24.1 Principle

When an accelerated electron strikes a substance, it sometimes ejects the inner shell electron of the atom existing in the substance and creates a vacancy. This phenomenon is called electron beam excitation. At this time, one of the electrons of outer shells falls into the vacancy. This generates the X-ray photon which has the energy equal to the binding energy gap between two shells. This X-ray is called characteristic X-ray, and its energy or wavelength shows a specific value to the element. Figure 24.1 shows an example of these processes. By examining the energy or wavelength of the generated X-rays, it is possible to identify the elements existing in the electron beam irradiation point. Since the amount of generated characteristic X-rays correlates with the concentration of the element, the element composition at the beam irradiation point can be found from the type and amount of X-rays to be detected. Since the electron beam can be focused by using an electromagnetic lens and can be two-dimensionally scanned by electromagnetic deflector, by measuring the signal amount of X-rays synchronously with beam scanning, the two-dimensional element concentration distribution can be obtained in a minute region.

Fig. 24.1
figure 1

Example in which characteristic X-ray is generated as the electron of the L shell drops into the vacancy generated by the incident electron ejecting the electron of the K shell

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24.2 Features

  • Composition analysis is possible for minute region up to about 0.1 μm.

  • A two-dimensional composition distribution can be obtained in a region with a side of several micrometers to the order of centimeters.

  • Heavier elements than Be can be analyzed with high quantitative accuracy.*

  • Trace elements can be analyzed, since the detection limits are on the order of 100 ppm for most elements.*

* Features when using wavelength-dispersive X-ray spectrometer.

24.3 Instrumentation

SEM + EDS, which is an electron microscope equipped with energy-dispersive X-ray spectrometer, is also included in the instruments using EPMA analysis method. But here we explain the instrumentation for the instrument which is aimed performing EPMA analysis, generally called electron probe microanalyzer. Figure 24.2 shows the instrumentation for electron probe microanalyzer. It consists of an electron optical system, a sample stage, more than one wavelength-dispersive X-ray spectrometer (WDS), an optical microscope, a secondary electron detector, a vacuum evacuation system that keeps the surroundings in a vacuum, various control devices, and a data processing device. The electron optical system consists of an electron gun, various lenses, scanning coils, etc., generates and accelerates an electron beam, then controls its current, focuses it, irradiates it on the sample, and scans it in two dimensions. By measuring the amount of secondary electrons in synchronization with beam scanning, you can observe the electron microscope image. The optical microscope is arranged coaxially with the beam, and its focal position is arranged to coincide with the analysis point of the instrument. In addition, the X-ray spectrometer is arranged so that X-rays generated from the analysis point can be spectrally separated appropriately. Therefore, by controlling the sample stage and observing the point of interest on the sample surface with an optical microscope and focusing the image, the center of the image becomes the analysis point, and X-rays can be measured under appropriate conditions.

Fig. 24.2
figure 2

Instrumentation for electron probe microanalyzer

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24.4 Applications

24.4.1 Conduction Failure Analysis of Electrical Appliances

EPMA is useful for migration and electrode corrosion assessment, which is a conduction failure factor in electrical appliances. In the example shown in Fig. 24.3, it can be seen that migration of Cu occurs from the cathode side (−) to the anode side (+). Also, you can see that chlorine ion is deeply involved in this migration.

Fig. 24.3
figure 3

Distribution of Cu and Cl in the region where Cu migration occurred between printed circuit board patterns. These data show that Cl ion is deeply involved in this migration

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