Scanning Electron Microscopy and X-Ray Microanalysis

By selecting the operating parameters of the SEM electron gun, lenses, and apertures, the microscopist controls the characteristics of the focused beam that reaches the specimen surface: energy (typically selected in the range 0.1–30 keV), diameter (0.5 nm to 1 μm or larger), beam current (1 pA to 1 μA), and convergence angle (semi-cone angle 0.001–0.05 rad). In a conventional high vacuum SEM (typically with the column and specimen chamber pressures reduced below 10−3 Pa), the residual atom density is so low that the beam electrons are statistically unlikely to encounter any atoms of the residual gas along the flight path from the electron source to the specimen, a distance of approximately 25 cm.

Close inspection of the trajectories in the Monte Carlo simulation of a flat, bulk target of gold at 0° tilt shown in Fig. 2.1 reveals that a significant fraction of the incident beam electrons undergo sufficient scattering events to completely reverse their initial direction of travel into the specimen, causing these electrons to return to the entrance surface and exit the specimen. These beam electrons that escape from the specimen are referred to as “backscattered electrons” (BSE) and constitute an important SEM imaging signal rich in information on specimen characteristics. The BSE signal can convey information on the specimen composition, topography, mass thickness, and crystallography. This module describes the properties of backscattered electrons and how those properties are modified by specimen characteristics to produce useful information in SEM images.

Secondary electrons (SE) are created when inelastic scattering of the beam electrons ejects weakly bound valence electrons (in the case of ionically or covalently bonded materials) or conduction band electrons (in the case of metals), which have binding energies of ~ 1–15 eV to the parent atom(s). Secondary electrons are quantified by the parameter δ, which is the ratio of secondary electrons emitted from the specimen, N_SE, to the number of incident beam (primary) electrons, N_B:δ=NSE/NB$$ \delta ={N}_{SE}/{N}_B $$

Energetic beam electrons stimulate the atoms of the specimen to emit “characteristic” X-ray photons with sharply defined energies that are specific to each atom species. The critical condition for generating characteristic X-rays is that the energy of the beam electron must exceed the electron binding energy, the critical ionization energy E_c, for the particular atom species and the K-, L-, M-, and/or N- atomic shell(s). For efficient excitation, the incident beam energy should be at least twice the critical excitation energy, E₀ > 2 E_c. Characteristic X-rays can be used to identify and quantify the elements present within the interaction volume. Simultaneously, beam electrons generate bremsstrahlung, or braking radiation, which creates a continuous X-ray spectrum, the “X-ray continuum,” whose energies fill the range from the practical measurement threshold of 50 eV to the incident beam energy, E₀. This continuous X-ray spectrum forms a spectral background beneath the characteristic X-rays which impacts accurate measurement of the characteristic X-rays and determines a finite concentration limit of detection. X-rays are generated throughout a large fraction of the electron interaction volume. The spatial resolution, lateral and in-depth, of electron-excited X-ray microanalysis can be roughly estimated with a modified Kanaya–Okayama range equation or much more completely described with Monte Carlo electron trajectory simulation. Because of their generation over a range of depth, X-rays must propagate through the specimen to reach the surface and are subject to photoelectric absorption which reduces the intensity at all photon energies, but particularly at low energies.

Why worry about electron optical parameters?

After leaving the electron source, the beam follows the central (optic) axis of the lens system and is sequentially defined by apertures and focused by the magnetic and/or electrostatic fields of the lens system. Within the final (objective) lens, a system of scan coils acts to displace the beam off the optic axis so that it can be addressed to a location on the specimen, as illustrated schematically for single deflection scanning in Fig. 6.1. At any particular time, there is only one ray path (solid line) through the scanning system and the beam reaches only one location on the specimen, for example, position 3 in Fig. 6.1. The SEM image is a geometric construction created under computer control by addressing the focused beam to a sequence of discrete x-y locations on the specimen and measuring the effect of the interaction of the beam with the specimen at each location. For a single gray-scale SEM image, this interaction could be the output from a single electron detector, such as the Everhart–Thornley detector. It is also possible to measure the output from more than one detector simultaneously while the beam is addressed to a single x-y location. When this is done, multiple gray-scale SEM images are built up at the same time during the scan. It is essential to realize that even when these multiple signals are being collected simultaneously and multiple images are produced, only a single scan is needed; the parallel nature of the acquisition arises from parallel detection, not parallel scanning. Note that no “true image” actually exists within the SEM in the same sense as the image created in a light optical microscope, where actual ray paths extend from each point on the specimen through the lens system to a corresponding point on the image recording medium, whether that is the eye of a human viewer or the positionally sensitive detector of a digital camera. In the SEM, at each location sampled by the incident electron beam, each signal is measured with an appropriate detector and the analog measurement is converted to an equivalent digital value (using an analog-to-digital converter, ADC). The beam x-y location and the intensity(ies) I _j of the signal(s) of interest generate a digital stream of data packets (x, y, I _j ), where j represent the various signals available: backscattered electrons (BSE), secondary electrons (SE), absorbed current, x-rays, cathodoluminescence, etc.

