SEM-EDX GUIDE FOR BUYERS

Chapter 4 - Electron sources and high voltage
Tungsten and CeB6 filaments

In addition to classic tungsten technology, table and compact laboratory instruments are also equipped with CeB6 filaments. Almost all major SEM manufacturers use tungsten cathodes, which are consequently not only widely used but also very mature. The filaments are cheap and can be replaced with a few simple steps by the user himself.

CeB6 filaments offer longer life and higher beam intensity, which can be an advantage in low voltage imaging. However, the filaments are much more expensive and usually not to be replaced by the user, which entails considerable maintenance costs.

typical tungsten filament

High voltages up to 30 kV

In order to excite the characteristic X-rays of an element, the 2 to 3-fold electron energy of the respective line is necessary. For example, an excitation voltage of 20-25 kV is needed to excite the Kα line of zinc at 8.6 keV. For the X-ray element analysis by means of EDX therefore the highest possible voltage is ideally a clear advantage up to 30 kV.

The majority of SEM compact devices are limited to a maximum accelerating voltage of 15 kV, forcing the user to confine themselves to the spectral peaks at energies less than 5 keV, where elements often overlap. Therefore, when selecting a suitable system, pay attention to the highest possible voltage in order to operate the EDX analysis optimally.

EDX spectra of a solar cell at 15 kV (blue) and 30 kV (red)

Low voltages below 10 kV

For sensitive samples, setting the accelerating voltage below 10 kV is an essential parameter because the sample is destroyed at high energies. Furthermore, fine structures can be outshone if the acceleration voltage is too high and thus become invisible.

When choosing a SEM device, pay attention to how flexible the high voltage can be set. Some tabletop devices offer adjustment in 1kV steps, others offer only preset levels for 5kV, 10kV and 15kV.

Coxem-SEM-EDX-high-voltage

Surface structures at low voltages

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