in the imaging gas. The proposed signal generation mechanism is used to explain contrast reversal in images produced using the GSED and ion current signals and accounts for discrepancies in contrast observed, under some conditions, in these types of images. Combined with existing models of signal generation in the ESEM, the proposed model provides a basis for correct interpretation of ESEM images.     

Applications of the environmental scanning electron microscope to the analysis of pharmaceutical formulations

Electron microscopy has been used for several years as a routine tool for the study of pharmaceutical formulations. However, it is usually desirable to obtain information on these systems in the wet state, and there are concerns regarding the interpretation of information provided by conventional electron microscopy where samples are subjected to preparation techniques which may include freezing, drying, fracturing, and coating. The environmental scanning electron microscope (ESEM) has been used to analyse a number of pharmaceutical samples in their natural state. Results obtained from these samples, including biodegradable matrices, microparticulate systems (both degradable and non-biodegradable), and bioadhesive matrices, will be discussed and the merits and limitations of the ESEM will be highlighted.     

A new correction method for high-resolution energy-dispersive x-ray analyses in the environmental scanning electron microscope

Spurious x-ray signals, which previously prevented high-resolution energy-dispersive x-ray analysis (EDS) in the environmental scanning electron microscope (ESEM), can be corrected using a simple method presented here. As the primary electron beam travels through the gas in the ESEM chamber, a significant fraction of the primary electrons is scattered during collisions with gas molecules. These scattered electrons form a broad skirt that surrounds the primary electron beam as it impacts the sample. The correction method assumes that changes in the width of the electron skirt with pressure are less important than changes in the skirt intensity; this method works as follows: The influence of the gas on the overall x-ray data is determined by acquiring EDS spectra at two pressures. Subtracting the two spectra provides us with a difference spectrum which is then used to correct the original data, using extrapolation, back to the x-ray spectrum expected under high-vacuum conditions. Low-noise data are required to resolve small spectral peaks; however, the principle should apply equally to x-ray maps and even to low-magnification images.     

The effects of space charge on contrast in images obtained using the environmental scanning electron microscope

We present experimental evidence for the existence of a space charge in the environmental scanning electron microscope. Space charge formation is attributed to differences in the mobilities of negative and positive charge carriers in the imaging gas. A model is proposed for the behavior of space charge during image acquisition. The effects of space charge on images acquired using the gaseous secondary electron detector, ion current, and backscattered electron signals are interpreted using the proposed model.     

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.     

Experimental data and model simulations of beam spread in the environmental scanning electron microscope

This work describes the comparison of experimental measurements of electron beam spread in the environmental scanning electron microscope with model predictions. Beam spreading is the result of primary electrons being scattered out of the focused beam by interaction with gas molecules in the low-vacuum specimen chamber. The scattered electrons form a skirt of electrons around the central probe. The intensity of the skirt depends on gas pressure in the chamber, beam-gas path length, beam energy, and gas composition. A model has been independently developed that, under a given set of conditions, predicts the radial intensity distribution of the scattered electrons. Experimental measurements of the intensity of the beam skirt were made under controlled conditions for comparison with model predictions of beam skirting. The model predicts the trends observed in the experimentally determined scattering intensities; however, there does appear to be a systematic deviation from the experimental measurements.     

Application of an environmental scanning electron microscope to micromechanical fabrication

Current advanced methods of micromechanical fabrication require expensive tooling or are restricted to the fabrication of lateral-shaped microstructures. To overcome these limitations, recent efforts have usedmicroscale additive freeform fabrication (AFF) methods to prototype micromechanical structures. However, these laser-based methods are limited in resolution. To improve the resolution of microscale AFF methods, an environmental scanning electron microscope (ESEM) was used to prototype several electron-beam (EB)-based microscale AFF processes. The results showed that the ESEM is capable of demonstrating process feasibility for EB-based microscale AFF.     

Manipulating biological samples for environmental scanning electron microscopy observation

Biological samples having different characteristics were observed by environmental scanning electron microscopy (ESEM). The environmental conditions for untreated biological samples was determined by optimizing sample temperature and chamber pressure. When the temperature was at 4 degrees - 6 degrees C and chamber pressure was 5.2-5.9 Torr, the relative humidity in the specimen chamber was about 85%. Under these conditions, the surface features of the sample were completely exposed and did not exhibit charging. The images obtained from the untreated samples at different ESEM conditions were also compared with fixed and coated samples observed under high vacuum.     

Direct measurement of electron beam scattering in the environmental scanning electron microscope using phosphor imaging plates

Phosphor imaging plate technology has made it possible to directly image the distribution of primary beam electrons and scattered electrons in the environmental scanning electron microscope. The phosphor plate is exposed under electron scattering conditions in the microscope chamber. When processed, the electron intensity distribution is displayed as a digital image. The image is a visual representation of the electron probe and skirt and may provide the basis for a more accurate model.     

