EDAX Phoenix Training Course - X-ray Signal Generation - page 1 X-RAY SIGNAL GENERATION
Signal Origin
The interaction of the electron beam with specimen produces a variety of signals, but the most useful to electron microscopists are these: secondary electrons (SE), backscattered electrons (BSE) and x rays. The SE signal is the most commonly used imaging mode and derives its contrast primarily from the topography of the sample. For the most part, areas facing the detector tend to be slightly brighter than the areas facing away from the SE detector, and holes or depressions tend to be very dark while edges and highly tilted surfaces are bright. These electrons are of a very low energy and very easily influenced by voltage fields.
The BSE signal is caused by the elastic collision of a primary beam electron with a nucleus within the sample. Because these collisions are more likely when the nuclei are large (i.e. when the atomic number is large), the BSE signal is said to display atomic number contrast or “phase” contrast. Higher atomic number phases produce more backscattering and are correspondingly brighter when viewed with the BSE detector. X-ray signals are typically produced when a primary beam electron causes the ejection of an inner shell electron from the sample. An outer shell electron takes its place but gives off an x ray whose energy can be related to its nuclear mass and the difference in energies of the electron orbitals involved. The Kα x ray results from a K shell electron being ejected and an L shell electron moving into its position. A Kβ x ray occurs when an M shell electron moves to the K shell. The Kβ will always have a slightly higher energy than the Kα and is always much smaller. Similarly, an Lα x ray results from an M shell electron moving to the L shell to fill a vacancy (see Figure below). The occurrence of an Lβ x ray means that an N shell electron made the transition from the N shell to the L shell. The Lβ is always smaller and at a slightly higher energy than the Lα. The L-shell x rays are always found at lower energies than the K lines. Because the structure of the electron orbitals is considerably more complex than is shown below, there are actually many more L-shell x-ray lines that can be present (it is not uncommon to see as many as 5 or 6). M-shell x-ray peaks, if present will always be at lower energies than either the L or K series.
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In energy-dispersive spectroscopy (EDS), the x rays are arranged in a spectrum by their energy and (most commonly) from low atomic number (low energy) to high atomic energy (higher energy). Typically, the energies from 0 to 10 keV will be displayed and will allow the user to view: the K-lines from Be (Z = 4) to Ga (Z = 31); L-lines Ca (Z = 20) to Au (Z = 79), and M-lines from Nb (Z = 41) to the highest occurring atomic numbers. From the interpretation of the x-ray signal, we derive qualitative and quantitative information about the chemical composition of the sample at the microscopic scale.
Spatial Resolution
The spatial resolution of these signals are significantly different from each other. The figure below is a representation of the depth of penetration of the electron beam into a sample. No scale has been placed on this image, but the depth of penetration increases as the accelerating voltage of the primary beam is increased. It will also be deeper when the sample composition is of a lower density and/or is of a relatively low average atomic number. All three of the signals discussed above are produced throughout this interaction volume provided the beam electrons still have enough energy to generate it. However, some electrons or x rays may be of lesser energy and generated at a considerable depth in the sample. Thus, they may be absorbed and not generate a signal that can escape from the sample.
The SE signal is one that is readily absorbed and therefore we are only able to detect the SE signal that originates relatively close to the surface (i.e. less than 10 nm). The BSE signal is of higher energy and is able to escape from a more moderate depth within the sample as shown. The x-ray signal can escape from a greater depth, although the x-ray signal absorption is actually variable depending upon its energy. For example, oxygen is of relatively low energy and can only escape from the near-surface region of the sample, while iron is of significantly higher energy and would escape from a greater depth. In quantitative x-ray analysis, it is possible to compensate for these effects with the absorption correction.
