Scanning electron microscopy (SEM) – spectroscopy

About this technique

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There exist a number of spectroscopic techniques available in the scanning electron microscope (SEM). In this section we deal exclusively with X-ray analysis techniques, namely energy dispersive X-ray spectroscopy (EDS or EDX) and wavelength dispersive X-ray spectroscopy (WDS or WDX). Both of these techniques involve the measurement of X-rays emitted from the sample due to the interaction between the incident electron beam and the sample surface. The volume from which the X-rays are emitted, and hence the spatial resolution of both techniques, is strongly dependent on both the acceleration voltage of the electron beam and the atomic number of the sample – values can range from a few 10s of nm to about 10 mm. In both techniques the characteristic energy (or wavelength, which is inversely proportional to the energy) is measured and compared to the background X-ray signal, allowing the abundance of individual elements to be measured. All elements from beryllium to uranium can be measured using EDS and WDS, although the measurement accuracy is dependent on the element in question.

The best results for EDS and WDS require some specimen preparation; ideally samples should be ground and polished (and if necessary, mounted beforehand) so that X-rays are collected from a flat surface. Insulating samples should be coated with a conductive coat, such as carbon, beforehand. Metal coating materials (such as Au or Au-Pd) are not ideal due to the multiple X-rays that they produce and the increased absorption of both incident electrons and any X-rays generated from the sample.

Energy dispersive X-ray spectroscopy

Energy dispersive X-ray spectroscopy (EDS or EDX) is a very common analytical accessory to an SEM. The EDS detector measures the energy of individual X-rays that are emitted from the sample, and the software plots these energies in a histogram or “spectrum” (usually with the X-ray energy on the x-axis, and the frequency or “counts” on the y-axis). The spectrum is made up of individual peaks corresponding to characteristic X-ray energies, superimposed on a background signal. By measuring the number of X-ray counts within each peak, removing the signal due to background radiation, and then correcting the results for effects due to atomic number, X-ray absorption and secondary fluorescence, the system can measure the abundance of individual elements to a relative accuracy of about ±1%. It is a parallel spectroscopy technique, meaning the whole energy range of X-rays can be measured simultaneously: a single spectrum can be collected in a matter of a few seconds using the latest silicon drift detector (SDD) technology.

EDS has several drawbacks, notably the poorer accuracy of quantitative measurements of light elements (in the range from Be to F), limitations in the detection of trace elements (realistically the element concentration should be greater than 0.1 wt%) and the differentiation between X-rays with similar energies. The energy resolution of EDS varies with the X-ray energy, but it is typically in the order of 50–150 eV. This means that elements with X-ray energies differing by less than 50 eV can be difficult to measure together (such as Mo and S, or W and Si). The main advantage of EDS is its ease of use and speed. High-speed X-ray measurements make the technique perfect for element mapping, in which the qualitative distribution of elements across the surface of a sample is mapped and plotted in colour-coded images. Sometimes EDS spectra are stored at every pixel in the map, allowing offline qualitative and quantitative measurements to be performed at a later date. This is known as spectral imaging, spectral mapping or position tagged spectroscopy (PTS).

Additional applications of EDS include the combination of fast X-ray measurements with image processing techniques. This approach allows the rapid detection and characterization of micrometre-scale particles and features in centimetre-scale samples, and is used extensively to characterise non-metallic inclusions in steels, gun shot residue (GSR) particles in forensic science and ore-mineral samples in the mining industry.

Wavelength dispersive spectroscopy

Wavelength dispersive spectroscopy (WDS or WDX) is a less common technique to measure the X-ray energies. Although WDS detectors can be fitted to an SEM, they are more commonly fitted to an electron probe microanalyser (EPMA), and the technique is dealt with in more detail in the EPMA section. A WDS detector measures the wavelength of the X-rays by using diffracting crystals coupled with X-ray detectors. The wavelengths can be measured with great accuracy, equivalent to an energy resolution of between 2 and 50 eV (i.e. an order of magnitude better than EDS), and the sensitivity of the technique can be as low as 10ppm for some elements, although routinely it is about 100 ppm (or 0.01wt% - once again about an order of magnitude better than EDS). The main problem with WDS is that it is a serial technique – each detector can only measure the X-rays at a particular wavelength (or energy) at a given time, and so many successive measurements are necessary in order to measure the overall composition of a sample. This is why an EPMA is usually equipped with 4–5 individual WDS detectors, whilst WDS systems on SEMs are typically integrated with EDS: the EDS system is used to measure major elements, whereas the WDS system measures trace elements or elements with overlapping X-ray peaks.

WDS is also a more difficult technique to use for several reasons: careful calibration of the system using known reference materials is necessary, detailed measurement of the background X-ray signal is critical, and in general higher electron beam currents are needed in order to generate sufficient X-rays (and the precise beam current needs to be measured regularly). This last factor makes WDS measurements on beam sensitive samples very difficult and so WDS is most commonly applied to measurements of metals, ceramics and geological materials.

The following table summarises the main differences between EDS and WDS:

	EDS	WDS
Element range	Be-U	Be-U
Detection levels	>0.1 wt%	>0.01 wt%
Spectral resolution	50–150 eV	2–50 eV
Light element measurement	Moderate	Good
Speed	Fast	Slow
Ease of use	Easy	More difficult