Gaseous Secondary Electron Detector Research Articles

Metrology of crystal defects through intensity variations in secondary electrons from the diffraction of primary electrons in a scanning electron microscope

Understanding defects and their roles in plastic deformation and device reliability is important for the development of a wide range of novel materials for the next generation of electronic and optoelectronic devices. We introduce the use of gaseous secondary electron detectors in a variable pressure scanning electron microscope for non-destructive imaging of extended defects using electron channelling contrast imaging. We demonstrate that all scattered electrons, including the secondary electrons, can provide diffraction contrast as long as the sample is positioned appropriately with respect to the incident electron beam. Extracting diffraction information through monitoring the modulation of the intensity of secondary electrons as a result of diffraction of the incident electron beam, opens up the possibility of performing low energy electron channelling contrast imaging to characterise low atomic weight and ultra-thin film materials. Our methodology can be adopted for large area, nanoscale structural characterisation of a wide range of crystalline materials including metals and semiconductors, and we illustrate this using the examples of aluminium nitride and gallium nitride. The capability of performing electron channelling contrast imaging, using the variable pressure mode, extends the application of this technique to insulators, which usually require conducting coatings on the sample surface for traditional scanning electron microscope based microstructural characterisation.

Charge Contrast Imaging (CCI): Revealing Enhanced Diagenetic Features of A Coquina Limestone

Abstract: Charge Contrast Imaging (CCI) is a low-vacuum scanning electron microscope (LV-SEM) technique that can be induced through partial surface charge suppression of uncoated nonconductive samples, imaged with a suitable detector such as a gaseous secondary electron detector (GSED). The technique commonly produces results similar in style to that of SEM-cathodoluminescence (SEM-CL), providing information on zoning, twinning, annealed fractures, and subtle chemical changes. The current work outlines an example from a Brazilian Lower Cretaceous coquina limestone, in which both optical and SEM-CL imaging produces a limited response from much of the sample. Backscattered electron (BSE) imaging typically suggests only a hint of the cement present, whereas CCI clearly displays a rich and varied cement stratigraphy. The earliest cement displays strong CCI, but appears mainly dark under CL imaging conditions (SEM-CL and optical CL). Later-stage manganese-“enriched” carbonate cement displays luminescence with both optical and SEM-CL, as well as a CCI response. Therefore CCI can provide additional information on cement zonation in an area where CL cannot.

A Fast Readout Electronic System for Accurate Spatial Detection in Ion Beam Tracking for the Next Generation of Particle Accelerators

This paper presents the design, implementation, and measurements of a complete electronic frontend intended for high-resolution spatial detection of ion beams at counting rates higher than $10^6$ particles per second (p/s). The readout system is made up of three main multichannel building blocks, namely, a transimpedance preamplifier, a signal-conditioning line receiver, and a charge-to-digital converter, as well as some off-the-shelf components. The preamplifier and the line receiver have been specifically designed and optimized to minimize the overlapping probability of ion beams tracking, at high counting rates, in low-pressure gaseous secondary electron detectors. Experimental results are shown, considering $\alpha $ particles sources and particles beams, featuring an adaptive shaping time frame of 170–230 ns with a peak signal-to-noise ratio of up to 25 dB. These performance metrics are competitive with the state of the art, demonstrating the suitability of the reported data acquisition and instrumentation system for precise and fast particle tracking detection.

Throttling Aperture as the Gaseous Secondary Electron Detector in the Variable Pressure/Environmental SEM

The authors analysed the conditions of the secondary electron detection with the use of the lower throttling aperture as the electron collector positioned close to the sample, at the distance less, or comparable to the aperture opening. For computer simulations of electron ow under various gas conditions, the software combining the commercial programme SIMION 3D v. 7.0, enabling computations of charged particles trajectories in the electric and magnetic elds, combined with the Monte Carlo one written by the authors was used. The results of the simulations show that the gas ampli cation of the electron signal and the noise to signal ratio are suitable for imaging at working gas pressures exceeding 1 hPa (100 Pa). The upper limit of the pressure de ned by the electron beam scattering by far exceeds 10 hPa and is a question of the accelerating voltage and vacuum units design. The detector unit combined also with the di erential pumping system has been designed in the form of an attachment to the classic scanning electron microscopy and applied in the JSM840 (JEOL) microscope. The simulation results have been proved experimentally.

Design and experimental results of a preamplifier for particles tracking in secondary electron detectors

This paper presents the design and experimental characterization of a preamplifier used in the electronic front-end of low-pressure gaseous secondary electron detectors. The circuit—implemented in a printed circuit board as a proof of concept has been designed to cope with the specifications of the readout electronics used in spatial (beam particle position) measurements. Experimental results show a transimpedance gain of 80dBΩ, an overall voltage gain of 18dB, a peak signal-to-noise ratio of 36.5dB and a shaping time frame of 140–170ns. These features improve the performance of previous reported approaches to the problem, and allow us to minimize the overlapping probability in secondary electron detections for radioactive ion beams tracking, achieving a counting rate higher than 106 particles per second.

