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
𝑆𝑒𝑐𝑛𝑑𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝑦𝑖𝑒𝑙𝑑 =
𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑒𝑚𝑖𝑡𝑡𝑒𝑑
𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
 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
𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝑒𝑛𝑒𝑟𝑔𝑦
= 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝑒𝑛𝑒𝑟𝑔𝑦 − 𝐼𝑜𝑛𝑖𝑠𝑎𝑡𝑖𝑜𝑛 𝑒𝑛𝑒𝑟𝑔𝑦
 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 𝐼0
 A current 𝐼+ is measured at a positive
potential (50 V)
 A current 𝐼− 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 (𝛿) is given as
𝛿 =
𝐼−−𝐼+
𝐼0
Where
𝐼− is the current at negative bias
𝐼+ is the current at positive bias
𝐼0 is the incident current
Drawback
Auger electron measurement (partially)

10. THANK
YOU
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