Hello Mahdi!

Could you specify the question you have towards your sample material a bit more and what have you tried in SEM to get that answer? What do you want to find out with your material? I understand it is about the detected secondary electron amount when putting the primary SEM beam either on conductive or non-conductive material? Is that correct?

Dear Tom

i am talking about the emmission of the secondary e from the material being obseved.

which has an average energy of 50 ev,

my question is 

whay the insolator materials generate more quantitavely the secondary e

than conductive materials

which alow us to see more brithness on insolator than conductor

Dear Mahdi,

I would even say the opposite: less SEs should be emitted by non-conductive materials. Or you have a very special case of material composition?

Secondary electrons are produced when an incident electron excites an electron in the sample and loses some of its energy in the process. The excited electron moves towards the surface of the sample undergoing elastic and inelastic collisions until it reaches the surface. Here it can escape, if its energy exceed the surface work function, Ew, which defines the amount of energy needed to remove electrons from the surface of a material. One of the major reasons for coating a non-conductive specimen with a conductive materials is to increase the number of secondary electrons that will be emitted from the sample (decrease Ew). The volume of production is very small compared with BSE and X-rays. Therefore, the resolution using SE is better than either of these and is effectively the same as the electron beam size. The shallow depth of production of detected secondary electrons makes them very sensitive to topography and they are used for scanning electron microscopy (SEM).

The average number of SE produced per primary electron is called the secondary-electron yield, δ , and is typically in the range 0.1 to 10 (varying between different materials). For a given sample material, δ decreases with increase in incident energy E0 since the probability of inelastic scattering of a primary electron within the escape depth decreases. SE yield depends on the angle of tilt of the specimen relative to the primary-electron beam, φ. The value is lowest for perpendicular incidence (φ = 0) and increases with increasing angle between the primary beam and the surface-normal. This effect may make cracks appear bright in SE images.

From the above discussion one might conclude that SE production will be independent of the electron beam accelerating voltage. However, only part of the SE signal, the SE1 component, comes from the sample surface. Other components arise from SE produced by backscattered electrons as they exit the specimen (SE2) and when BSE strike the walls of the specimen chamber (SE3). The SE2 component depends on a sample's backscattered coefficient, η, which reflects chemical differences well below the surface. This effect increases with increasing penetration depth, so the sample will appear more "transparent" at higher E0, and less so at low E0. Finer surface structure images can generally be obtained with lower accelerating voltages,

To conclude: non-conducting materials, like oxides appear most likely darker in the SE contrast, since further secondary electron production will be decreased due to charging (stacking up negative electrons in the material). The heavier atom species will appear brighter than lighter elements.