NetNotes


Edited by Bob Price University of South Carolina School of Medicine Bob.Price@uscmed.sc.edu


Everhart-Thornley Detector and Auger Electrons Microscopy Listserver I was wondering if an Everhart-Tornley detector also measures


Auger electrons from the sample? I don’t see a reason why it should not. Can somebody shine some light on my miserable ignorance? Best regards, Stephane Nizet nizets2@yahoo.com


If the low energy electrons make it to the detector, they would


register as noise. Without the right detector these energies are too low to separate for any useful purpose. Plus, at those vacuum levels of an SEM system the adsorbed gases (contamination) would dominate the Auger electron emission. Kimball Skinner kls6_30@yahoo.com


Detect them, yes; analyze them, not so much. How would


you measure their abundance and energy? Te ETD detector was not meant for spectroscopy. I suppose another problem would be differentiating the Auger electrons from other low energy electrons. Warren Straszheim wesaia@iastate.edu


You would need a retarding field analyzer (RFA), cylindrical


mirror analyzer (CMA), or similar setup. Ten a ramp generator to scan the analyzing voltage. Next a function generator and lock-in amplifier to differentiate the signal and to isolate the tiny Auger signal. And yes, the 50eV to 1000eV (0.05 to 1KeV) Auger signal would be dominated by the carbon and oxygen atom adsorbate on the surface. Jim Quinn james.quinn@stonybrook.edu


Consider that the ET detector is made of 2 elements: a polarized


front grid and a photoluminescent screen to convert the energy of the detected electron into light. To increase the signal intensity, the screen is positively polarized (≈ +10keV) to post-accelerate the electrons once they cross the grid, and a photomultiplier (or any suitable type of light detector) behind the screen to convert light to an electric signal. Te front grid is polarized to choose which of the emitted electrons will be allowed to reach the luminescent screen: a positive grid polarization (≈ +200…+300V) will let all emitted electrons to reach the screen. However, the electric field created between the grid and the sample will attract most of the (low-energy) secondary electrons (SE) toward the grid regardless of their emission direction, while backscattered electrons (BSE) (higher energy) will be less sensitive to the field and not affected by this “pumping” effect. Tis is the SE contrast mode, even if some BSE and Auger are also detected. A negative potential (≈ -200…-300V) will repel most of the low-energy SE electrons and let only BSE+Auger+some high energy SE electrons enter the detector if they were emitted toward the detector (a quite low solid angle) and with an energy larger than 200…300eV. Tis is the back-scattered contrast mode. Te absence of “pumping” leads to a much lower signal/sensitivity. Conclusion: the Everhart-Tornley detects Auger electrons, but not only them.


60 doi:10.1017/S1551929521001243


Te relative proportion of SE, BSE and Auger electrons reaching the detector depends on the “pumping” effect due to the grid polarization at low energy (that is, on the selectivity). Terefore, there is a world between “detect” and “measure”. A more detailed answer would require knowledge of the energy range of the Auger electrons being considered. Philippe Buffat philippe.buffat@epfl.ch


Question about the Resolution Limit of Field Emission-Scanning Electron Microscopes (FE-SEM) Microscopy Listserver Although I have some experience in analytical SEM, I am completely


new to the field of FE-SEM and I am not really aware of the true limits of the system. I inserted a gold-on-carbon specimen into the chamber with high vacuum, brought the specimen as near to the pole piece as I could, used the smallest probe current setting and aperture, and imaged using the in-lens detector. I worked at 1–5kV to limit penetration of the beam into the specimen. Using a magnification of 300,000X, I did my best to correct the astigmatism but could not get a sharp image. Is there something I still need to consider, or did I simply reach the limits of the system? Many thanks in advance. Stephane Nizet nizets2@yahoo.com


It depends on the FE-SEM you have. When I used a Hitachi


S4700 at 1kV, the best we could get was around 80,000X. On a Hitachi S4800, at 1kV, we get 800,000X (1nm). On the Hitachi SU9000 the resolution is at least 0.2nm. It sounds like you need a bake-out and perform a series of bombardment sessions to get a better picture. Elaine Humphrey ech@uvic.ca


I would suggest you contact the applications team from your


microscope manufacturer so you can understand the limits and best methods to tune the system for optimal imaging. Tey will be able to provide this advice in general, but it also sounds like you could benefit from a training session. Most of the manufacturers also offer live remote training options. Kimball Skinner kls6_30@yahoo.com


I don’t know what platform you are on: Hitachi, JEOL, TESCAN, or


TermoFisher, but all should give crisp images at 300,000x. With a field emission gun source, all manufacturers should be able to approach 1.0nm resolution at 5keV with the in-lens detector. Te 300,000X relative mag should be easily achievable with good crisp images on a FE-SEM. Have you performed a lens alignment to make sure there is nothing amiss? In a TermoFisher instrument, a lens alignment can show a host of issues including a bad lens supply and control boards that have gone bad. Tere is the lens bottom test where charging sphere is placed in the SEM and imaged at low mag. An image of the pole piece should be seen and appear


www.microscopy-today.com • 2021 November


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