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Much of the success of the scanning electron microscope (SEM) can be attributed to the fact that the images that it produces appear similar to those seen by our eyes. Consequently the interpretation of SEM micrographs is perceived as a straightforward process. Although this casual approach works well enough for many qualitative purposes, when the SEM is to be used to make precise dimensional measurements of micron-sized features, such as resist strips or conductor pads, then more care must be paid to the details of image formation.Two types of electron signal, backscattered (BS) and secondary (SE), are usually available in an SEM. In many respects quantitative image interpretation is more straightforward for backscattered than for secondary electrons because Monte Carlo trajectory tracing techniques can be employed to compute the expected variation of signal with beam position.

In transmission electron spectroscopy the fundamental formula for elemental quantitation using inner-shell excitations gives the number n of atoms per cm2 contributing to the K-edge aswhere Ik and Io are the integrated intensities in the K-edge and zero-loss peak, respectively. Both of these integrals are measured for a spectrometer acceptance angle 2α and an energy interval ΔE. The parameter αk(α,ΔE) is the ionization cross-section for the same angular and energy parameters. The variation of αk with α and ΔE is a function of the generalized oscillator strength and little detailed information on this quantity is available. Therefore it is necessary to proceed empirically and the simplest assumption is thatwhere σk is a saturation (x-ray) cross-section and ηα,ηΔEcan be identified with efficiency factors.The accuracy of Equation (1) for K-edges from light elements (Li ≤ Z ≤ Al) is being tested by computer curve fitting and background stripping (see Fig. 1).

Personal computers (PCs) are a powerful resource in the EM Laboratory, both as a means of automating the monitoring and control of microscopes, and as a tool for quantifying the interpretation of data. Not only is a PC more versatile than a piece of dedicated data logging equipment, but it is also substantially cheaper. In this tutorial the practical principles of using a PC for these types of activities will be discussed.The PC can form the basis of a system to measure, display, record and store the many parameters which characterize the operational conditions of the EM. In this mode it is operating as a data logger. The necessary first step is to find a suitable source from which to measure each of the items of interest. It is usually possible to do this without having to make permanent corrections or modifications to the EM.

The resolution of a secondary electron image is determined by the spatial distribution of the secondaries leaving the surface. This distribution results from two components, SE1 which are the secondaries generated by the incident beam and which carry the high resolution information, and SE2 which are the secondaries produced by exiting electrons and which carry information mimicing that in the backscattered signal. The integrated intensities of these two components are comparable, but their spatial characteristies are quite different. In order to study the factors which limit resolution it is therefore necessary to model the events which give rise to the two groups of secondaries.

Although the "cold" field emission gun has been used successfully for both transmission and scanning electron microscopy it requires ultra-high vacuum which is not obtained easily when such a gun is interfaced to a conventional microscope system. Recently, the "thermal" field emission gun (TFEG) in which the emitting tip is held at around 1700°K has been proposed as an alternative electron source for such applications. Under this condition the tip is cleaned continuously, and surface asperities are smoothed, therefore stable operation is possible in a high vacuum. In this paper we report on the build-up characteristics, current stability and brightness of a TFEG which has been interfaced to a JEOL JEM 100B microscope equipped with a scanning attachment. The gun consists of a (111) tungsten emitter set on a rhenium filament, three anodes and a two stage magnetic alignment system. The gun chamber is ion pumped to a pressure in the range 6xl0-8 to 2xl0-9 torr.

The image obtained in a scanning transmission electron microscope (STEM) is related to that obtained, under otherwise identical conditions, in the conventional transmission electron microscope (CTEM) by the principle of reciprocity. This principle is only applicable under rigorously defined mathematical conditions which are difficult to achieve in practice, nevertheless its general validity has been demonstrated experimentally for both amorphous and crystalline materials. The principle of reciprocity states, in part, that if equivalent beam cone angles are utilized then the STEM and CTEM images should be similar. Conversely, for a given set of electron-optical conditions in the STEM mode, the expected form of the images can be deduced from a knowledge of the CTEM image under reciprocally related conditions.

