I have read some literature about electron irradiation in TEM. The unit of electron dose in these literature is A/cm2, but in our electron microscope, the unit from the viewing screen is pA/cm2. There is a huge difference of magnitude. So, I thought it might be related to the magnification, but I am not quite sure.
Hi Wang,
Here is an excerpt from http://literature.cdn.keysight.com/litweb/pdf/5991-2414EN.pdf (page 7):
"The charging effect is also a function of imaging magnifications in SEM. In practice, reducing the magnification is a common approach indeed to overcome any possible image distortion and harsh contrast related to the charging. Changing the magnification is actually achieved by varying the scan size on the specimen. Assuming the same beam current is used for scanning, a higher current density is the result of a smaller scan size. Experimental measurements suggest that the surface potential is closely related to the beam dose, and the effect is evident when working at a beam with its energy close to the E2 value of the specimen. Thus lowering magnification results in a decreased surface potential and a minimized dynamic charging, and thereby creates less charging effects"
Regards
@Stéphane Breton
Question was about TEM, not SEM
@Wang Wei
May be it will be of some help:
http://iubemcenter.indiana.edu/estimatingElectronDose.html
https://books.google.com/books?id=dXdrG39VtUoC&pg=PA65&lpg=PA65&dq=electron+dose++TEM&source=bl&ots=rOdtOIAklF&sig=xhYuRUPxItggxDcUNQcZ0jSAyJ0&hl=en&sa=X&ved=0ahUKEwjlu-Cgw73UAhUh7YMKHSsaCpQQ6AEIPzAD#v=onepage&q=electron%20dose%20%20TEM&f=false
@Vladimir Dusevich
Thank you for your reply.
When checking out https://books.google.fr/books?id=dXdrG39VtUoC&pg=PA65&lpg=PA65&dq=electron+dose++TEM&source=bl&ots=rOdtOIAklF&sig=xhYuRUPxItggxDcUNQcZ0jSAyJ0&hl=en&sa=X&redir_esc=y#v=onepage&q=electron%20dose%20%20TEM&f=false (section 4.6.C), it appears that one suggested solution to minimize the beam damage is to use STEM imaging. STEM combines SEM scanning principle with TEM detection principle by focusing the beam more finely, so reducing the current density per area unit.
Do you agree with this analysis?
Once again, question was not about SEM (or beam damage), but about TEM electron dose.
As a synthesis of my previous posts, the following results apply both to TEM/STEM (and probably to SEM too):
- "In practice, imaging at higher magnifications correlates whith higher current densities", excerpt from https://www.researchgate.net/profile/Adrian_Balan/publication/258512270_Toward_Sensitive_Graphene_Nanoribbon-Nanopore_Devices_by_Preventing_Electron_Beam-Induced_Damage/links/57dd221408ae4e6f1849a3b3/Toward-Sensitive-Graphene-Nanoribbon-Nanopore-Devices-by-Preventing-Electron-Beam-Induced-Damage.pdf
- Woehl et al. suggest, as a solution to derive the electron dose rate, to divide the beam current by the size of the viewing area ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3482139/pdf/nihms-407982.pdf )
- Smith et al. estimate the electron flux based on the calibration of magnification to current density on the viewing screen ( https://research.physics.unc.edu/lcqin/group/qin/refs/2001-JAP-Luzzi.pdf )
Depending on the equipment and the imaging system, a linear model may probably be convenient as a "rough" approximation.
Regards
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