NetNotes
to put a uniform layer down. We have a Leica ACE 600, and I just measured the thickness of carbon using a double carbon thread using it all up. It gave a thickness of 390 nm, which is plenty. At any rate, putting the carbon layer next to the thin layer gives better contrast between the layer of interest and the protective top layer. Te high Z of Pt or W can make it difficult to see a very thin layer at the top of the surface of your sample. When preparing the sample by FIB, there will be an amor-
phous damage layer on the top and bottom with a thickness that is dependent on the energy of the beam used. Te new- est FIBs are capable of “polishing” with a 2 keV (or lower) Ga beam to minimize the damage layer. It is done by exposing the two surfaces at an angle of 3°. Samples prepared this way are pretty good, but remember, there is still an amorphous layer on both the top and bottom surfaces. Tere are also other ways of removing FIB damage from the surfaces of FIB lamellae using Ar ion milling. In decreasing order of cost: E.A. Fischione makes the NanoMill and the PicoMill, which use a scanning Ar beam to polish the surfaces with low energy Ar. Several manufacturers of ion mills, specifically Gatan and Technoorg Linda, have recipes for removing damage using their tools. Another technique that I patented while working at South Bay Technology is the Plasma Trimming method, which uses the shape of the FIB sample and a bias to create a field that causes ions from a plasma to polish the two surfaces of the sample. Te plasma is generated using Ar gas in a plasma cleaner. I can provide a Plasma Trimming presentation upon request. BTW, you can estimate the amount of damage and depth for differ- ent ions, energies, and angles by using SRIM and TRIM calcu- lations. See
www.srim.org for more information. Scott Walck
s.walck@comcast.net
In regard to your question, Valery and Scott gave you
some good advice. I’ve always referred students to the follow- ing paper:
https://www.sciencedirect.com/science/article/abs/ pii/S030439911200006X. Tere are a lot of ways to skin a cat, as they say, but it can be challenging to get good FIB samples if you’re used to the results generated by tripod polishing and low energy, angle ion milling. Te best results I’ve seen are from FIB samples prepared similarly to the paper above, fol- lowed by a cleanup in a low energy ion mill (Fischione, Gatan, Technoorg-Linda—all are fine). However, the ion mill cleanup is not magic. You can start to etch the liſtout if the angle and voltages are not ideal, or if you mill for too long to compen- sate for a thicker liſtout. Depending on the materials, the liſt- out may also be dipped in a dilute acid or base to thin it, or by using flash electropolishing to reduce the liſtout thickness and remove damaged layers. Regardless of what post-FIB step you use, a thin, flat liſtout with a minimal amount of sputter redeposition is needed, and that’s where that paper gives some valuable tips. Chris Winkler
crwinkler@ncsu.edu
EDS in SEM Microscopy Listserver I have a simple question regarding the detector area of EDX
(EDS) systems in SEM: 2020 May •
www.microscopy-today.com
Clearly the larger the detector the higher the sensitivity if every- thing else is equal, but what if everything else is NOT equal? One clear example: – detector 1: 30 mmÂ2 – detector 2: 150 mmÂ2
, optimal distance to pole piece 10 mm , optimal distance to pole piece 15 mm
Here the large detector is also the furthest from the pole piece! I don’t know how to appreciate the influence of the 2 factors, detector area and working distance, on the final sensitivity of the system. Are the extra $$$ for the larger detector worth it if the working distance is also increased? Stephane Nizet nizets2@
yahoo.com
To compare detector collection efficiency the collection
solid angles of the detectors should be compared. Tese depend on the geometry and size of the detector area and specimen distance. In principle, a larger detector area leads to lower energy resolution. Not exactly sure what you mean by “sen- sitivity.” Higher sensitivity would generally be defined by the lowest X-ray energy a detector can detect, which is also a func- tion of the detector window characteristics. By the way, the solid angle would be the active area of the detector divided by the square of the distance to the specimen. Krassimir Bozhilov
bozhilov@ucr.edu
Let me be sure I understand your question correctly. Your first option of a 30 mm2
working distance. Your second option of a 150 mm2
detector would allow a 10 mm detector
would require a 15 mm working distance to the pole piece. Tat does not say anything about the sample to detector distance which is required to calculate the solid angle. Te 15 mm working distance might be your better option. Tat is similar to the situation we ran into. I wished I had posed the question to the list 9 years ago. We were using a 10 mm2
detec-
tor on a Hitachi 2460N which was setup for a 25 mm working distance for the eucentric height and analytical distance. We were able to bring the EDS detector in below the pole piece so that it was quite close to the sample which gave us a pretty good solid angle. It did mean that the EDS detector was the first thing in harm’s way. A user scratched up one of our alu- minum sample platens when they raised it into the end of the detector and then proceeded to move the sample around. We upgraded to a FEI Quanta with field emission. Its eucentric height was 10 mm which was better for imaging. We knew we wanted to get a large detector in order to maintain as much image resolution as possible when doing EDS so we opted for an 80 mm2
detector figuring we would get 8×more counts at
the same beam current. It turned out that we only got about 3×more. We had ordered the system configured to work at 10 mm working distance. With the design of the pole piece, that meant our detector had to stand about 70% further off than before. So, we gained 8×from the detector area but lost almost 3×due to the greater distance. I have wondered about redoing our system to operate at 15 mm working distance and bring the detector in much closer. It would greatly improve our solid angle. If only doing EDS, I would probably go for it. It would mean different operating conditions for EDS versus just imaging. Tat might confuse some of our users. We also
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