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.     

Contact angle analysis on polymethylmethacrylate and commercial wax by using an environmental scanning electron microscope

The environmental scanning electron microscope (ESEM) represents one of the most exciting breakthroughs in electron microscopy since the invention of the electron microscope. Its ability to observe uncoated and hydrated samples enhances the possibility for investigating the wettability of surfaces at a microscopic level; by varying the relative vapour pressure or the temperature inside the chamber, it is possible to condense water drops on a micron scale. A large problem in measuring contact angles by ESEM is that the observation angle is not parallel or perpendicular to the surface; thus, the study of the droplets profile using the common algorithms such as spherical approximation or axisymmetric drop shape analysis (ADSA) is not possible, because only a spherical cap shape is commonly observed. In this paper we provide a useful mathematical model to calculate the real contact angle from the initial images. Initially, some simulated spherical caps with different contact and observation angles were created by an appropriate graphic package in order to test the mathematical model. Some real drops obtained by ESEM on wax and polymethylmethacrylate (PMMA) were then studied and the results compared with contact angles measured by common methods on the same materials.     

Mechanical scanning of the specimen in the scanning electron microscope

A scanning electron microscope (SEM) system equipped with a motor drive specimen stage fully controlled with a personal computer (PC) has been utilized for obtaining ultralow magnification SEM images. This modem motor drive stage works as a mechanical scanning device. To produce ultra-low magnification SEM images, we use a successful combination of the mechanical scanning, electronic scanning, and digital image processing techniques. This new method is extremely labor and time saving for ultra-low magnification and wide-area observation. The option of ultra-low magnification observation (while maintaining the original SEM functions and performance) is important during a scanning electron microscopy session.     

Tensile tests of polymers at low temperatures in the environmental scanning electron microscope: an improved cooling platform

The investigation of the fracture behavior of polymers in the environmental scanning electron microscope (ESEM) can provide information about the correlation between the microstructure of a specimen and the macroscopic stress-strain characteristic. As the mechanical properties of polymers change dramatically at the glass transition temperature, cooling of the specimens during the tensile tests can yield very valuable information about the influence of individual components of polymer blends on the fracture behavior of the material. A serious problem in this connection is the poor heat conductivity of polymers. A commercially available cooling platform, which can be mounted on the tensile stage used for the tests was substantially modified to both enhance the heat transfer between platform and specimen, and to minimize the temperature gradient along the specimen.The first experiments on modified polypropylene specimens already delivered some unexpected results. Fibril-like structures appeared at the crack tip that would not be expected at temperatures below the glass transition temperature of the polymer blend.     

Charging compensation of alumina samples by using an oxygen microinjector in the environmental scanning electron microscope

A gas microinjector system was set up in an environmental scanning electron microscope (ESEM) to create an oxygen atmosphere around the alumina samples for the charging compensation under a pressure between 2 x 10(-5) Pa approximately 2 x 10(-2) Pa. At low pressures, the skirt effect of the electron scattering can be degraded, which results in improvement of the imaging contrast and increase of the signal/noise ratio. The sample current (I(SC)) and the Duane-Hunt limit were measured to evaluate the charging effect.     

Use of backscattered electron detector arrays for forming backscattered electron images in the scanning electron microscope

The backscattered electron (BSE) signal in the scanning electron microscope (SEM) can be used in two different ways. The first is to give a BSE image from an area that is defined by the scanning of the electron beam (EB) over the surface of the specimen. The second is to use an array of small BSE detectors to give an electron backscattering pattern (EBSP) with crystallographic information from a single point. It is also possible to utilize the EBSP detector and computer-control system to give an image from an area on the specimen--for example, to show the orientations of the grains in a polycrystalline sample ("grain orientation imaging"). Some further possibilities based on some other ways for analyzing the output from an EBSP detector array, are described.     

Use of Monte Carlo modeling for interpreting scanning electron microscope linewidth measurements

A scanning electron microscope (SEM) can be used to measure the dimensions of the microlithographic features of integrated circuits. However, without a good model of the electron-beam/specimen interaction, accurate edge location cannot be obtained. A Monte Carlo code has been developed to model the interaction of an electron beam with one or two lines lithographically produced on a multilayer substrate. The purpose of the code is to enable one to extract the edge position of a line from SEM measurements. It is based on prior codes developed at the National Institute of Standards and Technology, but with a new formulation for the atomic scattering cross sections and the inclusion of a method to simulate edge roughness or rounding. The code is currently able to model the transmitted and backscattered electrons, and the results from the code have been applied to the analysis of electron transmission through gold lines on a thin silicon substrate, such as is used in an x-ray lithographic mask. Significant reductions in backscattering occur because of the proximity of a neighboring line.