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.     

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.     

Development of environmental scanning electron microscopy electron beam profile imaging with self-assembled monolayers and secondary ion mass spectroscopy

A method for demonstrating the scattering of the primary electron beam in the presence of a gas has been developed. A self-assembled decanethiol monolayer is damaged by primary beam electrons. The damaged portion of the mono-layer is exchanged with another thiol-containing molecule by immersion in solution. The resulting film is imaged using a secondary ion mass spectrometer. Three-dimensional reconstruction of the data yields a representation of scattered electrons in the gaseous environment of the environmental scanning electron microscope.     

The effect of some instrument operating conditions on the x-ray microanalysis of particles in the environmental scanning electron microscope

The objective of this investigation was to evaluate the practical effects of electron beam broadening in the environmental scanning electron microscope (ESEM) on particle x-ray microanalysis and to determine some of the optimum operating conditions for this type of analysis. Four sets of experiments were conducted using a Faraday cage and particles of copper, glass, cassiterite, andrutile. The accelerating voltage and chamber pressure varied from 20 to 10 kV and from 665–66 Pa (5.0 to 0.5 torr), respectively. The standard gaseous secondary electron detectors (GSED) and the long environmental secondary dectectors (ESD) for the ESEM were evaluated at different working distances. The effect of these parameters on the presence of artifact peaks was evaluated. The particles were mounted on carbon tape on an aluminum specimen mount and were analyzed individually and as a mixture. Substrate peaks were present in almost all of the spectra. The presence of neighboring particle peaks and the number of counts in these depended upon the operating conditions. In general, few of these peaks were observed with the long ESD detector at 19 mm working distance and at low chamber pressures. More peaks and counts were observed with a deviation from these conditions. The most neighboring peaks and counts were obtained with the GSED detector at 21.5 mm working distance, 10 kV accelerating voltage, and 665 Pa (5.0 torr) chamber pressure. The results of these experiments support the idea that the optimum instrumental operating conditions for EDS analysis in the ESEM occur by minimizing the gas path length and the chamber water vapor pressure, and by maximizing the accelerating voltage. The results suggest that the analyst can expect x-ray counts from the mounting materials. These tests strongly support the recommendation of the manufacturer to use the long ESD detector and a 19 mm working distance for EDS analysis. The results of these experiments indicate that neighboring particles millimeters from the target may contribute x-ray counts to the spectrum.     

Redesign of the scanning electron microscope for parallel energy spectral acquisition

This paper presents a scanning electron microscope (SEM) design that is compatible with parallel electron energy spectrum acquisition. The SEM should in principle be capable of capturing the energy spectrum of all scattered electrons simultaneously, from low energy secondary electrons to elastic backscattered electrons. Preliminary simulation results predict that the beam separator spectrometer will have a relatively high transmission-energy resolution performance, comparable or better than the cylindrical mirror analyzer (CMA), while at the same time being able to capture the entire energy range of scattered electrons.     

Electron optical property of a charged foil,observed with a transmitted electron detector device in an SEM

Experimental evidence is presented for the electron optical behaviour of a charged foil area, using the transmitted electron detection device of the scanning electron microscope JSM 50 A (JEOL). The primary electron beam scanning a thin pioloform foil on the one hand produces a charged foil region which on the other hand acts as an electron lens to the primary and scattered electrons. Scanning electron microscopical investigations of air particulates in the submicron size range can be eased by using a transmitted electron detection device both of the bright and dark field operation mode. The image contrast thus may be improved by orders of magnitude, also allowing on line operation of an image analysis system. Using a special preparation technique, depositing the particles on a thin supporting foil which is also used for LAMMA analysis – Wieser et al. 1980, the x-ray spectra of single particles provided by an energy dispersive x-ray spectrometer may be quantitatively interpreted on the basis of the peak-to-background method (Statham and Pawley 1978, Small et al. 1979). Figure 1 shows a schematic of the transmission detector device of the JSM 50 A when operated in the dark field mode. Geometrical dimensions and apertures also are given in Fig. 1. The dark field diaphragm (DFD) on the optical axis of the microscope blocks all electrons (primary electrons and scattered electrons) within an angle of about 10^?2 rad from contributing to the video signal. As long as magnifications above about 350 × are used the primary electron beam hits the DFD thus yielding a transmission scanning electron micrograph in dark field mode. Below this limit or above the corresponding maximal scanning angle (about 7 × 10^?3 rad) of the primary electron beam the rim of the DFD becomes visible in the displayed image as shown in Fig. 2a. At the same magnification Figure 2b shows the sharpened contours of the DFD as obtained by focussing the primary electron beam to the plane of the DFD by lowering the objective lens excitation. By means of the thin bar attached to the DFD (left hand upper corner of Fig. 2b) the DFD may be centered to the optical axis or exchanged to the bright field aperture. Looking to the circular center of Fig. 2a, we recognize the black grid bars and a few black particles whereas the supporting foil looks bright. No video signal can be obtained, because both the grid bars, and to a lesser extent the particles, are not transparent to the primary electrons of 15 keV energy. On the other hand all electrons scattered by the thin foil to an angle of more than 10^?2 rad are seen by the scintillator and hence accumulate a measurable video signal: This is also favoured by the large solid angle outside the DFD, which is about 30 times the solid angle of the DFD itself.     

Initial experimental results on a magnetic beam separator spectrometer for the SEM

This article describes preliminary experiments to test the concept of scanning electron microscope (SEM) using a round magnetic beam separator to perform energy spectroscopy. Two experiments with an add-on attachment inside a conventional SEM were performed, one to estimate the sector image aberrations and the other to capture an energy spectrum of scattered electrons. The experiments show that the sector image aberrations lie well below 2 nm and that it is possible to capture the energy spectrum of secondary electrons.     

