Low voltage scanning electron microscopy

The scanning electron microscope (SEM) is usually operated with a beam voltage, V₀, in the range of 10–30 kV, even though many early workers had suggested the use of lower voltages to increase topographic contrast and to reduce specimen charging and beam damage. The chief reason for this contradiction is poor instrumental performance when V₀=1–3 kV, The problems include low source brightness, greater defocusing due to chromatic aberration greater sensitivity to stray fields, and difficulty in collecting the secondary electron signal. Responding to the needs of the semiconductor industry, which uses low V₀ to reduce beam damage, considerable efforts have been made to overcome these problems. The resulting equipment has greatly improved performance at low kV and substantially removes the practical deterrents to operation in this mode. This paper reviews the advantages of low voltage operation, recent progress in instrumentation and describes a prototype instrument designed and built for optimum performance at 1 kV. Other limitations to high resolution topographic imaging such as surface contamination, the de-localized nature of the inelastic scattering event and radiation damage are also discussed.     

A charge coupled device camera with electron decelerator for intermediate voltage electron microscopy

Electron microscopists are increasingly turning to intermediate voltage electron microscopes (IVEMs) operating at 300-400 kV for a wide range of studies. They are also increasingly taking advantage of slow-scan charge coupled device (CCD) cameras, which have become widely used on electron microscopes. Under some conditions, CCDs provide an improvement in data quality over photographic film, as well as the many advantages of direct digital readout. However, CCD performance is seriously degraded on IVEMs compared to the more conventional 100 kV microscopes. In order to increase the efficiency and quality of data recording on IVEMs, we have developed a CCD camera system in which the electrons are decelerated to below 100 kV before impacting the camera, resulting in greatly improved performance in both signal quality and resolution compared to other CCDs used in electron microscopy. These improvements will allow high-quality image and diffraction data to be collected directly with the CCD, enabling improvements in data collection for applications including high-resolution electron crystallography, single particle reconstruction of protein structures, tomographic studies of cell ultrastructure, and remote microscope operation. This approach will enable us to use even larger format CCD chips that are being developed with smaller pixels.     

Dual-plane ultrafast limited-angle electron beam x-ray tomography

Electron beam x-ray tomography is an imaging technique, which can provide cross-sectional images of an object of interest with about 1 mm spatial resolution at frame rates of up to 10,000 frames per second. As a non-intrusive method it is especially suited for studying multiphase flows. For this purpose we devised an experimental limited-angle scan setup which utilizes linear beam deflection to generate radiographic projections. This setup was employed in the study of gas–liquid flow in an experimental flow loop operated at different liquid and gas flow rates. Electron-beam tomography images were compared with image data of a wire-mesh sensor. The latter is a fast but intrusive imaging device which is commonly used in gas–liquid flow imaging and achieves comparable frame rates but at lower spatial resolution. As a novelty we implemented a dual-plane limited-angle electron beam x-ray tomography which allows us to gain information about the phase velocities using cross-correlation data analysis.     

A SVM-based pipeline leakage detection and pre-warning system

A SVM-based pipeline leakage detection and pre-warning system is presented in this paper. In the system an optical cable is laid in parallel with a pipeline in the same ditch and three single mode optical fibers inside constitute the distributed vibration sensor. The sensor is based on Mach–Zehnder optical fiber interferometer and can detect the vibration signals along a pipeline in real time. Then the eigenvectors of vibration signals are extracted by “energy-pattern” method based on wavelet packet decomposition. Subsequently the vibration signals are recognized by support vector machine (SVM) through the features so that it can judge whether any abnormal event is taking place. If any abnormal event is found along a pipeline, the location is thus calculated. A series of trials in situ have been done, showing that the system is of good accuracy and real time performance both in recognition and locating.     

