Beam transfer characteristics of a commercial environmental SEM and a low vacuum SEM

Two commercial instruments that permit a gaseous environment in their specimen chamber have been investigated, namely, a 'FEI Quanta 600 FEG' environmental scanning electron microscope and a 'LEO SUPRA 35VP FESEM' low vacuum scanning electron microscope. The gas flow field is first computed by the direct simulation Monte Carlo method and the gas density gradient, speed, Mach number and temperature are found in the transition region from high pressure to vacuum. The electron beam transfer characteristics are then derived for different accelerating voltages and pressures and a comparison is made among different situations and with some published works. Certain physical parameters are analysed and discussed to establish a figure of merit that can be used as a standard performance specification for commercial environmental scanning electron microscope and low vacuum scanning electron microscope.     

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.     

A new method for low-magnification in the environmental scanning electron microscope

A device has been developed and used successfully on two models of the environmental scanning electron microscope that allows low-magnification imaging of about 30x, significantly better than the original 200x low-magnification imaging limit. This was achieved by using an additional aperture to limit the pressure at a point where it will not block the electron beam, and a larger aperture plate for the combination final aperture/secondary electron signal collection surface that also does not block the electron beam significantly.     

Figure of merit for environmental SEM and its implications

A recently introduced figure of merit for environmental and low vacuum scanning electron microscopes has now been computed in the full operational pressure range for one commercial instrument. The direct simulation Monte Carlo method has been used in lieu of experimental measurements. The theory of this figure of merit is further consolidated. It is shown that a thin pressure limiting aperture can indeed be used as an optimum reference system for all instruments employing differential pumping in the transfer of an electron beam from high vacuum to high pressure. The implications of the results obtained are discussed both in relation to existing commercial instruments and associated literature to pave the way for future progress in the field.     

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

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

Developing dual-beam methodologies for the study of heterogeneous polymer-based systems

The use of a combined focused ion beam/environmental scanning electron microscope (FIB/ESEM) offers new possibilities for imaging the internal structure of complex heterogeneous polymeric samples. The use of the focused ion beam, using positively charged gallium ions in conjunction with a measured 'defocused' low-energy primary electron beam, has permitted milling through the heterostructure to be achieved in a controlled way, exposing the inner structure, without introducing significant ion beam damage/destruction into the sample. The subsequent use of the environmental scanning electron microscope for imaging the revealed internal structure has then enabled insulating polymer structures to be imaged, without charging problems. Cross-sections of a 900-nm-thick spun cast film of phase-separated polystyrene–polybutadiene blends have been successfully milled and imaged; the morphology agreeing with previous results produced using ultramicrotomy and transmission electron microscopy.     

Imaging and spectrometry of a microscopic gaseous target by SEM and TEM

Microscopic gas jets are experimentally investigated using a scanning electron microscope/electron microprobe (JXA 50A, JEOL, Tokyo) equipped with a transmitted electron detector, and the CEM 902 (Zeiss, Oberkochen) which allows electron spectroscopic imaging. Both capillary microjet devices and circular orifices have been used. Imaging of the microgas jet (argon, air, artificial gas mixtures) is done by transmitted electrons (dark field), electrons as collected by the conventional Everhart-Thomley secondary electron detector, and by electron spectroscopic imaging, both at the argon M- and L edges. X-ray spectra and electron energy loss spectra taken at an axis point just a few micrometers downstream from the orifice are discussed.     

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.     

Electron optical characteristics of a concave electrostatic electron mirror for a scanning electron microscope

Numerical computer calculations are used to explore the design characteristics of a concave electrostatic electron mirror for a mirror-attachment for a conventional scanning electron microscope or an instrument designed totally as a scanning electron mirror microscope. The electron paths of a number of set-ups are calculated and drawn graphically in order to find the optimum shape and dimensions of the mirror geometry. This optimum configuration turns out to be the transition configuration between two cases of electron path deflection, towards the optical axis of the system and away from it.     