Information in SEM images about specimen properties is conveyed when contrast in the backscattered and/or secondary electron signals is created by differences in the interaction of the beam electrons between a specimen feature and its surroundings. The resulting differences in the backscattered and secondary electron signals (S) convey information about specimen properties through a variety of contrast mechanisms. Contrast (C_tr) is defined asCtr=(Smax−Smin)/Smax $$ {C}_{\mathrm{tr}}=\left({S}_{\mathrm{max}}-{S}_{\mathrm{min}}\right)/{S}_{\mathrm{max}} $$ where is S_max is the larger of the signals. By this definition, 0 ≤ C_tr ≤ 1.

The detection in SEM images of specimen features such as compositional differences, topography (shape, inclination, edges, etc.), and physical differences (crystal orientation, magnetic fields, electrical fields, etc.), depends on satisfying two criteria: (1) establishing the minimum conditions necessary to ensure that the contrast created by the beam–specimen interaction responding to differences in specimen features is statistically significant in the imaging signal (backscattered electrons [BSE], secondary electrons [SE], or a combination) compared to the inevitable random signal fluctuations (noise); and (2) applying appropriate signal processing and digital image processing to render the contrast information that exists in the signal visible to the observer viewing the final image display.

SEM images are subject to defects that can arise from a variety of mechanisms, including charging, radiation damage, contamination, and moiré fringe effects, among others. Image defects are very dependent on the specific nature of the specimen, and often they are anecdotal, experienced by not reported in the SEM literature. The examples described below are not a complete catalog but are presented to alert the microscopist to the possibility of such image defects so as to avoid interpreting artifact as fact.

“High resolution SEM imaging” refers to the capability of discerning fine-scale spatial features of a specimen. Such features may be free-standing objects or structures embedded in a matrix. The definition of “fine-scale” depends on the application, which may involve sub-nanometer features in the most extreme cases. The most important factor determining the limit of spatial resolution is the footprint of the incident beam as it enters the specimen. Depending on the level of performance of the electron optics, the limiting beam diameter can be as small as 1 nm or even finer. However, the ultimate resolution performance is likely to be substantially poorer than the beam footprint and will be determined by one or more of several additional factors: (1) delocalization of the imaging signal, which consists of secondary electrons and/or backscattered electrons, due to the physics of the beam electron ̶ specimen interactions; (2) constraints imposed on the beam size needed to satisfy the Threshold Equation to establish the visibility for the contrast produced by the features of interest; (3) mechanical stability of the SEM; (4) mechanical stability of the specimen mounting; (5) the vacuum environment and specimen cleanliness necessary to avoid contamination of the specimen; (6) degradation of the specimen due to radiation damage; and (7) stray electromagnetic fields in the SEM environment. Recognizing these factors and minimizing or eliminating their impact is critical to achieving optimum high resolution imaging performance. Because achieving satisfactory high resolution SEM often involves operating at the performance limit of the instrument as well as the technique, the experience may vary from one specimen type to another, with different limiting factors manifesting themselves in different situations. Most importantly, because of the limitations on feature visibility imposed by the Threshold Current/Contrast Equation, for a given choice of operating conditions, there will always be a level of feature contrast below which specimen features will not be visible. Thus, there is always a possible “now you see it, now you don’t” experience lurking when we seek to operate at the limit of the SEM performance envelope.

The incident beam energy is one of the most useful parameters over which the microscopist has control because it determines the lateral and depth sampling of the specimen properties by the critical imaging signals. The Kanaya–Okayama electron range varies strongly with the incident beam energy:RK−O(nm)=(27.6A/Z0.89ρ)E01.67$$ {R}_{K-O}(nm)=\left(27.6\ A/{Z}^{0.89}\rho \right){E_0}^{1.67} $$where A is the atomic weight (g/mol), Z is the atomic number, ρ is the density (g/cm3), and E₀ (keV) is the incident beam energy, which is shown graphically in Fig. 11.1a–c.