A novel approach for correlative light electron microscopy analysis

Correlative light and electron microscopy (CLEM) is a multimodal technique of increasing utilization in functional, biochemical, and molecular biology. CLEM attempts to combine multidimensional information from the complementary fluorescence light microscopy (FLM) and electron microscopy (EM) techniques to bridge the various resolution gaps. Within this approach the very same cell/structure/event observed at level can be analyzed as well by FLM and EM. Unfortunately, these studies turned out to be extremely time consuming and are not suitable for statistical relevant data. Here, we describe a new CLEM method based on a robust specimen preparation protocol, optimized for cryosections (Tokuyasu method) and on an innovative image processing toolbox for a novel type of multimodal analysis. Main advantages obtained using the proposed CLEM method are: (1) hundred times more cells/structures/events that can be correlated in each single microscopy session; (2) three‐dimensional correlation between FLM and EM, obtained by means of ribbons of serial cryosections and electron tomography microscopy (ETM); (3) high rate of success for each CLEM experiment, obtained implementing protection of samples from physical damage and from loss of fluorescence; (4) compatibility with the classical immunogold and immunofluorescence labeling techniques. This method has been successfully validated for the correlative analysis of Russel Bodies subcellular compartments. Microsc. Res. Tech., 2010. © 2009 Wiley‐Liss, Inc.     

A Hadamard transform electron ionization time-of-flight mass spectrometer

We describe the first Hadamard transform time-of-flight mass spectrometer (HT-TOFMS) that incorporates an electron (impact) ionization source. This implementation was realized in an existent TOF instrument using commercially available components and simple modifications to the ion source. In the present apparatus, a Hadamard mask is expressed by modulating the ion generation process within the ion source; thus, the present approach differs from previous designs that use external electrostatic devices to modulate a continuous ion stream. The present implementation may be operated in conventional TOF mode at 12.5 kHz and in HT-TOF mode at 20-40 MHz. In Hadamard mode the design can operate using any circulant simplex code, allowing the operator much flexibility for optimizing resolution and mass range and for eliminating nonstochastic fluctuations, e.g., encoding errors and signal hum. We demonstrate typical performance of the HT-TOFMS in standard and reflectron geometries using sequences of three constructions and of varied length, generating HT-TOF mass spectra of molecules that match conventional reference spectra. The auxiliary material includes an electrical schematic for the floating high-speed encoding amplifier, which is also of use in other high-speed electrostatic optics applications, and a list of 537 validated vectors comprising the first row of each circulant simplex sequence (S(n)=3-8219) derived using maximal shift register (n=2(m)-1), quadratic residue (n=4m-3), and twin prime constructions [n=p(p+2)].     

Simulation of transmission electron microscope images of biological specimens

We present a new approach to simulate electron cryo‐microscope images of biological specimens. The framework for simulation consists of two parts; the first is a phantom generator that generates a model of a specimen suitable for simulation, the second is a transmission electron microscope simulator. The phantom generator calculates the scattering potential of an atomic structure in aqueous buffer and allows the user to define the distribution of molecules in the simulated image. The simulator includes a well defined electron–specimen interaction model based on the scalar Schrödinger equation, the contrast transfer function for optics, and a noise model that includes shot noise as well as detector noise including detector blurring. To enable optimal performance, the simulation framework also includes a calibration protocol for setting simulation parameters. To test the accuracy of the new framework for simulation, we compare simulated images to experimental images recorded of the Tobacco Mosaic Virus (TMV) in vitreous ice. The simulated and experimental images show good agreement with respect to contrast variations depending on dose and defocus. Furthermore, random fluctuations present in experimental and simulated images exhibit similar statistical properties. The simulator has been designed to provide a platform for development of new instrumentation and image processing procedures in single particle electron microscopy, two‐dimensional crystallography and electron tomography with well documented protocols and an open source code into which new improvements and extensions are easily incorporated.     

A momentum imaging microscope for dissociative electron attachment

We describe an experimental approach to image the three-dimensional (3D) momentum distribution of the negative ions arising from dissociative electron attachment (DEA). The experimental apparatus employs a low energy pulsed electron gun, an effusive gas source and a 4π solid-angle ion momentum imaging spectrometer consisting of a pulsed ion extraction field, an electrostatic lens, and a time- and position-sensitive detector. The time-of-flight and impact position of each negative ion are measured event by event in order to image the full 3D ion momentum sphere. The system performance is tested by measuring the anion momentum distributions from two DEA resonances, namely H(-) from H(2)O(-) ((2)B(1)) and O(-) from O(2)(-) ((2)Π(u)). The results are compared with existing experimental and theoretical data.     

