For the calculation of the wavelength of electrons the acceleration voltage applied at the SEM (scanning electron microscope) is required.
How big is the error of this value?
Is there any opportunity to estimate the effective landing energy of electrons?
Dear Gert, That is my guess: The kinetic energy of electrons just hitting the target surface is some what above the energy applied to the thermionic (hot) electron coming from the filament entering the hole of cathode and passing through the
anode plates under the action of the accelerating voltage eV , which is about equal to the ionization energy of then hot filament (cathode) material. The uncertainty should be therefore in the range of excess thermal energy kT above the ionization energy of the filament, which dictates the mean kinetics energy of the electrons entering the hole of cathode plate in the electron gun.
Since there are many physical obstacles interfering during the collimation of the beam and directing them towards the surface of the target, actual uncertainty is not east to compute. Any projection of these diverting electric field along the trajectory of the electron beam can change the kinetic energy of the arriving electron to the target surface.
Yes, you are right Tarik. The problem is only that any equipment only reads out the number shown at the SEM. Therefore my question: Does anybody know, how reliable is this parameter? It becomes worse for the pre-interaction with the sample caused by charging.
Dear Gert,
that's a very interesting question and I thought about it some time now. First of all, I thought about the energy spread of the beam. Since there are monochromators for SEMs available, the primary beam normally shows some energy distribution. I found a tutorial (link attached) in which on page 12 a typical energy spread of 0.3 - 1 eV is noted, depending on emitter type. Compared to the typical accelerating voltage this is not much.
Regarding losses (or gainings) during collimation or due to charging, I am not quite sure. I guess that in good conditions without charging the beam energy should be very near to the accelerating voltage, maybe again a few eV off. It would be interesting to design an experiment to test that. At least at low energies up to 5keV, this could be tested by an SEM with beam deceleration technique. When increasing the stage bias more and more, the probe current (current at the stage) should become zero when the bias equals the primary beam energy. However, before I test that, we should wait for an answer from someone who really knows what's going on.. ;)
http://mrl.illinois.edu/sites/default/files/AMC/downloads/AMC-Workshop-2012_Tutorial-7_SEM.pdf
Who calibrates the acceleration voltage? The magnification we are usually calibrating, but the high voltage we simply enter and believe it. I mean, I am not seriously talking about the physics. I simply have no idea, how trustful the entered number is? Like the tilt of the stage. You certainly also don't believe that if there is written 70° the sample surface has this tilt. There the mounting of the sample, or the tilt accuracy and precision of the stage are important factors.
Dear Gert, the acceleration voltage should be calibrated by a voltage transformer VT (Spannungswandler in German). So you have to find the accuracy (or the error) class of the specific VT. It is not smth standard, it depends on the manufacturer of the microscope and the VT used in the model. In the microscope manual and the data sheets has to be mentioned that a "VT class x% is used". For special purposes there are VTs with an error of 0,1% or even lower. I hope, my answer helped.
The typical value of voltage ripple in HV power supplies of SEMs is about 10 ppm. So, for HV of 30 kV, ripple in the voltage is about 300 mV.
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