The conventional SEM must operate with a pressure in the sample chamber below ~10−4 Pa (~10−6 torr), a condition determined by the need to satisfy four key instrumental operating conditions:

Software is an essential tool for the scanning electron microscopist and X-ray microanalyst (SEMXM). In the past, software was an important optional means of augmenting the electron microscope and X-ray spectrometer, permitting powerful additional analysis and enabling new characterization methods that were not possible with bare instrumentation. Today, however, it is simply not possible to function as an SEMXM practitioner without using at least a minimal amount of software. A graphical user interface (GUI) is an integral part of how the operator controls the hardware on most modern microscopes, and in some cases it is the only interface. Even many seemingly analog controls such as focus knobs, magnification knobs, or stigmators are actually digital interfaces mounted on hand-panel controllers that connect to the microscope control computer via a USB interface.

A conducting or semiconducting specimen must maintain good contact with electrical ground to dissipate the injected beam current. Without such an electrical path, even a highly conducting specimen such as a metal will show charging artifacts, in the extreme case acting as an electron mirror and reflecting the beam off the specimen. A typical strategy is to use an adhesive such as double-sided conducting tape to both grip the specimen to a support, for example, a stub or a planchet, as well as to make the necessary electrical path connection. Note that some adhesives may only be suitable for low magnification (scanned field dimensions greater than 100 × 100 μm, nominally less than 1,000× magnification) and intermediate magnification (scanned field dimensions between 100 μm x 100 μm, nominally less than 1,000X magnification and 10 μm × 10 μm, nominally less than 10,000× magnification) due to dimensional changes which may occur as the adhesive outgases in the SEM leading to image instability such as drift. Good practice is to adequately outgas the mounted specimen in the SEM airlock or a separate vacuum system to minimize contamination in the SEM as well as to minimize further dimensional shrinkage. Note that some adhesive media are also subject to dimensional change due to electron radiation damage during imaging, which can also lead to image drift.

When studying the topographic features of a specimen, the microscopist has several useful software tools available. Qualitative stereomicroscopy provides a composite view from two images of the same area, prepared with different tilts relative to the optic axis, that gives a visual sensation of the specimen topography, as shown for a fractured galena crystal using the anaglyph method in Fig. 15.1 (software: Anaglyph Maker). The “3D Viewer” plugin in ImageJ-Fiji can take the same members of the stereo pair and render the three-dimensional surface, as shown in Fig. 15.2, which can then be rotated to “view” the surface from different orientations (Fig. 15.3).

As illustrated in Fig. 16.1, the physical basis of energy dispersive X-ray spectrometry (EDS) with a semiconductor detector begins with photoelectric absorption of an X-ray photon in the active volume of the semiconductor (Si). The entire energy of the photon is transferred to a bound inner shell atomic electron, which is ejected with kinetic energy equal to the photon energy minus the shell ionization energy (binding energy), 1.838 keV for the Si K-shell and 0.098 keV for the Si L-shell. The ejected photoelectron undergoes inelastic scattering within the Si crystal. One of the consequences of the energy loss is the promotion of bound outer shell valence electrons to the conduction band of the semiconductor, leaving behind positively charged “holes” in the valence band. In the conduction band, the free electrons can move in response to a potential applied between the entrance surface electrode and the back surface electrode across the thickness of the Si crystal, while the positive holes in the conduction band drift in the opposite direction, resulting in the collection of electrons at the anode on the back surface of the EDS detector. This charge generation process requires approximately 3.6 eV per electron hole pair, so that the number of charge carriers is proportional to the original photon energy, E_p:n=Ep/3.6eV $$ n={E}_p/3.6\ eV $$ For a Mn K-L₃ photon with an energy of 5.895 keV, approximately 1638 electron–hole pairs are created, comprising a charge of 2.6 × 10−16 coulombs. Because the detector can respond to any photon energy from a threshold of approximately 50 eV to 30 keV or more, the process has been named “energy dispersive,” although in the spectrometry sense there is no actual dispersion such as occurs in a diffraction element spectrometer.

Reading about a new subject is good but there is nothing like doing to reinforce understanding. With this in mind, the authors of this textbook have designed a number of practical exercises that reinforce the book’s subject matter. Some of these exercises can be performed with software you have available to you—either instrument vendor software or a spreadsheet like MS Excel or LibreOffice/OpenOffice Calc. Other exercises require functionality which may not be present in all instrument vendor’s software. Regardless, it is much easier to explain an exercise when everyone is working with the same tools.

