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my project update to 6_12_10 - Introduction Since the...

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Introduction Since the scanning electron microscopy (SEM) techniques start just before four decade it's already has used extensively to characterize the surface properties of different specimen, but this cute techniques will be useless if we do not recognize phenomenon like charging and how we should deal with it. Charging of an insulating specimen or surface is coming as a result of interaction between the beam electron and the specimen 1 . In other words when the beam bombard the specimen the electrons loss their energy and as a result they are absorbed in the specimen, so in the case that we have conductive specimen the electrons that are absorbed will go through the specimen stage, but in other case when we have nonconductive specimen the electron stop in it, and then the charging occurs. Fig (1) Electric flow in nonconductive specimen 2 We can determine the charging state of the specimen by SE (secondary electron) yield ,δ,, it's mean when SE yield is equal to one, the SE current is equal to the primary electron current, so the specimen is neutral ,no charging ,. And the specimen is positively charge when it's greater than one and negatively when it's less than one. For some kind of specimen the δ depend on two factors 1 , the first one is the energy of the primary electron in other words the accelerating voltage, and the second one is the incident angel of the primary electrons with respect to the normal of the surface of the specimen. The relationship between the SE yield δ and the primary electron energy on other words the accelerating voltage V PE is observed when we measure the SE yield δ as a function of the accelerating V PE we get 1 fig (2):
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Fig(2) Schematic diagram of the SE yield δ as a function of the accelerating voltage V PE pf primary electron. From the Fig(2) we see a maximum at (δ m , V m ) , and we have two critical voltage V cr1 and V cr2 , correspond to two situations of the SE yield when it is equal to one. We pay attention that we can get a situation that emitted SEs can be more than primary electrons when we work with accelerate voltage between V cr1 and V cr2 , therefore the specimen will be positively charged . On the contrary the specimen will be negatively charged when we work out of this rang (V PE >V cr2 or V PE <V cr1 ). Now we look at how the SE yield δ depends on the tilting angle. The SE efficiency , β, is a function of the tilting angle , , Ө 3 , β is given by Dekker 4 as follows: is a current ratio of SE to incident at the surface, ε an energy to excite the SE, (-dE 0 /ds) an energy loss per unit length along the incident electron path, a mean free path of SE and z a depth from the specimen surface 3 .
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