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.     

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.     

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.     

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.     

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.     

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.     

Low-cost image-capture system for a scanning electron microscope

We describe a PC-based active-capture system for recording digital images from a scanning electron microscope. The system is based on a National Instruments data-acquisition board and a Pentium computer, controlled by software that we have written in Visual Basic.     

扫描电镜潘宁规的维修过程详解

详细分析FEI Nova NanoSEM 230扫描电镜的潘宁规的维修过程,为积累处理扫描电镜故障的经验提供便利,对类似型号的维修有一定的借鉴意义。     

X-ray microanalysis of wet biological specimens in the environmental scanning electron microscope. 1. Reduction of specimen distance under different atmospheric conditions

X-ray microanalysis of non-biological and biological specimens was carried out in the environmental scanning electron microscope (ESEM) under different conditions of specimen distance (the distance travelled by the electron probe within the specimen chamber) and chamber atmosphere. Using both water vapour and argon atmospheres, it was shown that reduction in specimen distance had no effect on atmospheric gas X-ray signal in either case. Unlike water vapour, increased levels of argon (up to 10 torr) caused a marked depression of specimen P/B ratios, with a decrease in both characteristic and background (continuum) counts. These effects in argon were not altered by reduction in specimen distance. Specimen distance was important in relation to beam skirting and elemental analysis. With an extended assembly (short specimen distance), beam skirting in a water-vapour atmosphere was much reduced – leading to enhanced element detectability in a discrete biological specimen (Anabaena cyclindrica).     

Signal components in the environmental scanning electron microscope

We demonstrate that the gas-amplified secondary electron signal obtained in the environmental scanning electron microscope has both desired and spurious components. In order to isolate the contributions of backscattered and secondary electrons, two sets of samples were examined. One sample consisted of a pair of materials having similar secondary emission coefficients but different backscatter coefficients, while the other sample had a pair with similar backscatter but different secondary emission coefficients. Our results show how the contribution of the two electron signals varies according to the pressure of the amplifying gas. Backscatter contributions, as well as background due to gas ionization from the primary beam, become significant at higher pressure. Furthermore, we demonstrate that the relative amplification efficiencies of various electron signals are dependent upon the chemistry of the gas.     

The scanning low-energy electron microscope: First attainment of diffraction contrast in the scanning electron microscope

Using small Pb crystals deposited in situ on a partially contaminated Si (100) crystal, we demonstrate that a commercial scanning electron microscope (SEM) can easily be converted into a scanning low-energy electron microscope (SLEEM). Although the contrast mechanism is much more complicated than that in nonscanning LEEM because not only one diffracted monochromatic beam and its close environment are used for imaging, but several diffracted beams and a wide energy spectrum of electrons of different origin (secondary electrons, inelastically andelastically scattered electrons) are used, SLEEM is a valuable addition to the standard SEM because it provides an additional structure- and orientation-sensitive contrast mechanism in crystalline materials, a low sampling depth, and high intensity at low energies.     

Some aspects of optimizing contrasts for the investigation of joint materials in the environmental scanning electron microscope

In the environmental scanning electron microscope, material joints of different atomic mass and different electrical conducting properties can easily be observed simultaneously without coating the specimen. For such heterogeneous materials, the quality of the image can be optimized with respect to contrast and resolution if the contrast types as well as their significance to the composition of the image are known.     

Better visualization inside the environmental scanning electron microscope through the infrared chamberscope coupled with a mirror

Clearances are tight inside the specimen chamber of the environmental scanning electron microscope (ESEM), and it is difficult to see the relative positions of detectors and specimens through the viewport. For example, the relative placement of the energy-dispersive spectrometer (EDS) and the specimen is critical for attaining reasonable x-ray efficiency while protecting the detector window from damage. An infrared chamberscope and mirror attachment were added to improve the visibility inside the chamber.     

Electron-gas interactions in the environmental scanning electron microscopes gaseous detector

The construction of high signal-to-noise, artefact-free secondary electron images in the elevated pressure conditions of an environmental SEM is a nontrivial process. The interactions of information carrying species, as well as probe beam electrons, with the chamber gas are the major reasons for such complications. In this paper, we discuss and review the present understanding of these phenomena. In addition, we outline procedures for assessing the signal-amplifying and charge-neutralising capabilities of an environmental gas. It is only with a knowledge of such parameters and an appreciation of the gas-electron collision processes that one can optimise the microscope's operating parameters. Moreover, such information enables the separation of topographic detail from artefactual features in the detected electron images.     

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.     

In situ laser heating in an environmental scanning electron microscope

The environmental scanning electron microscope (ESEM) offers improved capabilities for coupling a scanning electron microscope (SEM) with an in situ laser device compared with conventional SEMs. Such coupling generally enables, for example, the observation of laser glazing effects or high-temperature behaviour as well as thermal shock behaviour of materials and devices. In an ESEM, decomposition caused by high-temperature gas reactions can additionally be studied while monitoring the gaseous environment online with a mass spectrometer attached to the specimen chamber. In this work, we demonstrate the capabilities of an in situ laser system suitable for heating specimen in an in situ deformation stage, thus enabling the further study of the mechanical properties of materials far beyond temperatures accomplished by thermal heating stages.