Before attempting automatic or manual peak identification, it is critical that the EDS system be properly calibrated to ensure that accurate energy values are measured for the characteristic X-ray peaks. Follow the vendor’s recommended procedure to rigorously establish the calibration. The calibration procedure typically involves measuring a known material such as copper that provides characteristic X-ray peaks at low photon energy (e.g., Cu L₃-M₅ at 0.928 keV) and at high photon energy (Cu K-L₃ at 8.040 keV). Alternatively, a composite aluminum-copper target (e.g., a copper penny partially wrapped in aluminum foil and continuously scanned so as to excite both Al and Cu) can be used to provide the Al K-L₃ (1.487 keV) as the low energy peak and Cu K-L₃ for the high energy peak. After calibration, peaks occurring within this energy range (e.g., Ti K-L₃ at 4.508 keV and Fe K-L₃ at 6.400 keV) should be measured to confirm linearity. A well-calibrated EDS should produce measured photon energies within ±2.5 eV of the ideal value. Low photon energy peaks below 1 keV photon energy should also be measured, for example, O K (e.g., from MgO) and C K. For some EDS systems, non-linearity may be encountered in the low photon energy range. Figure 18.1 shows an EDS spectrum for CaCO₃ in which the O K peak at 0.523 keV is found at the correct energy, but the C K peak at 0.282 keV shows a significant deviation below the correct energy due to non-linear response in this range.

A k-ratio is the ratio of a pair of characteristic X-ray line intensities, I, measured under similar experimental conditions for the unknown (unk) and standard (std):k=Iunk/Istd $$ k={I}_{unk}/{I}_{std} $$ The measured intensities can be associated with a single characteristic X-ray line (as is typically the case for wavelength spectrometers) or associated with a family of characteristic X-ray lines (as is typically the case for energy dispersive spectrometers.) The numerator of the k-ratio is typically the intensity measured from an unknown sample and the denominator is typically the intensity measured from a standard material—a material of known composition.

This module discusses the procedure used to perform a rigorous quantitative elemental microanalysis by SEM/EDS following the k-ratio/matrix correction protocol using the NIST DTSA-II software engine for bulk specimens. Bulk specimens have dimensions that are sufficiently large to contain the full range of the direct electron-excited X-ray production (typically 0.5–10 μm) as well as the range of secondary X-ray fluorescence induced by the propagation of the characteristic and continuum X-rays (typically 10–100 μm).

«Trace analysis” refers to the measurement of constituents presents at low fractional levels. For SEM/EDS the following arbitrary but practical definitions have been chosen to designate various constituent classes according to these mass concentration (C) ranges:

The incident beam energy, E₀, is the parameter that determines which characteristic X-rays can be excited: the beam energy must exceed the critical excitation energy, E_c, for an atomic shell to initiate ionization and subsequent emission of characteristic X-rays. This dependence is parameterized with the “overvoltage” U_0, defined asU0=E0/Ec $$ {U}_0={E}_0/{E}_c $$ U₀ must exceed unity for X-ray emission. The intensity, I_ch, of characteristic X-ray generation follows an exponential relation:Ich=iBa(U0−1)n $$ {I}_{ch}={i}_Ba{\left({U}_0-1\right)}^n $$ where i_B is the beam current, a and n are constants, with 1.5 ≤ n ≤ 2.

There are two “zero-th level” assumptions that underpin the basis for quantitative electron-excited X-ray microanalysis:

SEM images that show the spatial distribution of the elemental constituents of a specimen («elemental maps») can be created by using the characteristic X-ray intensity measured for each element with the energy dispersive X-ray spectrometer (EDS) to define the gray level (or color value) at each picture element (pixel) of the scan. Elemental maps based on X-ray intensity provide qualitative information on spatial distributions of elements. Compositional mapping, in which a full EDS spectrum is recorded at each pixel («X-ray Spectrum Imaging» or XSI) and processed with peak fitting, k-ratio standardization, and matrix corrections, provides a quantitative basis for comparing maps of different elements in the same region, or for the same element from different regions

While X-ray analysis can be performed in the Variable Pressure Scanning Electron Microscope (VPSEM), it is not possible to perform uncompromised electron-excited X-ray microanalysis. The measured EDS spectrum is inevitably degraded by the effects of electron scattering with the atoms of the environmental gas in the specimen chamber before the beam reaches the specimen. The spectrum is always a composite of X-rays generated by the unscattered electrons that remain in the focused beam and strike the intended target mixed with X-rays generated by the gas-scattered electrons that land elsewhere, micrometers to millimeters from the microscopic target of interest.