Environmental Scanning Ultrafast Electron Microscopy: Structural Dynamics of Solvation at Interfaces

On the surface: CdSe surfaces have been used as a prototype for the investigation of the spatiotemporal characteristics of solvation by environmental scanning ultrafast electron microscopy (see picture, GSED=gaseous secondary electron detector). This study has shown that the ultrafast relaxation dynamics of polar molecules, such as water, in the adsorbate layers exhibit a striking dependence on the CdSe surface structure.

The surface topography of the choroid plexus. Environmental, low and high vacuum scanning electron microscopy

Environmental scanning electron microscopy (ESEM) allows the examination of hydrated and dried specimens without a conductive metal coating which could be advantageous in the imaging of biological and medical objects. The aim of this study was to assess the performance and benefits of wet-mode and low vacuum ESEM in comparison to high vacuum scanning electron microscopy (SEM) using the choroid plexus of chicken embryos as a model, an organ of the brain involved in the formation of cerebrospinal fluid in vertebrates. Specimens were fixed with or without heavy metals and examined directly or after critical point drying with or without metal coating. For wet mode ESEM freshly excised specimens without any pre-treatment were also examined. Conventional high vacuum SEM revealed the characteristic morphology of the choroid plexus cells at a high resolution and served as reference. With low vacuum ESEM of dried but uncoated samples the structure appeared well preserved but charging was a problem. It could be reduced by a short beam dwell time and averaging of images or by using the backscattered electron detector instead of the gaseous secondary electron detector. However, resolution was lower than with conventional SEM. Wet mode imaging was only possible with tissue that had been stabilized by fixation. Not all surface details (e.g. microvilli) could be visualized and other structures, like the cilia, were deformed. In summary, ESEM is an additional option for the imaging of bio-medical samples but it is problematic with regard to resolution and sample stability during imaging.

Immunogold labelling in environmental scanning electron microscopy: applicative features for complementary cytological interpretation

We have combined environmental scanning electron microscopy (ESEM) and immunogold labelling (IGL) for the analysis of cell morphology and surface protein detection on human fine needle aspiration, which is processed in thin uniform monolayer (a single layer of cells) on a glass slide by Thin Prep technology. Among scanning electron microscopy techniques, we choose the environmental modality (ESEM) because it allows a slight manipulation of biological samples and an operational time comparable with cytological techniques. Moreover, the Thin Prep technology confirmed a reproducible cell monolayer on glass smear, minimizing problems for the determination of appropriate amount of material per slide. The first experimental data in ESEM-IGL on biological samples with fine needle aspiration Thin Prep, in human thyroid nodules, showed that cells retained their morphology and provided a clear IGL. The optimization of conditions (i.e. vacuum pressure, temperature and relative humidity) confirmed the possibility to observe an immunolabelled biological sample and morphological signal, joined with compositional informations, due to peculiar characteristics of gaseous secondary electron detector in ESEM. The ESEM-IGL and fine needle aspiration Thin Prep could be used in combination for the interpretation of cell morphology and cell surface immunolabelling. Our paper suggests this use as a powerful diagnostic tool in a pre-surgical evaluations, opening a new applicative window for electron microscopy.

Cell surface protein detection with immunogold labelling in ESEM: Optimisation of the method and semi‐quantitative analysis

In this work we used a combination of immunogold labelling (IGL) and environmental scanning electron microscopy (ESEM) to detect the presence of a protein on the cell surface. To achieve this purpose we chose as experimental system 3T3 Swiss Albino Mouse Fibroblasts and galectin-3. This protein, whose sub-cellular distribution is still under discussion, is involved in a large number of cell physiological and pathological processes. IGL technique has been utilised by many authors in combination with SEM and TEM to obtain the identification/localisation of receptors and antigens, both in cells and tissues. ESEM represents an important tool in biomedical research, since it does not require any severe processing of the sample, lowering the risk of generating artefacts and interfere with IGL procedure. The absence of metal coating could yield further advantages for our purpose as the labelling detection is based on the atomic number difference between Nanogold spheres and the biological material. Using the gaseous secondary electron detector (GSED) compositional contrast is easily revealed by the backscattered electrons component of the signal. In spite of this fact, only few published papers present a combination of ESEM and IGL. Hereby we present our method, optimised to improve the intensity and the specificity of the labelling signal, in order to obtain a semi-quantitative evaluation of the labelling signal.