EDAX Phoenix Training Course - X-ray Signal Generation - page 3
Although the discussion thus far has only mentioned the depths from which the signal can emerge, the width of the signal is proportional to its depth and provides an estimate of the signal resolution. Because the SE signal that is generated at even relatively shallow depths is absorbed, its resolution depends primarily on the position of the entering electron beam which has a spread related to the electron probe diameter or the spot size of the electron beam. The x-ray signal can emerge from greater depths (especially high-energy x rays) and the lateral spread of the primary beam electron can be quite large relative to the beam diameter. The only effective way to improve the resolution of these signals is to decrease the accelerating voltage which will decrease the beam penetration. The BSE signal is in between these two extremes and its resolution can be improved by decreasing the spot size to some extent, but the relationship between spot size and resolution is not as direct for the BSE signal as for the SE signal. The resolution of the BSE signal can also be improved by lowering the accelerating voltage, although this usually means having to increase the gain of the BSE detector and may result in a degradation of the signal-to-noise ratio of the image.
Directionality of Signals
All of the signals that emerge from the sample can be considered to be directional to at least some extent. A directional signal can be recognized in a photomicrograph of a sample that displays topography because there will be a very harsh contrast such that surfaces that face the detector will be bright and surfaces that face away from the detector will be dark. If the trajectory of the signal can be altered to favor detection, or if a symmetric array of detectors is employed, the effect of directionality is minimized. The trajectories of the SE signal are influenced by a positive voltage on a wire mesh network in front of the detector which attracts the SE from the sample, even from surfaces that face away from the detector. The BSE detector is typically arranged in an array such that they collect signals from a large, symmetrically arranged area. Some BSE detectors consist of two or more segments and the appearance of illumination direction in the imaged area changes drastically depending on which detector or segment is used for imaging. When all segments are selected, the result is a balanced, symmetric image that does not show an apparent directionality in its illumination.
The x-ray signal is effectively the most directional of all the signals because there is only one detector and it is usually at a 35 degree angle to the surface of the sample. There is certainly no simple way to influence the trajectory of x rays to increase the efficiency
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of the detector. As a result, if one is trying to collect the x-ray signal from a surface that slopes away from the detector, the x-ray count rate will be greatly diminished. If a topographic high area is between the imaged area and the x-ray path to the detector, there will also be few detected x rays. The effects of directionality for the x-ray signal are greatly diminished when working with polished samples rather than samples with a large topographic variation.
The Analysis of Rough Surfaces or Particles
There are difficulties associated with trying to analyze samples with anything other than a smooth surface. The figure on the next page shows a cross-sectional configuration with a detector at right and a small sample or portion of a sample at the left. The interaction volume is shown when the beam is at three different locations (‘A’, ‘B’ and ‘C’) and the shaded region represents the area from which a low-energy signal may escape from the sample without being absorbed. The arrows represent the flux of these low-energy signals which will be highest at ‘A’ and at ‘C’ but relatively low at ‘B’.
If the low-energy signal is the secondary electron signal and the detector at the right is the secondary electron detector (with a positive grid voltage), then the edges near ‘A’ and ‘C’ will appear brighter than the center of the sample at ‘B’. Alternatively, if the low- energy signal is some relatively low energy x ray (such as aluminum in a nickel alloy, or oxygen in a mineral, or the copper ‘L’ x-ray signal as compared to the copper ‘K’ x rays), and the detector is the EDS detector, then the height of the low energy peak would be highest at ‘C’ and lowest at ‘A’. Note that even though there would be a high flux of low-energy x rays leaving the sample at ‘A’, that these will not be detected because there is no detector positioned so as to intercept the signal.
Another difficulty in analyzing particles or rough surfaces is when the surface of the particle has a slope which is not parallel to the surface of the stage. In the figure shown above, the upper surface is shown parallel to the stage and our consideration was really centered on what might be regarded as “edge” effects. However, when the sample surface is inclined toward the detector at a greater angle than the stage surface, then the take-off angle will be greater than that of a parallel surface. If our sample consists of a single, non-parallel sloping surface, then its take-off angle could be determined and an accurate analysis performed, provided that we have some way of knowing the local surface tilt of the sample. If the sample is rough and consists of many surfaces of variable orientation, then it is unlikely that a reliable analysis can be performed.