Detectors for low voltage scanning electron microscopy

Two simple electron detectors (low and high take-off angles) for low-voltage scanning electron microscopy were built and tested. They contain large area scintillators with an applied high voltage and are able to detect backscattered electrons with high efficiency. These detectors also can record the sum of backscattered and secondary electrons. The primary beam of the microscope is screened from the scintillator high voltage by grids, which also permit switching from the BSE to the (SE + BSE) mode. The circular symmetry of the grids minimizes the influence of applied potentials on the primary beam. The use of the low and high take-off detectors permits the detection of back-scattered electrons emitted from the specimen surface into different ranges of take-off angles.     

A double detector system for BSE and SE imaging

The double detector system described here is a simple device suitable for any SEM. It permits efficient imaging of specimen surfaces in either the secondary electron (SE) or backscattered electron (BSE) mode. The BSE detector is an annular single-crystal scintillator made of yttrium aluminium garnet (YAG) and the SE detector has a scintillator of the same material. Both detectors have their own light guides which are connected to a single photomultiplier. The choice of signal is made with a mechanical diaphragm mounted on a flange between the light guide and the photomultiplier. The SE detector may be replaced by a second BSE detector to allow the detection of “low” take-off angle BSEs to provide information which differs from that given by the annular BSE detector which operates to detect BSEs with a “high” take-off angle. In this way it is possible to image either material or topographic contrast with high resolution and to take advantage of the choice of detected electrons.     

A new method of compositional image formation by a four-element backscattered electron detector in an SEM

A special mixing procedure for signals from a four element backscattered electron (BSE) detector is proposed for compositional image formation when a sample with a rough surface is examined by a scanning electron microscope (SEM). The new method allows appreciable suppression of the influence of the sample surface topography in a compositional mode for take-off angles less than about 30°, relative to the microscope axis. The theoretical approach based on the analysis of BSE angular distribution is compared with the experiment. The mixing procedure uses a dimensionless parameter, which depends mainly on take-off angle. Photographs of the Ge-Zn structure with its rough surface were taken in conventional and proposed compositional modes for take-off angle 11° and electron energy 20 keV and show a considerable suppression of the topographic effect when the new method is used.     

Influence of the angular distribution of backscattered electrons on signals at different take-off angles in low-voltage scanning electron microscopy (LVSEM)

It is well known that the differential Mott cross section for large-angle elastic scattering shows maxima and minima at angles depending on material and electron energy. For electron energies of 10–30 keV, the averaging by frequent elastic scattering processes results in approximate Lambert angular distributions of backscattered electrons (BSE). However, the present Monte Carlo calculations for electron energies E = 1–5 keV and different angles of incidence show strong deviations from a Lambert distribution which increases with decreasing energy. The signals of the BSE detector with five annular segments for different take-off directions show good agreement with the calculations for normal electron incidence.     

High-resolution backscatter electron imaging of colloidal gold in LVSEM

High‐resolution backscatter electron (BSE) imaging of colloidal gold can be accomplished at low voltage using in‐lens or below‐the‐lens FESEMs equipped with either Autrata‐modified yttrium aluminium garnet (YAG) scintillators doped with cerium, or with BSE to secondary electron (SE) conversion plates. The threshold for BSE detection of colloidal gold was 1.8 keV for the YAG detector, and the BSE/SE conversion was sensitive down to 1 keV. Gold particles (6, 12 and 18 nm) have an atomic number of 79 and were clearly distinguished at 500 000× by materials contrast and easily discriminated from cell surfaces coated with platinum with an atomic number of 78. BSE imaging was relatively insensitive to charging, and build up of carbon contamination on the specimen was transparent to the higher energy BSE.     

