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

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

Helium ion microscopy for high-resolution visualization of the articular cartilage collagen network

The articular cartilage collagen network is an important research focus because network disruption results in cartilage degeneration and patient disability. The recently introduced helium ion microscope (HIM), with its smaller probe size, longer depth of field and charge neutralization, has the potential to overcome the inherent limitations of electron microscopy for visualization of collagen network features, particularly at the nanoscale. In this study, we evaluated the capabilities of the helium ion microscope for high-resolution visualization of the articular cartilage collagen network. Images of rabbit knee cartilage were acquired with a helium ion microscope; comparison images were acquired with a field emission scanning electron microscope (FE-SEM) and a transmission electron microscope (TEM). Sharpness of example high-resolution helium ion microscope and field emission scanning electron microscope images was quantified using the 25-75% rise distance metric. The helium ion microscope was able to acquire high-resolution images with unprecedented clarity, with greater sharpness and three-dimensional-like detail of nanoscale fibril morphologies and fibril connections, in samples without conductive coatings. These nanoscale features could not be resolved by field emission scanning electron microscopy, and three-dimensional network structure could not be visualized with transmission electron microscopy. The nanoscale three-dimensional-like visualization capabilities of the helium ion microscope will enable new avenues of investigation in cartilage collagen network research.     

Measurement of total gas scattering cross‐section

Experimental measurements are presented of the total scattering cross‐section of four gases: He, Air, CH₄, and Argon as a function of electron beam energy. The method used was that of Gauvin. A comparison of theoretical estimates of the total scattering cross‐section with the experimental data indicates that the model used gives generally reliable results. The data confirm that the gases tend to be molecular rather than atomic in nature in this pressure range (1–300 Pascal). The factors affecting the accuracy of these results are examined.     

Direct measurement of electron beam scattering in the environmental scanning electron microscope using phosphor imaging plates

Phosphor imaging plate technology has made it possible to directly image the distribution of primary beam electrons and scattered electrons in the environmental scanning electron microscope. The phosphor plate is exposed under electron scattering conditions in the microscope chamber. When processed, the electron intensity distribution is displayed as a digital image. The image is a visual representation of the electron probe and skirt and may provide the basis for a more accurate model.     

Helium ion microscopy and its application to nanotechnology and nanometrology

Helium ion microscopy (HeIM) presents a new approach to nanotechnology and nanometrology, which has several potential advantages over the traditional scanning electron microscope (SEM) currently in use in research laboratories and manufacturing facilities across the world. Owing to the very high source brightness, and the shorter wavelength of the helium (He) ions, it is theoretically possible to focus the ion beam into a smaller probe size relative to that of the electron beam of an SEM. Hence, resolution 2 × – 4 × better than that of comparable SEMs is theoretically possible. In an SEM, an electron beam interacts with the sample and an array of signals are generated, collected and imaged. This interaction zone may be quite large depending upon the accelerating voltage and materials involved. Conversely, the helium ion beam interacts with the sample, but it does not have as large an excitation volume and, thus, the image collected is more surface sensitive and can potentially provide sharp images on a wide range of materials. Compared with an SEM, the secondary electron yield is quite high—allowing for imaging at extremely low beam currents and the relatively low mass of the helium ion, in contrast to other ion sources such as gallium, potentially results in minimal damage to the sample. This article reports on some of the preliminary work being done on the HeIM as a research and measurement tool for nanotechnology and nanometrology being done at NIST. SCANNING 30: 457–462, 2008. Published 2008 by Wiley Periodicals, Inc.     

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.     

Reproducibility and accuracy of spatial measurements from digitally captured images in the environmental scanning electron microscope

Accurate spatial measurements in a scanning electron microscope (SEM) require calibration of the magnification as a function of working distance and microscope operating conditions. This work presents the results of the calibration of an environmental SEM for the accurate spatial measurement of dimensions and areas in experiments, both for the measurement of strain in steel specimens under applied loads and the measurement of dimensional changes in timber with changes in relative humidity.     

Charging compensation of alumina samples by using an oxygen microinjector in the environmental scanning electron microscope

A gas microinjector system was set up in an environmental scanning electron microscope (ESEM) to create an oxygen atmosphere around the alumina samples for the charging compensation under a pressure between 2 x 10(-5) Pa approximately 2 x 10(-2) Pa. At low pressures, the skirt effect of the electron scattering can be degraded, which results in improvement of the imaging contrast and increase of the signal/noise ratio. The sample current (I(SC)) and the Duane-Hunt limit were measured to evaluate the charging effect.     