Quantitative electron probe microanalysis of rough targets: testing the peak-to-local background method

Rough samples with topography on a scale that is much greater than the micrometer dimensions of the electron interaction volume present an extreme challenge to quantitative electron beam x-ray microanalysis with energy-dispersive x-ray spectrometry. Conventional quantitative analysis procedures for flat, bulk specimens become subject to large systematic errors due to the action of geometric effects on electron scattering and the x-ray absorption path compared with the ideal flat sample. The best practical approach is to minimize geometric effects through specimen reorientation using a multiaxis sample stage to obtain the least compromised spectrum. When rough samples must be analyzed, corrections for geometric factors are possible by the peak-to-local background (P/B) method. Correction factors as a function of photon energy can be determined by the use of reference background spectra that are either measured locally or calculated from pure element spectra and estimated compositions. Significant improvements in accuracy can be achieved with the P/B method over conventional analysis with simple normalization.     

An experimental model of beam broadening in the variable pressure scanning electron microscope

In the variable pressure scanning electron microscope (VP-SEM) the incident electrons pass through a gaseous environment and the beam is scattered by these interactions. We show here that the experimental intensity profile of the scattered beam can be described as Gaussian in form to a high level of accuracy. This provides a rapid means of accounting for the effects of beam scatter in imaging and microanalysis because the standard deviation of the Gaussian is a simple function of parameters such as working distance, beam energy, gas type and pressure.     

Measures for spectral quality in low-voltage X-ray microanalysis

Characteristic x-ray production with energetic electrons depends strongly on the overvoltage, the ratio of the incident beam energy to the critical excitation energy for the atomic species of interest. Low-voltage x-ray microanalysis (beam energy < or = 5 keV) is especially susceptible to artifacts due to sample charging because the overvoltage is low and even slight charging can strongly affect peak intensities. The Duane-Hunt bremsstrahlung limit is a good diagnostic to detect sample charging. Dynamic charging effects, however, can influence spectra despite an apparently satisfactory Duane-Hunt limit. Dynamic charging effects must be examined by time series experiments, or through use of dynamic energy windows continuously measuring count rates placed across the spectrum. When charging is a problem, conductive surface coatings can eliminate the effects. When pristine surfaces must be examined without coating, the use of a conductive grid can control charging so that useful x-ray spectra can be obtained.     

A method to measure the effective gas path length in the environmental or variable pressure scanning electron microscope

A simple method is described to determine the effective gas path length when incident electrons scatter in the gas above the specimen. This method is based on the measurement of a characteristic x-ray line emitted from a region close to the incident beam. From various experimental measurements performed on various microscopes, it is shown that the effective gas path length may increase with the chamber pressure and that it is also often dependent of the type of x-ray bullet.     

The use of helium gas to reduce beam scattering in high vapour pressure scanning electron microscopy applications

Both image quality and the accuracy of x-ray analysis invariable pressure scanning electron microscopes (VPSEMs) are often limited by the spread of the primary electronbeam due to scattering by the introduced gas. The degree of electron scattering depends partly on the atomic number Z of the gas, and the use of a low Z gas such as helium should reduce beam scattering and enhance image quality. Using anuncoated test sample of copper iron sulphide inclusions in calcium fluorite, we show that the reduction in beam scatter produced by helium is more than sufficient to compensate for its reduced efficiency of charge neutralisation. The relative insensitivity to pressure of x-ray measurements in a helium atmosphere compared with air, and the consequent ability to work over a wider range of working distances, pressures, and voltages, make helium potentially the gas of choice for many routine VPSEM applications.     

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.     

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.     

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.     

Radiation damage of water in environmental scanning electron microscopy

Specimen damage from the electron beam poses a considerable problem with electron microscopy. This damage is particularly acute in environmental scanning electron microscopy (ESEM) for two reasons. Firstly, owing to its ability to stabilise insulating and hydrated specimens, ESEM lends itself to polymeric and biological materials that are typically highly beam-sensitive. Secondly, water acts as a source of small, highly mobile free radicals, which accelerate specimen degradation.
By taking the results of single-particle simulations of electron–water interactions, we determine the concentration of reactive species in a water specimen under ESEM conditions. We consider 12 species, which are produced in a Gaussian distribution, and annihilate according to a second-order reaction scheme. Self-diffusion along the concentration gradient is also modelled.
We find that the dominant reactive species is the hydroxyl (.OH) radical. Annihilation of this species is suppressed due to the lower concentration of reactants. The relatively stable hydrogen peroxide is also found at large concentrations. By comparing two beam energies, 5 and 25 keV, we find a drastic increase in the quantities of reactive species produced with beam energy. The longer range of 25 keV primary electrons spreads reactive species over a wider region, which then decay far more slowly.     

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.     

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.     

Imaging deep holes in structures with gaseous secondary electron detection in the environmental scanning electron microscope

The gaseous secondary electron detector (GSED) in the environmental scanning electron microscope (ESEM) permits collection of electron signals from deep inside blind holes in both conducting and insulating materials. The placement of the GSED as the final pressure-limiting aperture of the ESEM creates a situation of apparent illumination along the line of sight of the observer. In principle, any point struck by the primary beam can be imaged. Image quality depends on the depth of the hole. In brass, features at the bottom of a 1.5 mm diameter hole that was 8 mm deep were successfully imaged.     

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.