Material-specific contrast in the ESEM and its application in dentistry

The Environmental Scanning Electron Microscope (ESEM) equipped with a Gaseous Secondary Electron Detector (GSED) was used to image and analyze materials of different density, composition and structure applied in dentistry. Under ESEM conditions (at a H2O vapor pressure of 1–10 Torr) the hydrated surfaces of native teeth, which were coated with different polymers, generated a topographic and also a material specific contrast. The backscattered (BSE) and the secondary (SE) electrons involved into the imaging process produced a cascade-dependent mixed signal at the GSED. The material-specific contrast, generated by the BSE cascade, depends mainly on the atomic number z of the investigated material. The topographic contrast is based principally on the SE cascade. For the exact differentiation of the specific signal components inside of the ESEM, we additionally used a backscattered electron detector (BSED), the application of which allowed us to detect pure BSEs and no signals from cascade-dependent electrons. Conventional scanning electron microscopy (CSEM) used to investigate and image the structures of teeth and applied dental materials needs time-consuming and often artifact-inducing preparation steps before the partially hydrated specimen can be investigated, whereas the ESEM technology permits the imaging of hydrated organic structures with no prior specimen preparation. In the ESEM the interfaces between the hydrated organically structured tooth surfaces and the artificially applied polymer materials with its specific bond characteristics can be analyzed very fast and repeatedly (e.g. after etching series) at a reproducible high quality level.

Transient analysis of gaseous electron‐ion recombination in the environmental scanning electron microscope

Most of the work carried out in relation to contrast mechanisms and signal formation in an environmental scanning electron microscope has yet to consider the time dependent aspects of image generation at a quantitative level. This paper quantitatively describes gaseous electron-ion recombination (also known as 'signal scavenging') in an environmental scanning electron microscope at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector images of alumina (Al2O3) immediately after a region of enhanced secondary electron emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron-ion recombination that takes into consideration transients caused by the time constant of the gaseous secondary electron detector electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks are then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients and (iii) electron drift velocities, as well as absolute values of the total time constant of the gaseous secondary electron detection system and external circuitry, to be determined as a function of microscope operating parameters such as gaseous secondary electron detector bias, sample-electrode separation, imaging gas pressure, and scan speed. The results revealed, for the first time, the exact dependence that the effects of secondary electron-ion recombination on signal formation has on reduced electric field and time in an environmental scanning electron microscope. Furthermore, the model implicitly demonstrated that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought.

Direct Comparison of Various Gaseous Secondary Electron Detectors in the Variable Pressure Scanning Electron Microscope

The conventional Everhart-Thornely scintillation-photomultiplier secondary electron (SE) detector cannot function at elevated pressures due to the high voltage (~ +12kV) involved in its operation. As a result, SE imaging in the variable pressure scanning electron microscope (VPSEM) has required the development of a new generation of SE detectors that operate under low vacuum conditions. To date, three different methods have been devised to measure the secondary electron (SE) emission signal in a VPSEM. Each of these approaches involves the excitation of the chamber gas by the placement of a low voltage (< +1000V) positively biased electrode in the vicinity of the specimen. A SE image can be obtained by measuring the current induced in either the positive electrode (the gaseous secondary electron detector) or the grounded stage (the ion current detector) or via a photomultiplier that detects light emission from the gas (the gas luminescence detector). In this work, the performance of each of these three low vacuum SE detector types has been compared under identical operating conditions using a Zeiss Supra 55VPSEM and FEI XL30 ESEM.

Time Dependent Study of the Positive ion Current in the Environmental Scanning Electron Microscope (ESEM)

The Environmental Scanning Electron Microscope (ESEM) is capable of image generation in a gaseous environment at sample chamber pressures of up to 20 torr. in an ESEM, low energy secondary electrons emitted from a sample surface, by virtue of the primary electron beam, are accelerated towards the positively biased metallic ring (typically +30 to +550V) Gaseous Secondary Electron Detector (GSED). As these electrons accelerate towards the ring they undergo ionizing collisions with gas molecules producing positive ions and additional electrons known as environmental secondary electrons. The environmental electrons further ionize the gas on their way to the ring producing a cascade amplification of the original signal. The amplified signal induced in the ring is used to form an image. The electric field generated between the GSED ring and the grounded stage causes the positive ions produced in the cascade to drift towards the sample, effectively neutralizing negative charge build up on the surface of a non-conducting sample.