Application of channel electron multipliers in an electron detector for low‐voltage scanning electron microscopy

An electron detector containing channel electron multipliers was built and tested in the range of low‐voltage scanning electron microscopy as a detector of topographic contrast. The detector can detect backscattered electrons or the sum of backscattered electrons and secondary electrons, with different amount of secondary electrons. As a backscattered electron detector it collects backscattered electrons emitted in a specific range of take‐off angles and in a large range of azimuth angles enabling to obtain large solid collection angle and high collection efficiency. Two arrangements with different channel electron multipliers were studied theoretically with the use of the Monte Carlo method and one of them was built and tested experimentally. To shorten breaks in operation, a vacuum box preventing channel electron multipliers from an exposure to air during specimen exchanges was built and placed in the microscope chamber. The box is opened during microscope observations and is moved to the side of the scanning electron microscope chamber and closed during air admission and evacuation cycles enabling storing channel electron multipliers under vacuum for the whole time. Experimental tests of the detector included assessment of the type of detected electrons (secondary or backscattered), checking the tilt contrast, imaging the spatial collection efficiency, measuring the noise coefficient and recording images of different specimens.     

Collection deficiencies of scanning electron microscopy signal contrasts measured and corrected by differential hysteresis image processing

Limitations of scanning electron microscopy (SEM) image resolution and quality were measured in digital image data and their effect on image contrasts was analyzed and corrected by differential hysteresis (DH) processing. DH processing is a mathematical procedure that utilizes hysteresis properties of intensity variations in the image for a segmentation of differential contrast patterns. These patterns display contrast properties of the data as coherent full-frame images. The contrast segmentation is revertible so that the original image can be restored from the sum of the sequentially extracted DH contrast patterns. DH imaging enhances weak contrast components so that they are more easily recognizable and displays SEM image data free of signal collection efficiency contrasts. Example image data include environmental SEM (ESEM) and SEM images of low and mediumhigh magnifications where collection deficiencies included charging of the specimen surface, obstructions from specimen topography, and uneven signal collection properties of the detector. ESEM low-vacuum image data, which appear to be of high quality, contained local areas of reduced contrasts due to residual surface charging. In such areas, signal contrasts were reduced up to 80%, which suppressed most of the weak short-range contrasts. In low-magnification SEM images, up to 93% of the local high precision contrast was lost from the various adverse effects which diminished the pixel-related contrast resolution of the microscope and resulted in images with low detail. Also, at medium magnification, surface charging effects dramatically reduced the image quality because contrasts resulting from local electron beam/specimen interactions were reduced by as much as 71%. DH imaging restored the local contrast losses by elimination of the collected distorted fraction of signal contrasts and reconstitution of the collected maintained fraction. Restored DH images are of superior quality and enhance the imaging capability of the conventional SEM. DH contrast segmentation provides an improved basis for the measurement of various signal contrast components and detector performances. The DH analysis will ultimately facilitate a precise deduction of specimen properties from extracted contrast patterns.     

Imaging colloidal gold labels in LVSEM

On biological samples, the topographic imaging capabilities of the new generation of scanning electron microscopes (SEM) (those having both field-emission guns and low aberration lenses) rival those of the replica techniques. In addition, they permit the localization of specific molecules on the sample surface using one of several labeling techniques utilizing heavy metal colloids. Normally, colloidal gold can be detected in the SEM both by the secondary electron signal (shape) and by the backscattered electron signal (BSE, Z-contrast). The new instruments seem to produce their best topographic images using low-beam voltage (1–5 kV) where topographic contrast is higher and the required thickness of the metal coating is less (Haggis and Pawley 1988, Ris and Pawley 1988). Although the detection of backscattered electrons is more difficult at low-beam voltage, we are able to show here that the secondary electron (SE) signal produced with a 2–5-kV beam permits the unambiguous detection of gold particles as small as 5 nm on carbon-coated specimens while a 1-kV beam produces a high-quality topographic image of the same sample.     

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.     

A BSE scintillation detector in the (S)TEM

A single crystal YAG: Ce³⁺ annular scintillator axially placed in a movable light guide forms the essential part of a new BSE detector. Comparison of properties of this detector with those of a semiconductor detector is made. The bandwidth, signal-to-noise ratio, capacitance effects, and relative efficiency are parameters which favour the scintillation detector. Its disadvantage is that it must be equipped with a photomultiplier and a light guide. The position of the scintillator above the specimen permits efficient detection at a large collection angle of BSE. For normal beam incidence, the signal homogeneity from any area of the scintillator ensures that images are obtained without shadow effects due to signal loss in the scintillator or due to detector geometry. The same probe current as for other detection modes can be used. Resolution of details is as high as for an SE image.     

