The image of our Amray SEM has a horizontal band across the center third of the image which is slightly out of focus. The top and bottom thirds are fine. This machine, while being fairly old, is new to our institution and I believe some of the maintenance practices have lapsed. I was wondering if it was possible that a weak filament or contaminated optical column or dirty apertures could cause a problem like this.
Any advice would be appreciated.
Thanks. Gary
Is it affected by settings like magnification? Have you ruled out stupid stuff like the video monitor itself? :) Is the change in focus sudden (with respect to vertical location) or gradual? Have you looked at any of the signals that control focus?
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The problem is only evident in the 400 to 1200 magnification range. The change in focus does seem to be sudden. We ruled out the video monitor on the basis that the image is digitized on a computer as well as being sent to the monitor of the SEM and both show the focus problem. We haven't as yet started looking focus control signals. At this point I'm trying to get a feel for whether the lack of maintenance could be the problem or an electronic failure.
Thanks, Gary
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Is this at TV-Scan rate and Slow scan too ! What is the specimen being examined, This is behaving like a Charging artifact. Try another type of specimen, Sounds more like a specimen charging problem to me. What is the Accelerating Voltage. Dirty Column and apertures usually just cause astigmatism problems
Yukio YANO
Retired SEM Tech.
Our operator doesn't use the TV-scan, only the slow scan. Her specimens are organic. She is familiar with what a charging artifact produces on the system. She describes it as horizontal streaks across the image at which point she deposits more gold on the specimen to correct the charging problem. This problem is a very subtle focus issue in a horizontal band across the center of the image. The band takes up acout 1/3 to 1/2 of the image in width. The accelerating voltage is 15KV.
I have an image showing the problem at the following link:
Thanks, Gary
Wow, that doesn't exactly jump out at you!
Is the location/severity magnification dependent?
What does adjusting the focus of the microscope actually do to the image?
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The focus is OK in the affected area but the image is stigmatic! I would hazard a guess the Column needs cleaning. I presume that this uses a Column Liner. I am assuming that the Apertures would affect the overall focus. Whereas the Column would affect a given part of the image, as the beam is swept across the column during the scan.
Anyway I don't think there is any problem with the Focus Current Control.
One other thought comes to mind is a feature on the Philips, SEM500 that varies the Focus Voltage on a Tilted Specimen so that the image remains in Focus despite the tilt of an awkwardly orientated specimen. There should be no way that it should affect the Stigmation of the Beam.
Just for diagnostics, Can you Correct the Stigmation in the middle band of the image, or is it WAY Beyond Normal Limits.
I am going to stare at "blurry.jpg" for a while longer.
Yukio YANO
My Long Stare makes me wonder if I am looking at a small fibre strand in the Column!
My next step would be to use a "ANOPORE Filter" as a Test object I think 0.01 micron would be best, although 0.2 micron would do. Check with the Biology Department for a sample. I think I would have recommended it for sample preparation where dehydration on a membrane filter was indicated.
This is an Aluminum Oxide Filter Media that I used for a High Mag Test Object Sputter Coat a fragment bonded to a stub using Double-coated Tape.
This will Give you an extremely Uniform Mesh that will allow you to see Focus/astigmatism problems over the entire Field of View.
I suspect that at lower Magnification~ 400X, a small segment will show a localized area of astigmatism, (dirty side of Column) At a higher Magnification ~ 1500/2500X the majority of the field would be astigmatic as the beam would sweep across a smaller and smaller portion of the Column, or maybe asymptomatic as it sweeps across an unaffected part of the column.
Anyway I don't really think it is an Electronic Problem, more likely Housekeeping, "Contaminated Liner Tube", Scrub with Aluminum Oxide. The Microtombe people use it for Polishing their Knives, and its cheaper than "Wenol".
Yukio YANO
I am the operator of this SEM in question, so I thought it might be helpful if I joined in on the group. I am currently using the largest spot size to image my specimens - do you suspect any connection with this to my problem? I will plan to obtain a test object such as the one you mentioned and image it to check for focu/astigmatism problems. Thanks for your advice. Glenda
Did you pinpoint the source of your problem ?
Yukio YANO
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