Things That Go “Bump” in the VPSEM - and How We Image with Them

Abstract Variable pressure scanning electron microscopes (VPSEM) differ from conventional SEM by operating at pressures ranging from the ‘high vacuum’ SEM levels of 10-6 torr up to typically around 2 torr. The environmental SEM or ESEM is a commercial variant which employs an unique multistage pressure-limiting aperture (PLA) system to attain specimen chamber operating pressures of up to 50 torr. Early instruments used air or argon as the imaging gas but more commonly today water vapour is used. A wide range of gases have been employed, including potentially explosive hydrogen-methane mixtures. The choice of gas is operator-based and can be varied during the imaging session. Early VPESM were restricted to backscattered electron imaging (BSE) until the development of the gaseous secondary electron detector in the ESEM. Gaseous secondary electron detectors are now available for all models of VPSEM and together with compatible cathodoluminescence and EDS XRay detectors, the full range of SEM-based imaging options is present. The principal distinguishing feature of VPSEM is, of course, that samples can be examined uncoated. Gas-electron interaction generates a positive ion supply that can minimise conventional charging artefacts, in a simple imaging model.

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.

Electron backscattered diffraction analyses combined with environmental scanning electron microscopy: potential applications for non-conducting, uncoated mineralogical samples

Operational techniques have been developed which combine electron backscattered diffraction (EBSD) analysis with orientation contrast and secondary electron imaging in environmental scanning electron microscopy (ESEM). This is specifically directed towards the analysis of uncoated, non-conducting materials such as ceramics and mineral surfaces, where surface charge effects have a severe detrimental effect on EBSD microstructural analysis. Initial experimentation to establish optimum chamber conditions was based on single Ge crystals, using both H2O vapour and N2 imaging gases. Reliable EBSPs with correct crystallographic solutions were collected up to pressures of ∼4 torr (1 torr ≈1.33 mbar) under H2O vapour conditions and ∼2 torr in N2, well above the minimal values for gaseous secondary electron detector imaging and surface charge neutralisation. Using these initial pressure and gas type guidelines, polycrystalline structures (with grain mosaics of 50–100 µm) in etched steel were analysed by automated, long duration crystal orientation mapping (COM). Optimum chamber conditions were established at ∼1 torr pressure in H2O vapour environments. At higher gas pressures, increased electron scattering generated more unsolved EBSPs, requiring advanced filtering to reduce map noise. For tests with non-conductors, a suite of single crystal and polycrystalline minerals (garnet, calcite, and olivine) were analysed. Optimum EBSPs were obtained under H2O vapour conditions at pressures of 0.6–1.8 torr, and beam conditions were sufficiently stable for the collection of manual COMs. This new method of combining EBSD and ESEM will greatly improve the potential for microstructural analysis of sensitive, non-conducting ceramic surfaces.

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.

Imaging charge trap distributions in GaN using environmental scanning electron microscopy

We present direct experimental evidence for a field assisted component in images acquired using the gaseous secondary electron detector (GSED) employed in environmental scanning electron microscopes. Enhanced secondary electron (SE) emission was observed in GSED images of epitaxial GaN bombarded with MeV He ions. The increase in SE emission is attributed to an electric field generated by electrons trapped at defects produced by ion implantation. The presence of nonradiative recombination centers and of trapped charge in implanted GaN was established by cathodoluminescence spectroscopy and energy dispersive x-ray spectrometry. The field assisted SE component is distinguishable from the “normal” GSED signal by characteristic pressure and temperature dependencies. The presented results demonstrate the utility of the GSED for imaging charge trap distributions in semiconductors.

Space Charge Artifacts in ESEM Images: Shadowing and Contrast Reversal

Abstract The environmental scanning electron microscope (ESEM) employs a series of pressure limiting apertures and a differential pumping system to allow for electron imaging at specimen chamber pressures of up to 50 torr. Images rich in secondary electron (SE) contrast can be obtained using the gaseous secondary electron detector (GSED) or ion current (Iion) signals. The GSED and Iion signals are amplified in a gas cascade. SEs emitted from a sample are accelerated through the gas in the specimen chamber by an electric field, EGSED, produced by a positively biased electrode located in the chamber, above the specimen. The accelerated SEs give rise to a cascade ionization process that can amplify the SE signal by up to three orders of magnitude. Electrons produced in the cascade are rapidly swept to the biased electrode and are efficiently removed from the gas. Positive ions produced in the cascade drift away from the electrode with a velocity that is at least three orders of magnitude lower than that of the electrons.

Distribution and Behaviour of Positive Ions in the Environmental Scanning Electron Microscope (ESEM)

Abstract Electron detection in the environmental scanning electron microscope relies upon the presence of a small pressure of a gaseous phase inside the microscope chamber. The presence of this gas controls two important mechanisms by which the microscope functions. The conventional gaseous secondary electron detector (GSED) applies a variable positive bias (0 to +600 V) directly above the specimen. Electrons ejected from the specimen surface due to the incident scanning probe are accelerated by this field towards the positively biased detector. Whilst traversing the gap between specimen and detector the electrons undergo ionizing collisions with gas molecules. These result in an amplification of the incident electron signal through the production of ejected ‘daughter’ electrons and leave behind positively charged ions. After production these ions are repelled by the detector bias and drift back towards the microscope stage where they aid charge neutralization on the surface of the specimen.