Tabletop SEM units tend to be restricted to 5, 10, 15 kV Accelerating Voltage steps. Full-size SEMs offer Accelerating Voltage range from 0.3 kV to 30 kV in order to allow users to choose the right conditions for different samples (bio, polymers, metals, ceramics etc.)
Operating the SEM at low accelerating voltages (below 5 kV) is important in the observation of non-conductive charging samples, imaging of beam-sensitive samples and obtaining more surface information from conductive as well as non-conductive specimens.
Low energy of 1 or 2 kV could help investigate organic contamination on various substrates. Thin layers of organic contamination are electron transparent to higher voltages and the SEM would not be able to see them at high kV.
Some materials may be too sensitive to 15 kV energy but have low electron yield at 10 kV and other materials may even be sensitive to 10 kV and offer very low electron yield at 5 kV. In such cases, a voltage in between (e.g. 8 or 12 kV) is useful to balance between beam damage and signal to noise ratio.
Tabletop SEM needs to offer this flexibility instead of being restricted to 5, 10, 15, 20 kV steps.
Why 20 kV is not enough
For decades, high Accelerating Voltages ranging from 15 kV to 30 kV have been used by Electron Microscopists for imaging and analysing materials because of the nm scale image resolution and the ability to excite the renowned K-lines in the EDS atomic spectra for the identification and accurate quantification of elements. This is critical for samples where materials with similar elemental compositions and/or containing elements with overlapping EDS energy peaks that can be misinterpreted due to mis-identification of elements and/or inaccurate quantification.
For example, in the EDS spectrum below, L-line of Zr overlaps with K-line of P at ~2 kV. It would be difficult to know which element was causing a peak near 2 kV or whether both elements were contributing to the peak (and with what ratio). With the capability of running EDS analysis above 15kV, user would be able to see whether a peak shows up at ~15.75 kV, which is the K-line of Zr. Quantitative analysis would reveal percentage of P hidden in the Zr L-line peak based on intensity of the Zr K-line peak at ~15.75 kV.
A common problem and a question on ResearchGate
Q: Abdelaziem, Ali. (2015). Can anyone help me identify the reason for disappearance of strontium element from lanthanum strontium manganese oxide thin film in EDX analysis?
A: Dusevich, Vladimir. (2015). Too thin film, too short acquisition time, too weak Sr peaks to detect. You can see that even relatively bigger La M-peaks were not identified on your film (around I keV). And your EDS from film was taken at too low voltage to detect Sr K-peaks (around 16 keV). I’d repeat EDS on film a bit differently.
- Use specimen tilt, at least 60 degrees, toward EDS detector (to increase effective film thickness).
- Make spectra acquisition at much longer time and at two different voltages:
- 5 kV – better for detection of low-energy Sr L-peaks at around 2 Kev.
- 25-30 kV – better for detection of K-peaks at about 16 kV.
Experts at ElementPi LLC in the USA did a comparison of 15kV and 30kV EDS analysis.
Their comments are mentioned verbatim below:
“The majority of tabletop SEM units are restricted to 15 kV maximum accelerating voltage for the electron beam. This limited accelerating voltage does not promote collecting satisfactory spectra in the range above 8-10 keV for many critical elements like Molybdenum, Bromine, Zinc and several others. Further, 15 kV forces the use of spectral peaks at energies less than 5 keV where there are frequently many overlapping elements. As shown in figure below, an electron beam with accelerating voltage up to 30 kV can generate better imaging and better EDS results by matching the beam to the composition of the sample.”
Conclusion:
1. For decades, 15 kV to 30 kV have been used by Electron Microscopists for material analysis
2. 30 kV provides the ability to excite the renowned K-lines in the EDS atomic spectra
3. 30 kV enables accurate element identification
4. 30 kV enables accurate quantification of their concentrations