What you will need to prepare.

Background: As part of a study into the in-service failure of the bearing surface of a large water pump, characterization was requested of the hard-facing alloy, which was observed to have separated from the stainless steel substrate, causing the failure.

Cathodoluminescence (CL) is the emission of low energy photons in the range from approximately 1 eV to 5 eV (infrared, visible, and ultraviolet light) as a result of inelastic scattering of the high energy beam electrons (Fig. 28.1). Materials that can emit such photons are insulators or semiconductors which have an electronic structure with a filled valence band of allowed energy states that is separated by a gap of disallowed energy states from the empty conduction band, as shown schematically in Fig. 28.2a. Inelastic scattering of the beam electron can transfer energy to a weakly bound valence electron promoting it to the empty conduction band, leaving a positively charged “hole” in the conduction band. When a free electron and a positive hole are attracted and recombine, the energy difference is expressed as a photon, as illustrated in Fig. 28.2b. Because the possible energy transitions and the resulting photon emission are defined by the intrinsic properties of a high purity material, such as the band-gap energy but also including energy levels that result from physical defects such as lattice vacancies, rather than by the influence of impurity atoms, this type of CL is referred to as “intrinsic CL emission.” Since the valence electron promoted to the conduction band can receive a range of possible kinetic energies depending on the details of the initial scattering, the photons emitted during free electron–hole recombination can have a range of energies, resulting in broad band CL photon emission. Because of the great mismatch in the velocity of the high energy (keV) beam electron and the low energy (eV) valence electron, this is not an efficient process and in general CL emission is very weak. The ionization cross section is maximized for electrons with three to five times the binding energy of the valence electrons, so that most efficient energy transfer to initiate CL emission occurs from the more energetic slow SE (>10 eV) and the fast SE (hundreds of eV) also created by inelastic scattering of the primary electron.

While amorphous substances such as glass are encountered both in natural and artificial materials, most inorganic materials are found to be crystalline on some scale, ranging from sub-nanometer to centimeter or larger. A crystal consists of a regular arrangement of atoms, the so-called «unit cell,» which is repeated in a two- or three-dimensional pattern. In the previous discussion of electron beam–specimen interactions, the crystal structure of the target was not considered as a variable in the electron range equation or in the Monte Carlo electron trajectory simulation. To a first order, the crystal structure does not have a strong effect on the electron–specimen interactions. However, through the phenomenon of channeling of charged particles through the crystal lattice, crystal orientation can cause small perturbations in the total electron backscattering coefficient that can be utilized to image crystallographic microstructure through the mechanism designated «electron channeling contrast,» also referred to as «orientation contrast» (Newbury et al. 1986). The characteristics of a crystal (e.g., interplanar angles and spacings) and its relative orientation can be determined through diffraction of the high-energy backscattered electrons (BSE) to form «electron backscatter diffraction patterns (EBSD).

The use of focused ion beams (FIB) in the field of electron microscopy for the preparation of site specific samples and for imaging has become very common. Site specific sample preparation of cross-section samples is probably the most common use of the focused ion beam tools, although there are uses for imaging with secondary electrons produced by the ion beam. These tools are generally referred to as FIB tools, but this name covers a large range of actual tools. There are single beam FIB tools which consist of the FIB column on a chamber and also the FIB/SEM platforms that include both a FIB column for sample preparation and an SEM column for observing the sample during preparation and for analyzing the sample post-preparation using all of the imaging modalities and analytical tools available on a standard SEM column. A vast majority of the FIB tools presently in use are equipped with liquid metal ion sources (LMIS) and the most common ion species used is Ga. Recent developments have produced plasma sources for high current ion beams. The gas field ion source (GFIS) is discussed in module 31 on helium ion microscopy in this book.

Electron beams have made possible the development of the versatile, high performance electron microscopes described in the earlier chapters of this book. Techniques for the generation and application of electron beams are now well documented and understood, and a wide variety of images and data can be produced using readily available instruments. While the scanning electron microscope (SEM) is the most widely used tool for high performance imaging and microanalysis, it is not the only option and may not even always be the best instrument to choose to solve a particular problem. In this chapter we will discuss how, by replacing the beam of electrons with a beam of ions, it is possible to produce a high performance microscope which resembles an SEM in many respects and shares some of its capabilities but which also offers additional and important modes of operation.