Application of transmission electron microscopes to nanometre-sized fabrication by means of electron beam-induced deposition

Electron beam‐induced deposition was carried out using a scanning transmission electron microscope with a field emission gun to fabricate nanometre‐sized structures. A small amount of a metal–organic gas was introduced near the substrate in the microscope chamber, and focused electron beams were irradiated. Two‐ and three‐dimensional structures were fabricated by scanning the beam position. The minimum line width of the freestanding structures was 8 nm at a constant gas flux used. This line width of 8 nm is considered to be achieved by employing a high accelerating voltage, which leads to a small probe size, and the optimum scanning speed.     

Improvement of four anode rods ion source

In this work, an improved form of a saddle field ion source has been designed and constructed. It consists of four anode rods made from copper and two copper cathode discs. The two cathode discs are placed symmetrically on both sides of the four anode rods. The electrical discharge and output ion beam characteristics were measured at different pressures using argon gas. The optimum distance between each two anode rods was determined. Also the optimum distance between the four anode rods and any cathode disc was obtained. It was found that the optimum distance between each two anode rods equal to 6 mm, while the optimum distance between the four anode rods and any cathode disc equal to 16 mm, where a stable discharge current and maximum output ion beam current can be obtained. The effect of negative extraction voltage applied to both the extractor electrode and Faraday cup on the output ion beam current was studied. The sputter yield of copper and aluminum targets using argon ions of different energies was determined.     

Electron beam blanking systems

Electron beam blanking in the scanning electron microscope (SEM) by deflection over a chopping aperture is reviewed. The first part is concerned with electron beam deflection structures and driving methods, the second part with electron optics of deflection blanking systems in the SEM.     

Aperture contrast in thick amorphous specimens using scanning transmission electron microscopy

The contrast observed in thick amorphous specimens using a scanning transmission electron microscope (STEM) can be considerably improved by the use of an optimum collector aperture angle. The size of this angle can be calculated by considering the variation of electron current transmitted through the specimen as a function both of the specimen thickness and of the angle of collection subtended at the specimen. Typically these calculations predict optimum angles to be several times the half-width of the elastic scattering distribution, often 10(-1) rad or more. Observations of biological sections of up to 2 micron in thickness using scanning attachments of commercial transmission microscopes have verifie these results at beam voltages of 50, 100 and 200 kV. Wide angle convergent beam diffraction patterns were used to give accurate values of the effective angles represented by the various collector apertures. Once the linearity of the detector-amplifier system had been established, operation in a line modulation mode enabled quantitative measurements to be made of the image contrast. Such measurements also offer a quick effective method of comparing electron beam penetrations.     

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.     

An electron beam lithography and digital image acquisition system for scanning electron microscopes

A low‐cost microcontroller based control and data acquisition unit for digital image recording of scanning electron microscope (SEM) images and scanning electron microscope based electron beam lithography (EBL) is described. The developed microcontroller low‐level embedded software incorporates major time critical functions for image acquisition and electron beam lithography and makes the unit an intelligent module which communicates via USB with the main computer. The system allows recording of images with up to 4096 × 4096 pixel size, different scan modes, controllable dwell time, synchronization with main power frequency, and other user controllable functions. The electron beam can be arbitrary positioned with 12‐bit precision in both dimensions and this is used to extend the scanning electron microscope capabilities for electron beam lithography. Hardware and software details of the system are given to allow its easy duplication. Performance of the system is discussed and exemplary results are presented.     

Beam characteristics for various sizes of annular aperture on scanning electron microscope.

Using an analogy between light optics and electron optics, we have calculated beam characteristics such as the beam profile and the optical transfer function for several sizes of annular and circular apertures on a scanning electron microscope (SEM). It has been found that an annular aperture improves the image quality with regard to several kinds of image resolution and the depth of focus at the price of good low-frequency (nu) contrast. In contrast with conventional circular-aperture SEM images, a combination of a low-nu-pass filtered, circular-aperture SEM image with a high-nu-pass filtered, annular-aperture SEM image has the potential to enhance the image quality in terms of both the image resolution and the depth of focus.     

