CM12 Alignment 2

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CM12Alignment 2
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Column schematic
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Lens and Deflector Schematic
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Focus

• With no objective aperture, best focus is at
  minimum contrast
• Image wobbler works at low mag.
• Fresnel fringes are used at intermediate mag.
  (30kX 200kX)
• Amorphous grain (power spectrum/FFT) is used at
  high mag.

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Fresnel fringes
Under focus
Over focus
focus
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Focus

• Fresnel fringes are best seen when Obj. aperture
  is in place.
• Underfocus less current than focus (CCW)
• Overfocus more current than focus (CW)
• Underfocus gives high contrast edges but not best
  sharpness. Watch out!

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Over/Under Focus
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Focus knob
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Focus video
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Deflectors in the Column
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Deflector coil
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Deflectors

• Lorentz force accelerates electrons.
• Deflectors impart a change of direction

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Double Deflectors
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Pivot point concept

• 1st deflector - Translate
• 2nd deflector - Tilt
• We wish pure tilt and translate ? What do we do?

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Beam Deflection Coils
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Double Deflection Coils
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Pivot Point Balancing

• Focus image - IMPORTANT
• Focus C2 and center illumination (undersaturating
  the filament helps).
• Select ALGN and beamcoils Pivot Pt X.
• Use MF knobs to superimpose spots.
• Select beamcoils Pivot Pt Y and repeat.

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ALGN button, MF knobs
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Pivot Wobblers
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Pivot point video
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Objective - Optic Axis centering

• Objective lens is the most important lens for
  resolution.
• Most important aberrations (spherical, coma,
  etc.) are generally 3rd order (i.e. ? a3)
• We want the beam to be on the optic axis of the
  objective lens (i.e. a0)

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Optic Axis Centering

• Several methods
• Current centering
• Voltage centering
• Coma-free
• We use Current (Rotation) centering
• If the beam is inclined to the optic axis (a?0),
  the image will shift when varying the objective
  current (focus).

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Rotation Centering

• Set mag to 60kX.
• Focus image and set a distinctive feature to
  center.
• Center illumination Important!
• Select ALGN ROT CENTER
• Minimize image movement using MF knobs. (Center
  of image should be stationary.) FOCUS STEP
  adjusts wobble magnitude.
• Refocus image.

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Objective Lens Centering
Rotation Centering
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Rotation Centering video
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Caution

• While in DIFF mode, the MF knobs act as
  diffraction shift (post-specimen image
  deflectors). These do NOT change the beam angle
  through the Objective lens.
• Do NOT use the MF knobs to center the
  diffraction pattern unless Rotation Centering has
  been done first.

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Objective Aperture

• Located in Back Focal Plane (BFP) where the
  diffraction pattern is located.
• Improves contrast
• Limits spatial frequencies (i.e. resolution)
• Allows Bright Field (BF) and Dark Field (DF)
  imaging

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Obj. Aperture insertion

• Insert SA aperture (limits field of view)
• Select D mode (diffraction)
• Insert and center Obj. aperture around central
  spot (not screen center)
• Focus the Obj. aperture (sharp edge)
• Release D mode
• Remove SA aperture

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Henks aperture rule

• Never move an aperture you cannot see.
• (i.e. dont work blind)
• Objective aperture is in the BFP, therefore you
  see it in diffraction mode.
• SA aperture is in the intermediate image plane,
  you see it in image mode.

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TEM Imaging Optics
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Apertures
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Aperture Drive
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Mode selector
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Objective aperture video
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Objective Stigmation

• Same concept as condenser stigmation, just harder
  to see. (i.e. azimuthal variation of focus)
• Use what your sample gives you.
• Objective aperture contributes to astigmatism,
  thus insert it before stigmation.
• Use Fresnel fringes, amorphous grain (power
  spectrum/FFT)

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Objective Stigmation

• Set microscope to high mag (60 100kX).
• Center illumination and overfocus C2.
• Find line foci and set focus between (circle
  of least confusion) with obj. focus.
• Select STIG Objective and adjust with MF
  knobs.
• Readjust focus and look for image distortion.
  Readjust stigmators

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Fresnel fringes
Under focus
Over focus
focus
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Fringes show astigmatism
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STIG button, MF knobs
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Obj. stigmation
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Objective stigmation video
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Basic alignment

• Should do every time you start scope
• Should take only a few minutes to check/tweak.
• Essentially the same on all scopes. Memorize!

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Standby condition I

• Filament off, HT off at 120kV.
• Zero a and b, center sample tilt drives, center
  stage
• Objective and SA apertures out, condenser
  aperture at 1 (150µm).
• Set Mag to 6-10kX

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Standby condition II

• Spot size 1, BF mode
• Film camera loaded and fresh film in desiccator.
  Film supply counter reset.
• Panel lights and room lights off.
• Log time on sheet

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Lab 2

• Familiarize yourself with the controls and
  operation of the microscope and practice setup
  and alignment.
• Double tilt operation and balancing
• Fresnel fringes, focusing and stigmation
• Objective aperture