Monte Carlo simulation of scanning electron microscope signals for lithographic metrology

Two computer codes for simulating the backscattered, transmitted, and secondary-electron signals from targets in a scanning electron microscope are described. The first code, MONSEL-II, has a model target consisting of three parallel lines on a three-layer substrate, while the second, MONSEL-III, has a model target consisting of a two-by-two array of finite lines on a three-layer substrate. Elastic electron scattering is determined by published fits to the Mott cross section. Both plasmon-generated electrons and ionized valence electrons are included in the secondary production. An adjustable quantity, called the residual energy loss rate, is added to the formula of Joy and Luo to obtain the measured secondary yield. The codes show the effects of signal enhancement due to edge transmission, known as blooming, as well as signal reduction due to neighboring lines, known as the “black-hole” effect.     

Mc3D: A three-dimensional Monte Carlo system simulating image contrast in surface analytical scanning electron microscopy I—Object-oriented software design and tests

The main features of a three-dimensional (3-D) Monte Carlo software system (Mc3D), designed for the simulation of electron scattering and image contrast in a scanning electron microscope, are reported. Before simulating electron trajectories in the sample, impingement of the incident electron beam is described by introducing the idea of a virtual scan path in 3-D space. A general and concise algorithm is given for simulating the intersection of electron trajectories leaving the sample onto multidetector entrance apertures distributed in 3-D space. By optimising the object-oriented design in conjunction with the use of a process-oriented and data-oriented code structure, Mc3D is capable of simulating microscopic analysis of a sample with a 3-D geometry or structure that can be expressed with formulae. Three examples of the use of Mc3D are given. The first is for linescans across a block of SiO₂ on top of a Si substrate; the second is for a stripe of SiO₂ embedded in a Si substrate. Finally, the simulation of Auger linescans across an Au overlay on Si is compared with experimental results. The relationships between experimental linescans and the true beam impact positions on the sample are revealed through the virtual scan path. An edge effect, parallel-edge enhancement, is predicted when the incident electron beam size, the distance of impact position to the terrace edge, and the inelastic mean free path of the Auger electron from a given element are comparable, and the linescan is parallel to the terrace edge. All three examples demonstrate the sensitivity of image contrast to the disposition of the sample with respect to the electron column and the detector position.     

Nanotip electron gun for the scanning electron microscope

Experimental nanotips have shown significant improvement in the resolution performance of a cold field emission scanning electron microscope (SEM). Nanotip electron sources are very sharp electron emitter tips used as a replacement for the conventional tungsten field emission (FE) electron sources. Nanotips offer higher brightness and smaller electron source size. An electron microscope equipped with a nanotip electron gun can provide images with higher spatial resolution and with better signal-to-noise ratio. This could present a considerable advantage over the current SEM electron gun technology if the tips are sufficiently long-lasting and stable for practical use. In this study, an older field-emission critical dimension (CD) SEM was used as an experimental test platform. Substitution of tungsten nanotips for the regular cathodes required modification of the electron gun circuitry and preparation of nanotips that properly fit the electron gun assembly. In addition, this work contains the results of the modeling and theoretical calculation of the electron gun performance for regular and nanotips, the preparation of the SEM including the design and assembly of a measuring system for essential instrument parameters, design and modification of the electron gun control electronics, development of a procedure for tip exchange, and tests of regular emitter, sharp emitter and nanotips. Nanotip fabrication and characterization procedures were also developed. Using a "sharp" tip as an intermediate to the nanotip clearly demonstrated an improvement in the performance of the test SEM. This and the results of the theoretical assessment gave support for the installation of the nanotips as the next step and pointed to potentially even better performance. Images taken with experimental nanotips showed a minimum two-fold improvement in resolution performance than the specification of the test SEM. The stability of the nanotip electron gun was excellent; the tip stayed useful for high-resolution imaging for several hours during many days of tests. The tip lifetime was found to be several months in light use. This paper summarizes the current state of the work and points to future possibilities that will open when electron guns can be designed to take full advantage of the nanotip electron emitters.     

Simple modifications for accurate high-temperature gas exposures using scanning electron microscopy

Relatively low-cost modifications to standard commercial scanning electron microscopes (SEM) that allow accurate exposure of sample(s) to noncorrosive gases at ambient and high temperatures are outlined. Energy-dispersive spectroscopic analysis of sample(s) exposed to noncorrosive gases at high temperatures is demonstrated. Gas exposure is limited to pressures of less than 10^?4 torr (1.33 × 10^?2 Pa) as a result of limitations on SEM system operation and SEM safety interlocks. Gases are limited to noncorrosive types as a result of potential damage to system detection devices and internal mechanical parts.     

Use of Monte Carlo modeling for interpreting scanning electron microscope linewidth measurements

A scanning electron microscope (SEM) can be used to measure the dimensions of the microlithographic features of integrated circuits. However, without a good model of the electron-beam/specimen interaction, accurate edge location cannot be obtained. A Monte Carlo code has been developed to model the interaction of an electron beam with one or two lines lithographically produced on a multilayer substrate. The purpose of the code is to enable one to extract the edge position of a line from SEM measurements. It is based on prior codes developed at the National Institute of Standards and Technology, but with a new formulation for the atomic scattering cross sections and the inclusion of a method to simulate edge roughness or rounding. The code is currently able to model the transmitted and backscattered electrons, and the results from the code have been applied to the analysis of electron transmission through gold lines on a thin silicon substrate, such as is used in an x-ray lithographic mask. Significant reductions in backscattering occur because of the proximity of a neighboring line.     

