SEM Imaging and the Benefits of Using Low kV

insights from industryVern RobertsonEPMA Product ManagerJEOL USA

In this interview, AZoM speaks to Vern Robertson, EPMA Product Manager at JEOL USA, about the benefits of using a low kV in SEM imaging.

Can you give a brief overview of your company and the types of products you produce?

JEOL is a world leader in the manufacturing of scientific equipment. At its core are scientific instruments including its more widely known: TEM & SEM, as well as NMR, Mass Spec, Scanning Auger, XPS, EPMA, FIBs, semiconductor equipment (mask and wafer), and sample prep equipment. JEOL Ltd. was founded in Japan in 1946 and employs roughly 3500 employees worldwide in 32 countries. In 2020 JEOL celebrated 54 years of SEM production, 60 years of EPMAs, 64 years of NMR, 56 years of Mass Spec, and in 2019, 70 years of TEMs. JEOL USA serves and supports all customers in The Americas.

What is the difference between a good SEM image and a great SEM image?

A good image provides information that will answer the question. That is the reason that the image is taken. It also should have all of the data information that allows the client or end-user to interpret the image such as kV, probe current, magnification, detector and its position, and most importantly a scale bar.

What makes a great image is all of the above plus the right set of conditions of the image. A good image can have nothing wrong with it, but not answer the question you are trying to answer! The other quality of a great image is an aesthetically pleasing image with no artifacts (charging, edge effect, contamination, beam damage, bad sample prep, poor brightness & contrast, poor signal to noise ratio) and visually “framed” well.

What advantage does low kV bring to SEM images?

Low kV minimizes the artifacts normally seen at higher kVs: charging, edge effect, contamination, and beam damage. Most importantly, it provides surface-sensitive imaging. Murphy’s Law says, 'If you can’t see it you can’t image or analyze it, and at high kV, just because you see it doesn’t mean that it is all you are analyzing.'

Why does a low kV lead to less sample damage on biological or polymer samples?

In the past, theoretical calculations said that there is more sample damage at low kV due to the smaller interaction volume. That has been shown not to be true. Low kV electrons are typically traveling at a lower velocity and have a lower beam current density than high kV. Therefore, you are putting fewer and less energetic electrons into the sample. This also prevents heat build-up in the sample which can melt polymers and destroy some biological samples. 

Can low kV improve EDS and WDS spatial resolution? If so, how?

Absolutely yes! As the accelerating voltage lowers linearly, the X-ray generation volume decreases exponentially, so even a small reduction in kV can produce an increase in spatial resolution. There are three caveats: 1) at very low kVs the number of X-ray lines decreases 2) Low kV isn’t the only way to improve spatial resolution. Choosing the correct overvoltage is key (beam kV vs. energy of the X-ray line). 3) The type of SEM (W/LaB6/ FEG). When you get to low kV and high beam currents which are needed to prevent a signal intensity decrease with a W or LaB6 SEM, the probe diameter becomes bigger than the X-ray generation volume so you reach a point of no return.

The solution to this is an FEG source. This provides low kV with a very high probe current and maintains a very, very, small probe diameter overcoming all but the limited number of X-ray lines you can analyze. 

Why would using a low kV for SEM imaging get overlooked?

In the 'old days' SEMs didn’t have low kV imaging and analysis capabilities. Also, higher kVs produce more signal so it was easier to image. Many people who occasionally use an SEM don’t want to be bothered to change the settings of the SEM that they were trained on. (Typically, 15 kV, moderate beam current, medium OL aperture, medium working distance, etc.). Also, in the past, SEMs had significantly lower imaging resolution at lower kVs, so the user shied away from it. Today’s SEMs, even entry-level, can image and perform microanalysis at low kV without much effort. We just need to convince them that this is the right thing to do. 

Which instruments are best for taking SEM images at a low kV?

While FEG SEMS are “the best” for low kV and ultra-low kV (<1 kV, down to 10 V) all currently manufactured SEMs are very capable of low kV imaging. (1-5 kV). 

What’s next for JEOL? 

JEOL will continue to push the envelope in developing microscopes, imaging and microanalysis detectors, and software that will make this technology available to every research scientist and technician that would benefit from analyzing their samples with an SEM and continue to provide sales, service, and applications support to all of our customers.

About Vern Robertson

Vern has been with JEOL USA for 34 years and was appointed EPMA/SA Product Manager in 2016 and will continue as SEM Technical Sales Manager, providing in-house and in the field, technical product support and customer applications support. Vern served as the Senior SEM Applications Specialist at JEOL beginning in 1986,  was appointed National Laboratory Manager in 2004, and FEG SEM Product Manager in 2005. Vern received his B.Sc. in Geology from the University of New Hampshire. His prior industrial experience included eight years of consulting in an independent testing lab specializing in industrial and environmental problem solving, with responsibilities including polarized light optical microscopy, and atomic emission and absorption spectroscopy, SEM with EDS/ WDS, and X-ray diffraction. Vern was a Corporate Liaison member of the MAS (Microanalysis Society) Council for over a decade.