For many years I am using an SEM (scanning electron microscope) with variable pressure (i.e. low vacuum) which dramatically helps if any kind of charging occurs. Often only a very small chamber pressure of 7Pa is enough to stabilize the scan so that no remarkable image shift appears during a long-time investigation by EBSD. Unfortunately, for high-resolution investigations a major argument is the reduced spatial resolution caused by interaction of the beam with the N-atoms which are responsible for decharging of the surface.
An older and newly reinvented technique are local gas injection. Is this technique really better, or only better for very small scans?
I'm using both techniques for many years. I have VP mode on Zeiss, SUPRA 35 VP machine (fill the whole chamber with N) and Charge compensator on Zeiss Ultra+. CC works similar like any other GIS, you bring your sample close to the end of the needle and you expose just the near surrounding of the sample to N flow. You can tune the parameters with precise N flow and GIS needle distance. It is also possible to change the N gas to O for instance or something else....
To compare both techniques: in case of VP, the whole chamber is filed with gas and you can not use all the detectors. Usually just dedicated VP detector and BSD detector, it is also possible to do EDX, but with less signal. Any other detector, like Inlens, TLD or ET-SE will show (due to presence of N atoms all over the chamber) too much noise to get any reasonable signal out. But this technique is very effective regarding charge compensation and sample surface protection.
The local gas introduction system, like charge compensator, works very locally. It has advantage that you can use all the available detectors also ET-SE and InLens (TLD) and it will reduce X-ray signal much less than VP mode. It can be very effective for smaller region of interests and higher magnification imaging and it will give you freedom to chose any of detector and even mix between them. But it has also drawbacks. If you have some really nasty nonconductive material, like paper fibers, textile fibers or something like that, this technique will not be very effective regarding charge neutralization in compare to conventional VP mode. It is possible to play a little bit with flow and increase the pressure, but in any case you introduce gas just from one side and if the surface is very rough you will have some unreachable areas to which gas will not reach.
I attached one chamber snapshot with GIS inserted to have a feeling. And yes you must be very close to the sample with needle.
Regards, Gregor Kapun
Gregor, is the reduce EDS signal not theresult of the booster which is switched of in VP mode? This reduces the beam current by a factor of about 2.
For flat surfaces GIS works always comparable good?
Gert, yes you are right beam booster is off during VP mode in case of Gemini gun and at the same time current is reduced and practical this is one the mayor contribution regarding EDS signal reduction. However i did a comparison measurement on Co standard years ago and luckily i found them in my files. There are listed above:
21 kV, HV, BB=OFF --> 21kV, VP (20Pa), BB=OFF ---> 2,4% less EDS signal
20kV, HV, BB=ON --> 20kV, VP (20Pa), BB=OFF --> 20% less EDS signal
10 kV. HV, BB=ON --> 10 kV, VP (20 Pa), BB=OFF --> 28,6% less EDS signal
All the other parameters (WD, Aparture, EDS acquisition parameters,...) were constant.
Using GIS technology I hardly noted any EDS signal reduction even at 5kV. But of course in that case BB is all the time ON in range up to 20kV (Ultra+).
In case of using GIS on flat surface works comparable good only for user friendly nonconductive samples from magnifications 500x up. For nasty samples like polished polymer composite samples or even worse lacquered papers (quality control and failure analysis of passports for instance) GIS didn't worked well. In such a case charge is perfectly compensated locally, but it is still present outside the GIS flow area and such situation will give you hard time for your analysis. The same samples were also challenging in conventional VP mode, but with increase of chamber pressure from 20Pa to 40 or 50Pa the problem was easily solved.
So to bring story short. From charge compensating view, in practice GIS works comparable good on most nonconductive samples with flat surface which you can at the same time easily process using conventional VP mode with 20 or maybe max 30 Pa of nitrogen inside the chamber. But GIS charge compensator can not compete with 60Pa or 120Pa nitrogen pressure in VP mode.
Regards, Gregor
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