Advantages of stereo imaging of metallic surfaces with low voltage backscattered electrons in a field emission scanning electron microscope

High emission current backscattered electron (HC-BSE) stereo imaging at low accelerating voltages (≤ 5 keV) using a field emission scanning electron microscope was used to display surface structure detail. Samples of titanium with high degrees of surface roughness, for potential medical implant applications, were imaged using the HC-BSE technique at two stage tilts of + 3° and − 3° out of the initial position. A digital stereo image was produced and qualitative height, depth and orientation information on the surface structures was observed. HC-BSE and secondary electron (SE) images were collected over a range of accelerating voltages. The low voltage SE and HC-BSE stereo images exhibited enhanced surface detail and contrast in comparison to high voltage (> 10 keV) BSE or SE stereo images. The low voltage HC-BSE stereo images displayed similar surface detail to the low voltage SE images, although they showed more contrast and directional sensitivity on surface structures. At or below 5 keV, only structures a very short distance into the metallic surface were observed. At higher accelerating voltages a greater appearance of depth could be seen but there was less information on the fine surface detail and its angular orientation. The combined technique of HC-BSE imaging and stereo imaging should be useful for detailed studies on material surfaces and for biological samples with greater contrast and directional sensitivity than can be obtained with current SE or BSE detection modes.     

Automated in‐chamber specimen coating for serial block‐face electron microscopy

When imaging insulating specimens in a scanning electron microscope, negative charge accumulates locally (‘sample charging’). The resulting electric fields distort signal amplitude, focus and image geometry, which can be avoided by coating the specimen with a conductive film prior to introducing it into the microscope chamber. This, however, is incompatible with serial block‐face electron microscopy (SBEM), where imaging and surface removal cycles (by diamond knife or focused ion beam) alternate, with the sample remaining in place. Here we show that coating the sample after each cutting cycle with a 1–2 nm metallic film, using an electron beam evaporator that is integrated into the microscope chamber, eliminates charging effects for both backscattered (BSE) and secondary electron (SE) imaging. The reduction in signal‐to‐noise ratio (SNR) caused by the film is smaller than that caused by the widely used low‐vacuum method. Sample surfaces as large as 12 mm across were coated and imaged without charging effects at beam currents as high as 25 nA. The coatings also enabled the use of beam deceleration for non‐conducting samples, leading to substantial SNR gains for BSE contrast. We modified and automated the evaporator to enable the acquisition of SBEM stacks, and demonstrated the acquisition of stacks of over 1000 successive cut/coat/image cycles and of stacks using beam deceleration or SE contrast.     

Charge contrast imaging of material growth and defects in environmental scanning electron miscroscopy--linking electron emission and cathodoluminescence

An electron-based technique for the imaging of crystal defect distribution such as material growth histories in non- and poorly conductive materials has been identified in the variable pressure or environmental scanning electron microscope. Variations in lattice coherence at the meso-scale can be imaged in suitable materials. Termed charge contrast imaging (CCI), the technique provides images that correlate exactly with emitted light or cathodoluminescence in suitable materials. This correlation links cathodoluminescence and an electron emission. The specific operating conditions for observation of these images reflect a complex interaction between the electron beam, the positive ions generated by electron-gas interactions in the chamber, a biased detector, and the sample. The net result appears to be the suppression of all but very near surface electron emission from the sample, probably from of the order of a few nanometres. Consequently, CCI are also sensitive to very low levels of surface contaminants. Successful imaging of internal structures in a diverse range of materials indicate that the technique will become an important research tool.     