Image simulation, as a means of image interpretation, has been an important component of high resolution transmission electron microscopy for many years. By contrast scanning electron microscopists have relied almost exclusively on simple visual analogies to understand the micrographs that they produce. However with the development of SEMs capable of near atomic levels of resolution, and with the requirement for accurate dimensional metrology of sub-micron features in semiconductor devices using the SEM, a more quantitative approach to image formation and analysis is needed.Image simulation in the SEM encounters several special problems. Firstly, the signal displayed to form the micrograph is not well defined in terms of either the contrast mechanisms which give rise to it, or the energy and origin of the electrons actually collected to produce the signal.

The formation of glassy (amorphous) metallic alloys by rapid quenching from the liquid state was discovered in 1960 (1), and subsequent work revealed that certain metallic glasses are ferromagnetic (2). Recently, transition metal based alloys, which possess Curie temperatures as high as 700°K and saturation magnetizations up to 15,000 Gauss, have been shown to be both mechanically very hard and magnetically quite soft (3). We have applied the SEM technique of Fathers et al. (4) as well as electron transmission Lorentz microscopy to a study of roller quenched (5) metallic glass ribbons of composition: TM0.75P.16X.06A1.03, where TM denotes Fe, Co, Ni, or an alloy of these and X denotes either B, C or Si (6).

Measurement of thermionic electron emission values for pointed LaB6 single‐crystal emitter cathodes has shown that 〈110〉 axial orientations yield emission values ten times higher than 〈100〉 orientations at 1545 °K. Minimum values were obtained for the 〈510〉 directions. These findings seem encouraging in achieving perhaps two orders of magnitude higher emission fluxes as compared to tungsten emitters.

Electron beam irradiation of organic molecular crystals almost invariably leads to a rapid loss of crystallinity, followed by gross mass loss at higher doses. One interesting exception to this sequence of events was described, by Jones, Thomas and Williams who showed that the irradiation of p-terphenyl, at temperatures below 130°K, converted the stable triclinic phase to the characteristic room temperature monoclinic structure. For doses above 0.16 coulomb.cm-2 this transformation was permanent and could not be reversed by thermal cycling. We report here another electron-beam induced transformation, the end product of which has technologically useful properties.3,4,9,10-Perylenetetracarboxylic di-anhydride, C24H8O6 (PTCDA), the structure of which is shown in figure 1, when vapor deposited to a thickness of a few thousand angstroms forms a red transparent organic film. In this form it is stable to relatively high temperatures (300°C) does not appear to degrade after several months of room temperature storage and is electrically insulating.

In a crystalline solid the regular arrangement of the lattice structure influences the interaction of the incident beam with the specimen, leading to changes in both the transmitted and backscattered signals when the angle of incidence of the beam to the specimen is changed. For the simplest case the electron flux inside the specimen can be visualized as the sum of two, standing wave distributions of electrons (Fig. 1). Bloch wave 1 is concentrated mainly between the atom rows and so only interacts weakly with them. It is therefore transmitted well and backscattered weakly. Bloch wave 2 is concentrated on the line of atom centers and is therefore transmitted poorly and backscattered strongly. The ratio of the excitation of wave 1 to wave 2 varies with the angle between the incident beam and the crystal structure.

Electron channeling patterns (ECP) were first found by Coates (1967) while observing a large bulk, single crystal of silicon in a scanning electron microscope. The geometric pattern visible was shown to be produced as a result of the changes in the angle of incidence, between the beam and the specimen surface normal, which occur when the sample is examined at low magnification (Booker, Shaw, Whelan and Hirsch 1967).A conventional electron diffraction pattern consists of an angularly resolved intensity distribution in space which may be directly viewed on a fluorescent screen or recorded on a photographic plate. An ECP, on the other hand, is produced as the result of changes in the signal collected by a suitable electron detector as the incidence angle is varied. If an integrating detector is used, or if the beam traverses the surface at a fixed angle, then no channeling contrast will be observed. The ECP is thus a time resolved electron diffraction effect. It can therefore be related to spatially resolved diffraction phenomena by an application of the concepts of reciprocity (Cowley 1969).