Application of ion-beam etching techniques to the fine structure of biological specimens as examined with a field emission SEM at low voltage

The term “etching,” in electron microscopy, refers to the removal of specimen surface layers and includes chemical, electrolytic, and ion-beam methods. The ion-beam etching process is used to remove layers of a target material by bombarding it with ionized gas molecules. Recently, the method has been applied to the field of biological specimens; however, the practical procedures for such organic materials have not been developed. In the present study, we used an apparatus in which a beam of argon ions is collimated and focused by electrostatic lenses onto an appropriate target. We demonstrated the optimum conditions to observe biological specimens that were treated with osmium tetroxide and tannic acid. The specimens were examined uncoated at low accelerating voltage using a field emission scanning electron microscope. According to our experiments, when a biological specimen was observed under high-resolution conditions at over 50,000x magnification, the optimum condition of ion-beam etching consisted of an accelerating voltage of E = 1 keV and an ion-beam dose of It = 360 ～ 400 μA. min, depending on parts of the specimens. In order to decrease overetching, we had to choose factors such as E = 1 ～ 2 keV and It = 500 μA. min.     

Application of the focused ion beam technique in aerosol science: detailed investigation of selected, airborne particles

The focused ion beam technique was used to fabricate transmission electron microscope lamellas of selected, micrometre‐sized airborne particles. Particles were sampled from ambient air on Nuclepore polycarbonate filters and analysed with an environmental scanning electron microscope. A large number of particles between 0.6 and 10 µm in diameter (projected optical equivalent diameter) were detected and analysed using computer‐controlled scanning electron microscopy. From the resulting dataset, where the chemistry, morphology and position of each individual particle are stored, two particles were selected for a more detailed investigation. For that purpose, the particle‐loaded filter was transferred from the environmental scanning electron microscope to the focused ion beam, where lamellas of the selected particles were fabricated. The definition of a custom coordinate system enabled the relocation of the particles after the transfer. The lamellas were finally analysed with an analytical transmission electron microscope. Internal structure and elemental distribution maps of the interior of the particles provided additional information about the particles, which helped to assign the particles to their sources. The combination of computer‐controlled scanning electron microscopy, focused ion beam and transmission electron microscopy offers new possibilities for characterizing airborne particles in great detail, eventually enabling a detailed source apportionment of specific particles. The particle of interest can be selected from a large dataset (e.g. based on chemistry and/or morphology) and then investigated in more detail in the transmission electron microscope.     

Focused ion beam scanning electron microscopy in biology

Since the end of the last millennium, the focused ion beam scanning electron microscopy (FIB‐SEM) has progressively found use in biological research. This instrument is a scanning electron microscope (SEM) with an attached gallium ion column and the 2 beams, electrons and ions (FIB) are focused on one coincident point. The main application is the acquisition of three‐dimensional data, FIB‐SEM tomography. With the ion beam, some nanometres of the surface are removed and the remaining block‐face is imaged with the electron beam in a repetitive manner. The instrument can also be used to cut open biological structures to get access to internal structures or to prepare thin lamella for imaging by (cryo‐) transmission electron microscopy. Here, we will present an overview of the development of FIB‐SEM and discuss a few points about sample preparation and imaging.     

Real-time image quality assessment with mixed Lagrange time delay estimation autoregressive (MLTDEAR) model

A proposal to assess the quality of scanning electron microscope images using mixed Lagrange time delay estimation technique is presented. With optimal scanning electron microscope scan rate information, online images can be quantified and improved. The online quality assessment technique is embedded onto a scanning electron microscope frame grabber card for real‐time image processing. Different images are captured using scanning electron microscope and a database is built to optimally choose filter parameters. An optimum choice of filter parameters is obtained. With the optimum choice of scan rate, noise can be removed from real‐time scanning electron microscope images without causing any sample contamination or increasing scanning time.