In SEM, it is written that the Number of secondary electrons can be greater than the number of incoming (primary) electrons.
Again, I read in various sources that the Maximum yield (No. of emitted electron per primary electron) is 1.
In Goodhew's Book "Electron microscopy and Analysis", total electron yield is shown more than 1 between the accelerating voltage 1 and 5 keV (Fig. 5.8 (b), page number 128).
Kindly explain which one is correct?
The secondary electron yield depends on material and the accelerating voltage.
In insulators at a voltage around 1 kV the secondary yield can be larger than 1. However, when the secondary yield is larger than 1 a positive charge is left on the sample and the electric field generated by this charge reduces the electron emission until the neutrality is reached. In this situation the yield is 1. In practice if you send only 1 electron the yield is larger than 1 but in steady state when a constant flux of electrons is impinging on the sampe the observed yield is 1 owing to the effect of the electric field generated by the Surface charge.
@ Marco: Considering BSE, is there not in general the opportunity that the overall electron yield (BSE and SE) can be higher than 1, e.g. if you tilt your sample to a higher angle?
Probably in conducting samples by enhancing the Bse yield by tilting a sample made of heavy elements it is possible to have an overall yield larger than one. I think that with insulating samples this is hindered by the surface charging.
Yes, as Marco alluded to, as the primary electron energy decreases, the SE yield increases. This is due primarily to the decreased penetration depth which means more SE are produced near the surface where they can escape (remember every PE can make many, many SE). This yield can exceed a yield of 1 with a peak typically between 1 and 3 keV for most materials. The BSE yield is relatively independent of primary electron energy. However, as the PE energy is decreased further the yield decreases as the number of SE produced per primary electron decreases dramatically.
This is not strictly limited to insulators, but you can do a simple exercise to "see" this with an insulating sample. Image the sample at say 5000X and then rapidly drop the mag to 2000X. If the sample is above the charge neutrality point (yield of 1), you will see a bright square indicating negative charging. If it is below, you will see a dark square indicating positive charging. This can be of practical benefit if you want to image an uncoated insulator in a high vac SEM.
The Goldstein SEM text covers this pretty extensively BTW.
Some supplementary electrons are also going to ground generating the absorbed current.
- Badges
- Science method