Mechanical scanning of the specimen in the scanning electron microscope

A scanning electron microscope (SEM) system equipped with a motor drive specimen stage fully controlled with a personal computer (PC) has been utilized for obtaining ultralow magnification SEM images. This modem motor drive stage works as a mechanical scanning device. To produce ultra-low magnification SEM images, we use a successful combination of the mechanical scanning, electronic scanning, and digital image processing techniques. This new method is extremely labor and time saving for ultra-low magnification and wide-area observation. The option of ultra-low magnification observation (while maintaining the original SEM functions and performance) is important during a scanning electron microscopy session.     

Measurement of crystal parameters on backscatter kikuchi diffraction patterns

Electron backscatter Kikuchi diffraction patterns (BKDPs) recorded in the scanning electron microscope (SEM) require measurements on the plane of the photographic film or on the recording screen. The parameters that require measurements are the equivalent electron source point on the pattern, or pattern centre, specimen-to-film distance, true interzonal angles, true interplanar angles, Bragg angles, and interplanar spacing. In this paper, the geometry and the methods of calculation of these parameters on BKDPs recorded directly on film are described in detail. The methods described are suitable for practical purposes, providing speed of calculation but limited accuracy. The inherent factors that limit the accuracy of any measurements on BKDPs are the limitations of the gnomonic projection, resulting in projected distortions in Kikuchi bands and diffuseness of Kikuchi band edges originating from inelastic scattering of electrons. The methods described are applied to crystallographic analysis of BKDPs recorded from silicon and polycrystalline copper.     

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

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

A case of Leukonychia with scanning electron microscope observation

Leukonychia is a medical term for white discoloration appearing on nails. The pathophysiologic mechanisms that cause white discoloration are not entirely clear. We processed a case of leukonychia with scanning electron microscope observation and found many crispy, obviously dissociated "layers" in the lower part of the white nail plate. The dissociated "layers" were composed of thick, loose, coarse keratin bundles intertwined with each other. We believe the dissociated "layers" are related to the clinically noted white discoloration appearing on the nails.     

Scanning reflection electron microscopy of surface topography by diffusely scattered electrons in the scanning electron microscope

In this paper, the technique of scanning reflection electron microscopy (SREM) by diffusely scattered electrons in the scanning electron microscope is described in detail. A qualitative account of the formation of image contrast in SREM is also described. It is assumed that, for grazing geometry, forward-scattered electrons reflect from regions close to the surface, following a few scattering events within the first few atomic layers, and lose very little energy in the process. The penetration depth of the primary electrons is very limited, resulting in strongly peaked envelopes of forward-scattered electrons. It is also assumed that a surface containing topographic features presents a range of tilt angles, resulting in different reflection coefficients. Tilt contrast results because each facet has a different scattering yield, which is dependent upon local surface inclination. Full details of the instrumentation designed for SREM are described, and to illustrate the technique, results recorded from an epitaxial GaAs on GaAs crystal, Pb₂(Zr,Ta)O₆ thin film on silicon, and SiO₂ amorphous film on silicon are presented.     

Optimum beam transfer in the environmental scanning electron microscope

The gas density of argon along the axis of a pressure-limiting aperture in an environmental scanning electron microscope is found by the direct simulation Monte Carlo method. The aperture is made on a thin material plate, producing the sharpest possible transition region between the specimen chamber and the differentially pumped region downstream of the gas flow. The entire regime from free molecule to continuum flow has been studied, which covers any size of aperture diameter and any pressure from vacuum to one atmosphere. The amount of electron beam transmitted without scattering at any point along the aperture axis is found in the range of accelerating voltage between 1 and 30 kV for argon. The electron beam transmission is further computed for helium, neon, hydrogen, oxygen, nitrogen and water vapour. This study constitutes the basis for the design and construction of an environmental scanning electron microscope having an optimum electron beam transfer, which is the primary requirement for an optimum performance instrument.     

Monte Carlo simulation of secondary electron and backscattered electron images in scanning electron microscopy for specimen with complex geometric structure

A new Monte Carlo technique for the simulation of secondary electron (SE) and backscattered electron (BSE) of scanning electron microscopy (SEM) images for an inhomogeneous specimen with a complex geometric structure has been developed. The simulation is based on structure construction modeling with simple geometric structures, as well as on the ray-tracing technique for correction of electron flight-step-length sampling when an electron trajectory crosses the interface of the inhomogeneous structures. This correction is important for the simulation of nanoscale structures of a size comparable with or even less than the electron scattering mean free paths. The physical model for electron transport in solids combines the use of the Mott cross section for electron elastic scattering and a dielectric function approach for electron inelastic scattering, and the cascade SE production is also included.