We have applied the scanning electron microscopy (SEM) technique of Fathers et al. to a study of domain structure in metallic glasses of the form TM0.75P0.16X0.06Al0.03, where TM denotes Fe, Co, Ni, or an alloy of these and X denotes either B, C, or Si. The domain structure is dominated by the presence of a spatially nonuniform uniaxial anisotropy which is related to the flow processes induced by quenching the molten alloy. The existence of both a magnetostrictive and a directional ordering source of anisotropy is indicated.

The image produced by the SEM is not an intrinsic property of the object viewed but is also a function of the electron-optical conditions used to generate the image as well as of the chemical and geometric characteristics of the specimen itself. The image is therefore not a simple representation of the specimen, but must be interpreted to yield the desired information. A general purpose method for image interpretation is to perform a detailed contrast simulation for the specified object and to then examine the influence of various factors such the beam energy or signal mode on the form of the image so as to deduce the rules which relate the form of the image to the form of the specimen.In the computations shown here a Monte Carlo simulation is used to model the interaction of the incident beam. The generation of the secondary electron signal is incorporated using the method proposed by Salow, and the escape probability of these secondary electrons is computed using a simple exponential approximation.

A variety of standard specimens is needed in order to systematically investigate the instrumentation, specimen, data reduction and quantitation variables in electron energy-loss spectroscopy (EELS). Pure single element specimens (e.g. various forms of carbon) have received considerable attention to date but certain elements of interest cannot be prepared directly as thin films. Since studies of the first and second row elements in two- or multicomponent systems will be of considerable importance in microanalysis using EELS, there is a need for convenient standards containing these species. For many investigations a standard should contain the desired element, or elements, homogeneously dispersed through a suitable matrix and at an accurately known concentration. These conditions may be met by the technique of implantation.Silicon was chosen as the host lattice since its principal ionization energies, EL23 = 98 eV and Ek = 1843 eV, are well removed from the K-edges of most elements of major interest such as boron (Ek = 188 eV), carbon (Ek = 283 eV), nitrogen (Ek = 400 eV) and oxygen (Ek = 532 eV).

Micro-computers conveniently provide the basis of a powerful data collection, display and storage system for an electron microscope. In the example described here a Vacuum Generators HB501 STEM has been interfaced with an APPLE II micro-computer. The APPLE, with 48k of available core memory and a single mini-diskette system of 256k. byte capacity, was fitted with 16 input channels of analog-to-digital conversion (Interactive Structures AI-Ø2) and 8 output channels of digital-to-analog conversion (Interactive Structures AO-Ø3).Figure (1) shows a block diagram of the system and its interconnections to the microscope. ADC channels Ø to 5 monitor the excitations of the six lenses (two condenser, one objective and three post specimen) of the HB501. These inputs are taken directly from the feedback shunt resistors in the lens power supply and fall directly in the range 0 to +5 volts required for the ADC. Channels 6 to 9 measure the X,Y coordinates and X,Y tilt-coordinates of the specimen stage, by monitoring the voltages derived from the position-sensing potentiometers on the stepping drive motors. These signals in the range ±0.5V are converted to the required 0 to + 5V level by operational amplifiers.

High-resolution images of the surface topography of solid specimens can be obtained using the low-loss technique of Wells. If the specimen is placed inside a lens of the condenser/objective type, then it has been shown that the lens itself can be used to collect and filter the low-loss electrons. Since the probeforming lenses in TEM instruments fitted with scanning attachments are of this type, low-loss imaging should be possible.High-resolution, low-loss images have been obtained in a JEOL JEM 100B fitted with a scanning attachment and a thermal, fieldemission gun. No modifications were made to the instrument, but a wedge-shaped, specimen holder was made to fit the side-entry, goniometer stage. Thus the specimen is oriented initially at a glancing angle of about 30° to the beam direction. The instrument is set up in the conventional manner for STEM operation with all the lenses, including the projector, excited.