On-chip integration of high-frequency electron paramagnetic resonance spectroscopy and Hall-effect magnetometry

A sensor that integrates high-sensitivity micro-Hall effect magnetometry and high-frequency electron paramagnetic resonance spectroscopy capabilities on a single semiconductor chip is presented. The Hall-effect magnetometer (HEM) was fabricated from a two-dimensional electron gas GaAsAlGaAs heterostructure in the form of a cross, with a 50 x 50 microm2 sensing area. A high-frequency microstrip resonator is coupled with two small gaps to a transmission line with a 50 Omega impedance. Different resonator lengths are used to obtain quasi-TEM fundamental resonant modes in the frequency range 10-30 GHz. The resonator is positioned on top of the active area of the HEM, where the magnetic field of the fundamental mode is largest, thus optimizing the conversion of microwave power into magnetic field at the sample position. The two gaps coupling the resonator and transmission lines are engineered differently--the gap to the microwave source is designed to optimize the loaded quality factor of the resonator (Q相似文献   

Surface structure imaging by electron microscopy

Reconstructed structures at monolayer level on ‘clean and well-defined’ surfaces can be imaged by transmission electron microscopy in fixed beam illumination mode. The specimens are cleaned in-situ in the electron microscope in ultra high vacuum. Transmission electron diffraction pattern intensities can give useful information for determining the surface unit cell size of the structure, and the atom positions (geometric arrangement of atoms in the unit cell) especially those with a large unit cell, since the diffraction intensities are interpreted kinematically. High resolution surface imaging which gives directly the atom positions is tested here for a single monolayer terrace on Ag (111) surface. The result shows the value of HREM for studies of surface crystallography.     

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.     

Detective quantum efficiency of electron area detectors in electron microscopy

Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology. We present MTF and DQE measurements for four types of detector: Kodak SO-163 film, TVIPS 224 charge coupled device (CCD) detector, the Medipix2 hybrid pixel detector, and an experimental direct electron monolithic active pixel sensor (MAPS) detector. Film and CCD performance was measured at 120 and 300 keV, while results are presented for the Medipix2 at 120 keV and for the MAPS detector at 300 keV. In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated. We also show that part of the response of the emulsion in film comes from light generated in the plastic support. Computer simulations of film and the MAPS detector have been carried out and show good agreement with experiment. The agreement enables us to conclude that the DQE of a backthinned direct electron MAPS detector is likely to be equal to, or better than, that of film at 300 keV.     

A three axis stereocomparator for scanning electron microscopic photogrammetry - RS3

Despite the topographic appearance of the image derived from the scanning electron microscope operated in the secondary electron emission mode it is impossible to make linear measurements from single two dimensional micrographs and relate these accurately to distances in the third dimension. This paper describes a stereometer specifically designed for the examination and measurement of stereopair scanning electron micrographs. The new instrument uses rotary encoders attached to the movement of the photo-carriage to register the coordinates of image features in both members of a stereoscopic. These raw data are passed to a microcomputer programmed for the reduction and conversion to real three dimensional values according to the geometry of the projection in scanning electron microscope image. This allows large quantities of data to be collated without the errors and tedium involved in manual collection. The need for rapid data acquisition arose during a study on characterisation of resorption of bone by osteoclasts, where the collection and correlation of large quantities of data were required to permit a statistical interpretation to be made.     

Analysis of FIB‐induced damage by electron channelling contrast imaging in the SEM

We have investigated the Ga⁺ ion‐damage effect induced by focused ion beam (FIB) milling in a [001] single crystal of a 316 L stainless steel by the electron channelling contrast imaging (ECCI) technique. The influence of FIB milling on the characteristic electron channelling contrast of surface dislocations was analysed. The ECCI approach provides sound estimation of the damage depth produced by FIB milling. For comparison purposes, we have also studied the same milled surface by a conventional electron backscatter diffraction (EBSD) approach. We observe that the ECCI approach provides further insight into the Ga⁺ ion‐damage phenomenon than the EBSD technique by direct imaging of FIB artefacts in the scanning electron microscope. We envisage that the ECCI technique may be a convenient tool to optimize the FIB milling settings in applications where the surface crystal defect content is relevant.     