Fundamental problems of imaging subsurface structures in the backscattered electron mode in scanning electron microscopy

In‐depth imaging of subsurface structures in scanning electron microscopy (SEM) is usually obtained by detecting backscattered electrons (BSE). For a layer‐by‐layer imaging in BSE microtomography, it is preferable to use an energy filtering of BSE. A simple approach is used to estimate the contrast by using backscattering coefficients of bulk materials and the maximum escape depths of the BSE. The contrast obtained by BSE energy filtering is about twice that of the standard BSE method by varying the acceleration voltage. The contrast decreases with increasing information depth. The information depth is about four times smaller than the electron range. The transmission of the spectrometer influences the minimum current of the order of 10^?8 A that is needed to get a contrast of 1%, for example.     

A combination of topographical contrast and stereoscopy for the reconstruction of surface topographies in SEM

The reconstruction of three-dimensional surface topographies can be done in two principally different ways: conventional stereoscopy and ‘shape from shading’. In conventional stereoscopy the depth information is obtained from two perspective views of the specimen. For that, perspectively corresponding features have to be identified in both views to determine depth from perspective shift. Conventional stereoscopy normally results in a relatively sparse set of irregularly distributed points whose elevations are known precisely. The shape-from-shading approach determines the local surface orientation from the local surface luminosity. Over a limited range of surface inclinations the emission of secondary (SE) and back-scattered (BSE) electrons depends uniquely on the angle between electron beam and local surface normal of the specimen. Shape from shading uses this relationship to determine the surface normal with multiple detectors mounted in different take-off directions. Contrary to conventional stereoscopy shape from shading yields depth information from each surface point, but this method is less accurate than stereoscopy. In this paper we propose a combination of both approaches, in which the dense, but less accurate results from shape from shading are used to fill the gaps in the sparsely distributed, but very accurately known, depth information obtained from stereoscopy.     

Backscattered electron imaging of titanium dioxide in frozen hydrated preparations

Backscattered electron (BSE) imaging was used to study ultrafine TiO₂ crystals distribution in a test cream. The cream was fast frozen, cryofractured and observed uncoated at low temperature. The BSE detector was a microchannel plate. The results demonstrate that up-to-date photoprotective preparations can be investigated by this technique.     

Mineral content changes in bone associated with damage induced by the electron beam

Energy-dispersive x-ray (EDX) spectroscopy and backscattered electron (BSE) imaging are finding increased use for determining mineral content in microscopic regions of bone. Electron beam bombardment, however, can damage the tissue, leading to erroneous interpretations of mineral content. We performed elemental (EDX) and mineral content (BSE) analyses on bone tissue in order to quantify observable deleterious effects in the context of (1) prolonged scanning time, (2) scan versus point (spot) mode, (3) low versus high magnification, and (4) embedding in poly-methylmethacrylate (PMMA). Undemineralized cortical bone specimens from adult human femora were examined in three groups: 200x embedded, 200x unembedded, and 1000x embedded. Coupled BSE/EDX analyses were conducted five consecutive times, with no location analyzed more than five times. Variation in the relative proportions of calcium (Ca), phosphorous (P), and carbon (C) were measured using EDX spectroscopy, and mineral content variations were inferred from changes in mean gray levels ("atomic number contrast") in BSE images captured at 20 keV. In point mode at 200x, the embedded specimens exhibited a significant increase in Ca by the second measurement (7.2%, p < 0.05); in scan mode, a small and statistically nonsignificant increase (1.0%) was seen by the second measurement. Changes in P were similar, although the increases were less. The apparent increases in Ca and P likely result from decreases in C: -3.2% (p < 0.05) in point mode and -0.3% in scan mode by the second measurement. Analysis of unembedded specimens showed similar results. In contrast to embedded specimens at 200x, 1000x data showed significantly larger variations in the proportions of Ca, P, and C by the second or third measurement in scan and point mode. At both magnifications, BSE image gray level values increased (suggesting increased mineral content) by the second measurement, with increases up to 23% in point mode. These results show that mineral content measurements can be reliable when using coupled BSE/EDX analyses in PMMA-embedded bone if lower magnifications are used in scan mode and if prolonged exposure to the electron beam is avoided. When point mode is used to analyze minute regions, adjustments in accelerating voltages and probe current may be required to minimize damage.     

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.     

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.