Secondary electron yield.pptx

A small review of secondary electron yield while bombarding with electrons.

1. Secondary electron yield Factors affecting the yield, energy distribution and methods used A talk given by Muhammad Hassaan Saleem, Ali Raza, Adnan Ashraf, Naeemullah

2. What are secondary electrons Electrons resulting from ionization of the target material by primary electrons. Their emission happens near the surface of the target material.

3. Secondary electron yield Defined as Can be lesser than 1 and can be greater than 1 If greater than 1 then it is sometimes brought to 1 by the positive charge that is induced on the target surface. Depends on target material ( particularly on Z ) and primary electron energy ( i.e. accelerating voltage).  

4. Dependence on incident energy Too low energies can‘t ionize the atoms. Too high energies penetrate instead of ionizing. Most fruitful energies are around 100 to 500 eV (depends on target).

5. Secondary electron energy distribution Typically the energy distribution of secondary electrons look like the following. The energy is around 30eV. The energy is significantly sharp for SE.

6. Why so sharp energy? The secondary electron energy is given as follows The little span in energies is because of the scattering with atoms in the sample target. The span is little because secondary electrons come out near the surface.  

7. Methods of measuring secondary electron yield Everhart – Thornley detector The voltage of faraday cage is kept low so only the SE are attracted. Back scattered electrons would not be attracted (except the ones whose line of travel takes them to the cage). Disadvantage Some BSE are detected, causing us to overestimate the yield.

8. Methods of measuring secondary electron yield The contribution from BSE is eliminated by taking two measurements. Incident electron current is A current is measured at a positive potential (50 V) A current is measured at negative potential (-50 V) * Image from “Measuring the back scattering coefficient and secondary electron yield in scanning microscope ” by L.Reimer and C.Tollkamp (online wiley )  

9. Methods of measuring secondary electron yield The secondary electron yield is given as Where is the current at negative bias is the current at positive bias is the incident current Drawback Auger electron measurement (partially)  

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