Smith predictor based-sliding mode controller for integrating processes with elevated deadtime

An approach to control integrating processes with elevated deadtime using a Smith predictor sliding mode controller is presented. A PID sliding surface and an integrating first-order plus deadtime model have been used to synthesize the controller. Since the performance of existing controllers with a Smith predictor decrease in the presence of modeling errors, this paper presents a simple approach to combining the Smith predictor with the sliding mode concept, which is a proven, simple, and robust procedure. The proposed scheme has a set of tuning equations as a function of the characteristic parameters of the model. For implementation of our proposed approach, computer based industrial controllers that execute PID algorithms can be used. The performance and robustness of the proposed controller are compared with the Matausek-Mici? scheme for linear systems using simulations.     

A wet stage modification to a scanning electron microscope.

A modification to the vacuum system of a JSM2 scanning electron microscope has enabled hydrated specimens to be placed inside the specimen chamber of the instrument and to be surronded by water vapour at a pressure up to approximately I 3-kPa (10 Torr). The surface topography was observed by detecting the backscattered electrons using a wide angle backscattered electron detector placed close to the specimen. The microscope was operated in the normal scanning mode which allowed the examination of the surface topography of the specimens, whilst still retaining the depth of focus which is a feature of the SEM. This modification has enabled a resolution of approximately 0.2 mum to be obtained from biological specimens partially immersed in water at temperatures just above 0 degrees C.     

Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy

With the development of monochromators for (scanning) transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) is developing into a unique technique to study the band structure and optical properties of nanoscale materials. This article discusses practical aspects of spatially resolved VEELS performed in scanning transmission mode and the alignments necessary to achieve the current optimum performance of approximately 0.15 eV energy resolution with an electron probe size of approximately 1 nm. In particular, a collection of basic concepts concerning the acquisition process, the optimization of the energy resolution, the spatial resolution and the data processing are provided. A brief study of planar defects in a Y(1)Ba(2)Cu(3)O(7-)(delta) high-temperature superconductor illustrates these concepts and shows what kind of information can be accessed by VEELS.     

Transmission electron microscopy at 20 kV for imaging and spectroscopy

The electron optical performance of a transmission electron microscope (TEM) is characterized for direct spatial imaging and spectroscopy using electrons with energies as low as 20 keV. The highly stable instrument is equipped with an electrostatic monochromator and a C_S-corrector. At 20 kV it shows high image contrast even for single-layer graphene with a lattice transfer of 213 pm (tilted illumination). For 4 nm thick Si, the 200 reflections (271.5 pm) were directly transferred (axial illumination). We show at 20 kV that radiation-sensitive fullerenes (C₆₀) within a carbon nanotube container withstand an about two orders of magnitude higher electron dose than at 80 kV. In spectroscopy mode, the monochromated low-energy electron beam enables the acquisition of EELS spectra up to very high energy losses with exceptionally low background noise. Using Si and Ge, we show that 20 kV TEM allows the determination of dielectric properties and narrow band gaps, which were not accessible by TEM so far. These very first results demonstrate that low kV TEM is an exciting new tool for determination of structural and electronic properties of different types of nano-materials.     

Linear least-squares fit evaluation of series of analytical spectra from planar defects: extension and possible implementations in scanning transmission electron microscopy

In a previous paper, a new technique was introduced to determine the chemistry of crystallographically well‐defined planar defects (such as straight interfaces, grain boundaries, twins, inversion or antiphase domain boundaries) in the presence of homogeneous solute segregation or selective doping. The technique is based on a linear least‐squares fit using series of analytical (electron energy‐loss or energy‐dispersive X‐ray) spectra acquired in a transmission electron microscope that is operated in nano‐probe mode with the planar defect centred edge‐on. First, additional notes on the use of proper k‐factors and determination of Gibbsian excess segregation are given in this note. Using simulated data sets, it is shown that the linear least‐squares fit improves both the accuracy and the robustness to noise beyond that obtainable by independently repeated measurements. It is then shown how the method originally developed for a stationary nano‐probe mode in transmission electron microscopy can be extended to a focused electron beam that scans a square region in scanning transmission electron microscopy. The necessary modifications to scan geometry and corresponding numerical evaluation are described, and three different practical implementations are proposed.