Lens Aberrations(??)

1
Lens Aberrations(??)
?????? ???? ??? ???? ??? ???? ???? ?? ??? ?? ???
???. ????(spherical aberration) ???(chromatic
aberration) ??(astigmatism)
2
Spherical Aberrations(????)
??? ??? ??? ??(center)? ?? ?? ???(edge) ??? ?? ?
?? ?? focal length? ???. Center long focal
length Edge short focal length Peripheral ??
??? ??? ??? ??? ?? ???? ? ????.
3
Spherical Aberrations(????)
??????? ??? ???? ????? magnetic field? ??? ????
??? ?? ???(peripheral) ???? ??? ? ?? magnetic
field? ???? ? ???? focal length? ????.
4
Spherical Aberrations(????)
????? ???? focal point ? ??? ?? focal point ? ??
?? electron beam diameter? ??? ??? disc of least
confusion(circle of minimum confusion)??
?? ?????? ???? electron beam? ?? ?? diameter?
focal point? ?? ? ?? ??.
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Spherical Aberrations(????)
Disc of least confusion? ??? ?? electron beam?
??? caustic curve? ??. Disc of least confusion?
?? ?? ???? ???? ????.
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Spherical Aberrations(????)
????? ???? ??? ?????? ???? ???? ?? ????. ?? ??
???? ???? aperture? ???? ??? ??? ????? ????? ??
????? ??. ? ?? disc of least confusion? ???? ???
??.
7
Spherical Aberrations(????)
??? ???? ??? ??? ??? magnetic field? ???? ???
focal length? ?? ???? ????? ???.
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Chromatic Aberrations(???)
???? ??? ??? ??? ?? ???? ?? ?? ? ????. ? ??? ??
???? ??? ?? ??? ????, ?? ??? ??? ?? ?? focal
length? ????. ?? ??? ??? ?? ??? ?(blue, green)? ?
?? ?? ??? ? ???.
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Chromatic Aberrations(???)

• ?????? ??? ??
• ??? ??(monochromator)? ???? ?? ?? ?? ??? ?? ???
  ??? ???? ????.
• 2. ?? ?? ???? ??? ???(converging and diverging
  lens)? ?? ???? ??? ??

10
Chromatic Aberrations(???)
?? ?? ??(??)? ??? ??? ?? ??? ???? ?? ?? focal
point? ????. ?, ??? ?? ??? ??? magnetic field??
??? ??? ???? ??? ?? ??? focal point? ??.
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Cause of Chromatic Aberration in Electron Lenses
??????? ???? ???? ??? ????. ???? ??? ?? ??? ???
???? ?? ???? ?? ??? ???? ?? ??? ? ???, ??? 3??
??? ?? ?????.

1. Electron gun?? ??? ?? ??? ???? ??
2. ????? ??
3. Lens current? ??

12
Astigmatism(????)
Stigma point Astigmatism ??? ??? ??? point? ??
? ?? ??, ? ???? ??? ??? ?? ??? ??? ? ??. ???????
???? ??? ?? ??? ?? Stigmator? ???? ??? ?? ??? ?
??.
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Astigmatism(????)
??? ?? ?? magnetic field? ??? ???? ??, magnetic
field? ??? ???? ?? ???? ???? ?? ???? ??.
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Astigmatism(????)
????? ?? ???? ?? focal point? ?? ??? ???? point?
???, disc of least confusion?? ?? ?? focus? ????,
???? ???? ??? ????(blur).
15
Astigmatism(????)
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Cause of Astigmatism

• Nonsymmetrical fields in electron lens
• - ??? ???? field strength? ?? ???? ???
• - ???? ???? ????, winding? ????, pole piece?
  iron? ???? ?
• 2. ??? aperture, lens
• - column, aperture, lens ?? ???? ??? ????.

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Electrostatic Stigmator

• Stigmator ???? ????(astigmatism)? ???? ??
• ???? ???? circular beam?? ???.
• ????(electrostatic), ???(magnetic) stigmator?
  ????

18
Magnetic Stigmator
Electromagnetic field ? ??? ?? ??? ?? ??? ?. -
by right-hand-thumb rule
19
Magnetic Stigmator