Electron microscopy
  
Comparison between Different Electron Sources
- Practical Electron Microscopy and Database -
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Table 1409 lists the general requirements and characteristics of various electron sources used in EMs. For instance, it is not possible to generate enough current in the beam to produce a satisfactory image if the beam size with W filaments used in SEMs drops below 10 nm.

Table 1409. Comparison between different electron sources.

Type of source
Cathode material
W
LaB6
ZrO/W(100)
W(310)
Cathode radius (ds, µm)
~40 - 50
~1 - 10
0.01 - 0.02
0.005 - 0.01
Cathode regeneration
Not required
Not required
Not required
Every 6 to 8 hours
Effective source radius (d's, µm)
≈15
≈5
≈0.015
≈0.0025 (Virtual source)
  15   5
Energy spread @ cathode
(ΔE, eV)
0.59
0.40
0.31
0.26
Energy spread @ gun exit (ΔE, eV)
3 eV at 2800 K
1.0
0.5 - 2 & 0.5 in most cases
0.3
Work function φ (eV)
4.5
2.7
2.8
4.5
Operating temperature ( T, K)
2700
1800
1800
300
Electric field E (V/m)
Low
Low
≈108
>109
Emission current density
(Je, A/m2)
≈104
≈106
≈107
≈109
Total emission current (µA)
200
80
200
5
±0.05
±0.1
±<0.3
±3
Maximum probe current (nA)
1000
1000
10
0.2
Beam noise (%)
1
1
1
5 - 10
≈109
≈1010
≈1011
≈1012
Vacuum (Torr)
~10-6 - ~10-3
<10-6
<10-7
~10-10
Lifetime (hours)
100
1000
104
104
Sensitivity to external influence
Minimal
Minimal
Low
High
Lowest Low High (Then, better HRTEM imaging) Highest (Then, better HRTEM imaging)
Year of the first SEM system
1972
 

 

 

 

 

 

 

 

 

 
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