High resolution scanning electron microscopy is still a relatively new and unfamiliar concept because in the early days of the SEM it was expected, that secondary electron imaging would be limited to a resolution of between 5 and 10nm at best. Now, however, because of improvements in instrumentation and technique based on advances in the understanding of electron beam interactions with solids current SEMs can demonstrate spatial resolutions below 1nm, rivaling those obtained by transmission instruments.High performance scanning electron microscopes always incorporate two advanced items of instrumentation. Firstly they use field emission guns (FEGs). The high brightness, low energy spread, and small source size of the FEG makes it possible to produce an electron probe of sub-nanometer size which contains sufficient current for secondary electron imaging (i.e 10-12 amps or more) and which can maintain this performance over a wide energy range (3 to 30keV). Secondly, the new high performance instruments place the specimen within a high excitation, immersion, probe forming lens.

Measurements have been made of the functional form of the energy‐differential cross sections for carbon, using transmission electron energy‐loss spectroscopy. It is shown that both the characteristic K edge and the background, prior to the K edge, can be represented accurately by functions of the form AE−r, where E is the energy loss and A and r are constants. The values of the exponent in this expression for the K edge have been compared with calculated values derived from a hydrogenic model and good agreement has been obtained.

The increasing integration of mi­ croelectronics into the submicrometer region for VHSIC and VLSI applica­ tions necessitates the examination of these structures both for linewidth measurement and defect inspection by systems other than the optical micro­ scope. The low beam-voltage scanning electron microscope has been recently employed in this work due to its po­ tentiaIly high spatial resolution and large depth of field. This paper dis­ cusses applications of the scanning electron microscope to microelectronics inspection and metrology in light of the present instrument specifications and capabilities, and relates the scan­ ning electron microscope to the con­ trols required for submicrometer processing.

“Second best no more” was the title of an article written by David C. Joy for Nature Materials. The article was for highlighting a breakthrough made by a team formed between Brookhaven National Laboratory (BNL, USA) and Hitachi High Technologies Corporation (HHT, Japan). Sub-angstrom secondary electron (SE) images were obtained on a Hitachi HD-2700, which is a combined scanning electron microscope (SEM) and scanning transmission electron microscope (STEM) operating at a maximum accelerating voltage of 200 kV. For the first time, single atoms and atomic lattices on crystal surfaces are unambiguously presented in SE images.

Table of Contents 1. Low Voltage Scanning Electron Microscopy In Biology--Historical Preface : The Early Development of the Scanning Electron Microscope to 1965...1 Dennis McMullan 2. LVSEM For Biology...27 James B. Pawley, PhD 3. The Aberration-Corrected SEM...101 David Joy Science and Engineering Research Facility, University of Tennessee 4. Noise and its effects on the Low Voltage SEM...121 David C Joy 5. High-Resolution Low Voltage Field-Emission Scanning Electron Microscopy (HRLVFESEM) Applications for Cell Biology and Specimen Preparation Protocols...146 Heide Schatten, PhD 6. Molecular labeling for Correlative Microscopy: LM, LVSEM, TEM, EF-TEM and HVEM...173 Ralph Albrecht and Daryl Meyer 7. Low kV and video-rate, beam-tilt stereo for viewing live-time experiments in the SEM...209 Alan Boyde 8. CryoSEM of Chemically Fixed Animal Cells...228 Stanley L. Erlandsen 9. High resolution and low voltage SEM of plant cells...249 Guy Cox, Peter Vesk, Teresa Dibbayawan, Tobias I. Baskin and Maret Vesk 10. High Resolution Cryo-Scanning Electron Microscopy of Biological Samples...268 Paul Walther 11. Developments in Instrumentation for Microanalysis in Low Voltage Scanning Electron Microscopy...305 Dale E. Newbury